Safety Valve Tail Pipe Design

[72Hokie]

The main steam crossunder safety relief valves lifted at our power station, which resulted in blowing off the Turbine Building siding directly above the tail pipe discharges. The discharge tail pipes for these relief valves exit the side of the Turbine Building about 40 feet from the roof. Nearby windows were also broken out (probably from the sonic shock of the discharge?). The bank of 3 – 24”, 2 - 10” and 1 – 8” tail pipes for these safety relief valves have very little extension (less than 1 foot horizontal distance from the outside wall at the top of the 45 degree tail pipes) from the exterior wall. I am not having any success in finding a reference for calculating the negative pressure resulting from the discharge, or a reference for the amount of extension needed for the tail pipe to ensure damage will not occur to siding/windows from the negative pressure at discharge. Does anyone have references or other guidance for the proper design of safety relief valve tail pipes extending out of the side of a building?

 

[BigInch]

Did the glass blow out or into the building?


BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[72Hokie]

The glass blew out.

 

[BigInch]

Sorry for the 20 questions, but.. I'm thinking...
do you have known flowrates from those valves and their relief pressure settings? I suppose its possible that surrounding air was accelerated upward with the stream effect from the valves.

What did you have there about 155 dBa?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[72Hokie]

2 - 16"x24" SRVs at 1,426,360 lbs/hr and 236 psig
1 - 16"x24" SRVs at 1,403,408 lbs/hr and 232 psig
2 - 8"x10" SRV at 845,000 lbs/hr and 230 psig
1 - 6"x8" SRV at 210,000 lbs/hr and 225 psig.

Don't know what the sound levels were.

 

[BigInch]

I really don't have any particular knowledge about this, but I will say, if I was doing the vent design, I'd run them up to about 10 ft above the top of the roof.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[katmar]

I saw this happen on a plant many years ago. We didn't try to calculate the negative pressure developed - we just rerouted the tail pipes well away to where our "engineering intuition" (aka gut feel) told us it would be safe. Be careful of any additional back pressure you may introduce on the SRV's with longer tail pipes.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

After I got to thinking about it, I realized I've never seen relief vent outlets located below a building's eve level, which is natural, since the pipe outlets should be placed above the roof to radiate and dissipate noise up and away, rather than echo and possibly reinforce sound pressure waves bouncing between buildings. Bill your engineering company for the broken glass and the extension work.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Latexman]

72Hokie,

Did anyone see what really happened? Since the tailpipes did not extend much past the exterior wall, especially the bottom of the 45 degree miter cut, is it possible the tailpipes were "thrust" into the building where they would have damaged the building exactly like you described?

By the way, if the top of the 24" tailpipe was 1 foot from the exterior wall, wouldn't the bottom of the 45 degree miter cut be 1 foot inside the exterior wall?

Good luck,
Latexman

 

[72Hokie]

Latexman,

The tail pipes are not mitered, they are 90 degrees with the axis. The tail pipes go through the building wall at 45 degrees. (Sorry about my description not being clear enough.)

We have a security camera video of the event and the tail pipes do not appear to have "thrust" back inside the building wall. The video shows an initial discharge of water, followed by the steam discharge during which the siding pulls off. The event occurred on a day when we had approximately 12 inches of rain fall. We found some of the drains in the tail pipes clogged after the event, thus we believe quite a bit of water was in the tail pipes when the RVs lifted.

Thanks,

72Hokie

 

[Latexman]

Then the momentum of the exiting steam induced surrounding air near the exhausts (i.e. at exterior wall), creating a low pressure region, and the walls and windows blew out. Like a huge venturi.

Good luck,
Latexman

 

[72Hokie]

Latexman,

I agree with your assessment of what happened. Do you have any references for calculating the negative pressure developed by the discharge?

Thanks!

 

[Latexman]

No, not for an unconfined free jet induced flow case with a condensing media. There may be research out there that is applicable, but I think the search will be a difficult one.

Good luck,
Latexman