Selection of Pressure transmitter


#1

The most important parameters for selection of Pressure transmitter:

  1. Decide you wish to select a Gauge or an absolute pressure transmitter
  2. Know the max Design and operating pressure ranges so that you can take care of sensor span and accuracy.
  3. Know the minimum and max process temperatures
  4. Understand the MOC (Material of construction) of wetted parts required (SS316/ Hastelloy/ Monel, etc)
  5. Fill fluid required (Silicone oil/ Inert)
  6. Process connection (1/2 NPT (M/F), Din, angle) at both transmitter side and manifold valve side
  7. Material of housing and electrical connection/ Cable entry (SS316/ Aluminium and M20/ NPT)
  8. Transmitter output (HART/ FF/ Profibus)
  9. SIL-2 (Safety application or not)
  10. Requirement of display/ LCD (Display or without display)
  11. Requirement of hazardous area certifications (Exia/ Exd, etc)

It is always recommended to select a transmitter sensor with lesser turn down ratio for better performance.

As a safe practice, always select transmitter diaphragm material and the process connections of the same material of construction (SS316 or Hastelloy or Monel). Suitable manifolds of the same material to be offered. Using different trims might lead to galvanic corrosion.

Most often a common mistake done is when selecting a pressure transmitter, people dont bother the process temperatures. If you are choosing Silicone oil as fill fluid, you must keep in mind the range of silicone oil is only between -40 to +121 degC. It cannot withstand temperatures below and above this. Positive temperatures can be met with impulse tubing, where as negative/ cryogenic services need heat tracing/ selection of a different fill fluid which can withstand negative process temperatures beyond this limit. There are some manufacturers in the market who says silicone oil can withstand more temperatures than stated above, but there will be a hidden clause somewhere that your ambient temperatures to be in a specific range.

Trust the above helps.


#2

Additional Info:

(1) If you’re looking for operating and design conditions, check both the process data and line list to make sure. Perform inter-dicipline coordination if there’s a mismatch.

(2) Rule of thumb if not define in specs: Instrument Range selection shall be selected in such a way that operating conditions are only in between 10% to 90% of the said ranges.

(3) IP rating is also important.

(4) The wetted material is mostly 316L.

(5) If the fill fluid is within the boundary you can use 15 degC temperature drop per ft.

(6) Ambient capacity of the transmitter is also important. You may require adding a sunshade.