附录(一):Optimizing centrifugal pump operationCentrifugal pump operation is more than switching the pump on and directing the discharge flow to the required delivery point. This holds true even when a control value is installed in the discharge line for the purpose of flow or level control .A few very essential operating guidelines must be adhered to if early bearing or seal failure, premature erosion of internal wetted surfaces, or metal-to-metal contact of internal rotating and stationary surfaces, is to be avoided. In this article, Stan Shiels addresses the fundamentals of proper centrifugal pump operation and the most common areas of improper centrifugal pump operation. It is assumed that the pump has been successfully commissioned.The most common cause of centrifugal pump failure, separate from those causes associated with maintenance and/or design, but associated with how the pumps are operated may be summarized as follows:1. Insufficient suction pressure to avoid cavitation.2. Excessively high flow rate for the net positive suction head available (NPSHA).3. Prolonged operation at lower than acceptable flow rates.4. Operation of the pump at zero or near zero flow rate.S. Improper operation of the pump in parallel.6. Failure to maintain adequate lubrication for the bearings.7. Failure to maintain satisfactory flushing to mechanical seals.Insufficient suction pressure to avoid cavitation.While the provision of sufficient suction pressure to avoid cavitation may seem straightforward, requiring only that the net positive suction head available(NPSHA) be always greater than or equal to the net positive suction head required (NPSHR) by the pump, some misconceptions need to be revealed. When the pump manufacturer develops the pump's NPSHR curve, the calculated values of NPSHR are for conditions where theexpected head (as determined by the previously performed pumphydraulic performance test) has fallen of by 3% for that specific flow rate. This means that mild cavitation will exist when the NPSHA equals the NPSHR. Because of this a comfort margin of 3 feet, or 1meter, isrecommended, when determining the minimum acceptable NPSH margin:(NPSHA-NPSHR).The following piping faults will require an NPSH margin greater than 3 feet(1 meter) to ensure the absence of cavitation.~Where the inlet piping configuration is such that a number of 90 degree turns occur, in different planes, fairly close to the pump suction, the resultant fluid swirl will cause cavitation to occur, on occasion, even when the NPSH margin is 2 to 3 feet.~Where an eccentric inlet reducer is positioned in the pipe such that itforces asymmetric flow into the suction of a double suction pump. The side of the double suction impeller that receives the disproportionately higher percentage of the total flow will incur cavitation at an NPSHA much higher than the NPSHR indicated on the manufacturer's test curve.Excessively high flow rate for the NPSH availableBased upon the simple fact that NPSHA decreases as flow rate increases, and NPSHR increases as flow rate increases, there should be considerable effort place on ensuring that the point of intersection of these two parameters is not reached. This concept is illustrated in Figurel.Many centrifugal pumps are installed in a system, which cannot pro vide adequate suction pressure to avoid cavitation, if operated at flow rates much above their best efficiency point (BEP). It is probably never intended to operate these pumps in such a region, but transient conditions can often lead to intended operation at much higher than expected flow rates. One example is the case where a recycle control value has been installed to ensure that the pump is not operated below a minimum acceptable flow rate. At very low or zero process flow the recycle control value may attain a position of greater than 50% open. This of itself is not a problem, but the extremely high flow rate may occur when the process control value is again asked to open, and does so rather quickly. When the process control valve is faster acting than the recycle control valve, the combined process and recycle flows may necessitate a higher NPSHR than the suction system can provide.Manual operation must also be handled carefully when simple transfer operations between vessels is called for. At the start of such operation the receiving vessel may have a very high level. This leads to a negative static head, which can lead to an initial high flow rate until the level rises in the discharge vessel. This is a particularly risky operation where piping system resistance is low on the discharge side, but a relatively long section of suction piping exists.Prolonged operation at lower than acceptable flow ratesA centrifugal pump is most comfortable operating between 85% and 110% of its best efficiency point (BEP). However, by far the great majority of centrifugal pumps