×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

RV vent dP calculation - effect of exit

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.

RE: RV vent dP calculation - effect of exit

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

RE: RV vent dP calculation - effect of exit

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.

RE: RV vent dP calculation - effect of exit

I always use an exit loss.

RE: RV vent dP calculation - effect of exit

sailoday,

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

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

RE: RV vent dP calculation - effect of exit

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.

 

RE: RV vent dP calculation - effect of exit

sailoday,

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

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

RE: RV vent dP calculation - effect of exit

(OP)
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?

RE: RV vent dP calculation - effect of exit

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.

RE: RV vent dP calculation - effect of exit

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.     

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources