AXIAL FLOW PUMP:
An axial flow pump is one in which a screw propeller is used to create an increase in pressure by causing an axial accelaration of liquid within its blades, the incidental rotation imparted to the liquid is converted into straight axial movement by suitably shaped outlet guide vanes.
Axial flow pumps are sometimes classed with centrifugal although centrifugal force plays no useful role in the pumping action. A comparison of discharge characteristics shows that H/Q and working efficiency characteristic for the two pumps are quite different.
The discharge characteristic drawn in each case for constant speed, show those for the axial flow pump with a solid line and a broken line for centrifugal pump. Starting from the point of normal duty, throttling of the discharge of an axial flow pump reduces the flow but also cause a rise in pressure and power. With the valve closed and "zero discharge, the head can be about three times greater and absorbed power about doubled. The action of throttling to reduce throughput would overload the motor and cause operation of protective devices. The throughput needs to be controlled in another way or the pump motor must be over - rated. In fact, the closing of the discharge tends to cause
water hammer. If the discharge of centrifugal pump is closed in, the pressure rises by a moderate amount and power demand actually drops.
Beyond the point of normal duty, the axial pump power requirement decreases with lower head anri delivery increases by a modest amount. For the centrifugal pump operating at lower head, the greater throughput is matched by a rise in absorbed power (although there is a marginal fall off beyond normal duty). The centrifugal pump electric motor can be overloaded at low head and high throughput - the opposite condition for overload of an axial pump.
The axial pump retains reasonable efficiency over a wider head range, than the centrifugal pump. There are three other features of the axial flow pump which are of particular importance in their application.
1. Under the low head (2.5 to 6.2 m), high throughput (2800 -9500m3/hr) conditions commonly required by main condensers, an axial flow pump with a higher speed than an equally matched centrifugal pump can be used. The electric motor for the pump, can be of smaller size.
2. The pump can idle and offer little resistance when a flow is induced through it by external means, ideally suited to scoop intake for condensers.
3. It can be reversible in operation (a friction clutch between motor and pump is required).
The combination of characteristics makes the pump ideally suitable for condenser circulating duties, especially in conjuction with a scoop injection, where the motion of the ship under normal steaming condition is sufficientto induce a flow through the idling pump and the condenser.
Reversibility and high throughput make an axial turbine ideal for heeling and trimming operation. An axial flow pump may be fitted on the straight transfer pipe between tanks installed for this purpose.
When used for sea - water circulation, the pump will normally have a gun - metal casing, for heeling and trimming operation, it is more usual to find pump with cast iron casings. Impellers are of aluminium - bronze (76% Cu, 22% Zn & 2% Al) guide vanes of gun - metal, shaft of stainless steel, with a removable stainless steel sleeve in way of the bush.
AXIAL FLOW PUMP CAVITATION & SUPER CAVITATING PUMPS
The hydrofoil section of conventional axial pump blades, like those of propeller, promote a severe drop in pressure at the leading edge and on the surface of the suction side, while producing positive pressure on the discharge surface. The blade shape enhances operation but cavitation can be a problem mainly on the suction side surface of pumps rotating at very high speed.
Cavitation ocurs when a drop in pressure at the leading dege and across the blade surface is sufficient to produce vapour (and air if present) from the liquid. Pressure of the liquid increases as it passes through the pump, and the vapour pockets and bubbles, subjected to positive pressure, then collapse, as vapour is re-absorbed back into the liquid. Erosion is caused because of the collapsing of the bubbles and the impact damages the metal surface.
Axial Flow Pump
Centrifugal Pump
An axial flow pump with hydrofoil shaped blaeds, which operates at excessive speed also creates a demand of rapid inflow of the liquid being pumped. Flow may not match the demand with resulting loss in efficiency which may be compounded by cavitation and erosion of blade surface.
Supercavitating pumps have blades of a wedge cross - section to produce a different cavitation pattern. A drop in pressure is not pronounced at the leading edge, it is minimal over the full area of the suction side of the blades and exaggerated at the trailing edge. Vapour pockets persists until clear of the blade and collapse where they cause no damage. The supercavitating type of axial flow pump operates at high speed but generats low discharge head. As a small, light - weight high speed pump, it is used for fuel transfer in aircraft and rockets. It is also used as a suction booster or inducer for conventional centrifugal cargo pump in liquefied gas carriers because it can handle liquids at or near their vapour pressure with higher speed than can conventional pumps.
MATERIAL:
Axial flow pump casing would be cast iron or gun - metal. Impeller aluminium bronze. Guide vane is of gun - metal. Guide vanes guide water without turbulence to the discharge. Pump shaft stainless steel with solid and flexible coupling driven. A water cooled thrust of the tilting pad type is required because of the considerable thrust generated. Prime mover or motor capacity required less compared to a centrifugal pump. The mechanical seal is water cooled as is-the composition bush for the shaft. Water cooling through multi - leaf filter, in the case of condenser circulating, because of the possible ingress of sand.
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