AECL-6968

ATOMICENERGY & £ X l L'ENERGIE ATOMIQUEOF CANADA LIMITED V j S V DU CANADA LIMITEE

INVESTIGATION OF PERMEABILITY OF CARBON-GRAPHITEMATERIALS: RESULTS AND CONCLUSIONS

Etude de la permeabilite des graphites au carbone:resultats et conclusions

C.A. KITTMER

Chalk River Nuclear Laboratories Laboratoires nucle'aires de Chalk River

Chalk River, Ontario

October 1980 octobre

ATOMIC ENERGY OF CANADA LIMITED

INVESTIGATION OF PERMEABILITY OF CARBON-GRAPHITE MATERIALS:

RESULTS AND CONCLUSIONS

by

C.A. Kittmer

Mechanical Equipment Development BranchChalk River Nuclear LaboratoriesChalk River, Ontario KOJ 1JO

1980 October

AECL-6968

L'ENERGIE ATOMIQUE DU CANADA, LIMITEE

Etude de la perméabilité des graphites

au carbone: résultats et conclusions

par

Ce travail avait pour but d'étudier la perméabilité de diversgraphites au carbone couramment employés dans les joints des systèmes àeau sous pression. On a employé une technique de décroissance à hautepression afin d'obtenir des valeurs pour le coefficient de perméabilité( $0 ) résultant d'un écoulement visqueux. Des cylindres d'essai ont étépressurisés intérieurement. Des relevés de pression ont été enregistrésà mesure que le gaz s'échappait à travers la paroi des spécimens. L'analysegraphique des données d'essai a donné les valeurs recherchées pour 3o.

Douze qualités de graphites au carbone provenant de troisfournisseurs ont été mises à l'essai. Pour ces matériaux (Union Carbide:qualités CCP, CCP72 et CNFJ; Morganite: qualités MY3B, MY3K, CY10 et CY10C; PureCarbon: qualités P-19, P-49, P-69, P-5733 et P-658RC) les valeurs de 60se situaient dans la gamme allant de 10" 1 1 à 10" 1 6 cm2. Les résultatsobtenus indiquent que &o peut varier légèrement en fonction de la teneur enhumidité et de la température du spécimen ainsi que de la pression employéedans les essais.

Des spécimens CCP ont été imbibés d'un produit d'imprégnation pourcompléter la gamme des graphites au carbone trouvés dans le commerce. Lesrésultats ont montré que la résine phénolique (le principal produit d'im-prégnation employé) peut pénétrer dans des composants ayant jusqu'à 2.6 cmd'épaisseur. Il se produit, cependant, une décroissance graduelle dans leproduit d'imprégnation à mesure qu'il s'approche de la paroi du composant.Par contre, pour P-658RC, la matière ayant pour but de réduire la perméabilitéa une très faible profondeur de pénétration (de l'ordre de 0.3 cm). Cesessais ont également montré que l'on peut facilement détecter la perméabilitéanormale des rondelles d'étanchéité trouvées dans le commerce.

Laboratoires nucléaires de Chalk RiverChalk River, Ontario KOJ 1JO

Octobre 1980

AECL-6968

ATOMIC ENERGY OF CANADA LIMITED

INVESTIGATION OF PERMEABILITY OF CARBON-GRAPHITE MATERIALS

RESULTS AND CONCLUSIONS

by

C.A. Kittmer

SYNOPSIS

The object of this work was to study permeability of variouscarbon-graphites commonly used for seals in pressurized watersystems. A high-pressure decay technique was used to obtainvalues for the permeability coefficient (So) resulting fromviscous flow. Test cylinders were internally pressurized.Pressure readings were recorded as the contained gas was allowedto flow through the specimen walls. Graphical analysis of thetest data gave values for $o.

Twelve grades of carbon-graphite from three suppliers weretested. For these materials (Union Carbide grades CCP, CCP72,CNFJ; Morganite grades MY3B, MY3K, CY10, CY10C; Pure Carbongrades P-19, P-49, P-69, P-5733, P-658RC) go values were in therange 10" 1 1 to 10~ 1 6 cm^ . Results indicate that 3ocan vary slightly with specimen moisture content andtemperature, as well as with test pressure.

