CHAPTER-3

Testing:

Testing means to check instrument in on and off condition.

Calibration:

Calibration means comparison between measured value of instrument and standard value of instrument or the corresponding known value of the measured quantity.

Maintenance :

The term ‘maintenance’ means to keep the equipment in operational condition or repair/replace it to its operational mode.

Analog Temp. Indicating Instrument

Analogue thermometer

Test Procedures

Analogue Indicating Instrument

Set the Analogue thermometer instrument to zero.

Apply the load and record the indication.

Remove the load.

Reset Analogue thermometer instrument to zero if the indication is not showing zero.

Repeat steps 2 to 4 two more times.

Determine whether the instrument has passed or failed.

Record results on the test report.

Calibration:

Method 1: Ice Water

Fill a glass with ice cubes, then top off with cold water. 

Stir the water and let sit for 3 minutes.

Stir again, then insert your thermometer into the glass, making sure not to touch the sides. 

The temperature should read 32°F (0°C) which is measured value of instrument. 

Ice point of water is 0°C which is standard value.

Record the difference.

Use zero and span adjustment if measured and standard reading are not equal.

Method 2: Boiling Water

Boil a pot of distilled water.

Insert your thermometer, making sure not to touch the sides or bottom of the pot.

The temperature should read 212°F (100°C) which is measured value of instrument. 

Boiling point of water is 0°C which is standard value.

Record the difference. 

Use zero and span adjustment if measured and standard reading are not equal.

Maintenance procedure

Install it in easily visible place for proper reading.

instrument shall:

o be clearly and permanently marked with the capacity and scale interval.

o be clearly and permanently marked with the manufacturer’s name or mark and serial number;

follow manufacturer’s warranty procedure

Thermometers are very sensitive and will break, or lose their accuracy if they are dropped or handled roughly. You must make sure that the thermometer is fixed or replaced if it breaks.

Indicator correctly protected from heat,vibration.

Thermocouple testing and calibration

“A thermocouple circuit is formed when two dissimilar metals are joined at both ends and there is a difference in temperature between the two ends. This difference in temperature creates a small current and is called the Seebeck effect after Thomas Seebeck who discovered this phenomenon.”

Seeback Effect

“When there is a difference in temperature between the two ends of circuit of two dissimilar metals, a small voltage is formed within the circuit.”

This voltage or EMF (electro motive force) is usually measured in the 1/1000th of a volt (millivolt). Most people’s body produces more voltage than that! The higher the difference in temperature, the higher the voltage. If the right pairs of materials are used, these thermocouple circuits can be used to measure temperature.

Measurement:

Set your controlled temperature source to the specified temperature and allow it to adequately stabilize. Immerse the test assembly into the test temperature medium and provide sufficient time for the test assembly to stabilize. Once the test assembly is stable the EMF generated between the test specimen and the reference standard can be recorded. Avoid soaking the test

assembly at temperature for a prolonged period of time, as it can cause permanent changes to occur in the thermo elements. Once the reading is taken, raise the test temperature to the next higher temperature, first removing the test assembly from the temperature source, or advance the test assembly to the next temperature source. Allow the temperature source and the test assembly to stabilize as before, and take a second set of readings at the new temperature. In all cases take the reading in sequence from the lowest to the highest temperature. A base metal reference standard shall be used for one series of temperature changes only.

Calibration

The set of operations that (under specified conditions) establishes the relationship between the indicated or nominal value of an instrument and the corresponding known value of the measured quantity.

Thermocouple Calibration Methods

Some calibrations consist of copper as one of its metals. Thermocouple, an industrial thermometer consisting of two dissimilar metals joined together, operates on the principle that heating the metals to a certain temperature produces a certain voltage. By measuring the voltage, the thermometer, with a capability of reading up to 2,600 degrees Celsius, determines the temperature. Thermocouple refers to the joining of two metals, while calibration denotes the combination of metals, with different combinations utilized for different temperature measurements. Calibration also refers to the method of testing the thermocouple for accuracy.

Controlled Temperature Source According to Branom Instrument Company, one method for thermocouple calibration involves the use of a controlled temperature source. This source must maintain a constant temperature, normally 32 degrees Fahrenheit, for a minimum of 20 minutes. An ice bath, consisting of melting shaved ice and water, produces that constant temperature at which ice melts.

Comparison

According to the National Institute of Standards and Technology, taking a thermocouple that has been proven to be accurate, known as a reference, and using that device as a means to calibrate another thermocouple, provides a method of calibration that is as accurate as the reference calibrated instrument.

Calibration Furnace

Another method involves a microprocessor and computer software that perform the calibration, which is based upon the constant temperature of the furnace. Placing probes into the furnace allows that temperature to be the means by which calibration occurs, with the software producing the readouts.

How to Calibrate a Thermocouple

A thermocouple can be any junction between two different metals and may be used to measure temperature. Each metal produces a different electrical potential that varies according to changes in temperature. This rate of change is different for each of the metals in the thermocouple, so a thermocouple produces a voltage that increases with temperature. You can calibrate a thermocouple by plotting the thermocouple's voltage-temperature curve.

Instructions

Things You'll Need:

Multimeter Digital thermometer Electrical device with a thermocouple Thermo bath Thermocouple table Water

1. Fill the thermo bath container with water and turn the thermo bath on. Heat the water to 30 degrees Celsius and turn the thermocouple device on. Connect each lead of the multimeter to one end of the thermocouple. This multimeter should be able to measure a voltage of 1 microvolt.

2. Place one junction of the thermocouple into the water and allow the voltage to stabilize. This occurs when the voltage stops fluctuating except for the last digit. Record the stable portion of the voltage from the multimeter.

3. Increase the water temperature to 35 degrees Celsius and record the stable voltage on the multimedia again. Repeat this procedure for each 5-degree increase in temperature from 35 to 60 degrees Celsius.

