Design conditions for pipe before PSV

[garfio]

I am not sure that this is the right forum for my question, so direct me to the right one if its not.

I have a steam pressure reducing stations that includes a block valve, an emergency valve a pressure control valve and a safety valve. The pipe segment between the discharge of the pressure control valve and the safety valve should be design per the upstream conditions or the downstream conditions? Design code for this case is B31.1

Thanks for any comment.

 

[Chance17]

In the facility you described, you can take a spec break at the pressure control valve exit flange.
Basically design the pipe for downstream conditions.
The downstream pipe design pressure = (safety valve set) + (safety valve accumulation) + (safety margin - say 25-psig or 10% extra)

 

[JoeWong88]

You have not uploaded the sketch, it is difficult to visualize.

If the block valve is located downstream of pressure control valve but safety relief valve is between the pressure control valve and block valve, then the spec break can be located just Pressure control valve exit flange.

If the block valve is located downstream of pressure control valve and safety relief valve is downstream of the block valve, then the spec break may have to extended to block valve exit flange.

The safety relief valve can be used to protect gas blowby when pressure control valve failed to open. The set pressure can be same as design pressure of downstream piping.

JoeWong
Chemical & Process Technology