Introduction to Hydraulic Pumps

Hydraulic pumps are omnipresent and used in applications ranging from mud pumping (drilling site) to movement of the antenna in space stations. They convert the mechanical energy supplied by the motor to hydraulic energy. It develops the flow of fluid at the required pressure to overcome the external load. A hydraulic pump is a critical component of any hydraulic system and its selection is very sensitive to the efficiency of the system.

Type of Hydraulic Pumps

There are two main types of Hydraulic pump:

• Positive displacement pump
• Centrifugal (Hydrodynamic) pump

Positive-displacement pump

A positive displacement pump operates by forcing a fixed volume of fluid from the inlet of the pump to the discharge of the pump regardless of the system pressure. In a positive displacement pump, the amount of fluid displaced per cycle is completely dependent on the geometry of the pump and the outlet pressure has no influence on it.

Positive displacement pumps can be classified as,

i) Reciprocating pumps

1. Piston Pump(Single/double acting)
2. Plunger Pump
3. Diaphragm Pump

In a reciprocating pump, a constant flow of liquid is drawn into the cylinder on the intake stroke of the piston. This is discharged under pressure on the discharge stroke. Since the delivery of fluid happens at the end of every stroke, creating a discharge which is pulsating in nature.

Reciprocating pumps can be further classified as single acting pump and double acting pump. A plunger pump is similar to a piston pump with one exception. Due to the friction between the piston and the wall in the piston pump this demands a high surface finish of the cylinder bore. Where as in plunger pump, there is no physical contact between the cylinder wall and plunger, and hence the surface finish need not be very high. This results in a reduction in the price of the pump. However, compared to plunger pumps, piston pumps can supply more flow per unit volume occupied by the pump.

ii) Rotary pumps

1. Vane Pump
2. Lobe Pump
3. Gear Pump
4. Screw Pump

Unlike the linear motion of the piston in the reciprocating pump, rotary pumps trap the liquid in a confined space. It is then forced to the outlet by rotary elements. Because of this, the delivery will be continuous (less pulsating) in nature. The major reason behind the selection of rotary pump is to eliminate the inertia involved with reciprocating pumps. In a reciprocating pump, the piston accelerates to the predefined velocity, and at the end of the stroke, decelerates back to zero. As a consequence, this acceleration and deceleration consumes a part of energy used per cycle and leads to energy loss. Rotary pumps operate on constant velocity and hence save the inertial energy loss.

Rotary hydraulic pumps are rated by their “capacity” i.e. the total liquid displacement from suction to outlet in one minute. The capacity depends upon the geometry of the pump and clearance between the rotating elements and pump housing. The clearance creates a leakage pathway from the high pressure discharge side to the low pressure suction side. Another important point is that, in order to reduce the leakage, clearances should be kept to a minimum. This demands precision manufacturing and minimum tolerances; leading to higher pump costs. Hence, rotary pumps are generally not used for high pressure applications.

Centrifugal (Hydrodynamic) Pump

A centrifugal (hydrodynamic) pump has a rotating impeller mounted on the driving shaft and a case. The suction side is connected at the core of the pump and delivery side is connected to the discharge.

Centrifugal hydraulic pumps operate on a simple principal. When the liquid is made to rotate by an external mechanical force, due to the centrifugal force developed, it is thrown away from the central axis. This centrifugal force creates centrifugal head in the liquid and helps it rise to higher levels. Due to the centrifugal action, the liquid is thrown out thereby creating a vacuum at the core of the pump. As a consequence, if connected to the liquid reservoir, it will suck additional liquid into the pump, making it possible to continuously deliver fluid at higher pressure.

Compared to positive displacement pumps, centrifugal pumps can deal with higher flow rates. However, discharge is dependent on the delivery pressure. Centrifugal pumps are more suitable for water/less viscous liquids due to the friction between impeller blades. If the centrifugal pump is filled with air, upon starting it will not develop sufficient suction pressure to suck the liquid, hence, the pump will not function. In order to avoid this issue, the centrifugal pump should always be filled with water.