Troubleshooting Guide for Pneumatic Field Transmitters

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Pneumatic instruments are still a critical component in most instrumentation systems of today even though recent technology is making them to become somewhat obsolete. For plants where pneumatic instruments especially pressure transmitters are still being used, there are common problems with these instruments that are encountered in their everyday use. These problems are highlighted below so that when troubleshooting any potential problem with such instruments, we know what the culprits are.

Pressure Transmitter  Problem	Possible Cause(s)
No Output	Bent flapper No air supply; plugged restrictor (this is  very common) Corroded pneumatic relay or components Dirty pneumatic relay seats Flapper is away from the nozzle due to freezing, improper adjustment, bent "C" flexure or transmitter has been dropped Leak in the feedback bellows Leak in the nozzle circuit Leak in the sensor pressure circuit Disconnected or broken links in a motion balance pressure transmitter
Partial Output	Plugged low pressure leg on a DP cell Worn pneumatic relay parts Partially plugged supply screen or filter Burr on the flapper assembly Hole in the flapper assembly Damaged feedback bellows Worn capsule diaphragms Warped or distorted "C" or "A" flexure on a DP cell Wrong range-sensing unit Pin hole leaks in the control relay diaphragm
Full Output	Plugged nozzle Ballooned capsule diaphragm Loose nozzle lock nut Blocked pneumatic relay vent Sensing capsule impacted with process solids Flapper assembly distorted or bent
Zero shift diaphragms	Dirty flapper assembly set point capsule problems - coating, fatigue, warped Temperature changes -  either ambient or process temperatures Process static pressure changes. Worn zero or span adjustments Flapper is depressed or hollowed on the surface Pin hole leak in the flapper Flashing and/or condensate on either leg of a DP cell installation
Output Oscillates	Liquid in the feedback bellows - water, oil etc "C" flexure look nut loose Close coupled pneumatic system Loss of capsule fill fluid Hole in the feedback bellows Loose bleed/vent valves Flashing due to pressure variations

The list of pneumatic field transmitters problem and their probable cause in the table above is by no means exhaustive. It is however a useful troubleshooting guide for pneumatic transmitters.

Read Also: Troubleshooting Guide for DP Transmitters

Calibration Practice For Pressure and Flow Transmitters.

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Smart or intelligent transmitters are now the industrial standard for almost all pressure and flow measurement. These transmitters are required to be calibrated regularly to guarantee that they deliver the set measurement objective. What are the recommended calibration practice for these type of transmitters?

Calibration Cycle of a Typical Process Transmitter

Right from when the smart transmitter leaves the factory floor to the plant environment, below is illustrated the typical calibration cycle when applying the transmitters for pressure and flow measurement.

How to Calibrate a DP transmitter

Factory Calibration

Factory calibration is usually done at the factory as part of standard industrial  

practice and to conform to relevant standards and codes. A transmitter has an inherent factory Sensor characterization curve similar to the one below:

How a 2 Wire Transmitter 4 – 20mA Current Loop Works

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In two-wire 4-20mA control loops which are the most popular these days, the 2-wire transmitters convert various process signals representing flow, level, temperature, pressure, etc., to 4-20mA DC current for the purpose of transmitting the signal over some distance with little or no loss of signal.

Relationship between the Components in the 4 – 20mA Control Loop
There are three key components in the 4 – 20mA loop as shown below:

They are:

How to Troubleshoot the 4 – 20mA Output of Rosemount 3051 Transmitter

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One of the most versatile and commonly used transmitters is the Rosemount 3051 pressure transmitter. This transmitter can be used to measure pressure, differential pressure, gage pressure, absolute pressure or a liquid level. The Rosemount 3051 transmitter provides an analogue output (4 – 20mA) with a superimposed HART signal on the 4-20 mA output.