are forced to operate outside of this range. The degree to which it is acceptable to operate outside of this range is a function of two primary parameters:Suction Specific Speed (Ss) and Specific Speed (S). These parameters may be calculated for a pump provided the following data is known: For maximum diameter impeller:BEP FlowNPSHR at BEP FlowHead at BEP FlowPump Operating Speed (RPM)The following formulae allow Ss and S to be calculated:()0.50.750.50.75()()()bep bepbep bep SpecificSpeed S REP Q H SuctionSpecificSpeed Ss RPE Q NPSHR=⨯÷=⨯÷Where: QPM is pump rotational speed in revolutions per minute.bep Q flow at BEP for maximum diameter impeller.bep H is pump head at BEP for maximum diameter impeller.bep NPSHR is the pump net positive head required at BEP for maximum diameter impeller.The acceptable minimum continuous flow rate, which a centrifugal pump can be expected to endure without incurring damage or premature failure is also affected by a number of other parameters: such as, impeller head, fluid specific gravity , and percentage of total running time spent at the lower flow rate. When the pump's specific speed and suction specific speed are known the approximate percentage of the pump's maximum impeller diameter BEP flow at which suction recirculation can be expected to occur may be estimated.From this estimate the approximate minimum acceptable continuous flow rate for that specific pump may be estimated. Impeller geometry can be optimized to improve on initial estimates and, when conditions are marginal, it is always advisable to consult with the manufacturer's application engineer before deciding on the acceptable minimum continuous flow rate.A subsequent article can expend further on this much debated area of centrifugalpump operation. In the interim it is suggested that pump users request the minimum "stable" continuous flow rate recommended from the pump manufacturer, based upon the onset of flow instability .Impeller geometry can impact the "Cavitation Coefficient" which is defined by the following formula:211(2)u g NPSH u δ=⨯÷Where1u δ is the cavitation coefficientg is the gravitation constantNPSH is net positive suction head1u is fluid circumferential velocity at impeller entryThis is a dimensionless coefficient and would be valid for all geometrically similar pumps, independent of their size and speed of rotation.Operation of the pump at zero or near zero flow rateCentrifugal pumps are often used to pump out vessels, often to zero level. The pumps can, at times, be left unattended. If the level is allowed to fall to the point where the pump is allowed to run dry , failure of the mechanical seal will often follow rapidly. In such applications even a small recycle flow back to the suction vessel will not alleviate the problem, as the pump will still be able to empty the vessel. Many such applications are best protected by the installation of dual pressurized mechanical seals which will remain lubricated even during periods of completely dry running. In non hazardous applications a pump sealed with packing will survive better if the packing is lubricated from an external source; the source must, of course, be compatible with the fluid being pumped.Where batch delivery is normal operation, such that the pump will operate for long periods, but delivery is intermittent, a small continuous recycle flow back to the suction source will help to protect the pump during periods when delivery stops (typically by closing the delivery valve). Lack of some of the recycle protection for a centrifugal pump, which will see periods of zero flow, will certainly cause frequent pump failure. Any centrifugal pump operated at zero flow for even a few minutes will vapour temperature. It is worth remembering that once a pump has vapour-locked it will not generate any noticeable discharge pressure differential. Usually it is necessary to stop the pump and allow the gas to condense back into liquid, before pumping can resume.Improper operation of the pump in parallel.This topic has been covered in detail in a previous "Pump Academy" article, but the basics are worthy of mention in terms general centrifugal pump operating philosophy. The following principles should be applied to any operation of centrifugal pumps, which requires either continuous or intermittent parallel operation.~Shut-off heads of all pumps operating in parallel should be comparable typically as close as possible, with difference of no more than 2%or 3% recommended~V ery flat performance curves in one or more of the pumps operating in parallel is to be avoided;a drop of 10% to 20% between the shut-off and rated points is recommended. The point of hydraulic shut-off of the more worn pump of two initially identical pumps operating in parallel is shown in Figure 2, for high head rise to shut-off and low head rise to shut-off.