Short Answer:

The main function of an air vessel is to reduce pressure fluctuations and ensure a uniform flow of water in a reciprocating pump. It acts as a cushion for pressure by storing energy during high-pressure strokes and releasing it during low-pressure strokes.

An air vessel also helps in reducing the acceleration head, friction losses, and power required to run the pump. It makes the flow smooth, continuous, and efficient, which increases the overall performance and life of the pump. It is a vital component for improving the working of reciprocating pumps.

Detailed Explanation :

Function of an Air Vessel

An air vessel is a closed, air-tight chamber fitted to the suction and delivery pipes of a reciprocating pump. It contains compressed air at the top and water at the bottom. The main function of an air vessel is to smoothen the flow of water, reduce energy losses due to acceleration and retardation of water, and save power. It also minimizes vibration and ensures more uniform discharge.

In a reciprocating pump, water is not discharged continuously because the piston movement is intermittent — it delivers water only during one part of the stroke. This causes fluctuations in pressure and flow, resulting in jerky movement, high power requirement, and mechanical stress. To overcome these problems, an air vessel is introduced near the cylinder on both suction and delivery sides. The air vessel uses the compressibility of air to absorb pressure variations and provide steady water flow.

Working Principle

The air vessel works on the principle of energy storage and release through the compression and expansion of air. During operation, the vessel helps maintain a balance in pressure and flow throughout the cycle.

1. During Suction Stroke:
   When the piston moves outward, the pressure in the suction pipe falls below atmospheric pressure, and water is drawn into the cylinder. The air vessel connected to the suction pipe allows part of the water to flow from it into the cylinder, reducing the acceleration head and pressure drop.
2. During Delivery Stroke:
   When the piston moves inward, it pushes water into the delivery pipe. The air in the delivery-side vessel compresses as water enters it. This compressed air stores potential energy. During the next suction stroke, when the piston moves outward and delivery pressure drops, the compressed air expands and pushes water into the delivery pipe, maintaining continuous flow even when the piston is not delivering.

Thus, the air vessel helps to maintain a nearly constant discharge rate and minimize pressure variation throughout the cycle.

Main Functions of an Air Vessel

1. To Reduce Acceleration Head:
   The acceleration head is the additional head required to accelerate and decelerate the water column in suction and delivery pipes. The air vessel reduces the effective length of the fluctuating water column, thereby reducing acceleration head significantly.
2. To Ensure Uniform Flow:
   Without an air vessel, water is delivered only during part of the cycle, making the flow pulsating. The air vessel absorbs pressure fluctuations and supplies water even during non-delivery strokes, making the flow almost uniform.
3. To Reduce Power Requirement:
   Since acceleration head and friction losses are reduced, the pump requires less power to operate. This improves efficiency and reduces energy cost.
4. To Reduce Pressure Fluctuations:
   The air inside the vessel acts as a spring, compressing and expanding to absorb pressure variations. This prevents sudden rises or drops in pressure that could damage valves or pipes.
5. To Increase Pump Life:
   The smoother and steadier operation reduces wear and tear on moving parts, valves, and pipelines, thereby extending the service life of the pump.
6. To Reduce Vibration and Noise:
   Continuous and smooth water delivery minimizes vibrations and noise that commonly occur due to jerky flow in reciprocating pumps.

Energy Saving by Air Vessel

An air vessel plays a major role in reducing the work done by the pump. Theoretical analysis shows that in a single-acting reciprocating pump without an air vessel, the water in the entire length of suction and delivery pipes is accelerated and decelerated in each stroke. This requires extra work and power.

However, with an air vessel, only a small part of the water near the pump is subjected to acceleration, as the vessel supplies or absorbs flow variations. This reduces the work required to overcome acceleration head by as much as 80%, leading to considerable energy savings and better mechanical efficiency.

Importance in Suction and Delivery Pipes

• On Suction Side:
  The air vessel reduces pressure drop during suction stroke and prevents separation of flow or air binding, which can occur when the pressure falls below vapor pressure.
• On Delivery Side:
  The air vessel stores energy during the delivery stroke and releases it during the suction stroke, ensuring continuous discharge and steady flow.

Both positions play a key role in achieving balanced performance.

Practical Benefits in Operation

1. Smooth operation without sudden pressure surges.
2. Reduction in energy consumption and wear on moving parts.
3. Uniform delivery of water at constant rate.
4. Prevention of water hammer and damage to the system.
5. Better mechanical efficiency and reliability.

These benefits make the air vessel a crucial attachment for reciprocating pumps, especially in high-head and long-pipe systems.

Conclusion

The function of an air vessel in a reciprocating pump is to provide smooth and uniform flow by absorbing pressure fluctuations and reducing acceleration head. It stores energy during compression and releases it during expansion, ensuring continuous water discharge. The air vessel reduces power consumption, vibration, and wear, thus improving the efficiency and life of the pump. It is an essential component for achieving steady operation and reliable performance in reciprocating pumping systems.