What Are the Installation, Routine Maintenance and Failure Case Analysis of Pressure Transmitters

Pressure Transmitter

Opinions about Pressure Transmitter

The difference between pressure gauge and pressure transmitter is ……

A: Pressure gauge is generally installed directly in the pipeline, using the internal expansion tube to sense the pressure, while driving the gear mechanism to make the pointer rotate to achieve the effect of displaying the pressure value.

B: Pressure transmitter is generally used in industrial control automation. It is installed in the place where the pressure needs to be read, which may be a pipeline or a storage tank. And it converts the pressure signals of gases, liquids, etc. into current or voltage signals. These current or voltage signals are then provided to instruments such as recorders, regulators, alarms, etc., so as to achieve the functions of measurement, recording and regulation.

The difference between sensor and pressure transmitter is ……

A: Nowadays, people say the sensor is composed of two parts, namely, the sensitive element and the conversion element.

The sensitive element is the part of the sensor that can directly sense or respond to the measurement;

The conversion element is the part of the sensor. It converts a sensitive sensory or response measurement into an electrical signal suitable for transmission or measurement.

Because the output signal of the sensor is generally very weak, it needs to be modulated and amplified.

With integration technology developing, people, the power supply and other circuits are also installed inside the sensor. In this way, the sensor can output a usable signal that is easy to process and transmit.

B: The so-called transducer is the sensitive element above, and the transmitter is the conversion element above. A pressure transmitter is a pressure sensor whose output is a standard signal. And it is an instrument that accepts a pressure variable and converts it proportionally to a standard output signal.

Exterior view of commonly used transmitters

Exterior view of commonly used transmitters

Bar Pressure Transmitter

Bar Pressure Transmitter

Transmitter sign

Transmitter nameplate should be complete, clear, and has the following information: product name, model specifications, measurement range, accuracy level, rated working pressure, and other major technical indicators; the name or trademark of the manufacturer, the factory number, the year of manufacture, manufacture of measuring instruments license sign and number; explosion-proof products should have the corresponding explosion-proof sign. Differential pressure transmitter high and low pressure chamber should be clearly marked.

Transmitter sealing

The measuring part of the pressure transmitter shall not leak when it is subjected to the upper limit of the measuring pressure (rated working pressure for differential pressure transmitter).

Transmitter insulation resistance

In the ambient temperature (15 ~ 35) ℃, relative humidity of 45% ~ 75%, the transmitter terminals (including the shell) between the insulation resistance should not be less than 20MΩ.

Note: two-wire transmitter only output terminals on the shell test.

Transmitter insulation strength

In the ambient temperature (15 ~ 35) ℃, relative humidity of 45% ~ 75%, the transmitter terminals (including the shell) between the frequency of 50Hz test voltage applied to the table below, 1min should be no flying arc and breakdown phenomenon.

10 notes on the installation of pressure transmitters ……

  • Ensure that the meter is not damaged and avoid
Causes Results
Corrosion Corrosion of the wetted diaphragm
High temperature, capillary burn Aerosolization of the filling oil, damaging the diaphragm
Low temperature Icing of the receiving medium, damaging the diaphragm
Solid particles, impurities Damage to diaphragm due to high pressure, high speed flow
Vibration Damage to transmitter
Scraping or rubbing of diaphragm Damage to diaphragm
Poorly centered gaskets, squeezed diaphragms Damage to diaphragm
Spike pulse pressure shock Damage to instrumentation
Measured pressure exceeds the range of the transmitter Damage to instrument
  • Ensure normal pressure transfer and avoid

plugging

The medium contains solid particles and impurities;

Pressure-relief pipe is too thin, valve opening is not enough;

Too many right-angle bends in the pressure-relief pipe.

plugging

low temperature

The medium in the pilot tube condenses or even freezes;

Icing can cause damage to the instrument.

low temperature

Capillary tube folded, bending radius too small, etc.

Capillary tube folded

  • Ensure accurate meter measurements and avoid
Reason Result
Different temperatures between the high and low side of the pilot tube Differential pressure due to different density of the medium in the tube, resulting in measurement deviations that cannot be corrected.
The length of the pilot tube is very long Long response time
When using isolation fluids, the heights in the pilot tubes of the high and low pressure measurements are not the same. Differences in height, resulting in differential pressures and thus measurement deviations, which cannot be corrected.
Multiple media in different forms: gas or solid in liquid; liquid or solid in gas. Differential pressures are not predictable and cannot be compensated or corrected.

