Marine Engineering Articles

Air Drawing

If a pump draws in air while in operation the noise is produced and the amount of sea water discharged decreases, significantly lowering the pump's operating efficiency. Also, the amount of sea water being discharged will drop to zero in an instant once a pump is drawing a large amount of air. Why? When a small amount of air is drawn into the pump, the air passes out of the pump together with the sea water.

But, if a large amount of air has entered a pump, it gathers and gradually spreads to the pump periphery, decreasing the centrifugal force the sea water receives from the impeller and disabling the pump from discharging. If the discharge amount is extremely small, there are cases where unnoticeably small amounts of air gather within the pump and build up to a level that could disable the pump. If left as it is, sea water in the pump will run out resulting in burn.

There are several ways to prevent air drawing. Purge air thoroughly before starting the pump. Decrease the suction flow from the bell mouth by reducing the discharge valve opening to decrease the discharge amount. Note, beforehand, the water level at which pump begins to draw air and stand on the alert as the water approaches the critical level. You may use other tanks with a suitable water level to continue discharging. With some ships, cargo planning is based on the hull with trim at the stern. It is the suitable condition to discharge all the remaining ballast.

First, let us look at what ballast operations are and their purpose. Since the ship's draft is shallow while in ballast condition, it is susceptible to wind and wave impact - as a result, rolling increases and hull stability deteriorates. To improve hull stability, the ship's ballast tanks are filled with sea water. When loading the ship with cargo, sea water is discharged. Such sea water loading and discharging are called ballast operations which are conducted either by gravity or ballast pumps. Improper handling of pumps and/or valves may result in various accidents.

Valves are provided where the pipeline is connected to ballast tanks. The pipeline is fitted with many such valves. Improper valve opening or closing can cause accidents. Improper valve opening or closing can cause accidents. This is an example of an accident caused by valve rupture. In this accident, corrosion of the valve body combined with the sudden pressure change due to pumped stuck ruptured the valve body during the stroke. To prevent such accidents, you must have good understanding of the procedures for proper handling of pumps and valves.