Hydraulic pumps.

Positive displacement - what it sucks in it must push out. Hence positive. In theory it is a constant volume pump at a given speed and flow is independent of the system. Centrifugal pump (non-PD) "adds energy" to the fluid due to centrifugal action. Volume varies with differential pressure of the system. Gong Xi Fa cai

Here is a link to a great site providing you at least a good direction.

http://www.pumpschool.com/intro/pd%20vs%20centrif.pdf

When to use a Positive Displacement Pump

When to use a centrifugal or a Positive Displacement pump ("PD Pump") is not always a clear choice. To make a good choice between these pump types it is important to understand that these two types of pumps behave very differently.

Flow rate versus pressure

By looking at the performance chart you can see just how different these pumps are. The centrifugal has varying flow depending on pressure or head, whereas the PD pump has more or less constant flow regardless of pressure.

Flow rate versus viscosity

Another major difference between the pump types is the effect viscosity has on the capacity of the pump. You will notice in the flow loses flow as the viscosity goes up but the PD pump's flow actually increases. This is because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. Remember, this chart shows effect of viscosity on the pump flow; when there is a viscosity change there is also greater line loss in the system. This means you will also have to calculate the change in pump flow from the first chart for this pressure change.

The pumps behave very differently when considering mechanical efficiency as well. Looking at the efficiency chart you can see the impact of pressure changes on the pump's efficiency. Changes in pressure have little effect on the PD pump but a dramatic one on the centrifugal.

Viscosity also plays an important role in mechanical efficiency. Because the centrifugal pump operates at motor spefficiency goes down as viscosity increases due to increased frictional losses within the pump. Efficiency often increases in a PD pump with increasing viscosity. Note how rapidly efficiency drops off for the centrifugal pump as viscosity increases.

Another consideration is NPSHR. In a centrifugal the NPSHR 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 speof a PD pump, the lower the NPSHR.

Another thing to keep in mind when comparing the two types centrifugal pump does best in the center of the curve. As you move either to the right, additional considerations come into play. If you move far enough to the left or right, pump life is reduced due to either shaft deflection or increased cavitation. With PD pump you can operate the pump on any point of the curve. In fact the volumetric efficiency as a percent actually improves at the high speed part of the curve. This is dto the fact that the volumetric efficiency is affected by slip, which is essentially constant. At low speed the percentage of slip is higher than at high speed.

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thats what prof said

The question was about hydraulic pumps. Centrifugal pumps are very seldom used in hydraulic systems. Positive displacement pumps are such as gear, vane, piston etc. Variable displacement pumps as the name suggests vary the output of the pump not based on input speed. Variable vane has some use but by far the most popular is the variable piston pump. Controls can be pressure compensating, Horse Power limiting, machanical, servo and others.

Very simply, there's no direct connection between the inlet and the outlet of a positive-displacement pump, and there is one in all other pumps.

Fluid that enters the inlet follows a flow path into a cavity, which it completely fills up. The inlet-to-cavity path then gets closed so the fluid can't find its way pack to the inlet. Then, the flow path between the cavity and the outlet opens and the fluids gets pushed out of the cavity.

In the case of a piston pump, when the piston's moved to increase the working volume of its cylinder cavity, the inlet-to-cylinder check-valve opens and lets fluid flow from the inlet to fill the cavity. When the piston reaches bottom, the inlet-to-cylinder check valve closes to prevent fluid from flowing back to the inlet. The piston then pushes against the fluid, which reduces the working volume of the cylinder cavity, forces open the cylinder-to-outlet check-valve, and forces the fluid to flow to the outlet. The cycle then repeats ...

In non-positive-displacement pumps, not all of the fluid inside the pumps gets pushed out to the pump's outlet during every cycle. In the case of a centrifugal pump, for example, some of the fluid inside the pump at any given time just sloshes around without getting ejected through the outlet.

Cheers! DZ

A hydraulic pump that uses an impellor or propellor to move fluid by momentum, as opposed to a positive displacement pump, which moves discrete quantities of fluid with each rotation. A typical application of a non-positive displacement pump is the coolant pump, or water pump, on a radiator-cooled engine.

Even though the pump generates flow, the pressure is build up by the resistance generated thro the spring tension of the relief valve. In case of positive displacement pumps its capacity to generate higher pressure is utilised by the hydraulic systems. In case of non possitive displacement pumps its the head that is responsible for pressure genarated. In short the pumps generate the flow. In case of positiv displ pumps the pressure genearted is much higher than non positive displacement pumps.

Hi mansilakhani,

I think that you are probably seeking a simple answer to how best to power a hydraulic system.

As has already been suggested the positive displacement device ,lets call it a piston pump will create an ouput pressure equivalent to the input torque applied whilst its volume will be a function of the rotational speed.

The non positive displacement device depends on the speed of rotation to develop its pressure and there is then a defined relationship between pressure and flow once the delivery valve is opened.

Small installations requiring finite amounts of fluid to achieve say a positional control function are going to spend a lot of time literally stalled doing nothing and in either pump type will require the pump to operate in a stand-by mode in the case of the PD version spilling excess volume thru' a pressure relief valve or 'skidding' in the case of the centrifugal type; either way wasting energy, generating heat and wear.I am thinking here of the hydraulics providing the energy to control the lift arms on a tractor. This may be the only economic solution for very small individual applications.

However to overcome this drawback and where powers of 10+ kilowatts are used a variable displacement version of the piston pump was evolved some time ago, one such device often referred to as the 'swash-plate pump' also finds an application in the diesel fuel injection pump (in fact a very high pressure hyraulic pump).

The displacement of the swash-plate pump is controlled by means of changing the inclination of the displacer with reference to the driving shaft. This is an extremely simple and robust solution to the pressure/volume relationship and avoids wasting energy through dumping un-used volume thru' the pressure limiter.The pump can in fact become its own pressure control device with the inclination of the deflector plate being controlled by the delivery pressure. An elegant solution to a difficult problem.

These V/V pumps are used in transmission systems such as are found on large agricultural tractors and earth movers where the pump supplies high pressure oil directly to wheel mounted motors, thereby simplifying the drive train.

Best wishes,

Massey

"What is the difference between Positive displacement pump and non – positive displacement pump and why are they called so?"

One way of answering this question is:

Don't run a Positive displacement pump with the discharge valve closed.

Don't run Centrifugal pump (non – positive displacement pump) with the discharge valve wide open.

If you find out what would happen, if you don't follow the above, I think the primary question is well answered…

A positive displacement pump is one which will pump out a definite volume of fluid per each revolution until its pressure and qty handling capacity is reached.( Both suction and discharge).Operating conditions of pump as per spec is to be maintained.

Non positive displacement pumps will pump out the qty proportional to viscosity,pressure( Both discharge and suction) , temperature and RPM.

There are other minor factors but not very significant .

Positive displacement pumps are normally screw or gear pumps or Diaphram or piston pumps and non positive are the general centrifugal type.

Application wise, Non positive displacement pumps are the general water pumps or other fluid pumps .

Positive displacement pumps are Hydraulic system pumps and metering pumps of all type.