Impulse piping

[Lc85]

We have a tail end SCR Gas/Gas heater and wish to measure the pressure at the cold and hot end of both the treated and untreated sides.The problem is that the treated gas hot end is 170'C and the pressure transmitter is only good for 120'C. is there a way to work out how much impulse tubing is need to reduce the gas temperature reaching the instrument. I know the gas is static if that helps?

thanks very much

 

[Sycorax]

you can try blowing air on the tubing to transfer more heat and reduce the gas temperature.

For formula,
heat transfer is given by q=m*Cp*delt, m= flow rate of gas, cp = specific heat of gas and delt = 170-120=50

Once u have got the amount of heat(q) to be removed, use it in q=h*a* delt, h= 25 W/m2-K ( still air) a= to be determined and delt= ( average wall temperature - ambient air). take a swag at wall temperature to be average gas temperature( 170+120/2= 145C)

The surface area is the outer surface area of the tubing. You can calculate length based on your tubing size.

Hope this makes sense.

 

[davefitz]

There is no significant air flow thru the tubing to a sealed pressure transmitter. Maybe a 0.3 m long run of capillary tubing would suffice. No heat transfer calcs required.

If there are fast pressure transients, then you can also fashion the tube to act as a helmholz resonator or snubber to filter out the transients.

"Nobody expects the Spanish Inquisition! "

 

[MisterE2]

I like dave's solution, maybe clamp off the tubing at .2 meters and release after you feel its cooled down a bit. You have to remember as soon as the pressure from the process starts, it rushes down the tube at about the same temperature.

 

[Sam654]

I would install an isolation manifold just before the xmtr. I do this as a general practice and have used these on boiler steam applications and 500 degF Dowtherm heat transfer fluid flow xmtrs where operators periodically blow down the impulse lines to remove trash. The manifold works well especially if you can get them with a bypass valve to equalize across the sensing element when doing maintenance blow downs or bringing them online. Both sides pressurize equally. Pressurize up to the manifold and allow the line to cool. Then open the isolation valves and close the equalization bypass to put the instrument online. Gases cool much faster than liquids.

There are a bunch of them out there. Rosemount, Anderson-Greenwood, Hoke, Swagelock, Parker among others. I have my favorite but I don't think I can say on this forum.

 

[Sam654]

The problem with capillary lines is they tend to plug easily. No system is clean unless you're using glass lined pipe.

 

[moltenmetal]

1/4" OD tubing, 1 foot long, uninsulated, will lose more than 100 C to ambient. Do not use CAPILLARY tubing- these are impulse lines, NOT liquid-filled capillaries.

I doubt there's any realistic risk of plugging, but you do have to concern yourself with condensate formation. You can either ensure that the legs are self-draining, or you can try to drain them periodically- as long as you're in a non-freezing environment.