effect of surface characteristics on compressive stress and leakage rate in gasketed flanged joints

Int J Adv Manuf Technol (2003) 21:713–732Ownership and Copyright 2003 Springer-Verlag London Limited

Effect of Surface Characteristics on Compressive Stress andLeakage Rate in Gasketed Flanged Joints

J. Arghavani, M. Derenne and L. MarchandDepartment of Mechanical Engineering, Applied Mechanics, Ecole Polytechnique, Montreal, Qc, Canada

The effect of surface characteristics on compressive stress andleakage rate in gasketed flanged joints is discussed qualitativelyand quantitatively based on experimental verifications. Thesensitivity of the sealing phenomenon to the sealing surfacecharacteristics, gas pressure and stress level is presented.Sealing surfaces produced with grinding, turning and millingprocedures of different roughness values were the subject ofthe tests with three types of gasket, namely, PTFE(polytetrafluoroethylene), graphite, and spiral wound. Theexperimental results indicated that the roughness value Ra(average arithmetic height) has no effect on the sealing per-formance of the gasket, except for the ground type sealingsurfaces. The platen (flange) surface forms were found to bethe determining factor on the leakage rate and flow regimefor gasketed flanged joints. It is shown that the surface charac-teristics have no effect on the leakage rate and on the gasketperformance for the PTFE gasket at high stress levels. How-ever, at low stress levels the rougher ground and milled sealingsurfaces having radial channels on the surface produce largerleakage rates. The effect of surface characteristics, stress level,and gas pressure was more evident on the graphite and spiralwound gasket types, at both low and high stress levels.

It is also shown that the leakage rate follows an exponentiallaw as a factor of surface stress and gas pressure, for allsealing surface characteristics and gasket types. While thereis little difference in the leakage rate for the PTFE gasketwhen the gasket stress level is increased by a factor of two,from S3 to S5, a substantial leakage rate reduction by a ratioof 17.5 times was observed for the graphite and spiral woundgaskets for most sealing surfaces. On the other hand, whenincreasing the helium gas pressure from 200 to 800 p.s.i., theleakage rate increased by 5 times for the PTFE, and up to10times for the graphite and the spiral wound gaskets. It wasdetermined that while molecular flow regimes can be achievedfor the PTFE gasket for all sealing surfaces, the two other

Correspondence and offprint requests to: Dr J. Arghavani, Departmentof Mechanical Engineering, Section of Applied Mechanics, EcolePolytechnique of Montreal, Montreal, Qc, Canada, (CP 6079, succ. A,H3C 3A7). E-mail: [email protected] 28 December 2000Accepted 28 December 2001

gasket types provided laminar flow under the same operatingconditions.

Keywords: Compressive stress; Flow regimes; Gas leakagerate; Surface roughness and form

1. Introduction

Gaskets are used in many industrial applications to preventgas leakage through the flange surfaces by filling the spacescaused by surface imperfections and roughness. Many investi-gators have examined the performance of static seals with theview to reducing the leakage rate. There are many publicationsdealing with gasketed bolted flanged joints, test procedures,fluid effects, gasket characteristics, and sealing performance;however, only a few deal with the surface roughness Ra effecton the gasket performance. In most papers, it is shown thatthe lower the surface roughness is, the better is the gasketperformance, and the lower the leakage rate, this being mainlyvalid for metallic gaskets. Bazergui and Marchand [1] presenteda test procedure for the determination of gasket properties atroom temperature. Bazergui and Marchand [2] further presenteda work on the development of tightness test procedures forgaskets used at elevated temperatures. Bazergui and Louis [3]presented experimental results on the effect of various gaseson gasketed joint performance. Bazergui et al. [4] worked onthe effect of fluid on the sealing behaviour of gaskets. Marc-hand and Derenne [5] presented experimental results on thefugitive emission characteristic of gaskets. Chivers et al. [6]worked on the relationship between gas properties and surfaceroughness, and discussed flow regimes. In the case of metallicgaskets it was found that a radial scratch along the matingface would greatly increase the leak rate [6]. Mitchell andRowe [7] worked on the influence of asperity deformation onthe gas leakage between the contacting surfaces. Tuckmantel[8] presented experimental results on the effect of surfacepressure on the leakage rate. Rathbun [9] presented work onthe sealing performance of metal-to-metal and metal-gasketedseals. Bazergui et al. [4] presented work on the effect of fluidon the sealing behaviour of gaskets. Payne [10,11] worked on

714 J. Arghavani et al.

the effect of the surface roughness of stock finish turnedsurfaces on the gasket constants. Matsuzaki and Kazamaki [12]worked on the effect of surface roughness on the compressivestress in static seals. Derenne and Bouzid [13] have performedpreliminary experimental studies on the effect of flange surfacefinish at room temperatures on gasket tightness and emissions.Greenwood and Williamson [14] and Greenwood and Tripp[15] presented work on the contact of nominally flat surfaces,and two nominally flat rough surfaces, respectively. Shimomuraet al. [16] studied the relationship between the frictional charac-teristics and the surface condition of mechanical seals. Theconcepts of tribology and the properties of materials are funda-mental in the analysis of the contact mechanism in this domainand can be found in many publications similar to thosepresented in the works of Halling [17] and Askeland [18],respectively. The concepts of the physics of flow throughporous media is very well presented in the work of Scheid-egger [19].

It is worth mentioning at this point that, based on thetheory of elasticity, the stress distribution and stress patternfor different contact areas can be defined as in Fig. 1, [17].Consequently, in gasketed flanged joints, depending on theplaten surface characteristics, the leakage paths on the matingsurfaces and those due to the gasket porosity will be affectedby the platen surface characteristics. Likewise it affects thepressure distribution through the leakage paths from the highto the low pressure side, Fig. 2, [17,19].

In general, the leakage rate and the flow regime dependprimarily on the size of the leakage path, and the equivalentdiameter D and length L, and the mean free path length �which is defined as the average distance a molecule of aspecified type travels in the gas phase at the prevailing pressurebefore colliding with another molecule of the same type.Mean free path increases with the increase in temperature anddecreases with the increase of pressure and molecule size.Other factors also affect the sealing performance, such as,

Fig. 1. Pressure distribution and stress pattern for (a) a flat, (b) a groove with flat edges, (c) a cylinder, and (d) a sphere [17].

fluid type, gasket material, gas pressure, temperature, gasketcompressive stress, and flange face roughness and form. Surfaceform and roughness have been analysed to some degree inwork on static sealing gasket performance.

In this paper, we examine the effect of surface characteristicson the leakage rate for static gasket sealing performance forpolytetrafluoroethylene (PTFE) sheet, graphite sheet G2, andgraphite-filled spiral wound gaskets. It is shown experimentallythat for a given gasket, platens identical in material and surfaceroughness value Ra produce different leakage rates, dependingon whether the surfaces are machined by turning, milling, orgrinding procedures. Experiments were carried out by applyingdifferent levels of gasket compressive stress and differentgas pressures.

