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Are you getting the optimum performance from your pressure transmitter

27th July 2009
Installation tips for improving the measurement accuracy of pressure transmitters and extending the operational service life

For maximum pressure transmitter life and ease of maintenance the following guidelines are suggested:
 
If possible the pressure transmitter should be mounted at ground or walk way level to ease access for installation, maintenance and pressure calibration.
 
When the process conditions allow, pressure transmitters should be connected to the measurement point by suitable isolation valves at the process tapping point and also have isolation valves such as a directly mounted three way valve manifold for DP cells terminating the impulse pipe work to the transmitter.
 
Avoid mounting pressure transmitters in locations subject to vibration.
 
Ensure that the thermal operating limits of the pressure transmitter electronics are not exceeded and for the longest life and highest stability run the pressure transmitter well within its rated temperature limits.  Use impulse pipe work that is not lagged to provide better thermal transfer with the ambient temperature.  This will help to either cool down or heat up the process media in direct contact with the pressure transmitter so that its operating temperature limits are not exceeded.

Ensure that the wetted parts of the pressure transmitter including ’O’ ring seals and gaskets are chemically and thermally compatible with the process media.
 
Any impulse piping between the process and the pressure transmitter should accurately transfer the pressure present at the process taps to the transmitter.  Possible sources of error in this transfer are leaks, friction losses (especially in purged systems), trapped gas in a liquid line, or trapped liquid in a gas line causing a head error and temperature induced density variations between impulse line legs causing a head error in differential pressure applications.
 
The following recommendations are made to minimise pressure transmitter errors: 
 
Make the impulse piping as short as possible, taking into account the temperature limitations of the pressure transmitter when used with very hot or very cold processes.

Piping should have a slope of at least 1 in 12 up towards the process connection for liquid and steam service and down towards the process connection for gas service. Avoid high points in liquid lines and low points in gas lines.  On initial commissioning ensure all gas is vented from liquid impulse legs and all liquid is drained from gas impulse piping legs. On steam service, fill the impulse lines with water to prevent damage to the pressure transmitter from live steam causing the thermal operating limits of the pressure transmitter to be exceeded. 
 
For differential pressure applications keep both impulse pipe legs at the same temperature. 
 
When a sealing fluid is used, ensure that the system is installed such that both piping legs for differential pressure applications may be filled to the same level and will stay at the same level under operational conditions. 
 
Use impulse piping of sufficient diameter to avoid frictional losses and if purging is used, make the purge connection close to the process taps and for dp measurements purge through equal lengths of the same pipe size. Avoid purging through the pressure transmitter.

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Understanding pressure transmitter accuracy

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Site Last Updated:  07/02/12

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