0262 1762/04 © 2004 Elsevier Ltd. All rights reserved WORLD PUMPS January 2004 2 R otary lobe pumps, like eccentric screw pumps, are positive displacement pumps that are often used for pumping highly viscous, sludge-type media. With respect to operational characteristics, both are used in similar pressure ranges and have a volumetric flow rate that is relatively pressure-independent and proportional to the rotational speed, making them easy to regulate. Also the pumping efficiency at an identical pressure differential is comparable. As the stroke volume increases, the specific advantages of rotary lobe pumps become more evident when compared to other positive displacement pump types. Large volume, rotary lobe pumps can attain flow rates of over 900 m³/hr and are known for their extreme durability when running dry, which enables them to be employed as self-priming pumps. Unlike eccentric screw pumps, rotary lobe pumps require minimal installation space, maintenance and servicing (Figure 1). Incorrect pump selection The concept of a rotary lobe pump with elastomer-coated lobes is relatively new. Because rotary and screw pumps are used for similar applications, designers often employ the selection criteria without modification and without giving consideration to the distinct differences between the two pump types. As a result, selections are made that fail to consider the advantages of the excellent characteristics of rotary lobe pumps. One example is the rotary speed requirement. Eccentric screw pumps, due to their design, have a greater imbalance and are subject to high levels of wear. To counter the effects of wear, a low rotational speed is often specified. For rotary lobe pumps there is a specified gap during operation, meaning that they work without contact. Furthermore, the rotary lobes are symmetrically designed, so vibration cannot be a cause of imbalance. At one time rotary lobe pumps, particularly those with three or more vanes, had problems with strong pulsations. Now, as a result of advanced HiFlo technology, problems with pulsation in rotary lobe pumps no longer exist. Applying theory Doubts concerning the use of rotary lobe pumps at high rotational speeds still persist, even though their recommendation is based on test results already outlined and proven experience in other areas. It is often feared that high rotational speeds, as is the case with eccentric screw pumps, will also lead to increased wear in a rotary lobe pump. However, the test results are indisputable and demonstrate that the volumetric efficiency of a rotary lobe pump increases as the rotational speed increases. Using reverse logic, it is only at high rotational speeds that it is possible to attain optimum levels of efficiency, particularly in respect of advanced wear. In practice this means it is better to use a small pump at high rotational speeds than to use a large pump at slow rotational speeds. For identical flow volumes, a small pump with high rotational speeds is generally more efficient, which means that it operates more effectively. At the same time investment and operational costs are significantly lower as a result of reduced parts wear. Field trials Having conducted laboratory tests, pump designers looked for an environment in which to conduct field tests under realistic conditions to support their claims. The chosen site was a sewage treatment facility where three centrifugal pumps were pumping statically thickened primary sewage from pre-acidifying concentrators to a sludge digestion vessel. Running rotary lobe pumps at high speeds Rotary lobe pumps are claimed by manufacturers to be most economical when run at high speeds. Having previously been examined in scientific trials, this claim has now been subjected to field trials. Carsten Wenner Dipl.-Ing (FH) Hugo Vogelsang Maschinenbau GmbH Co. reports on the most significant results of this field test. feature running costs Figure 1: Size comparison
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0262 1762/04 © 2004 Elsevier Ltd. All rights reserved WORLD PUMPS January 20042
Rotary lobe pumps, likeeccentric screw pumps, arepositive displacement pumps
that are often used for pumping highlyviscous, sludge-type media. Withrespect to operational characteristics,both are used in similar pressure rangesand have a volumetric flow rate that isrelatively pressure-independent andproportional to the rotational speed,making them easy to regulate. Alsothe pumping efficiency at an identicalpressure differential is comparable.