4. Measure the room temperature and look up the voltage for your thermocouple type at the room's temperature. For example, the voltage for a type K thermocouple at a temperature of 25 degrees Celsius is 1 milivolt. Add this value to each of the voltages you recorded in Steps 2 and 3.

5. Use the curve-fitting method of your choice to find the line that best fits your recorded data. The slope of this line provides the voltage increase for each degree of temperature increase. The voltage on a standard type K thermocouple should increase about 40 microvolts for every degree Celsius increase in temperature.

Maintenance procedure

General

Every instrument shall:

(a) be clearly and permanently marked with the capacity and scale interval, on or in the vicinity of any mass-indicating device;

(b) be clearly and permanently marked with the manufacturer’s name or mark and serial number;

(c) manufacturers warranty procedure.MAINTENANCE:Thermocouples will deteriorate due to contamination from their environments. The quality and frequency of calibration checks must be determined for each individual application by noting the drift rate of each thermocouple at individual installations. Calibration is usually made by comparison to a primary standard. The thermocouple may be removed from its installation and checked in a tube furnace against the primary standard. Return thermocouples that were removed for tests to the same location and immersion depth for reliable and repeatable readings. Do not use a thermocouple to measure a very low temperature if it has been used to measure very high temperatures previously. Make sure the protection tubes and thermo-wells are in good condition when protecting thermocouples with them.

Preventative Maintenance - Thermocouples, protection tubes, and extension wires should be checked periodically. Experience largely determines the frequency of inspection but once a month is normally sufficient .Check out extension wire by making certain that it meets the established external resistance required. Damaged or burnt out protection tubes and thermo-wells should be replaced to prevent damage to the thermocouple. should be checked in place if possible. If removing the element is necessary, it should be to the same depth or deeper to avoid errors arising from placing an inhomogeneous segment of wire in a steep temperature gradient.

Recorders Circular Chart Recorders A circular chart recorder records data on a paper disc rotated beneath one or more pens, which, as in a strip chart recorder, deflect with fluctuating electrical signals. The difference is that the resulting chart is circular rather than linear. Circular chart recorders are ideal for batch processes that operate within a known timeframe. They can be configured so that each rotation of the chart covers a standard time period—1 hour, 24 hours, 7 days, etc. Some recorders will also accommodate non-standard periods. The advantage of a circular chart is that, at a glance, it gives a complete history of one or several variables over the specified period.

Description(YANTRIKA) Circular Chart Pressure /Temperature Recorders are technically proven instruments for the performance, ruggedness, durability & accuracy and are used for recording pressure / temperature variations in any process industry.

Working Principle

Pressure: Rise in pressure deflects the sensing element which is connected by mechanical linkage to the Pen arm, which records the Pressure on suitably calibrated chart. This chart will be rotated anti-clockwise with the help of chart drive at a constant speed. This helps us to find out pressure variations at any point of the time period.

Temperature: Rise in temperature causes mercury in the capillary to expand which is connected by mechanical linkage to the Pen arm, which records the temperature on suitably calibrated chart. This chart will be rotated anticlockwise with the help of chart drive at a constant speed. This helps us to find out temperature variations at any point of the time period.

Pressure /Temperature Recorders can be manufactured with single or double pen arrangements and are driven either by a Mechanical / Electrical Chart Drive or Quartz Drive. The Mechanical Chart Drive is recommended for hazardous applications. Every pressure recorder is calibrated to ensure the guaranteed accuracy of ± 1%.

Input Calibration Procedure

Calibration procedure sequenceThe calibration procedure sequence for all inputs is listed in Table 5-9. The calibration procedure for inputs 1 and 2 is identical. The displays indicate the input number.Press the FUNC key to change display as required (INP1 or INP2).

Current Output Calibration

Introduction

Calibrate the recorder so that the output provides the required amount of current over the desired range. The recorder can provide an output current range of from 0 to 20 miliamperes and can be calibrated at 4 mA for 0 % of output and 20 mA for 100 % of output, or at any other values between 0 mA and 20 mA.

Equipment needed

You will need a standard shop type milliammeter, with whatever accuracy is required, capable of measuring 0 milliamps to 20 milliamps.

Calibrator connectionsRefer to Figure 5-6 and wire the recorder according to the procedure given in Table 5-10.

Procedure

The procedure for calibrating the current proportional output is listed in Table 5-11. Make sure “LOCK” in the Set Up group is set to “NONE.”For display recorders with 2 pens, be sure the correct input is on display—INP 1 or INP 2 indicator is lit.Press FUNC key to change input on display to agree with the control loop output to be calibrated.

Routine Maintenance

Introduction

Unless the recorder’s location will expose it to dust, the DR4300 recorder does not require any periodic maintenance except the replacement of the chart and ink cartridges.This section provides instructions for replacing the chart and ink cartridge. It also contains tips for maximizing the life of the pens, including recommended practices if the recorder is used in a dusty area.

ATTENTION

Humidity can affect the size of the chart, resulting in the pen being offset from the proper chart increment. Instructions for aligning the pens are provided in Section 7 – Troubleshooting and Pen Alignment for Recorder Without Display and Section 8 – Troubleshooting and Pen Alignment for Recorder With Display.

What’s in this section?The following is a list of the topics covered in this section.

Routine Maintenance of chart recorder

Replacing the Chart

Strip Chart Recorders Strip chart recorders consist of a roll or strip of paper that passes linearly beneath one or more pens. As the signal changes, each pen’s deflection records the process being measured in the form of a chart. Well suited to recording of continuous processes, strip chart recorders are commonly used in both laboratory and process-measurement applications. For future reference, sections of the paper can be torn off and archived.

For Testing, calibration and maintenance see pdf file

What is a Transmitter?

