WATER ORIGIN AND STORAGE

There are 8 tube wells and two pumping stations (SHAHPUR and SAWAN) which fulfill the water demand of ARL. Some of these tube wells are within the refinery and some at outside refinery but within the refinery land. The water is pumped from tube wells by means of multistage submerged pumps. Total water pumped from these tube wells and water pumping station is estimated to be within the range 7,50,000 – 9,00000 gallons/day, Whereas, the pumping rate is estimated to be 30,000 gallons/hour. All the water inlet lines are metered and readings are noted to keep the record on the amount of water being stored at the refinery. Direct pipelines from water pumping stations are also provided where needed (other than storage), for example, cooling towers.There are 4 water reservoirs at ARL to store this water.

Reservoir 1The reservoir stores the water for the purpose plant service. The stored water is sprayed in the reservoir as well to lower the temperature naturally. The height of the reservoir is 18ft 6inch and water capacity of every inch is 9,000 gallons, so the total capacity of the reservoir is approximately 2,000000 gallons. Both reservoir 1 and 2 are located at water pump house.

Reservoir 2Water stored in this reservoir supplies water to the fire network. It has the maximum capacity among all the reservoirs at ARL. It has the total depth of 18ft 6inch and every inch of this reservoir can store 27,000 gallons. So the total capacity of reservoir is approximately 6,000000 gallons. The water stored in this reservoir, due to its large capacity, may be used for some other purposes like flushing the tank, checking the capacity of the tanks and to replace the service of reservoir 3.

Reservoir 3The reservoir is located near boiler house and offers water to Plants, Boiler house, Cooling towers, Compressors and AGL (Attock generation limited). The total height of the reservoir is 14ft 3inch and the water stored in every inch is 10,000 gallons. The total capacity is approximately 1,700000 gallons.

Reservoir 4Drinking water is stored in this reservoir. It is 10ft 4inch high and 2,500 gallons in every inch can be stored. The total capacity is 310,000 gallons. It is located at DWTP (Drinking water treatment plant).

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STEAM GENERATION BY BOILERS

Water is relatively cheap, and to use water for energy generation would make process economical. Steam generation by boilers is an effective way of using water as the source of energy and it is common in almost all the industrial operations. Boilers are fired by fuels like natural gas, oil, refinery gasses, wood, coal and etc. At ARL, Gas and oil are used. The heat value of natural gas is 1000 BTU/ft3 and 19,000 BTU/lb of furnace fuel oil.A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes steam. The steam or hot water under pressure is then usable for transferring the heat to a chemical process, electrical power generation and heating. When water is boiled into steam, its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder.  This causes the boiler to be an extremely dangerous item that must be treated with utmost respect.A boiler works on the simple mechanism of heating water till it becomes steam but it is not as simple as it looks. The water to be used must be free of impurities, dissolved solids, suspended solids and gases as these impurities may harm the structure and will reduce the boiler life in turn. Therefore, the boiler feed water treatment is an important aspect in this regard. Raw water from reservoir 3 after getting some treatment gets into the boilers.

Raw Water TreatmentThe salt of calcium and magnesium are present in the raw water. These salts produce hardness. They are present in the form of bicarbonates, sulfates, chlorides and nitrates. If these salts are not removed from raw water, they decompose on heating and form scale deposits in the internal surface of boiler water tubes, pipe lines and other equipment, thus creating restriction in the flow due to increase in pressure drop. Silica is also present in the raw water in a very small quantity. Even a few parts per million of silica can cause scaling problem in the boilers. These scale deposits reduce the rate of transfer of heat. The heat will increase the temperature of the tubes and a tube rupture takes place in the end.The oxygen in the boiler feed water becomes extremely aggressive when heated, in addition to boiler tube corrosion, further damage may occur to steam. Dissolved oxygen in boiler feed water intensifies the corrosion of boiler tubing, by accepting free electrons liberated from the surface of the boiler tubing.

Fe - 2e ----> Fe2+

O2 + 2H2O + 4e- --------> 4OH-

Fe2+ + 2OH- --------> Fe (OH) 2

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F ILTRATION Filtration is a technique by which we remove the suspended solids in the raw water and it is the first step of boiler feed water treatment. Water from reservoir 3 is pumped by means of two centrifugal pumps out of which one is running and the other is on standby. Water is pumped to the top of PERMUTED filter. There are 3 filters in parallel and all the filters are in operation all the time. The filters have anthracite (activated carbon or activated charcoal) bed in it. Activated charcoal is used for this purpose because its surface is porous and it can adsorb suspended particles efficiently. Open pipe is provided at top for proper distribution of water during operation and back washing while nozzles at the bottom of the filter. The water obtained from the bottom of the filter is free of suspended solids but it will have no effect on PH and dissolved solids. The filters are kept in service till the pressure drop across it is 5psi. When this pressure difference of inlet and outlet increases from 5psi it means that the filter requires backwashing.

W ATER S OFTENING The removal of impurities such as calcium, magnesium, iron and silica which can cause scale is known as water softening and the equipment used for this purpose is called water softener. Major disadvantages faced because of hardness are

Hinders the ability of soap and detergents to form foam. Precipitate at the surface of hot body and causes scaling Corrosion.

There are 3 water softeners out of which one is kept in service, one is on standby and one (exhausted) is going through the process of regeneration. Water from filters is directed to the water softeners to remove the hardness of water. The softening process is based on the phenomenon of ion exchange to remove the hardness present in the filtered water. The softening media used is called resin or sodium zeoilite. The resin has the ability to attract positively charged ions because of the presence of zeolitie, acquires a negative charge itself, along with sodium having positive charge. Nozzles are provided at the bottom of the softening vessel for water outlet which make sure that resin retains in the vessel and do not wash away during any operation. 88 nozzles are provided in each vessel.The resin is in the form of small beads. Half of the vessel is filled with clear water before charging resin. 1826 liters of AMBERLITE (resin) is added into the half water filled vessel. Addition of resin must be after filling of vessel to make sure that the beads remain un-ruptured. Water to be treated adds to the vessel from the top and passes through the resin bed. While passing, calcium and magnesium ions are strongly attracted by the beads. The hard ions (i.e. calcium, magnesium) replace the soft ion (sodium) during this passage of water from top to bottom.

NaX + MgCO3 --------> MgX + NaCO3

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This is how magnesium or calcium ion keeps on taking the place of sodium ion. Accumulation of these hard ions continues. Test of the water outlet from the softening vessel are carried out on regular intervals of time. The softener is kept in service till the outlet PPM of hardness does not increase by 0.2 to 0.3. As the resins become loaded with undesirable cations and anions they gradually lose their effectiveness and must be regenerated. When hardness gets higher than 0.2 to 0.3 PPM, the softener is subjected to regeneration and the standby softener is taken into service. This is done by Programmable Logical Control (PLC). Regeneration is the process in which sodium ions are refilled in the vessel by replacing hard ions. 1st step to this regeneration process is the backwashing. Vessel is backwashed at 20m3/hr for 15min. In backwash, water is forced up through the resin bed to remove dust accumulated during the service run. It also redistributes the resin bed to prevent channeling.

Back washing is followed by the addition of 20 to 30% NaCl solution. This facilitates the regeneration of the sodium ion.

MgX + NaCl --------> MgCl + NaX

This process continues for 30 to 40 min at 3.3m3/hr. Next step is the rinsing. Rinsing is the process in which water is forced over the resin bed to remove excess salts from the surface. Slow rinsing for 15 to 20 min and fast rinsing for 10 to 15 min. Rinsing is carried out at the regular flow (top to bottom). Total time taken for regeneration is one and a half hour approximately and the softener is ready to be used again. Drain of the softener is directed to the horizontal vessel near condensate tank.

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There are some certain things which should be always taken into account to improve the life of the resin. Water of suitable quality (free from suspended material and turbidity) must be used. Dehydration must be prevented by keeping some level of water in the softener vessel. For microbiological protection, the factors like organic matter, temperature and PH must be taken into account. NaCl to be used for regeneration must be on the recommended quality specifications by the designer. The soft water is now considered to have oxygen impurities in it only therefore, it will be treated in the de-aerator but before that, some heat is exchanged with the soft water in the heat exchangers to increase its temperature to about 90oC because the solubility of water with oxygen is minimum at this temperature and it would facilitate the operation of the de-aerator. From softener, some of the soft water is stored in the tanks for emergency use while some water is directed towards the heat exchangers by means of low lift pumps. Low lift pump, pumps the water to HCU, power plant and cooling tower, in addition to the heat exchangers. Heat exchanger uses steam of LUMMUS and flash vessels to exchange heat with the soft water and the condensate of these heat exchangers is transported to the Reservoir 1 (for LUMMUS). Soft water is finally injected at the top of the de-aerator.