2. Experimental Sealing Investigation Set-up

In order to isolate the sealing problem from the substructureproblem of the test system, apparatus was used which allowsthe measurement of the leakage rate at room temperature. Theleakage rate is measured as a function of the applied stressand of the gas pressure for the given gasket type, and theplaten material and the platen surface finish, taking into con-sideration the importance of load uniformity and the eliminationof the bending moment effect.

The apparatus used permits the performance of the roomtemperature tightness test (the ROTT test) on gaskets of 4in nominal diameter. After installation, a test is performedautomatically, without the intervention of the user. The programcontrolling the test database and its different components iswritten using LabView. The test program starts based on acommand file that lists all the test steps. Such a program andthe command files allow great flexibility in controlling andmodifying parameters such as applied stress, pressure variationand control, and duration of the test. Furthermore, the program

Effect of Surface Characteristics 715

Fig. 2. Machined surfaces and their effect on gasket pore deformation and gas pressure distribution. (a) Characteristic of ground, milled andturned surfaces. (b) Gasket pore and surface deformation with respect to the sealing surface form [19]. (c) Gas pressure distribution throughthe gasket and sealing surfaces [17].

allows for a graphical interface for the duration of the test,and the evaluation of parameters and the measured results. Itis also possible to interrupt the test and carry on with manualinstructions for certain operations. Essentially, the test gasketis installed between two platens with the desired surface finish,which are then placed in the centre of the apparatus (or testchamber). The entire apparatus and hydraulic ram are installedinto a compression testing structure, and loaded by means ofa shaft located on the centre-line of the top plate of theapparatus, thus assuring the concentricity of the applied load.A uniform load is applied on the chamber by the hydraulicram, which is then transferred onto the platens and on thegasket. The system is then charged using a pneumatic pressuris-ing system. The ROTT test structure has a capacity of 180ton, and is principally composed of two lateral plates, onehead and one base.

The applied stress on the gasket is measured by a pressure-meter consisting of a loading cell placed under the hydraulicpiston, and the internal pressure settings on the gasket arecontrolled and adjusted by an MKS controller. The maximumallowable internal pressure of the pressurisation system is 100bar (1.45 Kp.s.i.). The system is capable of measuring theleakage rate by three methods, through the flow-meter, througha pressure drop check and by mass spectrometers. Generally,the flow-meter leakage measurement starts if the leakage rateis larger than 0.8 mg s−1. If the flowrate ranges between0.8 mg s−1 and 0.018 mg s−1 the flowrate measurement is perfor-med by a pressure drop check, because any leakage rate largerthan 0.018 mg s−1 would not be safe for the spectrometer sincethe flow would saturate it easily. The leakage rate measurementperformed by the spectrometer takes place at intervals of 15min, 10 min of which is the pause and the other 5 min is themeasuring time, the whole lasting for a 2 h period. Then astatistical analysis of the last three measured points is perfor-med to determine whether the leakage rate has been stabilisedor not. The leakage rate is considered stable if the standard

variation of the analysed points is less than 0.008 mg s−1.Otherwise the measurement is continued until such a result isobtained for a maximum period of 5 h.

The gas leakage through the sealing surfaces enters a vacuumwhich is connected to a helium mass spectrometer AlcatelASM 180T with a precision of x × 10−10 mg s−1. The singularityof the leakage and of the leakage path is assured by a secondaryvacuum which precludes the leakage from going from thelower test platen to the apparatus’s lower platens by an O-ring placed between their mating surfaces.

The system permits highly flexible testing conditions forwhich the load imposed, and the internal pressure and thedesired testing points can be automatically adjusted and con-trolled to suit the required design. On the periphery of theapparatus, two linear variable differential transducers (LVDTs)are mounted at 180° apart. These LVDTs ensure the uniformityof the applied load and provide information on gasketdeflection/compression under the normal applied load, andultimately, provide the gasket stress.

In the present work, the common ROTT test procedure isnot used, but its steps are modified to suit the requirementsof this experimental research. The testing procedure considerstwo cycle stress levels for both soft and standard procedures.In each cycle the stress is increased incrementally, and thendecreased to the initial stress level, Table 1. This simulatesboth the gasket ageing and bolt load relaxation during service,and perhaps the most important information gained is thesensitivity of the system under investigation to the loss ofstress. The stress level at each stress point remains constantuntil the stabilised leakage rate is measured, then it isincremented or decremented to the next stress level, while thehelium gas pressure is kept constant during the entire experi-ment. Keeping the gas pressure constant ensures the eliminationof the gas pressure effect at each stress level during the test,and only the effect of stress level and platen surface character-istic is evaluated for the given test gasket.


Table 1. Stress levels on gasket for cycle 1 and cycle 2 test.

Gas pressure (p.s.i.)

Stress levels Soft stress values Standard stress values Test series 1 Test series 2

Cycle 1 Psi MPa psi MPa psi MPa psi MPaS1 1025 7.07 1025 7.07 200 1.38 800 5.52S2 3040 20.98 4560 31.46 200 1.38 800 5.52S3 5390 37.20 8090 55.82 200 1.38 800 5.52S2.5 4220 29.12 6325 43.64 200 1.38 800 5.52S1 1025 7.07 1025 7.07 200 1.38 800 5.52

Cycle 2 S1 1025 7.07 1025 7.07 200 1.38 800 5.52S4 7750 53.50 11630 80.25 200 1.38 800 5.52S5 10107 69.70 15160 104.60 200 1.38 800 5.52S3.5 6575 45.37 9860 68.03 200 1.38 800 5.52S2 3040 20.98 4560 31.46 200 1.38 800 5.52S1 1025 7.07 1025 7.07 200 1.38 800 5.52

Helium gas 1 atm = 0.101 T = 20°C � = �/� = 122.5 × 10−6

� = 19.9 × 10−6 (Mpa) (m2 s−1)(N.s m−2), (kg m−1 s−1)Mw = 4.0026 (g mol−1) 14.69 � = 0.1625 (kg m−3)

While incrementing the gasket stress, the leakage ratedecreases. At the decrementing loads, the leakage rate mustincrease; however, for the same stress level, as on the loadingside, the gas leakage rate must be somewhat less. This isbecause while the gasket stress level is increased to itsmaximum level in a cycle, the gasket compaction increases,thus reducing the overall pore size of the gasket and resultingin a reduction of the leakage through the gasket. The sameprocedure is repeated for the same gasket type (from the samebatch) using platens with different surface characteristics. Thus,the comparison of two or more tests provides information onthe effect of surface characteristics on the compressive stress,and gasket compaction, as well as their effect on the leakagerate.