A Transmitter is a device that converts the signal produced by a sensor into a standardized instrumentation signal such as 3-15 PSI air pressure, 4-20 mA DC electric current, Fieldbus digital signal etc., which may then be conveyed to an indicating device, a controlling device, or both. The indicating or controlling device is often located in a centralized control room. The transmitter often combines a sensor and the transmitter in a single piece. The sensor measures the process variable and generate a proportional signal. The transmitter then amplifies and conditions the sensor signal for onward transmission to the receiving or controlling device.

Transmitters Used in Process Instrumentation:

Transmitters can be broadly divided into two broad groups:(a) Electronic Transmitters(b) Pneumatic TransmittersElectronic transmitters can either be analog or digital/smart as the case may be.We can further group transmitters according to the types of signals they produce namely:(a) Pneumatic Transmitters(b) Analog Transmitters(c) Digital TransmittersPneumatic Transmitters

Pneumatic transmitters output a pneumatic signal corresponding to the process variable. The pneumatic signal range that is commonly used in industrial plants today is the 3 – 15psig. 3psig corresponds to the lower range value (LRV) and 15psig corresponds to the upper range value (URV). It is still commonly used today especially in remote locations where electric power is not readily available.The invention of electronic instruments in the later part of the twentieth century significantly brought down the costs involved in running electrical signal wire through a plant as opposed to running pressurized air tubes. This has made the pneumatic signal technology less popular.

As shown below, a pneumatic pressure transmitter is supplied with air pressure typically 20 – 30 psig depending on the application. Process pressure is applied to the High port of the transmitter. As the process pressure varies, the transmitter produces an output signal (3 -

15psig) that is proportional to the process pressure.

Analog TransmittersAnalog transmitters are mostly electronic in nature. They output an electrical signal (current or voltage) whose magnitude represents a physical measurement or a control quantity. The transmitter is classified as being analog by virtue of the fact that it uses an analog signal standard to communicate information. The most common standard for transmitting an analog signal is the 4-20 mA current signal. With this signal, a transmitter sends a small current, proportional to the physical measurement, through a set of wires. In this signal standard, 4mA represents the lowest possible measurement or the LRV (Lower Range Value) while the 20mA represents the highest possible measurement or URV (Upper Range Value).As shown below, the transmitter produces an output signal of 4 – 20mA when the process variable is applied to the transmitter:

Digital TransmittersDigital transmitters produce digital signals that are combined in a variety of ways to enhance communication with the devices; enhances diagnostic capabilities of the device and makes control of the devices and processes relatively easy and smooth. Digital signals are discrete levels or values that are combined in specific ways to represent process variables and also carry other important information, such as diagnostic information. Digital transmitters combine the digital signals in a variety of ways leading to various communication protocols

such as Fieldbus, HART etc. Most digital transmitters may be referred to as smart instruments. They have inbuilt microprocessors that helps in signal conditioning and processing and gives the devices some diagnostic capability.

How to Calibrate DP Pressure Transmitters: 8 Effective Tips that Works

Calibration of a DP pressure transmitter involves a process by which the output of the transmitter is adjusted to properly represent a known pressure input. Calibration is one of the most frequently performed maintenance operations on pressure transmitters. If well performed, the transmitter’s performance improves otherwise its performance could deteriorate with grave consequences. A pressure input is used to provide zero and span adjustments to the transmitter in the calibration process. Consult my previous post: How to Calibrate Your DP Transmitter for a detailed guide on how to calibrate a DP pressure transmitter.

Owing to the fact that a plant could go berserk, if one or two critical pressure transmitters are wrongly calibrated, it is important the calibration process and procedure be done properly. The following tips are general guides that you should have at the back of your mind when calibrating a DP pressure transmitter:Tip 1:

Read and understand the calibration procedure in the manufacturers’ instruction manual. The calibration procedures in the manual should be followed carefully to ensure a proper calibration.

Tip 2:

The use of proper calibration equipment is crucial. The pressure source and any readout device in use must be of greater accuracy than the instrument being calibrated. Some experts in calibration have posited that as a general rule, the pressure source and readout device should be at the minimum four times more accurate than the device being calibrated. High accuracy measurements cannot be obtained when the calibration is done with low-accuracy equipment. It should be a regular practice to check the accuracy of calibration equipment against a higher standard on a regular basis to maintain the accuracy of the calibration equipment.

Tip 3:

When doing calibration, leaks are a potential source of error. Eliminate all leaks in the calibration system. Use TEFLON tape on all pressure connections.

Tip 4:

Trapped liquids in the pressure transmitter are also a potential source of error. Drain all liquids from the transmitter and impulse piping before starting calibrating.

Tip 5:

Linearity adjustments are crucial in any calibration process involving transmitters. Linearity adjustments should only be made at one point. All other points should be used to check the adjustments only.

Tip 6:

Most DP pressure transmitters come with an electronic damping pot for curbing erratic output. Therefore, damping should only be set after the pressure transmitter is placed in service.

Tip 7:

Temperature is a critical parameter in transmitter calibration. Transmitter performance is affected by changes in ambient temperature. To minimize the effect of temperature change, calibration should be done at the expected ambient temperature. If temperature is expected to fluctuate, it will be good practice to calibrate between the extremes.

Tip 8:

Transmitter performance is also affected by changes in static pressure. We can reduce these effects if we calibrate at the line pressure. If this is not practicable then the pressure transmitter should be put in service after calibration and re-zeroed after the transmitter has reached the operating pressure.

How to Calibrate Your DP Transmitter

To calibrate an instrument involves checking that the output of the given instrument corresponds to given inputs at several points throughout the calibration range of the instrument. For the analog DP transmitter, its output must be calibrated to obtain a zero percent (4mA) to 100 percent (20 mA) output proportional to the DP transmitter’s zero percent to 100 percent range of input pressures. 

in other words calibration of the transmitter is required to make the transmitter’s percent input equal to the transmitter’s percent output. This is accomplished by adjusting screws located and clearly marked as ZERO and SPAN on the analog transmitter’s outer casing. The ZERO and SPAN screws may also be referred to as the ZERO and RANGE adjustment screws for some manufacturers of DP transmitters. Whatever the model/manufacturer of your DP transmitter, it can be easily calibrated according to the manufacturers specific instruction on how to calibrate it. For every calibration you need to do, consult your manufacturer’s specific instruction for calibrating the specific DP transmitter.

