Pump Control Valve vs. VFD
Pump Control Valve vs. VFD
(OP)
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.
thanks.
thanks.





RE: Pump Control Valve vs. VFD
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Kermit http://www.youtube.com/watch?v=hpiIWMWWVco
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RE: Pump Control Valve vs. VFD
"Scientists dream about doing great things. Engineers do them." -James Michener
RE: Pump Control Valve vs. VFD
**********************
Kermit http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy"
....
http://virtualpipeline.spaces.live.com/
RE: Pump Control Valve vs. VFD
Steve
RE: Pump Control Valve vs. VFD
One argument I've heard is that the Cla-Val pump control valve allows the pump to start against zero pressure, since the valve is still closed when the pump starts. This is true, as I've seen the valve and pump do exactly that.
Shouldn't the VFD be able to start the pump against the pressure of the main header?
Even a soft start can be ramped up and down (up to 4 minutes). I would think this would be gradual enough.
A soft start with a non-slam check valve seems like it should do the job just fine and, be much cheaper than a Cla-Val.
Thoughts?
RE: Pump Control Valve vs. VFD
A non-slam check won't protect the pump against water hammer if closes too slowly, or if it breaks. If it works, properly, it can protect the pump, but then it wouldn't do anything to lower the transient pressure that it itself creates in the pipe when it closes. A fast-acting check can cause a water hammer more easily than a slowly operated ball valve. Flow through a Cla is inversely modulated by its differential pressure, hence can control transient pressure changes when it is properly adjusted.
Pumping to a tower, I would think you'd not have so much of a problem starting (unless you're overpowered) and you'd be more likely to get a water hammer when the pump trips off and the flow in the line is subjected to a quick reverse by the action of gravity.
**********************
Kermit http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy"
....
http://virtualpipeline.spaces.live.com/
RE: Pump Control Valve vs. VFD
When the pump starts, even if it is slowly ramped up by a vfd or soft sart, there will be a point where the pressure will exceed the main header pressure and the check valve will open. At that time, the pump will be trying to suddenly push a given amount of water into the main. According to my pump curve, it will try to push 2500 gpm into the main at this pressure. I'm thinking this would be a water hammer issue.
Likewise, 2500 gpm will be suddenly not allowed into the main when the pressure drops below the main header pressure and the check valve closes. Another water hammer issue.
Hence the need for the Cla-Val, which gradually increases the flow into the header. The VFD is not the answer in the situation.
Thanks all for your comments, they really helped me think this through.
RE: Pump Control Valve vs. VFD
**********************
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy" ..Kermit the
http://virtualpipeline.spaces.live.com/
RE: Pump Control Valve vs. VFD
Second, VFDs will generally work fine. They don't have to be fast, they are cheaper than flow control valves, easier to install, and easier to maintain. You also get the advantage of running the pump at it's most efficient point unlike the valves. Soft starters are generally even a better deal but they can't vary the flow like a VFD.
Third, VFDs, soft starters, and flow control valves cannot help reduce tranisents during power outage. All depend on the pump to keep the water moving so the device can safely slow down the water.
You need to know how bad the pressure spikes will be if the pump sees an unprotected power failure. The resulting pressures might not damage anything or justify a surge anticipator.
Fourth, watch out for surge anticipator valves. They can cause low pressure spikes to get worse, which is often the major consequnce in a net worked distribution system. Customers will complain about no or low pressure water. The high pressure spike often never happens in a gridded system. Use a standard pressure relief valve instead if anything is necessary.
My recommendation is softstarter with quick closing check valve, but it is your responsbility to determine what's best in your situation.
Also think about how often power failures will happen.
Dan Barr, PE
Burgess and Niple