Conservation Vent Tailpipe

[RJB32482]

Is there any rule of thumb for conservation vent tailpipe sizing associated with pressure drop? Or does the tailpipe just have to pass the worst cast flow without having a pressure drop greater than the lifting pressure of the vent?

Thanks.

 

[Latexman]

"Does the tailpipe just have to pass the worst cast flow without having a pressure drop greater than the lifting pressure of the vent?"

No. The inlet pipe, conservation vent, and tailpipe (the entire system) have to pass the worst case flow without exceeding the back-pressure limits allowed by the Code the vessel was built to and it's MAWP.

Good luck,
Latexman

 

[RJB32482]

So the relief setting is 6" W.C. The MAWP of the baghouse its protecting is 8" W.C. and the worst relief case flow is 72,473 SCFM. The vent is on the piping to the baghouse. So will I have to make sure that from the beginning the inlet piping to the discharge will pass 72,473 SCFM of air at less than 8" W.C. pressure drop?

Thanks.

 

[djack77494]

RJB,
Be careful with these calculations. Know the characteristics of your particular conservation vent, because they will often require a significant amount of overpressure - perhaps double the expected pressure - before they can achieve their rated flowrate. If that were true in your case, you might need to set the C.V. at 4"W.C. in order to achieve the rated venting capacity as the pressure approached 8"W.C.

Also, keep in mind that a conservation vent is a differential pressure device. With a tail pipe, you will have some backpressure. That backpressure will serve to reduce the vent's capacity and increase its effective setpoint.

HTH,
Doug

 

[RJB32482]

Djack,
Thanks for the info. I do have the Protectoseal Sizing Program that states that the vent will relief the worst case flow rate at 7.5" W.C (MAWP is 8" W.C.) I know the 3% and 10% rule for conventional relief valves pertaining to pressure drop. Just wanted to know how to do this with the conservation vent. Do manufacturers have a K value so I can calculate it like a compressible pressure drop calculation needing < 7.5" W.C. drop through the relief inlet, vent, and tailpipe?

Or does just the tailpipe pressure drop come into play? If so, how (drop in tailpipe < 0.5" W.C.)?

Thanks.

 

[djack77494]

The 3% and 10% "rules" don't really apply here; they are for pressure vessels and relief valves, not atmospheric tanks and conservation vents. You're on the right track with the Protectoseal program. I am not aware of any K-value that would be supplied by a conservation vent manufacturer. What I think is that you must ensure that the hydraulic losses in the tail pipe do not exceed 0.5" W.C. That is assuming that the destination pressure is atmospheric; if it is above atmospheric, you need to lower even further the allowable losses.
Doug

 

[Latexman]

The flow characteristics of a vent unit is too complicated for a simple K relationship. Some do not open fully until 100% overpressure.

Vessel pressure - inlet losses - tailpipe losses can be used as the adjusted "sizing pressure" as long as you are dealing in small pressure drops, which you are.

I would hope Protectoseal's program does all this for you. I've never used it. We have an in-house program that does though.

Good luck,
Latexman