Pump Limits ```Name: Russell Status: Other Grade: Other Location: MA Country: United States Date: May 2008 ``` Question: I seem to recall early pumps in mines could only pump the water up so many feet, no matter how strong the pump. They used atmospheric engines. Is there a limit to how high a pump operated by a modern gas or electric motor can pump water? Replies: There exist today extremely large pumps that can move huge amounts of various materials. If you are pumping (pushing) 'up', the limits are more practical than theoretical (you could build a pump arbitrarily large, but it might be cost-prohibitive or too hard to manufacture, move, or install, etc., but not theoretically impossible). However, there is one issue that can arise if you are 'pulling' fluid from below, or with certain designs of pumps (such as a kind known as 'centrifugal' pumps). The force that a pump uses to move fluids is known as 'pressure drop' -- the difference between the pressure after and before the pump. 'Local pressure' is the pressure at a specific point (it can be a point inside or outside the pump). If the local pressure drops low enough, the fluid there can boil -- this is known as cavitation. The scientific definition of boiling is when the ambient pressure equals the vapor pressure of a fluid -- you can boil water at room temperature if you drop the pressure low enough. If a pump is 'pulling' fluid up from below it, the pull of gravity downward opposes the pull upward of the pump, creating very low pressure. The water in between can cavitate (think of it as being 'pulled apart' by the pump and gravity so that it boils). For this reason, pumps are usually placed at the bottom of a pipe to push, rather than at the top to pull. More commonly, the impeller blades in the pump can have very small areas of low pressure that cause cavitation. Tiny bubbles of water vapor form and then collapse in the pump, reducing efficiency and causing damage the pump due to vibration. This happens when the pump is operated too fast, or if certain operating problems occur. Hope this helps, Burr Zimmerman Hi Russell, The type of pump you are probably referring to is one that is located above the level of the water being pumped, and which "sucks" the water up from below to the pump above. An example would be a well pump that is located on ground level, and is sucking the water up from below. No pump (no matter how strong it is, or how big its motor is) can lift water up by suction more than 33.9 feet. The reason for this is that a column of water 33.9 feet high, exerts a pressure of 14.7 pounds per square inch, or exactly 1 atmosphere of pressure. When a pump tries to suck water up a pipe from below, what is really happening is that the pump creates a vacuum at the top of the pipe, and atmospheric pressure forces the water up the pipe toward the pump inlet. Since a column of water 33.9 feet high causes a pressure of exactly 1 atmosphere at the column's bottom, atmospheric pressure cannot push it up any higher, even if the pump could draw a perfect vacuum above. The only solution to this is to put the pump at the bottom (under the water you are trying to pump). In this case, the stronger the pump and its motor are, the higher the water can be pumped. An example of this is a water pump used to pump well water. Nowadays, so called "submersible" pumps are used for this. The pump is located under the water level in a well, and now there is no limit how high the pump can pump, providing the motor is strong enough. Regards, Bob Wilson If the pump is located at the bottom of the mine then it can push the water up to large heights. The height is limited only by the pressure capacity of the pump. If the pump is at the top of the mine, then the pump can reduce the pressure in the pipe so that atmospheric pressure pushes the water up from below. In this case, the water can be pumped only about 10 meters up because the pressure of the atmosphere is about 0.1 MPa, or 15 pounds per square inch. That is what is also occurring when sucking on a drinking straw. It is possible to pump and move water using capillary forces, but this phenomenon only works in limited circumstances. Robert Erck Click here to return to the Engineering Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs