What is an air vessel and What are the use of reciprocating pump?

An Air vessel is a closed chamber containing compressed air in the top portion and liquid (or water) at the bottom of the chamber. At the base of the chamber, there is an opening through which the liquid (or water) may flow into the vessel or out from the vessel. When the liquid enters the air vessel, the air gets compressed further and when liquid flows out of the vessel, the air will expand in the chamber. An air vessel is fitted to the suction pipe and to the delivery pipe at the point close to the cylinder of a single-acting reciprocating pump:

• to obtain a continuous supply of liquid at a uniform rate.
• to save a considerable amount of work in overcoming the frictional resistance in the suction and delivery pipes, and
• to run the pump at a high speed without separation

The single-acting reciprocating pump (Reciprocating pump is a positive displacement type of pump which includes a piston pump, plunger pump) to which air vessels are fitted to the suction and delivery pipes. The air vessels act as an intermediate reservoir. During the first half of the suction stroke, The piston moves with acceleration, which means the velocity of the water in the suction pipe is more than the mean velocity, and hence the discharge of the water entering the cylinder will be more than the mean discharge. The excess quantity of water will be supplied from the air vessel to the cylinder in such a way that the velocity in the suction pipe below the air vessel is equal to the mean velocity of flow. During the second half of the suction stroke, The piston moves with the retardation, and hence the velocity of the flow in the suction pipe is less than the mean velocity of flow. Thus, the discharge entering the cylinder will be less than the mean discharge. The velocity of water in the suction pipe due to the air vessel being equal to the mean velocity of flow and discharge required in the cylinder is less than the mean discharge. Thus, the excess water flowing in the suction pipe will be stored in the air vessel, which will be supplied during the first half of the net suction stroke.

             When the air vessel ( Air vessel is a small air chamber fixed on the discharge side of the reciprocating pump) is fitted to the delivery pipe, during the first half of the delivery stroke, The piston moves with the acceleration and force the water into the delivery pipe with the velocity more than the mean velocity. The quantity o water in the excess of the mean discharge will flow into the air vessel. This will compress the air inside the vessel. During the second half of the delivery stroke, The piston moves with the retardation, and the velocity of the water in the delivery pipe will be less than the mean velocity. The water already stored into the air vessel will start flowing into the delivery pipe and the velocity of flow in the delivery pipe beyond the point to which the air vessel is fitted will become equal to mean velocity. Hence, The rate of flow of water in the delivery pipe will be uniform.