survey of energy pump savings

7/30/2019 Survey of Energy Pump Savings

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Survey of Pump Energy Savings (2 PDH)

PDHengineer.com Course No. M-2020

Overview

This course is designed as a follow-on to Course No. M-3005, Centrifugal Pumps, alsoavailable through PDHengineer.com, however, it is also a stand-alone course for thosesomewhat familiar with pumps in general, and centrifugal pumps in particular.

The majority of technical articles and reports today dealing with energy and resourcesconservation involve nuclear power, redesign of industrial plants and infrastructure, and newfuels such as agricultural by-products and hydrogen. The purpose of this paper is to describesome potential cost and energy savings that are available today, and have been for years.

The design, selection, operation, and maintenance of centrifugal pumps is one of thesepotentials.

Introduction

The centrifugal pump is the second most widely used type of mechanical equipment in theworld; only the electric motor outnumbers it. There are literally millions of them in service inevery conceivable application. Fortunately, they are prime candidates for major energysavings, since the power required varies as the cube of the speed ratio; (i.e., cut the speed inhalf and reduce the power to one-eighth of the original value). The use of variable speed

drives alone has a tremendous potential for cost and energy savings. There are, however,many other areas where this is practical.

The following subjects are discussed in this paper:

System designSelection of pumpsPump construction and featuresPump drivesMaintenance

Field changes and operation

System Design

Careful attention to system design can eliminate unnecessary static head and pressure dropby minimizing fittings, valves, and strainers. Proper selection of heat exchangers can also


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reduce pressure drop, as can increased pipe size and selection of pipe materials. On a largesystem an economic analysis should be done to compare first the cost of larger piping,fittings, valves, and heat exchangers relative to the economic benefits of the power savedbecause of lower frictional losses. Special attention should be paid to selection of controlvalves, since they can be one of the largest pressure drop components in a system.

Reduce Specific Gravity and Viscosity Prior to the Pump

The power requirement of a pump is proportional to the specific gravity and the viscosity ofthe fluid being pumped. Many times it is possible to reduce both by raising the temperatureof the fluid. This often has to be done anyway as part of the process, so no there may be noincrease in energy required. Be careful, however, when raising the fluid temperature toensure that adequate NPSHa is maintained.

Increase available NPSH (NPSHa)

Most pump manufacturers have developed impellers with low NPSH required (NPSHr)characteristics, however, they generally have sacrificed efficiency to do this. By increasingthe NPSH available, a more efficient impeller may often be selected. Figure 1 shows theperformance of the same pump with a low NPSH and normal NPSH impeller

Figure 1. Normal and Large Ey ImpllrPerformance

400 to 600 FLOW(GPM)

SPEED 3550RPM

SPECIFIC SPEEDNs 1050

LARGEEYE

MOftMALIMPELLER

EFFICIENCY;NORMAL EYELARSE EYE

NPSHR: NORMAL

EYE LARGE EYE

300


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Note the difference in efficiency (5%) at the pump best efficiency point. Also, by increasingthe NPSHa, the speed of the pump may be increased, thus allowing the selection of a more

efficient specific speed pump. Care should be taken in this area, since the reverse can alsooccur; ie, an increase in speed could mean a less efficient specific speed, for example, goingfrom a radial to a mixed flow, or from a mixed flow to an axial flow impeller design.

Avoid Gas Entrainment

The presence of undissolved gasses, or gas that evolves during pumping reduces theperformance of the pump, and hence its efficiency. This reduced efficiency occurs becausethe hydraulic losses within the pump do not decrease at the same rate as the reduction incapacity and developed head. In fact, these gasses may increase the hydraulic losses due to

increased turbulence and hindered flow passages.

