RV vent dP calculation - effect of exit
RV vent dP calculation - effect of exit
(OP)
When calculating the back pressure in a tail gas line from a RV, should one incorporate the exit head in the friction loss estimate (i.e. Kexit = 1)?
The "Guidelines for Pressure Relief and Effluent Handling" text does not include the exit loss. I must be missing something, as this does not make sense to me.
I hope a knowledgeable contributor can set me str8.
The "Guidelines for Pressure Relief and Effluent Handling" text does not include the exit loss. I must be missing something, as this does not make sense to me.
I hope a knowledgeable contributor can set me str8.





RE: RV vent dP calculation - effect of exit
Good luck,
Latexman
RE: RV vent dP calculation - effect of exit
If the exit flow is subsonic, then
1-include exit loss if the exit station area (the receiver), is very large compared to pipe exit area. OR
2- There is no need for exit loss if the velocity at the exit is known. This velocity head should be combined with the pressure head. Discharge pressure in this case is the same as the pressure in the much larger area of the receiver.
RE: RV vent dP calculation - effect of exit
RE: RV vent dP calculation - effect of exit
If the exit loss is included, and flow is chocked, then you remove it and Ktotal reduces just enough to be subsonic, what to do?
Good luck,
Latexman
RE: RV vent dP calculation - effect of exit
If you know the flow is subsonic, then you have the exit velocity and exit pressure (pressure of receiver). A Fanno line approach should then work between the source and the exit.
Consider a single pipe. Having the entrance Mentrance, Pentrance and the fl/d (excludingexit losses) there is only one Mexit which will satisfy the fl/d of the pipe.
One may have to itterate on the mass flow to obtain the correct entrance Pentramce amd Mentrance to yield the correct Mexit for the specified fl/d.
regards
RE: RV vent dP calculation - effect of exit
For subsonic flow the pressure in the pipe exit is the same as the surrounding pressure (atmosphere). So simply start there and do a pipe pressure drop calc back to the PSV outlet.
RE: RV vent dP calculation - effect of exit
RE: RV vent dP calculation - effect of exit
Sorry, I'll be more thorough. Let say I include Kexit = 1 in the adiabatic, compressible flow calculation of a PSV tailpipe and it turns out to be choked at the end. With "If the exit flow is choked, there is no need for an exit loss", I take the Kexit = 1 out of the problem and rerun the numbers, but the difference is enough to make the flow subsonic at the end. With "If the exit flow is subsonic, include exit loss if the exit station area (the receiver) is very large compared to pipe exit area", I need to add the Kexit = 1 back into the problem and rerun the numbers again, but then I realize I am caught in a paradoxical loop! What to do?
Good luck,
Latexman
RE: RV vent dP calculation - effect of exit
Use Bernoulli engineering equation between station 1 and 2 and solve for (P1-P2)
When the delta p is obtained, what is the conclusion for subsonic exit flow?
Regards
RE: RV vent dP calculation - effect of exit
I do have a related issue I forgot to mention - the plant I am doing this work discharges through silencers. The manufacturer is not known. I was looking for a decent source to cite which might give me a range of K factors to use to compute the dP across the silencer. Any thoughts?
RE: RV vent dP calculation - effect of exit
Discuss your calculated upstream conditions with the silencer vendor.
RE: RV vent dP calculation - effect of exit