MICROFICHE REFERENCE LIBFUUW - Cd3wd.com

56Another significant difference between direct-acting and power pumpsis their discharge per time characteristics. Figure 3.13 illustrates sometypical Q-T curves for various reciprocating pumps. Power and direct-acting pumps are similar in that like other positive displacement devices,they have little variation of Q with H .Another feature of piston-, plunger-, and bucket-pumps is that dueto their positive displacement nature, they are self-priming. That is,within the limits of their structural soundness (e.g. cylinder wallstrength, air-tightness) and surrounding pressures (i .e. NPSHA) , theycan displace air in the suction line which causes a pressure drop andthus the suctionof water.3.2.3.4 Applications for Irrigation and DrainageSince the development of rotodynamic pumps (see Section 1.23,mechanically-driven, power and direct-acting pumps have lost popularityfor irrigationand drainage where mechanical prime movers are available.This is due primarily to the fact that rotodynamic pumps produce thehigh Q , low H performance usually needed for irrigation and drainagesituations, whereas positive displacement pumps are better suited forhigh H and low Q applications. Additionally, rotodynamic pumps con-tain far less moving parts which require less maintenance and cost.However, when these direct-actingand mechanical-power pumps areused for irrigation or drainage, they exist in three basic forms. Ineither a vertical or horizontal position, they can be mounted above thewater supply with a suction line extending to the water. In thisapplication, the dynamic suction lift must be considered so as not toexceed the NPSHR. When a conventional direct-acting or power pump cannotbe installed near enough to the water surface (e.g., in a well or mine