PRESSURE DESING VS WATER HAMMER

[ROMEUM2]

Dear all,

I have to design a bladder water hammer for a refinery. We have to achieve to avoid the water hammer between the buque and the storages tanks of the refinery.

I have a pressure design of 19 bar but with the water hammer we can increase the pressure until 22 bar. My question is that if it's correct define the design pressure 19 barg and include a note in order to advise to the manufacturer.

Anyone know if the asme VIII Div 1 include this situation.

Thanks in advance,

 

[davefitz]

I am not sure, but here are some thoughts.

Sect VIII div 1 rules apply if you meet the cyclic fatigue criteria and if the energy per water hammer pulse is less than that needed to be considered a primary stress. If the number of cycles exceeds perhaps 2000 over the life of the system, then you may need to use div 2 cyclic stress analysis.

If the time of each water hammer pressure pulse is long enough to indicate there is enough energy to fail any component on the system, then the max pressure of that pulse would of course be the design pressure . But if the energy of the pulse is much less than that needed to fail any part of the system, then I think a case can be made that it is a secondary stress and can be treated using cyclic fatigue analysis. This last decision is not clear in my mind, since even a short pulse imposed on a brittle , pre-existing crack can lead to failure if certain fracture mechanics criteria are met.

It is well known that many existing piping systems are exposed to water hammer and steam hammer pressures far in excess of the design pressure, yet last many years and are rarely modified to address the issue, much less de-commissioned or replaced based on this exceedance that is discovered after commissioning. Legally, there may be larger obligations if this overpressure is known during the design process. Also, when theorists try to predict what sort of catastrophic event may cause failure of a properly desing pressure vessle, the energy of the overpressure pulse( related to time of pulse times the overpressure )is one governing criteria.

"Nobody expects the Spanish Inquisition! "

 

[TGS4]

ASME Section VIII, Division 1 does not have a provision for pressures in excess of the design pressure (as ASME B31.3 does). Therefore, if the maximum pressure (instantaneous or sustained) that the vessel will experience is 22 bar, then the vessel must be designed to 22 bar.

I completely agree with davefitz that you will likely need to consider fatigue, per UG-22(e). You will also need to consider the fluid shock loads per UG-22(g).

I would add that I would make very certain what the maximum pressure might be. Water hammer pressure can be huge (but then again, you probably know that...).