ASME B31.1 App. II - Low pressure relief valve, open discharge piping

[OntarioMech]

Hi All,

Performing a relatively "routine" check of an open discharge for a steam PSV, and we have come to an unexpected result.

Some problem information:
Saturated steam operating at 12psig; relief valve is set to 15psig. PSV capacity is ~5500lb/hr. Discharge elbow is NPS 4, sch.40.

Using the equation II-2.2.1 (a) (1) yields P1=4.34psia. "a" and "b" values that were used are for 15psia < P1 < 1,000psia.

Are the calculations in this code section not applicable due to this low P1? How should we proceed with calculating the maximum open discharge vent pipe? The contractor is suggesting a discharge pipe routing that has a number of elbows and there is no direct path through a wall or roof.

We have used App.II methods routinely in past projects, but usually with PSV setpoints around 100psi, so this low pressure issue has not come up before.

Any suggestions are welcome.

Thanks.

 

[davefitz]

Not sure as I no longer have the code books handy, but most relief valve calculations assume that one has acoustically choked flow at the relief valve orifice , which implies the upstream ( absolute) pressure is at least twice the downstream pressure. The discharge will not be less than 15 psia (0.3 psig) unless it is discharging into a vacuum , as in a surface condenser. As I recall, the ASME relief valve flow and piping reactions are based on a 1980 era technical paper by Bechtel's Liao. Also, ASME section I does not cover such a vessel if the design pressure is less than 15 psig. That same pressure limit may be proscribed in the B31.1 pipig code as well- TBD. Check the introductory pages.

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

 

[KevinNZ]

Without choked flow at the PSV the valve capacity is going to be depend on the pressure drop in the discharge piping. The pressure drop in the whole path from vessel to discharge point will need to be worked out.