The pump operation is being changed
ChristyB has posted that the pump operation is being changed from a situation where the pump is running constantly to possibly an on/off scenario. It should be obvious that the surge pressures are going to be more of a problem with the new operating scenario.The much higher short term pressure rating of PVC pipe allows one to take advantage of plastic pipe's inherent property of being able to handle short term pressure surges. Non-plastic pressure pipes display insignificant difference betweent short term and long term design strength. Having said that, it is prudent to design an installation to minimize surge pressures.
I relise the change but presumably the pump has been turned on and off over its life to date and nothing as blown up. The frequency of surge pressure increase with the new scanario and PVC pipe is susceptable to fatigue failure so there may be a long term problem. Typical de-rating factors are: (UK Water research Council). Fatigue re-rating factors Hourly Total factor frequency cycles 50 years 0.2 73,000 0.7 1.0 438,000 1.3 2.0 876,000 1.5 5.0 2,190,000 2.0 10. 4,380,000 2.5 50. 22,000,000 4.0two starts per hour requires a derating of 1.5.For a pipe velocity of 0.7m/sec and a wave speed of about 360 m/sec the surge pressure by Joukowasky equation will be about 2.5 bar. If the pipe class is 12 bar and there are 2 starts per hour then, unless I was to make a more rigorous calculation, I would design for an allowable long term steady state pressure of 6.3 bar. (12-2.5)/1.5
first of all, transient analysis shall be done independently of the pump operating mode in the design stage, considering worst case scenario (stop can occur due to power failure, for example - don't expect that VFD will help you there). For this system, you shall go with start/stop mode to save water and energy, or design the VFD to keep constant level, or... consider also functioning of water disinfection system or other plant components. there are also many ways to relieve water hammer. VFD, Soft starters, PR and isolating valves are not GLOBE VALVES of much help, theoretically speaking. Some pressure relief valves, soft closing check valves, vacuum breaker valves may help. Routing of pipeline is crucial in water hammer calcs (vacuum can be more dangerous that overpressure for PVC pipes), many other factors shall be considered.However, a 0.5 m3 hydrobox i.e. surge tank (serially made, shall not be expensive) will clear any doubts you may have regarding transients, given that the system already works.
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