How deep will a vacuum pump water from?

Answer 1

i believe the question should be stated as "How high can a pump pull liquid when mounted above the liquid source". an old pump adage is that a pump doesn't suck. sounds dumb, but it refers to the necessity of having a positive pressure at the suction of the pump greater than the required net positive pressure req'd by the pump. NPSHa must be greater than NPSHr. in any system open to atmosphere the surface of the fluid will have 14.7psi (at sea level) X 2.31 ft/psi, or roughly 34' of head, or NPSHa, available. the manufacturers performance curves will show the NPSHr of the pump at any given flow for a given impeller trim. by subtracting this NPSHr from the calculated surface pressure you can arrive at a general maximum lift that the pump can run at. there will also be line friction losses that will reduce this height, and typically we subtract another 2-3' for a fudge factor as you would not want to run on the ragged edge. so, a pump with an NPSHr of 8' would be able to lift cold water approx 22' before cavitating. getting it primed is another issue, and having said all this, there is a type of centrifugal that can successfully trick this seemingly rigid restriction on lift. the typical home commercial jet pump can lift from many times this limited depth by taking a portion of the high pressure discharge and sending it down a separate pipe and into the suction pipe. this effectively increases the suction pressure and allows this type of pump to lift from quite a depth. a really neat way around having to install a down-hole pump submersible. of course the type of pump, what you are pumping, temperature, vapor pressure, specific gravity, and viscosity will all affect the height that a pump can lift a fluid.

If the question is ' at what height we can place the suction side of pump from the water level from where it is pulled up', then if we are not considering the NPSH (which is not practical of course) , then i think that the maximum height of the suction side will be the height which will balance the pressure which is on the water level below from where it is pumped. If the pressure there is atmospheric pressure (at the water level below suction side)then maximum height of water rising is 10.3 m around(will vary according to the fluid being pumped). Above it water will not rise whatever vacuum you create though pump.(Its just like in barometer where Mercury doesn't rise above 760mm, although there is vacuum above it in the tube. This is because at base of inverted tube of barometer, pressure is balanced). In practical of course the height is much less of course otherwise cavitation will take place when pressure falls below Vapor pressure of the Liquid being pumped.