Marine Auxiliary Machinery 7E Episode 5 ppsx

150 Pumps and pumping to mechanical seals from the lowest point on the pressure side of the pump, to ensure that some liquid reaches them, even when priming. Special header tanks have been installed for the seals in some applications. They must not ran dry and care must be taken to prevent ingress of foreign matter. Many mechanical seals incorporate a carbon face and there is a possibility of electrolytic action in the presence of sea water. Soft packing may be preferred in sea-water pumps, Stuffing box type glands (Figures 5.8c and 5.8d) may be packed with soft or metal foil type packing. Pump internal bearings may be lubricated and cooled by the pumped liquid in situations where liquid is always available when the pump is running. Lubricators for the application of grease are fitted in some circumstances. Pumps used for slurries or those for suction dredgers, have external bearings and a nozzle located to exclude solids from the gland area. Materials Pumps like pipelines, are used for high or low temperature liquids, those which are corrosive and some that carry abrasive particles. The materials chosen for pump construction must be suitable. (a) Pumps for engine cooling water, fresh or potable water: high grade cast iron casings with bronze internals — shaft of bronze or stainless steel {EN57. 18 Cr/2Ni) the latter material gives better wear life. (b) Sea-water pumps: (these must also handle harbour, river and canal water) may have gunmetal casing with aluminium bronze impeller (BS 1400 AB2 9: 5AI - 5 Fe - 5.5 Ni) - shaft either stainless steel (EN57) for soft packed stuffing boxes or EN58J (18Cr — lONi — 3 Mo) under mechanical seals or bearings (c) Boiler feed pumps: because of the high pressures and temperatures casings are of cast steel - shafts and impellers of stainless steel. (d) Cargo pumps: stainless steel casings, impeller and shaft suitable for most chemicals — nickel steels for low temperature liquefied gas. Priming Pumps may be mounted above the level of the liquid to be pumped even though placed low in the ship. Bilge pumps for this reason, must be self-priming or equipped with a means of priming, to create a vacuum in the suction pipeline. Ballast and other pumps (which may also be statutory bilge pumps) may need to be self-priming or equipped with a means of priming. Cargo pumps for oil tankers (see Chapter 6) are likely to be arranged for stripping the maximum amount of liquid from tanks. A centrifugal pump placed above the liquid to be pumped, is not self-priming because it cannot exhaust the air contained in its casing and suction pipe. A displacement pump can and is self-priming. A centrifugal pump Pumps and pumping 151 must be placed below the level of the liquid to be pumped (when it will flood if valves are opened) or it must be provided with an external device for the removal of air. Air handling methods The removal of air from pump suction pipes is usually achieved with a liquid ring primer. This is necessary in order to produce vacuum conditions, so that atmospheric pressure on the surface of the liquid to be pumped will promote flow into and priming of the pump, The liquid ring air pump (Figure 5.9) consists of a bladed circular rotor, shrouded on the underside, which rotates in an oval casing. Sealing water is drawn into the oval casing through a make-up supply pipe (in older types is was added through a plug). The water, thrown out to the casing periphery by the turning rotor, whirls around, to form a moving layer against the oval casing. The water seals the rotor blades and also recedes from and re-approaches the rotor boss twice in each revolution. The effect is to produce a series of reciprocating water pistons between the blades. As the water surface moves out from the rotor boss, it provides a suction stroke and, as it moves in, a discharge stroke. The shaped suction and discharge ports, provided above the elliptical core formed by the rotating water, permit air to be drawn in from the main pump suction pipe float chamber and expelled through the discharge ports, to atmosphere. A continuous supply of sealing water is circulated from the primer reservoir to the whirlpool casing, and discharged with the air back to the reservoir. The air passes to atmosphere through the overflow pipe. This circulation ensures that a full water-ring is maintained and the cooling coil incorporated in the reservoir, limits the temperature rise of the sealing water during long periods of operation. The supply for the cooling coil can be taken from any convenient sea-water connection. About 0.152 litres/s is required at a pressure not exceeding 2 bar. The air handling capacity of water ring primers is good, with air gulps being quickly cleared and small air leakages being handled without any fall in pump performance. This type of primer, replaced the now obsolete reciprocating pump primers. Eccentric vane primers have good air handling capability but vanes wear and they sometimes jam in the rotor slots. Ejectors are effective if sized correctly but their efficiency is low. Float chamber The water ring primer draws air from the pump suction pipe, through a float chamber (Figure 5.9). The float rises as liquid replaces air and as the level rises well above the pump the impeller and casing are flooded and the float spindle closes off the suction. This ensures that the primer itself is not flooded. 