It is valve slam or water hammer which commonly limits the highest speed at which a pump can be run. This water hammer may originate from the pulsations in a long or small suction pipe. The vacuum chamber lessens it, but there is commonly some point of high water in the vacuum chamber that will give much smoother action than any other. Valve slam in this style of pump is caused chiefly by the short rebound of the steam piston against the elastic steam cushion at the end of the stroke. This in turn snaps the valves down with a jump when the speed is high. Dividing this impact or slam on numerous valves of low lift, tends to break up and lessen the shock, therefore with valves of the size and style used in fire-pumps, other things being equal, the less they have to rise and drop to let the water through them, the less will be the valve slam. This height of rise and drop is governed by the circumference rather than the port area. Experience and practice has shown that a 1/2-inch limit of lift is reasonable and does ensure a smooth working pump under all ordinary conditions. _c._ The following table gives minimums for aggregate valve port area and aggregate valve outlet area, for the different size plungers, figured on a basis of 56% of plunger area for a 12-inch stroke, and 64% for a 16-inch stroke. ===+=======================+=========+========+=======+======== | =Size of Pump.= | =500= | =750= |=1000= |=1500= | | =Gal.= | =Gal.= | =Gal.=| =Gal.= +-----------------------+---------+--------+-------+-------- =1=| Diameter of plunger. | | | | | Inches | 7-1/4″ | 9″ | 10″ | 12″ +-----------------------+---------+--------+-------+-------- =2=| Area of plunger in | | | | | sq. inches | 41·28 | 63·62 | 8·54 | 113·10 +-----------------------+---------+--------+-------+-------- | 56% of plunger area, | | | | | or Minimum aggregate | | | | =3=| valve port area | | | | | allowed per section. | | | | 64% = | Square inches | 23·11 | 35·63 | 43·98 | 72·38 +-----------------------+---------+--------+-------+-------- | Minimum aggregate | | | | =4=| valve port | | | | | circumference, allowed| | | | | per section. Inches | 46·22 | 71·26 | 87·96 | 144·76 +-----------------------+---------+--------+-------+-------- | Minimum aggregate | | | | | valve outlet area | | | | =5=| allowed per section | | | | | for valves lifted | | | | | 1/2 inch high. | | | |