Anti-surge Control Valve English

Anti-surge Control Valvefor Gas Compressor

Oil & Gas Refinery

S&T S&T International Co.

Stanley Park (박시우 대표)

President, P.E. (기계기술사)S&T International Co.Mobile : +82(0)10-3840-0721Tel/ Fax : +82(0)2-400-8039E-mail : [email protected]

The article is summarized a presentation, taken for World famous Oil/Gas majors and Korea leading EPCs engineers, when I worked as SEVERN GLOCON Korea regional manager, so that it may be helpful for the engineers.

Compressor Systems

S&T International Co.

Compressor Recycle Valve Problems

Ø High pressure drop, noise & vibration

Ø Leakage across the valve

Ø Wear of the valve trim

Ø Clogging/sticking of the valve

Ø Inadequate response time

Ø Instability in valve operation

● The Gas Compressor must be equipped with a Recycle (Anti-surge) valve and a Blow-down to Flare valve in order to prevent surge and balance the pressure. In other words, if the recycle valve (anti-surge) is to balance the suction gas flowrate balancein the compressor, the blowdown gas to fare valve is installed to prevent the back pressure due to the flow fluctuation in the compressor downstream pipeline (i.e., kick back phenomenon). If the reverse flow of backpressure occurs in the downstream pipeline, it immediately causes a surge in the compressor. Gas Compressor is the most vital equipment in the gas plant. Compressor anti-surge valve and Gas to Flare valve in the downstream pipeline are valves to protect compressor by preventing compressor surge in advance.If a compressor has a problem, it will seriously affect the plant's overall operation, and in the worst case, this can lead to adisruption in gas production, which can be a big loss. Therefore, A-S Valve and Gas to Flare valve are usually separated and purchased as severe service valve inquiry. Especially, in the case of anti-surge valve, the customer prefers to purchase from companies with major delivery records.


● Normally, the seat leakage class is required to V, but when applied as class IV, the driving load equal to the difference in leakage of class V and IV is added to the electric motor, gas turbine and steam turbine which are the drivers.If the manufacturer of the gas compressor proposes a class IV instead of class V, Class IV can be used because there is no problem in compressor operation performance. Particularly the metal-to-metal seat for large valve has large diameter, so it is very difficult to meet the class V standard allowable leakage since there is difficulties at production.

● When a small amount of oil and water particles pass through the high delta P trim, the surface is hit and the surface is worn out.● Carbonized hydrocarbon particles, Black powder, cling to the trim hole or path, clog the cage hole, or the particles become like

tar, preventing the plug from moving properly. When opened, sticky black powder clogged the hole and plug, after a while, theball will open like a bounce. When the fluid is a dry gas, the shape of the black powder is somewhat different.This is made up of chemical and bio-particles, and clogging of the trim can cause serious problems.In this case, fluid flow, trim design, and material should be designed differently from conventional anti-surge valves, and thesedesigns should be referred to an experienced professional.

Anti-surge Control Valve for Black Powder- Samsung E&C. Takreer Jubail Export Refinery #4- Delayed Coker Process

Severn Glocon's Anti-surge Control Valve design was a completely unexpected idea.1. 30 "x 300# Angle Type, 1CC+Quick-opening Cage, Fluid: Black Powder2. Once the Black Powder is attached to the Drilled Hole, it is reattached

to the existing black powder and the hole is eventually clogged.3. Design is made of Dilled Hole Diameter with 13.5mm(?) in the bottom

area, Hole Diameter is made gradually larger (Linear Characteristic) inthe upper part, and Quick-Opening is applied at top area for fasterrecycling to the compressor suction. Wheel Plug applied as well.