152 Pumps and pumping Figure 5.9 Section arrangements of liquid ring primer (Weir Pumps Ltd.) 1. Air purnp casing pumps only) Motor half 9. Float gear cage 2. Air pump top cover coupling (long coupled 10. Needle valve 3. Rotor pumps only) Combined 11. Needle valve seat 4. Mechanical pump seal pump and motor shaft 12. Ball float spring (close coupled pumps only) 13. Pendant Stationary sealing ring 6. Separating chamber 14. Bridge piece Shroud 'O' ring 7. Sealing water tank 15. Roll pins 5. Pump shaft and coupling 8. Cooling coil 16. Spring Flange (long coupled Pumps and pumping 153 Central priming system This system may be used when more than four pumps require priming facilities. It gives a large air exhausting reservoir as well as capacity greater than individual pumps can carry. Pump casings can be filled with liquid before starting. The air exhausting units are usually of the liquid ring type. A typical schematic arrangement is shown in Figure 5.10. A float chamber arrangement is also used with central primers to prevent flooding of the priming unit. General purpose pumps Single entry general purpose pumps (Figure 5.11) are used for salt and fresh water circulating and also for bilge and ballast duties. The impeller is suspended from the shaft with no bottom support. A neck bush provides lateral location. The eye of the impeller faces downwards, to the suction inlet below the impeller. There are renewable wear rings, usually of aluminium bronze, located at the top and bottom around the collars or boss of the impeller. The clearance between the wear rings and collars is minimal to restrict the flow of liquid from the discharge side. A short circuit flow would reduce efficiency and pressurize the shaft seal. In this design access for maintenance is via the top cover. A distance piece arranged in the shaft, is removed to permit the impeller, shaft and cover, to be lifted out without disturbing the motor or the pipework. Figure 5.12 shows a different design of a pump intended for similar duties. The impeller is arranged with its eye uppermost, with the suction branch being elevated. This arrangement is claimed to give better venting to eliminate any possibility of vapour locking. Another significant design difference is that the casing is split vertically so that the impeller and shaft can be removed sideways. The wear rings and neck bush fitted in this design are stepped to abut with the removable part of the casing, to prevent them from turning. Where a single entry pump is to be employed to supply a large pressure head, an impeller of a greater diameter (Figure 5.13) can be used. The model shown also has a vertically split casing and an impeller eye which faces upwards. The added lower guide bush is deemed necessary for the larger diameter impeller. A novel design of single entry pump (Figure 5.14) was produced for ease of maintenance and adaptability. In this pump the impeller eye faces downwards but the impeller is open-sided, with the bottom of the pump casing effectively shrouding the vanes. This design allows the motor and cover of the pump to be tilted on a hinge so that the operation of a simple screw jack exposes the internal parts. A mechanical seal prevents water leakage or air ingress. This type of pump was designed for a wide range of capacities through simply fitting an impeller of suitable diameter and tip width. As previously stated, the performance of a centrifugal pump is dictated by speed of rotation, the impeller diameter and the area of the flow passage through the impeller (or width). The Figure 5,10 Layout showing central priming system (Hamworthy Engineering Ltd.) Pumps and pumping 155 Figure 5.11 Typical single stage centrifugal pump (Hamworthy Engineering Ltd.) first two variables basically control the pressure generated and the last the quantity of liquid delivered. For a constant speed pump, a set of performance curves (Figure 5.15) can be obtained, which reflect operation with different impeller diameters. A similar effect would be produced by using different speeds. Variation of capacity at constant speed and diameter, facilitated by fitting impellers of different widths (Figure 5.16) produces a different type of performance variation. These curves also have points where performance is at its highest. Obviously, by altering impeller diameters and widths, a pump can be tailored to requirements. A two-stage pump (Figure 5.17) may be installed as a fire and general service pump. Because both low and high head are available from the one