Mechanical Shaft Seals For Pumps - Grundfos

ABCDAtmosphericpressureFig. 1.12a: Pressure at standstill is eithersystem pressure orPumpatmospheric pressurepressureFig. 1.12b: At standstill, there isonly direct contactbetween the seal ticpressurepressureACDCDdistribution for seal withparallelseal facesBCDFig. 1.13b: Opening forces fromhydrostatic mosphericpressureABCDAtmosphericpressureFig.1.14a: Pressure distribution in theAAtmosphericpressureAsealing gap when theBCDhydrostatic and hydrodynamicpressures are addedBCFig. 1.14b: Opening forces fromcombined hydrostaticand hydrodynamicpressure distributionDWhen the shaft starts to rotate, the seal rings will separate and the pumped medium willenter the sealinggap. The pressure decreases linearly from pump pressure B, to atmosphericPumppressure C.pressureSee fig. 1.13a.Note: In this book, pump pressure means pressure in the seal chamber.PumppressureThe linearlydecreasing pressure is known as the hydrostatic pressure in the sealing gap. Theopening force is shown with red arrows in fig. tepressure [bar]ressure [bar]Absolutepressure [bar]When the pump runs, see fig. 1.14a, a pressure builds up in the lubricating film. This is similar toABCDa car hydroplaning on a wet road. This pressure is known as the hydrodynamic pressure in theAtmospheresealing gap.ABCDThe hydrostaticpressurecombined withthe hydrodynamicpressureproduces the pressure12Pumppressure10 red arrows in fig. 1.14b.distributionin the sealing gap. The opening force is shown withAtmosphere812WaterPumped medium pressureFull-fluid-film lubrication can be obtained if the pressure in6 the sealing gap is sufficiently high10VapourWaterto balance the closing force of the 48 802VapourPumped medium pressureNormal atmospheric pressureVapourWaterpressure90 100 110 120 130 140 150160 170 180VapourPumped medium pressure Temperature [ C]

Fc Ah x P Fs 179 mm2 x 1 MPa 45 N 224 NClosing forceThe parts of the seal inside the pumpare subjected to an axial force from the pressure in theAh π (Do2— Ds2) π (222 — 17.12) 150 mm24the spring force,4 the axial force creates the closing force on thepumped medium. Together withseal faces.If the differential pressure between the pumped medium and the atmosphere is aboveapproximately 20 bar, the closingforce becomes so strong that it prevents the formation of anAs π (Do2— Di2) π (222 — 172) 153 mm24 film. The seal4 faces begin to wear. Wear can be avoidedadequate hydrodynamic lubricatingby reducing the area where the hydraulic pressure affects the axial force on the shaft seal. Thehydraulic force of the primary seal faces as well as the closing force of the seal are reduced. Ah x P Fs shaft 150sealsmm2 x 1 MPa 45 N 195 NUnbalanced and balancedFcmechanicalThe balancing ratio, k, is the ratio between the hydraulically loaded area, Ah, and the sliding facearea, As.Ask AsAsAsAh 150 0.98As 153Formula 1:k AhHydraulically loaded areaA hSliding face areaAsAhFig. 1.15a: An unbalanced shaft seal, k 1AhAhFig. 1.15b: A balanced shaft seal, k 1The pump pressure acting on the area, Ah causes a closing force to be exerted on the seal. The area, Ah,of an unbalanced mechanical shaft seal is larger than the area, As, and the balancing ratio, k, is largerthan 1. The contact pressure in the sliding face area exceeds the pumped medium pressure.The spring force further increases the contact pressure. The balancing ratio is often chosen to bearound 1.2.In the low pressure range of the pumped medium, unbalanced mechanical shaft seals are sufficient.See fig. 1.15a.The area, Ah, of a balanced mechanical shaft seal is smaller than the area, As, and the balancing ratio,k, is smaller than 1. The area, Ah, can be decreased by reducing the diameter of the shaft on theatmospheric side. See fig. 1.15b.In the high pressure range of the pumped medium or at high speed, the balanced mechanical shaftseal is used. The contact pressure in the sliding face area can be smaller than the pumped mediumpressure. The balancing ratio is often chosen to be around 0.8.Balancing a mechanical shaft seal gives a thicker lubricating film in the sealing gap.A low k value can cause a higher leakage rate or can even cause the seal faces to open up.15

