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Monday, 30 September 2019

AIR VESSEL IN RECIPROCATING PUMP

We were discussing the basics of reciprocating pumpmain components of a reciprocating pumpworking principle of reciprocating pumpideal indicator diagram of reciprocating pumpeffect of acceleration and friction on indicator diagram of reciprocating pumpexpression for acceleration head in the suction pipe of a reciprocating pumpdouble acting reciprocating pump: working principle, discharge, work done and power required, slip and negative slip in reciprocating pump and maximum speed of a reciprocating pump in our recent posts. 

Today we will see here air vessel in reciprocating pump with the help of this post. We will first understand here the concept of an air vessel and further we will see here the functions and importance of air vessel in the operation of reciprocating pump and it could be easily understood by reading this post i.e. air vessel in reciprocating pump. 

Air vessel in reciprocating pump 

An air vessel is basically a closed chamber from its one side which will have compressed air in the top and water or liquid at the bottom of the chamber. At the base of the chamber, there will be an opening through which liquid may flow in to the air vessel or out from the air vessel. 

Air vessels will be installed at suction side and delivery side of the reciprocating pump as displayed here in following figure. 

These air vessels will be installed as close as possible with the cylinder of the reciprocating pump. 

Air vessel is installed with reciprocating pump in order to secure the following task. 
  1. To secure the continuous flow of liquid with uniform rate of flow
  2. To save the suction and discharge pipe
  3. To save the considerable amount of work in overcoming the frictional resistance in the suction and delivery pipes
  4. To run the reciprocating pump at a high speed without separation

Air vessel acts on the same principle on the basis of which hydraulic accumulator and flywheel work. It stores energy when there will be availability of additional energy and it releases the energy when it is required. 

Air vessel stores and releases additional energy with the help of compressed air contained in the top of the chamber of air vessel. 

When liquid will flow in to the vessel, air containing at the top of the chamber of air vessel will be compressed and when liquid will flow out from the vessel, air containing at the top of the chamber of air vessel will be expanded. 

During suction stroke

Let us consider the air vessel installed at the suction side of reciprocating pump. At start of suction stroke of reciprocating pump or during first half of suction stroke, acceleration head will be maximum and hence more water will be required through suction pipe during starting of suction stroke. In that case, air vessel will provide the additional water to the reciprocating pump. 

As air vessel will be installed very close with the cylinder of reciprocating pump, therefore water will flow with approximate same flow rate through the rest part of the suction pipe. Hence, there will not be any problem of cavitation in rest part of suction pipe. Rest part of suction pipe means the portion of suction pipe below the air vessel and it will be the larger portion of suction pipe.  

At the end of suction stroke, there will be retardation and hence less water will be required through the suction pipe and hence additional water will flow to the air vessel and air vessel will store the energy in terms of water by compressing the air contained in the top of the chamber. This stored energy of air vessel will be utilized during the start of next suction stroke. 

During delivery stroke

Let us consider the air vessel installed at the delivery side of reciprocating pump. At start of delivery stroke of reciprocating pump or during first half of delivery stroke, acceleration head will be maximum and hence more water will be discharged to delivery pipe of reciprocating pump during starting of delivery stroke. 

In that case, additional water will flow to the air vessel and air contained above the water in the top of chamber will be compressed or we can say that energy will be stored in the air vessel. This stored energy of air vessel will be utilized during the second part of delivery stroke. 

As air vessel will be installed very close with the cylinder of reciprocating pump, therefore water will flow with approximate same flow rate through the rest part of the delivery pipe. Hence, there will not be any problem of cavitation in rest part of delivery pipe. Rest part of delivery pipe means the portion of delivery pipe above the air vessel and it will be the larger portion of delivery pipe.  

At the end of delivery stroke, there will be retardation and hence less water will be discharged through the delivery pipe and hence additional water will be given by the air vessel and air vessel will release the energy in terms of water by expanding the air contained in the top of the chamber. 

Therefore, we have seen here the basic principle of air vessel in reciprocating pump and we have also seen here that how air vessel helps to maintain the uniform discharge in the operation of reciprocating pump. 

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Further we will find out, in our next post, working principle of centrifugal compressor

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google    

Also read   

Fundamentals of thermal engineering

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