Manual valves upstream and downstream PSV's

[AtleK]

Does anybody know if there are rules to where the manual valves should be placed around a PSV? "Inside" the reducers or "outside"? What I mean is that there would be a (significant) difference in manual valve size whether we place them close to the PSV's, upstream and downstream, where the pipe is narrowed in, or we place them where the pipe is larger.

 

[chemebabak]

The rules are in API 520/521. They do not directly specify if the valves should be on the smaller or larger diameter pipe, however API 521 limits the pressure loss from the vessel to the PSV to 3% of the set pressure in psig (not psia). That is, if the PSV is set at 100 psig, then the maximum pressure loss due to friction from the vessel to the PSV should not exceed 3 psi. Also, the maximum allowable back pressure should not exceed 10% of the PSV set pressure.

I have always seen the valves on the larger diameter pipe and the reducers right at the PSV, but I have never had to choose based on economics. How much money is significant? $1000, $10,000 difference? Is this for a refinery? Refineries typically have deep pockets for safety. Unless it were a really significant percentage of the project, I would put the valve on the larger diameter pipe, I would place the reducers right at the PSV and I would forget about it.

In either case, do not forget to lock open or car seal open those manual valves.

 

[AtleK]

Thanks for your reply, chemebabak. This is for an FPSO, and I agree with you that economy shall not govern our choice. That said, it shouldn't be a problem to optimize the design, as long as we can prove that the pressure loss will be low enough.

I am reviewing a system , and the engineer has placed the upstream manual valve upstream the reducer - no worries there. The downstream manual valve is placed between the PSV and the reducer. I would prefer, as you, the valve to be placed on the larger diameter pipe, but I don't think it is an actual problem to leave it where it is, if it's not restricting the flow. Or...?

The valves used are gate valves.

And yes, the valves are CSO/CSC .

 

[BenThayer]

the biggest issue is the procedure used to control the physical position of the two valves.

you MUST close the inlet first and then the outlet to ensure there are no issues with the class rating of the device.

many are higher pressure flanges on the inlet than the outlet.

don't go with a PSV that has a Class 300 inlet because it runs 500 psig and then have Class 150 outlet flanges and valves unless you can be assured that the Operators know to close the inlet piping first.

i have seen gaskets that have blown out on the discharge side because folks are closing Class 150 flange valves on the outlet BEFORE they close the Class 300 flange valve on the inlet.

lucky it was just the gasket.....

then the reverse to return to service. Open the outlet first and then the inlet, etc.

 

[MJCronin]

All of these scenarios and thier consequences should have been evaluated in the system HAZOP.

The HAZOP should be conducted after the propoposed design alteration but before construction.

Somewhere in the process, there should be a buy-in by operations

 

[dcasto]

Use whatever valve size that is cheapest and still meet the pressure drop requirments of 520/521