Fluid force on a pipe flange

[nycstev]

I am having an dispute with a plumber. They are installing water meters in buildings and the other day one of the fittings on an 8" line separated ( it was found the set screws on the flange were not tightened). The plumbers insisted there is only 50 pounds of force needed acing on the flange since the line pressure is 50 PSI. ( they don't understand the difference between force and pressure)

Here is the question. There is a run of 8" pipe about 15' long with elbows at both ends. 50PSI of pressure and low velocity 2-3 FTPS In the middle there is a gasketed flange. What is the total force required to separate the flange? Plumber says only 50LBS.
I plugged it into calculater (PXA) and it seems when the fluid is in motion it is acing on the elbows and essentially pulling apart the flange.
I got calculate results in the 2000 lb range. is this correct?
Thanks

 

[SlideRuleEra]

I plugged it into calculator (PXA) and it seems when the fluid is in motion it is acing on the elbows and essentially pulling apart the flange.

Assuming there are no external pipe supports to resist pipe axial force, P x A is approximately correct. Fluid flow will cause additional force, but at a flow rate of 3 feet per second, force due to flow is very small and can be neglected for most purposes.
"A" is based on the inside diameter of the pipe.

Even when there is no flow, fluid pressure is pushing the elbows apart... P x A is precisely correct.

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[LittleInch]

Total is quite large as assuming it's 8" ID, you have ~2,500 lbs force acting on the elbow , so the tensile force in the middle is 2,500 lbf.

Now in reality, some or all of that force will be taken by the pipe supports and anchors and without seeing how the pipes are suspended, supported or anchored, it's difficult to know how much axial force the flange in the middle is actually seeing.

Also there may be some bending moments on the flange as well which makes some bolts see less force and others more.

It's when the pipe is under pressure that you get the force. The movement will add a little bit but at that velocity I agree with SRE, it's basically negligible.

A 15 ft / 4.5m span on an 8" water line is quite a decent span and if it is suspended between the two elbows will have a reasonable bending moment in the middle.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

It is 50 lbs per square inch of pressure. Area of interior pipe end of the 8-Inch pipe = 50 square inches.

One would not expect a plumber to be knowledgeable to design a piping system, especially a large diameter one. The plumber should only be installing the system according to the design drawings. However, installers will give you the "this is how we always do these things" comment when they want to change the installation. If the plumber is unconvinced of what you are saying, ask them to accept responsibility and sign off on the change.

Area of interior pipe end of the 8-Inch pipe = 50 square inches. Total force = 50 * 50 psi = 2,500 lbs. thrust. Agree with SlideRuleEra post that the effects of velocity can be ignored.

Buried piping utilizes unrestrained joints so that the piping accommodates the movement of the earth. The pipe thrust is resisted by design of the piping including changes in pipe direction, sliding resistance of the pipe against the soil, restrained joints at key locations, etc.

Above ground piping generally incorporates restrained joints. The joint should be restrained even if the pipe supports are capable of resisting the pressure. If some plumber disconnected a pipe support to make a piping medication in the future, one would not expect the joint to pull apart.

 

[Compositepro]

"you have ~2,500 lbs force acting on each elbow in opposite directions, so the tensile force in the middle is 5,000 lbf."

You might want to rethink that statement, LittleInch.

 

[LittleInch]

I'm thinking this is like a U shaped pipe but with sharp elbows and long down pipes.

But you are correct, that was a schoolboy error but I was a bit tired yesterday... I've corrected the initial post as well.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1gibson]

Ask the plumber how the 40 psi in his tires holds up his 2 ton truck, when they can only exert a force of 4x40=160 pounds per his reasoning.

 

[Hurmanetar]

Like others have said, the force acting to pull the flanges apart would be P x A so if the ID is 8", ~2500 lbs neglecting any supports.

I was curious about flanges with set screws so I googled that and found Zip Flanges... is that what is being used?

http://www.sigmaco.com/product/zip-flange-set-screw-adapter-flange-for-ductile-iron-and-steel-pipe/

The literature says the set screws and bolts are to be torqued to 50 ft-lbs... just thought maybe the plumber was confusing the torque setting with the pressure containment. These are rated for 200 PSIG.

 

[nycstev]

Thanks Everyone for all the response. This what i assumed all along I just wanted to be sure . Now I am sure. Great analogy with the tires Gibson I will have them measure the contact patch on the ground. Hurmanetar yes they are Zip Flanges

Steve