In the September 1998 issue of World Pumps, Stan Shiels discussed the system eflects on rotodynamic pump performance and efficiency. This month, Brian Nesbitt looks at bow positive displacementpumps react to systems and clarifies the important dinerences. In two respects PD pumps behave exactly the same as rotodynamic pumps. Firstly, the pump can only operate at the intersection of the pump and system characteristics. Secondly, overesti-.H4K#+.LU#-X 'IH@UYI,cj t,5e AJfrk/lurrn tm#nCCr#Mn ~,OCINGv,CjG y, ClOOcy, G or adding safety ma@as reduces operating e~i?ciency.
Positive displacement pumps, PD pumps, can be divided into two broad categories. Not 'rotary' and 'reciprocating'; this is too simplistic and glosses over the important differencesThe categories are 'physically-sealed' pumps and 'viscously-sealed' pumps. Physically-sealed pumps use solid material seals to prevent internal leakage.These pumps tend to have the steepest pressure-flow characteristics and liquid compressibility can be a signit?cant factor. Pump performance usually changes very little with temperature. Viscously-sealed pumps rely on the process liquid viscosity to prevent internal leakage. Pressure-flow characteristics are very variable and generally not so steep as