~Multistage pumps, or expensive single-stage or two-stage pumps designed to operate in parallel, should be protected by low flow shut-down devices to avoid severe damage from occurring at transient low combined flow conditions. While the pumps may be identical, performance differences occur over time, and the better performing pump will effectively "shut-off" the weaker pump below a specific combined flow rate. The low flow shut-down will prevent the major damage that often results from such occurrences.~inlet and discharge piping configurations and lengths should be comparable between the pump and the suction and discharge headers. Proper piping configuration for pumps operating in parallel should include suction and discharge headers of larger diameter than the lines leading to and from the individual pumps. Differences in suction and/or discharge piping configuration will always lead to a disparity in pump flow rates.Failure to maintain adequate lubrication for the bearingsThis is a simple statement and sounds too fundamental to be ignored, but, even among those diligent pump operators who strive to maintain adequate lubrication, some inadvertent mistakes are made. Lubrication of rolling element bearings is the subject here. They may be oil lubricated or grease lubricated. The complete topic requires more attention than can be afforded in this text, but the basic essentials always hold true and can only be ignored at the risk of early pump failure.Oil lubrication is of three basic types: forced feed with a pressurized filtered and cooled oil supply to the bearings in a closed loop system; oil bath; and oil mistThe forced feed system is the most complicated and usually incorporates a low oil pressure shutdown protection; it is normally only present in large multistage pump installations. The key areas of concern are:~Maintenance of the correct viscosity of the oil and periodic monitoring for changes in viscosity and/or Total Acid Number (TAN) and the presence of water in the oil.~On occasion ferrographic analysis for the presence of wear particles helps to diagnose impendent bearing damage.~Lube oil cooler performance decline may lead to elevated temperatures and periodic oil temperature monitoring is also essential.~Oil filter differential pressure and main oil supply pressure to the bearings also requires frequent attention.A housing for oil bath lubrication normally incorporates a constant level oiler, although some pump users have chosen to seal their bearing housing and use synthetic oil, changing the oil only once every two or three years. For those bearing housings incorporating a constant level oiler the following basic principles apply:~Use of the correct viscosity oil for the operating temperature of the bearing (refer to the bearing manufacturer's literature).~Maintenance of a reserve of oil in the constant level oiler.~Clean storage of the make-up oil to prevent foreign material and/or moisture from entering the bearings.~Observance of the deterioration in the oiler level一remember, oilers whose level never falls over time may have a blocked connection between the oiler and the bearing housing.Oil mist lubrication requires that the same viscosity oil be used as for oil bath lubrication. On occasion a grade lighter oil may be used as the oil is being constantly replenished, although this approach should be taken with caution. While most oil mist systems incorporate an alarm for loss of oil mist pressure, each bearing housing should be periodically checked to confirm adequate venting of oil mist from the housing, and for proper condensation and drainage of bearing housing oil mist condensate collection pots.Grease lubrication of rolling element bearings requires proper attention to the following:~The type of grease used. A stiffer grease for higher speeds (3000 RPM and above) and a softer grease for lower speeds.~The grease should essentially contain the same viscosity of oil in it as that required for oil bath luibrication.~The amount of grease. This is of particular importance where pump operators are responsible for regreasing of bearings, as a major cause of failure of grease lubricated pump bearings is overgreasing. The idea that more is better is very wrong here.