The following should be observed when selecting a pressure point for the pressure transmitter:

  1. When measuring gas pressure, the pressure point should be in the upper half of the process pipe.
  2. When measuring liquid pressure, the pressure point should be in the lower half of the process pipe and the horizontal centerline of the process pipe into the range of 0 ~ 45 ° angle.
  3. Measuring steam pressure, the pressure point should be taken in the upper half of the process piping and the lower half of the process piping and the horizontal center lineof the process piping within the range of 0~45°angle.
  4. The installation location of the pressure source components, should be selected in the process medium flow beam of stable pipe section.
  5. Pressure source components and temperature source components are in the same pipeline. The pressure source components should be installed in the temperature source parts of the upstream side.
  6. The end of the pressure source components should not exceed the inner wall of the process equipment and process piping.
  7. In the vertical process piping to measure the pressure with dust, solid particles or sediment and other turbid media, take the source components should be installed at an angle upwards, and the angle with the horizontal line should be greater than 30 degrees, in the horizontal process piping should be installed at an acute angle downstream beam.
  8. Pressure transmitter installation location should be light, easy to operate and maintain. It shouldn’t be installed in the vibration, moisture, high temperature, corrosive and strong magnetic interference.
  9. Pressure transmitter installation location should be as close as possible to the source components. Measurement of low-pressure transmitter installation height should be consistent with the height of the pressure point, especially for measuring liquid media and considerable gas media.
  10. When measuring the pressure of gaseous media, the installation position of the transmitter should be higher than the pressure-taking point. And when measuring the pressure of liquid or steam, the installation position of the transmitter should be lower than the pressure-taking point, with the purpose of reducing the additional facilities of exhausting and draining.

14 Points of Attention for Routine Maintenance of Pressure Transmitters ……

The pressure transmitters use process should pay attention to consider the following situations:

  1. The transmitter should not be used on the voltage higher than 36V, which can easily lead to damage.
  2. The transmitter should not touch the diaphragm with hard objects, it will damage the diaphragm piece.
  3. The measured medium can not be frozen, otherwise the sensor element isolation diaphragm is easily damaged, resulting in damage to the transmitter.
  4. In the measurement of steam or other high-temperature media, its temperature should not exceed the limit temperature when the transmitter is used, otherwise it must use a heat sink.
  5. In the measurement of steam or other high-temperature media, in order to make the transmitter and pipeline connected together, should use the heat pipe, and use the pressure on the pipeline to the transformer. When the measured medium is water vapor, the heat pipe should be injected with the appropriate amount of water to prevent superheated steam directly in contact with the transmitter, resulting in damage to the sensor.
  6. In the pressure transmission process, should pay attention to a few points: transmitter and heat pipe connection must not leak; be careful when opening the valve, so as to avoid direct impact of the measured medium, damage to the sensor diaphragm; must keep the pipeline smooth, to avoid sediment in the pipeline pop-up and damage to the sensor diaphragm.

The transmitter manufacturers general warranty of one year, some warranty two years, but no manufacturer will often go to you to maintain the pressure transmitter, so we still need to know:

  1. To prevent dross from being deposited in the conduit and the transmitter from contacting corrosive or overheated media.
  2. Measuring gas pressure, the pressure port should be opened at the top of the process pipe, and the transmitter should also be installed in the upper part of the process pipe, so that the accumulated liquid can be easily injected into the process pipe.
  3. When measuring liquid pressure, the pressure take-off port should be opened at the side of the process pipe to avoid the deposition of slag.
  4. Pressure guide tube should be installed in the place with small temperature fluctuation.
  5. When measuring the liquid pressure, the transmitter should be installed in a position to avoid the impact of the liquid (water hammer phenomenon), so as to avoid overpressure damage to the transmitter.
  6. When freezing occurs in winter, the transmitter installed outdoors must take anti-freezing measures to avoid the liquid in the pressure-inducing port expanding in volume due to freezing, resulting in transmitter loss.
  7. Wiring, the cable through the waterproof connector or winding tube and tighten the sealing nut to prevent rainwater leakage through the cable into the transmitter shell.
  8. Measurement of steam or other high-temperature media, need to be connected to add a buffer tube (coil) and other condensers, should not make the transmitter’s operating temperature exceeds the limit.