Eliminate Fixed Orifice in Recirculation or Bypass Lines

Most recirculation or bypass lines use fixed orifices that pass a constant flow from the pumpsuction source. Many of these orifices have valves upstream that open and close at presetflows to protect the pump. Thus there is a constant or intermittent flow through the bypassline with a commensurate power loss. There are available today modulating valves thatregulate the bypass flow so that the total flow through the pump is always above theminimum flow requirement of the pump.

Eliminate Oversizing by Proper Use of Margins

Of all of the poor design practices, this is probably the most prevalent. There are thousandsof pumps installed in systems with margins added to margins. Not only do the pumps runless efficiently because they require excessive throttling, but many times they cost morethan the properly sized pump because they are larger, and the off-peak operation increasesmaintenance on the pump and the control valve. This is graphically shown in figure 2 on thenext page.

Hydraulic Turbines

Centrifugal pumps have long been used as hydraulic turbines where there is a source of highpressure liquid that must be reduced to a lower pressure or to atmosphere. Pumps are quiteefficient energy recovery devices when used as a hydraulic turbine to convert high pressurefluid power into electricity to drive other rotating equipment.


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NOTE:A* DESIGN SYSTEMCURVE WITH 13YEAR OLD P!P

B* THROTTLED SYSTEM CURVE WITH EXCESS MARGIN

C* THROTTLED SYSTEM CURVE WITH NORMAL MARGIN

D* ACTUAL SYSTEMCURVE WITH NEW PIPE

Figure 2. Effect of Excess MarginonEnergy Consumption

Use Booster Pumps

Many times a system will have a number of users of the fluid, only a few of which require thefull operating pressure of the system. The remainder use valves to reduce the pressure totheir needs. It is often possible to design systems that use booster pumps to raise pressurefor those portions of the system that require it. Usually the power savings and the cost

reduction for the lower pressure main pumps more than offset the additional capital,installation and power consumption costs of the booster pumps.

Proper Pump Selection

In addition to selecting the proper pump from an application service standpoint, many timesit is possible to select the most efficient category of pump. The major pump categories are

IOOO


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centrifugal, rotary, and reciprocating. For instance, almost all reciprocating pumps of anycapacity are over 90% efficient, while very few centrifugal pumps are over 90% efficient, nomatter how large its capacity. In the normal area where a reciprocating pump is used; (i.e.,low flow /high pressure), the difference in efficiency between a centrifugal and areciprocating pump can be 40 or 50%. Of course, there are other considerations, such as

maintenance, first cost, and availability, but efficiency is a very important consideration.

Optimum Specific Speed

For the normal range of flows, say 200 to 100,000 gpm, the 2000 to 2500 specific speedrange will give the optimum efficiency and pump selection. It should be checked to see if it ispossible to be in or close to this range without sacrificing other desirable pump features, suchas minimum number of stages. See Figure 4 below.

Ns = 1000 2000 3000

Figure 4 Pump Specific Speed vs. Efficiency

Select the Most Efficient Pump


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These days seem to be times of standardization, and much of that is good; however,sometimes we decide on a type of pump such as ANSI chemical or AP 610 heavy duty processpump without evaluating the consequence in other areas such as cost and performance.Many times, a pump may be perfectly suitable for the service, but a more efficient pump maybe selected if preconceived ideas such as speed limitation and standardization are set aside,

or at least reconsidered.

Select Closed Instead of Open Impellers

A pump with a closed impeller is more efficient than a pump with an open impeller, all otherthings being equal. That is why it is not necessarily a good idea to use ANSI chemical pumpsindiscriminately, since most of them are furnished with open impellers as standard. Somemanufacturers do, however, offer open and closed impellers for the same pump.

Limit the Use of Specialty Pumps

Almost all specialty pumps, such as non-clog, self-priming, dredge, and so forth, compromisethe hydraulic performance because of the nature of the specialty. An example would be asmall non-clog pump in which the impeller is designed to pass a sphere of a substantial sizeso it will pump solids readily. The efficiency of this pump is substantially lower than the samesize pump designed for clean, non-corrosive liquid service. Therefore, the use of thesepumps should be limited to services where their specific design is required.

