New stainless steels for casting,not only for centrifuges

Many centrifuge components are pro-duced of so-called duplex steels. These aresteels with a 2-phase texture in which theproperties of stainless ferritic chromiumsteels are combined with those of stainlessaustenitic chromium-nickel steels. Duplexsteels thus have both high corrosion resis-tance and also high ductility and strength/1/. The essential alloy elements of duplexsteels are nickel and molybdenum besideschrome. The contents of these three ele-ments are summarised as a measure of thecorrosion resistance into the so-called PREnumber (%Cr + 3.3%Mo +16%Ni). AsGeorg Wilhelm Overbeck, Schmidt +Clemens, explained, the prices of duplexsteel are determined particularly by theprices for molybdenum and nickel, which,in the past, were not only high, but werealso subject to large fluctuations. Theendeavour to produce cost-effective, yetfirm and corrosion-resistant materials hasled to the development of the so-calledLean Duplex and/or Ultra-Lean-Duplexgrade. Already in May, 2012, the casting material Centralloy® G21-02(GX3CrMnNiN21-5-2, an Ultra-Lean-Duplex steel) was launched on the market.The material has good strengths similar toconventional austenitic Cr-Ni steels,which are used in pumps, centrifuges andrelated equipment, as well as very goodresistance to intergranular corrosion.

Dr. Henrik Astemann presentedCentralloy® G45Mo (G-NiCr35MoCuN),an austenitic cast steel, as the latest mate-rial development, which has not, like manyother cast steels, been developed throughmodification of a comparable rolled andforged steel, but rather, the special require-ments for processing form casting weretaken into consideration in the develop-ment. In addition, certain requirements formechanical strength and corrosion stabili-ty also had to be fulfilled, and, moreover,the material should be cheaper than exist-ing nickel-based steels. G45 Mo contains45 wt.% of nickel and 35 wt.% of chrome,and hence is planned for use in highly cor-rosive surroundings. Spin, shape and pre-cision castings can be produced from G45Mo. The steel was developed within the

scope of a BMU research project for therealisation of geothermal power plants,which is being coordinated by theGeoresearch centre in Potsdam. Thepotential to gain geothermal energy and toproduce electric power from it, too, is har-boured by territories with aquifers whosewater temperature is in excess of 100°C.The Gross-Schönebeck research stationlies in such an area. There, water horizonsare being opened at about 4000 m depth.The hot liquid which is thereby extractedto the surface is a salt brine with an over-all concentration of about 300 mg/l and apH value of 5.3 - 5.8. With this medium,different materials that are known as beingcorrosion-resistant were and are beingtested in a specially installed bypass pipe,among them also the new Centralloy® G45

32 F & S International Edition No. 14/2014

Highlights 2013

Centrifuge technology: Materials,machine design and applicationsReport from the 4th International Separation Technology Symposium of theSchmidt+Clemens Group H. Lyko*

There are industrial centrifuges with a huge number of construction types and specifications, which are optimallyadapted to the separation task to be solved, the prevailing ambient conditions and the substance properties of themixtures to be separated. The bandwidth in single components and materials from which these are made is accordinglylarge. Important centrifuge components, like drum bodies and screw bodies, are produced with centrifugal castingmethods. Furthermore, there is a great need for customized components as mould or precision castings. The Specialproducts business unit of the Schmidt + Clemens Group with the Department of separation technology specializes inthe production of all relevant stainless steel components that are used in centrifuges. The separation technologysymposium offers to all parties involved a good opportunity for communication on one hand about new materials, aswell as about important application fields, and on the other hand about their requirements, machine models, designmethods and operating conditions. In June in Cologne, Wolfgang Unterbusch, Vice President of Sales, and ChristianLenz, Sales manager of the Special products business unit, welcomed about 50 participants of all the leading centrifugemanufacturers of Europe and of important supplier companies of the centrifuge industry. In addition, a representative ofa municipal wastewater disposal company, as well as one from an enterprise which deals with sewage sludge drying,respectively supplied interesting information from the environment of a classical application area for centrifuges.

Fig. 1: Sewage sludge treatment in the 2.7 million PE sewage treatment plant of Hamburg(image: Hamburg Wasser)

*Dr.-Ing. Hildegard LykoDortmund / Germany, Phone +49 (0) 231-730696

02_fs_international_2014_seite_03-98__ 21.05.14 20:09 Seite 32

Mo in the form of pipes, flanges, etc. The probability of generalcorrosion or pitting corrosion is determined on-line with elec-trolytic methods. In parallel, additional laboratory measurementson corrosion resistance are carried out in the Federal Institute forMaterials Testing (BAM). The results of both measures will lead infuture to finding the most suitable materials and possibly operatingparameters for such demanding applications.

