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45-degree cut on Open Pipe

45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe

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"?

RE: 45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe

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.  

RE: 45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe

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???

RE: 45-degree cut on Open Pipe

...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

RE: 45-degree cut on Open Pipe


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.

RE: 45-degree cut on Open Pipe

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.


RE: 45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe

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¢

Houston, Texas

RE: 45-degree cut on Open Pipe

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.

RE: 45-degree cut on Open Pipe


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.

Houston, Texas

RE: 45-degree cut on Open Pipe

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...


RE: 45-degree cut on Open Pipe


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.

RE: 45-degree cut on Open Pipe

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.

Katmar Software
Engineering & Risk Analysis Software

RE: 45-degree cut on Open Pipe

from my experience in HVAC packages design...i used the 45 cut pipe inside a storage tank ..where it goes to the pump (as a suction line).. i believe this procesure has to do with the Flow ... not the pipes stress .. i mean in the suction process...the 45 cut pipe has a larger suction area..and at the same time the 45 cut will reduce the sudden change in the suction area ( as in straight cut pipe)which mean reduce the turblency and air in the flow.

RE: 45-degree cut on Open Pipe

Guess I can add an un-educated 2 cents worth. As a pipefitter we were told the 45 degree cut was simply to minimize the whistling when discharging. As a designer, I've never asked why, and have never been told not too.

RE: 45-degree cut on Open Pipe

Good dialog...  For those who use API 520, you will notice there's a straight cut on the discharge pipe diagram.  As a member of API 520, I have been asked to come up with either scientific or experimental evidence for why we should change this diagram.  I have solicited the assistance of some university professors for the scientific evidence.

From what I'm reading, B31.1, Non-Manditory Appendix II does show that a cut will change the direction of the loads on the discharge piping and inlet nozzle.  I think there's other benefits.

I have also heard this is called a whistle cut.  I'm not sure if this reduces noise or just re-directs it.

Keep up the comments.  

RE: 45-degree cut on Open Pipe

Chamfers on arguably "open pipe ends" are of course also used frequently for various inlet and outlet pipe works (e.g. drainage, tank influenet or effluent purposes, etc.) generally with angle that matches the slope of the bank or sloping tank bottom/wall etc.(and it appears for arguable other purposes in such applications),"45 degree" and otherwise, and in these application sometimes fitted with grates etc.  

RE: 45-degree cut on Open Pipe

For what it's worth, I have not used the "whistle cut" on PSV discharge piping on any system in over 10 years (specializing in high pressure steam). There are a few reasons;
1) verticle whistle cuts due to increased cross section allow more rain.
2) placing whistle cuts on vertical vents that may face to the predominate windward side of facility allow increased back pressure of that vent.
3) no difference observed of efficiency of vent with either style as long as there is no back pressure.
4) As for stresses, correct routing of pipe and the supporting of same reduce 95% of the stress variables.

RE: 45-degree cut on Open Pipe

Interesting reading...

For my part, I have, over the years, worked on many hydraulic power units of all shapes and sizes. Designing, assembling, commissioning testing and servicing.

The one common feature across all of them, good ones and bad ones is the profile on the end of the tubes inside the tank.

Main return lines, pump suction lines, high velocity relief valves return lines...all had been cut at 45 degrees.

Why..? Because that's the way it is.

It must be worth while because pipes cut at 45 degrees are a pig to de-burr.


RE: 45-degree cut on Open Pipe

Pipefitter wins.

Its for sound dissipation.  Nothing to do with force components.

As zdas says, ever seen the hot gases leave an exhaust.  Of course most of us have.  They go straight out, hence reaction is straight in.

Sound transmission on the other hand is a minute vibration, with no net mass-momentum vector components remaining at the end of the cycle, as at the end of the cycle velocity is 0.  Its free to go in any direction it wants to at the beginning of the next pulse.  As soon as a pulse encounters a discontinuity, it can begin to change direction.

Prove it to yourself with a candle at the end of a cardboard (square cut) tube with rubber from a balloon stretched tight and glued to the other end.  Drum on the balloon with the tube end at various distances from the flame and with the tube axis at various lateral offsets from the flame and observe the flame's response to the drum beats.  Remember, or take videos.