The effect of gas pressure under different stress levels anddifferent surface characteristics can be determined by changingthe helium gas pressure while repeating the same test pro-cedure, on the same gasket type with different platens. Keepingthe gas pressure constant during the test will ensure the singu-larity of both the gas pressure effect and the characteristic ofthe leakage flow regimes. While increasing the stress level,the amount of stress required to ensure sealing at differentpressure levels can also be determined.

2.1 Platens Surface Characteristics

In order to examine the effect of the sealing surfaces on theleakage rate for the given gaskets, eight different surfaceswere produced by grinding, turning and milling machiningprocedures. The platens are made of stainless steel 304,commonly used in industrial applications for flange materials.Two ground surfaces with roughness values of Ra 17 � 25 �inand 75 � 85 �in were machined. Two milled surfaces withsurface roughness values ranging from Ra 135 � 315 �in and350 � 870 �in were machined using two different toolingand milling procedures. The milled surfaces of roughness

135 � 315 �in were machined with two linear pass cuts. Theplatens were fixed stationary, and a milling cutter tool of2.362 in, diameter was used. The milled surfaces of roughness350 � 870 �in were machined with a one pass cut, where theplaten was set on the rotating platform, and a milling cuttertool of 1.969 in, diameter was used.

Four types of turned surface were machined, two were spiralcut stock finished and two were concentric cut stock finished,with roughness values ranging between Ra 630–720 �in and840–900 �in. The spiral cut stock finished surfaces of rough-ness 650–720 �in were produced with a continuous tool feedr-ate of 0.044 in per revolution, using a tool tip radius of 1/16in (1.6 mm). The spiral cut stock finished surfaces of roughness860–900 �in were produced using a 1/32 in (0.8 mm) tool tipradius with a continuous feedrate of 0.031 in per revolution.

Concentric cut stock finished surfaces were produced byrepositioning the tool after cutting each groove in a completerevolution. To produce a concentric stock finished surface formof roughness value Ra 630 � 710 �in, a cutting tool with tipradius of 1/16 in (1.6 mm) was used. The depth of the cutwas 0.020 in, with a groove spacing of 0.080 in, and thespeed of the cut was kept constant at 265 r.p.m. The concentriccut stock finished surfaces of 840–900 �in roughness weremachined using a tool of tip radius 1/32 in (0.8 mm). Thedepth of the cut was 0.031 in, at 265 r.p.m., with a groovespacing of 0.075 in. The platens described above are presentedin Table 2 and their surface profile is given in Figs 3 and 4,along with their identification number, roughness values, sur-face form, and the machining parameters used to produce them.

2.2 Gasket Types

Three gasket types were selected for the present study, namelyPTFE GYLON 3504 sheet, G2 UCAR graphite sheet, andspiral wound with graphite filling. The PTFE gasket providesa high degree of flexibility for filling in the existing gaps


Table 2. Machining parameters and procedures used to produce platens.

Machining Surface Ra (�-in) Platens ID Platens ID Tool tip Depth of Tool feed Speed of Feedprocedure form code radius (in) cut (in) groove cut (r.p.m.)

spacing (in)

Turning Spiral stock 650–720 I3 SSI3–700 1/16 �0.020 �0.080 265 Continuousfinish 860–900 I1 SSI1–900 1/32 �0.031 �0.075 265 ContinuousConcentric 630–710 J3 SCJ3–700 1/16 0.020 0.080 265 Retracting toolstock finish 840–900 J1 SCJ1–900 1/32 0.031 0.075 265 Retracting tool

Milling Milled two 135–315 I12 MI12–315 �2.362 N/A ContinuouspassesMilled one 350–870 I13 MII3–870 �1.969 N/A Continuouspass

Grinding Grind 17–25 III1 GIII1–20 N/A N/A N/A N/A N/AGrind 70–85 III2 GIII2–80 N/A N/A N/A N/A N/A

generated by the machining process on the platen surfaces.This gasket has a low degree of porosity. The G2 graphitegasket is typically made of graphite layers and provides thenecessary filling for the platen surfaces irregularities. However,it is considered to be a highly porous type, thus it is likely toproduce higher leak rate. The spiral wound gasket is metallicwith in-filled graphite material. It provides initial filling of theirregularities on the flange faces at the initial loadings. Noporosity occurs for this type, thus the leakage through thegasket can be ignored. It provides low leakage through thegasket, but the interfacial leak on the mating surfaces may belarge owing to improper mating of the sealing surfaces andthe gasket.

In order to provide consistency in the experimental data, allPTFE and graphite gaskets were cut from the same respectivesheets. The PTFE gaskets were cut to the dimensions of insidediameter 4.875 in, outside diameter 5.875 in, from a sheet0.064 in thick. The graphite gaskets G2 UCAR were cut tothe dimensions of inside diameter 4.875 in, outside diameter5.875 in, from a sheet 0.62 in thick. The spiral wound gasketswere made to the standard size Flex 4–600, having dimensionsof 4.75 × 5.875 × 0.177 in3.

2.3 Tests Conducted

A total of 48 tests were performed on eight surface finishesand for three types of gasket at 200 and 800 p.s.i., heliumgas pressure levels. Platens of the same identification numberrepresenting the same surface finish were mated always inpairs with gaskets, no direct metal-to-metal seal was used(Table 2, Figs 3 and 4). The gaskets were inspected before thetests to ensure that they were uniformly thick and free ofdefects (surface defect, radial scratch, damage due to bent,pre-stressed surface, etc.) that could affect the leakage results.The gaskets were never reused. Each gasket material was usedwith each of the eight different sealing surfaces, so, for agiven gasket material, the effect of the different surface charac-teristics could be apprised.

For every test, the gasket was initially compressed betweena pair of platens, the surfaces of which have some asperitydistribution, so that the higher asperities on the sealing surfaces

(platens) would mate. When the gasket stress was increased tothe low normal stress level S1, and sufficient time was givenfor gasket stabilisation, the surface asperities penetrated intothe gasket. As a result of normal stress, the area of contactincreased slightly. Then the chamber was pressurised to apredetermined constant pressure, and some time was given forpressure stabilisation before the leak measurement. The leakagebehaviour and leakage rate assessment at the low stress levelat the beginning of cycle one was very distinct for each surfacefinish. This might be due to the fact that the gaskets, beingprimarily compressed, did not fill-in the platen surfaceirregularities.

For all tests, with the gasket configuration maintained as aconstant, the leakage rate at each stress level was plotted fordifferent platen surface finishes. The reduction in leakage ratedue to the increase of normally applied stress level on thegaskets can be observed by comparing the consecutive plots.The increase in leakage rate due to the increase of helium gaspressure can be observed from the plotted results for eachplaten surface finish. The abscissa of the plot is the platenidentification, for the surfaces. The stress level is indicated onthe plot headings, and, where appropriate, the gas pressure isindicated either for the plot type or on the plot headings.

