Contact US

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

Set Pressure < MAWP

Set Pressure < MAWP

Set Pressure < MAWP

I have an existing relief valve:

relief valve set pressure               = 15.1 psig
ASME vessel MAWP                        =  200 psig
required relief capacity                =   78 gpm (water)
valve rated capacity (@ 15.1 psig)      =  546 gpm

The relief valve is obviously oversized but it is desired to remain if at all possible.  Can I base the inlet pressure drop on 3% of the MAWP, or is it strictly to be 3% of the stamped set pressure of the valve?

RE: Set Pressure < MAWP


The ultimate goal behind the PSV inlet line max allowable pressure drop is to avoid chattering. Since the required relief capacity is way below the 25% rule of the rated capacity, Chattering is most likely to take place.


RE: Set Pressure < MAWP

First, with such a large difference between set pressure and vessel MAWP, and if this is a "safety" device, I hope you aren't using this relief valve as a pressure regulator.

From another perspective, is the source of liquid overpressure great enough to cause a safety concern for the referenced vessel?  If not, you may could make an arguement the relief valve isn't needed for that case.  However, you need to make sure there aren't other components in the protected system that require the low set point.

Otherwise, HDIMR gives good advice but as I understand the 25% rule relates more to valves in vapor service that tend to pop open to near full capacity upon reaching set point.  The pop action is related to the sudden expansion of the vapor inside the "huddling chamber" of the valve disc.

I have seen at least one company's practices that suggests that valves in liquid service are characterized more by a progressive increase in lift with increase in inlet pressure rather than pop action.  A similar discussion can be found in the following reference...
"Pressure Safety Design Practices for Refinery and Chemical Operations" by Cheremisinoff, N.P.

As an excerpt from the above reference...."Liquid service valves are, therefore, less liable to chattering at low relieving rates, and they will modulate down to about 10% of design flow."

The limitation on the inlet pressure drop is a direct function of the blowdown pressure of your relief valve.  You may want to discuss your issues with the relief valve manufacturer for possible options.

RE: Set Pressure < MAWP

EGT01, I've recently read some recommendations indicating that safety valve chatter in liquid service, though less likely, is potentially more serious than in vapor service  because of the liquid hammer effect (In Ostrwski's presentation here: http://www.sache.org/workshop/2003Faculty/presentations.asp)

RE: Set Pressure < MAWP

The previous valve was only sized for maximum R-123 pressures (< 10 psig according to operating temperatures and a P-H diagram).  I am  re-evaluating the valve for tube rupture which has been calculated to have a liquid (water) relief of ~78 gpm.  The valve is a Consolidated 1905N, 4" x 6".  I can only assume that the valve was fit minimally to the 4" evaporator outlet flange.

If the valve set pressure had been = to the MAWP (200 psig) then I would have no worries as the max water pressure is ~80 psig.  But since the set pressure is 15.1 psig, I can't get the inlet piping to pass (3% losses).  My question was in regards to whether code allows to accept inlet pipe losses using 3% of the MAWP if the valve set pressure is sig. lower.

I have also checked with the on-site mech. and he ensures that there is no other need for such a low set pressure.

Thanks for the help so far.

RE: Set Pressure < MAWP

The requirement recommended by API RP520,PartII of a max inlet pressure loss to a relief valve of 3% of set pressure is mainly to avoid excessive pressure loss and then serious overpressure resulting from late response of PSV.
In your case with set pressure far far less than the MAWP of the vessel, the severe situation you can get is if your vessel has, due to any reason in the process, high pressure close to MAWP then the PSV must open to relief wherein the resulting high operating pressure will overcome the pressure loss in the suction piping to PSV.
In the worst case where suction pressure loss is too high and with no 3% rule, the PSV will open at all cases without reaching the vessel MAWP.
Then, I don't see any concern at all and the PSV can be taken out as I guess the vessel is protected somewhere in your system.
I hope above is clear and not confuse you


RE: Set Pressure < MAWP

The safety relieve valve is to protect the vessel. If the vessel is good to 200 psi, why wouldn't you set the PSV to 200 psi?

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


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. 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