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Psv Inlet And Outlet Pipe Size

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#1 Yusuf 245

Yusuf 245

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Posted 04 April 2019 - 01:36 AM

Hello everyone,


Hope this finds everyone well. 


I am trying to install a relief valve on a fuel gas knock out drum to protect downstream gas turbine internal fuel gas skid. The below table presents the conditions.


Case                      Fluid State                Relief Capacity lb/hr                Relief State                 Orifice Size


Blocked                 Vapor                         24284                                      130                              "J", 1.287 sq.in

Discharge                                                                                                                                     effective. 

(isolated) fuel                                                                                                                     (Required 1.079 sq.in)

gas to turbine skid.




I have sized the PSV following API 520.


To facilitate the modification we have installed - during our recent turnaround, a tie-in with a blind flange on the flare header in preparation for connecting the outlet of the PSV. Since the flare header is always live, we cannot perform any hot work on it for 5 years and hence the reason for installing the tie in in advance.


My question is regarding the inlet and outlet pipe size to the PSV. The set pressure for this PSV is 310 PSIG and the site conditions favor a 2J3 PSV configuration. However, API 526 specifies the maximum set pressure for a 2J3 configuration is 285 PSIG. I want to know the implications on installing a 2J3 PSV with set pressure 310 PSIG.


The only implication I can think of is chattering. But then again, I do not see how the set pressure will affect this.


Appreciate your input

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#2 Raman Ramanjaneyulu

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Posted 04 April 2019 - 07:45 AM


In my opinion:


1) verify dP inlet line should be less than 3% of Pset

2) verify dP outlet should be less than 10% of Pset, and  check Mach number

3) still Pset = 285 psig satisfies all the criteria?

4) then go for size which meets set pressure criteria 310 psig

#3 Yusuf 245

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Posted 09 April 2019 - 01:58 AM

Thanks Raman for your reply.


Like you mentioned I believe the API 526 limit on the set pressure for a 2" inlet pipe to the RV has to do with the dP across the inlet pipe. API 520 part 2 states that " The total no recoverable pressure loss between the protected equipment and the pressure relief valve should not exceed 3% of the set pressure"


I have evaluated the pressure loss of the inlet piping assuming 0.5 piping with a gate valve and observed the following:


1. Scenario A 

Set pressure: 310 PSIG

Flowrate = 24284 lb/hr


2" Inlet piping

Pressure loss as % of set pressure = 5.86%, hence exceeding the recommended allowable pressure loss.


2. Scenario B

Set pressure: 310 PSIG

Flowrate = 24284 lb/hr

3" Inlet piping

Pressure loss as % of set pressure = 1.12%.


3. Scenario C - Aim is to calculate the minimum inlet piping size to maintain maximum 3% pressure loss for a set pressure of 285 PSIG

Flowrate:285 PSIg

Maximum % pressure loss of set pressue = 3%


RV inlet size = 2.4", approx. coinciding with API recommendation pipe size for a maximum set pressure of 285 PSIG of 2".

#4 PRVBoss


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Posted 10 April 2019 - 01:16 PM

The 285 psig set pressure limit is based on the inlet flange rating. You are looking at a 150# inlet flange, which has a design pressure of 285 psig at 100°F. 


I'm not sure what edition of API 526 you're using, but the 6th edition (April 2009) with Errata 1 (May 2009) and Errata 2 (October 2012), does not include the second row of your snippet which appears to allow a set pressure to 740 psig. I believe the 300# inlet flange on a 2J3 is actually a 150# flange, but with a bolt pattern to fit 300# connections and therefore only for low pressure applications.


For the 310 psig set pressure, you will most likely end up with a 3J4, which will have better inlet loss and backpressure characteristics regardless. 

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