Improving Pumping System Performance

Section 1: Pumping System BasicsSection 1: Pumping System BasicsOverviewPumps are used widely in industry to providecooling and lubrication services, to transfer fluidsfor processing, and to provide the motive force inhydraulic systems. In fact, most manufacturingplants, commercial buildings, and municipalitiesrely on pumping systems for their daily operation.In the manufacturing sector, pumps represent 27%of the electricity used by industrial systems. In thecommercial sector, pumps are used primarily inheating, ventilation, and air-conditioning (HVAC)systems to provide water for heat transfer. Municipalities use pumps for water and wastewatertransfer and treatment and for land drainage. Sincethey serve such diverse needs, pumps range in sizefrom fractions of a horsepower to several thousandhorsepower.In addition to an extensive range of sizes, pumpsalso come in several different types. They areclassified by the way they add energy to a fluid:positive displacement pumps1 squeeze the fluiddirectly; centrifugal pumps (also called “rotodynamic pumps”) speed up the fluid and convertthis kinetic energy to pressure. Within theseclassifications are many different subcategories.Positive displacement pumps include piston, screw,sliding vane, and rotary lobe types; centrifugalpumps include axial (propeller), mixed-flow, andradial types. Many factors go into determiningwhich type of pump is suitable for an application.Often, several different types meet the sameservice requirements.Pump reliability is important—often critically so.In cooling systems, pump failure can result inequipment overheating and catastrophic damage.In lubrication systems, inadequate pumpperformance can destroy equipment. In manypetrochemical and power plants, pump downtimecan cause a substantial loss in productivity.1 TermsPumps are essential to the daily operation of manyfacilities. This tends to promote the practice ofsizing pumps conservatively to ensure that theneeds of the system will be met under allconditions. Intent on ensuring that the pumps arelarge enough to meet system needs, engineersoften overlook the cost of oversizing pumps anderr on the side of safety by adding more pumpcapacity. Unfortunately, this practice results inhigher-than-necessary system operating andmaintenance costs. In addition, oversized pumpstypically require more frequent maintenancethan properly sized pumps. Excess flow energyincreases the wear and tear on system components,resulting in valve damage, piping stress, andexcess system operation noise.Pumping System ComponentsTypical pumping systems contain five basiccomponents: pumps, prime movers, piping, valves,and end-use equipment (e.g., heat exchangers,tanks, and hydraulic equipment). A typicalpumping system and its components are illustratedin Figure 1 on page 4. PumpsAlthough pumps are available in a wide range oftypes, sizes, and materials, they can be broadlyclassified into the two categories describedearlier—positive displacement and centrifugal.These categories relate to the manner in which thepumps add energy to the working fluid. Positivedisplacement pumps pressurize fluid with acollapsing volume action, essentially squeezing anamount of fluid equal to the displacement volumeof the system with each piston stroke or shaftrotation. Centrifugal pumps work by addingkinetic energy to a fluid using a spinning impeller.As the fluid slows in the diffuser section of thepump, the kinetic energy of the fluid is convertedinto pressure.in bold type are defined in the glossary in Appendix A.A Sourcebook for Industry

Section 1: Pumping System BasicsECDBIHGAKFJKeyABCDEFGHPumpLevel IndicatorsTank, Liquid SupplyPump MotorMotor ControllerThrottle ValveBypass ValveHeat Exchangers(End-Use Equipment)I Instrumentation LineJ Pump Discharge PipingK Pump Suction PipingFigure 1. Typical Pumping System ComponentsAlthough many applications can be served byboth positive displacement and centrifugal pumps,centrifugal pumps are more common because theyare simple and safe to operate, require minimalmaintenance, and have characteristically longoperating lives. Centrifugal pumps typically sufferless wear and require fewer part replacementsthan positive displacement pumps. Although thepacking or mechanical seals must be replacedperiodically, these tasks usually require only aminor amount of downtime. Centrifugal pumpscan also operate under a broad range of conditions.The risk of catastrophic damage due to impropervalve positioning is low, if precautions are taken.Centrifugal pumps have a variable flow/pressurerelationship. A centrifugal pump acting against ahigh system pressure generates less flow than itdoes when acting against a low system pressure. A centrifugal pump’s flow/pressure relationshipis described by a performance curve that plotsthe flow rate as a function of head (pressure).Understanding this relationship is essential toproperly sizing a pump and designing a systemthat performs efficiently. For more information,see the fact sheet in Section 2 titled CentrifugalPumps.In contrast, positive displacement pumps have afixed displacement volume. Consequently, theflow rates they generate are directly proportionalto their speed. The pressures they generate aredetermined by the system’s resistance to thisflow. Positive displacement pumps have operatingadvantages that make them more practical forcertain applications. These pumps are typicallymore appropriate for situations in which thefollowing apply:Improving Pumping System PerformanceA

