The follower type of water piston can readily be packed without removing it from the cylinder, providing rings of the proper depth and length are at hand. The old packing rings can be removed with a packing hook. Take the new ring and start one end with a soft stick, and push the remainder of the ring firmly against the collar or flange at the inner end of the piston, as shown in the engraving, Fig. [672]. Arrange the several rings so as to break joints.
Method of packing a follower piston. Coat the sides of the rings with a thick paste of cylinder oil and Dixon’s Flake graphite, which will prevent the rings from sticking together.
Fig. 671.
Pump packing cannot be readily examined and is liable to fail at any time, therefore several rings, cut to the proper length, should be kept on hand. This may be easily done by making a pattern ring, which is nothing more or less than a ring of packing which has been fitted into the piston and is known to be of the proper length. The extra rings can then be cut at odd times, and when occasion demands it the water piston can be packed very quickly and the pump started.
Care should be taken when about to pack a boiler feed pump or other pump subjected to high pressure to see that the cylinders are relieved before loosening the cylinder head bolts. This may be accomplished by closing the valve in the delivery pipe, and also in the suction pipe; if the pump receives water under pressure, open the air cock on the air chamber and cylinder cocks.
Fig. 672.
Pump slip or slippage represents the difference between the calculated and the actual discharge of a pump, which is generally expressed as a percentage of the calculated discharge. Thus, when the slippage is given as fifteen per cent. it indicates that the loss due to slip amounts to fifteen per cent. of the calculated discharge. Slippage is due to two causes, the time required for the suction and discharge valve to seat, due to excessive speed. When the piston speed is so high that the water cannot enter the pump fast enough to completely fill the cylinder only a partial cylinder full of water is delivered at each stroke. High speeds also increase slippage, due to the seating of the valves.
Graphite as a lubricant is almost without a rival. It is one of the forms under which carbon appears in nature; it is also known under the name of plumbago and black lead; it is soft and oily to the touch; it is a conductor of electricity; it is a lubricant that allows pipe joints to be screwed up to the tightest possible fit. Graphite remains upon the threads preventing rust, and it so preserves its peculiar properties that pipe can be unscrewed without effort eight or ten years after the joints have been made.