4. The flow is over-the-plug to prevent the residual black powder fromattaching to the hole when the valve is closed.

5. Noise was calculated as 110dBA, but 100mm rock-wool was used asan acoustic insulation at the site to reduce the level by 90dBA.(20 dBA noise absorption predicted)

Wheel Plug

Black Powder

Large Dia. Drilled Hole Cage

Compressor Costs Over a 30 Year Life

Ø Costs are for a 20,000

HP compressor

Ø 30 year projected

costs are:

– $4.0million procurement

– $9.0million maintenance

– $260 million

power costs

Procurement MaintenancePower Cost


Compressor Downtime Costs

Ø Natural gas production, 100MM SCFD costs

$18,500/hour in lost sales

Ø Repair costs (20,000 HP axial compressor)

- Seal replacement : $120,000

- Bearing service : $100,000

- New rotor assembly : $2,000,000

The figure shows a typical 30,000 total cost of a 20,000 horsepower (15MW) Compressor spending $273 million ($4 million for procurement, $ 9 million for maintenance and $ 260 million for power cost occurs).The cost of the compressor went to $ 18,500 per hour for gas production losses and $ 222,000 for repairs, which could be astronomical for 30 years.

● To protect the compressor, surge protection systems, control equipment and recycle valves should be provided as measuring

devices. A key function of the surge protection system is to quickly respond to process upset and to recycle the process fluid to

prevent surge on the compressor. The CCC controller reads all input values within 10 to 15 ms.The controller output value is

the average value of the signal for 3 cycles, or about 40 ms, just before it is sent to the valve.

● A valve that is not fast enough in the stroke causes the machine to cause a surge. This surge can shut down the compressor.

The compressor can be tripped due to surge protection of the compressor controller or due to axial shaft movement in the

mechanical package (usually 25μm to 75μm axial displacement: 0.001≤ to 0.003≤).

Compressor Surging

Ø Actual flow reversal occurs within 2 to 5 secØ Surging cycles at 2 to 3 cycles/sec.Ø Compressor vibration occursØ Surging may not be readily detected by conventional

instrumentsØ Most compressors surge at low energy during start-upØ Unstable flow & pressureØ Damage to seals, bearings, impellers & shaftØ Increased seal clearance & leakageØ Lower efficiencyØ Reduced compressor life


The Centrifugal Compressor compresses the gas in such a way that when a long string is hung on a stone, a centrifugal force is generated, which is equivalent to throwing the stone away because of this rotational force. Opening the casing of the Centrifugal Compressor and placing the impeller on the plane and looking at the top, the gas shown in the figure below is centrifuged while rotating along the impeller blade surface. In other words, by moving and damping the momentum using a fluid, the kinetic energy is converted into pressure to realize compression. The momentum is generated in the double curved blades of the impeller mounted on the rotating shaft, and the momentum is generated in the annular passages of the semicircular rings (so called “Diffuser”) which increase along the impeller. The gas compressor normally rotates at 3,000 rpm at 14,000 rpm, and each performance curve moves up and down depending on the number of revolutions. Surge is defined as the maximum head of the compressor and the operating point at which the minimum flow limit is reached, and the compressor loses its ability to maintain its head at the time of surge and becomes unstable. The gas compressor normally rotates at 3,000 rpm at 14,000 rpm, and each performance curve moves up and down depending on the number of revolutions. Surge is defined as the maximum head of the compressor and the operating point at which the minimum flow limit is reached, and the compressor loses its ability to maintain its head at the time of surge and becomes unstable. Centrifugal compressors have one thing in common.

Centrifugal force by rotational motion


Because of the high external circuit resistance at low mass flow, orderly gas flow through the compressor is not possible. The reason is that if the gas mass flow is too low, it will not be possible to convert the kinetic energy required in the discharge line to the pressure energy. It can be thrown away if we hang a stone with a certain weight. However, if we put a light stone on it, theturning itself is not smooth and it is difficult to throw it away. This can be likened to Surge with unstable driving in Gas Compressor. In other words, a certain amount of mass flow must be introduced to the impeller in order for the impeller of the gas compressor to generate kinetic energy and convert it into a pressure head.During normal operation, the compressor operates from the right sideof the surge line. The operating point moves to the surge line due to fluctuations in the flow rate, or a decrease in flow rate during start-up/emergency shutdown. When the operating point approaches the surge line, the impeller and diffuser are operated in a stalled state and flow recirculation occurs. The separation of the flow eventually decreases at the discharge pressure and the flow resumes from suction to discharge.

Centrifugal Compressor Performance Curve Stall - Kite

If the flow of wind is not sufficient whenflying a kite, the kinetic energy of the windcan not be converted into static energy(stalling) and the kite can not fly properly.


