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Monday, 23 September 2019

SLIP AND NEGATIVE SLIP 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 pump and double acting reciprocating pump: working principle, discharge, work done and power required  in our recent posts. 

Today we will see here another important topic i.e. slip and negative slip in reciprocating pump with the help of this post. Before going ahead, we must have brief introduction to reciprocating pump, single acting and double acting reciprocating pump in order to understand the slip and negative slip in reciprocating pump. 

Reciprocating pump

If the mechanical energy is converted in to stored mechanical energy or pressure energy by sucking the liquid in to a cylinder in which a piston is reciprocating backward and forward, which exerts the thrust on the liquid and increases its hydraulic energy or pressure energy, the hydraulic machine will be termed as reciprocating pump.  

Single acting reciprocating pump

We have already seen reciprocating pump with single piston and single acting, where we have discussed in detail how water is being sucked through suction pipe during suction stroke and how water is being discharged through delivery pipe during delivery stroke.  

In case of single acting reciprocating pump, there will be only one stroke in first half of revolution of crank and second stroke will be done during second half crank revolution. Hence, in case of single acting reciprocating pump, there will be one suction stroke and one delivery stroke for one complete revolution of crank. 

Double acting reciprocating pump

Water will be acting on both side of the piston in case of double acting reciprocating pump. Therefore, there will be two suction and two delivery pipes for double acting reciprocating pump. 

When there will be a suction stroke on one side of the piston, same time there will be a delivery stroke at the other side of the piston. 

Hence, in case of double acting reciprocating pump, there will be two suction strokes and two delivery strokes for one complete revolution of crank and water will be discharged through the delivery pipes during these two delivery strokes.

Slip in reciprocating pump

Slip in reciprocating pump is basically defined as the difference between the theoretical discharge and actual discharge of the reciprocating pump. 

Actual discharge of a reciprocating pump will be less than the theoretical discharge of the pump due to leakage of water during operation of pump. 

The difference of the theoretical discharge and actual discharge will be called as slip of reciprocating pump. 

Mathematically, we can express the slip in reciprocating pump as mentioned below

Slip = Qth – Qact 


Slip is usually expressed as percentage slip and it will be given by following expression. 

Where, Cd = Co-efficient of discharge 

Theoretical discharge of a single acting reciprocating pump and double acting reciprocating pump will be given by following equation as mentioned here and it is already derived in our previous sessions. 

Theoretical discharge of a single acting reciprocating pump and double acting reciprocating pump could be secure here. 

Theoretical discharge of a single acting reciprocating pump

Discharge of the pump per second = A L N/60 

Theoretical discharge of a double acting reciprocating pump 

Negative slip in reciprocating pump

As we have discussed above that slip in reciprocating pump is basically the difference between the theoretical discharge and actual discharge of the reciprocating pump. If actual discharge is more than the theoretical discharge, slip of the reciprocating pump will be negative and it could be concluded by considering the equation of slip of reciprocating pump. 

Negative slip will occur when suction pipe is long, delivery pipe is short and pump is running at high speed. 

So, we have seen briefly describe the reciprocating pump and basically we have discussed here the slip and negative slip in reciprocating pump.

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Further we will find out, in our next post, maximum speed of a reciprocating pump.   

Reference: 

Fluid mechanics, By R. K. Bansal 

Image courtesy: Google   

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