Pump Start-Up Pipe Stress
Pump Start-Up Pipe Stress
(OP)
In pipe stress analysis is it typical to take into consideration pump start-up thrusts or is it rare that start-up thrusts cause more stress than already operating systems?
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Pump Start-Up Pipe Stress
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RE: Pump Start-Up Pipe Stress
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RE: Pump Start-Up Pipe Stress
When I was talking about a thrust I was actually thinking in the axel direction rather than torsional.
thanks for you input though
RE: Pump Start-Up Pipe Stress
I read your comments a second time and realized that you did address the axial thrust.
regards
RE: Pump Start-Up Pipe Stress
http://virtualpipeline.spaces.msn.com
RE: Pump Start-Up Pipe Stress
Now that I understand the lingo a little better. Is surge analysis common practice for determining pipe stresses? Do most pumps create a pressure pulse at start-up that can be a cause for concern or is it usually insignificant when compared to stresses on an up and operating system?
Regards
RE: Pump Start-Up Pipe Stress
The codes don't help much with the many and often confusing cases to analyze, but do attempt to deal with this apparent discrepency by setting surge pressure limits slightly above design pressures and temperature stress limits well below yield stresses. Yup, good old safety factor. Surge pressures (usually) add directly to the design pressure as the maximum operating pressure case is also the case normally taken as the base event when initiating a surge event too, so the end result of the hydraulic analysis is design pressure + surge. But, that's the end of it. Hydraulic analysis does not consider much more than pressure allowable, ignoring even the axial pressure pipe stresses (closed end stresses) created by the surge and the base case axial stresses and bending moments caused by imposed loads and temperatures.
The pipeline codes for example allow a 10% overpressure (above design pressure, or more appropriately the Maximum Allowed Operating Pressure) before other measures must be taken, such as adding a pressure relief system to limit overpressures to 10%, or increasing the wall thickness and consequently upping the the MAOP until design + surge pressures are less than the new MAOP + 10%.
Most centrifugal pumps with electric motors start fast enough to create a pressure surge of some kind. Electric motors generally have a starting torque 50% higher than the running torque. That fact shouldn't be too terribly significant for the pipe stress engineer, since the hydraulics engineer should have limited the pressure to 10% of the design pressure, or recycled the design back to the pipe stress engineer if he could not hold it within that range.
Note that there is at least one "if" in there, so its probably a good thing for the pipe stress guys to keep in the back of their minds that a 10% allowance is given for surge pressures (in the case of ASME B31.4 & .8 pipeline codes, check your code if different from those). Its also common that surge pressures are still not considered at all in some cases, even though surge analysis is required by most all design codes.
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RE: Pump Start-Up Pipe Stress
Additional piping Stress Analysis may be required using other software application like BOSS FLUID.
L S THILL
RE: Pump Start-Up Pipe Stress
Do you have any refences that you could recommend for calculating surge forces that I can apply to stress analysis? I would think the pump manufacturer should have some of the info I need.
RE: Pump Start-Up Pipe Stress
http://virtualpipeline.spaces.msn.com
RE: Pump Start-Up Pipe Stress
ASME B31.3 allows for an increase in longitudinal stress up to 1.33 times design under a certain number of conditions. One condition is that the allowance is subject to the Owner's approval. All other stresses have to be taken into account. The conditions are too long to include here.
For pipeline with rubber ring joints the surge pressure is taken into account in design of concrete thrust blocks.
The question of fluid of fluid structure interaction is complex. T Some sophisticated software can provide answers but generally it needs experience to apply the boundary conditions.
Books by Casti and Becht cover the use of this code much better than I can explain in one posting.
As for surge analysis I would recommend Fluid Transients in Pipeline Systems by Thorley.
I strongly suggest you engage some one who understands the analysis and the design for surge . It is not an area for the enthusiastic amatuer.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au