improving pump performance and efficiency

8/13/2019 Improving Pump Performance and Efficiency

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CHOOSING THE BESTFLOWMETER

P LASTIC SEALS FORIMPROVED PERFORMANCE

B Y J ONATHAN P LEDGER

L ATEST NEWS ON API SEAL STANDARDS

T HE I NTERNATIONAL J OURNAL FOR P UMP U SERS

Number 420September 2001

Condition Monitoring

inliaisonwith

IMPROVING PUMPPERFORMANCE & EFFICIENCY

WITH COMPOSITE WEARCOMPONENTS

www.worldpumps.com

ELSEVIERADVANCEDTECHNOLOGY


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FEATURE SEALS

0262 1762/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved WORLD PUMPS September 2001

For years, pump manufacturers have applied metallicparts for wear rings, sleeves, and bushings insingle stage and multistage petroleum processingAmerican Petroleum Institute (API) pumps. Since steelmating wear parts have the capacity to gall and possiblyeventually seize, large wear ring clearances are requiredto insure the pump's rotor and stator do not touch off.Although this precaution is taken, galling and seizingproblems still plague pump users during slow roll, steamturbine driven pump starts and in applications wherethe suction pressure of the pump is very close to thevapour pressure of the media. Even greater wear partrunning clearances are required when stainless steel

is used due to its galling tendency. Stainless steel is requir-ed in water services and other services that would causeother steels to corrode.

In recent years, thermoplastic composite materials havebeen applied to either the rotating or the stationarywear part, with the mating component remaining insteel. The use of thermoplastic composite materialsprovides a greater hardness deferential between wearparts, the composite material serving as the sacrificialwear component.

Like all materials, there are limitations to thermoplastic

composite materials as well. Yet in numerous applications,the limitations are less prominent and the gains aresubstantial.

Problems due to pumprecirculation

API pumps are designed and built with devices to controlthe amount of flow recirculation from the discharge side of the impeller back to the suction side of the impeller. Theseseals are referred to as wear rings. Generally they aremounted as pairs. One is mounted on the rotating impeller(impeller wear ring) and the other is mounted in the pumpcase (case wear ring). These wear rings also provide rotorstability.

Historically wear rings have been manufactured fromvarious grades of steel. Friction caused by steel running

on steel is so high that large wear ring clearances arerequired to insure the pumps rotor and stator do nottouch off. Furthermore, most steel has a substantialtendency to gall when contacting mating surfaces, thusrequiring additional clearance. While API 610 givesrecommendations on minimum diametrical clearances forwear rings, most facilities have their own clearancecharts. Many times these have been increased to try toimprove reliability by reducing the risks of galling orseizing of pumps.

Unfortunately these large clearances lead to internal fluid

recirculation with the following consequences:

Efficiency losses large clearances allow a greateramount of internal recirculation having a direct effectupon pump efficiency.

Recirculation cavitation excessive wear ringclearances can impact the tendency for formation of

Improving pump performance &efficiency with composite wearcomponentsThough common, steel mating wear parts can gall and possibly eventually seize, requiringlarge wear ring clearances to insure the pump's rotor and stator do not touch off.Nevertheless, galling and seizing problems still plague pump users. Jonathan Pledgerdiscusses how the use of thermoplastic composite materials can provide a greaterhardness deferential between wear parts.

Figure 1. A double suction pump.


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WORLD PUMPS September 2001 www.worldpumps.com

both classical and recirculation cavitation. Increasedleakage across the wear ring from discharge to suctiondisturbs the flow pattern and can raise the NPSHR of agiven impeller pattern.

High vibration levels the velocity of the fluid acrossthe wear ring surfaces has a destabilising effect upon therotor.

Composite wear ringsThermoplastic composite materials are non-galling, have alow coefficient of friction, and demonstrate excellent wearresistance in clean fluids. More recently, carbon fibrereinforced, high performance thermoplastic compositeshave been developed that provide mechanical propertiesmore competitive with those of metals. As a result, thesematerials have been used to replace metal wear rings in anumber of pump applications.

Reduced friction and galling tendencies of these

performance composites allow the pump user to greatlyreduce wear ring clearances. For example, Table I shows

the API 610 recommended diametrical runningclearances for cast iron, bronze, hardened 11 to 13%chromium, and materials with similar galling tendencies;it also lists the diametrical clearances possible with theWR materials.

The benefits of reducing the diametrical runningclearances on pump wear parts include:

Pumps operating at greater efficiency-reduction ininternal recirculation that reduces horsepower

Recirculation cavitation opportunity is minimised Reduction of vibration levels

These enhancements in performance translate intotangible returns. For example, a pump OEM repair shopupgraded a 9-stage horizontal split case boiler feed waterpump from stainless steel impeller wear rings and sleeves tocontinuous carbon fibre, thermoplastic compositecomponents. This pump has been in service for over twoyears with no loss of efficiency gains. The pump particularsare as follows:

Capacity: 2200 GPMDifferential Head: 5100 ft.Specific Gravity: 0.96Delta P: 2119 PSIG

Pump efficiency increased from 81.2% to 83%. Based onelectricity cost for the region of $0.12kW/hr, savings

resulted in $57,248 per year. The overall cost to pull,transport, repair, and reinstall the pump was $30,000, sothe pump user achieved payback in 6.3 months.

