Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Pilot operarted PSVs and rated flow.

Pilot operarted PSVs and rated flow.

Pilot operarted PSVs and rated flow.

I am working with a new installation where we will install Pilot Operated PSVs with a maximum back pressure that is close to the set pressure of the PSVs. After contacting our normal PSV supplier, I got the information that the max capacity for the PSVs will be reduced to 80% when we have the maximum back pressure. But my question now is when calculating the rated flow (it will be popping PSVs) should I use the full capacity for the valve (without the 80% factor) due to the risk that I don’t have full back pressure in the system when I get a relief from my new valve.
In the end we will have the sizes calculated supplier but in this early stage we need some data for sizing of piping.

RE: Pilot operarted PSVs and rated flow.

The proper PSV valve sizing should be considered with the back pressure resulted from the maximum relief flow in the discharge piping. In order for PSV to reach the required relief capacity, any undersized piping, as a result from an engineering error in the early design stage, should be replaced.

RE: Pilot operarted PSVs and rated flow.

Hi mk3223
Thanks for your response.
I will clarify my question a bit.
I am working with a new installation and we have will be connected to a large flare system with a maximum specified backpressure of 7 barg from the client, my question is when calculating the maximum capacity for the valve should I use the flow with low backpressure (the capacity will be 20% larger than with max capacity with 7 barg in back pressure). This max pressure will be used for the piping calculations.

RE: Pilot operarted PSVs and rated flow.

Sounds like a good question for the technical team of the flare.

Why not report both. Both are valid depending on what else is going to the flare.

Good Luck,
Pats' Pub's Proprietor

RE: Pilot operarted PSVs and rated flow.

Thanks for responses.
I decided to go for the most conservative alternative and use the flow with low pack pressure to calculate the rated flow to size my piping. after finished the calculations and documents the client informed that they have recalculated the flare system and ended up with a lower back pressure so I have redo the calculations again.

RE: Pilot operarted PSVs and rated flow.

you are saying if you use pilot psv the piping flare system shall have been larger? approximately 20 percent? what is important for you?

RE: Pilot operarted PSVs and rated flow.

Hi Daria
Th e important thing for me is to get a correct estimate of the maximum flow in the piping as input to the pipe sizing. It is important to get the sizing accepted by the client because changes in the pipe sizing will afect many othe rdocuments.

RE: Pilot operarted PSVs and rated flow.

in my experience finally we should trust to manufacturer/vendor/supplier data.

RE: Pilot operarted PSVs and rated flow.

When reading API 520 part II on this topic, it does not stipulate that the rated capacity shall always be used to size the discharge piping. It allows the designer to select either the rated capacity or the actual process design relief rate. In the latter case, the actual calculated max superimposed backpressure on the common header is used to estimate the backpressure on the PSV. This would always < this 7barg upper limit specified by the Plant Owner.
But of course, the exit piping dia should be no less than the PSV exit nozzle dia, and the max velocity in the exit lateral should not exceed 0.7Mach at the max calculated backpressure in the common discharge header at this PSV.
So in your case, use a derated flow (which should = to the process design required flow) corresponding to the lower, actual value of the max backpressure quoted by your client.

RE: Pilot operarted PSVs and rated flow.

If your PSV is a pop up type then rated capacity to be considered as a conservative approach an the same is inline with API 520 PII.

For modulating type pilot operatded you may use the required capacity for calculating the relief piping (inlet and tail pipe) However, so many consultants dont tend to favor that and use rated capacity instead in order to account for any future expansion and increase in the required relieving rate.

RE: Pilot operarted PSVs and rated flow.

At the end of the day the rated capacity will only occur as a transient phenomena. After a while the flow will drop but you may enter a cycle where the valve will pop and the close again after a while, and then repeat: Thats why a modulating pilot operated PSV may be your best choice.

I take it that your set pressure is rather low and close to the max back-pressure and that why:

A) You want a pilot operated valve (its a really big and expensive valve and with the pilot you may go closer to your operating pressure in the protected vessel)?
B) Thats why the downstream pressure impacts the capacity - because its no longer choked flow?

Best regards, Morten

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Solutions Brief - Protecting and Rescuing On-Ground Personnel
Keeping our warfighters safe and delivering them a competitive advantage is a key goal of departments of defense around the world. It’s a goal shared by embedded computing manufacturers like Abaco: we never forget who we serve.This case study describes how a major international contractor integrated an Abaco single board computer at the heart of its CAS/CSAR solution. Download Now
Datasheet - Top Enhancements Creo 7.0
PTC's Creo 7.0 has breakthrough innovations in the areas of generative design, real-time simulation, multibody design, additive manufacturing, and more! With Creo 7.0, you will be able to design the most innovative products faster than ever before, keeping you on the cutting edge of product design and ahead of your competition. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close