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Force at Pipe Bends - Pipe Support
2

Force at Pipe Bends - Pipe Support

Force at Pipe Bends - Pipe Support

(OP)
Let me start by apologizing for my lack of knowledge in this area. I am not designing the lateral resisting system for the pipe in this case only the vertical forces. However my interests have been sparked. My questions are probably pretty academic.

A few questions:
Lets consider a 16" diameter pipe with a pressure of 40psi and the forces at a 90 degree bend.
First if you have the pressure and diameter and you neglect losses, you can determine the velocity and flow rate. Using V=sqrt(2*g*p/unitweight)
Next you apply a free body to the pipe bend a solve for the reactions.
I've attached a calculation that shows this and I'm wondering if this is correct. Seems like the forces are quite large (although 40 psi is pretty high I suppose). Maybe this is due to neglecting losses?

http://files.engineering.com/getfile.aspx?folder=f...

Any feedback is greatly appreciated. And I realize this is basically straight out of the text book but I'm wondering how this get applied in practice? Is it similiar, but need to account for losses and change in pipe size?

Thanks!

EIT
www.HowToEngineer.com

RE: Force at Pipe Bends - Pipe Support

(OP)
@desertfox - Thanks this was helpful and it appears to be the same as I had derived.

So I guess my questions are: in practice, do you guys normal derive the velocity using the pump output values then accounting for reductions? If you were to assume a velocity similar to what I had assumed in my calculation, this would be pretty conservative for finding forces at bends I suppose?

Thanks again!

EIT
www.HowToEngineer.com

RE: Force at Pipe Bends - Pipe Support

(OP)
I guess my other question is that -
If you have a pressurized system you always have a change in momentum force and a pressure force. At first this seems like you're accounting for the pressure twice. Meaning that the velocity is is affected by the pressure and the force at the bend is affected by velocity, but then you also add in the pressure force again to account for the force of the pressure...

This was probably explained my fluid mechanics class at some point..

EIT
www.HowToEngineer.com

RE: Force at Pipe Bends - Pipe Support

The Bernoulli equation is used for fluid flow. The equation demonstrates conservation of energy in fluid flow where no work is being done.

http://www.engineersedge.com/fluid_flow/simplified...

Are you asking if there are forces at pipe bends that require some type of restraint?

The force from the moving fluid is constrained by the piping so there is no outward force at a pipe bend except for the weight of the pipe and fluid.

If you have a nozzle on an open end of the pipe, then you would be have to account for the fluid force.

In the case of unrestrained pipe such as bell and spigot piping, there is force at the pipe bend. This is usually dealt with by restraining the pipe adjacent to the pipe bend.

RE: Force at Pipe Bends - Pipe Support

I hadn't opened up your file before now and I hadn't realised that what you appear to be doing is calculating the forces from a slug of liquid - at what appears to be a very high velocity - ~ 23m/sec(!). I am unsurprised that you will get a large momentum force. However, unless your bend then exist to atmosphere, I don't really think that pressure has anything to do with experience of an external force. The force from pressure is essentially held within the pipe and creates axial stress in the pipe. Unless you don't want this pipe to move in the axial plane then it won't have any real impact. also you can't ignore the Poissons effect if the pipe is anchored somewhere else close to this elbow or the temperature effects.

without seeing the actual orientaton of what you're trying to do it's difficult to see whether some forces would act together or cancel each other out. If you have a vertical leg which exits to atmosphere, fair enough you have a force from the pressure, but if the vertical leg than truns horizontal again, that pressure force is nearly cancelled out by the upwards one on the next elbow. Results in axial stress in the pipe, but not the support.

What about weight of pipe and contents?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Force at Pipe Bends - Pipe Support

(OP)
Ahhh... I see now, thank you. I was interested in the forces on an external restraint -

The pressure is going to cancel out and is really a force on the pipe. So if you had a welded on or bolted elbow then there would be a force at this joint. However the force on an external restraint would be the force from a change in moment, right?

As for gravity loads, wind, EQ, temperature - that I can account for. It is the fluid mechanics portion that had me wondering.


Thanks again!

EIT
www.HowToEngineer.com

RE: Force at Pipe Bends - Pipe Support

Sort of. If the force is big enough, the pipe will move and if you don't want it ot move then this is the force your support needs to resist.

As I said above - are you talking steady state (though 22mmsec is rather fast) or a wholly solid slug (normally doesn't exist) in what is usually an empty pipe?? In the latter case you need a whole lot of support with forces or the pipe literally jumps off your support and chaos follows.

If you give us the whole picture then we might be able to offer some advice....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Force at Pipe Bends - Pipe Support

(OP)
Sorry. My posted velocity was too high. I found out the output of the pump which was around 5200 GPM in a 36" pipe. I've re-attached the calculation and set the pressure to 0psi. The force due to the change in moment is pretty small.

Not exactly sure what you mean by solid slug. Is that like opening the value fully open rapidly (I know there is a term for this....) or closing rapidly? I am referring to steady state I believe.

The whole picture -
Various size pipes carrying fluid for waste water treatment plant. Sizes range form 16" to 36" with flow rates ranging from 2000 gpm (gallons per minute) to 5,200 gpm. The pull fluid from tanks and transport to other tanks. Really our design is EQ and vertical force related. But I was also curious about the transfer of the momentum of the fluid flow at bends.
Is there other information that would be helpful?

EIT
www.HowToEngineer.com

RE: Force at Pipe Bends - Pipe Support

Slugs are when you have an empty pipeline or one full of pressurised gas and then a lot of liquid suddenly arrives. Modelling this as a solid "slug", kind of like a shell inside a gun barrel, is rather conservative, but makes the maths easier. if the pipe is full all the time then it doesn't apply.

what you refer to in opening / closing rapidly is surge aka water hammer, which can result in sudden large forces on supports and piping and again can lift pipes off simple supports quite easily, not to mention overstress the pipe from internal pressure spikes.

Thinking up the transient events / things that happen at initial fill / refilling after drain down is key to your occasional loads. Holding down the pipework or providing restraints on movement to stop it jumping off your supports is usually a good idea...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Force at Pipe Bends - Pipe Support

The piping for water / wastewater applications is designed with low velocities so water hammer transient effects are not common.

You only need to be concerned when you are working with unrestrained pipe (bell and spigot).

RE: Force at Pipe Bends - Pipe Support

for velocities in the range of 1.6 fps and pressures of just 40 psi, like you posted, changes in momentum due to transients are largely negligible. also don't forget about expansion and contraction due to changes in temperature. However, if you wish to get into this further, Caesar II or AFT Impulse might be useful for doing your analysis

RE: Force at Pipe Bends - Pipe Support

Good example of "designing by computer program input" rather than from company guidelines, experienced company mentors, and post-college training.

True, very, very few colleges would have ever done this training, but too few get it nowdays from anyone. Then again, perhaps too few realize that they need it.

RE: Force at Pipe Bends - Pipe Support

(OP)
Whoa whoa, I derived these equations from a dusty textbook. I wish I had a program. It probably woulda told my that I didn't need to add the pressure component unless I was design an elbow connection or something along these lines. :)

Thanks for all the help and the link by rconner was helpful similar problem and thanks for the further info CVG. I do see now that the momentum forces are pretty small for the velocities I'm dealing with.

EIT
www.HowToEngineer.com

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