A: The design point is the basis of the impeller design. When the compressor's IGV (Inlet Guide Vane) is opened to its maximum, it is the reference pressure to flow rate. The pressure and flow rate at the design point are used to represent the performance of the compressor.

B: Compression ratio of compressor due to increase of system resistance is higher than design point, and flow rate is decreased.E: Compression ratio of compressor due to decrease of system pressure resistance is lower than design point, and flow rate is

increased.C: As the system pressure resistance continues to increase, it reaches the intersection of the performance curve and the surge

curve, peaking the performance curve. The 'C' point is called the surge point, and a surge phenomenon occurs in which the compressor flows backward.

Surge, one of the important characteristics required to understand turbo compressors, comes with compressor control and various operating constraints.When the surge occurs, the pressure and flow rate oscillate, which is because the compressor does not produce a higher pressure than the system pressure resistance, and the backflow occurs repeatedly.

Pressure Fluctuation when Surge Occurs


Surge Process

As described above, the fluctuation width increases with each successive repetition.C → D: Due to the large system pressure resistance, reverse flow occurs and the flow rate decreases. The flow reaches D point

at zero (0).D → E: The pressure produced by the compressor is greater than the system pressure resistance, the flow changes to the

forward direction (reverse flow → normal flow) and then to point E. At this time, the pressure change is constant and only the generated flow increases.

E → C: Moves to point C along the performance curve due to excessive pressure of the compressor.C -> D defined to process of one cycle. If it occurs within about 5 seconds, one-time Surge is specified. If it goes over 5 cycles (Surge 5 times) continuously, it damages the bearing and rotating blade of the equipment.In case of surge, it should be put in unloading state so that it will be released immediately. At this time, open BOV (Blow off Valve) or anti-surge valve and increase suction flow rate.

Surge Line and Surge Parameter of Rotator


In the above example, there are two conditions for the compressor to cause surge. Assuming that the compressor runs at point A, there are two cases of moving to the left (flow reduction) or moving up by Y (compression ratio increase) in order to reach the surge curve. This is why the speed of IGV (Inlet Guide Vane) should be adjusted appropriately in order to avoid the surge during operation of the compressor, and the IGV Minimum Position is set to avoid surge when the compressor is running.

The basic parameter of the surge is expressed as the ratio of the volume flow to the compression ratio.

The surge occurrence curve is expressed by the energy versus flow curve as follows.


Typical Compressor Surge Control System Layout with Single Recycle Valve

There are a number of applications for the surge control system and briefly mention the correlation and variation in the following basic applications:

Pipeline Compression ApplicationIn this case, a typical application of a centrifugal compressor surge system is that the compressor is installed in the pipeline and the compression ratio varies in the compressor. Surge is caused by a reduction in flow on the pipeline, which occurs over theentire surge line (i.e. over a given compression ratio range of the compressor). Changes in gas composition and operating temperature will affect when surges occur, and surge margins should account for these uncertainties. The temperature of the gas is very constant throughout the day, but may vary with the seasons. In this application, the gas component is predominantly methane (75-95%), plus heavy hydrocarbons and N2 and CO2. When multiple compressors are not used in parallel, the pressure and gas composition changes slowly over several hours or more due to the large gas flow in the pipeline.


Re-Injection Application

The compressor is used to re-inject the gas into the production field, and this application requires a variety of discharge pressures

depending on the gas field pressure (at field pressure changes). The composition of the gas differs depending on the various gas

wells, which appear in the shape of a reduced head and flow (separate components) on the surge line. The liquid slug in the gas

makes the fluid flow generally unstable and the surge control system alleviates such fluctuations and the pressure ratio in this

case is very high because the discharge gas pressure is excessively high (3,000 psig = 200 barg), Typically because a multi-

stage compressor is used. Excessive temperature changes in these applications are not typical, but may vary from season to

season. Suction flow rate changes can occur suddenly due to valve operation and process fluctuations under certain conditions.

Storage and Withdrawal Application

In this case, the Compressor is used for loading or unloading into the storage field or reservoir. Suction gas pressure varies with

time (pulling out is very high at the beginning, but rapidly decreases, and at the beginning of the injection is very low and gradually

increases).