In a second example, two horizontal split case pumps (300hp 8-stage and 200 hp 4-stage) running diesel fuel andgasoline at ambient temperature were retrofitted withcomposite case wear rings and bushings. The originalbronze components were replaced with components madefrom a carbon fibre reinforced thermoplastic.

Replacement of the wear rings and bushings with low

friction, non-galling material allowed the tolerancesto be reduced from API clearances to .0015 inch/inchon rotating diameter. The impact on vertical vibrations wassubstantial. Vibrations on the four-stage pump werereduced from .525in/sec to 0.084in/sec. Vibrations on the8-stage were reduced from .600in/sec to .094in/sec. Forboth pumps, vibrations were reduced 84%. These pipelinepumps have now been in service for over three years.

Galling and seizing of pumps

As long as there is metal wearing on metal, there is anopportunity for galling and seizing of wear components andpumps. In high corrosion environments this problem isgreatly magnified since stainless steel components arerequired, which are prone to galling. There are many causesfor wear parts rubbing, even when large diametricalclearances are used. Among these causes are:

Figure 2. A small vertical turbine pump.

TABLE 1

Diameter of Rotating Minimum Diametrical Clearance (Inches)Member at Clearance Joint(Inches) API 610 Clearance using w525*

4.500 to 4.999 .016 .006

5.000 to 5.999 .017 .006

6.000 to 6.999 .018 .007

7.000 to 7.999 .019 .008

8.000 to 8.999 .020 .009

*Greene, Tweed & Co. WR composite material

FEATURE SEALS


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WORLD PUMPS September 2001 www.worldpumps.com

Radial bearing failureWhen a radial bearing fails thisallows the rotor to drop and the wearcomponents to come into hard con-tact. This scenario often leads to severe

galling and complete pump seizure.

Product vaporisationWhen a pump suffers traditionalcavitation there is a definite oppor-tunity to loose the lift generated bythe hydrodynamic film created by thefluid thereby allowing the rotor todrop and the wear surfaces to rub.

Shaft deflectionExcessive shaft deflection is another

cause of wear components coming incontact with one another. When acentrifugal pump operates away fromits best efficiency point a radial forceis generated that will attempt to bendthe shaft. This can cause a rotatingcomponent such as a wear ring ormechanical seal to contact a station-ary component causing damage toeither or both of them.

Slow roll starts requiredby pumps being steamturbine drivenThis problem is especially evident inlarger multistage horizontal pumpswhere the rotor must achieve acertain speed to get adequate lift

otherwise the wear components in thepump are contacting.

Lack of radial bearingsupportThis problem is prevalent in oldermodel double suction pumps (seefigure 1) and 2-stage overhung pumps.When these pumps were originallydesigned, they were sealed with ropepacking that provided radial bearingsupport for the shaft. But due to morestringent emission standards, many of these pumps have been converted tomechanical seals. The loss of thepacking bearing support leads togreater shaft deflection, short

mechanical seal life, and possible wearring galling.

Reducing gallingand seizure

Though there are many thingsthat may be done to minimise thepossibility of galling and seizing of pumps, there is one consistent way toavoid this concern. That is to usenon-galling, non-seizing wear mat-erials. High performance thermo-plastic composites are available thatwill not gall or seize the mating steelwear components even when highload friction occurs under dry runningconditions. These composites func-

tion as the sacrificial element. In aworst case scenario, these componentswill wear or surface melt out beforegalling or seizing of a rotor to thepump housing.

A great deal of technical data andresearch is available to demonstratethe property differences betweenmetal and composite wear surfaces.However, there exists a large amountof field data and successes thatdemonstrate the practical improve-ments resulting from thermoplasticbased wear components.

Field success with

compositecomponents

Radial bearing failureA gulf coast refinery using acontinuous carbon fibre, thermo-plastic composite impeller wear ringsin a large diameter, single stage brinewater pump experiencedradial and axial bearing seizure,leading to the breaking of the shaft at1800rpms. The composite wear ring

clearances opened up to the pointthat they had to be replaced, butthere was no wear or damage to thestainless steel impeller or case wearrings. As a result the pump repairwas less than half the cost of what itwould have been, if the stainless steelwear rings would have been used.The pump had been converted tocomposite impeller wear rings dueto constant seizure of the pumpusing stainless steel on stainless steel

wear rings.

Product vaporisationThis type of problem often occurs inpumps moving product close to itsvapour pressure such as light endhydrocarbons and boiler feed waterpumps. A west Texas power plant wasexperiencing the seizure of three 4 x 6x 9C, 12-stage horizontal split case,boiler feed water pumps. These pumpswere operating at 266F (130C).Galling and seizure was occurringduring short-term upset conditionswhere the pumps were dropping belowthe NPSH required. Carbon fibrereinforced thermoplastic wear insertswere installed in all stationary wear

Figure 3. A large vertical turbine pump.


