Pumps used on a ship

Pumps used on a ship| Difference between centrifugal pump and positive displacement pump|

What is a pump?

A pump is a device which produces liquid/slurry movement or flow and due to the resistance faced it develops pressure which will be converted into potential energy. Pumps generates pressure to deliver the liquid from one place to another place. 

Consider a pump at the bottom of your house used to deliver the water into the rooftop water tank. Here the pump is started with electricity and this electric power is converted into mechanical energy in order to move the water into the overhead tank which now possesses potential energy.

Types of pumps used on a ship

There are mainly two types of pumps.

  1. Rotodynamic pump.
  2. Positive displacement pump.

Rotodynamic pumps

With the help of the rotatory motion of the pump which produces dynamic action on fluid to move the fluid that is called a rotodynamic pump.

There are two types of rotodynamic pumps.

  1. Centrifugal Pump.
centrifugal pump
Fig: Centrifugal pump

2. Axial Flow Pump

axial flow pump
Fig: Axial flow pump

Positive displacement pumps

A positive displacement pump moves a fluid by repeatedly enclosing a fixed volume and moving it mechanically through the system. The pumping action is a cycle and can be driven by a piston, screw, gears, rollers, diaphragms or vanes. With this type of pump we can get a high-pressure delivery head.

There are many types of positive displacement pumps

  1. Reciprocating Pumps

a) Diaphragm pump

diaphragm pump

b) Piston pump

piston pump

2. Rotary pump

a) Screw pump

screw pump

b) Gear pump

gear pump

Difference between a centrifugal pump and a positive displacement pump

              MechanicsImpellers pass on velocity from the motor to the liquid which helps moves the fluid to the discharge portTraps a certain amount of liquid and forces it from the suction to the discharge port.
PerformanceThe flow rate varies with a change in pressureFlow rate remains constant with change in pressure.
ViscosityThe flow rate decreases as the viscosity increases due to frictional losses inside the pumpThe internal clearances allow higher viscosity handling.
EfficiencyPump efficiency peaks at a specific pressure – any variations decrease the efficiency dramatically.Efficiency is less affected by pressure. Positive displacement pumps can be run at any point on their curve without damage or efficiency loss
Suction liftStandard centrifugal pumps cannot create suction lift.Positive displacement pumps create a vacuum on the inlet side, making them capable of creating a suction lift


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