Lesson10MarinePumps第10课船用泵PPT课件

1、Lesson 10 Marine Pumps第10课 船用泵 A pump is a machine used to raise liquids from a low point to a high point. Alternatively it may simply provide the liquid with an increase in energy enabling it to flow or build up a pressure . The pumping action can be achieved in various ways according to the type o

2、f pump employed .The arrangement of pipework(管管道道), the liquid to be pumped and its purpose will result in certain s y s t e m r e q u i r e m e n t s o r characteristics that must be met by the pump. The pumps, which are used on board ship, can be divided into two main groups: displacement pumps(容积

3、式泵容积式泵) and centrifugal pumps(离心泵离心泵). They can be broken down into three classes: (1) reciprocating(往复式往复式), (2) rotary(回转式回转式) and (3) centrifugal. Under reciprocating are the direct acting(直接作用式直接作用式), power, and crank-fly wheel(曲柄飞轮式曲柄飞轮式) types. Under rotary are the gear(齿轮齿轮), vane(叶片叶片), scre

4、w(螺螺杆杆) and piston(柱塞柱塞) types. Under centrifugal are the volute(蜗壳蜗壳), diffuser(导轮导轮), mixed-flow(混流混流), axial flow(轴流轴流) and turbine or regenerative(旋涡旋涡) types. Displacement pump The displacement pumping action is achieved by the reduction or increase in volume of a space causing the liquid (or g

5、as) to be physically moved. The method employed is either a piston in a cylinder using a reciprocating motion, or a rotating unit using vanes, gears or screws. Fig. 10-1 Diagrammatic reciprocating displacement pump A reciprocating displacement pump is shown diagrammatically in Fig.10-1, to demonstra

6、te the operating principle. The pump is double-acting(双作用的双作用的), that is, liquid is admitted to either side of t h e p i s t o n w h e r e i t i s alternately(轮流地轮流地,交变地交变地) drawn in and discharged . As the piston moves upwards, suction takes place below the piston and liquid is drawn in, the valve

7、arrangement ensuring that the discharge valve cannot open on the suction stroke. Above the piston, liquid is discharged and the suction valve remains closed. As the piston travels down, the operations of suction and discharge occur now on opposite sides. A relief valve(安全阀安全阀) is always fitted betwe

8、en the pump suction and discharge chambers to protect the pump should it be operated with a valve closed in the discharge line. Reciprocating displacement pumps are self-priming(自吸式自吸式的的), will accept high suction lifts, produce the discharge pressure required by the system and can handle large amou

9、nts of vapor or entrained(夹带的夹带的) gases. They are, however, complicated in construction with a number of moving parts requiring attention and maintenance. When starting the pumps the suction and discharge valves must be opened. It is important that no valves in the discharge line are closed, otherwi

10、se either the relief valve will lift or damage may occur to the pumps when it is started. Screw, Vane and Gear pumps a r e e x a m p l e s o f r o t a r y displacement pumps(as shown in Fig.10-2, Fig.10-3, Fig.10-4). All types of rotary pumps continue to force liquid into the system regardless of th

11、e opposition(阻力阻力) to the transfer. Due to this reason, additional protection to these pumps is required or else the pump will continue to build up pressure and finally lead to rupture or damage to the pump. To avoid this relief valves are fitted in the pumps. These relief valves are capable of bypa

12、ssing (旁通旁通) entire throughput of the pump. Rotary pumps are highly sensitive pumps and therefore special care is needed in order to prevent any abrasive(磨蚀性的磨蚀性的) material from entering the pump which can damage the helices(螺旋线螺旋线) and clearances(间隙间隙) in the pump. The starting procedure of rotary

13、displacement pumps is similar to that for the reciprocating displacement pump. Fig. 10-2 Screw pump Fig. 10-3 Vane pump rotorstatorvanepump bodyFig. 10-4 Gear pump Centrifugal pump Centrifugal pumps are those where an impeller(叶轮叶轮) rotating at high speed throws the liquid by centrifugal force from

14、the center to the periphery(边缘边缘) of the impeller where the liquid is discharged through the delivery outlet.A vertical, single stage, single entry, centrifugal pump for general marine duties is shown in Fig.10-5 The pumping element is made up of a top cover, a pump shaft, an impeller, a bearing bus

15、h(轴承套轴承套) and a sealing arrangement(密封密封装置装置) around the shaft. The sealing arrangement may be a packed gland(填料密封填料密封) or a mechanical seal(机械轴封机械轴封) and the bearing lubrication system will vary according to the type of seal. Replaceable wear rings(承磨环承磨环,阻漏环阻漏环) are fitted to the impeller and the

16、casing. Centrifugal pumps, while being suitable for most general marine duties, are not self-priming and require some means of removing air from the suction pipeline and filling it with liquid. When starting a centrifugal pump the suction valve is opened and the discharge valve left shut: then the m

