Safety Valve Discharge Pressure

[EngineerRed]

Hi
Is it possible to estimate the pressure at the outlet of a Safety Valve at discharge using the Upstream data . For Example : A safety valve discharging steam at Set pressure 1000 psig and relieving Temperature 650 F , what will be the back pressure at the outlet ?

 

[LittleInch]

No, it is not possible to calculate this knowing only the upstream pressure - You need to know what the system is downstream of your valve to calculate the back pressure.

A large short pipe exiting to atmosphere will be a lot different to a small long pipe discharging to a header.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[goutam_freelance]

A detailed procedure is available in ASME B31.1 'NONMANDATORY APPENDIX II
RULES FOR THE DESIGN OF SAFETY VALVE INSTALLATIONS'

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[davefitz]

The above referenced B31.1 code section is based on a 1974 paper by Bechtel's Liao method, which is based on Fanno flow , a subset of compressible flow.Basically, the Mach number =1 at the outlet of each separte sized pipe section.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[The Obturator]

EngineerRed Can you clarify your question.

Are you asking for a method to determine pressure at safety valve outlet when fully open?

This is not Back Pressure, though depending on the outlet piping arrangement, there will be a built up back pressure.

Superimposed back pressure variable and/or constant, is where there is pressure in the outlet piping from another source.

The actual pressure at valve outlet would be calculated from the ACTUAL (not API-Effective) nozzle area of the valve, using fully open pressure (Set Pressure PSIG + overpressure % + 14.7 = PSIA) against the safety valve outlet size at atmospheric pressure. You'll find that a pressure drop in the area of 90% (from set pressure) results. This is why Pressure Relief Valves have much lower outlet flange ratings than the inlet rating. I just can't get my hands a equation I used to use.

ASME B31.1 is for power and the calculation is based on steam which may give you the answer.

API-521 May give some indication. I'll try to dig up my notes.



Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[EngineerRed]

The Obturator - thanks for the response

I'm looking towards estimating the pressure that is displayed on the pressure gauge at the outlet of a Safety valve when its fully open and discharging to a header that relieves to atmosphere . By the way , what is the pressure displayed by the gauge fixed at this location when the valve is fully open called ? (is it backpressure or outlet pressure ) ?

And Thanks to all for the responses

 

[LittleInch]

Usually what you do is calculate the back pressure caused when your relief flow rate is exiting via the connecting pipe and the header before discharging to atmosphere. The only fixed pressure is the end pressure ( atmospheric pressure) and you work backwards from there. Hence the volumetric flow from what is now very low pressure steam can be very big and hence frictional losses can also become significant.



The normal aim is for that pressure to be less than 10% of the set pressure of the relief valve.

Try having a look through this

https://www.emerson.com/documents/a...-valve-engineering-handbook-en-us-3923290.pdf

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[goutam_freelance]

Relevant procedure is given in Appendix-II, B31.1 Cl.II-2.2.1 'Design Pressure and Velocity for Open Discharge Installation Discharge Elbows and Vent Pipes'. You should get hold of the code and do the calculation straightaway.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[davefitz]

Refer to :" Analysis of Power Plant Safety and Relief Valve Vent Stacks" by G.S. Liao, Bechtel Power Corp., Transactions of the ASME, november 1974 . For fluids other than steam, use the appropriate Cp/Cv value.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[The Obturator]

After some searching, I have now found what I believe is a method to calculate the outlet pressure.

Note that this is pressure at a Pressure Relief Valve outlet based on a fully open valve with no superimposed back pressure. This pressure is called the Static Outlet Pressure - built up back pressure is a bit different and not discussed here.

The reference to ASME B31.1 in an earlier post is related and is referenced in Emerson's method - See attached.

I located this formula in Emerson;s Pressure Relief Valve Handbook, which LittleInch earlier cited, where the attached extract comes from. It is in fact a requirement to calculate first before the reaction force calculation on what the article is about. Both metric and imperial versions are available.

Outlet Static Pressure is P2 in the equations.

Hope this helps.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[davefitz]

The above ISO-4126 method does not appear to include the effect of a long discharge pipe, ie the fL/d effect, which is explicitly included in the Liao method. Conventional compressible flow theory , applied to most releif valve cases ( ie, sonic flow at the discharge pipe exit, Mach no=1 ) uses the fanno flow relationships, the pressure at the inlet of the discharge pipe ( exit of relief valve nozzle) is found by the pressure ratio relationship, which is a function of the fL/d of the discharge pipe, mass flowrate, pipe flow area,temperature , and ratio of specific heat.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[The Obturator]

Davefitz - ISO-4126 has not been discussed.

The question also is not talking about any pipe from a PRV outlet, just the pressure at the PRV outlet.

Did you even look at the last attachment?

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[davefitz]

I looked at the last attachment, and there is no fL/d relationship in that method.

The pressure at the discharge of a relief valve ( assuming it is discharging into an exhuast pipe to atmosphere )is found from the fanno relationships. Given the mass flowrate ( and CP/Cv, T) the exhaust will be at Mach =1.0, the exhaust pipe outlet pressure is then determined. Given the fL/d for the exhaust pipe, the pressure ratio across the exhaust pipe Pi/Po is then determined , and the Pi for the exhaust pipe is also the exhaust pressure for the releif valve. QED quite easily done. This procedure does not work for low pressure systems for which the final exhaust is not at sonic critical flow ( mach no < 1.0)

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick