Relief scenario - mechanical PCV failure

[GWright303]

Hi everyone,

I would appreciate advice on the following:

I am currently producing datasheets for some N2 relief valves. I have analysed the system and decided that the governing case for relief sizing is the scenario where the mechanical PCV (pressure reducing valve) upstream of the relief valve fails.

The Supply to the PCV is 5 barg and is normally reduced to 0.5 barg. The relief valve is to be set at 0.6 barg. This means that when the PCV fails, the governing flow will be that produced in the system by a pressure drop of 4.4 bar (supply pressure - relief valve set pressure).

My query is - how do I simulate the effect of the PCV presence in my pressure drop calcs? Even though the device has failed, it will still have some effect wrt resistence to flow, given it is still present in the line.

Can I assume a K valve, or is it typical to assume it has the same effect as an orifice for example?

Many thanks for any help!

 

[insult2injury]

Use the full open Cv.

I2I

 

[Homayun]

Each valve has a maximum CV-value. So when a PCV fails fully open, it is assumed that it reached its maximum CV-value. Use this Cv in combination with your pressure drop of 4.4 to calculate how much N2 will be flowing to your system. The relief load will be this new N2 flow minus the normal N2 flow out if any.

Hope this helps

Regards

 

[GWright303]

Thanks very much for your advice!

I have managed to find vendor data for the PCV (a Norgren PCV) and so will contact them wrt the Cv value.

The amount of information stored locally seems very scant, so originally I wasnt sure if there would be any vendor info - thought could maybe estimate/approximate the effect as per an orifice or similar.

Thanks again, you have been a great help.

 

[JoeWong88]

You may not need to consider full dP of 4.4 bar as choke flow occur in the PCV (when full open)...

JoeWong
Chemical & Process Technology

 

[GWright303]

You may not need to consider full dP of 4.4 bar
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Thanks Joe - a check for sonic velocity/choking will be included.

Many thanks!