As the stroke volume increases, thespecific advantages of rotary lobepumps become more evident whencompared to other positivedisplacement pump types. Largevolume, rotary lobe pumps can attainflow rates of over 900 m³/hr and areknown for their extreme durabilitywhen running dry, which enablesthem to be employed as self-primingpumps. Unlike eccentric screw pumps,rotary lobe pumps require minimalinstallation space, maintenance andservicing (Figure 1).
Incorrect pumpselection
The concept of a rotary lobe pumpwith elastomer-coated lobes isrelatively new. Because rotary andscrew pumps are used for similarapplications, designers often employthe selection criteria withoutmodification and without givingconsideration to the distinctdifferences between the two pumptypes. As a result, selections are madethat fail to consider the advantages ofthe excellent characteristics of rotarylobe pumps.
One example is the rotary speedrequirement. Eccentric screw pumps,due to their design, have a greaterimbalance and are subject to highlevels of wear. To counter the effects ofwear, a low rotational speed is oftenspecified. For rotary lobe pumps thereis a specified gap during operation,meaning that they work withoutcontact. Furthermore, the rotary lobesare symmetrically designed, sovibration cannot be a cause ofimbalance.
At one time rotary lobe pumps,particularly those with three or morevanes, had problems with strongpulsations. Now, as a result ofadvanced HiFlo technology, problemswith pulsation in rotary lobe pumps nolonger exist.
Applying theory
Doubts concerning the use of rotarylobe pumps at high rotational speeds
still persist, even though theirrecommendation is based on testresults already outlined and provenexperience in other areas.
It is often feared that high rotationalspeeds, as is the case with eccentricscrew pumps, will also lead toincreased wear in a rotary lobe pump.However, the test results areindisputable and demonstrate that thevolumetric efficiency of a rotary lobepump increases as the rotational speedincreases. Using reverse logic, it isonly at high rotational speeds that it ispossible to attain optimum levels ofefficiency, particularly in respect ofadvanced wear.
In practice this means it is better touse a small pump at high rotationalspeeds than to use a large pump at slowrotational speeds. For identical flowvolumes, a small pump with highrotational speeds is generally moreefficient, which means that it operatesmore effectively. At the same timeinvestment and operational costs aresignificantly lower as a result ofreduced parts wear.
Field trials
Having conducted laboratory tests,pump designers looked for anenvironment in which to conductfield tests under realistic conditions tosupport their claims. The chosen sitewas a sewage treatment facility wherethree centrifugal pumps were pumpingstatically thickened primary sewagefrom pre-acidifying concentrators to asludge digestion vessel.
Running rotary lobe pumps athigh speeds
Rotary lobe pumps are claimed by manufacturers to be most economical when run at highspeeds. Having previously been examined in scientific trials, this claim has now beensubjected to field trials. Carsten Wenner Dipl.-Ing (FH) Hugo Vogelsang Maschinenbau GmbHCo. reports on the most significant results of this field test.
f e a t u r e r u n n i n g c o s t s
Figure 1: Sizecomparison
WORLD PUMPS January 2004 www.worldpumps.com 3
The existing application involvedpumping a media with a solid materialcontent of 4% at a flow rate of about55 m3/hr and at 3 bar. Variations inthe solid material contentunavoidably led to fluctuations involumetric flow rates. Regulation waspossible, but only with considerabledifficulty. Efforts were also made topump media containing a higher solidmaterial content previously notpossible using the centrifugal pumps.Eventually, it was decided to employpositive displacement pumps.
A medium-sized rotary lobe pump(pump 1) operating at the relativelyslow rotational speed of 272 rpm wasused. On average a solid materialcontent of 9% could be pumped,where peak values of 12% werereached. In general, the rotary lobepump performed well, but from thebeginning satisfaction with the pump’soperational lifetime was a concern.
The result.
This application provided the idealsetting for a field trial. Three rotarylobe pumps were installed one afterthe other. One large-size pump (5l/rev) with a low rotational speed of218 rpm (pump 2) was followed by twosmaller medium sized pumps (2,5 l/rev,1,9 l/rev) with the significantly higherspeeds of 414 rpm and 550 rpm(pumps 3 and 4). The result, shown inFigure 2, was very clear.
