PSV Sizing Flowrate? Actual Vs Design

[Berenger]

Hi experts,

I have a few questions about PSV sizing. I got some drawings and datasheets from a vendor for a fuel gas skid and I don't think the vendor calculations are correct. I will appreciate if you can help me with answers to the following questions:

(1) Fuel gas skid is designed for a maximum flowrate of 2 mmscfd BUT the actual usage is 0.5 MMscfd. Do you size PSVs based on 2 mmscfd figure OR 0.5 mmscfd figure? Keeping in mind that usage might increase due to possible plant expansion.
(2) When sizing PSV for the fuel gas scrubber, do you size based on flowrate OR do you size based on scrubber volume?

I have always sized PSVs based on actual flowrate to prevent oversizing and/or compromising integrity of the valve (e.g. chattering)

You inputs are welcomed and appreciated.

Berenger.

 

[georgeverghese]

Can you post us a process / controls sketch which also shows fuel gas source pressure control letdown into this scrubber and also describe this fuel gas unit.

 

[mk3223]

The PSV protection is for the most critical scenario of several overpressure conditions for each process. And then, the PSV sizing is based on the flow of the most critical case.

(1) You need to clarify:
- What's is the basis of the actual flow vs maximum flow.
- Is the flow of the PSV sizing adequate to protect the system in the most critical scenario?

(2) The PSV size should be based on the fuel gas flow rate, not the volume of the Gas Scrubber.

 

[zdas04]

Another way of saying what mk3223 said is to look at the list of possible scenarios in API 521 and determine which is credible. In those scenarios that are credible in your case, determine the maximum credible flow rate in those conditions and size the PSV for that flow rate.

So if the source of gas to the skid is a gas well (for example), then the usual controlling scenario for the fuel skid is either shut block valve or failed regulator. If it is shut block valve (e.g., the meter run outlet gets shut by the gatherer without any intervention on the wellsite), then you need to know the flow rate of the well at the PSV set pressure and size the PSV for that flow rate.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[gwalkerb]

You need to size to the highest flow credible overpressure condition. If you undersize your PSV to avoid chattering for lower flow cases, you run the risk of overpressuring your piping and vessel systems, and having catastrophic failure. If you size for the highest flow case, while you might have chattering in lower flow relieving conditions, you put yourself in a much safer spot.

And a PSV relieving event should be infrequent enough that chattering doesn't occur often enough to damage the PSV. If you have conditions where you have a high flow relieving case, and a low flow relieving case that you are expecting to happen much more frequently, you might want to look at including multiple PSVs, with the lower flow PSV set at an appropriately lower pressure. Then it can relieve at the appropriate rate for the low flow relieving case, but in the case of a higher flow overpressure, the larger PSV will open when your smaller one becomes overwhelmed, and system pressure continues to rise. Or perhaps you should include some sort of relief valve set lower than your PSV, designed for more continuous operation, if you are expecting to need to relieve pressure on a regular basis.

 

[don1980]

Here are my comments on your two questions:
(1) The sizing basis for the PSV can't be known until one does a detailed scenario analysis. One at a time, list all the potential failures and human errors which could cause system overpressure (e.g. control valve failure, blocked outlet, etc.). Then for each credible scenario calculate the PSV orifice size. The scenario that requires the largest orifice is the controlling scenario (the sizing basis for the PSV). One doesn't just look at the nominal capacity of the process system and say that that is the PSV sizing basis.
(2) This answer is the same as #1. Do a scenario analysis to identify the failures and errors that could cause the scrubber to be overpressured. Then calculate the necessary PSV orifice for each scenario, and chose the one that is the largest. Does this scrubber have an open vent to the atm? If so that open pipe can be the "relief device" as long as calculations show that it is large enough.