An important market: Sewage sludge treatment

Sewage sludge dewatering is a standard application fordecanters. Here, dewatering performance (throughput and achiev-able solid matter content) and energy consumption are generally inthe focus of the users. How sewage sludge treatment is handled inGermany’s largest municipal sewage treatment plant in Hamburg,was described by Hendrik Schurig of Hamburg Wasser. Thesewage treatment plant is designed for 2.7 million PE (populationequivalents) and consists of two biological treatment stages. Thewhole system of sludge treatment is shown schematically in Fig. 1.In this sewage treatment plant, about 1.5 million m3 fouled sludgeoriginate during the cleaning of about 150 million m3 of waste-water annually that is dewatered with 5 decanters and afterwards isdried and combusted on site. In addition, 8 decanters provide forthe thickening of the surplus sludge since static thickening pro-duces insufficient dewatering. More over, the sludge combustionaccepts dry sludges from other plants. Altogether the energyrequirements of the sewage treatment plant can be covered up to75%, and the remaining 25% are provided by wind power. The bio-gas originating with the sludge digestion is processed and fed intothe gas grid.

The low solid matter content of the dewatered sludge of 21%before the dryer is noteworthy here. Because of the combustion onsite, there are no transport costs for the sludge, and therefore poly-mer consumption for dewatering was crucial for the treatmentcosts. A stronger dewatering would have led here to the increase ofpolymer consumption.

Sewage sludge dewatering with decanters down to only 21% TSis far below what these machines are able to perform and is also noteconomical, since for disposal one has to factor in about 40 to 80euros per ton of dewatered sludge, dependent on location. Here,one calculates for the reduction of water content by 1%, with areduction of the sludge volume to be treated and to be transportedby 4 to 5%. This was also highlighted by Wolfgang Steiger of thecompany Flottweg, which was also the supplier for the Hamburgsewage treatment plant. His lecture was about the design ofdecanters for sludge dewatering, inter alia. The basic principle ofthe design is based on the calculation of the required clarificationareaaccording to the so-called Sigma principle. The equivalentclarification area Σ is the area for static dewatering multiplied bythe acceleration factor (expressed as a multiple of gravitationalacceleration g). In case of consideration of the centrifuge bowl asa cylinder, one can accordingly convert to the diameter and thelength and/or the centrifugal acceleration - the same value Σ willthen always mean the same dewatering performance. That is thetheory, because decanter drums consist not only of a cylindricalpart but have, according to make and construction form, differentlong and steep conical elements (see Fig. 2). How one includes thenot cylindrical parts of the drum in the calculation of Σ is not estab-lished uniformly and, much to the regret of the lecturer, is solvedby every manufacturer with another formula. Into the formulae, thelength of the conical part and the diameter of the solids dischargeare included with different factors. If one carries out the Scale-Upof the pilot up to industrial scale in each case with the same for-mula, one receives similar magnification factors. It becomes diffi-cult for various manufacturers in preparing their tenders, becausethe different calculation methods mean that different enterprisesoffer machines of different sizes for the same dewatering perfor-mance, whereby distortions of competition can occur. With thesame external dimensions, dewatering performance is increased

Highlights 2013

F & S International Edition No. 14/2014

02_fs_international_2014_seite_03-98__ 21.05.14 20:10 Seite 33

through a steeper cone (synonymous with a longer cylindricalzone) and through a deep pond (corresponding to a bigger processvolume). The calculation of the equivalent clarification area doesnot include any specific design parameters of a machine like thescrew design, the inlet geometry or the drive system. However,these play also an important role, inter alia, for the compactionpressure and hence the achievable dewatering.

In practice scaling-up is performed according to the requiredmaximum capacity. For applications on wastewater and sewagesludge, Flottweg offers its standardised decanter series C4E. It waspointed out that maximum dewatering does not always mean themost efficient operating mode, but rather one must consider therelationship between water content of the solid matter and thethroughput capacity. The dryness of the solid matter drops approx-imately linearly with increasing throughput.