Use Multiple Pumps

On systems with a large range of flow requirements it is often desirable to install severalsmaller pumps than one single large pump. That way pumps can be shut down and restartedas the flow requirements decrease and increase, allowing the running pumps to operate atmore optimum conditions. An example is a cooling water system where the watertemperature changes with the seasons, and hence flow requirements vary over a large range.

Use of Mechanical Seals Instead of Packing

The use of mechanical seals instead of packing will result in a two-fold energy savings:

1. Reduced outleakage will result in improved efficiency. The lower leakage improveshousekeeping,and also is a savings if the liquid is of significant value. Even in a caseof a liquid such as demineralized water, the reduction in leakage with a mechanicalseal will result in a measurable savings over a years time. You can go through thecalculations yourself, using the estimated leakage for seal and packing suppliers.

2. Generally speaking, a mechanical seal will have lower friction losses than packing.With pressures in the seal above 350 psig, balanced seals should be considered, notjust for increased seal life, but to reduce the friction losses.


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Use grooved wearing rings and bushings

On any pump developing more than 400 psi per stage, whether new or old, serrated wearingrings and bushings should be investigated. Not only does this grooving reduce the internal

leakage from high to low pressure, assuming the same clearance as with straight rings, it alsoreduces the likelihood of seizure in event of contact. In some cases it is even possible toreduce the clearances, further reducing the leakage. This should not be done without carefulconsultation with the pump manufacturer, discussing such items as type of pump, service,duty cycle, wear ring and bushing materials.

Select Proper Materials

Generally it is possible to achieve a smoother surface finish with cast iron or bronze ascompared to cast carbon steel or stainless steel. In fact, some pump manufacturers reduce

their efficiencies up to five points when quoting pumps fabricated of steel. When it issatisfactory from a corrosion/erosion temperature and safety standpoint, it is preferable touse cast iron or bronze. Even if the pump manufacturer does not reduce the quotedefficiency, you can be sure a pump will be more efficient if constructed of cast iron or bronze.This is particularly true in smaller pumps where the surface roughness has a greater influenceon the hydraulic losses.

Consider using Coatings

In the same way, in many cases, coated or plastic pumps will be more efficient than their cast

metal counterparts. For instance, many vertical or turbine type pump manufacturers havefurnished enameled or porcelainized casing bowls for many years. Not only do these coatingsimprove efficiency to start with, but their efficiencies hold up because the enameled bowlsremain smoother for a longer period than the normal cast iron bowls.

Use of Low Capacity Impellers

Many pump manufacturers have several impeller patterns available for the same pumpcasings. It is possible, for instance, to select a low capacity impeller for the initial start andoperating phase, which may extend to several years, and then replace it with a normal

capacity impeller when the system capacity is increased. The same principle holds true whena low capacity impeller is selected for normal operation, then in the future when the plantcapacity is stretched, replace it with a normal or forcing impeller that is more efficient at thestretch rating.

A possible field-fix is to replace a normal capacity impeller with a low capacity impeller whenit is discovered that excessive margins have been added, and the actual pump requirements


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Speed increasers are normally used on motor drives to allow pump speeds in excess of thosepossible at 60 Hz. (Maximum is 3600 rpm with a two-pole motor). This generally allows theuse of a smaller, less expensive pump. It can also allow the selection of a pump with a moreoptimum specific speed, and a pump that operates closer to its best efficiency point.

Speed decreasers are generally used where the driver, such as a steam or gas turbine, run atmuch higher speeds than is optimum for the pump selected for a particular application, suchas an oil pipeline, or a low-head system where high speed is not required or desirable. Again,a more optimum specific speed may be selected, and the pump can operate closer to its bestefficiency point. Most times the increase in pump efficiency more than offsets the losses ofthe speed decreaser (Nominally 3%).

