Guidance on selecting a Pressure Safety Valve (PSV) from API 526 "Valve Size" column 1

[samuel-1]

I am reaching out to seek guidance on manually selecting a Pressure Safety Valve (PSV) from API 526 "Valve
Size" column.

Here are the details of my scenario:

- **Operating Pressure:** 122.7 barg

- **Operating Temperature:** 29.2°C

- **Set Pressure:** 240 barg

- **Relieving Pressure:** 290.4 barg (fire case)

- **Relieving Temperature:** 244.2°C (fire case)

- **Valve Type:** Carbon steel, pilot-operated valve with orifice type D (Effective Orifice Area = 0.307 in²)

I am particularly confused about the use of Table 17 in API 526. It mentions “Inlet Flange (Set) Pressure Limit,” and I am unsure whether the relieving temperature and corresponding pressure should be considered when choosing the PSV.

I need clarity on this matter.


Thank you for your time and assistance.

 

[The Obturator]

You should use the valve Set Pressure and incidental Relief Temperature. However, you should also ensure that the selected size/ratings parameters are also suitable for your design/extreme - pressure/temperatures (not normally the fire case temp).

What API-526 does not inform, is the pressure ratings in between the stated temperatures.

You are best to actually use the manufacturers catalogues which show the ratings graphically - note manufacturers ratings may be different to API-526 dependent on their design.

Your conditions normally require a Class 2500 inlet (F orifice) x Class 300 Outlet (if carbon steel) - 1" x F x 2" or 1.1/2" x F x 2" can be used

Note your orifice size stated as 0.307in2 is a 'F' orifice not 'D'.

Also consider what elastomers are to be used at your high temperatures.

Question: What is reason to select pilot operated and not spring operated? Spring selection would need to be 1.1/2" F x 3" size.

My advise. Select your vendor and talk to them.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[samuel-1]

Thank you Obturator,

I truly appreciate

Yes you are right the Effective Orifice Area of D” Orifice = 0.110 in.2 (I was only talking about 'D' orifice table 17, but mistakenly pulled information from another table). My spreadsheet selected 1.1/2" x D x 2", but I really wanted to know why (and not rely too much on softwares/spreadsheets)

ok so if I understand you correctly, I would select the "Valve Size" column, based on a Set Pressure of 240 barg and temperature of 244.2°C, while ensuring that the selected size/ratings parameters are also suitable for my extreme temperatures.

Re: So I am on a project and the client wanted a pilot PSV (because its relieving mass flow = rated mass flow) in other to optimise the size of the maintenance KO drum & flare stack (The client wants it to be as small as possible. Infact the KO drum is mobile & would be hooked up when needed for maintenance, as the facility is fully rated.)

[COLOR=]At the present stage of the project, Client & EPC is yet to engage the vendors.[/color]

Thank you once again.

 

[The Obturator]

You followed my notes well. You say that a pilot is selected for the PSV to be as small as possible. At these sizes D - F most designs are not much smaller than their spring counterparts - some manufacturers designs are even bigger! Personally, I would be offering a spring loaded design which is much more economic at that size and the maintenance much less due to the robustness of a spring loaded design.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[georgeverghese]

Firecase relieving temp is only relevant to selection of orifice size. Use the process-mechanical upper design temp for selection of flange rating for all nozzles.