However there are general guidelines you need to follow before you calibrate any transmitter:

Step:1Ensure all the materials needed for the calibration are within reach e.g meters, pressure source, pressure gauge, Digital multimeter, power supply module(24V) etc 

Step 2:Record and put down the following(can easily be sourced from transmitter nameplate): (a) Transmitter make and model (b) Transmitter calibration range (c) Transmitter span (d) Transmitter MWP(Maximum Working Pressure)

Step 3:Connect all the equipment needed for the calibration exercise in the appropriate manner. To ensure you don’t make any mistake, you should draw a connection diagram for all equipment involved paying particular attention to polarity of transmitter and power source!  Then connect them according to your connection diagram. A typical DP cell transmitter calibration diagram is shown below:

For your application, this could be modified slightly. For example if the pressure source is a hand pump, you can easily control the pressure applied to the DP cell. However, if you are doing field calibration that requires the use of the actual process pressure, you will need a pressure regulator in conjunction with a pneumatic calibrator to help you control the pressure applied to the DP cell. 

Step 4: 

Most transmitter calibration done is a five point calibration. That is for 0%, 25%, 50%, 75% and 100% of input span or range (in this case pressure input into the DP transmitter). This should correspond exactly to 0%, 25%, 50%, 75% and 100% of the transmitter output span (4- 20mA). The graph below illustrates the correlation between input and output values.

Readings are taken for both increasing and decreasing input values and the corresponding transmitter output values are recorded

Step5:In most calibration done, you will be doing either a bench(shop) calibration – A bench calibration is a procedure where the device is calibrated at a calibration bench using calibration devices to simulate the process, – or a field calibration where the actual process is used. Whether you are doing a bench calibration or a field calibration, the low port of the DP transmitter cell is vented to the atmosphere(as shown in the connection diagram above) and the high port of the DP transmitter connected to a pressure source e.g a hand pump or any other suitable pressure source in a bench calibration or the actual process pressure through a pressure regulator and a pneumatic calibrator in a field calibration. So once your equipment is well setup, power it up and pressurize the high port of your DP transmitter. Record the current reading in (m A) which will be your first data point. Continue pressurizing the transmitter and recording your readings for the five points (0%, 25%, 50%, 75% and 100% of input pressure). All the reading obtained will be the as found readings. If you calibrate the DP transmitter before first testing and recording the as found data, the history of the device performance data will be lost.

You are now ready to calibrate the DP transmitter. Note that during the calibration process, the transmitter’s zero percent, (LRV), is to be calibrated to the , LRV, of the calibration range and the transmitter’s span is to be calibrated to the, URV, of the calibration range. For

example, suppose a DP transmitter with output 4 – 20mA is to be used to measure pressure in the range 0 – 300 psig, then the transmitter zero percent (LRV) is 4mA and will be calibrated to 0psig and the transmitter’s 100%, URV, which is 20mA will be calibrated to 300psig. Locate the manufacturer’s instruction manual and with it locate the transmitters ZERO and SPAN adjustment screws sometimes called Zero and Range adjustment screws. Note that these screws each connect to a variable resistance (potentiometer) and can be turned indefinitely. That is the potentiometer is of a type that once fully adjusted clockwise or counter clockwise the screw may continue to turn without further varying the resistance for either direction.The potentiometer has a maximum of 20 turns between minimum and maximum resistance therefore turning the ZERO or SPAN screws clockwise or counter-clockwise for 20 turns will cause the potentiometer to be at either maximum or minimum. Please note that for the analog DP transmitter,  the ZERO and SPAN adjustments are interactive. That is, adjusting one has an effect on the other. Specifically, changes made to the span adjustment almost always alter the DP transmitter’s ZERO point. This back and forth adjustment of the ZERO and SPAN is what makes the DP transmitter calibration sometimes tedious.

Step7:Turn the ZERO and SPAN screws both 20 turns clockwise. Next turn both screws10 turns counter clockwise to approximately adjust the potentiometer to the mid resistance point (50%).

Step8:Apply the 0% (LRV) pressure to the transmitter’s High side, and the transmitter’s low side vented so that there is no differential pressure acting across the transmitter’s DP cell.

Step9:Adjust the ZERO screw on the transmitter while observing the current meter to cause the indication to be 4m A, which is the transmitters LRV output. This may not be exactly 4mA but depending on your plant’s acceptable margin of error, you should get a value very close to4mA.

Step10:Next pressurize the high side of the DP transmitter to cause the pressure applied to the high side to increase to the 100 percent value (URV) of the calibration range.

Step11:Adjust the SPAN screw while observing the meter’s current indication to cause the meter to indicate 20 mA, which is the 100% (URV) output value signal for the DP transmitter.

Step12:100% input to the transmitter (pressure) exactly equals the transmitters 0 % through 100 % output (4 - 20 mA current). A correctly calibrated DP transmitter can be described as one where the % input equals the % output for all values between 0 and 100 percent.

Once you are satisfied with the level of accuracy of the calibration result, you are done with the calibration of the DP transmitter otherwise you will have to continue fine tuning the calibration process until a reasonable accuracy is achieved.