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www.worldpumps.com WORLD PUMPS September 2001

components including all case wearrings and bushings. These pumps wereretrofitted and reinstalled in Januaryof 1996 and have now been in servicefor over four years. The plant-

engineering manager has reportedthat system upsets continue to plaguethe area, but no galling or seizing of the pumps has occurred.

Shaft deflectionShaft deflection causes highvibrations that shorten the life of mechanical seals and create noise. Asingle stage, vertical in-line pumpwas experiencing such a problem.Mating steel wear rings wereconverted to continuous carbonfibre, thermoplastic compositeimpeller wear rings on stainless steelcase wear rings. A carbon throatbushing was also converted to acontinuous carbon fibre,thermoplastic composite throatbushing. As a result, clearances werecut in half. A substantial drop invibrations was accomplished byrunning tighter wear ring clearancesand turning the wear rings intoadditional hydrodynamic bearings.

Closing down of clearances waspossible since the wear ring materialswere constructed from a non-gallingcomposite. This pump has now beenin service for well over one year withno increase in vibrations. The tighterclearances clearly minimise shaft runout.

Slow roll startsWhen a pump is being driven by asteam turbine, often the need to get

the rotor up and running at operatingspeed is set directly against therecommended slow roll starting warmup of the turbine. One plant wasexperiencing difficulties in the startup of an eight-stage 2CNTA8 boilerfeed water pump. The turbine drivingthis pump was a single stage, 260horse, steam turbine (60-PSI Steam). Normal speed for this turbine wasfrom 3550 to 3600. The turbinescapabilities required that it be slowroll started at 500rpms forapproximately 30 minutes. Thecontact occurring between thestainless steel wear parts caused thepump to gall several times duringattempted start up. The pump was

retrofitted with carbon fibrereinforced thermoplastic stationarywear parts. The clearances wereactually reduced having the positiveeffects of increasing pump efficiency

and lowering vibrations, whileallowing slow roll starting with nogalling.

Lack of radial bearingsupport

Short mechanical seal life as well asgalling problems have been solved onmany double suction pumps as well asin many 2-stage overhung pumps. Ondouble suction pumps thermoplasticcomposites have been applied veryoften as tight running case wear ringsas well as in throat bushings in bothstuffing boxes when space hasallowed. The small clearance casewear ring provides additional bearingsupport in the centre of the shaftwhere deflection is a problem. Theuse of thermoplastic composite casewear rings or impeller wear rings andthroat bushings when possible hasgreatly improved mechanical seal lifewhile eliminating galling. Two-stage

overhung pumps are inherentlypredisposed to high deflections,galling, and short mechanical seallife. There are two impellersoverhung with a bushing in betweenthem but with no bearing support atthe end of the shaft. Thermoplasticcomposite components allowthe running of tight clearances onthe wear rings and thebushing that is between theimpellers. Shaft deflection is

minimised thereby greatly improvingmechanical seal life and eliminatinggalling.

Fracturing of carbonand graphite alloys

Carbon and graphite alloys aresometimes used for wear componentsin vertical sump pumps and turbinepumps. Often, the primary driverin selecting these materials istheir excellent self-lubrication prop-erties. However, these materialsare susceptible to cracking and/orfracturing. As the clearances onbushings and line shaft bearings onvertical pumps start opening up,

vibrations increase and impact of shaft against the wear parts becomesmore violent causing the carbon orgraphite alloy parts to fracture. Also,during plant start up, many times

there are solids that were not flushedout that must pass through the pumps.The velocity of the solids as theycontact the wear parts will often causedamage to these brittle components.

Thermoplastic composites are avail-able that have excellent impactresistance, in addition to being non-galling and non-seizing. Thousand of pumps retrofitted with these materialshave not reported cracking orfracturing of these materials.

Conclusion

Thermoplastic composite materialshave been applied to either therotating or the stationary wear partsin a broad range of productionpumps. The use of these compositematerials provides a greater hardnessdeferential between wear parts, thecomposite material serving as thesacrificial wear component. This

reduces the risk of galling and seizing,thus allowing reduction in clearancesthat offer the following benefits:power reductions through greaterpump efficiencies, lower pumpvibrations, greater pump reliabilitythrough the ability to survive dryrunning pump episodes. Thesebenefits result in tangible savings inthe operation and repair of APIpumps.

As with all materials, thermoplasticcomposites have their limitations.Maximum life is obtained whenthey are applied as wear partsin pumps moving clean media.There are also chemical com-patibility limitations that shouldbe discussed with the materialmanufacturer.

CONTACTJonathan Pledger is product managerfor WR composite materials andfluid handling sales manager forGreene, Tweed and Company, aglobal manufacturer of seal andbearing solutions for a variety ofindustries including fluid handling,chemical processing, petroleum,and more.


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