IntroductionCalculation example, unbalanced andbalanced shaft sealA π (Do2— Ds2) π (222 — 162) 179 mm24 closing force of4 a liquid-lubricated mechanical shaft seal.In this example, we shall look ath theThe data below apply to an unbalanced Grundfos type A shaft seal. For more details on thisshaft seal type, see Chapter 2, type A, page 27.DoDiDsFig. 1.16: UnbalancedGrundfos typeA shaft sealDiDoDsFig. 1.17: Balanced Grundfostype H shaft sealπ D 2— D 2 ππ 222 — 162 179 mm2A(D o2— Dis2D))s 16(222 — 172)) 153 mm2Ahs π4diameter,Shaft4( o44(mmSliding seal face, inner diameter, Di 17 mmSlidingseal face, outside diameter, Do 22 mmAh Aπ (Do2— Ds2) π (222 — 162) 179mm2Spring179Fs 45 N44h force,k π 2 1.17As Aπs (Do153— Di22) ππ (2222— 172)2 153 mm2 22Thisresults:4π (DotheAh gives— followingDs2) 4π (22— 162) 179 mm224π (DDo22—AAh πD 2) 4π(222— 16 2) 179 mm 2222 — Dss2 ) π2 — 162 ) 179 mm2((44hoA D—D 22— 16 ) 179 mmHydraulicallyloadeds )area: 4(h44( o4ππ222222— D 2) π (22 2— 17 )AA Aπ (DD179— Dis ) 4 (22 — 162) 153179mmmm2k sh 44h oo 1.174153AAhs 179Slidingface222kAs π Darea:— D1.17 π— 1722) 153 mm22(πππ(22oi2)22153πAAs A4D—D 422222— 17 2) 153 mm 2()(sππo2 — Di2 ) 4D22— 17 153 mm2((4Ao — D i ) 4 (22 — 172)) 153 mm2A s D179i4k s 4h4 ( o 1.17A1532As Aπs (D—accordingDi2) πto222 — 172 153 mm2Balancingo4h ratio,4 ( formula 1,)page 15:179k AAhh 179 1.17179 1.17k AA h 179153 AAhss x P 1.17mm2 x 1 MPa 45 N 224 NFkkc Fs 1.17179153hsA153A s 153Ahs 179k closing 1.17A 179Theforce,F , at a 10-bar pressurek Ahs 153 1.17c(P 1AMPa)iscalculatedas follows:153sA1792 xhxFkc A Fs 179 mm 1 MPa 45 N 224 NAAhh P 179179 1.17h 153kk AA1.17πhs 179221.17— D1.17 π (222 — 17.12) 150 mm2kAh aAA D153o153 Grundfoss )For4ss ( 1534 type H shaft seal for a Ø16 shaft, theAbalancedsAh 179calculationisasfollows:xk 1.17Fc Ah P Fs 179 mm2 x 1 MPa 45 N 224 NAs 153Sleeve diameter, Ds 17.1 mm2 xFc Aπh xsealP 2 Fface, 179mm 45N mm 224 2Nπ diameter,Sliding 172inner21 MPa D222 mm17.1 2)i45150)(FA A4h x(xPD o —FFss D179MPa mm224NN2 xx1—s179ch AFP mmMPa D45NN2222442 diameter,πx2 11MPa2 outside2 452 Ns 179Sliding mmFAcc Aπhhx(sealPD 2—Fface,mmN 22422 — 17 ) 153o mms Di ) (so4 force, F 45 N4SpringFc Ah x P Fs s 179 mm2 x 1 MPa 45 N 224 NAh π (Do2— Ds2) π (222 — 17.12) 150 mm2Hydraulicallyloaded area:44π 22222 — 17.1222 2 150 π x(PDo o222— ππ ((22—MPa172) 22 )45153x 21FAFs D150Nmm mm195 Nis2)) mm24hπ (DDo 2—4π(22AAchsh A4πD2) 42— 17.1 2) 150 mm 2ππs—D 22—17.1 150mm(Darea:o2— D s2)) 4 ((222 — 17.12)) 150 mm2Ahh 4face(44osSliding44π D 2— D 2 ππ 222 — 17.12 150 mm2A π22()Ahs 4 (Doo — Dis ) 4((222 — 172) ) 153 mm2442 xBalancingFc Aπh x Pratio: Fs 150 mm1 MPa 45 N 195 N2As π (Do22— Di22) π22— 1722) 153 mm22(π244AππAAs πh (DD1502— Di 2) π (22 2— 17 2) 153 mm 2o— DD0.982i ) 4 (222 — 172 ) 153 mm2kAs s 44 ( (Doo2— 22 — 17 ) 153 mmA4s 153 i ) 44(2 xπFAc closingAπh x PD 2—Fforce, 150 ,mm (P1952 1 MPaTheat a2210-bar 2 1NMPa)s D 2) F— 172pressure) 45153Nmmcsi4( o4(is calculated as follows:Fc A Fs 150 mm22 x 1 MPa 45 N 195 NAhh xx P150xFkFc AP 0.9815045 195xhAhx PP FFFss 150mmmm22 xx111MPaMPa 45N195NFcc AA 150mmMPa 45NN 195NN153hss16Fc Ah x P Fs 150 mm2 x 1 MPa 45 N 195 Nloaded areaAAHydraulically150k h h 0.98Sliding face areaAsAs 153Ah 150k AAh 1500.98150 153kk AAhsh 150 0.980.98k As 153 0.98