D E -A ERATOR De-aeration is used to remove the non-condensable gases such as oxygen, carbon dioxide and ammonia from boiler feed water and the equipment used for this purpose is called de-aerator. In particular, dissolved oxygen in boiler feed water will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). It also combines with any dissolved carbon dioxide to form weak acid that causes further corrosion by lowering the PH of the system. The removal of these gases makes boiler feed water less corrosive.De-aeration is based on two scientific principles. The first scientific principle that governs de-aeration is the relationship between gas solubility and temperature. Gas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature. The second principle is Henry's Law. Henry's Law verifies that gas solubility in a solution decreases as the gas partial pressure above the solution decreases. A de-aerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feed water. The feed water is sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation. Spraying feed water in thin films increases the surface area of the liquid in contact with the steam, which in turn provides more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes it from the feed water. The liberated gases are then vented from the de-aerator. Here at ARL we have tray type de-aerator. The horizontal tray type de-aerator has a vertical domed de-aeration section mounted above a horizontal boiler feed water storage vessel. Boiler feed water along with condensate return enters the vertical de-aeration section above the perforated trays and flows downward through the perforations. Low pressure de-aeration steam enters below the perforated trays and flows upward through the perforations.

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Vigorous mixing of water along with the scrubbing by steam removes the non-condensable gases. Water is broken down into small droplets by means of trays which increases the surface area of water exposed to stream. This allows the gases to escape easily and the heated water gathers in the storage below the dome of de-aerator. There are three alarms for the level indication of de-aerator storage in the vessel below the dome shape.

o 88% High Level Alarm

o 50% Low Level Alarm

o 30% Low Low Level Alarm

Pressure of 0.05 to 0.07 kg/cm2 is maintained in the de-aerator to make sure that the separation is efficient. The oxygen level at the outlet of de-aerator is reduced to 5-10PPB.

Advantages Because of minimal pressure drop through the tray, less steam is required. Operates efficiently under high load.

Disadvantage Sale price can be 15% higher. Must maintain a level for proper operation.

The water from the bottom of the de-aerator is pumped by means of “multistage boiler feed water pumps” to the boilers after passing through control valve. The mechanical operation is followed by chemical treatment. Two chemicals are added at the suction line of feed water pump. One is NALCO 72210 (alkaline, phosphate based) and the other is NALCO 19 PULV (sulphite based) . The function of NALCO 72210 is to act as an anti-scalent for boiler and for the pipelines as well. While the NALCO 19P is used for the removal of any other PPM of water left in the boiler feed.

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Addition of both of these chemicals is in the diluted form. Its solution with water is prepared in the small containers and is pumped to the suction line by means of diaphragm pumps. As estimated quantity of 17.6PPM of NALCO72210 and 5PPM of NALCO19P is added to the boiler feed water but this quantity is just an approximation. It solely depends upon the flow rate. Moreover, NH3 is also added to the boiler feed water to lower the PH as NALCO72210 is strongly alkaline (PH>13). NH3 is used so that, on its way back, the condensate should not corrode the lines due to the higher alkalinity. The treatment ends here and the feed water is ready to be injected in the boiler. Test are conducted in the water lab and it must comes in the following limits to be suitable for treatment in the boiler

Test Description Specifications

PH 8.0-9.5Total Hardness <1.0

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BOILERS

ARL is equipped with 5 boilers at boiler house to meet the requirement within the refinery premises, 3 of which are known as DESCON boilers and 2 are called B&W boiler (BABCOCK & WILCOX). DESCON boilers are 3 pass, forced draft, fire tube, saturated steam boilers which produce steam at the temperature of 190oC while the B&W boilers are induced draft, water tube, super heated steam boilers which produce steam at 270oC. The steam requirement in summer is 18ton/hr while in winter it increases to 22ton/hr. The capacity of DESCON boiler is 12ton/hr while B&W can produce 9ton of steam per hour. Usually, two DESCON boilers are kept on standby (out of which, one may be going through maintenance) and one in service whereas, both B&W boilers are kept in service.

D ESCON B OILER These boilers have three passes and it may be charged with oil or gas but only one at a time (i.e. either oil or gas). The air for combustion in these types of boiler is forced in due to which these are called forced draft boilers. These are named fire tube boilers because the fire, flame and flue gases travel in the tubes while the water is at the shell side.

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Water from the feed water pumps comes to the DESCON boiler by means of control valve, so that to maintain a water level in the drum which is very critical in boiler operation. The water do not comes in the drum directly. Water is provided with a preheat coil which takes some turns within the shell side to preheat the feed water which is then directed to the ECONOMIZER. The flue gases, having passed through the main boiler will still be hot. The energy in these flue gases can be used to improve the efficiency of the boiler. To achieve this, flue gases are passed through an economizer from which the feed water is also passed. The remaining heat contents in the flue gases are transferred to the feed water.

An economizer removes additional energy from the stack gasses by circulating the de-aerated boiler feed-water through a series of bent tubes in the stack. This is a simple diagram of a boiler from which flue gases on their way to chimney transports its heat to the feed water. The economizer is a heat exchanger through which the feed water is passed. The feed water thus, arrives in the boiler at a higher temperature. Less energy is required to raise the steam. This results in a higher efficiency. In general, a 10°C increase in feed water temperature will give an efficiency improvement of 2%.From economizer, the water comes into the shell side of the fire tube boiler. Flame extends in the first pass in the main flue duct followed by the internal turning chamber which have 2nd pass tubes after it. The 2nd pass tubes ends at the external turning chamber and the hot gasses after external turning chamber, are carried away with 3rd pass tubes. These tubes lead the flue gases to the flue outlet box which channels it to the economizer. There are two manholes, one at the water side and the other at the steam side. There is a peep hole as well to look at the flame behavior. This peep hole is provided with an air line to reduce the temperature so that there may not be any rupture to the glass because of a high temperature.

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General Design DataSome general design information of DESCON boiler is given below

Parameter ValueBoiler Steam Pressure 12Kg/cm2

Boiler Steam Temperature 193oCDrum Level 60%

Fuel Gas Pressure 2.5Kg/cm2

Fuel Oil Temperature 75-90oCEconomizer Inlet Temperature 94-97oC

Economizer Outlet Temperature 105oCFlue Stack Temperature 165-168oC

Thickness of Boiler Drum = 20mmThickness of Flue Duct = 14mmThickness of End Plates = 28mmMaterial of Flue Duct = 17Mn4

Type of Flue Duct = CorrugatedHeating Surface Area = 315m2

Flue Gases at Furnace Outlet = 1207oCFlue Gases at Boiler Outlet = 235oCFlue Gases at Economizer Outlet = 150oc (on gas) & 175oC (on oil)

Blow DownThe boiler feed water entering the boiler does have some PPM’S of TDS (Total Dissolved Solids). This amount of TDS keeps on increasing as more and more water is converted into steam and TDS accumulates at the bottom whereas the steam goes up. If the amount of TDS keeps on increasing, it can cause scaling. Blow Down is the technique to overcome this problem. It is the removal of some quantity of water which will take the TDS with it and hence lowering the total amount of dissolved solids in the drum. Blow downs are of two types. One is the continuous blow down and the other is intermediate blow down. In continuous blow down, the water keeps on running out continuously whereas intermediate blow down is used when the amount of TDS shoots suddenly because if this amount increases suddenly continuous blow down will take a lot of time to settle the issue. In intermediate blow down, the blow down valve is opened manually for a period of time in which a large amount of water with TDS is taken away. Blow down depends entirely on the amount of TDS which is obtained by the test conducted in the water lab.The blow down of the boiler has the PH of 13. All the blow downs gather in the flash vessel from where it is subjected to the blow down pit and to the shell side of the heat exchanger. PH of blow down is maintained in the pit by means of addition of H2SO4. For this purpose, 98% acid solution is pumped by means of a diaphragm pump to the blow down pit. It is drained from the pit after the treatment with the acid in order to meet the environmental and quality standards and it must satisfy the following limits

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Test Description SpecificationsPH 9.5-12.5

Total Hardness <0.2Caustic alkalinity 500-1500

Total Dissolved Solids 2000-3500Fe <1.0

SO3 25-60PO4 20-60

CarryoverThe boiler water that escapes from the boiler along with the steam is called carryover. Carryover of contaminants into the steam can be caused by

Foaming (Formation of foam bubbles due to certain chemical condition). Misting (fog like condition due to the breakage of bubbles, like carbonated beverage). Dissolving or vaporizing of specific contaminant.

Steam separation equipment is designed to remove entrained boiler water from steam. This is accomplished by baffling. This knocks the water droplet out and allows the dry steam to continue out of the boiler.

Soot BlowingSoot blowers are used to give off steam through a rotating pipe with holes (nozzles) on it and this steam is used to clean the surface of boiler tubes. The pipe keeps on circulating to remove any kind of ash particle present on the surface of the pipes having water in it. The nozzles are positioned in such a way to blow between the lanes of the economizer tubes to give maximum cleaning. The tube giving off the steam is rotated by means of a motor and an air line is provided to cool down that motor. Soot blower is present at the economizer which is the only part where the water is present in the tubes and the tubes are directly exposed to the flue gases.

Automatic TrippingThere are some set parameters for the safety of the boiler and the boiler will trip in accordance to it. The following are the automatic tripping

Drum Pressure High: Its reason may be the reduced demand of steam on the plant side while the steam is produced at the same rate. It can be prevented by lowering the firing rate or by blow down.

DESCON 1 - 13.5Kg/cm2

DESCON 2 - 13.2 Kg/cm2

DESCON 3 - 13 Kg/cm2

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Drum Level Low: It can be caused by the same reason as for drum pressure but another reason may be the problem in the control valve (time delay). For low level, manually control the valve.

