×
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

Pressure relief valve

Pressure relief valve

Pressure relief valve

(OP)
Pressure relief configuration

Dear colleagues,

I am designing a pressure relief configuration for a vessel. The relief system consists of a bursting disk followed by a relief valve and it is designed basically to prevent the overpressure in the vessel in case of an external fire.

After doing the calculations I obtain the following results:

The vessel operates normally at 8.7 psia  (0.6 bara).

The set pressure is 15.5 psig (1.07 barg) so for the calculations I use 15.5 *1.1 +14.7 =31.8 psia.

Reading from API 520 3.5.3 Fire Contingencies I understand that I could use a value of 121% of the MAWP instead of the 10%. Is this correct? The design pressure of the vessel is 17.4 psig (1.2 barg)  

The pipe that discharges the vapours is a 2” pipe that vents into an atmospheric  dump tank. The pressure drop in this line is 4.6 psig (0.32 barg)

The Critical flow pressure is

 PCF=0.6 * (15.5+14.7) = 18.12 psig (1.2 bara)

As the back pressure (4.6+14.7= 19.3 psia) is bigger  than PCF (=>critical flow) I use the equations of sizing for subcritical flow.

For the calculations I use a Kd=0.62 for the bursting disk and 0.975 for the relief valve (plus a 0.9 of combination factor) and the results are a diameter of 1.37” and 1.15 “ respectively.

So I take the next size which would be 1.5 in for both. However the pipe is 2 in.

Would the normal configuration be a reduction before the bursting disk and a expansion after the valve or are there any other possibilities (valve of 2” body but with an 1.5” orifice for example).

What about the kind of relief valve?

According to API 520, “in a conventional pressure relief valve application, built-up back pressure should not exceed 10% of the S.Pressure at 10% allowable pressure”, in my case 0.1 * (15.5*1.1) = 1.7 psig.

My pressure drop is 4.6 psig so I should use a balanced one.

A balanced valve can be used when

(Built-up pres. + superimposed) < 50 % Set pressure.

This number can be seen in two ways: gauge or absolute.

1)  4.6 psig    < 0.5 *15.5 psig   =>TRUE  or
2)  4.6 + 14.7(superimposed) psia < 0.5*(15.5+14.7) psia =>  FALSE
         19.3                     <          15.1

In figure 30 from API 520, the backpressure correction factor only arrives until values of 50 (percent of gauge pressure), so I understand that the superimposed pressure refers to a pressure different to the atmospheric (say that that my dump tank was at 0.5 barg).

So I should use a balanced valve, but what would happen if my pressure drop was of 8 psig (>50%) what kind of relief valve should I use then, would it be a pilot operated?

It is one of my first calculations and I would appreciate your comments to have a correct vision of how to design relief systems.

Cheers.

RE: Pressure relief valve

imatasb,

Your first effort is most excellent!  Congratulations!

Yes, for fire scenarios (with no subsequent runaway reactions), accumulations of up to 121% of MAWP are allowed.  I have been trained to call this the “sizing pressure” of the scenario.  In your case, it’s 21.054 psig (1.21 x 17.4).

The critical flow pressure should be based on the sizing pressure, not the set pressure.  The set pressure is where your hardware opens.  The sizing pressure is the maximum pressure of the fluid at the relief device inlet.  Btw, this may shift you to critical flow equations, and be careful of units, the units on your calculated PCF should be psia, not psig.

Your logic on conventional, bellows, and pilot operated relief valves is fine.

Be sure you size for the worst credible scenario.  Sometimes this is NOT the fire scenario.

Keep up the good work!

Good luck,
Latexman

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