The smaller the pump size selected,the smaller the investment costs and
the higher the rotary speed, the longerthe operational lifetime of the pump.Using the standard requirements ofpump No.1, the smallest pump withthe highest rotational speed, thisbeing pump No.4, attained a lifetimethat was 1.4 times the standardlifetime (Figure No.2) and representedthe least investment.
This advantage becomes even morepronounced when consideration isgiven to the spare parts costs for atypical repair and replacement of wornparts. At a cost difference of between56% and 115%, pump No.4 performedthe best (see Figure 3, yellow bars).
Ignoring the investment costs, butcorrelating the average spare partscosts in relation to the attained servicelife, the operator can also realiseenhanced financialadvantages whenconsidering the costs ofwear per hour of operation,or per cubic meter ofpumped medium. Here,the difference ranges from40% to 115% (see Figure 3,red bars). The smallestpumps not only representthe lowest investment andworn component costs, butalso give the longestoperating lives.
Theoreticalsimplification
A very interesting factorfor the operator is
provided by a simple theoreticalsimplification. This takes as its basisthe investment costs and the costs ofrepairing and replacing worn partsfor the first three pumps. Theseoperating costs are compared as afunction with service lifetime. Usingthese reported figures pump No.4clearly performed the best with 52%. If one adds up the investmentcost and the costs of replacing wornparts for three jobs calculated withrespect to running time, the totalcosts for pump No.4 are just overhalf of those when compared againstpump No. 1, which was selectedaccording to the standard prescribedrequirements. As can seen in Figure4, pump No.4 is far superior to pumpNo.1.
Viscosity
Under actual operating circumstances,the decision was taken to opt for pumpNo.3 and this was done for otherreasons. This pump is to be operatedwith an even higher solid materialcontent and where an under-pressureof 0.5 bar is needed in order to drawthe medium into the pump. If therotational speed is too high, there is apossibility that the chamber may notbecome completely filled, whichwould lead to a drastic drop inefficiency.
f e a t u r e r u n n i n g c o s t s
Figure 2: Operational lifetime and investment costs.
Figure 3: Costs of worn parts and repairs.
0262 1762/04 © 2004 Elsevier Ltd. All rights reserved WORLD PUMPS January 20044
A high viscosity medium issignificantly more effective in closingthe gaps inside the pump. As a result,the efficiency of the pump increases asthe viscosity of the medium increasesuntil the chamber, depending on therotational speed, is no longer filledcompletely. This means that selectionof the pump must always give
consideration to theviscosity of the medium. Inother words: the optimumrotational speed is verymuch dependent on theviscosity of the medium.As a general rule, asviscosity increases, therotational speed must bereduced accordingly.
Elastomers
In other trials, variouselastomeric materials forcoating the rotary lobeswere tested. Developments
in this area have yielded significantprogress. However, the rule thatshould be applied is that the cheapestsolutions are not always the mosteconomical. For example, by using anelastomeric material that is 20% moreexpensive, a service lifetime in thiscase could be increased by 70%.
Summary
The figures given in this report do notof course apply one hundred per centfor every case. The trials, however, doconfirm the scientific trial results andclearly indicate that the pump typeshould be selected according to itsspecific characteristics. For elastomer-coated rotary lobe pumps, this meansthat running at high rotational speedsprovides the best efficiency and thelongest working life, which in turnsleads to the most economic operation.However, all relevant factors must betaken into consideration, which iswhy installation designers andoperators are advised to seek theguidance of specialists when selectingpumps. ■
CONTACT:Carsten Wenner Dipl.-Ing Vogelsang, +49 5434 / [email protected]
f e a t u r e r u n n i n g c o s t s
Figure 4: Theoretical operating costs of 3 repair jobs.
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