Relief Valve Reaction Forces - Open and Closed Systems

[DSB123]

Hi All,
Just wanted to solicit some responses to the old subject of Relief Valve Reaction Forces. Over the years I have worked at Companies who use the standard Formula from API 520 for calculating the reaction force when a RV opens. However they tend to neglect the second Pressure times Area term of the equation citing either a)for open systems - that the pressure at the end of the tailpipe is atmospheric or b)for closed systems -that the pressure term can be ignored because it says so in the API Code and the forces in the system all balance.

Recently I did a check on the reaction loads from various publications a) Peng's book on Pipe Stress Analysis, b) Crosby Catalogue, c) Diers Workbook and a few others. In doing the calcs to the various approaches all gave at least double the value obtained by the API Formula (without the Pressure term). This is not due to the effect of the dynamic load factor.

Any comments on the above? Also what methods/approaches do others adopt?

 

[DSB123]

No views on this old subject??

 

[tr6]

I trust you saw this earlier thread?

http://www.eng-tips.com/viewthread.cfm?qid=290973&page=2

That being said, We used SolidWorks flow simulation software to model a 6" relief valve discharge pipe 6' long with an elbow. To simulate the valve, we took a converging nozzle and adjusted the throat diameter to until we had the capacity, which was previously calculated for a given set of inlet conditions and a given valve orifice.

The model came out pretty close to what was expected, except that the velocity at the discharge was greater than sonic. We are investigating with SolidWorks to see if that is a program deficiency or not. Perhaps it has a problem handling any shock wave. I also posted this on the SolidWorks Simulation FEA forum.

Anyway, the discharge pressure came out to be 16.7 psia, which is pretty much what was expected. IMHO, the outlet pressure is NOT atmospheric. It is greater. How much has been the subject of discussion.

Attached is the model for reference.

 

[DSB123]

Hi tr6,
Thanks for the reply. I wondered if anyone actually had any opinions on this.

Yes I saw the earlier thread - I actually posted on it.

The problem I have is trying to convince people that you cannot use the API 520 Equation and neglect the pressure term for either an "open" discharge or a "Closed" discharge otherwise the reaction loads are miles away from the actual RV load. There are numerous methods/approaches, even Vendor software, out there which all give different results, however without exception all give reaction loads well above the "basic" API 520 formula (without the pressure term). Most give at least double the value (and thats not due to the DLF). However Companies still insist on using the API 520 values for design which concerns me.

My thought are that you cannot use the API 520 formula without calculating the pressure throughout the system and including the pressure term in the computation of reaction loads - even for closed systems. The Peng "Pipe Stress Engineering" book suggests this is the case also.

Any comments?

 

[DSB123]

No views/discussions out there?

 

[DSB123]

Well I am overwhelmed by the "responses/view" from the community on this subject.

 

[DSB123]

Hi,
Just wondered if there are any views ??

 

[mariog123]

Dear DSB123,

Please see my post on

http://www.eng-tips.com/viewthread.cfm?qid=290973&page=1

Best regards.

 

[DSB123]

Hi Mariog123,
Yes I saw the earlier post. I actually posted on the thread. However I was really after direct opinions/views on my questions and also what approaches others use to determine the discharge force values in both "open" and "closed" systems.

 

[mariog123]

There are a lot of approaches/formulas and I use them only when I'm obliged to follow them. When I have the freedom to think, I consider the reaction force must be linked to the existence of one-dimensional free jet.

The "free jet effect" is the 3rd law of dynamics.
If a free jet is released in atmosphere or in a large volume, the piping system will receive a reactive force.
This is the force that API counts and is:
Reactive_Force= [mass flow-rate]*[jet_velocity]+ [p_jet]*[area_jet]
where
- mass flow rate must be the actual value (it is greater than the designed flow rate, because the actual PSV orifice is larger than minimum required)
- jet_velocity is the critical speed when the jet gas flow has Mach=1 feature and is counted as jet_velocity= sqrt(2*R*k*T/ ((k+1)*M)), where notations are as in API, R is the universal perfect-gas constant , in SI is R=8314.5 J/kg mol/K.
- p_jet is the gauge pressure in the released jet
- area_jet is the internal area of piping at the point where the jet is released

In brackets, this is exactly the API formula, where the numerical coefficient is sqrt(2*R), in SI units sqrt(2*8314.5)=129

What is missing in API formula is a "p_jet" evaluation. I think I made a correct evaluation valid for Mach=1.

I consider we have this force exactly where there is a FREE jet.
IMO, that means:
- in an open system, where the free fluid jet is released into atmosphere
- [maybe] in a closed system, at the header connection, presuming the PSV is not pressurizing the header; that means the header is considered as a large volume receiving the jet rather than a path for flow.

Best regards.