Pumps

Question 1

1. List eight reasons why a bilge pump may not be able to remove water from an engine room bilge.

Answer 1

Strainer or filter fouled.
Pump damaged or obstructed.
Insufficient level within bilge to maintain suction.
Pipeline damaged inducting air.
Pump Discharge Valve Closed.
Fluid too viscous for the pump to handle.
Other bilge suction valve open or damaged on the line.
Electrical supply isolated.
Empty bilges.

Question 2

2. With reference to of a gear pump suitable for use with fuel or lubricating oil: a) Sketch a cross-section indicating flow of fluid (8)
   b) Explain the operating principle. (8)

Answer 2

b) The sketch above shows a gear pump
suitable for use with fuel or lubricating oil on a small 4 stroke engine. One shaft is motor driven and is fitted to the gear (the driver gear), this drives the other gear (the driven gear). During rotation, oil is carried around the casing trapped in the space between the gear teeth. When the teeth of both wheels squeeze together the oil is directed towards the discharge ports. The pump id s type of positive displacement pump and should have a pressure relief valve on the casing that can’t be isolated by valves.

Question 3

3. With reference to centrifugal pumps:
   a) State why the pump can be started with the discharge valve closed; (8)
   b) State four possible reasons for the pump failing to achieve suction from a double bottom tank. (8)

Answer 3

a) As the impeller of the pump rotates at high speed, a centrifugal force is created. The force  gives kinetic energy to the fluid as its thrown from the eye of the impeller to the outer part.  The impeller rotates inside a volute casing; whose cross-sectional area increases as it nears  the pump outlet. This acts as a diverging nozzle that converts the kinetic energy in the fluid 
John Willison  IAMI Revision 40 
to pressure energy. As energy cannot be created or destroyed, the rise in pressure energy is  limited to the amount of velocity in the fluid. Therefore, it is perfectly safe to start the pump  with the discharge valve closed, without causing over pressure in the system.
 b) Strainer or filter blocked. 
Insufficient level within tank. 
Pump wear-rings and seals damaged. 
Pipe-work damaged inducting air. 
Hole in suction pipes. 
Impeller coupling failure.