pump suction lift in negative suction?

Question

why normal pump's suction lift is limited to certain value and what value , need mathematical expression for the same

Answer 1

There is an absolute theoretical limit depending on physics and perhaps a practical limit depending on pump design. One atmosphere supports a column of mercury 30 inches high (with a vacuum above the mercury column) as in a barometer. Actually it is not a perfect vacuum because the space contains mercury vapor which is nil at normal temperature. In like manner, one atmosphere can support a column of 'cold' water about 34 feet high (with water vapor in the vacuum space above the water column). As you raise the temperature of the entire column of water and the vapor temperature above the column, the water column height will decrease by the amount of feet equivalent to the water vapor pressure at temperature. One atmosphere (14.7 psia = about 34 feet of water). Vapor pressures for water can be found from steam tables and converted to feet of water. Because the rapidly moving vanes of a centrifugal pump can create local pressure drops, the actual (negative) lift that the pump can attain will depend on pump design and pump speed, usually determined in bench tests, etc. Engineers use required NPSH (Net Positive Suction Head) to match pump requirements to system conditions such as water supply temperature and lift.