Level Limit Switch

Operation/testing

Another type of float switch uses a magnetic-activated switch. From the two diagrams in ill. 2 notice the float in this type of switch has a rod connected to it just like the previous type float-level sensor. This type of sensor has a permanent magnet connected to the end of the rod. ill. 2b shows that when the float raises with the liquid level, the magnet is moved into position so that it's near the magnetic-activated switch that is mounted in the head of the sensor. When the permanent magnet on the rod is in the correct position, it will pull the movable magnet that activates the switch. When the movable magnet is pulled to the magnet on the rod, the switch contacts close. Notice in ill. 2a that when the liquid level drops and the float allows the rod to be lowered, the magnet will no longer have any attraction to the switch. Small springs will then cause the contacts to move to their normally open position. (Notice that the switch has a single-pole. double-throw configuration so that the common terminal can be connected for normally open or normally closed operation.) The switch is typically connected to a pump motor a solenoid valve that is used to change the amount of material entering or leaving the tank.

Calibration

Calibration is simple and easy. The Delta Controls Type 107 can be calibrated using any two levels in the tank. The tank does not have to be completely filled or emptied. You need only change the level to get a required second known elevation.

Calibration Procedure (model: Delta Controls Type 107)

1. Measure the level of material in the tank or set the level in the tank to a known level. Press the NEXT button until CAL PT 1 is displayed. Press the INCREASE or DECREASE buttons until the actual level in the tank is displayed. Then press SAVE.

2. Change the level of material in the tank to another known level. The amount of change is not critical, however the farther apart the two calibration points are, the more accurate the calibration will be. Press the NEXT button until CAL PT 2 is displayed. Press the INCREASE or DECREASE buttons until the new level in the tank is displayed. Then press SAVE.

3. To set up the relays, Press the NEXT button until the relay setpoints ("IN" & "OUT") are displayed. Press the INCREASE or DECREASE buttons until the "IN" setpoint is reached. Then press SAVE. Repeat for the "OUT" setpoint. This allows a reverse or direct action of any amount.

Level Switch Maintenance

Switch parts are subject to normal wear and must be inspected and replaced when necessary. The frequency of inspection and replacement depends on the severity of service conditions.

MAINTENANCE

1. Switch Adjustment – The switch is set at the factory for correct operation. Over time it may be necessary to adjust it due to wear or other factors. There are two different switch adjustments and they are done as follows:

1.1 Lateral Adjustment

1.1.1 Loosen the 2 nuts that secure the switch mounting bracket to the plate.

1.1.2 Move the bracket left or right to center the magnet holder (on the end of the switch arm) on the flat on the bottom of the fitting.

1.1.3 Tighten the nuts.

1.2 Vertical Adjustment

1.2.1 Loosen the 2 screws that secure the switch to the bracket.

1.2.2 Move the switch up or down so that the switch arm is parallel with the flat on the fitting and the magnet holder is against the flat. You should feel resistance from the internal spring in the switch, but it must not be pushed down so far that the switch has operated.

1.2.3 Check for correct operation of the switch, then tighten the screws.

2. Float Removal & Replacement – If it becomes necessary to replace the float, the float can be unscrewed from its shaft. To remove the float:

2.1 Heat the end of the float shaft with a heat gun, or equivalent, to break the adhesive bond.

2.2 Put a 7/32” open-end wrench on the flats at the float end of the shaft and grasp the float with a strap wrench (or similar tool) and un-screw the float,

turning one tool against the other.

2.3 When reinstalling the float, apply LocTite 242 or a suitable thread locking compound to the threads before assembly.

3. Disassembly & Cleaning – In applications where there is a large amount of contamination in the fluid, it may be necessary to clean the switch mechanism periodically.

This is done by:

3.1 Removing the retaining ring from the float side of the fitting.

3.2 Then, work the Float and Pivot assembly out of the fitting, being sure to catch the locating pin (1/8 dia. X 1/4” long).

3.3 If necessary, the pivot shaft and bushing are disassembled by removing the pivot pin and separating the 2 parts (Be sure to note the orientation of the notch in the pivot bushing to the flat side of the inner end of the shaft).

3.4 Clean the parts and the inside of the switch body with a suitable cleaning solution.

3.5 Re-assemble all of the float and pivot components.

Make sure that the mechanism moves freely after reassembly. Be sure that the notch in the pivot bushing is toward the float and the notch is on the same side

as the flat on the inner end of the shaft.

3.6 Insert the pivot assembly into the fitting, being sure to align the notch in the pivot bushing with the mating notch in the fitting.

3.7 Insert the locating pin into the notch and reinstall the retaining ring.

3.8 Test operation as described in Installation step 6.

Pressure Limit Switch

What is a Pressure limit Switch?

This is a device designed to monitor a process pressure and provide an output when a set pressure (setpoint) is reached. A pressure switch does this by applying the process pressure to a diaphragm or piston to generate a force which is compared to that of a pre-compressed range spring.

A pressure switch is used to detect the presence of fluid pressure. Most pressure switches use a diaphragm or bellow as the sensing element. The movement of this sensing element is used to actuate one or more switch contacts to indicate an alarm or initiate a control action.

Pressure switches have different designs with different sensing elements. One of the most common is the one with diaphragms or bellows as the sensing elements. The one I will discuss here uses a piston as the pressure sensing element. In any case, the operating principle for this piston type is the same with a diaphragm or bellow type pressure switch.

Basic Parts of a Pressure limit Switch:

The basic parts of a typical pressure switch are shown in the schematic diagram below:

A sectional view of the pressure switch showing all the basic parts of the switch is shown above.

Also shown below is a pictorial view of the pressure switch:

Operating Principle of a Pressure limit Switch

As shown in the sectional view of the pressure switch above, the inlet pressure is applied to the bottom of the operating piston.  This piston is forced upwards by the inlet pressure against the range spring.   The tension of the range spring can be adjusted so that it is compressed at a certain pressure or setpoint.  When this pressure is reached, the operating pin will hit the trip button on the micro-switch and change it over.  The normally open contacts (NO to C) will become closed and the normally closed contacts (NC to C) will open.  The pressure at which the micro-switch changes over is set by adjusting the trip setting nut.  This nut adjusts the tension of the range spring (e.g. if the nut is turned clockwise the trip pressure will be higher).

  pressure limit switch calibration:

Setpoint:

This is the pressure at which the pressure switch is required to operate. A pressure switch may be set to operate on either a rising pressure (high level alarm) or a falling pressure (low level alarm). Most switches are designed to operate at a 'gauge' pressure setpoint i.e. relative to atmospheric pressure. Some applications require an 'absolute' pressure setpoint i.e. relative to absolute zero pressure, and an absolute pressure switch is required for these. Ideally the range of the switch should be chosen such that the setpoint is between 25% to 75% of this range.

