×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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!
  • Students Click Here

*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

Jobs

Safety Relief Valves

Safety Relief Valves

Safety Relief Valves

(OP)
If I have a safety relief valve where the set point is at 250 psig and the maximum allowable back pressure is 52.2 psi, is the pressure at the valve discharge at 250 psig and the maximum allowable pressure drop along the line 52.2, or is the  pressure at the valve discharge 52.2?

RE: Safety Relief Valves

In your case, the discharge pressure will be no more than about half of the inlet pressure or less due to sonic flow through the valve.  See API RP 520 for a detail explanation.

If the discharge system backpressure exceeds 52.2 psi (assuming the number is correct) then the capacity of the relief valve will decrease unless hardware provisions are made in the valve such as a balanced bellows.

RE: Safety Relief Valves

What is the fluid type? is it a gas or a liquid?

If it a gas the 52.2 limit is not reasonable because up to ~125 psi the flow will be constant (sonic) when the upstream pressure is 250 psi. If it is a liquid then

 flow=constant*SQRT(P_up-P_down)

and this will be affected if the P_down is larger then 52.2 psi.

RE: Safety Relief Valves

Given your statement "If I have a safety relief valve where the set point is at 250 psig and the maximum allowable back pressure is 52.2 psi", the pressure at the valve outlet can not exceed 52.2 psi during relief.

That said, API 520 restricts conventional relief valves where the built-up back pressure is not more than 10% of set pressure.   Static backpressure can be higher than 10% but affects the pressure the relief valve opens open.

Where the built-up backpressure is greater than 10% of set pressure, then bellow relief valve or pilot operated relief valves are options.

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!


Resources