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KLINGERtop-chem-2000The pressure resistant PTFE material for narrow-section gasketsKLINGER GmbH, Germany4Typical applications can be found inthe plumbing and heating sector, in otherwords with water as the medium and amaximum temperature of approximately86 °C. But threaded unions are alsoused in gas supply. In industrial plant andmachine tool construction, supply linesfor water, nitrogen, oxygen, fuel gas, andalso oil, heating media and coolants, areoften closed off using threaded unions.Threaded unions are also one of the fittingsused for connecting solar collectors,where the temperature of the heattransfer medium can exceed 180 °C, tothe pipe system.Threaded unions with flat gaskets arethus exposed to the most varied media,temperatures and stresses.The Behavior of threaded unionsincorporating flat gasketsTwo different types of threaded union canbe examined:n Connection of a male and a femaleend using a screw-thread connection,where the two sealing surfacesare rotated in opposite directions whenfitting.n Connection of two components withflanged sealing surfaces using a unionnut, with manual resistance eitherreducing or preventing rotation of thesealing surfaces (Fig. 1).A look at the special requirements for achieving a leak-freeseal with narrow-section gaskets - for example in threadedunions - reveals that although these connections have been inuse for many decades, problems are always reoccurring in anumber of application areas.A common feature of both types is theconnection’s relatively rigid behavior. Thiscan best be explained with a contrastingexample - a flange connection (Fig. 2) -where the long bolts and the flange platehave a sprung effect.Fig. 2: Flange connection with spring componentsIn the case of a loss of thickness ofthe gasket (e.g. through heat-relaxation),spring action will result in the maintenanceof some degree of surface pressure- and thus sealing effect and mechanicalstability.With a threaded union, only the pipeor the tensioned section of the union nutcan be regarded as a sprung element,although it’s clear that component lengthsof only a few millimeters are involved.In the illustration (Fig. 3), below it can beseen that the length of the portion undertension merely equates to the thicknessof the gasket plus the flange on the righthandcoupling. The connection surface ofthe union nut can also flex slightly.In practice, the options for compensatinga possible loss of thickness arevery restricted.A further problem with threadedunions is the width of the gasket. Often,the actual sealing width is merely the wallthickness of the component (pipe). Inthis situation, wider gasket rings merelyappear more secure. Moreover, dependingon the component, damage can becaused to the ring gasket during assembly,while rotational movement during assemblycan also lead to considerable shearstresses. And with assembly often beingcarried out using pump pliers, a specifictorque loading cannot be relied on and theresultant forces can varyconsiderably.Fig. 1: Typical screw connectionGasket + Nipple = approx. elastic lengthFig. 3: Elastic component length of a threaded connection
The requirements on thegasket materialThe types of stress described above meanthat special demands are placed on thegasket material. They include:n Low shrinkage, even under temperaturestress, in order to maintain thesealing forces.n Edge stability, despite unfavorable ringwidth (thickness/width ratio)n Resistance to shear or frictionalforces arising from possible relativemotion of the gasket surfaces duringassembly.Additionally:n The gasket material must be ableto conform to the seal counterpart(metal surfaces are standard) sothat no leaks occur on the contactsurfaces.n The sealing material must also beimpermeable, so that this leakageroute is also eliminated.The sealing sequenceIn the case of standard ring gaskets madefrom soft fibrous materials (filler, fibers,elastomer as the binder), the sealing processensues as follows:The gasket is pressed:n It adapts to the surfaces of the sealingcounterpart and seals the contactsurfaces.n In so doing, the cross section of thegasket is placed under mechancalpressure and the existing capillariesand cavities are closed.n This process requires a certain time,however, and is associated with a lossof thickness in the gasket.On first heating, the gasket materialagain becomes slightly softerand resets - with additional loss ofthickness.If, therefore, higher demands are to beplaced on the security of the threadedunion, also from a temperature andduration point of view, standard materialsare no longer able to satisfy thesedemands.The alternativesWhich gaskets can be seen as alternatives,and how are they to be assessed?n Rubber gasket rings - these are veryimpermeable and have an excellentability to adapt; nevertheless, theypossess low stability when faced withhigher demands relating to temperatureand screw forces.n Metal gasket rings - these arevery stable and impermeable, butadapt insufficiently to the sealingsurfaces.n Pure graphite gasket rings - thesehave very high adaptability, but aretoo fragile during assembly and copebadly with the unfavorable thickness/width ratios.n PTFE gasket rings - these exhibit highconformability and are impermeablebut standard materials still possessexcessive flow properties.5Rich. KLINGER Ind e Com Ltda., BrazilKLINGER GmbH lectures at the 15 thInternational Sealing ConferenceKLINGER GmbH took advantage of the 15 th ISC in October 2008 in Stuttgartto showcase their PTFE gasket materials and to lecture on the stabilityof PTFE-material for narrow gaskets. Sales Manager Norbert Weimer, KLIN-GER GmbH, presented the latest results of research and development andthe experiences of applications in practice for KLINGERtop-chem-2000.The prospects for PTFE gaskets aretherefore exrtremely good if the problemsassociated with flow characteristics canbe solved.The following features and measurementswill allow an evaluation of the flowcharacteristics:n Compressibilityn Residual surface pressure on thegasket after temperature cycling
Page 1 and 2: KlingerNewsFebruary 2009www.klinger
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