Dead-band or Reset:

This is a setting that determines the amount of pressure change required to re-set the switch to its normal state after it has tripped. The dead-band or reset or switching differential is the difference in the rising and falling pressures at which the pressure switch operates. For a fixed differential output switch this is typically about 1% to 3% of the switch range. For an adjustable differential output switch it may be adjusted from about 5% to 12% of the switch range.

The pressure switch is a ubiquitous device. It is practically everywhere in your plant. But how do you calibrate this simple device? The answer is here. Just follow the simple steps that I have outlined below.

Before you calibrate your pressure switch, confirm the following:

The setpoint of the pressure switch The dead-band of the switch

Also depressurize and isolate the pressure switch from the process. If opening the switch exposes voltages or energy that is not intrinsically safe, please follow the specified procedure for your plant. For example, if in an explosive environment, use a continuously monitoring gas detector to monitor for the presence of explosive gasses.

Calibration Procedure of the Pressure Switch

Step 1:

Connect the pressure switch to a pressure source e.g air supply via a hand pressure regulator and test gauge, as shown in the diagram above.

Step 2:

Use an Ohmmeter or a Digital Multimeter (DMM) set to the continuity range to check and verify that the switch contacts are as indicated: NO (Normally open) and NC (Normally close).

Step 3: 

Connect the Ohmmeter or DMM between the normally open contacts(NO) and the common terminal (C) of the switch. The meter should read "open circuit". Adjust the hand pressure regulator to increase the pressure to the setpoint of the pressure switch until the contacts change over. The meter should now read "short circuit". Note the pressure reading and write it down. This pressure is the switch setpoint for a "rising" pressure.

Step 4:

Increase the pressure to the switch to its maximum rating. Slowly reduce the pressure to the switch until the switch changes over from closed to normally open again. Note and write down this pressure reading. This pressure is the switch setting for a "falling" pressure.Step 5:

From the readings you have taken work out the pressure difference between the rising and falling pressure settings. This is called the "dead-band" of the switch. The dead-band calculated should be equal to or less than the manufacturers’ dead-band.

The maximum dead-band is usually stated by the manufacturer. The switch is unserviceable if the maximum dead-band is more than the manufacturer's recommendation (dead-band on the nameplate of the switch)

To calibrate the switch for a low pressure, go through the steps in this order:

Step 1 to Step 2 to Step 4 to Step 3 to Step 5

MAINTENANCE(company:Argus Pressure Switch)

Preventive maintenance program be implemented to ensure the longevity of the Argus Pressure Switch.1) Argus recommends that the operator periodically (quarterly) test each Pressure Switch to ensure that it is working properly.2) Tests may consist of a pressure test and verification of unit shutdown.a) For a Pressure Switch that is set for increasing pressure the operator may close a valvedownstream and check that the Switch trips off at the appropriate set point. If the set pointis incorrect, the operator should adjust it to the original setting. (Pressure Setting).b) For a Pressure Switch that is set for decreasing pressure the operator may close a valveupstream and check that the Switch trips off at the appropriate set point. If the set point isincorrect, the operator should adjust it to the original setting. (Pressure Setting).c) The Pressure Switch may also be taken off the line and sent to an Argus Service Center fortesting and adjustment.3) Visually check for excessive moisture in the Bottom Sub, by looking in the Access Port. If excessive moisture is present the Pressure Switch should be cleaned and serviced by Argus or one of their approved Service Centers.4) Argus also recommends that the Pressure Switch be tested after a service rig has done any work on the well site.5) The operator should visually check the Pressure Switch for any leaks or physical damage each time that he/she works on it. The Bottom Sub has two weep holes that will indicate when the internalgasket or diaphragm has failed. Ensure these weep holes are free of debris or foreign material. Ifexcessive leaking (small amounts of moisture that is trapped inside the Bottom Sub may leak outnormally) is evident the Pressure Switch should be replaced as soon as possible. The PressureSwitch may then be sent to an Argus Service Center for assessment and/or repair.6) Argus recommends that due to the sensitivity of the Microswitch the customer should considercarrying one (1) spare Microswitch sub-assembly in inventory for every ten (10) Pressure Switches inthe field.

MICROSWITCH REPLACEMENTIf the Microswitch should fail during normal operation it can be easily replaced in the field using thefollowing procedure:1) Make sure that the power supply to the Pressure Switch is turned off.2) Remove the Cap from the Switch Housing (use break-out slots in the Cap if required).3) Disconnect all of the electrical wires from the Microswitch and tuck them out of the way.4) Using a 3/16" T-Handle hexagonal wrench, loosen and remove the Switch Block Locking Screw.5) Lift out the entire Microswitch sub-assembly.

6) Place the new Microswitch sub-assembly in the Switch Housing and tighten the Switch Block Locking Screw with the 3/16" T-Handle hexagonal wrench.CAUTION: Do not over-tighten, as damage may occur.7) Re-attach all the electrical wires to the Microswitch as they were before. (Refer to Argus WiringInstructions Bulletin No. TB-PS-002).8) If necessary, make adjustments to the Leaf Spring. Use a flat blade screwdriver to tighten the LeafSpring Adjusting Screw clockwise until it shoulders. Turn the Adjusting Screw counter-clockwiseuntil you hear the Microswitch trip off (approximately 3 turns). Turn the Adjusting Screw 1 1/2 turnsclockwise to set.9) Install the Cap on the Switch Housing and tighten until the Cap shoulders with the Switch Housing.10) Turn on the power supply to the Pressure Switch.

