Pipe Size for Thermowell

[mb44]

why in natural gas sytems (say 98% methane 30 barg 5000 kg/h 50C) does there seem to be an engieering standard for 4inch line size for thermowells, thermowells with RTD or T/C conected back to a transmitter only measure at the tip so why cant the line size be 2 or 3inch?

 

[EGT01]

MB44,

Instrumentation is not my expertise but I've worked for some time in the petrochemical industry and all company standards I've seen have called for thermowells in a minimum of 4" pipe line size. I believe this is related to the area of sensitivity of the measuring device and to help ensure there is no "outside" effect on the measurement.

Look at the "Insertion Length—The Accuracy Factor" in the following link...

http://www.newportus.com/Products/IndustTC/Intro_TW.htm

I would be curious to know if anyone knows of other reasons.

 

[hacksaw]

It is an immersion length issue and a piping matter in dealing with small bore piping. Beyond that as a practical matter, you only need temperature measurement at the point of custody transfer for high volume industrial users. Small volume application cannot justify cost of temperature monitoring. Small user contracts (gas sales) are written with that in mind.

 

[Ashereng]

mb44,

I am a bit fuzzy on your question. Are you asking:
a) why are thermo wells used on 4" lines and larger, or
b) why are the thermowells themselves in a 4" pipe/nozzle going into a pipe/vessel

 

[jsummerfield]

Many standards exist. A common piping practice enlarges small bore piping for theremowells. Others insert the wells in elbows pointed into the flow. The shortest insertion length available for the thermowell is typically 2.5-inch. With smaller piping this is often accommodated with a short run of larger pipe with one-foot of the larger pipe between the reducers. API RP551 reflects the elbow installation, the straight installation with six-inch pipe and contrasts the threaded connection with the flanged arrangement. Other applicable standards include the API MPMS 7 and ASME PTC 19.3.

John

 

[augiedog]

Its also a matter of blocking the flow - draw out to scale a 2" sch 40 or 80 pipe with a ¾ or 1" TW in it and see how much Xsectional area is left