For further use of the sewage sludge as fuel, this is dried. In theHamburg sewage treatment plant, this happens in disc dryers,which are heated with steam generated during the sludge combus-tion. Also here there are now alternative drying systems on themarket, like solar drying alone or in combination with waste heatrecovery. Dr. Steffen Ritterbusch, Thermo-System GmbH, report-ed about such methods. The drying happens in climate- controlledhalls of glass or PE sheeting, which resemble greenhouses and,when required, are also equipped with a waste air treatment facili-ty. According to sludge volume, the process is carried out intermit-tently (loading and unloading with a wheel loader), semi-continu-ously (automatic loading, unloading with a wheel loader) orcontinuously (automatic filling and emptying). In the meantime, alarge number of plants exist worldwide. The biggest solar sludgedrying plant, with a capacity of 140,000 t/a, is currently in con-struction in Dubai. The fact that solar sludge drying can be operat-ed not only in southern countries is shown by the example ofOldenburg, Lower Saxony. Here, the sludge of a 400 - PE sewagetreatment plant (corresponding to 40,000 t/a sludge) is dried byutilisation of solar heat and waste heat. The use of waste heatreduces the space requirements of the plant to about 20% of thatfor pure solar drying, for which one calculates about 1m2 surfacearea for 1 t of sludge. The supply of the waste heat is done eithervia the supply of warm air or via underfloor heating. A componentespecially worth mentioning for small and medium-sized sludgedrying plants is the Electric Mole™, an automatically controlledappliance, made completely from stainless steel, for turning andaerating the sludge during drying.

Other construction type: Vertical decanter

As a known manufacturer of decanters and separators, thePieralisi Group has significantly expanded its product portfolio inrecent times. On one hand, the company’s own centrifuge pro-gramme is complemented by horizontal screen screw centrifuges.On the other hand, through cooperation with KyffhäuserMaschinenfabrik Arten GmbH (KMA) and the Japanese enterpriseTOMOE Engineering Co. Ltd., other machine models are avail-able. KMA is a manufacturer of vertical disc centrifuges andTOMOE is regarded as a world leader in large decanters for theprocess industry and wastewater treatment. Gert Bergjohann,Pieralisi Deutschland GmbH, presented the vertical decanter builtby TOMOE (see Fig. 3) as a special machine model. This con-struction type was originally developed more than 50 years ago atSharples in the USA and was for a long time the only form inwhich a decanter was feasibly gastight. Nowadays, verticaldecanters are used for separation tasks that take place at high tem-peratures and under high pressure. These include coal liquefactionand applications in the presence of organic solvents, such as forexample the recovery of catalysts. As is evident from Fig. 3, thespindle area and all technically sophisticated equipment, such asthe gearing and the main bearing for the drum, are outside the

34 F & S International Edition No. 14/2014

Highlights 2013

Fig. 3: Diagram of a vertical decanter (image: provided by PieralisiDeutschland GmbH)

Fig. 2: Dimensioning of a decanter (image provided by Flottweg GmbH)

02_fs_international_2014_seite_03-98__ 21.05.14 20:10 Seite 34

product chamber. Besides their suitabilityfor temperatures up to 400 °C and pres-sures up to over 30 bar, the vertical con-struction form is fascinating because of therelatively small installation surface. InAsia, this machine model takes on animportant role in the production of PET(Polyethylene terephthalate). The centrifu-gal acceleration in vertical decanters isnormally 2000 - 3000 *g. They are madewith diameters of 150 to 900 mm and canprocess 100 kg/h up to 20 t/h of dry solids.In addition, there are special designs with5000 - 10,000 *g centrifugal accelerationthat are not suitable, however, for highsolids concentrations.

Centrifuge drives

Different drive systems are available toput the drum and screw of a centrifuge intorotation at different speeds. Nils Ziegl -gänsberger from Sumitomo (SHI) CycloDrive Germany GmbH presented theCyclo drive whose operating principle hadbeen invented in the 1920s and had beenused first in 1949 in a centrifuge built bythe manufacturer Siebtechnik. A Cyclodrive is an excentric  drive whose outerprofile describes a cycloidal curve path. Itconsists of the components: high speedshaft with excentric bearing, cycloid discs,ring gear housing with pins and rollers, aswell as the slow speed shaft with pins androllers. The single-stage cycloid drive herecreates a transmission ratio of 1:87 (seeFig. 4). Higher transmission ratios are fea-sible in multistage construction methods,inter alia, in combination with a planetarygear in which mechanical transmissiontakes place via gear wheels in a toothedring gear member. Different constructionforms permit different operating modes ofscrew centrifuges, depending on whetherthe screw should turn more slowly orfaster than the drum. With a two-stage gearunit with differential planetary gear stage,the setting of a variable relative rotationalspeed with pre-triggered and also withretarding screw is possible in the entirecontrol range of the engine.