CCP samples were impregnated to complement the stock ofcommercially available carbon-graphites. Results showed thatphenolic resin (the main impregnant used) can penetratecomponents up to at least 2.6 cm thick. There is however agradual decrease in impregnant as the centre of the componentwall is approached. In contrast, for P-658RC, the materialpresent to reduce permeability has a very shallow depth ofpenetration (in the order of 0.3 cm). Testing also indicatedthat abnormal permeability in manufactured seal rings is readilydetectable.

Mechanical Equipment Development BranchChalk River Nuclear LaboratoriesChalk River, Ontario KOJ 1J0

1980 OctoberAECL-6968

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. OBJECTIVES 1

3. EXPERIMENTAL METHOD 1

3.1 Material 13.2 Equipment 2

3.2.1 Permeability Test Apparatus - InternalPressurization 2

3.2.2 Permeability Variation Test Apparatus 23.2.3 Permeability Test Apparatus - External

Pressurization 23.2.4 Permeability Test Jig for Full-Size Seal Rings 2

3.3 Test Procedures 33.3.1 Standard Permeability Test 33.3.2 go Variation Due to Procedure 3

3.3.2.1 Sample Moisture Content 33.3.2.2 Sample Temperature 43.3.2.3 Test Pressure 4

3.3.3 Permeability Variation Within Each Cylinder 43.3.4 External Pressurization Permeability Test 53.3.5 Impregnation Procedure 53.3.6 Test for Effect of Impregnant 63.3.7 Test for Effect of Impregnant Viscosity 63.3.8 Test for Penetration of Impregnant 63.3.9 Testing of Full-Size Seal Rings 7

4. TEST RESULTS 8

4.1 Standard Permeability Test 84.2 So Variation Due to Procedure 8

4.2.1 Sample Moisture Content 84.2.2 Sample Temperature 84.2.3 Test Pressure 9

4.3 Effect of Impregnant 94.4 Effect of Impregnant Viscosity 94.5 Impregnant Penetration 94.6 Permeability Variation Within Each Cylinder 94.7 External Pressurization Test 104.8 Testing of Full-Size Seal Rings 10

5. DISCUSSION 10

6. CONCLUSIONS 13

7. RECOMMENDATIONS 14

FIGURES 1-8

TABLES 1-10

APPENDIX

NOMENCLATURE:

A cross-sectional area of porous medium (cm^)

A1 logarithmic mean area of specimen wall (cm')

C,C integration constants

H specimen axial length (cm)

K total permeability coefficient (cm^/s)

Ko permeability coefficient for combined slip and

Knudsen flow (cm)

L length of porous medium In direction of flow (cm)

V internal volume of specimen and all associated tubing

(cm3)

h slope of straight line plot of ln(pm/Ap) versus

time

PI pressure at inlet side of specimen (Pa)

P2 pressure at outlet side of specimen (Pa)

Ap = Pi~P2> Instantaneous pressure difference

across the specimen (Pa)

pm = (pi + P2)/2, Instantaneous mean pressure

(Pa)

q total flow rate (cm^/s)

r^,r2 inside and outside radii, respectively, of

permeability specimen (cm)

t time (s)

v" mean thermal molecular velocity of gas (cm/s)

go permeability coefficient for viscous flow (cm^/s)

D • gas viscosity (Pa*s)

dp/dx pressure gradient along the direction of flow

(Pa/cm)

1. INTRODUCTION

Carbon-graphite is widely used throughout the nuclearindustry in rotary shaft seals. Experience at ChalkRiver Nuclear Laboratories (CRNL), however, has identi-fied many problems associated with its use. One of themajor problems is permeability, the ability to transmitfluid through a seal ring by way of interconnected pores.Work over the past three years has looked at the influ-ence of permeability on dimensional instability, erosionand inter-component leakage, and property variability offinished components made of carbon-graphite.

