pumping theory (principle of pump)

PUMPING THEORY (PRINCIPLE OF PUMP)

Normally the powered electric motor converts the electrical energy into mechanical energy which is used to drive the shaft of the pump. The pump in turn coverts the mechanical energy into hydraulic energy by creating a partial vacuum at the inlet, which induces the atmospheric pressure to force the fluid through the inlet line. Then the internal design of the pump mechanically pushes the fluid to the outlet line.

1. How a Pump Works ?

The working principle of the pump can be better explained by referring to the simple piston pump as shown in Fig.4.1.

• In contrast, the devices used to convert hydraulic energy into mechanical energy are known as turbines or motors.

• Instead of liquid, if gas is employed then other terms may be used. If the primary objective is to increase the pressure of the gas, the device is termed a compressor. On the other hand, a device used primarily for causing the movement of a gas is known as a fan or blower.

As shown in Fig.4.1, the pump has the following two ball-check valves : 

Inlet check valve: It is connected to the pump inlet to allow fluid to enter the pump. 

Outlet check valve: It is connected to the pump outlet to allow fluid to leave the pump. 

When the piston is pulled to the right, a partial vacuum is generated in pump cavity. The vacuum created in the cavity makes the outlet check valve to close the outlet line and allows the atmospheric pressure to push fluid from the reservoir into the pump via the inlet check valve.

When the piston is pushed to the left, the fluid movement forces the inlet check valve to close and open the outlet check valve. The quantity of fluid displaced by the piston is ejected out through the outlet line. Thus the pump produces flow of fluid.

2. Uses of Pump (Where Do You Need a Pump ?)

The two broad applications of pump are given below:

1. Pump is used to transport liquid as the case in pumps for the irrigation or pumps used in house. In this case, the outlet line is open to the atmosphere. There is no resistance to the fluid flow, so there will be no discharge pressure above atmosphere but there will be flow. These types of pumps will fall into positive displacement pumps (refer Section 4.3.1).

2. Pump is used to develop pressure in fluid-power systems. In fluid-power systém, outlet line is not exposed to atmosphere (output line is blocked). Liquid is ideally an incompressible fluid, so the resistance to the fluid flow delivered at the outlet line is the pressure developed by the pump. The hydraulic power developed by pumps is converted back into mechanical energy by hydraulic actuators connected to the output line. Theoretically we have infinite resistance to flow. The pressure will therefore rise until some component breaks unless pressure relief is provided. That's why the pressure relief valve is used in fluid-power systems. When the pressure reaches the set value, the relief value will open to allow the flow back to the oil tank.