The most common problems with the Rosemount 3051 transmitter are adapted from the transmitter manual and can be used to troubleshoot the 4 – 20mA output whenever the symptoms in the table below are observed during transmitter service:

Symptom	Corrective Actions
Transmitter miliamp is Zero	Verify power is applied to the signal terminals Check power wires  for reverse polarity Verify terminal voltage is 10.5 to 42.4 Vdc Check for open diode across test terminal
Transmitter Not Communicating with Field Communicator	Verify the output is between 4 and 20mA or saturation levels Verify terminal voltage is 10.5 to 42.4 Vdc Verify clean DC power to transmitter (Max. AC noise is 0.2 volts peak to peak Check loop resistance, 250 ohms minimum (PS Voltage - transmitter voltage/loop current Have field  communicator poll for all addresses
Transmitter milliamp reading is low or high	Verify applied pressure Verify 4 and 20mA range points Verify output is not in alarm condition Verify if 4 - 20mA output trim is required
Transmitter will not respond to changes in applied pressure	Check test equipment Check impulse piping or manifold  for blockage Verify the transmitter is not in  multi-drop mode Verify applied pressure is between 4 and 20mA set points Verify output is not in alarm condition Verify transmitter is not in loop test mode
Digital pressure variable reading is low or high	Check the accuracy of test equipment Check impulse piping for blockage or low fill in wet leg Verify transmitter is calibrated properly Verify pressure calculations for application
Digital pressure variable reading is erratic	Check application for faulty equipment in pressure line Verify transmitter is not reacting to equipment turning on/off Verify damping is set properly for application
Milliamp reading is erratic	Verify power source to transmitter has adequate voltage and current Check for external electrical interference Verify transmitter is properly grounded Verify shield for twisted pair is only grounded at one end

How to Calibrate a Rosemount 1151 Pressure Transmitter

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Calibrating the Rosemount Model 1151 Pressure Transmitter is simple and easy. With the aid of an accurate pressure source, an output meter, a power source and external span and zero buttons on the transmitter, the device can easily be calibrated. The zero and span adjustment screws are accessible externally behind the nameplate on the terminal side of the electronics housing. To calibrate the transmitter, setup the equipment required - pressure source, Rosemount 1151 pressure transmitter, Current meter to measure current output from transmitter, power source, pressure gauge (covering the LRV and URV of transmitter), a 250 ohm resistor if necessary for communication with transmitter - as shown below:

 

The output of the transmitter increases with clock wise rotation of the adjustment screws. The zero adjustment screw does not affect

How to Install Pressure Transmitters –Best Installation Practices

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Installing a pressure transmitter or a differential pressure transmitter is suppose to be a simple process but can become a problem if certain best practices are not imbibed. One critical aspect of transmitter installation is the impulse piping between the process and the transmitter.

The piping between the process and the transmitter must accurately transfer the pressure to obtain accurate process measurements otherwise measurement error will occur and compromise the process. There are five possible sources of error in any given pressure transmitter installation. They are:
(a) Pressure transfer leaks
(b) Friction loss
(c) Trapped gas in a liquid line
(d) Liquid in a gas line
(e) Density variations between high pressure and low pressure impulse lines

The best location for the pressure transmitter

How to Zero a Pressure Transmitter – Three and Five Valve Manifolds Operation

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I had discussed previously on DP Transmitter Valve Manifolds where I explained how to remove a transmitter from active service using 3 and 5 – Valve manifolds but zeroing a transmitter is slightly different.

Zeroing a transmitter involves bringing the transmitter to zero signal but does not remove the transmitter from service.The most popular transmitter manifolds are either a 3 or 5-Valve manifold and the operations involved in zeroing a transmitter using these manifolds are different.

How to Zero a Transmitter with a 3-Valve Manifold
Each step involved will be illustrated with a diagram. It is important to realize that over-pressure or transmitter damage could occur if the steps are not properly followed:
Under normal operation

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

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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:

Troubleshooting Guide for DP Transmitters

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Just like every other equipment, DP transmitters can malfunction while in service. The ability to troubleshoot and locate the possible cause of malfunction of the transmitter is crucial for easy start up of the process the malfunctioning DP transmitter may have upset.
When troubleshooting DP transmitters, it is always valuable to consult the manufacturer’s manual for your DP transmitter. By so doing, you should be able to locate the malfunctioning parts of the transmitter and proffer a remedy immediately. However there are some problems that are common to most DP transmitters. If you suspect a malfunction, follow the guidelines below to verify that the transmitter hardware and process connections are in good working condition. Under each of the five common problems with DP transmitters, you will find specific guidelines/suggestions for solving the problem. It is wise to always deal with the most likely and easiest problems first.