Pressure Transmitter Failure Case Analysis

Shell and fluid section failure

Failure: shell into the water

Reason: the transmitter cable inlet for the entire cable alignment of the lowest point, rain, condensation along the line pipe into; sealing is not good and so on.

Red plastic cap for the factory to prevent foreign objects, debris into, can not be used as a seal: (aging yellow) and does not meet the explosion-proof requirements.

Mechanical damage

Mechanical damage

Failure: oil filling and leakage

Under normal conditions, the diaphragm is elastic due to oil filling.

Can be felt by finger (not vigorously)

Oil filling and leakage

Note: When cleaning the diaphragm, it’s especially important not to remove it in a direct way with hard objects such as scraping or rubbing. Instead, it’s necessary to use a solvent to dissolve the adherents on the diaphragm before cleaning and removing them. Otherwise the diaphragm will be damaged, affecting the measurement.

Pressure Transmitter Practical Case Study

 What are the common reasons why the temperature measurement point on the DCS operation screen turns white?

(1) Card safety fence is not powered or faulty

(2) The site is not wired or wiring errors

(3) The measured temperature over range

There is a pressure transmitter that is used to measure the pressure inside the chimney. But how to determine whether the pressure transmitter is good or bad, what is its resistance value in general, and how to zero the calibration, are all questions.

The roughest practice in the field is:

  • Usually it’s to look at the output at zero pressure input and see if the output is at 4MA.
  • Does the output follow when the pressure changes!
  • The internal resistance parameter is used to calculate the voltage drop of the equipment. And the internal resistance is different at different pressures. Many manufacturers of internal resistance is the highest upper limit (conservative parameters). The actual factory products will not have this high internal resistance.
  • If you have the condition, you still need to pressurize and measure the output!

During winter startup, process personnel reported that one of the differential pressure transmitter displays was significantly different from the process reality and requested that it be addressed. Describe in detail the entire process of dealing with the fault. (This should include: communications, interlocks, freeze protection, safety, logging, and other relevant elements)

  1. Communicate with process personnel in detail, confirm the instrument position number, confirm the operating conditions. Fill in the work ticket, ready to start work.
  2. For instruments involving interlocks, an Interlock Release Form should be completed prior to processing and then the responding interlock should be released in DCS and ESD.
  3. After arriving at the site, you should check the accompanying heat condition. If it freezes, check the accompanying heat pipe first, then blow the accompanying heat and pressure pipe with low pressure steam. Then check the cause of freezing. If it’s caused by the steam accompanied heat stops steam, must immediately contact the process for steam accompanied heat treatment.
  4. If it is not frozen, check whether the root of transmitter can discharge liquid, so as to judge whether the pressure taking tube is open or not. If it is not possible, it should be dealt with by way of sewage or blowing out.
  5. Discharge may discharge toxic and harmful process gases, check the heat tracing to prevent scalding.
  6. Upon completion of machining, all insulation and site sanitation of the meter should be taken care of. Machinist should be required to pay more attention to the display of the meter and sign the work contact sheet.

Pressure Transmitter Common Failure and Analysis

Serial

number

Failure phenomenon Cause of malfunction Processing method

1

 

 

 

 

No output

 

 

 

 

 

 

 

Is the switch of the pressure-guide tube not turned on? Open the pressure-guide switch.
Pressure-conducting pipe is clogged Unclog the pressure-conducting tube
Is the power supply voltage too low? Adjust the power supply voltage to 24V
Is the output circuit of the instrument disconnected? Turn on the break point
The power supply is not connected correctly Check the power supply and wire correctly
Poor contact of internal connector Find the treatment
If the meter is equipped with a head, the head is damaged. Replace the meter head
Faulty electronics Replace the circuit board with a new one or find the fault according to the instruction of the meter.
2  

 

 

 

 

 

Output too

large

 

 

 

 

 

Residual liquid or gas in the conduit pipe Drain the liquid or gas from the pilot tube.
Output wires are reversed or connected incorrectly Check treatment
Sticking of main or sub levers ortest pieces, etc. Handling
Poor contact with internal connectors Handling
Electronic device failure Replace the circuit board with a new one or find the fault according to the instrument’s instruction manual.
Damaged pressure sensor Replace the transmitter
Whether the actual pressure exceeds the selected range of the pressure transmitter. Re-select a pressure transmitter with the appropriate range.
3 Output too small Transmitter power supply is normal If less than 12VDC, check whether there is a large load in the circuit, the input impedance of the transmitter load should be in accordance with the RL ≤ (transmitter power supply voltage – 12V)/(0.02A)Ω.
4  

