Negative NPSHa!?

[shimo1989]

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!

 

[BigInch]

A submersible pump is the best way out.
Floating the pump on a sort-of raft at the water surface might work.


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"Being GREEN isn't easy." Kermit

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[Artisi]

I agree, insane and unsafe.

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?

 

[BigInch]

Labour is cheaper than the pump.
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.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[TenPenny]

How much water are you talking about, and what is it for?

Is it clean fresh water?

 

[hydtools]

Consider an airlift pump.

Ted

 

[QualityTime]

swallow the pain and install the correct solution....it is bad for the nerves and creates issues if you try to engineer on the cheap.....submersible or vertical turbine?

 

[JJPellin]

Something seems to be wrong with your data. With a water level 50 feet below a standard centrifugal pump, it is impossible that the pump ever functioned at all. If it pumped even one drop of water, then you have something else than what you describe.

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

 

[BigInch]

I could imagine that the water table was once much higher than it is now and that the pump worked very well until the table dropped.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[KiwiMace]

You mount the motor at grade and the pump above the waterline, then put a long shaft with lots of intermediate bearings betwixt. Then get ready to replace the bearings and shaft seals every 3 months.

Thats what people did before submersibles made everything cheaper.

Really now, get submersibles.

 

[shimo1989]

Turns out I'm getting submersible pumps anyway. Thanks everyone for your help!

 

[BigInch]

Could have saved us some dreamtime, if you mentioned that first.

http://virtualpipeline.spaces.live.com

 

[stanier]

If you cannot afford a submersible pump then you are stuck with a rope and bucket.

 

[BigInch]

I was going to suggest in a previous post that he pull the motor to drive a bucket chain. Nice to know that great minds think alike!!!

http://virtualpipeline.spaces.live.com

 

[Artisi]

Seems common sense prevailed.

 

[BigInch]

Sometimes it happens.

http://virtualpipeline.spaces.live.com

 

[NGLENGR]

Despite the best efforts of engineers!

 

[chicopee]

If you have a lot of money, you could rig several pumps in series or you could have a vertical shaft pump with several impelers is series.

 

[waterpipe]

Well, now the problem is solved but I was going to mention to put your pump on a steel platform and suspend it with steel cables inside the well at a suitable depth.
Perhaps you can put the pump in the bucket and rope that stainer has mentioned and send it down near the water level!