45-degree cut on Open Pipe
45-degree cut on Open Pipe
(OP)
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.
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
RE: 45-degree cut on Open Pipe
RE: 45-degree cut on Open Pipe
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
RE: 45-degree cut on Open Pipe
RE: 45-degree cut on Open Pipe
RE: 45-degree cut on Open Pipe
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 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
RE: 45-degree cut on Open Pipe
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
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
RE: 45-degree cut on Open Pipe
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
RE: 45-degree cut on Open Pipe
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.
NozzleTwister
Houston, Texas
RE: 45-degree cut on Open Pipe
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
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
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
http://katmarsoftware.com
RE: 45-degree cut on Open Pipe
RE: 45-degree cut on Open Pipe
RE: 45-degree cut on Open Pipe
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
RE: 45-degree cut on Open Pipe
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
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.
Hydromech
RE: 45-degree cut on Open Pipe
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!
http://virtualpipeline.spaces.msn.com
RE: 45-degree cut on Open Pipe
Surely not sound dissipation...
RE: 45-degree cut on Open Pipe
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: 45-degree cut on Open Pipe
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: 45-degree cut on Open Pipe
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
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com
http://www.fluidpower1.us
RE: 45-degree cut on Open Pipe
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 functions...it wouldn't surprise me, then again there is little to do with fluid power that does...
Hydromech
RE: 45-degree cut on Open Pipe
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: 45-degree cut on Open Pipe
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)
"NozzleTwister,
....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."
Regards,
XHPIPE
RE: 45-degree cut on Open Pipe
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: 45-degree cut on Open Pipe
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
htt
Where's the advantage of the 45 degree pipe cut in these images?
Paul
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
You wouldn't be a UF Gator would you???
RE: 45-degree cut on Open Pipe
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.
Paul
RE: 45-degree cut on Open Pipe
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
RE: 45-degree cut on Open Pipe
What is your input on why the pipe is beveled at 45-degrees?? Is your application steam???
RE: 45-degree cut on Open Pipe
Thanks.
RE: 45-degree cut on Open Pipe
"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
1973-1991
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
1973-1979
ISBN 0-87201-759-1
RE: 45-degree cut on Open Pipe
NozzleTwister
Houston, Texas