MAINTENANCE OF GLOBE VALVES

Lubrication Valves need no internal lubrication. Lubrication may be required; however, for the valve stem threads and for the operator (if valve is equipped with an operator).

Maintenance (Packing Gland Type) Globe Valves should be operated through approximately 80% of travel at intervals of approximately one month depending on convenience. If not operated at intervals of one month or less, it is recommended that regular inspection of the stuffing box should be made to verify tightness. In adjusting the stuffing box, great care should be taken that the packing is not tighter than necessary to control leakage and that the packing nuts are adjusted equally. Uneven adjustment can cause damage to the stem and operating failure. If leakage is detected and cannot be stopped by tightening the packing nuts, the leakage may be attributed to one of more of the following reasons: 1. Incorrect packing used; replace with proper packing. 2. Packing may have become hard; replace with new packing. 3. Stem may be scored or deeply scratched; stem must be replaced. 4. Packing may have been improperly installed; packing ring splits should be staggered so that the splits are not positioned one over the other. 5. Stuffing box gland may be binding against either the stem or the stuffing box, due to uneven bolt pull-up. Stuffing box parts should be repositioned to assure even compression of the packing.

Maintenance (Injection Type) When put into service, a clockwise turn of the Hex Head Adjustment Screw may be required to compress additional packing from the reservoir into the sealing area. Adjustment can be made under FULL line pressure with the disc of ANY position. There is no need to backseat the disc; the specially designed Ball Check Valve eliminates the possibility of packing

extrusion. Additional adjustment turns may be made, as necessary, until adjustment screw “bottoms” in the injector fitting. In the event the screw “bottoms”, the packing reservoir must be replenished. Remove screw and insert new packing sticks. Replace adjustment screw. CAUTION: Do not remove packing injector unit while under pressure.

REPAIR OF GLOBE VALVE

1.Valve Disassembly Have replacement gasket available. Shut off line pressure and remove valve from line. . Remove bonnet bolting and carefully lift bonnet assembly with disc from body. 2. Procedure for Lapping Disc and Body Seat Ring Apply a small amount of valve grinding compound all around the seat-bearing surface then place disc into port and lap with alternate rotary motion. Lift disc at intervals and turn 180 ° and continue lapping. Remove compound occasionally while lapping, clean body seat and disc to remove compound residue. Apply Prussian Blue to disc and insert into seat. Applying normal pressure to disc, rotate 5° (approx.) to record seat bearing. If bearing periphery does not record a complete circle on disc face, repeat operation to remove high spots until a complete circle is obtained. 3. Procedure for Making Up Bonnet Joint Before reassembly of bonnet to body inspect bonnet flange and gasket surfaces for damage and/or any foreign material that will obstruct a good sealing surface. Such matter must be removed with emery cloth (Grade 80 or 120). Clean off all residue from gasket and gasket surface before reassembly. 4. Reassembly Place gasket on body. Apply high temperature thread lubricant to gasket, (on non-critical service applications only), bonnet bolting and spotface surfaces. Before bonnet assembly, raise stem to full open position. Install bonnet to body in proper position. Tighten bonnet bolting alternately (crossover method) with even pressure, up to a bolt stress of 15,000 lbs./In.². Should there be leakage at bonnet after valve is put into service, tighten nuts up to a maximum of 30,000 lbs./In.² bolt stress.

MAINTENANCE

Proper PPE should be worn when preparing to service a valve. Observe the following general

warnings:

A valve is a pressurized device containing energized fluids and should be handled with appropriate care.

Valve surface temperature may be dangerously too hot or too cold to the skin.

Upon disassembly, attention should be paid to the possibility of releasing dangerous and or ignitable accumulated fluids.

Adequate ventilation should be available for service.

Tools Required

Aside from standard wrenches (for bonnet cap screws and packing gland nuts) the only tool

needed for Smith valve maintenance is a packing hook.

Packing

Special care is to be placed in the tightening of gland nuts during installation, in order to get the proper packing adjustment and functionality. The packing gland should be checked periodically in service and tightened as necessary to stop leakage around the stem. Tighten in a manner to develop uniform loading on the gland. Tighten only enough to stop the leak.

Over tightening will cause the packing to fail prematurely as well as increasing the force required to operate the valve. If the leak cannot be stopped by tightening the gland nuts, it is necessary to add additional packing rings or completely repack the valve. While Smith gate and globe valves are

equipped with a back seat feature, it is NOT RECOMMENDED TO REPACKED THEM UNDER PRESSURE.

Back seating the valve and attempting to repack under pressure is hazardous and is not recommended. Rather than attempting to repack under pressure, it is preferable to use the backseat to control the stem leakage until a shutdown provides safe repacking conditionsThe end rings (top and bottom) of the standard Smith graphite packing set have a diagonal cut that will allow them to be installed around the stem of an assembled valve. However, the factory installed intermediate graphite packing rings are die formed and have no end cut. As a result, these rings cannot be replaced without removing the valve bonnet. If the valve is to be repacked without removing the bonnet (see re packing the valve in line below), care must be taken when removing the original packing not to scratch the valve stem sealing surface. Where it is necessary to repack the valve in line, a compatible ribbon packing system or equivalent braided packing stock should be used. The joints in the packing rings should be diagonally cut. When installing the rings, care should be taken to stagger the ring joints. Other specialty packing such as V ring Teflon will require that the valve be disassembled if repacking is required.