pump, it can be readily used for a double duty. Lower head is obtained by pumping through the first stage impeller only and a higher head when pumping through both impellers. Erosion by abrasives A pump handling liquids which contain abrasives, will suffer erosion on all internal surfaces, including bearings and shaft seals. The sea-water circulating pumps of ships operating in waters that contain large quantities of silt and sand 156 Pumps and pumping Figure 5.12 Single-entry pump giving throughput of 425 m 3 /hr against heads of up to 54m (Weir Pumps Ltd.) 1. Pump casing and cover 7. Gland 2. Impeller 8. Packing 3. Casing ring (bottom) 9. Lantern ring (split) 4. Casing ring (top) 10. Neck bush 4A. Locking pins 11. Water service pipe to stuffing box 5. Pump spindle 12. Motor stool 6. Coupling (motor half) 13, Pump foot require frequent renewal of shaft seals or packing, also of shaft sleeves in way of the gland and bearings. Impellers are sometimes extensively damaged with resulting perforations and massive enlargement of wear ring clearance. Pump casings suffer erosion on all internal surfaces. Impellers and wear rings may have to be changed frequently and casings may need to be renewed at longer intervals. Special provision can be made in such pumps or those employed in suction dredgers (Figure 5.18) to safeguard bearings and shaft seals. The protection is provided by a water service to the shaft which washes solids away from the shaft seal area. Bearings can be protected by being mounted external to the casing. The pumps designed for use in suction dredgers require frequent casing repairs: a solution has been provided by one manufacturer with a casing which is built from renewable parts. Erosion due to cavitation as opposed to the presence of abrasives is selective. The problem occurs, as with cavitation damage on propeller blades, in certain areas where cavitation pockets or bubbles collapse. Pumps and pumping 157 Figure 5.13 Large diameter impeller for large pressure head (Weir Pumps Ltd.) 1. Pump casing and cover 10. Lantern ring (split) 2. Impeller 11. Neck bush 3. Casing ring (bottom) 12. Motor stool 4. Casing ring (top) 13. Bottom bush housing 5. Locking pins 14. Bottom bush liner 6. Pump spindle 15. Water service pipe to bottom bush 7. Coupling (motor half) 16. Water service pipe to stuffing box 8. Gland 17. Pump foot 9. Packing Centrifugal pump cavitation During operation, if the drop in pressure created at the suction side of a centrifugal pump (by liquid moving radially outwards from the eye of the impeller) is greater than the vapour pressure for the temperature of the liquid being pumped, then vapour will be drawn from the liquid in this area. The phenomenon is likely to occur if there is a restriction in the suction pipe, if the liquid is volatile or has a higher temperature than anticipated, or if the impeller speed is excessive. A vapour cavity of this type is likely to cause loss of suction or erratic operation. A lesser cavitation problem occurs when NPSH required by the pump is only just matched by the NPSH available from the system, because centrifugal pumps have features which promote localized cavitation. The impeller entry on the side away from the shaft has a profile which resembles that of a hydrofoil, and local liquid flow creates a drop in pressure 158 Pumps and pumping Figure 5.14 Single-entry pump with open-sided impeller (Weir Pumps Ltd., 1. Pump casing 6. Shims 2. Casing cover 7. Mechanical seal 3. Impeller 8. Combined pump and motor shaft 4. Casing ring 9. Screw jack 5. impeller locking screw 10, Air release plug Figure 5,15 Performance curves for centrifugal pump impellers of different diameters but running at the same speed Pumps and pumping 159 Figure S.16 Performance curves for different width impellers of the same diameter and constant speed which starts at the effective leading edge and extends along the surface. A vapour pocket created by a drop in pressure at this surface would collapse in an area of higher pressure and cause cavitation damage. A change of flow direction from axial to radial causes the fluid to experience different velocities and at the extremity, a drop in pressure could produce cavitation, again with subsequent bubble collapse and damage. The types of cavitation described could cause surface roughening and generate some tell-tale noise. Corrosion wastage could cause impeller wastage but the cast iron casing of a sea-water pump is a prime target. Centrifugal pumps for lubricating oil duties Because of their self-priming ability, positive displacement pumps are widely used for lubricating oil duties. This practice is completely satisfactory in installations where the pump speed is variable but when the pump is driven by a constant speed a.c. motor it is necessary to arrange a bypass which can be closed in to boost flow. By using a centrifugal pump with an extended spindle, such that its impeller can be located at the bottom of the oil tank, the H/Q characteristics of the centrifugal pump can be utilized without the priming disadvantages. Known as the tank type pump this pump has a small open impeller. It can be driven directly by a high speed alternating current motor without capacity restrictions, whereas the permissible operating speed of the [...]