Machinery's Handbook, 31st Edition
PUMP SHAFT LOADING 2807 Pump Shaft Loading: Piston pumps carry the entire piston compression force on their drive rod. The axial load on the rod is easily calculated as force = pressure/area. Buckling of the load bearing members and/or piston rod may also need consideration. Many pumps are rated based on the maximum discharge plunger load that can be carried when the suc tion pressure is zero. If the piston is massive, or pump speeds high, the force to accelerate the piston must be taken into account. For radially driven pumps, the torque on the motor shaft driving a pump depends upon the type of pump being driven. Pumps which produce pulsating flow must accelerate the fluid during each cycle, and therefore experience variable shaft torque. Centrifugal pumps experience a steady torque on the shaft, which can be calculated either from brake power and speed or from the change in momentum of the fluid being pumped. Torque can be calculated using the following equations: T 63025 P n = ---------- where T is torque in in-lbf, P is brake horsepower, and n is rotation in RPM. T 9549 P n = -------- where T is torque in N-m, P is brake power in kW, and n is rotation in RPM. T m · g c --- v t d ( ) r v t i ( ) r i – ( ) = where T is torque, m · is mass flow rate, v t(d) and v t(i) are tangential velocity at the discharge and inlet, r is the radius of the impeller, and r i is the radius at the inlet. When the inlet is central to the impeller, the tangential velocity at the inlet is zero. If a pump is driven using a belt or chain, overhung loads on both the motor and the pump shafts should be calculated. Additional bearing support or larger shafts may be required. In centrifugal pumps and some rotary positive displacement pumps, axial thrust is generated that must be accommodated by the pump drive shaft. In some multi-stage cen- trifugal pumps, mounting the impellers in opposing directions can balance the thrust load and reduce axial load transferred to the drive shaft. In other cases, a hydraulic balancing device is used. This type of device is piped to the suction line or reservoir and exerts an opposing force to the thrust generated by the impellers. For axially driven reciprocating pumps, the axial loading on the drive shaft must be con sidered, as well as radial loading, moment loading, and torque. Axial and radial/moment loading on the motor shaft can be calculated using simple trigonometric relationships. Torque required to axially drive the motor will depend on how the pump is being driven. In some cases, a lead screw or equivalent mechanism is used and the lead screw equation for torque is applicable. Pump Curves: Pump performance can be shown graphically by plotting various perfor mance parameters as a function of flow rate. These graphs are usually supplied by pump manufacturers and usually include efficiency, power requirement, dynamic head, and NPSHR . These values are given as a function of the pump alone, and normally are mea sured in a way that minimizes the influence of suction/discharge line effects. Pump curves are also normally generated using water as the fluid. Since hydraulic power is a function of the fluid’s specific gravity, care must be taken when planning to pump a fluid with a differ ent specific gravity. In those cases, the pump’s power rating should be multiplied by the specific gravity of the planned fluid. The performance of a centrifugal pump is generally based on capacity (gal/min or m 3 /h), added fluid head (ft or m), input power (bhp of kW), efficiency (%), and speed (rpm). It is common to plot centrifugal pump curves for a range of impeller diameters together for a fixed speed. This allows selection of the diameter when looking at operating points.
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