~Compatibility of greases. When regreasing it is essential that the grease to be added is compatible with the originally installed grease.As is the case for oil type selection, it is recommended that the bearing applications engineering department be consulted, when in doubt about which grease is best for each specific application.Failure to maintain satisfactory flushing to mechanical sealsThe flushing plan for a mechanical seal is the means of controlling the environment in which the seal operates. It follows then that any event that alters the intended flushing flow parameters will alter the seal's environment and may lead to seal failure. The following parameters, for some of the most commonly utilized seal flushing arrangements, should be regularly monitored for mechanical seals:Pump discharge flush to the seal to confirm that the temperature is correct. The dischargeflush should be at the same temperature as the pump discharge flow. In hot service a cooler flush will indicate lack of sufficient flow; this is often the only means available of checking flow, as most discharge flush plans do not have a flow indication.Suction flush from the seal cavity to the pump suction should also be checked for temperature. This may require that the temperature be checked initially to set a "benchmark." Any noticeable increase in this flush return to the pump suction may indicate a flow restriction.Cooled discharge flushes require adequate cooling to attain the required flush temperature. Discharge flush coolers foul over time and often the flush temperature is allowed to rise to the point of seal failure before cooler fouling is discovered. Both the cooling water differential temperature and the flush supply temperature to the seal should be monitored and the cooler replaced of cleaned before fouling progresses too far.Dual unpressurized seals rely on a head of liquid in a seal pot to provide a small pressure differential between the seal interspace and atmosphere. Both the seal pot level and the seal pot pressure should be observed for signs of change. Dualunpressurized seals are often utilized in volatile fluid service, where the vapour pressure is above atmospheric and any normal primary seal vapour leakage can be vented through the seal pot orifice to a safe collection point; a noticeable increase in seal pot pressure would indicate a primary (inner) seal failure. A reduction in seal pot level would indicate an outer seal failure.Dual pressurized seals incorporating a seal pot similarly should be monitored. The seal pot pressure must be maintained at a pressure above the seal cavity pressure for satisfactory seal operation. A reduction in seal pot level will indicate either a primary or or outer seal failure; further observation for obvious leakage of the outer seal is required to determine which one has failed. Loss of seal pot pressure may indicate a failure of the pressure blanketing system. Alarms are usually incorporated in dual seal systems incorporating seal pots. It is still advisable to monitor these parameters to allow early warning of loss of adequate seal flushing.Dual pressurized seals utilizing a flow-through barrier fluid system should have regular checks carried out for barrier fluid inlet and outlet temperatures and pressures. If the barrier fluid return pressure is allowed to drop to a value below the seal cavity (pump side) pressure, seal failure will follow in a pump media containing abrasives. Not all such systems have flow indication and a rise in differential temperature will usually indicate an increase in leakage rate of the inner seal, but could signify a reduction in barrier fluid flow rate; where flow rate is not subject to change it normally indicates an increase in inner seal leakage rate. A check of barrier fluid inlet pressure to the seal will assist in the diagnosis. A drop in barrier fluid return pressure, usually accompanied by arise in return temperature, is almost always associated with an inner seal failure.A last wordWhile there are many more causes of operational failure of centrifugal pumps, this discussion has dealt with the most frequently contributing areas. The operators of centrifugal pumps must extend their scope into the area of "ownership: Operators need to become familiar with tow these pumps operate, their key areas to monitor, and what to look for. It could also be said that many large process plants, which operate a large number of centrifugal pumps, do not provide sufficient pump training to permit the operating staff to understand key operating principles and satisfactory monitoring guidelines. We enjoy driving our automobiles. When have maintenance carried out: oil change, engine tune-up, transmission checked, brakes overhauled,tires renewed, fluid levels checked, hoses replaced, etc. most of us do not drive our cars until the oil breaks down completely and seizes the engine, the coolant hose blows, the brakes fail, the tires lose their tread, or the transmission fails. Periodic checks allow us to avoid such failures. We own our cars and do not want to incur the cost and inconvenience of such failures. A similar frame of mind toward centrifugal pump operation, supported by a proper program of operator training will pay dividends in terms of pump reliability and the cost of pump operation, and avoid some costly process debits.附录（二）：优化离心泵的操作操作离心泵不仅指打开水泵阀门、接通排水管、导出排放液体到规定传输点还包括在排水管中安装控制阀以控制调节流量和液位等为了避免早期轴承或密封失效、内部潮湿表面过早浸蚀、以及内部旋转和固定部分金属表面发生摩擦等状况发生，在操作离心泵时须遵循一定的操作原则, 本文中，Stan Shiels给出了离心泵合理操作的基本法则和不正确使用离心泵的常见误区本文假设离心泵正常投入运行。
