45-degree cut on Open Pipe 10

[UKCats]

Does anybody have a scientific reference (or test data) to why the end of an open pipe is often cut at a 45 degree angle. My recollection is a straight cut could build up harmomics (standing waves) and reduce capacity. If you notice, organ pipes, and many automobile exhaust pipes are cut at an angle.

You don't see it on smoke stacks, etc so I think it's a phenomia that occurs at high velocities.

 

[metengr]

UKCats;
Yes, I would agree with your opinion. The Boiler Safety Relief discharge lines at each of our Power Plants are prepped with a bevel, in lieu of a straight cut, that vent to atmosphere.

 

[rconner]

I don't know the answer to this but also wonder if the intent of this is as expressed/and sort of like a relatively cheap to contruct (and more quickly dissipating than a square end?) "flare"?

 

[JJPellin]

There is a bit of a contradiction in your theory. If you cut at an angle to avoid a standing wave accoustic resonance, then you would definatly not want to do this with an organ pipe. An organ pipe only serves its purpose if it does generate a standing wave resonance. I don't know the answer to your question but will be very interested to see what everyone else comes up with. I suspect that the angle cut on some auto exhaust is purely cosmetic.

 

[pennpiper]

I have no proof, documentation or "scientific reference" for this, but I will share with you what I was told some 45 years ago when I started my piping designer training.
I was told that the 45 degree angle cut on the outlet end of a relief valve discharge (to atmosphere) was to dissipate the sound (db) level over a wider area.

 

[Armen75]

By cutting the pipe at an angle, you are effectively increasing the discharge surface of the pipe (same effect as putting a larger bit of pipe - but cheaper) and therefore reducing the venting velocity. This will have a positive impact on sound level, but the other advantage that I have heard is in reducing piping stress put onto it during the inital discharge.

 

[Frank1344]

Just to add that in case of a PSV discharge, the reaction force will be resolved into X and Y direction as .707*F which is lesser force and as Armen mentioned will be less stress. But you have to be careful about the force in X direction.

 

[UKCats]

I'm not convinced the 45-degree cut will reduce stress. It should just be a function of the x and y length of the pipe, shouldn't it???

 

[11echo]

...Hmmm Interesting, first time I've heard the stress issue. I was told, when working up on the North Slope, Alaska, that this helps eliminates snow and ice blockage on discharge of Press. Safety Valves. ...And later I heard it helps eliminate birds from nesting on/in the discharge opening. Actually I've seen it both ways in the field ...45 deg. & 90 deg. When I detail it out I call out the 45 deg. bevel. My $0.02! ...Mark

 

[Frank1344]

OK,

There are several types of stress in a piping system, the one which deals with the length and contents is Sustained stress or mainly weight of the system.
There is Thermal Stress which is due to the temperature of the fluid.
There is Occasional which is due to Wind, Earthquake and PSV discharge , etc. and they occur in a very very short time.

The 45 degree cut will not change the Sustained and Thermal stresses but will be a help to reduce the Occasional one.

I hope this helps.

 

[zdas04]

I'm not buying the "bigger hole, lower velocity" theory. Have you ever watched a sonic vent out a beveled pipe (either through a PSV or just a blowdown)? The visible flow is exactly in line with the axis of the pipe. For the larger hole to be a factor the flow would have to turn to be normal to the bevel.

I'm starting to think that the bevel is there because no one knew why the first bevel was put on a pipe and were afraid to stop doing it.

Engineers suffer from fear and superstition at least as much as anyone else.

I think that we're all just justifying something that we can't explain.

David

 

[Zapster]

My vote:
For a vertical discharge, to make it harder for a bird to build a nest across it.
For a horizontal discharge, to keep the rain out.

 

[johnp]

1 - My take is that the 45 deg cut has little to do with vibrations. A woodwind instrument such as the clarinet, flute or recorder, uses holes in the side of the tube to dissipate vibration lower down and generate higher frequencies in the upper section only. So the tone comes from the air column above the hole. A 45 cut would have no effect on vibrations. The tone would be set by the shorter side of the pipe.

The bell at the end of a woodwind, or brass instrument is an attempt to focus the sound in a given direction.

2 - I feel the issue is much more pragmatic. The cut prevents a concentrated drip eroding soil or concrete. Any outflow is dispersed over a larger area, and thus reduces the effect on the target area. This is particularly so if there is any chemical component, or even distilled water, in the flow.

In more serious cases this is done by providing a sparge at the end of the discharge pipe.

 

[nitrobeam]

In regards to pressure relief piping, I feel the 45 degree angle at the discharge is an attempt to keep (or at least make it more difficult) an ignorant person from plugging, capping, or otherwise restricting the flow of the released fluid.