Use Multi and Variable Speed Motors or Other Variable Speed Devices

There are a variety of multi- and variable speed motors, controls, and other devices available

to take advantage of the centrifugal pump and other centrifugal machines capable of runningat varying speeds and exactly matching the system requirements. This paper cannot addresseach of them, but will only list them so the reader can investigate the pros and cons appliedto his particular requirement.

Variable speed motors:

Wound rotorD.C. motorVariable voltageVariable frequency

Brushless synchronous motors with speed control

Variable speed devices:

Variable diameter sheaves on V beltsEddy current couplingsHydrokinetic couplings

` Hydrodynamic couplingsHydroviscous couplings

Use More Efficient Electric Motors

Several motor manufacturers offer a line of NEMA frame size motors that are approximately2% more efficient than their standard line. They sell for a premium, which should beevaluated against the energy savings by the purchaser.


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Maintenance

Renew Internal Clearances

All pumps have internal clearances that serve as a leakage path between high and lowpressure areas in the pump. As the pump runs in normal operation, these clearances wearopen and the leakage increases, reducing the outflow of the pump and lowering efficiency.Generally a good rule of thumb is to rework the pump and restore original clearances whenthey have doubled. This will give an efficiency improvement of from 1 to 5% on the averagepump.

Proper Packing Adjustment

The proper adjustment of the packing has a two-fold benefit, both of which will increase theefficiency of the pump:

1. Excessive outleakage is not only a housekeeping problem, it can be energy wasteful ifthe packing is sealing above suction pressure; (i.e, it is developed within the pump)This excess leakage reduces the outflow of the pump, and hence the efficiency.

2. Excessive tightening of the packing will cause the packing to wear rapidly as well asthe shaft sleeve. It also reduces efficiency by increasing the friction losses.

Field Changes and Operation

Shut Down Unnecessary Pumps

Pumps are among the most reliable pieces of mechanical equipment. Substantial energysavings can be effected on multi-pump installations by simply shutting down one or morepumps. The savings are two-fold: Elimination of the energy cost for the shut down pumpand in most cases running the operating pumps closer to their best efficiency points.

See Figure 6 below.


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Replace Oversized Pumps and Drivers

There are many pumps and drivers operating in existing systems that are oversized for onereason or another; for instance, too much margin designed in or a change in the process orsystem. It may be possible to replace the pump and/or driver and pay for the change with

energy savings.

Trim the Impellers

Although it may not pay to replace the pump or driver, it may pay to cut the impeller to moreclosely match the pump with the actual system requirements. The impeller diameter followsthe same affinity laws as the pump speed; (i.e., the pump speed varies directly as the squareof the diameter ratio, and the horsepower varies directly as the cube of the diameter ratio).

Replace the Impeller

Many times the pump manufacturer has a number of impellers that fit the same pump casing,so it may be possible to select an impeller that operates at a much higher efficiency than theone presently in use. This is true even on older pumps where the manufacturer may havedesigned a new impeller since the pump was originally purchased, and it may more nearly fitthe actual field operating conditions.

Eliminate or Reduce Cooling Water and Injection

Bearing and packing or mechanical seal cooling and injection water may cost money and

energy to supply it. In most cases it can be reduced or even eliminated. The adage that alittle is good, and more is better is definitely wrong in the case of bearing cooling. Excessivecooling to bearings is more times than not, harmful to them and reduces their life. Also,there are packing and mechanical seals available today so that cooling in many cases can becompletely eliminated in the design phase, and turned off in existing plants.

Throttle the Control Valve

Just like the simple decision to stop a pump, so can the decision be made to throttle flow toreduce the energy requirements. Most centrifugal pumps have horsepower required curves

that decrease with decreasing flow, so that partially closing the control valve reduces thehorsepower requirement and the energy input.


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Conclusion

Careful attention to these areas of design, selection, operation, and maintenance will reapsubstantial cost and energy savings in almost all industrial, process, marine, and utilityapplications.

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