DESCON 1 - 37% DESCON 2 - 26% DESCON 3 - 33%

Fuel Pressure: This problem may have many of the reasons. It can be a problem at the back end of SNGPL or it may be a line fault or because of a chock filter or by the malfunctioning of a control valve. Its prevention is to shift the load on the other boilers and switch to the second type of fuel (i.e. switch to oil if gas is in operation). The boiler will trip as soon as the gas pressure goes less then 1Kg/cm2.

Strength Of Flame: Strength of flame can be the result of the disturbed ratio of air to fuel. This strength of flame is observed by the flame eye which generates a typical value of voltage depending upon the luminosity of the flame and it trips the boiler if there is deviation from the set value of that voltage. It may be avoided by making sure the ratio of air to fuel does not get disturbed.

Fan Tripping: It can be because of the motor failure or any other abnormality and the remedial action will be the troubleshooting of that abnormality.

Leakage of Boiler TubeThe leakage in the boiler tube can be a big problem, especially in the case of fire tube boilers. It can even result in a small blast which may be severe for both property and workers. To check the leakage in the tubes, two types of tests may be conducted, one is hydro test and the other is dye penetrate test.In hydro test, the drum is filled up with water and the tubes are observed sharply to point out if there is any leakage, this method is commonly in practice. The other method is dye penetrate test and it is to pin point the type of the leak. The leak may be of the crack hole sort or it may be a pin hole. Dye is penetrated in the tubes and the color is observed.If there is some leakage in the tube then there can be two remedial actions commonly practiced. One is tapped head block, this means to block the head of the particular (leaking) tube from both ends by means of a tapped cork. The other way to solve the leakage problem is to replace the tube.

Safeties and AlarmsThe boiler is a very dangerous device if it comes to accident, that’s why the boiler is equipped with safeties and alarms to keep an eye on the performance and to make sure there is not any kind of mishandling. Safeties and alarms of the DESCON boilers are as follows

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DESCON 1o Pressure safety valve number 1 - 13.6 Kg/cm2

o Pressure safety valve number 2 - 13.7 Kg/cm2

o High water level alarm - 73%

o Low level alarm - 40%

o Low-low level alarm - 37%

DESCON 2o Pressure safety valve number 1 - 13.6 Kg/cm2

o Pressure safety valve number 2 - 13.7 Kg/cm2

o High water level alarm - 86.5%

o Low level alarm - 36%

o Low-low level alarm - 26%

DESCON 3o Pressure safety valve number 1 - 13.65 Kg/cm2

o Pressure safety valve number 2 - 13.75 Kg/cm2

o High water level alarm - 87%

o Low level alarm - 38%

o Low-low level alarm - 33%

Some Critical Parts of BoilerSome useful but critical parts of the DESCON boiler will be discussed here briefly. These parts are important because these may impart an important role in the boiler tripping. Moreover, for efficient and proper working of boiler one must have sufficient knowledge of its part.

Duoblock Dual Fuel Burner: Burner is the portion in which burning takes place. Fuel and air lines are joined with it. There is a tubular port for igniter and an eye for flame scanning. The primary air (atomizing air) comes to it by means of a flexible external pipe which is provided with a butterfly valve (damper) to adjust the flow. Secondary air is the combustion air which plays its part in combustion. Primary air is 15% of the required combustion air. Tertiary air is to prevent the carbon deposits on the burner and it comes from the small holes at the inner side of the burner head. Combustion is optimized by means of tertiary air. These holes are at the sides. Air is also provided to stabilize the burner flame. It comes into the flame core. The burner has the option of alternating firing (either gas or oil).

Rotary Cup Atomizer: There is a rotary cup atomizer which atomizes the primary air. The atomizer can swing to 80o. It forms a combustible oil and air mist right after leaving the rotary cup.

Axial Vane Ring unit: This unit is provided with the blades which can direct the combustion air. It is adjusted to control the NOx, CO, soot concentration. Flame length and shape is also controlled by it. The flame formed in DESCON boiler is of the shape of vortices.

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Vortex Meter: It is for the metering of fuel coming to the boiler. Flow is obtained by the vortex meter. Vortices are produced after the meter, which are called KARMAN vortices. Frequency of these vortices is calculated by means of a sensor and the flow rate may be obtained by the velocity (frequency is directly proportional to the velocity).

Oil Compound Regulator or Cam Strip Positioner: It is used to control the firing rate. Servomotor takes the signals and changes the oil accordingly to control the firing rate. The rotary valve may be operated manually or automatically. There is a locking lever, which locks the controller and it will not take any further action till it is unlocked. The spindle of rotary valve can move in both clockwise (less flow) and anticlockwise (more flow) directions. The oil regulator is connected with combustion air regulator and turn actuator (primary air volume regulating). All three are interconnected and may adjust air fuel ratio automatically.

Gas Electric Igniter: It is present in the burner. Gas air mixture after the nozzle is swirled into the area of ignition where high voltage spark ignites the mixture.

Pressure Switches: These are the type of electrical safety. The switches can transform signals further to a valve (solenoid) or it may play a role in tripping. Fixed values are set on the pressure switches and deviation from these values can trip the equipment. It is important that different make of pressure switches should be used so that malfunctioning of one switch could be cured.

Solenoid Valve: These valves are important in boiler operation because of there property to close the line suddenly. These valves are either fully open or fully close. In emergencies, these valves block off the line suddenly after getting signals from pressure switches or from the flame scanner eye.

Advantages of DESCON Boiler Spherical furnace gives increased radiant heating surface and is the ideal shape for

with standing pressure. Efficient (up to 80%) and robust. Easy to maintain and its upkeep is less costly than the water tube boiler. No furnace brickwork required. The fire tube boiler does not require greater skill in operating and maintaining.

Disadvantages of DESCON Boiler Steam can not be raised from cold water if immediately required. It takes more time

than the water tube boiler. In fire tube boiler the temperature difference between top & bottom parts of the boiler

causing unequal expansion resulting in mechanical straining. The steam and water drums are bigger in diameters compared with the small shell of

the water tube boiler therefore they are not stronger and suitable for much higher steam pressure.

More mass of water is carried in the boiler, hence an increasing weight.            

B ABCOCK & W ILCOX B OILER ( B & W B OILERS)

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The Babcock & Wilcox Company was incorporated in 1881, offering best class boilers to the US marker. Their patented design featured larger heating surfaces, increased water circulation, and improved safety over competing boilers. Their boilers powered the first central electrical generating station in the United States and in the 1920’s Babcock & Wilcox (B&W) began to develop larger boilers and water-cooled furnaces. These boilers were installed at ARL in 1922. These boilers have the option to be charged with oil or gas and even both at the same time. The air for combustion in these types of boiler is induced in due to which these are called induced draft boilers. These are named water tube boilers because the water travels in the tubes while the fire and flue gases are at the shell side. As water tube boilers require much purer feed water, their general acceptance was slow for a time due to lack of water treating knowledge in those days.

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This diagram is of the water tube boilers made by B&W initially. ARL’S B&W boiler is different from this boiler in the context that the boilers here have two cylinders on the top and an economizer is there to take away the heat of flue gases similar to that of the DESCON boilers. Soot blowers are present in a large quantity (8 in number for B&W boiler) because the tubes are exposed to the flue gases directly which may deposit carbon on the tube material but in B&W boilers, soot blowers are manually operated and no motor is mounted with it.Feed water from de-aerator is taken into the economizer of the boiler where the flue gases transfer their heat with the water to pre heat it. Then the water goes into the water tanks at the top of the boiler in which water is stored. 20 tubes are tilted toward the bottom mid of the boiler from the bottom of the water tanks. These tubes are exposed to radiant heat as 4 gas louvers and 5 fuel louvers are burning at the bottom of the boiler. These louvers are provided with steam line as well so that the flame does not come back and remains optimized. Water travels in these tubes and gets heated. It is directed back to the water tanks and from the top, saturated steam is obtained. A simple diagram to illustrate the process so far is given below.

But these boilers are designed to give off superheated steam. For this purpose, the outlet saturated steam is given a bend into the boiler once again to make it superheated. The difference between saturated and superheated is that there are no water vapors in superheated steam at all and vice verse. Superheated zone is nothing but a turn of saturated steam into the boiler again. This can be illustrated by the following diagram.

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The firebox walls are of high temperature firebrick to resist and hold inside 2000° F (approx) or more temperature of the burning fuel. The front wall around the oil burners is formed with special cone shaped high temperature refractory material. Unless the brickwork is treated properly it will soon crack, crumble and begin to tumble down. Baffles act as partitions between the tubes to slow down the hot gases and direct them over the entire tube heating surface. Those near the firebox are made of high temperature refractory material to withstand the heat while those between the tubes may be of cast iron. Because of burning fuel oil produces some soot, which travels with the hot gases and lodges on the outside of the tubes, soot blowers are required. Ordinarily this should be removed each day. Otherwise, the heat has difficulty getting to the tubes, resulting in fuel wastage. When operating, it must be made certain that dry steam is used, as wet steam will mix with the soot, setting up a condition that will cause rapid corrosion of the tubes.One of the most important things in trouble free water tube boiler operation is to keep the water side of the boiler clean. Any appreciable formation of scale or mud in a tube directly over the fire is almost certain to cause overheating with resultant tube failure. Modern methods of treating the water in the boiler practically eliminate this possibility if the treatment is properly kept up. Finally, as in all boilers, the water level in a water tube boiler must not be allowed to drop below the bottom of the gage glass.