Now cut a 45º bevel and do the drumming bit again.  When the bevel is facing away from the flame, there is almost no response by the flame to the drum pulses.  When it is directed towards the flame, there is a lesser response than with the square cut bevel.

Its the sound pressure, not the forces, not the birds, not the....


         Going the Big Inch!

RE: 45-degree cut on Open Pipe

So why do it to tubes submeged in oil inside a reservoir that usually has a pump running at 80+ DBa.

Surely not sound dissipation...

RE: 45-degree cut on Open Pipe

You must agree that my explanation of the exhaust gas bevel is valid before I answer your new question, but I will give you this tip,

Pumping liquid from a tank is a little different.  The momentum vector of fluid is about 1000 X more important in relation to that of a gas.

   Going the Big Inch! worm

RE: 45-degree cut on Open Pipe

The reason I heard for cutting Suction and Return lines in a Hydraulic Tank at 45 Deg. is to eliminate the possibility of blocking flow into or out of the pipes in case they are insatlled too deep or if they can come loose and drop to the bottom of the tank.

As anyone can see this would be catastrophic for any pumps Suction Line for sure. Could be almost as bad for return lines especially from the Relief Valve return.

Another thing I was told in my early days is that the Return Lines 45 Deg. cut, should always be aimed at the tank walls to attempt to get the most contact with the tank walls since the return oil is often at an elevated temperature. This alows for better heat transfer through the walls since more surface is involved.

It sounded logical to me and I thought it worthwhile to put it in my Basic Fluid Power training book.

Some of this might apply to other systems as well.

Bud Trinkel CFPE
fluidpower1 @

RE: 45-degree cut on Open Pipe

BigInch...I am in no way questioning your excellent description of the function of the bevel on the end of a gas exhaust pipe, indeed I have no reason not to agree with it.

Bud...I believe that is what you were told, but I don't buy it, but then again thinking about it you might be right. I've been in this game to long to discount anything.

All the power units I made had bevelled pipes in the reservoir and they were all cut to a length that kept the end of tube under the minimum oil level but well clear of the bottom of the tank.

Putting a bevel on the end of a suction tube that is the wrong length will only delay the death of a pump as it hoovers all the crap off the bottom of the reservoir.

I don't think that the bevel is a suitable insurance policy in cases of incorrect measurement.

I like to think that there is a more pragmatic reason for the shaping of the end of the tube. More to do with fluid velocity and pump priming ability than measuring skills...

Perhaps the bevel shape is a magic bullet that has many wouldn't surprise me, then again there is little to do with fluid power that does...


RE: 45-degree cut on Open Pipe

Right.  I have no personal knowledge about the noise reducing bevel, other than the candle experiment and my transient analysis experience with reflected pressure waves moving in long pipelines.  Reflected pressure wave amplitudes are dependent on end area(s).  At a pipe junction with different diameters, reflected pressure waves are distributed by area ratio.  It all makes great sense. My interest in transient analysis warrented doing some additional research and I have an obscure reference to this phenomenom from a engineering company in Holland that specializes in noise abatement of all kinds and I also inquired at a custom muffler manufacturer in the US who confirmed that it was for noise abatement purposes.

As for the hydraulic tank.  The amount of head loss when fluid leaves a tank and enters a pipe are very dependent on the geometric profile of the hole through the tank wall and exactly how the hole intersects the inside tank wall.  A flush square cut is the worst.  Just about any other shape is better.  There are no references I know of giving head loss coefficients for a bevel cut on a protruding pipe, however any increase in area at the immediate point where fluid acceleration is occuring would make a heck of a difference if it was done as slow and as smoothly as possible.  It would also tend to draw from the bevel direction which only IMO, confirms what you say about drawing cooler fluid from the bottom of the tank, while keeping away from the crud.  With slower velocities in that area, the fluid stream won't lift much of it, so seems like a great idea to me.  Likewise, I doubt that anybody can logically refute it.

   Going the Big Inch! worm

RE: 45-degree cut on Open Pipe

Well all in all this thread shows just how confusing this subject is.