The pressure ratio depends on the initial pressure of the storage field, and the gas composition varies with the change of the

storage gas. This application is particularly challenging for the surge control system and requires a quick response to normal

process control operation, but requires a more gradual response as the pressure ratio drops. Therefore, control signals for recycle

valves may need to be more varied than typical pipeline applications.


Gas Gathering Application

This is when collecting the product gas mixture for processing or transmission through the transmission pipeline. This application

can vary greatly in gas composition depending on the type of gas being treated and the level, separation and filtration of the

dehydration used in the particular installation. This may require extensive modeling or design effort to minimize piping volume.

The diversity of the gas mixture due to various gas fields depends on the characteristics of the compressor. causing a change in

the speed curve. The pressure ratio is typically lower than the re-injection and storage applications. At this time, discharge gas is

typically supplied to the pipeline and is maintained below 3,000 psig (200 barg).

High Pressure/Process Compression Application

High pressure compressors vary widely in pressure ratio and gas composition, depending on the application being applied.

High pressure typically spans from 3,000 to 5,000 psig (200-340 barg) in this application.

The process plant compressor is subject to changes in pressure, head and operating conditions, depending on the process type

(LNG plant, refinery, hydrocarbon and NGL removal process), and the gas composition and temperature vary from process to

process. The surge control system should be designed to avoid surge during normal operation, as higher pressures will cause

additional compressor damage due to surge.

The Compressor Anti-surge valve is controlled by the inlet flow rate and creates the compressor inlet / outlet pressure differential.

The fluid changes direction within 2 to 5 seconds and the surge occurs at 2 to 3 times/sec and accompanies vibration. It is not

easily detected by conventional instruments, but it is most likely to occur during initial operation as shown in the performance

curves. Fluid and pressure are unstable, vibration damages seals, bearings, impellers and shafts, gaps occur in seals and

leakage occurs, compressor efficiency decreases and life is shortened.


Ø Recycled valves are typically sized for normal flow,

with rated capacity of 1.8 to 2.2 times normal flow

Ø Overcapacity is required for:

- Safety margin in case valve opens slowly

- Additional venting of the downstream line

Sizing Compress or Recycled ValvesThe valve shall be selected by applying a normal flow rate of 1.8 to 2.2 times the rated capacity (or stem travel to 55%). This excess flow is required as a safety margin when the valve is opened slowly and is required for additional blow-down vent on the downstream pipeline. In addition, it is possible to flow 80% to 120% of margin in order to recycle 100% of the flow rate quickly from the compressor discharge to the suction, and then the margin flow rate is discharged to the atmosphere by the blowdown.

Valve Characterization for Compressor Recycle

In case of initial instable operation during surging, multi-path, multi-stage trim should be used because the recycle flow rate is small and pressure difference takes the suction / discharge pressure of compressor.Since close to the surge line, the flow rate must be returned to the suction side, the delta P is designed to be combined with asingle stage trim, which can handle a large amount of flow at a time.

Characterized Valve Trim


Ø MLT/CC trim for low noise, velocity & energy control

Ø Long valve stroke permits characterization of the disk stack or use of high capacity trims

Ø Characterized trim for good low flow control & high flow capacity protection

Ø Metal seal bonnet design retains the MLT stack in compression

Ø Internal stops in the valve to absorb impact load

Ø Expanded outlet to handle gas expansion

Ø Balanced trim for running forces

Ø High seat loading for tight shutoff

Ø Sizing for normal flow must be considered as potentially extended duty

Ø Sizing for emergency flow can be considered as intermittent duty

Ø Local HSE guidelines for worker exposure to noise

Ø Consideration for noise & damage to the downstream pipe system

Ø Consideration for potential damage to the valve or components from vibration

Control Valve for Compressor Recycle and Noise Considerations

● The valve selection should be based on the normal flow amount considering future capacity expansion operation, and valve sizing should be done with emergency flow amount considering intermittent operation. Noise should follow the Health Safety Executive (HSE) guidelines for the operator, consider noise in the downstream pipeline of the valve, and also consider valve and components caused by vibration and downstream pipeline damage.● Professional manufacturers typically apply multi-stage and multi-path / CC trim to Severe Service conditions, which are suitable for large-capacity disc stack valves with long strokes and can make characterized curves.This characterized trim shows very good performance from small capacity to large capacity. The metal seat supports the weight of the multi-stage / multi-path stack, and the valve seals absorb impact loads.Considering the gas expansion, the valve outlet is made into an expanded structure. The balanced trim is applied in consideration of the force during operation, and the seat which can withstand the large loading is applied for tight shutoff

