What is a Metering Pump On board a Ship?

Metering pump, as the name suggests, is a type of pump which is used on ship to pump a precise volume of liquid. Metering pump can be adjusted to provide different flow rates. They are generally used when the precision of volume to be delivered is very important.

The marine application of metering pump is in the form of chemical dosing pumps, which is used to transfer precise amount of dosing chemicals, especially in boilers. They are also used for dosing chemical additives to protect corrosion in the cooling water system. It is to note that the amount of dosing has to be precise. Over dosing or under dosing may cause corrosion and other damages inside the boiler, which may end up in heavy repairs.

The metering pump is connected to the system in which the chemicals are injected with the help of valve in the pipeline. The pressure produced by this pump should be higher than the pressure in the pipeline, or else there will not be any dosing and the level of chemicals in the dosing tank will be the same.

The metering pumps consist of a small motor which moves the plunger, in and out of the barrel, to provide pressure in the system. The check valves are provided in the suction and discharge side of the pump. Since the pump is of the plunger type, metering pump is a positive displacement pump.

Points to Note

For a positive displacement pump, the relief valve has to be provided in the discharge line. The reason for this is that the liquid is incompressible and there is no space in the barrel to accommodate the pressed liquid. A high pressure is created if the discharge valve is closed or if there is any other restriction/blockage in the system. The high pressure might completely damage the pipes connected. For this reason a relief valve is connected in the discharge line and is set at a particular pressure. When the pressure rises, the relief valve opens and relieves the extra pressure, thus protecting the pipes.

Generally metering pumps are connected to the dosing pumps and the whole unit is known as the dosing unit. The relief valve is connected to the dosing pump, which releases the excess pressure back to the dosing tank.

Working of Metering Pump

As the plunger moves away from the suction valve, a vacuum is created inside the pump because of which makes liquid flow inside the pump. This closes the check valve closes and the plunger again starts moving towards the valves. The discharge valve then opens and supplies the liquid to the system.

The seal arrangement is provided at the end of the plunger i.e. towards the motor side so that no leak should take place.
The volume of the liquid supplied can be altered by altering the movement of the plunger inside the barrel. This is generally done by a small knob provided on the pump and which thus allows adjusting the percentage of liquid supplied.
When the knob is turned to increase or decrease the flow rate, it alters the movement of the plunger, which means that it moves less in the plunger rather than along the full length of the plunger. Because of this the liquid enters the chamber and changes the plunger position along with the flow rate.