Hydraulic Pump Rating and Selection Factor.

Pump Rating and Selection Factor.

The performance of different pumps is evaluated on the basis of many factors, including:

Physic characteristics, Operating Characteristics and Cost. When selecting a pump, the following pump rating and selection factors are considered:

            Capacity

Pressure

Drive Speed

Efficiency

Reliability

Fluid characteristics

Size and weight

Control adaptability

Life

Pump Installation and maintenance costs.

Some of the selection factors for different pumps are given in Table 5 – 1. Each of these selection factors is described briefly in the following paragraphs.

 
Capacity 

The primary rating of a pump is its capacity. This is also called the delivery rate, flow rate, or volumetric output. The capacity is usually given in gallons per minute, cubic inches per minute, or cubic inches per revolution at specified operating conditions. Pump capacity ratings are usually given at standard atmospheric inlet pressure and various output pressures, as well as at approximate fluid service temperatures.

Pressure

The pressure rating of a pump is generally based on the ability of the pump to withstand pressure without an undesirable increase in its internal leakage (or slippage) or damage to pump parts. Pumps are pressure – rated under the same conditions (speed, temperature, and inlet pressure) at which they are capacity rated. Most pumps are pressure rated at 100, 500, 1000, 1500, 2000, 3500, and 5000 psi (pound per square inch).

Drive Speed

Pumps are often rated at the commonly available electric motor speeds of 1200 or 1800 rpm. They may also be rated at speeds other than motor speeds. For instance, higher speeds occur in mobile hydraulic pumps driven from internal combustion engines. These engines usually operate at a constant speed and include speeds of 2000 rpm. And higher. Some industrial hydraulic pumps are rated at speeds up to 4000 rpm.

The maximum safe speed for a rotating pump is limited by the pump’s ability to avoid cavitations and high outlet pressures. Most pumps also require a minimum operating speed. Although these speeds are usually not critical, pumps operating at high pressures require a minimum speed to prevent overheating or internal slippage.

Maximum speed and pressure ratings for pumps are often given for both intermittent and continuous operation. Continuous ratings describe the maximum speed and pressure at which a pump can be operated for a normal design life (about 10000 hours). Intermittent ratings are the maximum speed and pressure at which a pump can be operated safely for short times and still have a satisfactory service life. Operating a hydraulic pump beyond any ratings will usually reduce its service life.

Efficiency

As pointed out earlier, the pressure a system exerts on the hydraulic pump directly affects the delivery rate of the pump. As the pressure increases, the flow rate of the pump decreases. The amount of decrease varies depending on the type of pump used. This change in flow affects the pump’s efficiency. Pump efficiency is described in two ways:

VOLUMETRIC EFFICIENCY–the ratio of actual delivery rate to the theoretical displacement.

OVERALL EFFICIENCY–the ratio of its hydraulic power output to its mechanical power input.