Short Answer:

An air vessel is a closed chamber made of cast iron or steel, fitted to the suction and delivery pipes of a reciprocating pump. It contains compressed air at the top and water at the bottom. The air inside acts like a spring and helps to reduce pressure fluctuations during pump operation.

The main purpose of an air vessel is to make the flow of water smooth and uniform, to reduce the work done by the pump, and to save power. It also decreases acceleration head and friction losses, improving the overall efficiency and performance of the reciprocating pump.

Detailed Explanation :

Air Vessel

An air vessel is an important component used in reciprocating pumps to improve their performance and efficiency. It is a strong, closed, hollow chamber fitted on both suction and delivery sides of the pump. The upper part of the vessel contains compressed air, while the lower part is filled with water. The air inside the vessel acts as an elastic cushion that absorbs pressure changes in the pump system.

When a reciprocating pump operates, the flow of water is not continuous because the piston moves forward and backward, creating variations in velocity and pressure. These variations cause acceleration and retardation of water in the pipes, leading to energy losses, vibrations, and uneven delivery. To overcome these problems, an air vessel is attached, which helps to maintain a steady flow and reduce fluctuations.

Construction and Arrangement

The air vessel is generally made of strong materials like cast iron or steel to withstand high pressure. It is cylindrical in shape and connected vertically to the suction and delivery pipes near the pump cylinder. The vessel has two regions —

The lower part is filled with water, which communicates with the water in the pipe.
The upper part contains compressed air, which acts like a spring to absorb pressure changes.

A partition between air and water is not required, as the air is compressed naturally by the water entering the vessel. However, over time, the air may get absorbed into the water, so it needs to be replenished periodically.

Working of Air Vessel

The working principle of an air vessel is based on the compressibility of air and the continuity of water flow. The vessel stores energy during one part of the cycle and releases it during another, ensuring smooth operation.

During Suction Stroke:
When the piston moves outward, the pressure in the suction pipe drops. The air vessel helps maintain flow by allowing water from the vessel to move into the pipe, reducing the pressure difference and acceleration head.
During Delivery Stroke:
When the piston moves inward, water from the pump cylinder is pushed into the delivery pipe. During this period, the air in the delivery vessel compresses, storing some energy. In the next stroke, when the piston moves backward, this compressed air expands and pushes water into the delivery pipe, maintaining a continuous flow even when the piston is not delivering water.

Through this process, the air vessel minimizes velocity changes and pressure variations in the system, making flow uniform and reducing energy losses.

Functions of Air Vessel

To Reduce Acceleration Head:
The acceleration head is the pressure required to accelerate or decelerate the water in the suction and delivery pipes. The air vessel reduces this head by shortening the length of the fluctuating water column.
To Ensure Uniform Flow:
In a reciprocating pump without an air vessel, water flow is pulsating. With an air vessel, flow becomes almost uniform because the vessel compensates for fluctuations.
To Save Power:
Since acceleration head and friction losses are reduced, less power is required to operate the pump, improving efficiency.
To Reduce Vibration and Noise:
By maintaining steady flow, the air vessel reduces vibration and noise caused by irregular pressure changes in the system.
To Increase Pump Life:
Smooth operation and reduced stress on mechanical parts increase the life of valves, pipes, and the pump itself.

Advantages of Air Vessel

It makes the discharge of water nearly uniform.
It reduces pressure variations in the suction and delivery pipes.
It decreases acceleration head and friction losses.
It reduces the power required to drive the pump.
It helps in achieving smoother operation and higher efficiency.

Position of Air Vessel

On the suction side, it is fitted near the cylinder to minimize acceleration head in the suction pipe.
On the delivery side, it is fitted close to the pump delivery valve to reduce the pressure variations and to maintain steady flow into the delivery pipe.

The correct position of air vessels is important to achieve the best performance and efficiency.

Example of Working Effect

In a single-acting reciprocating pump without an air vessel, water delivery is intermittent, and pressure fluctuates widely. When an air vessel is fitted, the air cushion absorbs these fluctuations, resulting in almost constant pressure and flow. The work done per cycle becomes more uniform, and the energy saved can be significant — often up to 80% of the fluctuating energy losses.

Maintenance of Air Vessel

Over time, air dissolves into the water, reducing the air content in the vessel. This lowers its effectiveness. Therefore, air must be replenished periodically through an air valve or compressor to maintain proper cushioning effect. Regular inspection ensures that the vessel functions properly and safely.

Conclusion

An air vessel is an essential part of reciprocating pumps that helps to smoothen the flow of water and reduce pressure variations in suction and delivery pipes. It stores energy during one part of the pump cycle and releases it during another, ensuring steady operation. The use of an air vessel reduces acceleration head, friction losses, and power consumption, thus improving the efficiency and lifespan of the pump. Proper design, placement, and maintenance of air vessels are crucial for the safe and efficient performance of pumping systems.