In the examples above, where the areas of the sliding faces and the spring force are equal, theclosing force is reduced from 224 N to 195 N by reducing the balancing ratio from k 1.17 to k 0.98.A smaller closing force gives less wear on the sliding faces because improved lubrication isobtained. The result is also a higher leakage rate.LeakageThe lubricating film formed in the sealing gap during pump operation results in the escape ofsome of the pumped medium to the atmospheric side. If the mechanical seal works well andno liquid appears, the lubricating film has evaporated due to heat and pressure decrease inthe sealing gap. Therefore, no liquid seeps out of the seal.Fig. 1.18: Seal with excessive leakageNote that evaporation of water can take place at temperatures below 100 C, unless thesurrounding atmosphere is saturated with vapour. Think of how you can dry your clothesoutside on a clothes line.The leakage rate of a mechanical shaft seal depends of a number of factors such as: surface roughness of seal facesflatness of seal facesvibration and stability of pumpspeed of rotationshaft diametertemperature, viscosity and type of pumped mediumpump pressureseal and pump assembly.17

IntroductionCalculation of leakage rateThe leakage rate of a liquid-lubricated mechanical shaft seal with parallel seal faces throughthe sealing gap can be calculated by means of this approximate formula:Formula 2:Q π x Rm x h3 x p6xηxbWhereQ leakage rate per unit of timeRm average(22radiusthe sliding face 17) ofR 9.75 mmh gapmheight4between the sliding faces (thickness of the lubricating film)Δp differential pressure to be sealedh dynamic viscosity of the pumped mediumb radial (22extensionthe sealing gap (sliding face width).– 17) ofb 2.5 mm2The leakage rate, Q, is then linear to the radius, Rm, sliding face width, b, and pressure difference, Δp.The gap height, h, however, is extremely important. Note that twice the height causes eighttimes as much leakage, with all other conditions remaining the same.(22 – 17)decreasesb leakage 2.5 mm when viscosity, h, increases. But when viscosity increases, theIt seems as if the2lubricating film and thus the sealing gap increases, which may result in an increase in the leakagerate. The increase in sealing gap height with an increase in viscosity is not linear. This makes itdifficult to predict whether or not an increase in viscosity results in a higher or lower leakage rate.-63x62-3xxxxxQ π 9.75 10 m (0.2 10 m) 1 10 N/m 1.63 x 10-11 m3/s 0.06 ml/h2-3The roughness and flatnessof theslidingx s/mx 2.5 x faces6 x 0.001N two10 maffect the height of the sealing gap andx h3 x pπ x RmThethus theQleakage.hydrodynamic pressure increases with the speed. This can cause an xb6 x ηheightincrease of the gapand thus the leakage rate.A gap height between the sliding faces of 0.2 micron is typical for a mechanical shaft sealrunning in water. Consequently, the seal faces have to be very smooth and flat.Rm (22 17) 9.75 mm43xπ x Rm x h pThe calculation example below applies to a GrundfostypeH seal running in water at 20 C at aQ xxpressure of 10 bar. A sealing gap of 0.2 mm is assumed.6 η b(22 – 17) 2.5 mmΔpb 10 bar 1 MPa 1 x 106 N/m22x 3xxπRh pDo 22 mmmQ Rm (22 17) 9.75 mmDi 17 mm6xηxb4Viscosity 1 cst 0.001 N x s/m2h 0.0002 mm 0.2 x 10-6 mb (22 – 17) 2.5 mmRThus, m (22 2 17) 9.75 mm and b (22 – 17) 2.5 mm42Using formula 2, the leakage rate, Q, is as follows:-63x62-3xxxxxQ π 9.75 10 m (0.2 10 m) 1 10 N/m 1.63 x 10-11 m3/s 0.06 ml/hb (22 – 17)6 x 0.0012.5 mm2x-3xxN s/m 2.5 10 m2b (22 – 17) 2.5 mm2 in a sealing gap of 0.3 micron, the leakageIf the roughness of the seal faces is higher, resultingrate is 0.2 ml/h.18b (22 – 17) 2.5 mm2-63x62-3xxxxxQ π 9.75 10 m (0.2 10 m) 1 10 N/m 1.63 x 10-11 m3/s2-36 x 0.001 N x s/m x 2.5 x 10 m

