What is discharge head?

Short Answer:

The discharge head is the vertical distance between the pump centerline and the point where the liquid is discharged from the system. It represents the energy or pressure required by the pump to move the fluid from the pump outlet to the delivery point. In simple terms, it is the height to which the pump can raise the liquid on the discharge side, including the effects of pressure and friction losses in the discharge pipe.

The discharge head depends on factors such as the discharge pressure, the height of the delivery tank above the pump, and the resistance in the discharge piping. It is a vital part of the total head of a pump and helps determine the overall efficiency and power requirement of the pumping system.

Detailed Explanation :

Discharge Head

The discharge head is one of the important components used to describe the total head developed by a pump. It specifically refers to the height or energy required to move the fluid from the pump discharge flange to the final discharge point. This value includes both the vertical elevation difference and the pressure required to overcome any resistance or friction losses in the discharge line.

In pumping systems, the discharge head is expressed in meters or feet of liquid column. It helps in calculating the total dynamic head (TDH), which is the total energy needed to move the fluid from the suction source to the discharge point. Understanding discharge head is crucial in pump selection and operation because it directly affects the pump’s efficiency and power consumption.

Meaning and Measurement of Discharge Head

The discharge head represents the pressure head on the discharge side of the pump. It can be measured using a pressure gauge installed on the discharge line near the pump outlet. The reading from the gauge (in meters or feet of liquid) gives the discharge head directly.

Mathematically, discharge head (Hd) is given by:
Hd = Pd / (ρ × g)
Where:

  • Hd = Discharge head (m)
  • Pd = Pressure on discharge side (N/m²)
  • ρ = Density of liquid (kg/m³)
  • g = Acceleration due to gravity (9.81 m/s²)

This formula shows that the discharge head depends on the discharge pressure and the type of liquid being pumped. Higher pressure or denser liquid results in a higher discharge head value.

Factors Affecting Discharge Head

  1. Height of Delivery Point:
    If the delivery tank or outlet is located far above the pump, the discharge head will increase since more energy is needed to lift the liquid to that height.
  2. Friction Losses in Pipes:
    When the liquid flows through discharge pipes, valves, and fittings, it experiences resistance which leads to friction losses. These losses are included in the total discharge head.
  3. Flow Rate:
    Higher flow rates increase the velocity of the liquid, which increases friction losses and thus the discharge head.
  4. Pipe Diameter and Length:
    Narrow or longer discharge pipes increase resistance and result in higher discharge head values.
  5. Pressure at Discharge Point:
    If the discharge outlet is connected to a pressurized system or tank, the pressure at that point contributes to the total discharge head.

Importance of Discharge Head in Pump Systems

The discharge head plays a key role in the performance and design of pumping systems. Engineers use it to determine the total dynamic head and select a suitable pump that can deliver the required flow rate at the given head.

A properly designed discharge head ensures:

  • Efficient Pump Operation: The pump operates close to its best efficiency point (BEP).
  • Energy Saving: By optimizing discharge head, energy losses due to over-pressurization or excessive friction can be minimized.
  • System Safety: Maintaining proper discharge head prevents overloading and damage to pump components.
  • Smooth Fluid Delivery: It ensures a steady and adequate supply of liquid to the desired location.

Example

Consider a centrifugal pump that lifts water from a sump and delivers it to an overhead tank 10 meters above the pump. The discharge pressure gauge shows 150 kPa. Using the formula:
Hd = Pd / (ρ × g) = (150000) / (1000 × 9.81) = 15.28 m

Therefore, the discharge head of the pump is approximately 15.28 meters.

If the tank is located at a height of 10 meters, the additional 5.28 meters represent friction and pressure losses in the discharge system.

Relation with Total Head

The total head (H) of a pump is the sum of the suction head (Hs) and the discharge head (Hd).
So,
H = Hd + Hs

This relationship helps in calculating the total energy that the pump needs to provide for efficient operation.

Conclusion

The discharge head is the height or pressure energy required by a pump to push the liquid from its discharge outlet to the delivery point. It includes the effects of static height, friction losses, and discharge pressure. It is a crucial parameter in pump design and operation since it affects both efficiency and power consumption. Accurate calculation of discharge head ensures smooth and energy-efficient pumping system performance.