3. Experimental Results

Graphical results of the leakage rate and the gasket deflectionfor the PTFE, G2 graphite and SW gaskets tests, with respectto the platen surface characteristics are presented in Figs 5 to15. In each case, graphs are plotted for the stress applied onthe gasket and the internal helium gas pressure.

3.1 Results of PTFE Gylon Gasket Tests

The surface sealing performance results for the PTFE gaskets,at 200 and 800 p.s.i. gas pressure, are presented in Figs 5, 6,and 7. The leakage results obtained for the low stress S1 levelat the beginning of cycle 1, at 200 p.s.i. gas pressure revealthat, for the ground surface types, as the surface roughnessincreased from 20 to 80 �in, the leakage rate increased by a


Fig. 3. Surface characteristics (roughness and form) of sealing surfaces.

factor of 102. At stress levels higher than S3, both surfacesproduced approximately the same leakage rate. A similar resultis observed at a gas pressure of 800 p.s.i. Milled surfaces II2and II3 performed similarly at all stress levels, except at the

initial stress level S1, where the surface II2 (315 �in) had ahigher leakage rate by a factor of 102.

Analysing the turned surfaces performance, we note that thesurface roughness has no significant effect, but that the leakage


Fig. 4. Platen surfaces machining form, the sealing surface characteristics.

rate is attributed to the existence of radial channels on theplaten surface, as a result of peak-to-valley asperities on theapex edges of the grooves. For the concentric cut turnedsurfaces, a large drop in the leakage rate can be seen at thefirst stress level S1 that is attributed to the circumferential

concentric grooves. We noticed that the surface roughnessvalue Ra alone is not a critical factor for the leakage rate,except in the case of ground-type sealing surfaces, where atlower gasket stress levels, the rougher surface produces a largerleakage rate.


Fig. 5. Leakage rate versus stress level, for a PTFE gasket at helium gas pressure of 200 and 800 p.s.i., for all sealing surfaces.

In general, comparing the effect of platen characteristics atdifferent stress levels, Figs 6 and 7, we note that for platensmade by the same machining procedures, the leakage ratecurves corresponded to each other. For instance, for turnedsurfaces of spiral cut stock finish, both platen I1 and I3performed equally. Similar results are observed for other platentypes, under similar test conditions. We notice that the fourplatens of milled and ground-type surface finish also performedequally; and at stress levels above S2, little effect on theleakage rate change can be seen. Whereas the four turned

surfaces performed equally, as the stress level increased to themaximum, the leakage rate decreased. This trend is valid forall gasket types. Furthermore, unloading to S1 from higherstress levels in cycle 2, we notice that the ground sealingsurfaces produced a larger leakage rate compared to the milledsurfaces, in spite of the fact that the latter had more radiallydirected leakage paths on its surface. This is due to the factthat the apex of the grooves on the milled platen surfacesprovided higher local stress levels on the PTFE gasket, pen-etrating into the gasket and blocking some of the leakage


Fig. 6. Leakage rate and gasket deflection versus sealing surface characteristics, for a PTFE gasket, at helium gas pressure of 200 p.s.i.

Fig. 7. Leakage rate and gasket deflection versus sealing surface characteristics, for a PTFE gasket, at helium gas pressure of 800 p.s.i.

paths. The turned surfaces of concentric cut stock finish forms,J1 and J3, provided the lowest leakage rate, at all times,regardless of the stress level and gas pressure; these surfacesprovided sufficiently high circumferential stress patterns onthe gasket, and penetrated better into the gasket, resultingin blocking the leakage paths on their surfaces. Furthermore,the results presented in these graphs for stress levels S3

and S5, indicate that the surface characteristic had no orvery minimum effect on the leakage rate for PTFE gasketsat higher stress levels. We should mention that since theporosity level in the PTFE gasket type is negligible, thepermeation leak can be neglected, and the leakage rateobtained results from gas passing through the leakage pathon the sealing surfaces.


It is worth indicating that leakage rates were obtained at aninitial stress level of S1 and at the stress level S1 afterunloading from S3 and S5, owing to compression, cyclic, andageing effects. Although the initial S1 stress level may not beof great concern since we are unlikely to operate at that stresslevel, the cyclic effects as well as the conditions at low stresslevels after unloading from higher stress levels are of someinterest for gasket performance, for this simulates gasket ageingas a function of time under different operating conditions.Thus, ignoring the leakage rate at the initial S1 stress levels,for the PTFE gasket, all the surface finishes produced similarleakage results at all intermediate to high stress levels, andthat at stress levels higher than S3 the stress made littlecontribution to the leakage reduction (Figs 5, 6 and 7). How-ever, a higher gasket deflection was obtained as the stress levelincreased. Nonetheless, we remark that the milled surfacesprovided higher gasket deflection compared to the turned sur-faces, whereas the ground surfaces provided the lowest gasketdeflection of all under the same operating conditions of gaspressure and gasket stress. This might be due to both surfacepenetration and gasket flow.

3.2 Flow Regimes on PTFE Gaskets

Flow regimes were determined based on two different methods,for all sealing surfaces and at all test stress levels. The firstmethod determines the slope of the lines based on the leakagevariation as a result of gas pressure, where the remaining testconditions are kept the same [9]. Then by plotting log (Lrm)versus log P, with log P on the ordinate and log (Lrm) on theabscissa, a molecular flow regime is present if the slope ofthe line is equal to one, whereas, if the slope of the line isequal to two, the flow is viscous laminar [9]. The secondmethod is based on the determination of Reynolds and Knudsennumbers. The Reynolds number Re of the viscous flows canbe used to determine the flow in turbulent or laminar regimes.Knudsen’s number, Kn = �/D, the ratio of the mean free pathto the pipe diameter, can be used to distinguish between theflow in molecular (D/� � 0.1), transition (0.1 � D/� � 10), slip(10 � D/� � 100), or in laminar regimes (Re � 2000,D/� � 100) [6,19].

The first method is a generalised method and since the pathsize may very well be a function of the pressure and tempera-ture, the slope of the line from the plot of log (Lrm) versuslog P may not necessarily provide accurate results for determin-ing the correct flow regime [6]. For instance a slope of “one”indicating a molecular flow regime was found at a total massleakage rate as large as x × 10−2 mg s−1, and also as small asx × 10−6 mg s−1, where x is a real number smaller than or equalto one. Obviously, the first condition where the leakage ratewas found to be very large cannot be in the molecular regimeeven if the slope of the line is “one”. Furthermore, a slope oftwo indicating a laminar flow regime was found at lowerleakage rate values. This may be interpreted as a sealingsurface providing a large leakage rate at a low gas pressureor a low leakage rate at a higher gas pressure for the samestress level.