Section 1: Pumping System Basics The working fluid is highly viscousThe system requires high-pressure,low-flow pump performanceThe pump must be self-primingThe working fluid must not experiencehigh shear forcesThe flow must be metered or preciselycontrolledPump efficiency is highly valued.A disadvantage is that positive displacementpumps typically require more system safeguards,such as relief valves. A positive displacementpump can potentially overpressurize system pipingand components. For example, if all the valvesdownstream of a pump are closed—a conditionknown as deadheading—system pressure willbuild until a relief valve lifts, a pipe or fittingruptures, or the pump motor stalls. Althoughrelief valves are installed to protect against suchdamage, relying on these devices adds an elementof risk. In addition, relief valves often relievepressure by venting system fluid, which may bea problem for systems with harmful or dangeroussystem fluids. For more information on this typeof pump, see the fact sheet in Section 2 titledPositive Displacement Pump Applications. Prime MoversMost pumps are driven by electric motors.Although some pumps are driven by direct current(dc) motors, the low cost and high reliability ofalternating current (ac) motors make them themost common type of pump prime mover. Inrecent years, partly as a result of DOE’s efforts,the efficiencies of many types of ac motors haveimproved. A section of the Energy Policy Act(EPAct) of 1992 that set minimum efficiencystandards for most common types of industrialmotors went into effect in October 1997. EPAct hasprovided industrial end users with greater selectionand availability of energy-efficient motors.In addition, the National Electrical ManufacturersAssociation (NEMA) has established the NEMAPremiumTM energy efficiency motors program,A Sourcebook for Industrywhich is endorsed by the Hydraulic Institute; theprogram defines premium efficiency motors withhigher efficiency levels than those established byEPAct. In high-run-time applications, improvedmotor efficiencies can significantly reduceoperating costs. However, it is often moreeffective to take a systems approach that usesproper component sizing and effective maintenance practices to avoid unnecessary energyconsumption.A subcomponent of a pump motor is the motorcontroller. The motor controller is the switchgearthat receives signals from low-power circuits,such as an on-off switch, and connects or disconnects the high-power circuits to the primarypower supply from the motor. In dc motors, themotor controller also contains a sequence ofswitches that gradually builds up the motorcurrent during start-ups.In special applications, such as emergencylubricating oil pumps for large machinery, somepumps are driven by an air system or directlyfrom the shaft of the machine. In the event of apower failure, these pumps can still supply oilto the bearings long enough for the machine tocoast to a stop. For this same reason, many fireservice pumps are driven by diesel engines toallow them to operate during a power outage. PipingPiping is used to contain the fluid and carry itfrom the pump to the point of use. The criticalaspects of piping are its dimensions, material type,and cost. Since all three aspects are interrelated,pipe sizing is an iterative process. The flow resistance at a specified flow rate of a pipe decreasesas the pipe diameter gets larger; however, largerpipes are heavier, take up more floor space, andcost more than smaller pipe. Similarly, in systemsthat operate at high pressures (for example,hydraulic systems), small-diameter pipes can havethinner walls than large-diameter pipes and areeasier to route and install.Small-diameter pipes restrict flow, however, andthis can be especially problematic in systems with

Section 1: Pumping System Basicssurging flow characteristics. Smaller pipes alsooperate at higher liquid velocity, increasingerosion effects, wear, and friction head. Increasedfriction head affects the energy required forpumping. ValvesThe flow in a pumping system may be controlledby valves. Some valves have distinct positions,either shut or open, while others can be used tothrottle flow. There are many different types ofvalves; selecting the correct valve for an application depends on a number of factors, such asease of maintenance, reliability, leakage tendencies, cost, and the frequency with which thevalve will be open and shut.Valves can be used to isolate equipment or regulate flow. Isolatio

Figure 10. Centrifugal Pump Performance Curves 37 Figure 11. Family of Pump Performance Curves 38 Figure 12. Performance Curves for Different Impeller Sizes 38 Figure 13. Performance Curves for a 4x1.5-6 Pump Used for Water Service 39 Figure 14. Multiple Pump Operation 44 Figure 15. Multiple-Speed Pump Performance Curves 45 Figure 16.