... centrifugal pumps although centrifugal force plays no useful part in the pumping action A comparison of discharge characteristics (Figures 5. 20 and 5. 21) shows that H/Q and working efficiency characteristics for the two pumps are quite different The discharge characteristics (Figure 5. 20) drawn in each case for constant speed show those for the axial flow pump with a solid line and those for the centrifugal with... condition for overload of an axial pump The axial pump (Figure 5. 21) retains reasonable efficiency over a wider head range, than the centrifugal pump There are three other features of the axial flow pump not indicated by the graph but of particular importance in their application These are: 1 Under the low head (2 .5 to 6.2 m) high throughput (2800- 950 0 mVhr) conditions commonly required by main condensers,... pressure the volumetric efficiency should be 100% but as the differential pressure increases (Figure 5. 24) the amount of leakage (slip) through clearances will increase This Figure 5. 23 A two-screw displacement pump (Weir Pumps Ltd.) 1 Mechanical seal 2 Timing gear (driving) 3 Timing gear (driven) 4 Upper bearing 5 Lower bearing 6 Valve body 7 Relief valve spindle Pumps and pumping 167 slip (the terminology... keep the pitch within limits set by the field requirements for suction performance Screw pumps Both double-screw pumps, in which the screws are driven in phase by timing gears (Figure 5. 23), and triple screw pumps (Figure 5. 25) , in which the centre screw is driven and the outer screws idle are used at sea especially for pumping high viscosity liquids such as oil and some liquid cargoes Being self-prirning... designs incorporating outside bearings (Figure 5. 26) which can be independently lubricated would be used Pumps with inside bearings are shorter and lighter than their outside bearing counterparts and have only one shaft seal as against four 170 Pumps and pumping Figure 5, 26 Counterscrew pump for oil or water service Shaft sealing The double-screw pump (Figure 5. 23) shaft sealing is effected by either mechanical... Pumpheads are usually of the piston or plunger type (Figure 5. 29a) where the purnp is to be used against high pressures For lower pressure duties the diaphragm version (Figure 5. 29b) 174 Pumps and pumping Figure S.29 Metering pump (a) Typical plunger head for MPL Type Q pump; (b) Typical diaphragm head for MPL Type Q pump Pumps and pumping 1 75 is generally used The plunger model is more exact in its... pressure liquids They are suitable for operation at high rotational speed (units are in operation with speeds of 350 0 rev/min, delivering over 1000 litres/min) and can thus be easily matched with standard electric motors Performance characteristics of screw pumps are illustrated in Figure 5. 24 In the IMO triple screw pump the centre screw is driven mechanically, through a flexible coupling The two outer... Double-screw pump with timing gears This type of pump can be mounted either horizontally or vertically Pumping is effected by two intermeshing screws rotating within a pump casing Each Pumps and pumping Figure 5. 25 169 A triple-screw displacement pump (IMO Industh) screwshaft has a right and a left hand screw which ensures axial hydraulic balance, there being no load imposed on the location bearing Metal contact... centrifugal feed pump must not be operated unless it is fully primed The pump casing should be filled before starting, the suction pipe and pipe branch Pumps and pumping Figure 5. 18 161 Dredge pump (GEC-Elliot Mechanical Handling) Figure 5. 19 Comparison of centrifugal and positive displacement pump characteristics with respect to lubricating oil duties to the discharge stop valve must also be full If the... or erosion of parts, due to friction contact or the presence of abrasives, is avoided by employing this type Figure 5. 22 Hydrofoil and supercavltating blades 166 Pumps and pumping of pump for specialized rather than genera! duties Contact between elements in some screw pumps (Figure 5. 23) is made unnecessary by gear drives When used for lubricating oil and hydraulic systems, rotary displacement pumps . aluminium bronze impeller (BS 1400 AB2 9: 5AI - 5 Fe - 5. 5 Ni) - shaft either stainless steel (EN57) for soft packed stuffing boxes or EN58J (18Cr — lONi — 3 Mo) under mechanical. the impeller (or width). The Figure 5, 10 Layout showing central priming system (Hamworthy Engineering Ltd.) Pumps and pumping 155 Figure 5. 11 Typical single stage centrifugal. large quantities of silt and sand 156 Pumps and pumping Figure 5. 12 Single-entry pump giving throughput of 4 25 m 3 /hr against heads of up to 54 m (Weir Pumps Ltd.) 1. Pump casing