Based on the second method for the determination of theReynolds and Knudsen numbers, the following results wereobserved for the PTFE gaskets:

1. At a gas pressure of 200 p.s.i., all surfaces produced leakagein the slip flow regime, with the following exceptions: thesurfaces II2, II3, and III1 at the end of cycle two, whenunloading to stress level S1 = 1000 p.s.i., produced a leakagerate in the laminar regime. Whereas, at S2 = 3000 p.s.i.,and at the end of cycle one and two when unloading toS1 = 1000, the surface III2 produced a laminar flow regime.

2. At gas pressure 800 p.s.i., all surfaces provided a leakagerate in the laminar flow regime.

Obtaining the flow regimes based on the considerations ofthe Reynolds and Knudsen numbers is more accurate, in staticseals of gasketed flanged joints. Nonetheless, the accuracy ofthe application of this method in gasketed joints depends onthe assumptions made in the determination of the leakage pathsize on the sealing surfaces. This involves its own complexitiesand so far there exists no literature to correctly and preciselycover the determination of the leakage path size on the sealingsurfaces, due to the many undeterminable parameters involvingthe gasketed flanged joints.

3.3 Results of Graphite G2 Gasket Tests

The graphs of the leakage rate versus the stress for the g2Graphite gasket indicate that for all surfaces the leakage rateincreased by a factor of 10 as the gas pressure was increasedfrom 200 to 800 p.s.i. except for the ground surfaces wherethe leakage rate increased by a factor of 5, Fig. 8.

We note that at a low gas pressure of 200 p.s.i. (Figs 8 and9) in the case of turned surfaces, the platen I3 producedmore leakage than I1, whereas both sealing surfaces performedsimilarly in cycle two stress levels. Similarly, the concentriccut stock finished surface J1 produced a lower leak ratecompared to the J3, in both cycle one and two stress levels. Themilled sealing surface II2 of lower roughness value produced ahigher leakage rate in both cycle one and two compared withthe II3, whereas the ground surface III1 produced less leakagethan the III2 surface. This indicates that except for the groundsurfaces, the surface roughness value Ra, is not the controllingfactor for the leakage rate and sealing performance evaluationfor the graphite gasket types (Fig. 8). At higher gas pressure,800 p.s.i., similar results were also observed, but we notice aleakage rate increase by a factor of x × 10 (Figs 8, 9 and 10).This is a good indication of the effect of gas pressure on thesealing performance of the graphite type gaskets. Furthermore,a clear indication of the effects of surface roughness, Ra value,can be observed from the leakage rate results for the groundsurfaces, only at higher gas pressure, so that as the roughness ofthe sealing surfaces increased, the leakage rate also increased.

The effect of the compressive stress, due to surface character-istics, can be seen from the graph of the gasket deflectionversus the sealing surfaces (Figs 9 and 10). We observe thatmoving from the turned surfaces, with concentric grooves,towards the ground surfaces, the gasket deflection decreased.The LVDTs actually measured the axial movement of the


Fig. 8. Leakage rate versus stress level, for a G2 gasket at helium gas pressure of 200 and 800 p.s.i., for all sealing surfaces.

platens, indicating that the apex of the grooves penetrated intothe gasket media, indicating higher gasket deflection.

3.4 Flow Regimes on Graphite G2 Gasket

The analysis of the leakage results, at a helium gas pressureof 200 p.s.i., based on the Reynolds and Knudsen numbers,indicates that, in general, all sealing surfaces provided leakagerates in laminar flow regimes, whereas at stress levels higherthan 10 000 p.s.i., in cycle two, the flow tended to be in the

slip flow regime. The leakage rate for the turned sealingsurfaces I1 and I3 fell clearly into the slip flow regime atS5 = 15 000 p.s.i. At gas pressure 800 p.s.i., and stress levelshigher than 3000 p.s.i., all surfaces provided large leakagerates, but still the flow regime remained in the laminar state.However, at lower stress levels, especially in cycle one, theflow tended to be unstable and was in the transition laminarstate. In this case, it is found that the Reynolds number waswithin the defined range of laminar flow with Re � 1000, butthe Knudsen number was very small, i.e. �/D = 0.002 andlower (so the ratio D/� is very large, i.e. D/� = 500 and


Fig. 9. Leakage rate and gasket deflection versus sealing surface characteristics, for a G2 gasket, at helium gas pressure of 200 p.s.i.

Fig. 10. Leakage rate and gasket deflection versus sealing surface characteristics, for a G2 gasket, at helium gas pressure of 800 p.s.i.

higher). A clear definition on the state of such flow does notexist for gasketed flanged joints.

3.5 Results of Spiral Wound Gasket Tests

The graphs of the leakage rate versus the stress for spiralwound gasket (Figs 11, 12 and 13), indicate that by increasing

the gas pressure from 200 p.s.i. to 800 p.s.i. the leakage rateincreased by a factor of x × 10 for all surface types. Presentingthis in terms of leakage rate ratio [Lrm (P800)]/[Lrm (P200)],then the ratio values of 12 to 16 were obtained for the turnedsealing surfaces J3 (700 �in) and J1 (900 �in), respectively, athigh stress levels in cycle two, and a ratio of 8 was found athigh stress levels in cycle 1. Whereas, for the turned sealingsurfaces I1 (900 �in) and I3 (700 �in), the leakage rate ratios


Fig. 11. Leakage rate versus stress level, for a SW gasket at helium gas pressure of 200 and 800 p.s.i., for all sealing surfaces.

[Lrm (P800)]/[Lrm (P200)], of 10 and 44 can be seen. Suchresults indicate that the spiral cut stock finished surfaces pro-vide leakage paths outward which results in a higher leakagerate at a low gasket stress levels or at higher gas pressure.Milled surfaces produce a ratio of values 5 to 7 at high stresslevels in cycle one and ratio of value 4 at high stress levelsin cycle two. Similarly, both ground type sealing surfaces ofsmooth and rough, III1 and III2, respectively, produce leakagerate ratios [Lrm (P800)]/[Lrm (P200)] of values 3 to 4 in both

cycles. This indicates that these surfaces are leaky at both lowand high gas pressures, when used with a spiral wound gasket.

As can be seen from the graphs of the leakage rate formilled surfaces at both high and low gas pressure, the resultsobtained are almost equivalent, as shown in Fig. 11. A clearindication of the effect of the surface roughness can be seenat cycle one stress levels for the ground sealing surfaces, atboth low and high gas pressures (Fig. 11); whereas at cycletwo stress levels, the leakage graphs for both rough and smooth


Fig. 12. Leakage rate and gasket deflection versus sealing surface characteristics, for a SW gasket, at helium gas pressure of 200 p.s.i.