17、otor is started and the priming unit will prime the suction line. Once the pump is primed the delivery valve can be slowly opened and opening or closing the delivery valve can regulate the quantity of liquid. When stopping the pump the delivery valve is closed and the motor stopped. During the opera

18、tion of the centrifugal, several problems are faced which have to be looked into(了解了解,调查调查) during their operation in order to get trouble-free(无故障的无故障的) service from these pumps. The main problem, typical to centrifugal pumps, faced by these pumps is cavitation(汽蚀汽蚀), which is caused by the fact th

19、at these pumps operate with suction pressure much below the atmospheric pressure. This occurs mainly in the pumps used for transferring water. Other problems faced by these pumps are mainly due to normal wear and tear(磨损磨损) of the materials over a period of time. During the pump operation, as the wa

20、ter mass passes from eye of the impeller towards its periphery, it creates vacuum at the eye of the impeller and inside the suction pipe. If the temperature of the water passing through the suction pipe and the eye(中心中心) of the impeller happens to be higher than the saturation temperature(饱和温饱和温度度 )

21、 o f t h e w a t e r a t t h e prevailing(当前的当前的) vacuum, some of the water may evaporate (蒸发蒸发) to form tiny vapor bubbles(汽泡汽泡), which travel through the impeller. As the water mass passes from eye of the impeller towards its periphery, its pressure rises and the vapor in the tiny bubbles becomes

22、unstable(不稳定的不稳定的) and condenses(冷凝冷凝) back to liquid causing small implosions(向内破向内破裂裂) in these bubbles. These implosions in the tiny vapor bubbles in the water mass have enough energy to cause pitting(点点蚀蚀) in the adjoining(毗连的毗连的) material of the impeller as well as the pump casing. This process

23、 of impeller and the casing material wastage(损耗损耗) is called cavitation. As the pump operates at a particular available net positive suction head(净正吸入压头净正吸入压头NPSH,汽蚀余量汽蚀余量), the cavitation is found occurring at a particular location on the pump impeller and its casing. Available net positive suction

24、 head有效有效汽蚀余量，汽蚀余量， ha Required net positive suction head必需必需汽蚀余量，汽蚀余量，hrCritical net positive suction head临界临界汽蚀余量，汽蚀余量， hcTo prevent cavitation in any pump,ha 110% hr, ha hr+0.5m, hr hc+0.3m有效汽蚀余量ha 泵工作时实际所具有的汽蚀余量，取决于泵的吸入条件和液体的饱和压力Pv，与泵本身无关。它表示液体在泵进口处水头超过汽化压头的富裕能量。hagpsgvs22gpv=(+Z)mZ一般可忽略。 是为了使泵不

25、发生汽蚀，泵进口处所必需具有的超过汽化压头的能量即为必需汽蚀余量，也称最小汽蚀余量,用hr表示。也即为避免汽蚀所必需的汽蚀余量，取决于泵进口部分的几何形状以及泵的转速和流量，反映液体进泵后压力进一步降低的程度，它与与泵的吸入条件和液体的饱和压力Pv无关。临界汽蚀余量hr很难计算，用汽蚀试验确定：逐步增大吸入真空度，扬程或效率下降(2+K/2)%时的汽蚀余量称为临界汽蚀余量hc。hc加上不小于0.3m的余量定为必需汽蚀余量hr。使用时ha比hr具有大于10%(不小于0.5m)的余量。3 . 0crhhrahh%110(差值不小于0.5m)mhhra5 . 0The location may va

26、ry from pump to pump as different pumps operate at different net positive suction heads. After several years of use, the pitting causes the wastage in the impeller and the casing material sufficient to make it weak requiring replacement. The only remedy(解决办法解决办法) to prevent cavitation in any pump is

27、 to ensure that the temperature of the water passing through the suction pipe and the eye of the impeller is maintained below the saturation temperature at the prevailing vacuum. Apart from that, the inner surface of the casing may be coated with hard epoxy(环氧树脂环氧树脂) layer(层层) to minimize the effect

28、 of cavitation. 4 Other problems related to the centrifugal pump operation are mainly due to wear and tear in the moving parts due to friction . This will result in: Increase in the clearance between the impeller neck and the wear rings resulting in reduction in volumetric efficiency(容积效容积效率率). Repl

29、acement of wear rings is the only remedy for this. Damage to the impeller shaft in the way of gland packing(密封填密封填料料) which may be rectified by replacement of the shaft or, if the shaft is fitted with a sleeve(轴套轴套), by replacing sleeve. Wearing off(磨损磨损) damage to the bearings leading to increased

30、power consumed(消耗消耗) by the pump. This may be rectified(改正改正) by replacement of the bearings. Regular maintenance on the machine will involve attention to lubrication of the shaft bearing ensuring that the shaft seal or gland is not leaking liquid. Unsatisfactory operation or loss of performance may require minor or major overhauls(修理修理). Common faults, such as no discharge, may be a result of valves in the system being shut, suction strainers blocked or other faults occurring in the priming system. Air leaks in the suction piping, a choked(堵塞堵塞) impeller