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Here is the side view of a B&W boiler with the only difference (with ARL’S B&W boiler) of the air draft. At ARL, we have induced draft water tube boilers. It is very easy to understand the operation of the B&W boiler from this diagram. Other valves include the super heater drain valve, blow down valve, safety and check valves.

General Design DataSome general design information of B&W boiler is given below

Parameter ValueBoiler Steam Pressure 12Kg/cm2

Boiler Steam Temperature 280oCDrum Level 60%

Fuel Gas Pressure 5PsiFuel Oil Temperature 75-90oC

Economizer Inlet Temperature 95-100oCEconomizer Outlet Temperature 105-110oC

Superheat Inlet 220-230oC

Heating Surface = 4092 ft2

Number of Tubes = 20 x 9 (180)Diameter of Tubes = 4 inch

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Tripping, Safeties and Alarms of Babcock & Wilcox BoilerTripping of B & W boilers is on two parameters only, one is the low fuel pressure (fuel may be gas or oil) and the tripping of the fan. The reasons and remedies for both of these troubles are already described in the tripping of DESCON boiler. Safeties and alarms of B&W boiler are as followed B & W Boiler Number 4

o Drum safety valve number 1 - 220 Psi

o Drum safety valve number 2 - 219 Psi

o Drum safety valve number 3 - 218.5 Psi

o Drum safety valve number 4 - 218 Psi

o Super heater safety valve - 217 Psi

o High water level alarm - 80 %

o Low water level alarm - 35 to 40 %

B & W Boiler Number 5o Drum safety valve number 1 - 219 Psi

o Drum safety valve number 2 - 218 Psi

o Drum safety valve number 2 - 217 Psi

o Drum safety valve number 2 - 216 Psi

o Super heater safety valve - 215 Psi

o High water level alarm - 80 %

o Low water level alarm - 40 %

Advantages of B & W Boiler Savings in weight for a comparable heating surface area Possibility of using higher temperatures and pressures without increasing wall

thicknesses. More efficient combustion space allowed Greater flexibility of the structure and rapid circulation prevents the problems of thermal

stressing. In water tube boilers roof and floor tubes are sloped at 15o to ensure circulation.

Thinner tube materials allow rapid steam rising and faster heat transfer rates.

Disadvantages of B & W Boiler Lower reserve of water means a more efficient water level control is required. High quality feed required. Little allowance to corrosion. High initial capital cost Cleaning is more difficult due to the design No commonality between tubes Physical size may be an issue

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CONDENSATE RETURN SYSTEM The outlet steam of B & W boilers combine with the steam produced by DESCON boiler in a header and is distributed to fulfill the requirement of plant and offsite. When steam has performed its work in processes, turbines and transferring heat, it transfers heat and reverts back to a liquid phase called steam condensate.  However, not all the energy used in producing steam is lost when condensate is formed. The condensate system collects the combined vapor and liquids that has given up its useful energy, so that it can be returned to reuse as high quality water and a low energy source. There is a significant energy saving related to the heat required to raise the temperature of makeup water in the heat exchangers and not to forget about the additional cost in pre-treating (softening) the makeup.If the water has some gases dissolved in it, which admitted the system along with the steam, then on its way back as steam condensate, the gases would become a weak acid (i.e. carbon dioxide will become carbonic acid on its return by combining with water at high temperature and pressure). It will cause corrosion. This is the reason why ammonia is added in the system (at the boiler feed water treatment stage), which will make a layer with the pipelines and will prevent corrosion. Steam to liquid change occurs in a wide variety of equipments types.

o Condensers: Condensers and heaters force the change by using the steams

energy.o Traps: It collects the liquid form in the line of steam (water vapors).

o Flash Tanks: These collect high pressure condensate to make low pressure steam.

o Receiver Tanks: Maintains a constant level of the condensate for the pump

suction and all the vapors are vented to the atmosphere or converted to liquid condensate for easier handling and reuse.

At ARL, Two condensate return headers receive condensate from various sections of the plants combines at boiler house from where this condensate is stored in a storage tank for onward pumping to the boiler feed water system. The total condensate recovered makes up for 10 % of the total Boiler feed water. The condensate return is tested in the water lab and it must come on the following specifications to meet the requirements:

Test Description Specifications

PH 8.0-9.5Total Hardness <1.0

Magnesium alkalinity <10Chlorides <10

Fe <0.4

There are two, four stage centrifugal pumps which pump this condensate to the plants and to the heat exchangers. Moreover, these pumps help the circulation of the condensate in the tank itself. There are two other pumps, condensate water pump, for de-aerator. Water from low lift pumps is directed to the tank to facilitate the removal of steam vapors by lowering the temperature a bit.

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COOLING TOWER

To keep the refinery process units running smoothly water is needed for the purpose of cooling of products, condensing of vapors and cooling of equipment. ARL has three cooling towers, Cooling tower number 1 is forced draft and provides cooled water to HBU-1 and HBU-2. Tower number 2 is induced draft for HCU and Reformer while tower number 3 is forced draft, 2 cells for LUMMUS plant. To accomplish the above jobs about 288,000 gallon per hour of water is circulated through all the process areas of the refinery with the help of three cooling towers.Refinery uses millions of gallons of water and care must be taken in the regard of the out flowing water (i.e. discharging this amount of water may cause pollution problems) therefore, it is best to re-use this water. The water used in different processes comes back to the cooling tower at a higher temperature. Its temperature must be lowered in order to reprocess. This water is cooled in the cooling tower. The main scientific principle behind this cooling phenomenon is removing heat from water by evaporating a small portion of the water that is re-circulated through the unit. The heat that is removed is called the latent heat of vaporization.Cooling towers are provided with a Header which distributes the water considerably throughout the tower. There are Splash Bars (fill pack) which makes the water to spatter into droplets. This helps increasing the area exposed of water to air. Louvers are in some of the atmospheric cooling towers which help directing the wind into the tower and also provide protection to water losses. Drift Eliminators are there to avoid any water losses from the top due to the air flow (drift). An integral part is the Fan which induces or forces the air into the tower.

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Fan Deck is a horizontal surface enclosing the top of the cooling tower above the plenum which serves as a platform for inspection and maintenance and Plenum is the internal area of cooling tower between drift eliminator and the fan. Distribution System is the portion of the cooling tower which helps in the water distribution over the fill pack.

Factors Affecting Rate of EvaporationThere are some certain factors which affect the rate of evaporation. These are as followed

Area Exposed: In order for water to evaporate it needs to be in contact with air. The larger the exposed surface area of water the greater will be the rate of evaporation. Cooling towers are designed to expose large surface area of the water so the hot water evaporates at a quicker rate.

Humidity: Air can hold only a certain volume of moisture in it. Moisture content in air is called humidity. If the air becomes saturated with water it will no longer absorb water. This saturation of water in air will decrease the rate of evaporation. On a damp day, the water will not evaporate as much as it would in a dry day because of humidity difference.

Temperature of Water: The third factor is the temperature of water approaching cooling tower. If this temperature is low, cooling towers will work less efficiently (if we consider the function of cooling tower).

Earlier, the method of cooling water was spray ponds, as is the reservoir 1 at ARL. By spraying water into the air, the surface area of water exposed to the air increases and it will cause more evaporation and hence more cooling. The spray pond system was not very efficient because the water droplets could blow away with air, resulting in water losses, also these kinds of ponds depend upon the wind velocity as well.

Types of Cooling TowersCooling towers are divided into different types on the basis of the draft and flow pattern. On the basis of draft there are three types of cooling towers

o Natural Draft

o Induced Draft

o Forced Draft

And on the basis of flow, there are two typeso Counter Flow (The Air and water flows in the opposite direction. Air from the

bottom and water from the top)o Cross Flow (In this type, the water-air flow is crossways. Air enters from the

sides and water from the top)

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N ATURAL D RAFT Heat in a furnace causes the hot (light) gases to flow upward through the stack causing a natural draft. Hot water is sprayed in above the splash bars. These splash bars break the falling water into fine droplets which increase the surface area of water. Some of heat is transferred to the air and the air becomes lighter and leaves the chimney. Fresh air will enter to take the place of that hot air which left the stack. This difference created will cause a natural draft in the cooling tower. Louvers are some times installed to control the amount of air entering the cooling tower which in turn controls the rate of evaporation. These are crossway flow towers.

F ORCED D RAFT The air flow through the water, falling from the splash bars, is produced by a fan. The internal construction of a forced draft is usually similar to that of the natural draft with the only divergence that the walls of forced draft are close. Motor driven fan forces the air from or near the bottom of the tower. This fan must chip in a large quantity of air with low flow rate, so that the air to water contact must increase. The tower is provided with the drift eliminator to avoid the drift losses. Degree of cooling in this type of the towers depends upon the fans and the rate of water flow. ARL has two forced draft cooling towers (cooling tower number 1 & 3).

