Outlet Piping Pressure Drop - Steam 1

[RJB32482]

Hello,

We have a PSV that has a set pressure of 50 PSIG, 10% allowable accumulation, and 4,456 lbs./hr. relief capacity.

I'm looking to size the tailpiece piping to ensure we don't go over the 10% rule (5 PSI in this case) for the relief valve.

If I use normal pressure drop calculations, what density of steam should be used (since the lower steam pressure = lower steam density = higher pressure drop).

Or do other usually use the API standard method where you work backwards for vapor systems (find mach number at the outlet (ambient in this case), calculate the total K values, look up P2/P1 on a chart in API, then calculate P1).

Any help would be appreciated. I have just used the density of steam that was the average pressure between atmospheric and the set pressure in the past. But that doesn't mean I haven't did it incorrectly in the past.

Thanks in advance.

 

[Latexman]

Well, I put the data into my handy dandy, company proprietery software and let it crunch the numbers. That's not much help for you, especially since you did not give us any details on the tailpipe, like length, ID, fittings, K values, etc.

All I know now is you probably need a "K" nozzle PSV. It might be 3x4, 3x6, or 4x6.

My software uses rigorous, compressible flow calculations.

With 5 psi drop and the best case that the pressure at the end of the tailpipe is about 14.7 psia, thats 25% pressure drop, so you should be using rigorous compressible flow methods too.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Latexman]

Of course, simple incompressible methods can be used as long as the pipe is big enough diameter so the results are accurate, say pressure drop is less than 10% of absolute pressure at the PSV end of the tailpipe. Or, you can mathmatically break the tailpipe into multiple sections so each section can be accurately characterized by incompressible methods. "There must be 50 ways" to solve this type problem.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[RJB32482]

Thanks Latexman,
Biggest thing I was asking if I used the steam pressure drop formulas, what density would I use for the steam. Would I just be conservative and use the density of 14.7 PSIA steam?

Although the API 521 method looks better since it works backwards and we know the pressure and density of the steam at the outlet.

Bu the way, the PSV is a 3" outlet and the piping is estimated to be 20ft of Schedule 40 3" piping.

 

[TD2K]

I'd do a constant enthalphy expansion of the steam from the inlet to the outlet. Part of the expansion will be isentropic but isentalphic will give you a higher outlet temperature, a lower density and higher backpressure.

If you can have up to 5 psig back pressure, your steam, as a worse case, will flow from 5 psig on the outlet of the PSV to 0 psig. That's more of a pressure drop than I would want to use incompressible flow equations.

If you have Crane's flow of fluids, I would use that. Assume the outlet pressure on the PSV is 5 psig, use the temperature as above to calculate the density, you use the inlet density to the tailpipe. Calculate the flow you get through the pipe and fittings, exit, etc. If that flow is greater than the rated capacity of the PSV, you are good to go.

 

[Latexman]

Aha, it must be a 2J3. I mistook 4456 lb/hr as the sizing flow previosly; it is the RATED flow.

TD2K's suggestion is a good one.

You could also do an isentropic flash from 50 psig to the critical pressure. The resultant stream could then be isenthalphically flashed to atmospheric pressure. Then start with those exit conditions. If the exit velocity is subsonic, you are on the right track.

My calcs show exit Mach ~ 0.8, so you don't want to be overly conservative, or you may get a false indication of choked flow at the exit.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[chicopee]

I have an older issue of ANSI/ASME B31.1 power piping 1980 edition in which a full scale example of safety valve installation is presented. This example includes steps to check for blowback from vent pipe. More recent will probably have updated examples.

 

[MJCronin]

For a 50 psi setpoint, size the tailpipe to be one pipe size larger than the RV outlet size.

(This general "one-pipe-size" rule does not apply when a RV tailpipe is sized for HP boiler systems... two pipe sizes may be necessary)

Check for blowback and follow the sizing recommendations contained in ASME B31.1 Appendix II.

Respect us and complete this thread.. ! !..... come back with your calculations and results

MJCronin
Sr. Process Engineer