Please follow the precautions below before and during troubleshooting of your malfunctioning DP transmitter:
1. Isolate the failed DP transmitter from its pressure source as soon as possible. Pressure that may be present could cause death or serious injury to technicians or personnel if the transmitter is disassembled or ruptures under pressure
2. Do not use higher than the specified voltage to check the transmitter loop. This may damage the transmitter electronics.
3. If there is need to open your DP transmitter while troubleshooting, please follow your manufacturer’s specific guidelines for dis-assembly of your transmitter. If you don’t serious injury or death to personnel may occur or your transmitter may be damaged.
The table below itemizes some common problems with DP transmitters and their possible remedies:

Problem	Potential Cause	Corrective Action
Low output or No output	Primary Element	Check the insulation and condition of primary element
	Loop Wiring	Check for adequate voltage to the transmitter Check the mA rating of the power supply against the total current being drawn for all transmitters being powered. Check for shorts and multiple grounds Check for proper polarity at the signal terminal Check loop impedance (should not exceed the specification for your plant) Check wire insulation to detect possible shorts to ground
	Impulse piping	Ensure that the pressure connection is correct.  Check for leaks or blockage.  Check for entrapped gas in liquid service. Check for sediment or debris in the DP transmitter process flange. Ensure that blocking valves are fully open and that bypass valves are tightly closed. Ensure that density of fluid in impulse piping is unchanged.
	Sensing Element	The sensing element is not field repairable and must be replaced if found to be defective.  Disassemble the transmitter and probe further(check your manufacturer’s manual for instructions on how to disassemble your transmitter;  Check for  any obvious defects. At this point you may need to contact your manufacturer if there is any defects in the sensing element
DP transmitter does not calibrate properly	Pressure source/correction	Check for restrictions or leaks. Check for proper leveling or zeroing of the pressure source. Check weights/gauge to ensure proper pressure setting. Determine if your pressure source has sufficient accuracy
	Meter	Determine if the meter is functioning properly
	Power Supply	Check the power supply output voltage at transmitter
	DP transmitter electronics	Make sure the transmitter connectors are clean. If the electronics are still suspect, substitute with new electronics.
	Sensing Element	The sensing element is not field repairable and must be replaced if found to be defective.  Disassemble the transmitter and probe further(check your manufacturer’s manual for instructions on how to disassemble your transmitter. Check for  any obvious defects. At this point you may need to contact your manufacturer if there is any defects in the sensing element
High Output	Primary Element	Check for restrictions at primary element
	Impulse piping	Check for leaks or blockage. Check for entrapped gas in liquid service. Check for sediment or debris in the DP transmitter process flange. Ensure that blocking valves are fully open and that bypass valves are tightly closed. Ensure that density of fluid in impulse piping is unchanged.
	Power Supply	Check the power supply output voltage at transmitter
	DP transmitter electronics	Make sure the transmitter connectors are clean. If the electronics are still suspect, substitute with new electronics
	Sensing Element	The sensing element is not field repairable and must be replaced if found to be defective.  Disassemble the transmitter and probe further(check your manufacturer’s manual for instructions on how to disassemble your transmitter;  Check for  any obvious defects. At this point you may need to contact your manufacturer if there is any defects in the sensing element
Erratic Output	Loop Wiring	Check for adequate voltage to the transmitter. Check for intermittent shorts, open circuits and multiple grounds
	Process Pulsation	Adjust damping
	DP transmitter electronics	Make sure the transmitter connectors are clean. If the electronics are still suspect, substitute with new electronics
	Impulse piping	Check for entrapped gas in liquid lines and for liquid in gas lines.

How to Calibrate Your DP Transmitter

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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.