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I NDUCED D RAFT It is the function of the fan to induce air in these types of cooling towers. Its difference with the forced draft originates in a way that in forced draft towers, air is pushed into the tower by the fan while in induced draft towers, air is pulled into the tower. The fan acts like an exhaust fan which sucks the air from the tower and brings on a place for the fresh air entering from or near the bottom. Walls of these types of towers are also fixed and therefore, these are also counter flow. The tower has movable side louvers to regulate the air inlet near the bottom and the air intake is controlled by the louvers and the fan speed. ARL has one induced draft cooling tower (cooling tower number 2).

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M ULTICELL C OOLING T OWERS Large cooling towers are constructed in this way. The cooling tower is portioned into 2 cells and both individually act as a single cell cooling tower. Number of fans equals the number of cells. In single cell cooling tower, if the tower need a repair job or cleaning then the entire tower has to be shut down and hence the plant (to which it provides cool water) will also shut down. On the other hand, in multi cell cooling towers, any cell can be operated independently. If one cell needs to be shut down it will not affect the plants as the other cell will be providing the services. At ARL cooling tower number 3 has two cells.

Absolute and Relative HumidityHumidity expressed in terms of weight per quantity is called absolute humidity (i.e. 1 pound of water in 10 pound of air). Temperature does not play any role in the case of absolute humidity and it will not give any idea about how much more water air can absorb. In order to know how much more evaporation can take place, it is necessary to estimate the degree of saturation (amount of water present in the air). It is done by relative humidity. In contrast with absolute humidity, relative humidity gives the percentage of water in air or to be more exact, it is the percentage of maximum humidity that air can withhold. It depends upon the temperature. If air holds all the molecule of water which it possibly can then it is said to be 100% saturated air. With the increase in relative humidity, the rate of evaporation decreases (i.e. the air gets saturated with water) and in turn, the performance of cooling tower decreases.

Dry Bulb & Wet Bulb TemperatureDry bulb temperature is the normal temperature of the surrounding which we experience in daily life. It can be obtained by a thermometer whereas, the wet bulb temperature is the temperature caused by the evaporation and is obtained by hygrometer. Hygrometers bulb is covered with by a wet wick. Evaporation has a cooling effect so, the water on the wick will evaporate and will cause the temperature to decrease. Wet bulb temp is less then the dry bulb temperature on the dry day. The greater the difference between these two temperatures, the lower will be the relative humidity and the faster will be the rate of evaporation.Cooling tower performance is lowest when wet bulb and dry bulb temperatures are equal and this usually happens on a rainy day. When the outside air is cooler then the water being cooled, efficiency will be lowest but even then some cooling will take place due to conduction and convection. In winter operations, when air goes below 0oC, the water droplets starts freezing and will block the splash bars. This freezing can be fended by limiting the amount of air entering the tower and by changing the direction of rotation of the fans. Limiting the amount of air can be done by lowering the speed of the fan. The incoming water will make the ice to melt.Cooling towers are never 100% efficient. If the wet bulb temperature is (say) 60oF, the lowest possible temperature that can be achieved by the cooling tower will be above 60oF and this is the approach of the cooling tower.

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Approach, Range & Efficiency of Cooling TowerApproach of the cooling tower states, the tower can not achieve a temperature less then the wet bulb temperature. The difference between the temperature of the cold water leaving the tower and the wet bulb temperature of the air is known as the approach. Establishment of the approach fixes the operating temperature of the tower and is a most important parameter in determining both tower size and cost.Approach of cooling tower = Temp of cooling tower supply water - wet bulb temperature

Range of the cooling tower is just a difference of the inlet temperature to the outlet temperature to have an idea of the range of operation of the cooling tower, or to be more exact, it gives the ∆T of inlet and outlet.Range = Temp of return water to cooling tower- Temp of supply water from cooling tower

Since a cooling tower is based on evaporative cooling, the maximum cooling tower efficiency is limited by the wet bulb temperature of the cooling air. We have to take an account for the wet bulb temperature in calculating the efficiency. The cooling tower efficiency can be expressed as

μ = (ti - to) 100 / (ti - twb)Where,μ = cooling tower efficiency (common range between 70 - 75%)ti = inlet temperature of water to the tower (oC, oF)to = outlet temperature of water from the tower (oC, oF)twb = wet bulb temperature of air (oC, oF)

Some other important TERMINOLOGIES in cooling tower studies are as followed:

The amount of heat to be removed from the circulating water within the tower is called HEAT LOAD. Heat load is equal to water circulation rate (in GPM) times the cooling range (∆T) times 500. It is expressed in BTU/hr. Heat load is also an important parameter in determining tower size and cost.

The pressure required in pumping the water from the tower basin, through the entire system and return to the top of the tower is called PUMPING HEAD.

As a result of evaporation, dissolved solids concentration will continually increase. The circulating water in tower which is discharged to waste to help keep the dissolved solids concentration of the water below a maximum allowable limit is called BLEED OFF. It is also called BLOW DOWN.

The amount of water required in replacing normal losses caused by bleed off, drift, and evaporation is called MAKEUP WATER.

CYCLE OF CONCENTRATION is the ratio of solids in the circulating water of the cooling water system to the solids in the make up water. It generally gives an idea about the quantity of chemicals that are added to the cooling tower.

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Cooling Towers at ARLNow let us discuss the cooling towers at ARL briefly.

C OOLING T OWER # 1 This forced draft cooling tower provides services to HBU 1&2. Hot return from plant side comes in the hot side of the reservoir. Cold side and hot side are open to one another to make cooling more efficient. Level of cold water is always kept greater then the level of hot water, so that, water may travel from cold to hot side and not from hot to cold side. Water from hot side is pumped to the side stream filter and basin of the cooling tower by means of two centrifugal pumps. Side stream filters are equipped with gravels from inside to remove the turbidity of the water. Side stream filters are backwashed as well when it becomes less efficient. This happens due to the accumulation of the TDS which chocks the side stream filter. Back washing also facilitates the re-alignment of the gravel bed which discourages channeling within the bed. The filtered water is returned to the cold side of the reservoir.Basin is provided with sub channels to distribute the hot water equally in the cooling tower by means of nozzles present on the distribution lines. Water falls down from the splash bars which are made of PVC. Above the tower is the drift eliminator which is also made of PVC. Water falls down in the tower and comes in contact with the air supplied by the fan at the side wall of the tower near the bottom. This is an automatic fan which switches on when the water supply temperature from cooling tower increases 30oC (in winter the fan hardly moves). The water gathers in the bottom sump. From the sump, a channel is provided for the water to come into the cold side of the reservoir where chemical treatment is done (will be discussed later). 2 centrifugal pumps pump the water to the plant side.

C OOLING T OWER # 2 The difference, other then the draft, rises in the way that cooling tower two does not have any external reservoir. Its sump acts similar to the reservoir. This induced draft cooling tower has the fan on the top which sucks the air from the tower and allows the sides near the bottom to let fresh air come in. The angle of fan blade is 16o. It is used to give services to HCU and REFORMER. Water return comes to the basin of the tower and similar to cooling tower number 1 it is distributed in the tower. Water falls to the bottom giving off its heat to the air and gathers in the sump. Its chemical treatment is done in the sump. Cool water from the water is pumped to the plant side by means of three pumps out of which two are kept in operation and one on stand by. Discharge of these pumps is joined together by means of header. This header provides water to the side stream filter. And the outlet of side stream filter is sent back into the sump. Pipes for the suction line of these pumps is inserted into the sump after passing through concrete therefore, any vibration in the pump will cause a big damage (leakage) to the tower. Expansion pillows are provided to absorb the vibration of the pump (if any). Vacuum pump is used for the sump cleaning.

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C OOLING T OWER # 3 This forced draft, two cell cooling tower is used attend LUMMUS requirements. Hot water comes to the tower top. Both Fan forces air into the tower from the bottom. These fans are provided with water collector system on the fans surface to protect it from the falling water. Water after getting cooled comes into the sump from where it comes into the basin. It is the only cooling tower at ARL which has its basin on the bottom side. Chemical treatment takes place in the basin. From the basin, 3 centrifugal pumps equipped with expansion pillows pump it to a common header from where cooled water is supplied to the water and sent to the side stream filter. Side stream filter after lowering its turbidity sent it back to the basin. A metal copan is provided in all the cooling towers to check the corrosion rate. Condition of this copan will narrate the condition of the cooling tower. This copan is tested after regular intervals of time (2-3 months). This copan observes the corrosion rate as well, which may be caused by the excess of chlorine present in the water.