0102030405060708090

100

0 10 20 30 40 50 60 70 80 90 100

% Flow

% Stroke

Modified Linear

Linear

Modified Equal %


Disk Stack Characterization

Ø Linear stackThe trim has a stack or cages with the same

number and size of holes or passages on

the full length of travel.

- Flow is directly proportional to the valve’s

stroke at constant differential pressure

Ø Characterized stack- All disks are not the same

- Provides precise variable flow versus

pressure drop over the full range of the

valve

Ø Application of high unit loading to valve seat surfaces

Ø Finesse lapping of seating surfaces

Ø Use balanced trims to reduced hydraulic forces

Ø Use soft seats to minimize loading requirements

Ø Pressurized seat designs

The linear stack allows the flow rate to be proportional to the valve stroke under constant delta P conditions in all strokes by constantly adjusting the number of holes and the diameter of the cage trim. The characterized stack shows that all discs are not the same, which it is designed to be precisely matched to the change.Because of the large load on the seat surface, precision machining and polishing of the seat surface is required, a balance trim is applied to reduce the fluid force, a soft seat is used to minimize the load, and a pressurized seat structure must be designed.

Methods to Obtain Shutoff

Seat Leakage Ratings (Most Common)

Ø FCI-70

- Class II : 0.5 % of rated CV

- Class III : 0.1% of rated CV

- Class IV : 0.01% of rated CV

- Class V : 0.0005 cc/min * Dia.(plug) * shutoff,

psi

- Class VI : Defined in bubbles/min, gas test


Shutoff Loading Requirements- ISA Guidelines

Ø FCI-70

- CLASS IV : 300 pounds force/linear inch

- CLASS V : 500 pounds force/linear inch

- CLASS VI : 1000 pounds force/linear inch

(per MSS-SP-61)

● ANSI FCI-70 has provisions for seat leakage class andshutoff loading and is also mentioned in the ISA Guideline(Leakage Class at Control Valve Article for more information).

● As test media, air is usually applied rather than water because it is costly to use water and wipes and dries the interior to prevent rust after the test, but air can be easily used at the factory.As shown in the picture above, in the case of large valves, metal to metal seats should have almost no leakage at the balance seals and seat rings in order to achieve high class V. This requires precise coincidence between stellited stems / plugs and stellited seat rings, and accurate machining and straightness of the bonnet flange and body flange surfaces.

Actuator Design Features for Compressor Recycle

Ø Double acting “low volume” spring-less pneumatic cylinder for

- High force levels- High stiffness- Minimum air volumes for fast response- Wrought end plates, steel tie rods for impact

loading


Control Valve Response

Ø Opening response times

- Emergency opening (trip) : 1 to 2 sec

- Surge protection opening (modulate) : 2 sec

- Delay/lag time : 0.2 to 0.4 sec max.

Ø Closing response times can be slower depending on

surge control design

- Modulating : 3 sec

Ø Control valve stability

- Minimal overshoot & quick correction without

oscillation

With a double acting pneumatic piston actuator, powerful thrust force, high rigidity, fast response to minimum air, and sophisticated

assembly case and steel seams make it resistant to impact loads.

The reaction speed at the time of valve opening should be 1 to 2 seconds for tripping, 2 seconds for surge protection, and the delay

/ lag time should be 0.2 to 0.4 seconds at the maximum. The closing time of the valve is to be gradually closed according to the

surge control design, and is usually designed to be 3 seconds in the modulating mode. In order to ensure stable operation of the

valve, minimum overshoot and quick correction must be done without oscillation.


