Pumps (in general):

When 2 pumps are fitted in parallel and where one always needs to be kept ready as standby , it is necessary to fit nor-return valves on the discharge side of the pump to prevent back flow.

Strainer filters are fitted on the suction side of all pumps to prevent solid matter or debris from fouling the pump.

Depending on usage, low & high pressure alarms are fitted to monitor the operation of the pump.

Isolating valves are to be fitted on the suction and discharge side of the pump to allow for safe isolation of the pump when it requires to be taken out of service and for maintenance.

Air purging cocks are fitted. Pressure gauges for the pump & ammeters for the motor are necessary to monitor the

performance of the pump.

Solid displacement pumps: A full flow relief valve is always fitted at the discharge side of these pumps to prevent damage

due to increasing pressure in the pumped if the discharge side valves should be inadvertently fully or partially shut. Normally bypassed to the suction side of the pump.

A positive displacement pump must not be operated against a closed valve on the discharge side of the pump, because it has no shut-off head like centrifugal pumps. A positive displacement pump operating against a closed discharge valve will continue to produce flow and the pressure in the discharge line will increase, until the line bursts or the pump is severely damaged, or both.

A relief or safety valve on the discharge side of the positive displacement pump is therefore necessary. The relief valve can be internal or external. The pump manufacturer normally has the option to supply internal relief or safety valves. The internal valve should in general only be used as a safety precaution, an external relief valve installed in the discharge line with a return line back to the suction line or supply tank is recommended.

Plunger pumps - a reciprocating plunger pushes the fluid through one or two open valves, closed by suction on the way back.

In case of reciprocating pumps, an air bottle I fitted on the discharge line to act as a pressure pulse damper. As the discharge pressure rises the air is compressed in the vessel, and as the pressure falls the air expands. The peak pressure energy is thus stored in the air and returned to the system when pressure falls. Air vessels are not fitted on the reciprocating boiler feed pumps since they may introduce air into the de-aerated water.

Centrifugal pumps: The pump is not self priming and hence a priming pump is required to be fitted to remove the

air from the suction side of the pump casing when there is a negative suction lift.

Oil pumps & their system: Fuel oil & Lub Oil pumps are usually solid displacement pumps

Problems with centrifugal pumps pumps:

If a motor-driven centrifugal pump DRAWS TOO MUCH POWER , the trouble will probably beindicated by overheating of the motor. The basic causes may be (1) operation of the pump to excess capacity and insufficient discharge pressure;(2) too high viscosity or specific gravity of the liquid being pumped; or(3) misalignment, a bent shaft, excessively tight stuffing box packing, worn wearing rings, or other mechanical defects.

AC Motors are ignition protected with built-in thermal overload protection.

Fire pumps Two independently powered pumps must be provided in all cargo ships of 1000 tons gross and over and in passenger ships of less than 4000 tons gross. Larger passenger vessels and passenger ferries must have three such pumps. The pumps are fitted with non-return valves if they are of the centrifugal type, to prevent loss of water back through open valves when not running.

A relief valve is necessary in the system if the pumps are capable of raising the pipeline pressure so that it is greater than the design figure. With centrifugal pumps the relief valve is unlikely to be needed but it is essential for a positive displacement fire pump. Minimum line pressures and capacities are governed by the regulations.