 

[NozzleTwister]

What I've been taught is that the bevel is to deflect the release and noise away from the structure or operating area. In the old days it was common to see short tail pipes at the edge of a structure at mid-level. Now most vent pipes exit at a minimum of 7 or 8 feet above the highest nearby access level.

The bevel does create a horizontal component (on a vertical pipe) that serves to increase the bending moment on the tail pipe dramatically, increasing stresses and complicating the support/guide scheme. There is a good discussion of this in ASME B31.1, Appendix II.

The bevel does not increase the capacity and has outlived its usefulness. A bird nest is not going to slow down a release, though the bird may get the ride of his life.

As a Pipe Stress Engineer who is responsible for supporting and restraining these vent pipes, I always request the ends to be square cut at 90 degrees unless the client has dictated otherwise.

My 2¢

NozzleTwister
Houston, Texas

 

[UKCats]

Thank you Nozzle Twister for your reference to B31.1.

Looking around, some cars have tail pipes beveled and some don't. That would support your statement that there's no increase/decrease in capacity. Beveled underneather supports your noise theory and re-directing flow (i.e. down).

I don't understand why the bevel complicates the stress. I thought with most discharge piping with a 90-degree elbow turned up, there is always a horizontal force on the nozzle (e.g. the x and y force components of the resulting force), so why does this little extra x-direction force component casue so much problem. Perhaps the havic is with restraining the pipe and not so much the stress on the nozzle itself.

Can you provide more insight. The outcome is whether or not it's proposed for inclusing in API 520.

 

[NozzleTwister]

UKCats,

When the PSV Tail Pipe is 'Square Cut' the thrust force is opposite the flow in-line with the axis of the tail pipe. I perfer, if I can, to support directly under the elbow to absorb the force. The reaction can be 5000 lbf or more for high pressure reliefs and API 520 gives equations to calculate this as well as most PSV catalogs.

If the tail pipe is beveled on the outlet then the reaction force AT THE END OF TAIL PIPE is NORMAL to the bevel as illustrated in B31.1, Appendix II.

For a 45 deg. bevel the forces components would be the following for a 5000 lb reaction:
.707 x 500 = 3535 lbsf horizontal and 3535 lbsf vertical down.

For 7 foot long unguided tail pipe the horizontal force will create approximately 24745 ft-lbs of moment at the bottom of the tail pipe and even more at the PSV inlet connection. Depending on your pipe sizes and wall thickness, this may create an over stress situation.

It can be corrected by square cutting the tail pipe or adding steel to guide the tail pipe near the top.

As far as the forces and flows involved, car exhuasts are really not a good comparison.

NozzleTwister
Houston, Texas

 

[KernOily]

I've been told by more than one person that it's just sacred tradition and there's no science behind it at all.

One good thing about it - one can tell on a quick glance that an open pipe cut on a 45 bevel is likely a PSV discharge stack...

Thanks!
Pete

 

[Zapster]

NozzleTwister,

I looked briefly at Appendix II in B31.1 and do not agree with your assumption. First I am assuming the flow at the tip of the tailpipe is subsonic. The calculation example I saw in the appendix shows a cut of 60 deg with an assumed deflection of fluid being 30 degree. The example in B31.1 appendix II is highly conservative, if not just wrong, with the 30 degree deflection assumption. Your assumption that the force is transferred normal to the cut is incorrect, provided the flow is subsonic in the tail pipe tip, I would somewhat agree with zdas04 that the flow is very close to the axis of the tail pipe. The turning force on the flow would be the friction loss on the high side of the cut segment (note not the length of the tail pipe, just the cut tip). For a vertical tail pipe the pressure force in the horizontal plane would be from the friction loss on the side of the cut tip. I don’t believe these will add up to the flow turning normal let alone turning it enough to be visible. Take a garden hose and cut the hose at 45 deg angle and turn on the faucet, you will see the fallacy of the normal flow assumption for a cut tailpipe.

 

[katmar]

My vote goes with the "not too much science" votes.

The theory of it helping to spread drips is wrong. The drips still all come from one spot.

It may help discourage birds from nesting when the discharge is vertical, but it definitely does not help in this regard with horizontal discharges. I've seen a few unfortunate birds lose their nests this way.

The most useful function of the 45 degree bevel is that it shows that the pipe was meant to be this way, and it is not just an unfinished piece of pipeline dangling at the edge of the plant where nobody bothered to continue with it. It will also discourage some well meaning engineer from later continuing the line to a "safer location" and inadvertently increasing the back pressure on the safety valve.

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