MICROFICHE REFERENCE LIBFUUW - Cd3wd.com

54exhaust valves for the steam (or air) chamber. Although most direct-acting pumps have one steam piston per connecting rod and are referredto as simple, some designs utilize two (or three) tandem pistons. Withtwo piston surfaces to push upon, the same steam pressure can apply twice(theoretically)the power to the water piston.3.2.3.2 Power Pumps-“Power pump” is the popular nomenclature given to any piston-,plunger-, or bucket-pump which is not direct-acting, although of course,all pumps need power to operate. Also, this term usually denotes mechan-ically driven (e.g. gas engine, electric motor) pumps, however, since theprinciple is identical, manual-, animal-, and natural-driven piston pumpsare included in this category. In the basic “hand pump”, Figure 3.12, themanual power is normally transmitted by means of a reciprocating lever.3.2.3.3 CharacteristicsSeveral differences exist, other than the method of driving,between direct-acting and power pumps. The speed, i.e., strokes perminute, of a steam pump varies with the discharge head and when this head[on the water piston) equals the pressure on the steam piston(s), the pumpwill stall. The speed of the power pump does not vary significantly withdischarge head and will not stall at excess heads. Instead, it may over-heat or stall the prime mover, or if not governed, burst the pump cylinderor valves. Therefore, power pumps are usually specified for high headapplications and direct-acting pumps for lower heads. Similarly, sincea plunger simply protrudes into the water cylinder and need not slide andseal along the cylinder wall as a piston, plunger pumps are more suitablefor high discharge head. However, this sealing at the cylinder wallmakes pistons more efficient for high suction lifts.