Nitrogen Tank Blanketing

[cgw67]

Dear Memebers,
I have been asked to source some pilot operated pressure regulators for the above application. In the customer's P&ID's they have a 3 valve arrangement, whereby 2 PRV's are in series and the 3rd is in parallel.
The PRV in parallel is located in a larger line size 3" as opposed to the ones in series which is 1 1/2" line size. I'm assuming that the larger PRV will be for use when the tank is being emptied. What I don't understand is why they show 2 PRV's in series. Does anyone have an suggestions as to why they have designed the system like this?

 

[BenThayer]

look at the max inlet pressures of the downstream PCV. sometimes you cannot get proper control and deliver 1-2" water column with a 100 psig inlet.

then folks might have the first drop the pressure from say 100 psig to 20 psig and then have the second PCV drop it from 20 psig to 2" water column.

not sure what the big bypass PCV is for.....

 

[bimr]

A single PRV is normally used. Suggest you obtain the process requirements and talk to Fisher or Emerson.

 

[LittleInch]

Sounds like the right answer from benthayer.

If you check out the larger unit it should have its set point below that of the two in series so that it only opens when the tank is emptying, but the two in series are there for smaller volume changes and have a more precise set-point for static or small flows. Pressure regulators like this are best operating in a certain range and if your end pressure is less than 5-10% of what's coming in then you really need two in series for precise control.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Compositepro]

A self contained pressure regulator (pilot operated only means the set point is from a remote location) senses the pressure at its outlet. This severely limits the flow rate at 2" wc and can easily go into oscillation. For tank blanketing the pressure signal should come from the tank and not from the N2 inlet pipe. In this case a single stage valve is possible and desirable. The tank head space must be large enough that the valve cracking open does not cause an overshoot in pressure. But perhaps this is what you meant by "pilot operated".