Instrument Impulse Line Air Purge

[Weegie]

Looking for some guidance please on settings/calcs for air purging instrument impulse lines.
Situation is as follows.
Atmospheric Furnace (refinery)
Operating temp & pressure 1750 Deg F & 3.5psig
Max temp and pressure 2100 Deg F & 6 psig.
Refinery std is for a Fisher air regulator feeding air through
3/8"od tube via a 3/8" tubing union with a 0.125" in line restriction orifice and tee'd into the pressure transmitter impulse line.
Is there a rule of thumb as to how much higher the purge pressure is in relation to the prossess operating pressure ?
How would I calculate the flow rate and pressure drop due to the in line restriction orifice ?

Many thanks in advance.

 

[Scotsinst]

Impulse lines may be purged to have a velocity of 1 fps (you are just trying to keep dust out).
The pressure upstream of the RO should be sufficient to ensure critical pressure drop (roughly twice the outlet pressure) but it may be higher if desired so I'd set it at, say, 12 psig.
Calculate the flow through the RO with 12psig upstream pressure using standard orifice calc.
You can then calculate the resultant velocity. As long as it's not too high (causing pressure drop and interfering with process pressure reading) you should be okay (I assume you have 1/2" impulse tubing).

 

[Weegie]

Many thanks !