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
in relation to the process pipe is dependent on the process. To ensure the transmitter measures pressure or differential pressure accurately, the following best practices are adhered to during installation in determining transmitter location and placement of impulse piping.
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
in relation to the process pipe is dependent on the process. To ensure the transmitter measures pressure or differential pressure accurately, the following best practices are adhered to during installation in determining transmitter location and placement of impulse piping.
- Keep impulse piping as short as possible
- For liquid service, slope the impulse piping at least 8cm per meter or 1inch per foot upward from the transmitter toward process connection
- Avoid high points in liquid lines and low points in gas lines
- Ensure impulse lines are at the same temperature
- Use impulse piping large enough to avoid friction effects and blockage
- Vent all gas from liquid piping legs
- When using a sealing fluid, fill both piping legs to the same level
- When purging, make the purge connection close to the process taps and Purge through equal lengths of the same pipe size. Avoid purging through the transmitter
- Keep corrosive or hot process material out of direct contact with the sensor module and flanges
- Prevent sediment deposits in the impulse piping
- Maintain equal leg of head pressure on both legs of impulse piping
- Avoid conditions that might allow process fluid to freeze within the process flange