Fig. 13. Leakage rate and gasket deflection versus sealing surface characteristics, for a SW gasket, at helium gas pressure of 800 p.s.i.

ground surfaces show that the results obtained are equivalentfor high and low gas pressures. Among the turned surfaces,the sealing surface I3 provided the lowest leakage rate duringthe entire test. This is mainly because the distance betweenthe edges of the groove is wider and the apex of the grooveedge is smoother. Also the groove profile mated better withthe spiral wound gasket profile.

It can be observed from the graphs of the gasket deflectionversus the sealing surface characteristics that, unlike other

gaskets, the gasket deflection for all surfaces was the samewith the exception of the rougher ground surface form. Thereason why, in this particular test, the LVDTs read highergasket deflection is not known, but it may be that there wassome raised material on the sealing surface as well as on thespiral wound gasket mating surface that deformed due to theapplied stress. At higher gas pressure, the milled surfacesprovided higher gasket deflection compared to the others. Thereis no physical explanation for this phenomena except that the


value has been read by the LVDTs before the gasket stabilis-ation (Figs 12 and 13). Similar to the other gaskets, increasingthe gasket stress level from S3 to S5 resulted in highergasket deflection.

From the graphs of the leakage rate versus the sealingsurfaces at gas pressure of 200 p.s.i. (Fig. 12), we note thatafter unloading to stress level S1 in cycles one and two, allturned surfaces performed the same, with lower leakage rateat S1 of cycle two, whereas, the two milled surfaces performedthe same but produced the highest leakage rate compared tothe other surfaces. The smoother ground surface performedmuch better than the milled surfaces, but the rougher groundsurface produced a leakage rate close to those of the milledsurfaces. Similar results can be observed from the test at 800p.s.i. (Fig. 13).

3.6 Flow Regimes For Spiral Wound Gaskets

The analysis of the flow regime based on the evaluation ofthe Reynolds and Knudsen numbers indicated that at heliumtest pressure of 200 p.s.i., the four turned surfaces with concen-tric and spiral cut stock finish of roughness values 700 �inand 900 �in provided leakage in the slip flow regime at stresslevels S4, S5, as well as S3.5 after unloading from stress levelS5 in cycle two. The slip flow regime obtained at S4 and S5indicates the effect of the gasket stress level, whereas thereason for the slip flow regime in stress level S3.5 is thegasket compaction, and also the deformation of the surfaceasperities after being exposed to the high stress level, providinga higher surface area of contact. The turned surfaces, in general,provided a series of knife-edge grooves penetrating better intothe gasket medium, allowing better blockage of the leakagepaths on the sealing surface. The smoother the groove’s edge,the lower the micro-asprities on the edge apex, thus the betterthe sealing surface, which leads to fewer radial channels onthe sealing surfaces. The leakage rates obtained for the turnedsurfaces at stress levels other than those described above, andin particular at stress levels at cycle one, were in the purelaminar flow regime.

At a gas pressure of 800 p.s.i., it was found that the leakagerate obtained was in the pure laminar flow regime, at all stresslevels in cycles one and two, except for the flow obtained atlow stress level S1. The flow regime at S1 had a tendency tobe unstable and tended to move towards the transition laminarflow regime. This was because the rigid spiral wound gasketrequired high stress levels to maintain a high surface contactin order to block the leakage paths due to the surface asperities.

3.7 Comparing Sealing Surfaces and GasketPerformance

Performance of gaskets for the same operating conditions wascompared. As shown in Fig. 14, for the gas pressure 200 p.s.i.,we note that at low stress levels S1, all surfaces performedthe same for the G2 graphite type gasket, and provided aleakage rate of approximately 0.5 × 10−2 mg s−1, indicating thatthe leakage rate is independent of the sealing surface character-istics. As the gasket stress increased, a shift in the leakage

rate reduction could be seen, but little sign of a surface effectwas present. At the maximum stress level S5, the effect ofsurface roughness and surface form was more evident in thistype of gasket, owing to the fact that it is highly porous. Thus,depending on the surface macrostructure, the local stress levelson the gasket media provide leakage paths similar to thosepresented earlier (Fig. 2). At S1 stress level the spiral cut stockfinish forms I1 (900 �in), I3 (700 �in) provided the leastleakage rate at approximately 4.0 × 10−5 mg s−1, with milledsurface II2 (315 �in) being the leakiest at 2.3 × 10−4 mg s−1.This is because the former provided a series of circumferentiallocal stresses on the gasket, blocking the leakage path channelsbetter, whereas the latter had large radial grooves on thesurfaces, ultimately producing a larger leakage path both inthe gasket pore media and on the platen surfaces.

At the same gas pressure for the spiral wound gasket, atlow stress levels, the milled surface II2 of lower Ra value(315 �in), and the ground surfaces III2 (80 �in) produced thelargest leakage. The smoother ground III1 (20 �in) and milledII3 (870 �in) surfaces performed better, with lower leakagerates. It appears that the turned surfaces both with widergrooves, namely I3 and J3 with Ra = 700 �in, properly matedwith the spiral wound gasket, blocked the leakage paths onthe sealing surfaces and produced the lowest leakage rate.However, the turned surfaces I1 and J1 with Ra = 900 �in,having smaller peak distance between grooves, did not matewith the spiral wound gasket properly and thus produced largerleakage rates in comparison with I3 and J3. Furthermore, asthe gasket stress level increased, greater distinction betweenthe sealing surface effects could be seen. It can be seen thatboth the milled and the ground surfaces were very leaky,compared with the turned surfaces.

For the PTFE gasket, the effect of surface roughness canbe observed at low stress level S1 after unloading from higherstress level. The PTFE gasket released itself from the sealingsurfaces when unloading to low stress level after being com-pacted at higher stress levels. Thus, for sealing surfaces thathave radial grooves, such as milled surfaces, a substantialleakage rate increase can be seen. However, the surface charac-teristic had no or little effect on the leakage rate at stress levelshigher than S3. We must appreciate the better performance ofthe PTFE gasket over the other types, owing to the fact thatthe stress levels on that gasket, except for S1 level, are muchlower in comparison with the stress used for the two othertypes (Table 1). In fact, the effect of the gasket form and ofthe gasket roughness was evident at low stress levels at theend of cycle two, since the gasket had been compacted. It canbe deduced from the results that the turned surfaces providebetter local stress levels on the gasket media, penetrating intothe gasket and blocking any existing leakage path comparedwith the milled surfaces. On the other hand, even thoughmilled surfaces produce higher leakage due to the existence ofradial grooves on the sealing surfaces, they were to somedegree more efficient than the ground surfaces since they alsoprovided better local stress on the gasket. However, since theground surfaces could not provide any circumferential localstress on the gasket, the rougher surface with larger peaks-to-valley asperities produced the largest leakage rate.


