I am working with a local municipality to replace their aging water tower booster pumps.  The big question is whether to replace the pumps and Cla-Val booster pump control valves, or just put all the pumps on VFD.  We're worried about water hammer and I don't see how the VFD wouldn't work just fine.  Any experience or thoughts on this would be appreciated.

I take issue with that VFD controlling water hammer strategy anyway.  True waterhammer is too fast of a phenomenom for a VFD and pump to control.  The pressure wave hits the pump before the pressure sensor even figures out it isn't working anymore.  In water, transient pressures travel at near 3000 fps. If your PT is 1/2 mile away, you've got roughly 1/2 second to adjust the pump and its going to take another 1/2 second or so for the new discharge pressure to affect flow in the line between the pump and the PT, and that's if your pump has a low rotating inertia and can change speed that fast.  By that time, the transient wave has already traveled all the way to the pump.  You might be able to do something about the second wave, but that's only about 1/2 the amplitude of the initial wave's pressure.  Any damage that was going to be done, has already been done.

In Municipal Water System pump design/selection, very little attention is placed on analyzing transients. The common design is to ramp up/down the flow slowly so as to keep from causing pressure disturbances in the system. The "Cla-Val" and other hydraulic valves allow a Municipal Engineer to do this "safely". Now that VFD's are common-place, some salesmen are suggesting that they be used to do the same thing. As "BigInch" has pointed out, this doesn't hold-up when the power fails. The big problem with the control valve is that, typically, the operator does not understand the function/need for the valve, nor do they understand how the valve works. (They don't understand how the VFD works either, buts that's the electrical guy's problem.) When VFD's are used, a non-slam check is typically used. The VFD's are used as soft-start devices only. I tend to use the control valves, but am bucking the current trend.

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