If you got here looking for information   on smart transmitter calibration please see : How to Calibrate Smart 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:

Common terms Used in DP Transmitter Calibration

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Lower Range Limit (LRL)
This is the lowest value of the measured variable that a transmitter can be configured to measure. This is different from Lower Range Value (LRV)

Lower Range Value (LRV)
Lowest value of the measured variable that the analog output of a transmitter is currently configured to measure.

Transmitter Re-ranging
Configuration function that changes a transmitter 4mA and 20mA settings

Upper Range Limit (URL)
This is the highest value of the measured variable that a transmitter can be configured to measure. This is different from Upper Range Value (URV)

Upper Range Value (URV)
Highest value of the measured variable that the analog output of a transmitter is currently configured to measure

Span
Span is defined as the algebraic difference between the upper (URV)and lower range(LRV) values of the DP transmitter.

Span = URV – LRV

For example, if the DP transmitter is being used to measure a pressures in the range 0 – 300psig, then URV = 300, and LRV = 0

Therefore span = URV – LRV = 300 – 0 = 300

To have a better understanding of LRV and URV as used in instrumentation systems, please go through control signals

Calibration Range

The calibration range of a DP transmitter is defined as “the region between the limits within which a quantity is measured, received or transmitted, expressed by stating the lower and upper range values.” The limits are defined by

DP Transmitter valve manifolds

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An important accessory to the DP transmitter is the valve manifold. Most DP transmitters come with either a 3-valve manifold or a 5-valve manifold or a single block and bleed valve manifold depending on the application. The valve manifold is used:

• To isolate the DP transmitter from the process for maintenance and calibration.
• To ensure that the DP transmitter is not over-ranged

3-Valve Manifold: 

This device incorporates three manual valves to isolate and equalize pressure from the process to the transmitter, for maintenance and calibration purposes. It consists of two block valves - high pressure and low pressure block valve - and an equalizing valve. The schematic below shows the configuration of a 3-valve manifold:

DPT in the schematic above is the DP transmitter. During normal operation

Applications of DP transmitters

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The DP transmitter is a very versatile pressure-measuring device. This one instrument may be used to measure pressure differences, positive (gauge) pressures, negative (vacuum) pressures, and even absolute pressures, just by connecting the “high” and “low” sensing ports differently. 

In every DP transmitter application, there are means of connecting the transmitter’s pressure-sensing ports to the points in a process. Metal or plastic tubes (or pipes) are the means used for this purpose, and are commonly called impulse lines or sensing lines.

Let us now look at a few of the several applications using the versatile DP transmitter:

4 - 20mA Transmitter Wiring Types: 2 -Wire, 3 - Wire & 4 - Wire

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Today’s electronic process transmitters - pressure, temperature, flow and level are connected in different wire types or configurations. These connection methods are of great concern to the instrument engineer/technician. The 2 - Wire, 3 - Wire and 4 - Wire types are often used to describe the method of connection of electronic transmitters. However in today's rapidly evolving technological world, the 2 - Wire type transmitter is by far the most common. Evidently so because of the huge savings in wiring and other advantages it possess over the other transmitter wire configurations.

Two wire transmitters:

These are the simplest and most economical and should be used wherever load conditions will permit. They are often called loop powered instruments. In a 2 -wire system, the only source of power to the transmitter is from the signal loop. The 4 mA zero-end current is sufficient to drive the internal circuitry of the transmitter and the current from 4 to 20 mA represents the range of the measured process variable. The power supply and the instruments are usually mounted in the control room. The schematic diagram below shows the wire transmitter configuration:

An Introduction to DP Transmitters

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Differential pressure (DP) transmitters are one of the most common, versatile and most useful pressure measuring instruments in industrial instrumentation systems. A DP transmitter senses the difference in pressure between two ports and outputs a signal representing that pressure in relation to a calibrated range.

DP transmitters currently in use in most instrumentation systems are based on any of the following technologies: 

a) Force-balance principle

b) Strain gauge 

c) Differential capacitance 

d) Vibrating wire or mechanical resonance

The force-balance principle is utilized in pneumatic pressure transmitters while most of today’s electronic pressure transmitters that have practically replaced the pneumatic pressure transmitters, use the technologies (b) – (d).