N2 Pressure Regulators Analysis 1

[kevllt]

Hey all,

I was reading through this forum topic because I'm having some trouble with N2 regulators. And needless to say, I'm impressed with the responses.

My problem involves some instrumentation work, control valves failure analysis. My system contains a letdown regulator from 70 psig N2 supply reduced to 12 psig and a final regulator at the tank overhead, reducing it to 2" WC pressure. To tell you the truth I'm not exactly sure what would happen if the let down regulator failed. The final regulator can handle the higher pressure on the supply side without failing mechanically, however say for a Fisher S201-P2 with a 1 3/16" (1.1 in2 area) trim, the force on the trim trying to open it from the upstream pressure just went from 12 lbs to 70 lbs. The spring on this valve also tries to open the valve. The only closing force is the pressure in the tank acting on the diaphragm. The set pressure is typically 2 in WC. If the higher force of the supply pressure is linear acting then the tank pressure would increase to 11.6 inWC
(2*70/12) to close the regulator. If this line of thinking is correct, then it could cause an over pressure situation in some tanks. In other tanks it would cause a venting situation that may not be an over pressure situation. Looking forward for some input.

Thanks.

 

[Montemayor]

Kevilt:

I think you are correct in your reasoning. To confirm your belief, check your data with an expert (a guy who not only specializes in N2 blanketing, but who designed and holds patents on N2 pad supply valves) like Paul Ostand. Be sure to go to:

http://www.ostand.com

where you should download Paul’s 4-part Paper on the details of N2 blanketing. After reading Paul’s dissertation you’ll be feeling a lot more knowledgeable about your application. If you still have doubts and questions, I’d contact Paul himself in West Virginia via phone or email. He can give all the mechanical details about N2 regulators and how they can or shouldn’t be installed on N2 blanket systems. You should have the possibility of regulator failure totally mitigated for a storage tank.

I hope this helps you out.

 

[SeanB]

A little bit of a different situation but we load chemical storage tanks by pressuring off the truck with a nitrogen supply. We have a pressure regulator to maintain the pressure. However in the case of a regulator failure we have a PSV downstream of the regulator to prevent blow through to the storage tank.

 

[sshep]

kevilt,

Regulator failure is a tank relieving case, this is my practice, and I assumed it was so everywhere. The regulator port size(s) and N2 header pressure are the easiest basis to use to get a handle on this. The PVSV, emergency vent, or free vent capacity as applicable are checked against this case. It is my belief that this requirement could be superimposed on thermal breathing or working capacity requirements as a broken blanket system could go undetected for a while.

I would like to also express the belief that if high supply pressure on the final regulator is an overpressure consideration causing you anxiety, than you will have a heart attack to consider if the final regulator were to fail fully open (by any scenario). In that case the letdown regulator would try to hold the tank at 12psig which it surely could not take (implied by 2" normal setting).

best wishes, sshep