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General Design DataData of cooling tower 3 is available and is as followed

Design ParametersFlow Rate = 436m3/hr

Hot Water Inlet = 43.33oCCold Water Outlet = 32.22oC

Wet Bulb Temperature = 28.06oC

LossesDrift Losses = 0.01%

Evaporation Losses = 2.4%

Structural Material of Nuts & BoltsStainless Steel or Mild Steel

Fills or Splash BarsMaterial = PVCType = Cellular

Average Fill Height = 915mmSheet Spacing = 28mm

Support Material = Mild Steel

Drift EliminatorMaterial = PVC

Average Eliminator Height = 305mmType = Cellular

Number of Passes = 6Sheet Spacing = 28mm

Support Material = Mild Steel

Water DistributorType = Down Spray

Number of Headers = 2Diameter = 267mm

Distribution LateralsNumber of Distribution Laterals per Cell = 18

Diameter = 114mm

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Distribution NozzlesDesignation = Centrifugal Atomizer

Number per Cell = 216Orifice Diameter = 14mm

Mechanical EquipmentNumber of Fans per Tower = 2

Diameter of Fan = 3050mm Number of Blades = 4

Speed = 368RPMFan Ampere = 19AMP (for A) and 21 AMP (for B)

Blade Angle = 8o-12o

Emergency at Cooling TowerThere can be three types of major emergencies at cooling tower

Pump Emergency: There can be any sort of problem to the pump. In such situation, switch to the standby pump.

Level Low: Water level in the sump may decrease because of the increased water losses. In such state, make up water is added to the system.

Fan Tripping: If the fan gets cease due to any problem (i.e. motor load), make up water must be added and the bleed off valve must be open in order to create some value of ∆T.

Water Treatment & Chemical Injection at Cooling TowerMakeup water used in the refinery comes from natural sources and can have suspended and dissolved solids in it. Water circulates in the pipes, exchangers and cooling tower, it picks up more solids. These solids upon heating causes scale deposits on the hot surface. Discarding some water from the cooling tower and adding makeup water can a solution to this problem but it results in a lot of water losses. Another solution is to decrease the flow rate of water so that the suspended solids would settle down at the bottom of the sump and the basin. It may be cleaned by vacuum pump or any other means.Calcium and magnesium carbonates are les soluble in hot water then in cold. So upon getting heated up, these dissolved solids converts into the suspended solids and are the major reason of fouling (formation of scale deposits). Suspended solids also cause erosion (wear and tear) in the narrow pipelines. To avoid these troubles, water in the cooling tower is treated with different kinds of chemicals battle against the factors which affect the condition of the cooling water system.On the next page is a table, that includes the company name providing the services to ARL and the range of the factors that may be dangerous to the cooling tower system. This table also includes the agents that control these factors. In addition to these chemicals, DRIOSPURS and BIOSPURS are the two chemicals added to the cooling tower number 2. These chemicals react with water to form foam. Foam is the indication of the presence of oil in the system. This oil might have entered into a system by the leakage in the tubes of a heat

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exchanger or by similar other means. The water in the sump is overflowed (oil is lighter then water and comes at the surface of oil water mixture). After making sure that there is no oil present, treatment of BIOSPURS is done to handle the biological growth.H2SO4 used at the cooling tower is 98% concentrated and NaOCl is 21% concentrated. Testing of critical factors like PH and Chlorine (Arthotoludene is used to measure chlorine and phenol red for PH) is done on the spot to get the rough estimate and other tests are conducted in the laboratory. Samples are always taken from the return line coming back from the plant. Other chemicals added are NALCO 23212 (for cooling tower number 1) and PERFORMIX 2021-A (for cooling tower number 2 and 3). Function of both of these chemicals is to form a layer with the pipelines that will help protecting the material. Chemicals are dosed by means of diaphragm pumps but the facility is not the same for all chemicals. Some chemicals are shock dosed. Shock dosing means adding on the chemical on the basis of approximation (from the cans into the sump directly without the help of any pump).

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Cooling Tower # 01 for NDU

Cooling Tower # 02 for HCU & Reformer

Cooling Tower # 03 for Lummus

AIR COMPRESSORS

Compressors by definition are machines that by forced mechanical movement increase the pressure of a gas through volume compacting or reduction. The machine design and driving force (driver horsepower) determine the amount of pressure increase. Compressed Air is often described as the fourth utility, although not as common as electricity, petrol and gas, it plays a fundamental part in the modern world. Compressed air accounts for about 10% of the global energy used in industry today. With so many applications in different environments being dependant on compressed air, the compressors not only have to compress the air to a specific pressure, at a certain flow, it has to deliver air of the right quality. To obtain the right quality of the compressed air, more equipment is often needed. Filters and dryers are often needed to remove oil and water before it reaches the application.Air to be used at the plant side is of two types, instrument air and process air. Instrument air is pressurized air normally having pressure of 7 kg/cm2. A necessary characteristic of instrument air is that it should be free from moisture and particulate matter because instruments are very sensitive to these contaminants. Plant Air is simply compressed air .It may contains moisture and dust particles although Plant Air compressors contain a course filters at its suction. Plant Air pressure varies depending upon requirements.For instrument air one has to be serious because it is used in transmitting signals to the control valves operating at critical spots. Gauges are also operated by air (as at LUMMUS). Instrument air is also used for flushing and cleaning of instruments that can not work in the right way if subjected to moisture, regardless of how small the quantity of moisture is, whereas plant air is used for flushing and cleaning of the pipelines and other such tools. Plant air may also be used to drive pumps. Plant air can be used in processes if required but it can not take place of instrument air. Compressed Air is clean, safe, simple and efficient. There are no dangerous exhaust fumes of or other harmful by products when compressed air is used as a utility. It is a non combustible, non polluting utility.When air at atmospheric pressure is mechanically compressed by a compressor, the transformation of air from atmospheric pressure, into air at higher pressure is determined scientifically by the laws of thermodynamics. They state that an increase in pressure equals a rise in heat and compressing air creates a proportional increase in heat. Boyle's law explains that if a volume of a gas (air) halves during compression, then the pressure is doubled. Charles' law states that the volume of a gas changes in direct proportion to the temperature. These laws explain that pressure, volume and temperature are proportional, change one variable and one or two of the others will also change. Mathematically,

(P1 V1)/ T1 = (P2 V2)/T2

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Where,P = PressureV = VolumeT = Temperature 1 & 2 are for initial and final stages respectively.

Types of CompressorsThere are basically three types of compressors used in industries

o Reciprocating Compressor

o Rotary Compressors

o Centrifugal Compressors

ARL only have reciprocating compressors, so let us discuss it a bit. Reciprocating air compressors are positive displacement compressors. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by using a piston within a cylinder as the compressing and displacing element. Reciprocating compressors can produce very large pressure differences, but because they produce a pulsating flow, may require a receiver vessel (discharge snubber) large enough to damp the pulsation. Reciprocating compressors are furnished in either single or multistage types. It is considered single acting when the compressing is accomplished using only one side of the piston and double acting when compressor uses both sides of the piston.

The reciprocating air compressor uses a number of automatic spring loaded valves in each cylinder that open only when the proper differential pressure exists across the valve. Inlet valves open when the pressure in the cylinder is slightly below the intake line. Discharge valves open when the pressure in the cylinder is slightly above the discharge line.

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Compressors at ARLDifferent compressors are used for different purposes (i.e. some for plant air and some for the instrument air). Instrument air provided to the plant is at a pressure of 6.5Kg/cm2 and plant air supplied is usually at 100psi.In ARL, there are six compressors (C-1, C-3, C-10, C-11, C-13 and C-15) which are present to provide plant air to the plants. These compressors are present at boiler house (C-11) and water pump house (rest of the compressors). Out of all these compressors, the compressor C-1 is usually taken in operation and remaining compressors are on standby. Two compressors C-50 and C-51 are also present to provide process air to the LUMMUS. These compressors are also reciprocating compressors located near boiler house. The objective of these compressors is to supply process air to LUMMUS but these are now out of order and instrument air of C-102 is afforded for LUMMUS.ARL has six compressors (C-101, C-102, HB-C-101 A&B, HB-C-301 A&B) there to provide instrument air to plants. Out of these six compressors, compressors C-101 and C-102 are present at boiler house whereas, compressors HB-C-101 A&B and HB-C-301 A&B are present at HBU plant. These compressors provide moisture free and oil free air to the plant. Compressor C-102 is primarily taken in operation to meet the demand of refinery (400-450scfm) and others are on standby.

Compressor Number Flow Rate (scfm) Pressure (psi)C-1 666 100C-3 Not Available 45C-10 75 100C-11 240 300C-13 Not Available 45C-15 100 Not AvailableC-50 50 100C-51 50 100C-101 300 100C-102 565 100

HBC-101 100 100HBC-301 200 110

Nomenclature of Positive Displacement CompressorsLet us now discuss some important terminologies involved in the area of compressors

The rate at which a compressor compresses a gas is called its CAPACITY and RATE is the volumetric flow rate of gas or air per unit time. The volumetric flow rate may be considered as number of cubic feet handled by a compressor in unit time. SCFM is the units of capacity, standard cubic feet per minute.

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RATE OF COMPRESSION is the ratio of absolute discharge pressure to the absolute suction pressure. Since compression always increases the pressure at the outlet therefore, rate of compression or compression ratio will always be greater then 1.

As the compressor forces gas molecules closer together, it increases the speed of these molecules. Increase in speed will cause an increase in the temperature. This temperature is referred as HEAT OF COMPRESSION. The amount of heat added depends upon the suction temperature and the compression ratio. These factors directly affect the heat of compression (i.e. increasing suction temperature, increase temperature of gas and the same is true for compression ratio).