Fig. 14. Leakage rate versus sealing surface characteristics, for PTFE, G2 and SW gaskets, at a given stress level and helium gas pressure.


We note that, although the leakage rate increased as a resultof the increase in gas pressure, the leakage patterns were foundto be similar in the range 800–200 p.s.i. helium gas pressure.In addition, we have to indicate that we have experiencedsome gasket blow-out of the PTFE type, at low stress levelsand at higher gas pressures with ground type sealing surfaces.This was due to the smoothness of the sealing surfaces. Thus,in this case, at S1 stress level, the gas pressure was maintainedat 200 p.s.i., and then increased to 800 p.s.i. after the stresslevel was increased to the next higher level.

Figures 15(a2) to 15(a9) indicate the effect of the sealingsurfaces on the leakage rate of the PTFE gasket and comparethe gas pressure effect. It can be seen that, as the gasket

Fig. 15. Effect of gas pressure and the sealing surface characteristics, on the leakage rate, for PTFE, G2 and SW gaskets.

stress increased, the effect of surface characteristics becamenegligible. A clear leakage rate increase can be seen as aresult of the gas pressure increase. We note that the effect ofsurface roughness is more evident at a higher gas pressure andat a low stress level S1 after unloading from S3 level, sincethe gasket has been compacted and the surface stress has beenreleased. The sealing surface effect is even more evident atthe S1 level after unloading from the S5 level. In this case,we can observe that the milled and the ground surfaces pro-duced higher leakage rate. At 200 p.s.i., the rougher groundsurface compared with the milled surfaces resulted in a higherleakage rate, since it provided lower local stress on the gasket.However, at the gas pressure of 800 p.s.i., we see that the


milled sealing surfaces provided the highest leakage rate com-pared to the other surfaces, because they have large radialgrooves.

The effect of the sealing surfaces, as well as the gas pressure,on the leakage rate of the G2 graphite gasket is presented inFigs 15(b2) to (b9). We notice that unlike the PTFE gasket,at low stress levels, the effect of surface characteristic isnegligible for both test series at 200 and 800 p.s.i. An increaseof the leakage rate can be seen with the increase of gaspressure. At higher stress levels, however, the effect of surfacecharacteristics is more evident, and as the gas pressure increasesat higher stress levels, the effect of surface characteristics ismore marked. We note that surfaces with larger radial groovesproduced the highest leakage rate. As a result, both the milledsurfaces and the rougher ground surfaces produced larger leak-age rates at a higher stress level, especially at higher gaspressure, compared to the other sealing surfaces. The flow, inthis case, tended to be unstable and was in the transition oflaminar flow regimes.

Figures 15(c2) to (c9), present similar test results to thosedescribed above for the spiral wound gasket. The spiral woundgasket is considered to be a rigid type and requires a higherstress level to mate the sealing surfaces. However, because thegaskets are graphite filled, and the graphite encroaches on theirmating surfaces, they fill-in the irregularities of the platensealing surfaces. Nonetheless, owing to the rigidity of thisgasket, the effect of surface characteristics (roughness andform) on the leakage rate and sealing performance can beobserved easily, especially, at lower stress levels. The turnedsurfaces I3 and J3, mate with the spiral wound gasket better,producing a lower leakage rate than the I1 and J1 sealingsurfaces. The milled surfaces having largely radial groovesproduce higher leakage rates, whereas in the case of the groundsurface types, the rougher surface produces higher leakage. Asthe gasket stress increases, the higher peak asperities on thesealing surfaces deform producing a larger contact area andthe graphite media on the gasket surface also fills in thesurface irregularities better. Thus, the leakage rate slightlydecreases owing to their combined effects, but the leakagepattern remains the same. As the gasket unloads to stress levelS1, from S5 and S3, we notice that the leakage rate increasessubstantially for the milled surfaces, where the ground surfacesare less leaky. The turned surfaces maintain a better sealing per-formance.

It is found that the leakage mass flowrate follows anexponential form [8], Lrm = exp(−ps), and this is valid forall sealing surfaces, all gasket types and all gas pressures,(Tables 3), where, and [1/p.s.i.] are the constants ofsurface pressure, and R is the correlation factor. It is worthindicating that the variation of the leakage rate at the initialand intermediate stress levels (S1 and S2) compared with thoseat higher stress levels (S3, S4, S5) is much higher for milledand ground sealing surfaces II2, II3, III1, and III2 whencompared with other surfaces.

4. Summary and Conclusions

We investigated the effect of surface characteristics (roughnessand form) on the leakage rate and the leakage behaviour on

three different gasket types. It was noted that sealing surfaceforms provide different stress levels on the gasket, affectingthe leakage paths shape, size, and directions. The surfacecharacteristics effect on the static sealing performance waspresented along with experimental results as supporting evi-dence. It was shown that surface roughness expressed by(arithmetic average height) Ra value has little or no effect onthe leakage rate, when the sealing surfaces were produced byturning or milling procedures. However, surface form plays acritical rule on the leakage path patterns and size, affectingthe leakage rate and flow regimes.

Based on the experimental results, it was shown that theeffect of sealing surface characteristics varies depending onthe gasket type. Surface characteristics were determined to beinsensitive and have little effect on the sealing performance ofthe PTFE gasket, at stress levels higher than 3000 p.s.i.,whereas, at low stress levels, the rougher ground surface andmilled surfaces, having higher surface asperities and radialchannels on their sealing surfaces, produced higher leakagerates compared with turned surfaces that have circumferentialedges.

The sealing performance of the graphite G2 gasket wasdependent on the surface machining form as well as the surfaceroughness. Milled surfaces with large radial grooves and roughground surfaces were found to produce higher leakage rates atall stress levels, in comparison with other surface types.

Spiral wound gaskets with filled graphite also behaved differ-ently depending on the surface form and roughness. Resultsindicated that turned surfaces mate properly with the spiralwound pattern producing a low leakage rate, whereas themilled surfaces with radial grooves were leakier than others.Regardless of the surface roughness value Ra, it was determ-ined that turned surfaces provided a series of circumferentiallocal stresses over the gasket, blocking the leakage path sizebetter. In general, turned sealing surfaces with a concentricstock finish form provided better sealing and lower leakagerates, regardless of the gasket type used.

While PTFE gasket were insensitive to sealing surfacecharacteristics at stress levels higher than S3, the graphite andspiral wound gaskets indicated that as the gasket stress levelincreased by twice from S3 to S5, a substantial leakage ratereduction by a ratio of 17.5 times was observed. When increas-ing the gas pressure, the PTFE gasket produced a leakage rateincrease of 5 times, the graphite and spiral wound gasketsproduced a leakage rate increase of 10 times under the sameoperating conditions. Furthermore, the PTFE gasket producedleakage in the molecular regime at high stress levels, whereasthe two other gaskets produced leakage in the laminar state,even though the maximum stress level for the PTFE gasketwas 10 000 p.s.i., and for the graphite and spiral wound gasketsit was 15 000 p.s.i.

