Marine Centrifugal pumps and priming system
The simple centrifugal pump is used for sea water circulation and other duties where self priming is not a requirement. When installed for bilge pumping or ballast duty, these pumps require a primer i.e. some means of removing air from the suction pipe so that the liquid to be pumped is caused to flow into the pipe and so to the eye of the impeller.
SINGLE STAGE CENTRIFUGAL PUMP
For general duties the impeller is of aluminum bronze keyed and secured to a stainless steel shaft. The impeller shown is fully shrouded and of the single entry type. The renewable wear rings are of aluminium bronze and the casing is normally of bronze or cast iron. The cover has a hub containing the shaft bearing at the bottom and above either a packed gland or a mechanical seal. The shaft bearing is of phenolic resin asbestos, lubricated by the liquid being pumped except for pumps operating on high static lift. These have grease lubricated bronze bearings to ensure adequate lubrication during the priming period.
A spigotted coupling spacer connects the motor half coupling to the pump shaft. When this is removed, the pump casing for inspection or maintenance.
The fully shrouded single entry impeller in the pump shown in the type most widely used. It consists of a number of vanes curving backwards from the direction of rotation. The vanes are supported on one side by shrouding connected to the hub. The shrouding supported the vanes on the other side, has an entry at the centre. When the pump is operating, liquid in the casting is swirled by the rotating impeller. The swirling action causes the liquid to move towards the outside and away from the centre. The backward curving vanes and the rotation give the liquid a combined radial and circular motion.
The section of the volute casing shown in the sketch increases, thus allowing unrestricted flow from the impeller. The volute also acts as a diffuser, converting kinetic head into pressure head. Some pumps have a double volute casing which gives radial balance and reduced wear on the bearings.
Pumps designed to produce high pressure, have a diffuser ring so that a greater quantity of kinetic energy in the liquid can be converted to pressure.
When a centrifugal pump is operating, the liquid leaving the impeller produces a drop in pressure at the entry or eye of the impeller. This caused liquid from the suction pipe to flow into the pump. In turn, there is a movement of the liquid to be pumped. The latter is normally subject to atmospheric pressure. A centrifugal pump will maintain a suction lift of four metres or more once it has been primed, because of the water passing through.
The water in a pump acts like a piston for water in the suction pipe and an empty pump will not operate.
A pump which is required to initiate suction from a liquid level below itself, must be lifted with an air pump.
AIR PUMP ARRANGEMENT
The diagram shows a primer coupled to the top of an electric motor and centrifugal pump set. A pipe from the pump outlet, provides cooling water for the primer. This returns through another pipe to the pump suction.
The main pump suction pipe has a float chamber fitted. The float operates a valve on the pipe leading from the float chamber to the air pump suction. With no liquid in the suction, the float drops, opening the valve and allowing the air pump to evacuate the air from the suction pipe. This partial vacuum causes the atmospheric pressure to force liquid into suction pipe. The rising liquid will lift the float and close the valve on the air pump suction. Air pumped out, passes to atmosphere.
AIR PUMP PRINCIPLE
The air pump or water primer, as the simple plan view shows consists of an elliptical casing which a vaned rotor and has a covering plate with ports cut in it. The casting is partly filled with water. The rotor is coupled to the electric motor so that when the pump is running the water spins with the rotor and being thrown outwards, takes up an elliptical shape. The tips of the vanes are sealed by the water and the volume between them varies during the rotation. Beneath the suction ports, the volume increases so that air is drawn from the float chamber. Under the discharge ports, the volume decreases, forcing air out.
Cooling water is necessary to prevent overheating of the sealing water from the action of the vanes in the liquid. Interruption of the coolant supply results in vapour from the sealing water destroying the vacuum effect, so that air is no longer pumped.
The internal passages of a typical air pump are shown in the sectional sketch. The right side shows the operating passages and the path of the air being pumped. It is drawn from the suction float chamber of the main pump and through the pipe and passages to the suction ports of the primer. The discharge ports are not shown but the air from them is discharged into the top of the outer casing. From the outer casing the air is discharged to atmosphere.
The primer runs continuously but can be unloaded, when the pump has been primed, by the arrangement on the left. The shut – off handle rotated a ported chamber to the position shown, so that the partial vacuum is broken and the water is free to circulate.
Centrifugal pumps for blige, ballast and general service are usually fitted with primers. Before starting such pumps, the primers must be checked, to ensure that the sealing water is at the correct level. Fresh water is used for topping up. Suction valves between the liquid and the pump are opened and a check is made that other valves on the suction side of the system are closed. The delivery valve is kept shut and the pump is started. The centrifugal pump can be started with the discharge valve closed, but it is an exception. If priming takes a long time, the primer will become hot unless cooling water is passed through it. The sea water suction can be opened to allow cooling.
EXHAUSTER FOR CENTRAL PRIMING SYSTEM
Several centrifugal pumps can be primed from a central vacuum tank as an alternative to being fitted with individual water ring primers. The pumps are connectedto the vacuum tank through the same sort of float chamber arrangement as is used with the individual water ring primer. There is also a shut off cock for isolating the pump and a non – return valve.
The primer has two electricity driven air pumps which evacuated the tank. Starting is by means of pressure switches through suitable starters, and air pumps are automatically stopped by the switches when the required vacuum is reached. The pumps run intermittently as demand makes for a number of pumps and the running time for them.
The primer shown, consists of a gunmetal casting of oval shape with a rotor and ported plate, as described for the individual water ring primer. Water in the casing forms a seal and, because it takes up the elliptical shape of the casing when the rotor turns, produces a pumping action. The two suction ports are connected by passages in the cover to the suction pipe from the vacuum tank. The two discharge ports are connected via an aperture to the sealing water tank. Air from the vacuum tank, together with make – up water, is drawn into the suction and discharged to the sealing water tank. The water remains in the tank, while the air passes to atmosphere through the outlet pipe. The sealing water reservoir also keeps the air pump cool and is cooled in turn, by a sea water flow through the cooling coils shown. If the water ring temperature rises, then the function of the primer will be destroyed by the presence of water vapour.