 

 

 

 

 

 

 

 

 

Unstable

output

 

 

 

 

 

 

 

 

 

whether the actual pressure exceeds the selected range of the pressure transmitter Re-select the appropriate range of the pressure transmitter.
Damage to the pressure transducer (severe overload may damage the isolation diaphragm) Send the transmitter back to the manufacturer for repair.
There are liquid or gas residues in the conduit. Discharge the liquid and gas in the pressure guide tube.
Influence of pulsation of the measured medium Adjust the damping to eliminate the effect
Supply voltage is too low or too high Adjust the power supply voltage to 24V
Poor contact or intermittent short-circuiting in the output circuit Inspection
Loose wires, wrong power supply wires Check wiring
Multiple ground points in the circuit Check that one point of grounding is preserved
Poor contact with internal connectors Handling
Damaged pressure sensor Replace the transmitter
5  

Incorrect

pressure

indication

Transmitter power supply is

normal

If less than 12VDC, check for larger loads in the circuit. The input impedance of the transmitter load should be in accordance with RL ≤ (Transmitter Supply Voltage – 12V)/(0.02A) Ω.
The reference pressure value

must be correct

If the accuracy of the reference pressure gauge is low, it should be replaced by another one with higher accuracy.
The range of the pressure indicator is the same as the range of the pressure transmitter. The range of the pressure indicator must be the same as the range of the pressure transmitter.
The input of the pressure indicator and the corresponding wiring are correct. Pressure indicating instrument input is 4 ~ 20mA, the transmitter output signal can be directly accessed; if the pressure indicating instrument input is 1 ~ 5V must be in the pressure indicating instrument input and connected to a precision of 1 ‰ and above, the resistance value of 250Ω resistor, and then access to the input transmitter
The input impedance of the transmitter load should be ≤ (transmitter power supply voltage – 12V)/(0.02A)Ω. If it doesn’t meet the requirements, different measures can be taken depending on its situation. It includes increasing the supply voltage (but it must be lower than 36VDC), reducing the load, etc.
Whether the input terminal of the multi-point paper recorder is open circuit when there is no record. If the open circuit, can not take other loads; switch to other no record when the input impedance ≤ 250Ω recorder
Whether the corresponding equipment shell is grounded Grounding of the equipment shell
Whether to separate from the

AC power supply and other power lines

Separate wiring from AC power and other power sources.
Whether the pressure sensor is damaged (severe overload can sometimes damage the isolation diaphragm) Must be sent back to the manufacturer for repair
Whether the pipeline is clogged with sand, impurities, etc. (the

measurement accuracy will be

affected by impurities)

Clean the impurities, and add a filter before the pressure excuse
Whether the temperature of the pipeline is too high (the

operating temperature of the pressure sensor is -25~85℃, but it is better to use it within -20~70℃).

Add buffer tube to dissipate heat. It’ s best to add some cold water to the buffer tube before use to prevent overheated steam directly impact the sensor. It may damage the sensor or reduce the service life.

 Intelligent Pressure Transmitter Common Failure and Analysis

Serial

number

Failure phenomenon Cause of Failure Treatment
1 Output indicator reading zero

 

 

 

Whether the power supply electrode is reversed Correct wiring
Whether the power supply voltage

is 10~45VDC

Restore power supply 24VDC
The diode in the terminal block is damaged Replace diode
Damaged electronic circuit board Replace electronic circuit board
2 Transmitter cannot communicate Power supply voltage on transmitter (min. 10.5V) Restore power supply 24VDC
3  

 

 

Transmitter reading is

not stable

 

 

 

 

Load resistance (min. 250Ω) Add Resistor or Replace Resistor
Is the unit addressed correctly Re-addressing
Whether the measured pressure is stable Take measures to stabilize voltage or wait
Check damping Increase damping
Check for interference Eliminate source of interference
4  

Meter reading is not accurate

 

 

 

Is the instrument pilot tube free Unclog the pilot tube
Transmitter settings are correct Reset
System equipment is in good condition Safeguard system integrity
Meter not calibrated Recalibrate
5 Pressure change, no response from output Instrument lead pipe is clear Unclog the pilot tube
Transmitter settings are correct Check and reset
System equipment is in good condition Guarantee system integrity
Check transmitter safety jumpers Reset
Damaged sensor module Replace sensor module

 

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