It was found that determination of the flow regimes basedon the slope of the line from a log–log plot of leakage rateversus gas pressure did not provide a correct indication of theflow regimes. The molecular, transition, and slip flow regimescannot be clearly distinguished, since the slope of one may beobtained at large leakage rates. It was shown that, based onthis approach, a slope of one and two can be obtained atleakage rates as high as x × 10−2, which is an incorrect indi-


Table 3. Leakage rate as exponential law as a function of surface pressure at gas pressure (a) 200 (p.s.i.) and (b) 800 p.s.i.

(a) Platens Surface CharacteristicLrm = I1 I3 II2 II3 III1 III2 J1 J3exp(−,ps)

PTFE 0.0001 0.0001 0.00003 0.00003 0.00004 0.0014 0.00004 0.00004 0.0003 0.0003 0.0002 0.0002 0.0002 0.0007 0.0002 0.0002R2 0.730 0.77 0.84 0.79 0.81 0.68 0.81 0.80

G2 0.0189 0.425 0.0355 0.0347 0.0413 0.065 0.0303 0.0391 0.0004 0.0005 0.0003 0.0004 0.0004 0.0004 0.0004 0.0004R2 0.90 0.98 0.98 0.98 0.99 0.98 0.97 0.98

SW 0.0151 0.0123 0.0496 0.0097 0.0098 0.2213 0.0162 0.0084 0.0004 0.0005 0.0004 0.0003 0.0003 0.0005 0.0004 0.0004R2 0.98 0.96 0.64 0.80 0.75 0.82 0.95 0.95

PTFE 0.0001 0.0002 0.0015 0.0105 0.0145 0.0344 0.00009 0.0002 0.0002 0.0003 0.0005 0.0008 0.0008 0.0009 0.0002 0.0003R2 0.84 0.73 0.77 0.79 0.70 0.69 0.90 0.74

G2 0.2068 0.21 0.0617 0.0606 0.1514 0.1079 0.0606 0.0642 0.0004 0.0004 0.0002 0.0002 0.0004 0.0003 0.0003 0.0003R2 0.96 0.97 0.93 0.92 0.96 0.97 0.98 0.97

SW 0.0188 0.0115 0.18 0.0221 0.0181 0.1593 0.0402 0.0185 0.0003 0.0003 0.0004 0.0002 0.0002 0.0004 0.0003 0.0003R2 0.88 0.95 0.85 0.96 0.90 0.88 0.97 0.98

cation of flow regime. Flow regimes can be determined morerealistically based on the evaluation of Reynolds and Knudsennumbers. However, for gasketed flanged joints, this becomesmore complex when determining the exact leakage path size.In the present work, the leakage path size was assumed initiallyto be equal to the surface roughness value by consideration ofthe total leakage rate, after the gasket was given sufficienttime to stabilise between the sealing surfaces at the firststress level.

The results obtained highlight that the gasket sealing per-formance was system dependent and must be evaluated basedon the application requirements. In order to evaluate the surfaceroughness effect precisely, the surface form must be eliminatedfrom the sealing surfaces, and only one particular surface formmust be considered with different roughness values. Further-more, since it was determined that turned surfaces with concen-tric cuts provide the best sealing performance, to gain betterinsight into their effects and to standardise the sealing proper-ties, future research should consider the properties of the edgesof grooves and the apex shape, number of grooves, groovespacing as well as the micro-asperities on the apex edges,which in general are the controlling factors in the leakagechannels on the sealing surfaces.

References

1. A. Bazergui and L. Marchand, “A test procedure for determiningroom temperature properties of gaskets”, ASME, Proceedings ofthe Pressure Vessels and Piping Conference, 98(2), pp. 95–103,1985.

2. A. Bazergui and L. Marchand “Development of tightness testprocedures for gaskets in elevated temperature service”, WeldingResearch Council Bulletin, 339, September 1988.

3. A. Bazergui and G. Louis, “Tests with various gases in gasketedjoints”, Experimental Techniques, Society for Experimental Mech-anics, 12(11), pp. 17–21(s), November 1988.

4. A. Bazergui, L. Marchand and J. R. Payne, “Effect of fluid onsealing behaviour of gaskets”, 10th International Conference onFluid Sealing, Innsbruck, Austria, Paper H2, pp. 365–385, 3–5April 1984.

5. L. Marchand and M. Derenne, “Fugitive emission characteristicsof gaskets”, Pressure Vessel Research Council, Project no. 92–25,February 1997.

6. T. C. Chivers, R. P. Hunter, W. J. Rogers and M. E. Williams,“On the relationships between gas properties, surface roughnessand leakage flow regimes”, Proceedings of the 7th InternationalConference on Fluid Sealing, Nottingham, UK, Paper D3, pp. 13–24, 24–26 September 1975.

7. L. A. Mitchell and M. D. Rowe, “ Influence of asperity defor-mation mode on gas leakage between contacting surfaces”, Journalof Mechanical Engineering Science, 11(5), pp. 534–545, 1969.

8. H. J. Tuckmantel, “Leak rate as a function of surface pressure”,2nd International Symposium on Fluid Sealing, CETIM, La Baule,France, pp. 99–102, 18–20 September 1990.

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10. J. R. Payne, “Effect of flange surface finish on constants for steeljacketed and other gaskets”, 3rd International Symposium on FluidSealing, Biarritz, France, pp. 505–519, September 15–18, 1993.

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Nomenclature

surface pressure constants

surface pressure constants (1/psi)

� surface stress (p.s.i)

a,b,c width of contact surface

c1 constant, in � approximately equal to 2

D capillaries tube diameter

Dg gasket deflection (in)

Kn Knudsen number, �/D

L length of contact surface

Lrm total mass leak rate through the gasket and platens mating

faces, (mg s−1)

M molecular weight of the gas

p mean pressure

P helium gas pressure controlled in the rig test (p.s.i)

ps surface pressure or stress on the gasket surface area (p.s.i)

R gas constant

Ra arithmetic average roughness

Re Reynolds number, �VD/�

Sg gasket stress based on full gasket area (p.s.i)

T absolute temperature

W normal load

x a real number � 1.0� mean free path length, � = ci

�

p ��RTM �

� gas dynamic (absolute) viscosity (N.s m−2) = (kg m−1 s−1)

� kinematic viscosity, �/� (m2 s−1)

� gas density, �/� (kg m−3)

effect of surface characteristics on compressive stress and leakage rate in gasketed flanged joints

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