Relief Valve Discharge Piping

[DSB123]

Anyone out there can help me with this. In ASME B31.1 Appendix II there is a method for Relief Valve Discharge Piping Design. One of the equations is to determine the pressure at the vent exit P3. Under certain conditions P3 calculated by this equation can be less than atmospheric (14.7 Psia). From other sources I have come across, if this occurs in the pipework sizing then the exit pressure P3 must be set to atmospheric (14.7 Psia) and the process is then to work backwards along the pipe to determine if the calculated pressure drop causes a problem. Can anyone concur with my understanding or have I got it wrong.My problem is that ASME B31.1 does not warn the user that if the calculated value of P3 is less than atmospheric then P3 must be set to atmospheric.The example given is such that P3 is well above the atmospheric pressure hence no problem.
If I consider the other sources then by setting P3 to atmospheric, then calculate P2,V2 in accordance with ASME B31.1 and check for blowback then you get blowback. On the other hand if you use the calculated value of P3 from ASME B31.1 and calculate P2,V2 then blowback is not a problem. Please advise.

Basically in a constant diameter discharge pipe can the pressure just inside the exit point be less than atmospheric ?

Regards

DSB

 

[grt12]

The answer to the last question is no, the pressure cannot be less than atmospheric, becasue then you don't get flow. A rule of thumb to remember is that pressure is always backwards imposed so you should always start from the outlet and work back to the inlet. Therefore in your case you should set the outlet pressure to atmospheric and work backwards to the PSV outlet, e.g if you calculate a pressure drop in the tail pipe of say 100 kPa then the backpressure at your PSV is 200 kPaa (Using 100 kPaa as atmospheric pressure).

 

[DSB123]

Thanks very much grt12.

Your understanding of the process is exactly the same as my own, however I have a Vendor who think's otherwise and that the pressure just inside the vent exit can be less than atmospheric. The Vendor has followed the ASME B31.1 approach using a Mathcad application but has not recognised the situation where the Mathcad application calculates the downstream pressure to be less than atmospheric.

Again many thanks.

DSB123

 

[DeltaCascade]

Maybe the problem is just semantics. Standard atmospheric pressure is 14.696 psia, but, for example, here in Calgary, Alberta, Canada, atmospheric pressure is typically about 13.8 psia.

 

[ChemE001]

grt12 is corrct. The pressure at P3 can not be less than the atmosphric pressure what ever the atmosphric pressure is be it above or below sea level.

If the pressure inside the pipe is less then atmosphric than you will have a vacuum and the flow will be slightly reversed inside the pipe until it reaches equlibruim.

YOu can download from

http://www.dires.net

under software, the free Mach II Reader and Regression Tool along with over 200 DIERS benchmark examples for single and two-phase flow which includes inlet and outlet pipe design. The User Manual is also a great training manual to teach Jr.
Engineers relief system design and pressure drop calculaitons for compressible and incompressible fluids.

 

[hacksaw]

sounds like your vendor's calculation is incomplete. such results typically indicate that the assumed flow is too large.

 

[Hookem]

What is this 'DIRES.net'? This site does NOT come up on MSN.com or on Google.com as a valid site.

 

[StoneCold]

You can find the software at

http://www.iomosaic.com/diersweb/htdocs/software/software.html

 

[ChemE001]

DIERS which is the Design Institute of Emergengy Relief Systems is the research think tank for relief system design methodology especially for two-phase venting. DIERS is the premire source for designing pressure relief systems for reactive, non-reactive, homogeneous onset disengagement, bubbly flow, churn-turbulent flow selection and for flashing flow, frozen flow, hybrid mixtures of non-condensibles. They are the best "good engineering practices", as required by OSHA 1910.119.

The Institute site is being hosted by IOMosaic an engineering consulting firm who is also a DIERS member. DIERS is made up of over 200 member companies too numerous to list but looks like a list of the fortune 500, chemical, perto chemical, refinery, pharmacutical, and food and beverage industries.

DIERS has influence changes in API 520/521 Recommended Practices and is regonized by ASME as best engineering practices. Memebership into DIERS is free and open to any company or individual. It is a must for anyone involved in looking for the latest innovations and methodologies for designing relief systems.

The Mach II software has incorporated all the single and two-phase methodologies practiced by the institute as good engineering practices. The single-phase design uses API/ASME sizing equations and uses the HEM(Homogeneous equilibruim model) and the Omega method for two-phase flow.

The Institute has published over 200 single-phase and two-phase examples for almost every type of relief calculation you would encounter in the industry. These benchmarks can now be shared with the entire industry absolutely free by using the Free Mach II Reader and Regression Tool. The Reader allows you to fully view, and examine every single detail of the relief system from venting vessel, inlet piping, nozzle, outlet piping, to atmospher or effulent handling system.

The Mach II Raader is also an excellent training module to teach your junior engineers or those not as familiar with relief system design and documentation requirements. The software is very user-friendly and intuitive. The single phase engine is separated from the two-phase so that the more advanced calculations can be explored and enjoyed only by the experienced user.

Let us know what you think of the software and please give suggestions or comments on making it an even better tool for both training and design.

 

[ChemE001]

DIERS which is the Design Institute of Emergengy Relief Systems is the research think tank for relief system design methodology especially for two-phase venting. DIERS is the premire source for designing pressure relief systems for reactive, non-reactive, homogeneous onset disengagement, bubbly flow, churn-turbulent flow selection and for flashing flow, frozen flow, hybrid mixtures of non-condensibles. They are the best "good engineering practices", as required by OSHA 1910.119.

The Institute site is being hosted by IOMosaic an engineering consulting firm who is also a DIERS member. DIERS is made up of over 200 member companies too numerous to list but looks like a list of the fortune 500, chemical, perto chemical, refinery, pharmacutical, and food and beverage industries.

DIERS has influence changes in API 520/521 Recommended Practices and is regonized by ASME as best engineering practices. Memebership into DIERS is free and open to any company or individual. It is a must for anyone involved in looking for the latest innovations and methodologies for designing relief systems.

The Mach II software has incorporated all the single and two-phase methodologies practiced by the institute as good engineering practices. The single-phase design uses API/ASME sizing equations and uses the HEM(Homogeneous equilibruim model) and the Omega method for two-phase flow.

The Institute has published over 200 single-phase and two-phase examples for almost every type of relief calculation you would encounter in the industry. These benchmarks can now be shared with the entire industry absolutely free by using the Free Mach II Reader and Regression Tool. The Reader allows you to fully view, and examine every single detail of the relief system from venting vessel, inlet piping, nozzle, outlet piping, to atmospher or effulent handling system.

The Mach II Raader is also an excellent training module to teach your junior engineers or those not as familiar with relief system design and documentation requirements. The software is very user-friendly and intuitive. The single phase engine is separated from the two-phase so that the more advanced calculations can be explored and enjoyed only by the experienced user.

Let us know what you think of the software and please give suggestions or comments on making it an even better tool for both training and design.

 

[chicopee]

I think what the vendor is alluding to is the static pressure that develops along the inside wall of the discharge pipe. By virtue of the extreme velocity that develops when the medium is released thru the valve, there will be a negative pressure in the area I mentioned that's how you create bubbling effects in swimming pools by having air aspirated by the flow of fast moving water.
What the vendor is not mentioning is that you have a velocity pressure combined w/ the static pressure to a total pressure that far exceeds the outlet total pressure of nearly atmospheric value.