AtmosphericpressureABCDNon-parallel seal facesSystemIn practice,the seal faces become distorted due to temperature and pressure gradients. ThepressurePumpmost typical deformation is a tapered seal ureFig. 1.19: Converging sealing gapBCFig. 1.20: Diverging sealing gapDAtmosphericFor non-parallel seal faces, the hydrostatic pressurepressure no longer decreases linearly from theABCDvalid for calculatingpump side to the atmospheric side. In this situationformula2 is no longerthe leakage rate.PumpConvergingsealing gappressureWhen the sealing gap opens towards the pumped medium, as shown in fig. 1.19, the hydrostaticpressure increases. This is called a converging sealing gap. It appears as the blue curve in fig. 1.21.PumpDiverging sealing gappressureAtmosphericpressureWhen the sealing gap opens towards the atmospheric side, as shown in fig. 1.20, the hydrostaticpressureAdecreases.This is a calledaDdiverging sealing gap. It appears as the orange curve in fig. 1.21.BCThe pressure distribution in the sealing gap is obtained by adding the hydrostatic pressure andthe hydrodynamic pressure. This is shownAtmospherein fig. 1.22. Note the similarity with fig. 1.14 a, page 14.BCDAbsolute pressure mped medi6Converging4AtmosphereBCDFig. 1.21: Hydrostatic pressure distributionfor different sealing gapPump geometriespressure2Diverging0ABCD8090100110Fig. 1.22: Hydrostatic and hydrodynamicpressure distribution for different12sealinggap geometriesute pressure [bar]AAtmosphere1086WaterPumped medium pressureVapour19120

APump medium pressureIntroductionStationaryseatBCDRotatingseal ringThe frictional heat in the seal faces increasesthe temperature of the medium resulting in anincrease of the vapour pressure. This moves thestart of evaporation point to the pumped mediumside. See fig. 1.23.PressureLiquidpressureFor seals in cold water, the lubricating filmextends through the entire sealing gap. For a wellfunctioning seal, the only leakage escaping on theatmospheric side is vapour. The evaporation willoccur even in cold water due to leakages throughthe very narrow sealing gap, i.e. 0.0002 mm.VapourpressureSev tartap oforationExat it tm oosphereAtmosphericpressureEse ntryal inin tg ogapEvaporationThe absence or inadequate formation oflubricating film frequently causes damage to theseal faces. Evaporation of the pumped medium inthe sealing gap occurs where the pressure is belowthe vapour pressure of the pumped medium.DistanceA partial lack of lubricating film often occurs in thesliding seal faces towards the atmospheric sidewhen pumping water above 100 C. This is due toevaporation of the lubricating film.Fig. 1.23: Pressure distribution in a sealinggap with hot waterStationary seatDeposits and wear tracksWhen the lubricating film in the sealing gapevaporates, dissolved solids are left deposited onthe seal faces.If the thickness of deposits exceeds the necessarythickness of the lubricating film, the seal startsto leak.In case of hard deposits, wear tracks can develop inone of the seal rings, see fig. 1.24a. In case of soft andsticky deposits, a build-up can cause the seal faces toseparate, see fig. 1.24b.Rotating seal ringFig. 1.24a: Development of wear tracksdue to hard depositsStationary seatRotating seal ringFig. 1.24b: Deposits build-up on seal faces20

