45 degree bevel of tail pipe

[stressone]

What are the advantages and/or disadvantages in applying a 45 degree bevel at the end of the tail pipe of a safety valve? I am thinking about forces, noise,...

 

[KernOily]

Great question...! I went to Rip Weaver's piping design class some years ago and, at the time, he claimed that the 45° bevel on PSV tailpipes is strictly the result of sacred tradition amongst pipefitters and that it has no basis in science. If anyone should know, it would be him... IF you find out anything different I would love to know about it. I say save the 0.15 hours of labor it takes to make the cut and grind it smooth, and leave it out of the design. Pete

Thanks!
Pete

 

[jjtjr]

Like all sacred tradition. it has a basis in fact. It helps to keep rainwater from collecting in the discharge pipe

 

[TBP]

I don't know about that one, jjtjr. Cutting a bevelled end will give you a larger opening than just leaving it round. If anything, you'll collect more rainwater. But if drip-pan elblows have not been omitted by the designer or contractor, then it shouldn't matter.

I was always under the impression that the larger opening of the bevelled end was to minimize the velocity of the discharging steam (or whatever fluid is being handled).

If nothing else, it's the mark of a proper fitting job - that attention was paid to detail. It's like walking into a boiler room. At least 99% of the time, your first impression tells the tale. If it's a nice, clean plant, and looks like it's been taken care of properly, it likely is, in all respects. If it looks like a dirty junk-hole, it likely is, again, in all respects.

 

[Montemayor]

stressone (Mechanical):

As TBP states, it's mostly for "looks" and aesthetics. It probably looks more "engineered" to some people - when it really isn't. The beveled cut does not reduce the exit velocity any more than a 90 degree cut - both have the same effect and there is no difference in the results. Proof of this is that the type of cut NEVER enters into any PSV discharge calculations.

Additionally, if rain drops are falling perpendicularly (as they often do), the exposed open area is the same when the tail pipe is pointing directly up. Therefore, a beveled nozzle will not collect more rain water - and even if it did, the PSV outlet should have a drain hole in the bottom horizontal section anyway and water collection should not be a problem. So, in the end, the only advantage is "Looks".

Art Montemayor
Spring, TX

 

[TBP]

Dunno, Montemayor - it's been a long time since I figured the area on the end of a frustrum of a right cylinder, but I think it has a larger area than the circle on the other end. Correct me if I'm not remembering correctly. I do know that if you're putting a pipe vertically through a pitched roof, if you just mark out a circle on the roof, the same diameter as the pipe, and cut so-marked round hole, the only way the pipe will fit through, is if it's at a right angle to the roof. This leaves the pipe at a goofy angle. You have to hold a short piece of pipe plumb, and use that as a guide to mark-out the hole to be cut in the roof. That leaves an oval, which if cut out, will allow the pipe to pass through plumb.

 

[Montemayor]

TBP (Mechanical):

I'm afraid the cross-sectional area of the pipe that the rain sees, when the pipe is vertically straight up, is nothing more than a perfect circle - regardless of what angle the pipe nozzle is cut at. If you can visualize the vertical view of the nozzle, rotate it 90 degrees and obtain the "plan" view of the vertical nozzle (or a "bird's eye view&quot and you will see that the vertical, falling rain sees only a perfect circle - the same as if the nozzle were cut square. The rain does not "see" an oval or a cross-section other than that of a circle.

Art Montemayor
Spring, TX

 

[TBP]

OK - So the area of a bevelled pipe, exposed to rain falling vertically is the same as a square cut pipe of the same size. Kind of academic anyway, since the drip pan el should deal with it in any event. But the area of the bevelled opening is larger, as compared to the same pipe cut square, isn't it?

 

[robarts]

everyone is assuming a vertical pipe exposed to the rain. i do know that relief valves for water heaters are downturned to grade or other approved locations. the bevel cut on piping for water heaters relief piping discourages connecting other piping, hoses, etc... without cutting the pipe square first (solder joint and threading end fail).

 

[11echo]

When I was working on the Alaskan pipeline, we were told that with a 45 deg bevel it was harder to plug the end with ice & snow. And now that I'm in Calif. I've heard it's harder for bird to make nest at the end! (I'm afraid that animal/bird considerations are a BIG thing in these parts, you'd be surprised how big!) ...11echo