Medium for calculation of PSV relief rate

[Dulli2000]

Hello.
When going through standards as CGA I see that they provide a formular for calculation of PSV relieving rate which is based on air at standard conditions. What is actually the idea behind sizing the PSV for air even if we usually are working with other media and how do you use this value?
My idea would be calculating the volume flow according to the formula given by the standard (for air) and then the actual required massflow by using the actual relieving conditions for the actual used medium. Is this the way?

 

[shvet]

Is this the way?

No, this is not as a manufacturer testes and certifies the PSVs on an air bench.
They are not able to certify the PSV capacity on multiple fluids.

Massflow has nothing to do with PSV capacity.

 

[goutam_freelance]

Refer Mandatory Appendix 11 of Sec VIII-1 for correlating PRV capacities for any compressible fluid with the tested capacity with air as medium.

However, please check with latest code (i.e with Sec XIII) as I do not have the latest code.

 

[The Obturator]

@goutam_freelance ASME VIII-1 Mandatory Appendix is long deleted and replaced by ASME XIII (2023) Mandatory Appendix IV.

@Dulli2000 I believe you are trying to size a Pressure-relief Valve for a flow other than air? You might need to familiarise yourself with the requirements of Pressure-relief Valves by reviewing the basic (start point) standards API-520 Part I and ASME XIII.

 

[Latexman]

My idea would be calculating the volume flow according to the formula given by the standard (for air) and then the actual required massflow by using the actual relieving conditions for the actual used medium. Is this the way?

It’s not exactly the way I was taught. Close, but not exactly. Using the “worst case scenario“ and actual medium properties, I calculate the perfect nozzle (orifice) area needed for the sizing flow. Then, I get a real nozzle area by dividing by the certified flow coefficient of the PSV the plant prefers. Then, pick the next bigger PSV nozzle area. That’s how I do/did it.

 

[Snickster]

My understanding is that the CGA equations for required pressure relief device capacity are cook book formulars that just need to be complied with without any understanding of how they are derived. So if you have a cylinder of a particular water capacity and pressure, regardless of actual gas, you just plug into the capacity equations and determine the required air flow capacity. Then you select a CGA certified relief device that has been tested and rated for that air capacity. There is no indication of how the CGA equations were developed but it seems they looked at for a given pressure and maximum possible heat input to the cylinder of any possible contained gas what would be the required absolute maximum equivalent air relief capacity. The equations are not like API 520 equations where you can figure out the required relief valve orifice size for a specific gas by plugging into the API equations the exact properties of the actual gas you have such as MW, k (Cp/Cv), etc.

Someone had asked a related question a few months ago per this link:

Formula source of flow capacity of pressure relief deives for gas cylinders in CGA S 1.1 -2019 - Gas compression engineering

Hi everyone, I am doing some pressure relief device calculations to gas cylinders according to CGA S 1.1-2019. There are two equations for flow capacity of PRD, Qa=0.154Wc for other PRD except PRV, and Qa=0.00154PWc for PRV. Qa is the flow capacity in ft3/min of free air, Wc is water capacity of...

www.eng-tips.com

 

[Dulli2000]

Hello and thank you for replying.

I am sizing a PSV for a pressure vessel for firecase and with not air as fluid. I agree that the formular CGA gives is hard to understand (similar from other class societies). It calculates required flow rate of air at std. conditions that is dependent only on area ( Q=22,11*A(Outside surface area of vessel)^0,82 ). I am using a sizing software provided by the vendor following the API. Flow coefficients are taken into account and with specifying our fluid and the calculated required flow rate of air we get meaningful results, but I want to understand the background.

The required effective discharge area is dependent on the required flow through the device (API 520 5.6.3.1.1). According to my understanding it makes sense that it needs to be the flow at relieving conditions (p_set+dp_overpressure), but I am not certain if that is correct according to API. For the pressure it does not matter which fluid, but for the flow it matters.
So, how to get the required flow from the formula provided by the CGA (if possible)?