B31.1 Apprendix II Blow Back Calculation Experience

[REP2]

Everytime I try to apply B31.1 Apprendix II Rules for PSV installation it always seems my f(Lmax/D) values are off the Chart II-1. I am looking for some discussions on this analysis. I do not have access to the original Keenan and Kaye 1948 Gas Tabels and do not know if values for f(Lmax/D) are available outside the area in the published Chart II-1. Due to the limitation of Chart II-1 I would interpret this as the B31.1 Apprendix II rules do not apply to long vent lines sizing that do not fall within the chart area. Anyone have the same experience and would my conclusion be accurate? Are the B31.1 Safety Valve rules for short pipe runs only?

Having said the above, I am looking method for calculating blowback on a long PSV for a saturated steam (35 psig) vent line 10" in diameter, 75 ft of horitontal straight pipe, 10 ft of vertical with (2)45ELbows and (5)90Elbows. To me this vent line is too long for the rules of B31.1 Appendix II. I also think that the use of drip pan elbow can not be used unless no blowback can be confirmed and I am afraid that there will be blowback. Any suggestions on a method for determining blowback?

An alternative is to hard pipe the outlet but condensate in the line would be hard to manage but could be desinged. Any one with this experience have any comments to add?

 

[StevenHPerry]

Did ASME intend the non-mandatory appendix only for short runs? Maybe ASME just omitted data? Or... maybe ASME is guiding designers away from long vent lines?

I've only been involved in system that has experienced blowback. I found that the original designer called for drip-pan, vertical stacks for each RV, but construction decided to save money by tieing all the vent stacks on the same level together into one header, then traverse ~100 feet, snake around a pipe chase, then go up and out of the building. Thankfully, no one was nearby when the event happened.

Can you relocate the vent to minimize the distance you have to traverse or at least minimize the changes in direction?

- Steve Perry

http://www.linkedin.com/in/stevenhperry

This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.

 

[REP2]

Thanks Steven,

This is what I thought and since I have not experienced this I am hesitant to proceed. How did you resolve the blowback in your experience. Did you shorten the runs and go vertical? Did you run all lines independently?

 

[DSB123]

Why not hard pipe and place a small drain line off the discharge pipe close to the RV??