Check PSV Required Capacity 1

[C26M13]

Hi Engineers,

I'm looking for some guidance to verify the required PSV shown in the datasheet is correctly stated.

From PSV datsheet:
Component = Nitrogen Vapour
Required Capacity = 2,013kg/hr
Normal Operating Pressure = 8.65barg
Set pressure = 12.3barg
Over-pressure = 10%
Relief Temperature = 28oC
Calculated Area = 0.487in2
Selected Area = 0.503in2 (Orifice G)

As per attached drawing, my question is: if the control valve PCV upstream of the PSV failed, is the PSV adequately size to handle the case of PCV failed (please see attached drawing "Eng-Tips.xls")?

Has someone done any calculation or simulation for this case?

Greatly appreciate.

Regards,

 

[RVAmeche]

It depends on the regulator. You need to determine the maximum flow rate based on a failed open scenario (worst case supply pressure and downstream pressure). Then your PSV sizing will be based off this required flow, ensuring your capacity > worst case required flow.

So can your regulator supply more than 2,013 kg/hr? Some regulator manufacturers like Emerson include a failed open capacity equation for relief valve sizing in the literature. If yours does not, I'd ask the manufacturer how they propose you size it. Alternatively, if you know the the Cv you could do a compressible flow model if you have software available.

Furthermore, it should be obvious but still noted that you cannot modify the regulator or trim without verifying the PSV is still adequate, since a different regulator or internal trim may result in higher flow creating an unprotected system.

 

[Latexman]

I've done hundreds of these calcs over my career. Calc. the PCV throughput at 25.5 barg inlet and 13.53 barg outlet (PSV sizing pressure). As RVAmeche said, look to your PCV vendor for support, methodology, and information/data (Cv). Come back with your calcs and we'll check them out.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[C26M13]

Thanks RVAmeche and Latexman for your comments on this post. I will take these information from you to do the investigation. I will come back with the response or questions to clarify with you,

 

[Orsiz]

The link below provides an example of a product bulletin for the MR95 pressure regulator series:

https://www.emerson.com/documents/automation/product-bulletin-mr95-series-en-122532.pdf

On page 9, the wide open coefficients are given. These are used when sizing a PSV for this particular type of pressure regulator. By knowing the wide open Cv and pressure differential (max upstream pressure and set pressure), the relief flow rate can be calculated using the proper equations for compressible flow.

On page 35, some equations are given for calculating the wide open flow capacity in case of choked flow.

 

[casflo]

Additionally, you must take into account that the discharge piping, downstream the PSV valve, has enough capacity to pass an equal or greater flow rate tan the valve.