ChristyB has posted that the pump operation is being changed from a situation where the Chemical 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.
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 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.

2011-08-12

Back