Starting Centrifugal Pumps

Priming Centrifugal pumps usually are completely filled with the liquid to be pumped before starting. When so filled with liquid, the pump is said to be primed. Pumps have been developed to start with air in the casing and then be primed.44 This procedure is unusual with low-specific-speed pumps but is sometimes done with propeller pumps12. In many installations, the pump is at a lower elevation than the supply and remains primed at all times. This is customary for pumps of high specific speed and all pumps requiring a positive suction head to avoid cavitation.

Pumps operated with a suction lift may be primed in any of several ways. A relatively inexpensive method is to install a special type of check valve, called a foot valve, on the inlet end of the suction pipe and prime the pump by filling the system with liquid from any available source. Foot valves cause undesirable frictional loss and may leak enough to require priming before each starting of the pump. A better method is to close a valve in the discharge line and prime by evacuating air from the highest point of the pump casing. Many types of vacuum pumps are available for this service. A priming chamber is a tank that holds enough liquid to keep the pump submerged until pumping action can be initiated. Self-priming pumps usually incorporate some form of priming chamber in the pump casing. Section 2.4 and Reference 7 may be consulted for further details.

Torque Characteristics of Drivers Centrifugal pumps of all specific speeds usually have such low starting torques (turning moments) that an analysis of the starting phase of operation seldom is required. Steam and gas turbines have high starting torques, so no special starting procedures are necessary when they are used to drive pumps. If a pump is directly connected to an internal combustion engine, the starting motor of the engine should be made adequate to start both driver and pump. If the starter does not have enough torque to handle both units, a clutch must be provided to uncouple the pump until the driver is started.

Electric motors are the most commonly used drivers for centrifugal pumps. Direct current motors and alternating current induction motors usually have ample starting torque for all pump installations, provided the power supply is adequate. Many types of reduced voltage starters are available7 to limit the inrush current to safe values for a given power supply. Synchronous motors are often used with large pumps because of their favorable power-factor properties. They are started as induction motors and run as such up to about 95% of synchronous speed. At this point, dc field excitation is applied and the maximum torque the motor can then develop is called the poll-in torque, which must be enough to accelerate the motor and connected inertia load to synchronous speed in about 0.2 s if synchronous operation is to be achieved. Centrifugal pumps usually require maximum torque at the normal operating point, and this should be considered in selecting a driver, particularly a synchronous motor, to be sure that the available pull-in torque will bring the unit to synchronous speed.

Torque Requirements of Pumps The torque, or turning moment, for a pump may be estimated from the power curve in USCS units by

and in SI units by

n where M = pump torque, lb • ft (N • m) P = power, hp (kW) n = speed, rpm

Equation 25 makes no allowance for accelerating the rotating elements or the liquid in the pump. If a 10% allowance for accelerating torque is included, the constant should be correspondingly increased. The time At required to change the pump speed by an amount An = n2 — n1 is given by

I = moment of inertia (flywheel effect) of all rotating elements of driver, pump, and liquid, lb • ft (kg • m2)

An = change in speed, rpm k = 307 in USCS (9.549 in SI)

The inertia I of the driver and pump usually can be obtained from the manufacturers of the equipment. The largest permissible An for accurate calculation will depend on how rapidly Mm and M vary with speed. The quantity Mm — M should be nearly constant over the interval An if an accurate estimate of At is to be obtained. Torque-speed characteristics of electric motors may be obtained from the manufacturers.

Horizontal-shaft pumps fitted with plain bearings and packed glands require a breakaway torque of about 15% of Mn, the torque at the normal operating point, to overcome the static friction. This may be reduced to about 10% of Mn if the pump is fitted with antifriction bearings. The breakaway torque may be assumed to decrease linearly with speed to nearly zero when the speed reaches 15 to 20% of normal. Construction of torque-speed curves requires a knowledge of the pump characteristics at normal speed as well as details of the entire pumping system. Some typical examples taken from Reference 7 are given below. The following forms of the affinity laws (Eqs. 12) are useful in constructing the various performance curves:

where
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