Negative NPSHa!?
Negative NPSHa!?
(OP)
Hi guys,
I'm designing a pumping system to pump out water from a well, and the water level is around 50 ft below grade. Since 50 ft. is more than the atmospheric pressure in feet of water (around 32 ft of water), I get negative NPSHa, which means the pumps suffer from cavitation. How would I go about getting a positive NPSHa?
I've considered digging a 10 m hole next to the well to mount the pump (so the NPSHa is within what is supplied by the atmospheric pressure) and to mount the pump inside the well itself, on top of the water surface and in a sealed watertight container, but they both sound insane and kind of unsafe. I also thought I could install a submersible pump, but we haven't got enough funding to afford one. What's the best route out of this mess?
Thanks in advance!
I'm designing a pumping system to pump out water from a well, and the water level is around 50 ft below grade. Since 50 ft. is more than the atmospheric pressure in feet of water (around 32 ft of water), I get negative NPSHa, which means the pumps suffer from cavitation. How would I go about getting a positive NPSHa?
I've considered digging a 10 m hole next to the well to mount the pump (so the NPSHa is within what is supplied by the atmospheric pressure) and to mount the pump inside the well itself, on top of the water surface and in a sealed watertight container, but they both sound insane and kind of unsafe. I also thought I could install a submersible pump, but we haven't got enough funding to afford one. What's the best route out of this mess?
Thanks in advance!





RE: Negative NPSHa!?
Floating the pump on a sort-of raft at the water surface might work.
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
http://www.youtube.com/watch?v=hpiIWMWWVco
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RE: Negative NPSHa!?
What flow rates are you taking about and what is the well diameter ?
If you can afford to dig, line and install a pump 10 metres below ground I fail to see how you can't afford a submersible pump.
How about a windmill?
RE: Negative NPSHa!?
A submersible would be cheaper than a windmill plus a new lift pump, no?
Digging a 10 meter hole is really kind of insane too, isn't it.
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.live.com
RE: Negative NPSHa!?
Is it clean fresh water?
RE: Negative NPSHa!?
Ted
RE: Negative NPSHa!?
RE: Negative NPSHa!?
This is a common situation with water wells. I have one at my cabin in Wisconsin. If the water level is more than about 25 feet down, you have three options. You can install a submersible pump which is very common. You can install a jet style pump which uses an eductor fitting at the bottom of the well. You pump water down the well under pressure, the eductor fitting draws in water from the well which combines with the water flow you are pumping down and the combined stream that returns to the surface is a greater volume that what you pumped down. The last option for a larger, industrial application is a vertical turbine pump (line-shaft) with the motor on top and the impellers down in the bottom.
Any of these options would tend to be less expensive and more likely to be successful than the options that you list. I can't imagine that you could dig a 30 foot deep pit cheaper than you could install a submersible pump. I have never heard of any sort of floating box option.
If you provided more information about the flow and pressure required and the size of your well casing, you might get more useful suggestions.
Johnny Pellin
RE: Negative NPSHa!?
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.live.com
RE: Negative NPSHa!?
Thats what people did before submersibles made everything cheaper.
Really now, get submersibles.
RE: Negative NPSHa!?
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RE: Negative NPSHa!?
Perhaps you can put the pump in the bucket and rope that stainer has mentioned and send it down near the water level!