RV vent dP calculation - effect of exit

[Bill3752]

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.

 

[Latexman]

I'm not familiar with that reference, but the methods my company developed and uses has the exit loss on the tail pipe built into the calculations (software). In fact, when entering misc. velocity head losses on the tailpipe it says "excluding exit loss".

Good luck,
Latexman

 

[sailoday28]

If the exit flow is choked, there is no need for an exit loss.
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.

 

[psafety]

I always use an exit loss.

 

[Latexman]

sailoday,

If the exit loss is included, and flow is chocked, then you remove it and K[sub]total[/sub] reduces just enough to be subsonic, what to do?

Good luck,
Latexman

 

[sailoday28]

Latexman, I'm not sure if I follow you.
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

 

[CJKruger]

Bill3752, in general you should NOT include the exit loss of K=1.

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.

 

[EGT01]

Here's a link to thread378-126305 that you might find helpful.

 

[Latexman]

sailoday,

Sorry, I'll be more thorough. Let say I include K[sub]exit[/sub] = 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 K[sub]exit[/sub] = 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 K[sub]exit[/sub] = 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

 

[sailoday28]

Latexman: Let me respond to one part of your question. Use of K=1 with subsonic exit(station 1) flow to a large area receiver (station 2).
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

 

[Bill3752]

I want to thank everyone for their input. While I am still not understanding this issue with 100% confidence, it appears I should disregard the exit factor. I wrote an email to CCPS re: this issue; don't know if I will receive an answer as I am not a member.

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?

 

[sailoday28]

For a know mass flow(or mass flux) in the discharge piping and known (or reasonably obtained )fl/d, you should then be able to calculate the entrance conditions just upstream of the silencer.
Discuss your calculated upstream conditions with the silencer vendor.

 

[4Pipes]

The safety valve tail pipe exit loss is the pipe stress engineer's best friend. Literally hundreds of hours can be spent trying to justify oversized tailpipes. If your friend is a pipe stress engineer then include it. If not, bin the software and follow Sailoday.