The alternative to the energy transfer tothe screw of a decanter is a hydraulic dri-ving mechanism whose advantages com-pared with a mechanical transmission hadalready been shown by Marco Metzger ofthe Swiss firm Viscotherm AG at the 3rdSeparation Technology symposium /1/.

The heart of this driving mechanism is aso-called radial piston machine whose sta-tor has a cam disc with cylindrical wind-ing. The rotor consists of a cylinder blockin whose boreholes pistons are pressurizedalternately with the pressure medium andreturn medium via a distribution system.The radial forces are converted into tan-gential forces, through the incline of thecam disc over the piston parts as a joint,and generate therefore the necessary

torque between rotor and stator. This yearMetzger presented, inter alia, the mostrecently developed stepped piston withwhich an even higher torque can be gener-ated at the same dimensions because moreliquid is moved per stroke of each of thesepistons.

Screen screw centrifuges andhybrid forms

Screen screw centrifuges are screencentrifuges with forced transport of solids/2/. They are used for dewatering coarsematerial like coal or minerals (grain sizefor coal about 0.25 to 2 mm). BernwardFeldkamp, Andritz Separation GmbH,showed details of the design of this type ofcentrifuge. The cone angle of the screw,the height and spacing of the filaments andtheir rotation speed determine the dwelltime of the solid matter in the machine andwith it the levels of achievable dehumidifi-cation. With the help of these parameters,the speed that the solids cake onto the bas-ket, or the time-dependent position of thesolid particles on the transport path untildischarge, are calculated. The result is thecurve of the saturation of the filter cakeabove the basket diameter, which is hyper-bolic. This curve path helps in the designof a screen screw centrifuge for a separa-tion process, but cannot be carried out forevery substance system, which is why oneventures to dare a transmission.

The differential speed between thescreen basket and screw results from thespeed of the screen basket and the trans-mission of the gearbox. Here, Cyclo orplanetary gear drives are interchangeablebecause of their similar external dimen-sions.

Sedimentation and filtration centrifugesin many special designs make up the prod-uct portfolio of Siebtechnik GmbH, and inmost applications solid matter is the prod-uct. As Andre Adam elaborated, this most-ly concerns crystalline particulate solids,which sediment rapidly. In his presenta-tion Adam drew special attention to themachines in which sedimentation, as hap-pens in a solid bowl centrifuge, and filtra-tion in the centrifugal field, take place side

by side or successively with a drumdesigned as a screen basket. The motiva-tion for these combination machines lies,inter alia, in that a pure screen centrifuge,depending on the degree of separation andselectivity of the screen, generates particlelosses and that one wants to lower thehumidity of the dewatered solid matterand, nevertheless, one wants to get a clearcentrate. In the Conthick combination cen-trifuge, the feed suspension enters firstinto a conical bowl portion and is thentransported into the cylindrical screenmember. This arrangement allows a small-er separation limit compared to a purescreen centrifuge (as the minimum, stillseparable particle diameter at which noclogging of the screen occurs). TheTurboscreen model is a combination ofdecanter and screen centrifuge and thediameter of the screen member isincreased abruptly relative to the decantermember. A higher centrifugal accelerationis thereby achieved in this area and a thin-ner filter cake is generated. In both combi-nations one obtains a clear centrate of thedecanting part and a filtrate conditioned bythe separation limit of the screen, in whichparticles might still be present. TheTwinCone machine is a decanter with twoseparation stages of different diameters.The dewatering degree and the solidscapacity can be increased by this arrange-ment. As was shown on the basis of testresults from comparisons of a standarddecanter with Turboscreen and/ or TwinCone, the improvement of the materialseparation through the combinationmachines is not equally high for everyproduct, so that one must produce the jus-tification of the higher investment costs fora combination machine in individual casesby tests.

Literature:/1/ Lyko, H.: Industrial centrifuges for different applications:

Challenges for manufacturers and suppliers – Reportfrom the 3rd International Separation TechnologySymposium of the Schmidt + Clemens Group F&SInternational Edition No. 11 (2011) pp. 27 -31

/2/ Benz, M.: Siebschneckenzentrifugen, in: Stahl, W.:Industriezentrifugen – Band II: Maschinen- undVerfahrenstechnik; DrM Press, ISBN 2-9522794-0-4,2004

Highlights 2013

F & S International Edition No. 14/2014 35

Fig. 4: Structure of a Cyclo drive (images: SUMITOMO (SHI) Cyclo Drive Germany GmbH)

02_fs_international_2014_seite_03-98__ 21.05.14 20:10 Seite 35