INTERCOOLING is a technique of cooling the compressed gas prior charging it for the 2nd compression stage. It is usually installed after every act of compression (i.e. after 1st stage compression in a multi stage compressor). Intercooling may be done by means of air or water. It reduced the temperature of the gas before it goes to second stage compressor. Reducing the temperature will make it denser and therefore, more cubic feet of gas will be compressed in the same stroke. According to an estimate, 1% of horsepower is saved if 5oF is absorbed at the intercooling stage.

AFTERCOOLERS are a good first step in removing moisture and air contaminates. They lower the temperature to safe, usable levels, thus reducing the air's ability to hold water vapor, removing 70% of moisture approximately. However the air is still saturated. A further drop in temperature will cause additional condensation to occur in downstream airlines.

HORSEPOWER is the rate of doing work, or work done per unit time. When the time required to do a certain amount of work decreases, it means the horsepower requirement also decreases. Work by a gas compressor depends upon the capacity, compression ratio and the suction temperature (as the temperature can affect the volume of the gas).

In a reciprocating compressor, one forward stroke and one backward stroke are together addressed as REVOLUTION. In case of a double acting compressor, one revolution has two discharge strokes while a single acting compressor has only one stroke in one revolution.

VOLUMETRIC EFFICIENCY is the ratio of actual capacity of the compressor to the volume of the gas that it should theoretically handle. It is affected by the amount of clearance (i.e. increasing the clearance will decrease the efficiency). In this way, clearance provides an important part in determining the actual capacity of the compressor.

The space provided for valve recess is called as CLEARENCE VOLUME. It also provides a distance between piston and the cylinder of the compressor. It is the method to control the volumetric efficiency.

Another method of controlling the volumetric efficiency is THROTTLING. It is the partial closing or pinching of the suction valve. Since throttling reduces suction pressure, it always increases the compression ratio. The increase in compression ratio increases the power requirements but throttling reduces the volumetric efficiency at the same time that decreases compressor capacity and we know that decrease in the capacity decreases the horsepower requirements. It is estimated (by calculations) that

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ratio of compression below 2.0 increase power requirements and above 2.5 it decreases the power requirements. Throttling is an inefficient way of controlling the efficiency.

Information about the compressors is not available in refinery and due to the fact that these compressors are old aged, their information is not available on internet as well with the exception of C-102. It is a COMPAIR BROOMWADE VMD 500 SERIES 4000 COMPRESSOR. All series 4000 BROOMWADE compressors are double acting, heavy duty, reciprocating and dry. These are two stage compressors and has water cooled intercooler.

Air FilterThe air filter element can be either reusable or disposable depending upon the type of the air filter to be used. The surface of the filter must be capable of trapping the dust particles in the intake air.

Dry CylinderDry cylinder means that the machine is oil free, and the lubrication to the cylinder wall is supplied by the piston ring. The packing of the piston rod in non-lubricated.

Cooling System

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The cooling water system consists of water jackets in the cylinders and cylinder head, an intercooler and after cooler. Solenoid valves are there to save water and prevent condensation when the compressor is shut down. There is a thermostatic bypass valve, which lets the water into the compressor cylinder. It has a sensing unit which decides the amount of water to be sent into the compressor according to the heat generated during compression. The thermostatic bypass valve lets the water into the 1st cylinder and the outlet from the 1st cylinder is the inlet to the 2nd cylinders jacket. The cylinders are provided with drains to allow the cooling water system to drain for maintenance work.

Three Step RegulationThe compressor is controlled by regulating the quantity of air entering the cylinder by means of three-step regulation. This system utilizes two pressure switches, unloader devices and solenoid valves. When air demand falls to less then the compressor output, a pressure switch is operated to guide the excess pressure to unloader device and hence the compressor ceases to pump air at the outward stroke and runs on the half load. When air demand continues to fall, another pressure switch is operated to unload the excessive pressure. The compression ceases on the inward stroke as well and the compressor runs on zero load.Air is a colorless, odorless, tasteless mixture of many gases. Air is naturally contaminated with solid particles, such as dust, sand, soot and salt crystals. This contamination varies with environments. Water vapor is another natural ingredient, found in variable amounts in the air. The amount of water vapor and contamination of the air plays a vital role in the compression process and in the quality of the air delivered by the compressor. Water is highly corrosive in nature. Untreated air at atmospheric pressure contains large amounts of water and other contaminants. When the air is compressed the concentration of moisture and other contaminants increases. If allowed to remain in the system this corrosive mixture has a detrimental effect on pneumatic equipment, product spoilage and reduced equipment life.Air from the atmosphere enters to the first stage of compression by means of a filter. From the first stage it is directed to an intercooler. There is a moisture separator after the inter cooler in which the air gives off some of its moisture contents which are removed by the help of automatic drain. Then it goes to the second stage compression. Air from second stage compressor goes to the aftercooler. Compressed air enters the top of after cooler and passes around a water cooled tube stack inside the aftercooler. The condensate is collected into the well of the aftercooler from where it is discharged automatically after attaining a certain level. Aftercooler is followed by a receiver tank. It is a normal practice to install a receiver tank in the downstream. It eliminates the factor of pulsating flow. In addition to this, it avoids overloading as it can store a certain amount of compressed air. Usually the size of the receiver must be according to the fact that it should store sufficient amount of air that can be used for one or one and a half minute when the compressor is shut down. A rule of thumb is to provide a minimum of one gallon of receiver capacity for each cubic foot of compressor flow. Moisture, either liquid or vapor, is present in compressed air as it exits the compressor system. If this moisture is not properly removed, compressed air system can lose efficiency

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and require dramatically increased maintenance, which can result in costly downtime. Operation of compressed air (either instrument or plant) is the same till the receiver tank. Plant air from the tank goes into the header and is distributed to plants.Air dryers are used for the purpose of purification of air and to make it moisture free. Drying is a very critical operation because any moisture in the air can lead the instruments to a lot of maintenance. Dryers are of many kinds. Mostly used dryer is a simple refrigeration system which removes moisture from air by means of refrigeration. The other type of dryers (in use at ARL) is desiccant dryers. Desiccant dryers utilize chemicals beads, called desiccant, to adsorb water vapor from compressed air. Silica gel, activated alumina and molecular sieve are the most common desiccants used. Silica gel or activated alumina are the preferred desiccants for compressed air dryers.Two types of desiccant dryers are used, heatless regenerative and heat regenerative. The heatless type uses a percentage of the dried air (purge) for re-generation of the desiccant material, while the heat regenerative type uses an electric heat disk, which reduces the amount of purge air needed for regeneration. ARL uses heatless regenerative type. Regeneration is an important factor in the desiccant dryers. Water covers the surface of the beads which hinders the further adsorption of water on the surface of beads. Therefore, regeneration takes away the moisture from the beads and throws it out to atmosphere with a very large sound. The process of regeneration is automated and controlled by solenoid valves. When one dryer is undergoing regeneration the other one is taken in operation. The dryer undergoing regeneration is called dried out.These dryers continuously dry the compressed air by means of two identical towers having desiccant beads in it. Air, prior coming to the dryer is passed through an air line filter which is there to protect the desiccant bed from solid and liquid contaminants. Temperature of the air coming to the dryer must be at least 49oC. Increase use of aftercooler if higher temperatures are present. Cycle of operation may vary from 4 - 10 minutes. Dried air from the dryer goes to the header of instrument air which supplies air to all the pneumatic instruments at ARL. Manufacturers of compressors other then C-102 are as followed:

Compressor Name Manufacturers C-1, C-10, C-11 INGERSOL-RAND COMPANY

C-101, C-102, C-50, C-51 COMPAIR BROOM WADE COMPANY

C-3, C-13 THE SENTINEL ALLEY & MACLELLAN LTD C-15 REAVELL & Co LTD

FUEL SYSTEM

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Fuel system at ARL is to provide adequate supply of fuel to carry out refinery uses. Gases and oil is used for this purpose. Let us discuss it briefly.

G AS S YSTEM The gas gathering system was installed to optimize the use of fuel gas at plants, boiler house and colony. The main source of input fuel gas to gathering system is hydrogen gas exported from Reformer Unit. The other sources are off gases from plant side

HP gas, from HCU plant and field gas (SNGPL) which enters the gas gathering system.

LP gas, from HBU plants and LUMMUS.

Hydrogen is mixed with HP gas to form MIXED gas which is used in different purposes. LP gas is not mixed with other gases because it has some oil vapors in it that can damage the gas burners. The LP gas is stored in the vessels and oil is removed from the bottom of the vessel after some intervals of time. Whereas, mixed gas does not have any such characteristic. Tanks of gases for boiler house storage are located infront of boiler house (infront of B&W boiler). SNGPL gas has the calorific value of 975-1000 BTU/ft3 and calorific value of LP gas is 2500-3000 BTU/ft3. The SNGPL and HP gas mixture gives a calorific value of 600-900 BTU/ft3. Mixing of these gases is necessary because the combining ratio of SNGPL to HP is 55 to 45. If these gases are used separately, the quantity of SNGPL to be used will increase. This is not favorable, keeping in mind that HP is a product of refinery itself, there is no need to pay extra charges to SNGPL when refinery can fulfill its needs as by the mixture of these gases. Therefore, these gases are mixed for economical purposes. Another reason is the pressure of the gas, which may not be the same for SNGPL gas. The set value for pressure of the gas in refinery use is 30psi and for domestic use is 5-6psi. Refinery supplies gas for the domestic use as well, to the refinery colony. Gases are mixed and stored in the knock out drum. When the pressure of the gases exceeds a certain set value, the controller directs some amount of gas to flare in order to bring the pressure to the set value.

