Pressure drop to consider for Combination of Rupture disk and PSV 2

[Rhy8]

Hello...I was wondering what relief load (i.e. required or the capacity of the valve) to consider in calculating the inlet and outlet pressure drop for a PSV with a rupture disk upstream of this PSV. As per API, we consider the required relief load for stand-alone rupture disk. I am not sure if this is applicable for PSV with the rupture disk upstream. I have a case where considering the rated/capacity of the PSV would give a massive inlet pressure drop during fire. Also, in checking if the PSV is adequate, do we consider that it is stand alone PSV (except taking into account the flow resistance factor in the inlet hydraulic calculations)? This case seems weird. The rupture disk set pressure is set 50% of the set pressure of the PSV. Thanks for the thoughts...

 

[don1980]

If it's a pop-acting valve, then the rules require you to use the rated capacity for calculating inlet & outlet pressure drop. If it's a modulating valve, then you can use the required flowrate. For the PRV inlet loss and capacity calculations, the disk is just a flow resistance. Check the National Board "Red Book" for the "combination coefficient".

Unless the PRV has a very small orifice (~F or smaller) you'll generally find that you can't satisfy the inlet loss requirement without a makeing the disk one size larger than the valve inlet.

 

[Rhy8]

Thank you for the response don1980. The PSV valve is a conventional valve. Thus, rated capacity should then be used. Once the rupture disk is gone, then the system will be like stand-alone PSV. The existing valve is too big (i.e. 16 in2, R orifice) and the required is only ~6 in2. This is why using the rated capacity gives a very high pressure drop. Alternative solution is to install a smaller PSV enough to cover the required relief load or making the disk one size larger than the valve inlet.

 

[don1980]

I would replace this valve with a smaller one.

 

[Rhy8]

Thank you don1980...

 

[Latexman]

There's a button for that.

Good luck,
Latexman

 

[Bill3752]

Because this was not discussed, I presume the subject of RV capy derating and inlet dP are well understood. But just in case... (1) For a system with a RD ahead of the RV, be sure to include the Kccf = .9 factor in the RV calc, and (2) for determining the inlet dP, find the certified Kre for the RD, which is to be added to your other P&Fs; the default valve for a RD Kre = 1.5, but it can vary widely.

 

[RaRo]

The worst Kre I've seen for rupture disk is 5.6 (composite disks from Schlessinger).
Be careful with using the 'default' value. I personnaly consider 3 by defualt for metallic disks, 1.5 for graphite ones, and 5.6 for composite ones.

 

[Jim111]

Please tell me how to audit PSV. Thank you!

 

[TD2K]

Jim111, you should start a new thread with your question.