Short Answer:

Discharge capacity is the amount or volume of liquid delivered by a pump per unit time. It is generally expressed in terms of cubic meters per second (m³/s), liters per second (L/s), or gallons per minute (GPM). It shows how much liquid a pump can handle during its operation.

The discharge capacity depends on several factors such as the pump speed, size of the cylinder or impeller, stroke length, and suction or delivery head. A higher discharge capacity means the pump can transfer more liquid in a given time, which helps in achieving higher efficiency for the pumping system.

Detailed Explanation :

Discharge Capacity

Discharge capacity is one of the most important performance parameters of any pump. It defines the rate at which a pump delivers liquid from the suction side to the discharge side. It is also known as the flow rate or pump output. The discharge capacity helps engineers and operators determine how much liquid a pump can transfer within a specific period of time under certain operating conditions.

The discharge capacity is commonly expressed in cubic meters per second (m³/s), liters per second (L/s), or gallons per minute (GPM), depending on the system of units used. This parameter is crucial in designing and selecting pumps for applications in industries, water supply systems, and hydraulic machines, where continuous and steady flow is essential.

Formula for Discharge Capacity

The formula for discharge capacity depends on the type of pump being used. For reciprocating pumps, which operate based on the to-and-fro movement of a piston or plunger, the discharge is calculated as:

Where,
= Discharge capacity in m³/s
= Length of stroke (m)
= Area of the piston (m²)
= Speed of the crank (revolutions per minute)

If the pump is single-acting, discharge occurs only during one stroke (usually the forward stroke). If it is double-acting, discharge takes place during both forward and return strokes, effectively doubling the discharge rate.

For centrifugal pumps, the discharge depends on the impeller diameter, rotational speed, and head developed. The flow rate in such pumps is generally determined experimentally and represented on performance curves.

Factors Affecting Discharge Capacity

Several factors influence the discharge capacity of a pump:

1. Speed of the Pump:
   The discharge capacity is directly proportional to the pump speed. Increasing the rotational speed of the crank or impeller increases the amount of liquid delivered.
2. Size of the Cylinder or Impeller:
   Larger piston area or impeller diameter allows the pump to move a greater volume of fluid per stroke or revolution, increasing discharge capacity.
3. Length of Stroke:
   In reciprocating pumps, a longer stroke means a greater volume of liquid displaced per cycle, thus increasing discharge.
4. Head on the Pump:
   The discharge capacity decreases as the total head (suction + delivery) increases, because the pump has to work against more resistance.
5. Fluid Properties:
   The viscosity, density, and temperature of the fluid also affect the discharge rate. Highly viscous fluids flow slower and reduce discharge capacity.
6. Leakage or Slip:
   In real pumps, some fluid slips back due to imperfect sealing, reducing actual discharge. The difference between theoretical and actual discharge is known as slip.

Theoretical and Actual Discharge

1. Theoretical Discharge:
   Theoretical discharge is the ideal discharge assuming there is no loss or leakage. It can be calculated directly from pump dimensions and speed.
2. Actual Discharge:
   Actual discharge is the real discharge obtained during pump operation. It is always less than theoretical discharge due to losses like leakage, valve friction, and slip.

The efficiency of a pump can be evaluated by comparing actual discharge with theoretical discharge using the formula:

Importance of Discharge Capacity

The discharge capacity of a pump is very important in determining its suitability for a particular application. A pump with insufficient discharge cannot meet system requirements, while one with excessive discharge may cause overloading or inefficiency.

Some key points of importance include:

• Ensures proper fluid transfer for the intended process.
• Helps select appropriate pump size and motor power.
• Determines the rate of energy consumption.
• Influences pressure and velocity distribution in the system.
• Ensures system reliability and performance consistency.

For example, in irrigation systems, the discharge capacity determines the volume of water supplied to the fields. In industrial systems, it helps in selecting pumps that can handle process fluids effectively without cavitation or damage.

Improving Discharge Capacity

To enhance the discharge capacity of a pump, several measures can be taken:

• Use a larger cylinder or impeller.
• Increase the operating speed within safe limits.
• Reduce suction and discharge losses.
• Maintain the pump and valves properly to reduce leakage.
• Avoid air entrapment, as it can reduce efficiency and flow rate.

In practical situations, it is also important to ensure that an increase in discharge does not exceed the system’s designed pressure or flow capacity, as it may lead to pipeline stress or cavitation.

Conclusion

Discharge capacity is the measure of the quantity of liquid delivered by a pump per unit time. It is one of the main characteristics that define a pump’s performance. The discharge capacity depends on several factors like pump speed, head, fluid type, and size of the components. It can be expressed as theoretical and actual discharge, where the latter is always lower due to system losses. Maintaining proper discharge capacity is essential for efficient and safe operation of pumping systems in various mechanical and industrial applications.