Pump Characteristics

 The behaviour of a pump under varying conditions is shown graphically by the curves known as characteristic curves of the pump.

PUMP CHARACTERISTICS

1. What are Characteristic Curves?

• The behaviour of a pump under varying conditions is shown graphically by the curves known as characteristic curves of the pump.

• The characteristic curves are used to identify and interpret the following parameters of the hydraulic pumps:

• Delivery at various pressures (with constant speed).

• Delivery at various speeds (with constant pressure).

• Volumetric efficiency, mechanical efficiency, and overall efficiency (with constant speed).

• Volumetric efficiency, mechanical efficiency, and over efficiency (with constant pressure).

• Input power at various delivery and pressure conditions (with constant speed).

2. Pump Discharge Pressure Vs Rate of Discharge

• Fig.4.22 depicts the relation between the pressure (or head) produced by the pump and the rate of discharge when the pump is running of constant speed.

• From Fig.4.22, it is clear that less the pressure to be delivered, more the rate of discharge.

3. Pump Discharge Pressure Vs Power, Efficiency, and Rate of Discharge

• Fig.4.23 tells us how much power must be supplied to the pump to obtain different rates of discharge against different discharge pressures.

• From Fig.4.23, it is clear that more power is required to deliver higher discharge pressure. These curves also give information about the volumetric efficiency and mechanical efficiency at different discharge pressures.

4. Rotary Pump Characteristics

• Fig.4.24 depicts the typical efficiency curve, power curve, and rate of discharge curve for the rotary pump at constant operational speed.

• From Fig.4.24, we can interpret the following conclusions :

• Rotary pumps usually run more efficiently at lower delivery pressures.

• The power necessary to run a rotary pump increases almost directly, as does the pressure produced by the pump at constant speed.

• There is relatively little difference in the rate of discharge for delivery pressures.

5. Centrifugal Pump Characteristics

• Fig.4.25 depicts the typical efficiency curve, power curve, and rate of discharge curve for the centrifugal pump at constant operational speed.

• From Fig.4.25, we can interpret the following conclusions:

• Centrifugal pumps usually run more efficiently at higher rate of discharge.

• The rate of discharge of centrifugal pumps drops quickly as discharge pressure increases.

6. External Gear Pump Characteristics

• The typical characteristics curves of an external gear rotary pump is shown in Fig.4.26. These curves show the capacity and power input for an external gear pump at various speeds.

• The curve showing the relation between pump discharge pressure and pump capacity is often termed the head-capacity or HQ curve.

• The curve showing the relation between the power input and the pump capacity is termed the power-capacity or PQ curve.

• From Fig.4.26, we can interpret the following conclusions:

• At any given speed, the capacity characteristic is nearly a flat line. 

• The power input increases with both operating speed and discharge pressure. 

• As the speed of a gear pump is increased, its discharge rate also increases.