H-08 Bloqueo inversión

thereby preventing overheating and possible seizure. Modern designs of screws preclude the use of timing gears, ensure smooth efficient simple operation, eliminate turbulence and vibration.

SCREW DISPLACEMENT PUMP Fig. 9.1 1

PUMPS AND PUMPING SYSTEMS 367

Emergency Bilge Pump

The function of this pump (Fig. 9.12) is to drain compartments adjacent to a damaged (holed) compartment. The pump is capable of working when completely submerged. The pump is a standard centrifugal pump with reciprocating or rotary air pumps. The motor is enclosed in an air bell so that even with the compartment full of water the compressed air in the bell prevents water gaining access to the motor. The motor is usually dc operated by a separate remote controlled electric circuit which is part of the vessels emergency essential electric circuit. The pump is designed to operate for long periods without attention and is also suitable for use as an emergency fire pump. This design is particularly suited for use in large passenger vessels giving outputs of about 60

EMERGENCY BILGE PUMP Fig. 9.12

368 REED'S GENERAL ENGINEERING KNOWLEDGE

COMPARISON O F PUMPS, SUCTION LIFT, CAVITATION, ETC.

Consider first the performance of a reciprocating pump. If the pump could create a perfect vacuum in the barrel it should theoretically be able to lift cold fresh water from a height of 10.3

above the suction valve.

1.013 bar

-

-

Thus the pump lift depends on the barometer reading (for vacuum attainable) and also the fluid pumped oil below a density of 1.0 will be capable of being lifted a greater amount. Also the fluid pumped should be cold as warm fluids tends to vaporise and destroy the vacuum. In practice a good reciprocating pump will lift cold water from about 8 m with a high barometer. As the temperature of the fluid rises the suction lift decreases so that at 94°C the pump will not draw water. Above this critical temperature water must be supplied from a head to increase the pressure on the suction valve and prevent vapour lock. The following figures give an indication of the above points:

Barometer 750 mm Practical Lift 7.5 m

above figures for cold water.

Temperature 64°C Practical Lift 3 m

77°C ,, 2.1 m

94°C

Head required 3 m

123°C

,,

above figures for a 760 mm barometer reading.

Air vessels are usually fitted to reciprocating pump discharge lines to ensure uniform water flow velocity in discharge lines so reducing the inertia head required. The vessel is merely a cylinder forming an air space damping cushion with fluid entry at one side and discharge over a weir at the other side, or via an internal pipe.

In pumps carrying liquids a phenomenon known as cavitation occurs. Low pressure regions occur in the flow at points where high local velocities exist. If vaporisation occurs due to these low pressure areas then bubbles occur, these expand as they move with the flow and collapse when they reach a high pressure region. Such formation and collapse of bubbles is very rapid and collapse near a surface can generate very high pressure hammer

PUMPS AND PUMPING SYSTEMS 3 69 blows which results in pitting, noise, vibration, and fall off in the pump efficiency. This phenomenon is not usually very pronounced in reciprocating pumps. Incipient cavitation cavitation which is just beginning, can occur when suction lift capability cannnot meet supply requirements and the output reduces until the two coincide. Under these conditions of operation the pump runs noisily and cavitation damage can occur. By throttling the discharge, or reducing pump speed, rough running of the pump and possible damage can be avoided. Super cavitation occurs when the vapour bubbles collapse within the liquid after the impeller.

Inducers

These are sometimes fitted to centrifugal pump impeller shafts at suction. Their purpose is generally to ensure the supply of fluid to the impeller is at sufficient pressure to avoid cavitation at impeller suction, or it enables the pump to operate with a lower net positive supply head. Different types are used either scroll, screw or propeller.

The propeller (like a stub bladed fan) inducer is fitted to super cavitating pumps, pumps where the cavitation occurs between the inducer and the impeller. Such pumps can operate at about one third of the net positive supply head normally required for conventional centrifugal pumps and they are suitable for LPG and LNG carriers.

Considering the previous remarks with respect to centrifugal pumps the following points are applicable: the same remarks apply for pump suction head but as clearances in a fast running centrifugal pump are difficult to maintain then even with the good pump with facing clearances of a few millimetres it will probably lift about 7.3 m of cold water with a high barometer. (2) air vessels are rarely fitted as steady flow and air extraction is usual. (3) such pumps are very prone to cavitation especially at inlet to impellers and it may be advantageous to reduce the suction lift to prevent the formation of bubbles due to low pressure regions, incorrect attention to this point may cause severe cavitation and very poor pump performance.

Considering next the gear and screw displacement pumps then these pumps are also affected by changes of barometer pressure and fluid temperature. A reasonable mechanical clearance must be provided and any clearance will of course reduce the vacuum efficiency and hence suction head available. Summarising for these two types on the above three points it may be said