The lift of valves for moderate or low speed pumps is seen in [Fig. 190] and those for higher speeds in [Fig. 191]. These engravings clearly show the relative position of the suction and discharge valves during the movements of the piston.
Fig. 190.
Fig. 191.
Pump slip or slippage is a term used to denote the difference between the calculated and the actual discharge of a pump, and is generally expressed as a percentage of the calculated discharge. Thus, when the slippage is given as 15 per cent. it indicates that the loss due to slip amounts to 15 per cent. of the calculated discharge. Slippage is due to two causes, the time required for the suction and discharge valves to seat.
When pumps run very fast the piston speed is so high that the water cannot enter the pump fast enough to completely fill the cylinder and consequently a partial cylinder full of water is delivered at each stroke. High speeds also increase slippage, due to the seating of the valves. Fig. 191 represents a sectional view of the water end of a pump, showing the position of the valves during a quick reversal in the direction of the arrows, which illustrates the position of the valves corresponding to high speed. The valves in a pump, like almost every other detail in the operation of machinery, do not act instantaneously, but require time to reach the seats.
When pumps run at high speed the piston will move a considerable distance, while the valves are descending to their seats, and water flows back into the pump cylinder until the valves are tightly closed. The valves will remain in the raised position shown in Fig. 191 until the piston stops at the end of the stroke, and under high speed the piston will reach the position on the return stroke indicated by the dotted line L by the time the valves are closed. The cylinder will be filled up to this point with water from the delivery chamber so that no vacuum can be formed until after the piston reaches this position. The volume of water that can be drawn into the cylinder must necessarily be represented by the cubic inches cf space, minus the quantity which flows back during the time the valves are closing. It will thus be seen that the actual volume of water discharged is considerably less than a cylinderful, and the difference, whatever it may prove to be, is called, and is due to slippage.
Fig. 192.