Performance Characteristics

Abstract

Following the changing operational requirements, practically all pumps temporarily operate away from the design point which is defined by q* ≡ Q/Q_opt=1. Overload corresponds to q* > 1, while operation at q* < 1 is called “partload”. The pump characteristics describe the behavior of head, power consumption and efficiency as functions of the flow rate (the behavior of NPSH=f(Q) is discussed in Chap. 6). The shape of these performance curves over the range from shut-off (or zero flow Q=0) to the maximum possible flow rate is important for the operational behavior of the pump in the plant—for instance when operating in parallel or during start-up (see Sect. 1.1). The majority of applications require a Q-H-curve steadily falling with increasing flow rate, i.e. ∂H/∂Q < 0. This is termed a “stable characteristic”. In contrast, if the Q-H-curve has a range with ∂H/∂Q > 0, the characteristic is said to be “unstable”. Unstable or flat Q-H-curves can cause problems in parallel operation or with a flat system characteristic, see Sect. 1.1.