AtmosphereBCDPumpcurveVapour pressurepressureIn order to secure a proper liquid lubrication in themajor part of the seal gap, it is recommended tokeep the temperature around the seal at 10 to 15 Cfrom the vapour pressure curve. The curve forwater canAtmospherebe seen in fig. 1.25.ABCDAbsolute pressure [bar]A1210Water8Pumped medium pressure64Vapour20VapourpressureNormal atmospheric pressure8090100110120130140150160170180Temperature [ C]Fig. 1.25: Vapour pressure curve for waterFrictional heatA mechanical shaft seal generates frictional heat. If the lubrication is poor, the heat generatedcan be as high as 100 watts/cm2. Compared to this, a cooking plate generates around10 watts/cm2 at maximum power. To minimise the temperature increase in the sealing gap, itis important to remove the heat. The amount of heat removed is determined by these factors:···liquid flow in the seal chamberthermal conductivity of the machine partsconvection to the atmosphere.Sometimes the influence of these factors is not sufficient, causing the lubricating film in thesealing gap to evaporate. This results in dry running of the seal.The power loss, P, due to friction can be calculated by means of the following formula:P Fc x f x vWhere:Fc Closing forcef Coefficient of frictionv Sliding speedThe coefficient of friction (COF) depends on the lubrication and the pairing of the seal facematerials. For well-lubricated seal faces, the factor is between 0.03 and 0.08.In case of poorly lubricated seal faces, the COF depends on the seal face materials. Thus if thetwo seal faces are made of hard materials such as tungsten carbide, a COF up to 0.4 is possiblein hot water.For a balanced Grundfos type H shaft seal for a Ø16 shaft at 2900 min-1 and 10 bar, assumingf 0.04, the situation is as follows. See page 16:Fc 195 N, f 0.04, v 3.0 m/sP Fc x f x v 195 [N] x 0.04 x 3.0 [m/s] 23.4 [W]Turbulence loss in the seal chamber generates small amounts of heat when the sliding speedis below 25-30 m/s.Sometimes a narrow seal chamber requires additional precautions to remove the heat, forexample increased circulation of the pumped medium around the seal. See Chapter 2, page 31.21

Introduction1952Grundfos CP pumpwith unbalancedO-ring seal1971Grundfos CR pumpwith rubberbellows seal1982Grundfos CH 4 pumpwith unbalancedO-ring seal1991Grundfos CHpump withunbalancedO-ring seal withspring as torquetransmissionelement1992Grundfos CHIpump with rubber bellows sealFig. 1.26: Grundfos shaft seal development4. Historical developmentAt the beginning of the nineteenth century, many endeavours were made to develop areplacement for the conventional, braided packing used for piston pumps and rotating shafts.A more reliable system for different kinds of liquid-conveying rotating machinery was desired.By the 1930’s, the James Walker Group came up with a mechanical shaft seal for refrigerationcompressors. At the same time, the John Crane company invented the first automotivemechanical shaft seal. In the early 1940’s, the company developed and introduced the patentedelastomer bellows axial sha

Mechanical shaft seal types and sealing systems 25 1. Mechanical shaft seal types 26 2. Sealing systems 31 3. Selecting a mechanical shaft seal 42 Chapter 3. Materials 45 1. Seal face materials 46 2. Seal face material pairings 51 3. Testing of shaft seals 55 4. Secondary seals 59 5. Materials of other shaft seal parts 61 Chapter 4. Tribology 63 1.