MICROFICHE REFERENCE LIBFUUW - Cd3wd.com

65The third method of diaphragm pump operation is similar to thehydraulic-driven device mentioned above. The only difference is that thepiston is not attached to the diaphragm, but instead is utilized as a typeof secondary pump above the diaphragm. The piston alternately forces the"working fluid" in and out ti the cylinder, pulsating the diaphragm, whichin turn pumps another fluid. This method has the advantage of providingeven pressures across the diaphragm membrane as opposed to mechanicalstroking which produces nonuniform stresses with stretching or fatiguenear the piston (Pumping Manual, 1964) . This type of diaphragm pump isusually confined to small-discharge, industrial uses, however, it ismentioned to illustratethe use of secondary, working fluids.As with power pumps, capacity regulation of diaphragm pumps is normallyaccomplished by varying the driver speed.3.3 Rotary MethodsThe devices in this subclass increase head by displacing (positively)water in a rotary motion. They should not be confused with rotodynamicpumps which also use a rotary motion (see Section 4.2), but utilizeprimarily high-speed, kinetic energy to increase head. Rotary devices,however, can operate even at very slow speeds and still increase head. Aswill readily be seen in several of the early devices, (e.g., noria, paddlewheel),rotary water lifters "trap" the water and push it from one elevation(or head) to a higher one. Although, like reciprocating devices they displaceisolated units of water, rotary methods normally discharge a morecontinuous flow (i.e., at a given speed) because several water pushingcomponents (e.g., blades, buckets, pistons) rotate one behind another.