+Where to model the discharge load - at valve or at 1st elbow? 1

[Hansito]

Dear Coleagues:
We have a little controversy at office reggarding to where to model the discharge load in a piping stress model.

In my opinion it should be modeled at elbows, points B and D (please see my sketches) with dir arrow as indicated. Some people say that should be points A and D.

Please tell me your point of view.

Regards,
Hansito

 

[DSB123]

Hansito,
If you are not sure where to apply the loads how do you know what loads to apply also? There are loads at the valve initially then there are loads at the bend and then there are loads at the end of the tailpipe. The loads applied are dependant upon whether yo are doing a comprehensive time-history analysis or a simple "static" analysis with an applicable dynamic load factor.

 

[Hansito]

Thanks DSB123

We are doing a simple "static" analysis with dynamic load factor 2. I'm not in the office at this moment, so I don't remember the formula exactly but is a function of Flow, molecular weight and temperature. On that conditions I think the points are B & D. I agree during the discharge time, the forces are equilibrated but not when opening or closing.

Regards,
Hansito

 

[Greg1920]

The answer is both points will experince load. Initially there will be a thrust load in the A direction when the relief valve opens until the pressure in the horizontal pipe equalizes.

B (in the direction you state) will be loaded when the fluid first hits the elbow and is turned up.

Once the valve reaches steady state there should just be a vertical force downward at the elbow as the pressure in the horizontal pipe will be balanced.

 

[BigInch]

You do not have just one force that moves around.

Initially there IS only the end-cap force at the closed valve. Anywhere you have a pressure differential across a valve, the net force dP * A is applied axially to the pipe. If the valve is closed, it creates an "end cap force" at the valve with load equal to internal pressure x flow area. When a valve is opened, the end cap load changes to the flow pressure loss (dP) across the valve x area of flow.

Anywhere you have a moving fluid mass changing direction, or escaping from a container, forces are imparted. These dynamic forces begin when the valve opens and fluid starts moving.

Whether the pipe can balance static end cap forces, or dynamic forces with internal tension, compression, or moments, in the case of welded joints, or if you need a brace, clamp or a thrust block in the case of bell/spiggot joints, must be decided.

YOU must place the dP x A forces, change of momentum and thrust forces wherever they are applicable. See attached.

Please practice free body diagrams before you try to do more pipe stress analysis.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Hansito]

Thanks BigInch.

Your answer added a very nice graphic and description to the previous valuable Thread answers. The combination Graphic-description is always very good and demands big effort, as the combination wisdom-humility.
Regards,
Hansito

 

[BigInch]

Humility. I have it. That's the name of my yacht.

Seriously, pay attention to FBDs. They help you see those unbalanced moments where you might otherwise think forces simply balance.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/