I am doing stress analysis for 30 inch water piping it is used in chiller to cooling tower and it is basically building piping.
My client required to perform stress analysis with thrust force.
Pipe material is A 53 Gr.B, design Temp is 40 degree Celsius, ambient Temp is 55 degree Celsius and design pressure is 150 psi

I have calculated thrust force using formula Force=P×A
It is arround 471.637 KN I think this is huge force..

Can I apply the above force on each elbow point. is it correct way. Or any other technical issues are there for this problem.

Here how to consider thrust force in my stress analysis.

Kindly advice me how to ensure my system is safe with thrust force.

By

Rajiv

Dear Expert,

kindly reply me how to consider thrust force in my stress analysis.

by

rajiv

Dear Sir,

The longitudinal thrust force you are asking about is automatically considered in CAESAR II and requires no manual input. About the only time you would want to manually input the pressure thrust is when you are using an unrestrained expansion joint and even this case, CAESAR II will include the appropriate thrust force given the bellows ID. However, CAESAR II applies the thrust at the bellows center line rather than at the elbows where the force is actually developed. So, I'm not sure why you want to manually apply the thrust force, please let me know.

Thanks for your Reply,

I already checked some reference book related to thrust force, we were not able to find exact solution,

if you know tell us Where it is mentioned the thrust force shall be included automatically in CAESAR II,

I think big size pipe fluid change in direction at elbow points thrust force will come huge arround 471.637 Kn, so that I decide to include the thrust force also,

Kindly give me additional information about this problem.

By

Rajiv

What do you mean about the thrust force? Is it external pressure? Or are you using Expansion joint?

CAESAR II will apply pressure thrust load on either end of untied joint...

If not, we already do calculate stress from the internal pressure.

Check ASME code, I'm refering B31.1(if you use B31.3, please check 2012 edition)
In sustain stress Sl=(P*D0/4*tn) + (0.75i*Ma/Z) <= 1*Sh
First term is stress due to pressure, and latter is stress due to moments. But compare to the stress due to moments, stress due to pressure is quite small.

And at the components that change direction like bend, tee, etc. we use SIF to compensate it. But let's not get into that...

In addition, we only use longitudinal pressure stress, even it's only half of the hoop stress. Minimum thickness already take care of hoop stresses.

In a steady state flow through a regular piping system having only elbows, fluid exerts forces on elbows and we can split them in "pressure forces" and "dynamic forces".

Under normal circumstances the dynamic components are negligible and the forces acting in elbows are rather pressure forces, these forces are balanced and give the pressure longitudinal stress (commonly considered as PD/(4t)).
If the dynamic components cannot be neglected, Fluid Mechanics (by "Momentum equation") gives us the expression for forces acting in 90 degree elbows. I understand you calculated the dynamic forces and got the result of 472 kN. However, not the magnitude of force is important but the consequences (longitudinal stress) in pipe.

At a first approach, we can consider that the fluid has the same pressure and same velocity for all elbows, so all forces are balanced in such system, giving tension in pipes.
At a second analysis(more detailed and useless for practical purpose), one would observe that these forces are not identical because it is a loss of pressure in system and some changes in velocity, but in this case the "momentum equation" must be completed with supplementary friction and/or inertial forces, so again the forces appear as balanced. So the forces are balanced anyway, because the physical reality; it is also true that, for compressible fluids, it would be difficult to accurately calculate "by hand" the magnitude of forces.

As a conclusion: read again the post of Dan Edgar and consider his valuable advice...

You may like to read

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=40342

Best regards,
M

Check below link to calculate thrust force. Apply same into your pipeline.

Rajiv, I think you have confused the term "Slug force" with "Thrust force". In stress analysis terminology Thrust force is explicitly reserved for unbalanced force in unrestrained piping (A prime example for thrust is "Untied pressure bellows")

For the slug force what you are talking, you would need a different analysis using other software. Refer attached paper for some insight on the subject

Cheers,

Hi Rajiv,

You can also ask if there are sources of potential surge in your system which is one of the cause of unsteady flow in your system leading to force imbalance in elbow-elbow pairs. Common sources are sudden valve opening or closing, pump start-up or shutdown, etc. You can talk to your Process engineer and check your client specs if there are requirements for surge analysis. 472 KN is very high, usually if you use Joukoswky equation for sudden valve closure it would result in very conservative figure. Nowadays, there are many software packages dealing with surge which will give a more realistic result. Usual solution is to slow down the valve closure or opening to minimize the pressure spikes in your system. You can check the previous webinar presented by Caesar II together with the use of fluid software.

Any other opinion is greatly appreciated.

Cheers!!!