marți, 29 mai 2012

The Bilge Pumps Used on Board Ships


It is important to empty ship's bilge wells at regular interval of times to prevent flooding of the engine room. Bilge pumps are used for this purpose.It is important to know the construction and working of these pumps as they are used in emergency situations and also often face technical problems.

Introduction

We learnt what is bilge water, oily water and use of oily water seperator. Now we will talk a look at these pumps used on board various types of ships and boats and see how does a bilge pump work . A bilge pump found on a ship is generally a reciprocating type or a centrifugal type. The rate of flow provided by a reciprocating type is lesser than that of the centrifugal type. Also, the mechanism and construction of a reciprocating pump is extremely complicated for a pump used for such important purpose.
All the bilge pumps that are generally used on ships are placed below the sea level. Though this helps them to gain extra suction pressure, they often require priming before the start. Reciprocating pumps are self-priming but due to the drawbacks mentioned earlier, their use is getting reduced day by day.
Nowadays centrifugal pumps are generally used as bilge pumps as they provide greater output rate and take lesser time for the pumping process. The only drawback of these pumps is that they don't have a self- priming system. This has not been an obstacle in their use and the problem is compensated by using an external priming system.














Construction of Bilge Pump

A centrifugal pump consists of an impeller. The impeller is fixed at the center with a help of a shaft.It has vanes which are fixed radially and are strategically located.
Around the impeller, a diffuser or volute is fixed. A diffuser is a ring which also has fixed blades that are strategically placed and are used to increase the speed and convert the kinetic energy of the liquid into pressure.
The impeller and diffuser are contained in a casing or the main frame which also supports the motor of the pump.
Thus there are three main parts of a centrifugal pump :
  • Impeller
  • Diffuser
  • Casing
A sealing arrangement is provided around the shaft to prevent leaking of fluid. The sealing arrangement might consist of a gland or a mechanical seal.











Working of Bilge Pump
The liquid enters the pump through the center of the impeller also known as the eye of the impeller. The liquid after entering flows radially out and enters the vanes of the impeller. Due to the rotation of the impeller the velocity of the liquid increases. This high velocity liquid then enters the diffuser or the volute casing where the kinetic energy of the liquid is converted to pressure. This pressurized liquid is pumped out through the pump discharge.Many centrifugal pumps have more than one impeller for increased velocity.












Priming arrangement

As centrifugal pumps are not self priming, they require some additional arrangement to get rid of the air in the suction line. When the liquid that is to be pumped is at a level higher than the pump, then the air in the suction line can be removed by opening an air cock fitted at the pump suction. This will automatically allow the liquid to flow due to gravity.Bilge Pump Priming
If the pump is located below the sea water level and there is a provision made to utilize the pressurized sea water, then priming can be done by opening the sea water cock and the air cock simultaneously.
Alternate priming systems can be an external air pumping unit that provides pressurised air to different pumps simultaneously.



How to prime the bilge pump

So you want to prime a bilge pump before starting. How would you do it? Given below is the step by step procedure of doing it in the right way.
  1. Before starting the pump , open the suction valve and close the discharge valve.
  2. Start the priming unit to the suction line
  3. Start the motor
  4. Keep a watch at the priming process ( It will start when you start the pump)
  5. Once priming is done, open the discharge valve slowly , turn by turn.
  6. Adjust the amount of flow with the help of the suction valve.
  7. Keep the desired output flow by adjusting the discharge valve
  8. While stopping, stop the motor first and then close the discharge and suction valve.

What if the bilge wells overflow?

It is a governmental law that any liquid that has oil particles suspended in it should not be discharged in the open sea. Bilge wells are helpful to store this contaminated water but can sometimes be a threat to the engine room. If the bilge wells overflow it could cause a rise in the level of water up to or above the floor plates. This could lead to accidents, emergency situations or even disturbance in the stability of the ship. For this reason, bilge wells are periodically emptied by pumping out the bilge with the help of bilge pumps.
MARPOL states that bilge water cannot be directly pumped out into the sea. For this reason, bilge water is first passed through an Oily water separator where the level of suspended oil particles in the mixture is significantly brought down. When the PPM (parts per million) of oil particles in the mixture comes down to the permissible limit, it is allowed to be thrown overboard but only when the ship is en route.
Bilge wells are always provided with strainers to prevent solid particles from entering. This is done to prevent choking of the solid particle in oily water separator or bilge pump. Absence of strainers might lead to drop in bilge pump outlet pressure and also reduction in outlet flow of oily water separators.

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