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Pump characteristics, losses and selection

Short answer

Centrifugal pump losses: hydraulic (shock, friction), volumetric (leakage), mechanical (bearing, disc friction). Overall efficiency = manometric × volumetric × mechanical. Characteristic curves: H-Q falls, P-Q rises with flow; efficiency peaks at BEP. Selection considers duty point, liquid type, NPSH, specific speed, and cost.

Write short notes on (a) the efficiencies and energy losses in a centrifugal pump, (b) the main characteristic curves of a centrifugal pump, and (c) the factors considered in the selection of a pump.

(a) Losses and efficiencies

Energy losses in a centrifugal pump:

  1. Hydraulic losses: shock at impeller entry (off-design flow), friction in impeller passages, casing and volute, and eddy losses due to sudden changes of area.
  2. Leakage (volumetric) losses: liquid leaking back from the delivery side to the suction side through the clearance between the impeller and the casing.
  3. Mechanical losses: friction in bearings and stuffing box, disc friction on the outer faces of the impeller.
EfficiencyDefinition
Manometricηman=HmHe=gHmVw2u2\eta_{man}=\dfrac{H_m}{H_e}=\dfrac{gH_m}{V_{w2}u_2}
Volumetricηv=QQ+q\eta_v=\dfrac{Q}{Q+q}, where qq is the leakage
Mechanicalηm=power given to the impellershaft power\eta_m=\dfrac{\text{power given to the impeller}}{\text{shaft power}}
Overallηo=ρgQHmPshaft=ηman ηv ηm\eta_o=\dfrac{\rho gQH_m}{P_{shaft}}=\eta_{man}\,\eta_v\,\eta_m

(b) Characteristic curves

Curves are drawn for constant speed, with discharge on the x-axis.

 H, P,       H-Q (falls with Q)
 eta   \  .-------eta
        \/   ..  
        /\       P rises with Q
       /   \
      0---------------------- Q
 (shut-off head at Q = 0)
  • Head-discharge (H-Q): head is highest at shut-off (Q=0Q=0) and falls as discharge increases (for backward curved vanes).
  • Power-discharge (P-Q): power rises almost linearly with discharge; power at zero discharge is small, hence the pump is started with the delivery valve closed.
  • Efficiency-discharge (η\eta-Q): zero at Q=0Q=0, rises to a maximum at the best efficiency point (BEP) and falls again.
  • Main (constant-speed) and operating curves show the performance at different speeds; constant efficiency (Muschel) curves show the lines of equal efficiency in the H-Q plane. The system curve cuts the H-Q curve at the operating point.

(c) Selection of a pump

  • Required discharge and head (duty point), and the nature of the system curve; the duty point should be near the BEP.
  • Type of liquid: clean water, sewage, slurry, corrosive or viscous liquid (viscosity reduces head and efficiency and raises the power).
  • Suction conditions: NPSH available, to avoid cavitation.
  • Specific speed to choose between radial, mixed and axial types; choose multistage if the head is high.
  • Efficiency and power cost; the motor and speed; variation in demand (parallel units or variable speed).
  • First cost, maintenance, spare parts availability, space, and the reliability required.
  • For small discharge at very high head, or for viscous liquids and metering duties, a reciprocating or other positive displacement pump is selected.

✓ Answer checked by Claude Sonnet 5.5

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