Some food for thought:
B31.1 is discussing a steam relief where the  fluid (steam) is expansive as it changes state.  I would expect its behavior as it comes out the end of either a straight or beveled end pipe to vary significantly fro water... (Re:Zapster (Electrical)      

....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.")

Sound attenuation may be the reason lost in the sands of time I wish that...  UKCATS would share with us the benefit of his research... "Good dialog...  For those who use API 520, you will notice there's a straight cut on the discharge pipe diagram.  As a member of API 520, I have been asked to come up with either scientific or experimental evidence for why we should change this diagram.  I have solicited the assistance of some university professors for the scientific evidence."


RE: 45-degree cut on Open Pipe

On small reservoirs, on the upstream side of a projecting pipe outlet you cut a 45 deg with bevel going up and then weld a plate horizontal extending out to the reservoir.  This helps get full pipe flow quicker on steep slopes. ( from NRCS Tech Release 3 based on Oregon State College research) I have also seen it work somewhat with out the plate.  This may be true for some oil applications.

Otherwise for pipes, when cutting small pipes with a saw I am less likely to bend it shut.  Also with the bevel down I think there is something structural. If the bevel is down any force on the pipe from above has to bend more pipe to collapse the pipe than for a straight cut.  It is easier to install a trash rack without your bars decreasing inlet area and it looks better if it matches the slope.

RE: 45-degree cut on Open Pipe

Here's a sketch clarifying one example of what we're discussing:

Where's the advantage of the 45 degree pipe cut in these images?


PS I can probably provide graphics uploads at my site (for this forum only) if people need a picture to accompany less than 1000 words. Legible, hand-drawn sketches only though.

RE: 45-degree cut on Open Pipe

Gator... don't see any previous threads from you but from your sketch, I believe you're supporting the idea that an angle cut changes the forces and moments on the PSV nozzle???

You wouldn't be a UF Gator would you???

RE: 45-degree cut on Open Pipe

Hi UKCats.

Actually, I'm suggesting the opposite.

The eng-tips "Gator" handle is from 1999 when I first joined youse guys and was interested in sewergators. I just haven't posted a lot since then.


RE: 45-degree cut on Open Pipe

Ah...  Sounds like you had a job in the south.

Back to my first question, are you suggesting the 45 degree cut is to split the PSV discharge force into two smaller vectors?

RE: 45-degree cut on Open Pipe

No. Am I wrong?

RE: 45-degree cut on Open Pipe

ASME B31.1, Appendix ??? shows the benefit of cutting the end of a pipe to change the force vector and reduce bending moments on the boiler nozzle.  I notice a bending moment symbol on your sketch, so I put 2 and 2 together...

What is your input on why the pipe is beveled at 45-degrees??  Is your application steam???

RE: 45-degree cut on Open Pipe

OK, gotcha - II-


RE: 45-degree cut on Open Pipe

I knew I had seen comments on this somewhere and I just tracked it down (mind you, these were written by piping designers, not stress engineers):

"Pipe exhausting to atmosphere is cut square, not at a slant as formerly done, as no real advantage is gained for the cost involved."

Piping Guide
PP 94
David R. Sherwood
ISBN 0-914-08219-1


"There is a very old myth among piping designers that calls for a 45 degree bevel on  relief valve tail pipes discharging to the atmosphere. This myth has been handed down from generation to generation and in almost every plant today the beveled tail pipe is seen. The idea proposed by the myth is that beveling will direct the outlet velocity away from a platform or building. The squared cut end shown in Figure 5-12 will direct the discharge upward, away from platforms, etc., and will do it at the minimum cost. The 45 degree bevel is more expensive to cut and results in extra waste pipe. The real falacy [sic] of this myth is seen when observing the actual installation where bevels have been cut, which, in effect, direct the velocity toward operating platforms! This is caused by improper orientation of the bevel. the square cut end design eliminated all orientation problems."

Process Piping Design Volume 1
PP 90
Rip Weaver
ISBN 0-87201-759-1

RE: 45-degree cut on Open Pipe

Thank you for your valuable post Gator.

Houston, Texas

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