Fig. 188.
Fig. 189 represents the customary gland and stuffing-box in which the gland is adjusted by the nuts C and D upon two studs. After the adjustment has been properly made lock-nuts are tightened which leaves the gland free yet preserves the alignment.
It has been proven by practice—after long and costly experiments—that a number of small valves instead of one large one are far the most durable; durability being the question. Corliss, Leavitt, Holly and other leading pump builders had occasion to find the truth of this statement early in their careers. The “slamming” of large valves under moderate speeds proved itself an almost insurmountable difficulty until the principle of keeping the valve area as low as possible within reasonable limits had been fully demonstrated.
Fig. 189.
To illustrate the advantage of having a number of comparatively small valves instead of one large one, suppose a pump to be fitted with four 31⁄2-inch delivery valves at each end, the valves covering ports 21⁄2 inches in diameter. The area of each port is 4·9 square inches. In order to provide an equal area between the valve and the seat the valve must rise a distance equal to one-fourth the diameter of the port.
The combined area of the four ports is 19·6 square inches, which corresponds to the area of a circular opening 5 inches in diameter, one-fourth of which is 11⁄4 inches. It will be understood that the smaller valves can seat much more quickly and with less jar than the larger one, hence a larger number of small valves is not only better because of the great reduction in slippage, but they are also more economical, being subjected to less wear and tear.