LESSON 18. Pumps and pump classification

18.1 Introduction

Hydraulic pumps are the devices which converts the mechanical energy into hydraulic energy. Mechanical rotations are provided to the pump with the help of an electric motor , which creates a partial vacuum  in the inlet of the pump and allows the fluid into the pump through inlet pipe . The entered fluid is being pushed by the pump into the various components of hydraulic system or machine.

There is a misconnect that a pump generates pressure, it only make the flow of fluid. Pressure occurs when the flow from the pump is subjected to  a resistance. The mechanical action of the pump creates a vacuum at the inlet port, which allows atmospheric pressure to force liquid from the reservoir into the pump. The mechanical rotations of the pump then forces the liquid into the hydraulic system or hydraulic equipment.

18.2 Pump Classification

Classification

Pumps can be broadly classified into two categories-

  • Positive Displacement Pumps

  • Non Positive Displacement Pumps

18.2.1 Positive Displacement Pumps

Normally the  hydraulic pumps used in the hydraulic systems are positive displacement pumps. A positive displacement pump has an extremely close fit between the pumping element and the pump housing, which results  in very low or negligible slippage. A positive displacement pump causes a fluid to move by trapping a fixed amount of it and then forcing the trapped volume of fluid  into the discharge pipe. This type of  pumps work using an expanding cavity on the suction side and a decreasing cavity on the discharge side. Fluid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity vanishes. In the positive displacement pump the flow enters and leaves the pump at same velocity and no change in the kinetic energy takes place.

Advantages

This type of pump will have the following advantages –

  • It can produce high pressure

  • These are having wide application in the hydraulic systems

  • The volumetric efficiency is comparatively more

  • This will have less change of efficiency through out the pressure range

  • Size of pump is small

18.2.1.1 Positive displacement types

A positive displacement pump can be further classified depending upon the  mechanism used -

  • Rotary pump

Rotary type of  pumps are most commonly utilized in the hydraulic systems when low to medium pressures are there. These may be of various types like -  gear type, screw type, vane type etc. It works on the principle that a rotating gear, screw, or vane traps the liquid at bthe suction side of the pump and forces it to the discharge side of the pump. This type of pump is generally self type  priming because it has the capability to remove the air from the suction side of pump. This type of pumps are having close clearance between the rotary element and the housing and hence rotated at low speeds to have reliable operation.  

  •  Reciprocating Pump

This type of pumps are normally consisted of  piston, cylinder, crank with connecting rod delivery and suction pipe, etc. Operation of reciprocating motion is done by the power source ytglike electric motor or an engine, etc.

18.2.2 Non positive displacement

Non-positive displacement pumps are those in which fluid is transferred with the help of inertia of fluid in motion . These pumps include centrifugal pump, propeller pump etc. These are normally used in low pressure and high volume systems. With this pump, the volume of liquid delivered for each cycle depends on the resistance offered to flow. This type of pump costs less as the it has fewer moving parts. It has the following advantages-

Advantages

  • Its initial cost is low

  • Simple in design

  • Maintenance cost is low

  • Reliability is more

  • Operation is comparatively quiet

Last modified: Saturday, 15 March 2014, 10:53 AM