Closing time The client specifications typically require fast closing times (Aramco specification is three times the opening time),but the GE compressor required a closing time of less than 12 seconds. This is because, when the compressor returns to normaloperation in the surge state, the operation of the compressor becomes unstable if the valve is closed earlier or later than theclosing time required by the compressor for stable operation of the compressor, which may cause vibration of the compressorimpeller. These vibrations accumulate the fatigue strength and can cause the compressor to trip when a displacement occurs inthe shaft (usually 25μm to 75μm axial displacement: 0.001≤ to 0.003≤).Overshoot It is possible to apply the 3.0% relaxed GE specification which is less than the 2.5% tolerance of the client’sspecification. If modulating purpose is influenced by the downstream process, sophisticated valve throttling is required andrigorous overshoot is required. However, quick opening the required anti-surge valve can be applied by GE's relaxed 3.0%overshootStroke time The time it takes the actuator's piston to reach the required stroke length (from bottom to top, or vice versa), is mechanical.Response time It refers to the time it takes for the valve actuator to start moving by bringing the command to the positioner so that the valve can be operated by transmitting the amount detected from the suction and discharge of the compressor to thecontroller by the electronic control system. When the emergency shutdown valve located at the downstream of the compressor is closed, a kick-back phenomenon may occur, leading to a compressor surge, where the controller sends a signal to the positioner so that the valve can be opened accurately and quickly. The valve should open quickly (within 2 or 1 second) to prevent surge. On the other hand, when the valve is closed, it must be closed in conjunction with the compressor operation and then compressor returns to normal operation. At the same time, the command to the positioner to gradually close the valve will stabilize the compressor back pressure. The control logic for the response time should be programmed such that the closing time is usually 3 times the response time for valve opening. The operator specification for the response time should be applied to the controller supplier. This is very important for the anti-surge control system. Even if the stroke speed of the valve actuator is fast and accurate, it may lead to a compressor trip with the surge if the response time in the controller is late or the valve does not work properly due to malfunction. Therefore, the client designates a highly reliable controller company (CCC-USA, etc.) with high technology and sufficient references.

S&T International Co.Static PerformanceThe static performance criteria relate valve position to the input signal, and

are influenced by friction and valve hydraulic loads. All of these factors will

affect the accuracy of a valve and its ability to control the process.

Dead Band The measure of the smallest change in signal control required

to achieve any stem movement. It is sometimes referred to as sensitivity.

If you can imagine a linkage with a slot and a pin. As the actuator is moving in

one direction, the pin is pressed against the side of the slot, moving the valve.

When the actuator changes direction, the pin has to press against the other

side of the slot before the valve will change direction. If there is significant play

in the slot there will be a period when the valve does not move.

Hysteresis The difference in stem position is the same control signal when

approached from upscale and downscale. For it to be true hysteresis the valve

will be moving at all times. Hysteresis is most often caused by a high degree of

static friction within the valve.

Linearity A measure of the deviation from an ideal straight line along

the position versus signal graph. The valve in Table are typical in service

for DRAG® valves(IMI). The linearity of a valve is related to how close to

a straight line the graph of travel vs. signal


Resolution The smallest change possible at the valve stem position .An example of a typical test for dead band and resolution is shown in Figure 1. The sticky behavior of valves is often referred to as “stiction.” It is the result of the interaction between static friction and dynamic friction. Static friction is usually much higher than dynamic friction. As a result, a valve tends to stick in place until enough pressure builds up in the actuator to break the static friction, then the valve moves quickly to the new position. Resolution is a measure of the smallest movement that a valve is capable of in the same direction. This is called a static test,because one must always wait long enough after each step for any possible movement to take place. No measurements are made while the valve is moving, but only the valve’s static position is recorded after it has come to rest. The control signal is stepped in one direction in very small steps. After each step there is a waiting period to make sure there is time for the valve to make any move it is going to make before the next step is initiated. Observing the number of control signal steps that are required to make the move indicates how sensitive the valve is, commonly referred to as “resolution.” After a number of steps in the same direction, the direction of the steps is reversed. Observing the number of steps required to initiate reversal of valve motion indicates what the dead band is. In this example, the step size is 1/4%. In the same direction, this valve responds to each 1/4% step, so it has a sensitivity or “resolution” of at least 1/4%. Upon reversal, it took two of the 1/4% steps before the valve started moving in the reverse direction, so this valve has a dead band of no more than 1/2%. Dead band shows up in the process as dead time, which is destabilizing to control. Note that the scales for the input and position are different so that the two graphs will be easier todifferentiate from each other.

