b. The net suction valve port area and the total suction valve outlet area under valves lifted 1⁄2 inch high must not be smaller than the figures given in the table below.
(1) Length of Stroke (in inches) | 12 | 16 | |
(2) Greatest No. revolutions per minute. | 70 | 60 | |
(3) Corresponding Piston travel per minute. | 140 | 160 ft. | |
Approx. actual max. Piston velocity | (4) Feet per minute. | 308 | 352 |
(5) Feet per second. | 5.1 | 5.9 | |
(6) Net Suction Valve-port area speed regarded | 56% | 64% | |
(7) Total Suction Valve Outlet Area under Valves | 56% | 64% | |
(8) Discharge Valve Area. | 2/3 of Suction Valve Area. | ||
By “valve-outlet area,” we mean the vertical cylindrical surface over the outer edge of the valve ports, i.e., the distance L multiplied by the circumference at the outer edge of the valve ports C, [Fig. 8]. Thus for a 4-inch valve, with ports inscribed in a 31⁄2-inch circle, whose circumference is 3·5 × 3·1416 = 11 inches; the valve “outlet area” for 1⁄2-inch lift would be 51⁄2 inches.
Fig. 8.
The actual velocity of piston during the middle portion of stroke is from 2.0 to 2.4 (average 2.2) times as great as the piston travel per minute (as determined in experiments by Mr. J. R. Freeman on several duplex pumps of different manufacture). This is because each piston stands still nearly half the time, or while its mate is working, and, moreover, moves more slowly near start and finish of stroke. The words “piston speed” are commonly incorrectly used and refer to “piston travel.” A clear understanding that the actual piston speed is more than twice as great leads to more generous valve design.
Large aggregate valve areas are necessary for pumps designed to run as fast as these, and experience has shown that to prevent valve slam at high speed and to accommodate high suction lifts, it is just as important to have a large “valve outlet area” as to have a large area of valve port.
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.