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.

 

[fnanan2006]

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.

 

[kjnpiper]

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.

 

[UKCats]

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.

 

[rconner]

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.

 

[precisionengineer]

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.

 

[hydromech]

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.

Hydromech

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[hydromech]

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...

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[BigInch]

UKCats, Would it not be better to commission a CFD study?

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[budt]

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
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[hydromech]

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 functions...it wouldn't surprise me, then again there is little to do with fluid power that does...

Hydromech

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[xhpipe]

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)
"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

 

[BigInch]

Do the candle experiment.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[blueoak]

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. (

http://www.info.usda.gov/CED/ftp/CED/tr03.pdf

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.

 

[Gator]

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

http://www.pipingdesign.com/eng-tips-graphics/psv_45_outlet.jpg

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.

 

[UKCats]

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???

 

[Gator]

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.

Paul

 

[UKCats]

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?