Positive displacement pumps discharge curves

Positive-displacement pumps have an almost vertical head—flowrate curve the decline in capacity at increased pressure results mainly from increased internal leakage, a relatively small quantity in an efficient pump. It is not, however, normal to present the performance of a positive-displacement pump on a head—flowrate basis it is essentially a constant flowrate device (for constant speed), where the discharge pressure is determined by the discharge system only. Such pumps usually require a relief valve as protection against overpressure in the event of flow restriction (typically, a closed valve), and capacity is controlled by a bypass flow rather than by a series control valve. 

Similitude for centrifugal devices volumetric flowrate ratios = (diameter of impeller ratio) times (impeller rpm ratio) head ratios = (diameter ratio) times (the impeller rpm ratio) power ratio = (diameter ratio) times (impeller speed ratio). Positive displacement reciprocating pumps head"-capacity curve is almost vertical flowrate decreases only slightly with higher discharge pressure. Although we usually consider head for centrifugal pumps, pressure is more appropriate for positive displacement. Suction velocity < 1 m/s. 

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