Centrifugal Pump Suction and Cavitation Problem

If the reservoir top level is 12m below the suction of a pump, it will not matter if there is a foot valve on the end of the pipe, for the pump cannot prime on a suction lift of that height, no matter what type of pump it is.

One solution would be to use a submersible pump.

You give us absolutely no information on the type, size, head and capacity, of your pump/system and the liquid pumped assuming it isn't water. That info is almost always needed to answer a pump question in order to get a useful reply.

All we know at this time is that your vertical lift is more than can be supported by atmospheric pressure, as has already been pointed out. Assuming the pumping elements have to stay where they are you are going to have to figure out some other way to add energy to the liquid to give it enough NPSHA to meet the pump requirement.

Certainly a submersible pump, vertical turbine pump or a vertical sump pump might be a better choice. But that is likely a more expensive solution than modifying the system to use the existing pump.

The simplest way to do this may be with an eductor variant of an ejector pump. Also known as a jet pump. These are frequently used for small domestic well water systems where well levels can drop below 4-5 meters below the pump inlet. In the USA when you buy a "Jet pump" you usually get a conventional horizontal centrifugal pump with a suction chamber and two pipe connections on the discharge close coupled to a fractional size electric motor. Included would be an eductor and a foot valve to place at the inlet down in the well casing. The installation would require two pipes. One from the discharge to carry part of the flow back down the well to the eductor below. The other larger pipe would carry the larger volume of water back up to the suction chamber inlet.

Note another type of pump, a propulsion system for watercraft, is also referred to as a Jet Pump". This is a source of confusion that you will have to filter out as these devices for boat propulsion are becoming increasingly common.

This general approach can be applied to larger pumps. It's a cheap answer to an immediate problem; but take note the eductor process added to the pump system will decrease efficiency. This is not a big deal on a one or two KW pump. It is on a 25 or 50kw pump, especially one running on a high duty cycle.

Anyhow, I've given you a couple of good search phrases: "eductor" and "ejector pump". Follow them on your favorite search source and you'll find out more about these things. A good written reference source is Karassik, et. al., "Pump Handbook", Mcgraw Hill. Chapter 4, "Jet Pumps" written by Alex M. Jumpeter.

Some links that may be interesting:

http://www.animatedsoftware.com/pumpglos/jet_pump.htm

http://www.eductor.net/

http://en.wikipedia.org/wiki/Eductor-jet_pump

http://www.popularmechanics.com/home_journal/how_your_house_works/1275136.html?page=3

http://www.goulds.com/GP-Residential-Water.asp

Hope some of this helps.....

Ed Weldon