Low Temp. -29 to 232℃

High Temp. 232 to 566℃

Dead Band 1% 2%Hysteresis 1% 2%Resolution 1% 2%Linearity 3% 3%

Typical Valve Stroke(IMI-CCI)


Turbo Expander is used to expand the gas to lower the gas temperature by lowering the pressure, which is usually used to liquefyheavy-carbon gas. In the case of a compressor driven by a Turbo Expander, the closing time of the compressor A/S valve must be higher than 10,000 rpm (with a typical compressor operating at 5,000 - 7,000 rpm). Therefore, Aramco's anti-surge valve for Shaybah Gas Package # 2 carried out by Samsung Engineering was limited to 2 seconds. In addition, the Joule Thomson valve for Turbo Expander as bypass is connected to this valve and the compressor A/S valve, so the control mode of the CCC controller is much more complicated than the general gas / steam turbine or electric motor driven compressor.

Expansion brings 2 positive effects

through expansion by 2 phase turbine

♦ Power production

♦ Temp. is lowered with same pressure

drop due to less heat produced by

friction

Improved System for Turbo Expander

Dynamic ResponseDynamic response parameters measure the valve’s position as function of time when the signal is changing. Common parameters for dynamic response are over or undershoot, or to time to stabilize in response to a step input, and frequency response at -3dBfor sine wave testing. Specifying these parameters is not cost effective unless the dynamic response of the valve is crucial to the rest of the system.Stroking Speeds Typical stroking speeds for DRAG® valves(IMI) vary from 0.25in/sec. to 2in/sec/, with speeds up to 6in/sec. in modulating mode and 12in/sec. in on-off operation. These specifications more than 30 seconds.


Gas-to-Flare blow-down valve to prevent kick-back when surge occurs

Straight Pipeline Length for Anti-surge ValveThe valve downstream pipeline is critical for surge protection and the vortex is extremely harmful for essentially stable compressor operation. The linear pipeline length of the valve front end is not so severe compared to the rear end, and the inlet flange diameter is two to three times as large as that of the inlet flange diameter to prevent vortex flow.The length of the straight pipeline at the rear end of the valve is disadvantageous as the length is long, because the gas must be recycled quickly to the compressor suction, but the shorter the pipette length, the higher the vortex generation. In addition, the higher the valve outlet speed, the more likely the vortex will occur. The minimum straight pipe length should be at least four times the valve outlet diameter.

Hot Gas Bypass

Ø Severn Glocon has supplied many hundreds of high speed response compressor recycle valves.

Ø The total number of hot gas bypass valves is a small portion of the above number.

Ø Hot gas bypass normally involve on/off function for compressor protection in the event of power loss.


Hot gas bypass valve does not require precise control according to flow rate change like anti-surge valve. In the case of a high-pressure compressor with a large compression ratio or a large flow rate, the surge cannot be blocked by Cv of the anti-surge valve at start-up or sudden shutdown, in addition to the anti-surge valve. In addition, the installation position is shorter than the conventional anti-surge valve at the front of the cooler, so that the discharge gas can be quickly recycled to the suction without cooling the discharge gas. Therefore, this valve requires quick opening. Of course, in the case of a quick opening, valves with too large a Cv may cause another problem, so it is necessary to determine the valve opening characteristic through dynamic simulation. In addition to the basic single cooling recycle valve loop, designs with hot gas bypass with secondary cooling loop can be designed.

S&T International Co.Hot Has Bypass Valve Application■ Refrigeration Cycle : The high-pressure gas compressed by the compressor is sent to the condenser, and some of the

hot gas is bypassed and sent to the evaporator to remove the ice or frost caused by sub-cooling

■ Dehydration Process : Remove the acid gas from the natural gas during the LNG process, compress it, and supply the hot

gas to the dryer to remove moisture before entering the propane cooling unit (Refer to Severe Service Valve at LNG Process).

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Anti-surge Control Valve English