O IL S YSTEM Furnace fuel oil system provides the necessary fuel oil to the plants and boiler house. It is the product of refinery itself. Two tanks, tank number 184 & 185, with the capacity of 230800 liters each are dedicated to the utility operations. These tanks are filled by the ATG (auto tank gauging) officer. The tanks are equipped with 5 pipelines (i.e. Filling line for tank filling, return line for FFO return from plant side and utility, Suction line for pump suction of FFO, Steam line to give steam to the tank so that FFO do not clung the lines and one line is for the steam condensate return).

Oil from the tanks comes to the fuel oil pump house. There are four screw pumps at the fuel oil pumping house. Two pumps are to pump the fuel oil to plant side and B&W boiler while

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the other two pump the oil to the DESCON boiler. The pump for the DESCON boiler may also be availed for the pumping of oil to plants and B&W boiler. All the pumps are provided with filters at the suction line. Wherever are the filters in the fuel oil system, they are two in number so that the operation remains ineffective when one filter is subjected to cleaning. Cleaning of filters is done after every one or one and a half week. All the lines incorporating fuel oil system are provided with steam tracers to avoid chocking. Steam will keep oil warm enough to flow in the lines. Even tanks are provided with the steam to keep FFO warm. Temperature is of the tank is about 80 to 100oC. Pump 1 and 2, pumps the oil to the DESCON boiler. The pumped oil passes through a thermostat coil heat exchanger in which steam is used as a heating media. From the bottom of the heat exchanger, it travels towards the boiler after passing through filters. The line is confused by insulating it along with the DESCON return. These are actually two separate lines but insulated together. The return from DESCON and plants comes to a combined header and then leads to the tanks. Pump 3 and 4, pumps the oil to the B&W boiler and plant side. The pumped oil passes from the main line of former pumping arrangement (which had 4 steam driven PD pumps) to a heat exchanger with steam at the shell side. This exchanger is not in use now a day. Cock valves are provided in the system at some critical places, it is because the chances of clogging a line are less in the presence of cock valve rather then gate valve.

DRINIKING WATER TREATMENT PLANT

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Properly treated drinking water for the whole refinery and the attached colony is processed and transported by the drinking water treatment plant. Water coming from SAWAN station and other tube wells (i.e. 2, 4, 6 and 10) is directed to the drinking water treatment plant (DWTP). The water originating from these sources is used for other purposes like swimming and garden watering as well. Water from the sources comes into a Sedimentation Basin (clarifier). The purpose of this basin is to remove the suspended solids from water by means of sedimentation phenomenon. Prior to which, it experiences the de-aeration phenomenon. It is done to remove any dissolved gases in water by means of spraying it to the incoming tray. The inlet pipe to the clarifier tray has many holes on it which helps increasing the surface area of water. The greater the surface area exposed the more will be the de-aeration. Mechanical clarifier occupies very less space as compared to the sedimentation basins whereas, purpose of both is the same. At ARL, the clarifier (sedimentation basin) consist of 3 chambers to increase the rate of settling of the particles by increasing water’s residence time and decreasing its flow rate. The first chamber is smallest which have the most unclean water among all three chambers. Then comes the second chamber, which is bigger then the first chamber but smaller then the third chamber. Water from the third chamber, falls into a channel by means of pores in the weir of third channel. Weir holds up a certain water level in the chamber. Pores are provided to help the removal of dissolved gas formed by the chemical reaction taking place in the clarifier.Chemicals like Aluminum sulfate (alum) and sodium hypochlorite are added in the clarifier. These chemicals act as a coagulant to make the suspended solids heavy enough to settle at the bottom. Function of a coagulant is to combine small suspended solids together to form large FLOCS. These flocs settle at a greater rate as compared with the small one. Calcium and magnesium bicarbonates are the dissolved solids, less soluble in water and therefore they become suspended when reacts with the alum.

Al2SO4 + CaHCO3 -------------> Al(OH)3 + CO2 + CaSO4

Sodium hypochlorite is used for the purpose of addition of chlorine for the bacterial protection. These chemicals speed up the sedimentation process and the turbidity of the water decreases. There is a pipeline provided at the bottom of the clarifier which leads to the suction of a pump. This pump is used for sluding, it is the process for removal of settled particles at the bottom of the clarifier. Sludge is removed after regular intervals of time by opening the mud valve.Water from clarifier is directed to the Sand Filter. There are three sand filters at ARL. One is taken in operation while one is standby and the third is subjected to cleaning. Water enters at the top of the filters and seeps down from the sand to remove any of the turbidity remaining in it. Screws are provided in the sand filter for the purpose of removal of leaves, as the filters are open to atmosphere. Chlorine is added as sodium hypochlorite for the same purpose.

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Clear water is passed through charcoal filter and through a tank where again chlorine is added. This particular operation is not in service now a day and after sand filters, water is directed to the reservoir number 4 to store the water. Water in the reservoir is tested to maintain a chlorine level of 0.2 to 0.3 PPM. For this purpose, NaOCl is added in the reservoir as well. There are two pumps for pumping of drinking water and both of these pumps are taken into service at the same time. A monoblock pump (motor and pump are fixed together directly with no coupling) is there to pump water for AGL.Water of the reservoir is tested in the laboratory and it is make sure that test results come within the permissible range as given below

Test Description Specifications PH 7-8.5 Total Hardness 100-500 Calcium Hardness 75-200 Magnesium Hardness 50-150 Total Dissolved Solids 289 Chlorides <250 Turbidity Out <0.3 Cl2 <0.3

According to an estimation, the flow rate of water coming from source is at the rate of 90m3/hr. So the inlet flow rate of water that can be treated will be 4,75,000 gallons/day. The water that is pumped to colonies and other associated areas is 3,50,000 gallons/day and the rest of the water is stored in the storage tank.

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CAUSTIC HANDLING & STORAGE FACILITY

Caustic in the refinery, is used for the different purposes. Somewhere (HBU) to execute the neutralization reaction and somewhere (MEROX plant) it reduces the extent of sulfur in the sour crude. Storage, Handling and supply of caustic are responsibilities of utility operations. NaOH is comes into the tank lorries. The solution is 50% concentrated. It is transferred to the two horizontal vessels by means of two centrifugal pumps near the chemical storage at boiler house. These centrifugal pumps are also lined up with a conical vessel which is called mixing tank. 50% concentration is usually never required in any operation that is the reason for the presence of the mixing tank in which dilution of the solution up to 12-15% takes place.Safety lines are provided in the form of over flows from the horizontal vessel and the mixing tank. Caustic solution enters from the top and leaves from the bottom. To dilute NaOH, soft water is added to the mixing tank and then 50% caustic is added to it by means of the centrifugal pumps. Water line is provided at the top of the mixing tank for the circulation purposes. There is a line for air that facilitates mixing of caustic solution with water. Its concentration is lowered to 12-15% usually but it can be diluted even more, subjected upon the demand of the plant side. Diluted caustic is pumped to MEROX and HBU by means of a propeller and a centrifugal pump. There is another centrifugal pump to pump the solution to plan side but this pump may also be used for the purpose of pumping brine solution to the softener from the small brine pit in front of the horizontal caustic vessel.

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WATER PUMP HOUSE & FIRE NETWORK

Water pump house at ARL has 9 propellers. Four of which are equipped with gas engine and five are electrically operated. Pump number 9 is the only pump which has the option to be operated with either gas or electricity. A brief detail is as followed

Pump 1 & 2: Both pumps are for fire fighting purpose. These pumps maintain 80psi pressure in the lines for the emergency purpose. Pump 2 is usually on standby but both pumps can be used at the same time.

Pump 3 & 4: These pumps are to serve the plants. Pump 5 & 6: These pumps are used for the spraying purposes at reservoir number 1.

Usually pump number 6 is kept on standby. Pump 7: It is used to provide services to the plants. Pump 8 & 9: These may be used for fire network or to serve the plants.

Pump Number Flow Rate (GPM) Head (ft) Speed (RPM) 1, 2, 3, 7, 8 ,9 2000 145 1300

4 1300 145 1350 5, 6 4500 65 1000

There are two eye wash pumps at WPH (water pump house), these provide water to all eyewash baths installed to meet the emergencies (i.e. if a chemical gets spilled over any worker). The pumps take suction from drinking water and can pump 60 gallons per minute at 80ft head. There are two pumps and one propeller to facilitate gardens with water and are called garden water booster pumps. Booster pumps can pump 500 gallons per minute to 60ft head.

Firewater network is an underground piping network, designed to supply fire fighting water at all tight areas inside refinery to meet the safety requirements. The network is kept pressurized all the time at the pressure up to 40psi and in emergency situation the pressure is boosted up to 150psi. Network system is equipped with following pumps for system pressurization:

Diesel Engine Pump Electrical Motor Pumps Gas Engine Pumps Jockey pumps

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