Characteristics of positive displacement pump

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Characteristics of positive displacement pump

The positive displacement pump gives approx. a constant flow at a wide range of pressures. Mostly positive displacement pump use for deliver high viscosity fluid. This type of pump is three types rotary, reciprocating and linear. This type of pump operates at the initial stage with always turn the valve in open condition because It has not shutoff head and continues to discharge the liquids, the cause of leakage in discharge line.


Pressure vs flow rate chart

Now in figures you can see centrifugal pumps flow rate is varying with respect to pressure where, as in the positive displacement pump more or less constant flow regardless of pressure as showing line showing in the graph.

Pressure vs flow rate chart

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You can see in the flow rate vs viscosity graph loss of flow as the viscosity goes up in centrifugal but the positive displacement pump’s flow actually increases. This is because the higher the viscosity liquids fill the clearances of the pump cylinder body more as compared to other causing pump and give higher volumetric efficiency.

Net Positive Suction Head Requirements

In centrifugal pump, the NPSHR varies respect of flow which is determined by pressure and viscosity as discussed above. In a PD pump NPSHR varies as a function of flow which is determined by speed. The lower the speed of a PD pump, the lower the NPSHR required.


 Characteristics curve of positive displacement pump

The characteristic curve of (HQ curve) a positive displacement pump showing in figure. H(Head) Q(flow) characteristic curve when positive displacement pumps that we have taken as an example. you know the working principle of reciprocating the pump. because reciprocating pumps is also a category of PD pump rather than Positive Displacement pump. I said that apart from the reciprocating pump there are many other types of pumps like screw pump, vane pump, gear pumps that also you know falls under the category of positive displacement pump. So, here we are discussing the HQ curve of reciprocating pump.



 Characteristics curve of PD pump

We have discussed the working principle of reciprocating pump how this pump works?

The pump sucks fluid from the sump then it discharges to the delivery tank. You know suction stroke and delivery stroke which is connected by a crank and connecting rod mechanism so the linear motion of the piston created by the crank and connecting rod also depends upon the frequency of the rotation of the crank speed. We have discussed that if we try to draw the HQ curve of a PD pump then the HQ curve looks like as show in figure.

So, the discharge noted by Q and discharged is by H. We try to you know develop the HQ curve for this kind of pump, then how will we get it? We have discussed that the PD pumps you know positive displacement pumps are this HQ curve is like this normally this class of pumps are used to deliver high head against an almost constant flow rate.

This is a characteristic feature of the positive displacement pump. So, it is normally used to deliver a high head against a constant speed here that remains almost constant flow rate. We have discussed that normally whenever it discharges from the open end of the cylinder to the close end of the cylinder.

Then it discharges water to the delivery sump and then high pressure develops because of that high pressure there is a leakage of some amount through the glands and as a result of which we get an actual curve which looks like this in figure.

Because of the leakage through glands the reciprocating pump performance is totally dependent on volume capture in cylinders. But if the speed is higher or fluctuating in reciprocating pump leakage chances also increase. 

Slip factor is the difference between theoretical discharge and actual discharge of a same pump. Due to some losses during the operation the pump slip factor was affected.

When the discharge pressure is increasing with increasing the head of the fluid because of the losses in friction, like slip and other types of mechanical parts rubbed.

Discharge and speed graph

In this curve is a linear line showing because if speed is increased the discharge pressure after reach the beyond limit of speed the linear line going to Flat. While the speed of the pump increases, the chances of leakage are also increasing, with other losses. 


Efficiency and head graph

Efficiency and head curve lines sowing approx linear but efficiency is totally dependent upon the head of the pump. 



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