Centrifugal Versus Reciprocating Pumps

The competition between centrifugal and reciprocating pumps for hydraulic presses has been decisively won by reciprocating pumps. Under full load, centrifugal pumps have higher efficiency than reciprocating pumps (Figure 2). However, in press installations, the power plants are idle a significant portion of the time. The losses during idling are only about 10% of full load for reciprocating pumps and well over 60 to 70% for centrifugal units. The total combined losses in installations with reciprocating pumps are less than half the losses in centrifugal pump plants, as shown in Figures 3 and 4.

Reciprocating pumps for hydraulic presses that use water as the hydraulic medium are generally of the single-acting multiplunger type. Although there are large installations in operation using double-acting pumps, the faster vertical single-acting pump, which requires less floor space, offers a considerable saving in capital expenditure and has proved itself dependable in service. In most cases, this type of pump is used in conjunction with an accumulator, which allows the averaging of demand and thus a reduction in required pump capacity. There are, however, installations in which a vertical single-acting pump drives hydraulic cylinders directly.

FIGURE 4 Comparison of load cycling losses

Idling is controlled by remotely operated bypass valves or pump suction valve lifters that take command from the operator, accumulator level control, or some other sensormonitoring press action.

Oil pumps most commonly used for the power strokes of presses are

1. Vane rotary constant-delivery pumps, generally for pressures not in excess of 2500 lb/in2 (17,200 kPa)

2. Piston rotary constant-delivery pumps

3. Piston rotary variable-delivery pumps

The constant-delivery pumps require a bypass system for idling. This is generally accomplished by an arrangement of directional control valves or by relief bypass valves.

The variable-delivery pump has the advantage of providing efficient press speed control and idling by an adjustment of piston stroke. In cases of multiple pump application, constant- and variable-delivery pumps are often used jointly, their selection depending on the speed ranges required. It is possible to obtain these pumps designed for use with nonflammable liquid as a hydraulic medium.

Generally, both water and oil pumps in press applications are driven directly by electric motors, the motor speed matching the pump speed. Sometimes, on large water pumps, a geared speed reducer is installed between motor and pump.

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