Stem Thread LubricationThe operating Yoke Nut (Item 14) of Smith OS&Y Gate and Globe valves requires proper

lubrication to stem threads and or to bonnet. The recommended grease to be applied is

Loctite 77164 or equivalent. The following is the proper grease application method:

If valve is CLOSED:

o Apply Grease below the yoke nut onto stem threads

o Open Valve To the Full Open Position

o Apply Grease to the stem thread protruding above the Yoke Nut

o Close Valve to the Full Close position

o Cycle 1 additional time Full open to full close to evenly apply grease inside yoke nut

If valve is OPEN:

o Apply Grease above the yoke nut onto stem threads

o Close Valve To the Full Close Position

o Apply Grease to the stem thread below the Yoke Nut

o Open Valve to the Full Open position

o Cycle 1 additional time Full open to full close to evenly apply grease inside yoke nut.

Repairs

Due to the relatively low replacement cost of standard carbon steel valves, it is usually less

expensive to replace the complete valve than to have maintenance personnel effect repairs.

Additionally, in the case of a gate valve, it must be removed from the line in order to replace

seat rings. Generally, the only justifiable repairs are replacement of packing and gaskets as

previously described.

Always replace the bonnet gasket whenever a valve is disassembled. Gasket seating

surfaces should be scraped clean (avoid radial marks). Bonnet bolts should be tightened in a

diagonal pattern at several different increasing torque settings until the final recommended

torque value is attained.

BODY-BONNET/COVER BOLTINGS

Only proceed to this operation changing one bolt at a time to prevent losses of pressure on the gasket. If this is not possible, replace the body-bonnet gasket locking bolts in a crossed way (see figure) till torque are the same of Appendix C.

MAINTENANCE ON BOLTED BONNET GLOBE VALVES

7.1 DISC and SEAT

The seating surface is integral to the body. To check the seal characteristics between the disc and body seating area, we suggest the “BLUEING TEST”: a. Proceed opening completely the valve, assuring that the stem is brought to the backseat position. b. Loosen the body-bonnet bolting. c. Remove the bonnet-stem and disc assembly. Apply some prussic-blue on the body seating surface. d. Place the bonnet-stem and disc assembly in the original position, and tighten the bolts as described in section 5. e. Take the valve in the close position, wait 20 seconds, and repeat steps “a” and “b” above. f. Remove the bonnet again, and check that the blue trace on the disc and the body isuniformly present on the contact surfaces. If this has not occurred there are two possibilities: There are incisions or marks on sealing surfaces, either the disc or the body. Check and, if any, use fine sand paper or emery cloth to eliminate them, taking care that the original planarity of these surfaces is not modified. Repair is not possible because great damage has occurred. Contact our sales department giving details as described later to receive a new disc and replace it. g. Replace the body-bonnet gasket. h. Reassemble the bonnet-stem and disc assembly and tighten bolts as described in section 5.7.2 STEM

a. Proceed opening completely the valve, assuring that the stem is brought to the backseat position. b. Loosen the body-bonnet bolting. c. Remove the bonnet-stem and disc assembly. Extract the disc from the stem end. d. Disassemble the stem by turning it in the counter-clockwise direction. e. Make sure that the stem surface in contact with the packing is not damaged. If the stem is damaged beyond repair, call for a stem replacement or consider replacing the entire valve. f. Replace the stem by screwing it clockwise in the bonnet. g. Replace the gasket between body and bonnet, insert disc into the stem end. h. Bring the bonnet-stem assembly to its original position, and tighten the body-bonnet bolts as described in section 5.

HOW TO CALIBRATE A PNEUMATIC CONTROL VALVE?

For any pneumatic control valve the operating pressure is from 3 to fully opened at 15 psig. It means a pneumatic control valve opens fully at 15 psi and closes at 3 psig.To calibrate it, prepare the following, one mercury manometer 0 tp 1000mm,Hg, one air pressure regulator for 20psig supply and air hoses,3 to15 psi output gauge, micrometer screw pressure regulator and air pressure supply. Prepare a Tee with the air supply at the centre. Connect one leg to the mercury manometer and the other equalizer leg to the control valve and a tee on top of the valve and an output gauge connected to the tee.. Both the manometer and control valve should receive equal pressure from the pressure regulator. A 15psig pressure is 775 mm mercury . So, set the mercury manometer to an equivalent of 15 psig,775mmHg, and prepare a table of equivalent pressure from 3, 6, 9, 12 and 15psig. The valve should be fifty percent open at 9 psig and 25% open at 6 psig, likewise, it should close at 3 psig and 75% open at 12 psig fully opened at 15 psig. The pressure variation may be done with the micrometer type pressure regulator at the pressure supply.

Calibrate valve positioner

Calibration of the valve positioner can be performed at the same time as the I/P in a loop calibration. Simply tee in the pressure module at the I/P outlet in the I/P calibration. Record the valve position at each test point.

If calibrating the valve positioner separately, connect an input test pressure regulator or hand pump, and monitor the input pressure applied with a pressure standard. If there is no supply

air, connect the required supply air to the positioner. Apply the pressure for the desired test points and record valve position.

For example, assume our valve positioner is 3-15 psig input = 0-100% valve position. In this case, apply 3.0, 6.0, 9.0, 12.0, and 15.0 psig. The expected valve positions should be 0, 25, 50, 75, and 100%, respectively.

The valve position indicator on the stem usually marks off in 5% or 10% increments. Therefore, a best estimate of the valve position may be all you can obtain. In other cases, a valve position detector provides a remote indication to a DCS. In such cases, ensure both indicators are working properly.

Many organizations do not require calibration of valve positioner for these reasons. There's much documentation that control valve positioner performance is responsible for significant loss in system efficiency and, therefore, increased costs.

To provide guidance on methods for testing positioner and control valve performance, ISA has developed a standard, ANSI/ISA-75.25.01-2000, Test Procedure for Control Valve Response Measurement for Step Inputs.

As to control valve calibration, the process is similar to positioner calibration in that one applies a pressure signal to the actuator and then tallies the resulting valve position. This step can take place with the positioner calibration, if applicable, and it can happen in conjunction with I/P calibration.