This report deals with an investigative program lookingat permeability of various grades of carbon-graphite. Ahigh pressure decay technique was used to determine sam-ple permeability. Details of the theory involved, alongwith a sample calculation, are given in the Appendix.Work was done in an attempt to characterize carbon-graphite through impregnant distribution and permeabilityvariation within samples. This report summarizes allwork done to date at CRNL on permeability of carbon-graphite for nuclear rotary seals.

2. OBJECTIVES

The objectives of this investigation were:

a) to investigate permeability of carbon-graphites usedin rotary shaft seals;

b) to gain experience in permeability testing andimpregnation techniques.

3. EXPERIMENTAL METHOD

3.1 Material

Thiss series of tests was concerned with twelve materialsfrom three suppliers. These are as follows:

1. Union Carbide grades CCP, CCP 72, CNFJ2. Morganite grades MY3B, MY3K, CY10, CY10C3. Pure Carbon grades P-19, P-49, P-69, P-5733, P-658RC

Typical properties of these materials are given in Table• 1 .

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3.2 Equi pment

3.2.1 Permeability Test Apparatus - Internal Pressurizatlon

A cross section of this test jig is shown in Figure 1.The test cylinder is clamped between two steel plates andsealed at both ends by a rubber gasket. The cylinder isinternally pressurized using nitrogen gas.

Union Carbide and Morganite specimens were machined fromblank cylinders as close as was available to the finishedcylinder dimensions (8.9 cm OD x 5.1 cm ID x 3.8 cmlong). Pure Carbon samples were purchased as finishedcylinders. Table 2 gives rough blank sizes whereapplIcable.

3.2.2 Permeability Variation Test Apparatus

This test apparatus is shown in Figure 2. O-ring sealedcollection lines are clamped to the outer surface of thetest cylinder at six different locations. The cylinderis then clamped between two steel plates and Internallypressurized. Gas leaking through the wall at eachsampling location flows through plastic tubing to anInverted graduated cylinder where it Is collected bywater displacement.

3.2.3 Permeability Test Apparatus - External Pressurization

This apparatus is shown in Figure 3. The carbon-graphitesample Is clamped between steel plates and sealed byrubber gaskets as before; however, this time nitrogen gaspressure is applied externally to the test cylinder.

3.2.4 Permeability Test Jig for Full-Slze Seal Rings

This test fixture is shown in Figure 4. As with theprevious test, pressure is applied to the test ringexternally. The fixture is large enough to test sealrings used in primary circulating pumps in CANDU(Canad ian Deuterium Uranium) nuclear generating stations.Piston pressures of ~6.9 kPa were required to ensure therubber gaskets sealed the ends of the test ringprope rly.

-3-

3.3 Test Procedures

3.3.1 Standard Permeability Test

The following general procedure was used:

1) Each cylinder was washed in soap and water to removeany surface scale, grease or fine carbon particles frommachining. The surfaces were dried with blasts ofcompressed air (free from oil).

2) Just prior to testing, each cylinder was heated in anoven at 110°C for one hour (to remove any internalmoisture and provide a common starting place). Thecylinders were allowed to air cool at room temperature(~ \ hour).

3) The permeability test apparatus was assembled andchecked for leaks when pressurized (ensuring sufficientbolt torque or end load to cause the rubber gaskets toseal properly).

4) The test cylinder was internally pressurized to 1.4MPa (or the required test pressure), and isolated fromthe gas bottle.

5) The time was recorded for every 10% (approximate)change in pressure reading (the standard pressure gaugeused had small graduations).

6) Accurate measurements were made of the connectingtubing length, and the OD, ID and length of the sample.

The standard test was carried out initially on twosamples of each of the test materials.

3.3.2 3o Variation Due to Procedure

3.3.2.1 Sample Moisture Content

1) A CCP cylinder was completely submerged indistilled water for one-half hour. The cylinder wasthen tested a number of times using the standardpermeability test of the previous section omittingsteps 1 and 2 (washing and drying).

2) The CCP cylinder was impregnated with distilledwater using the vacuum-pressure technique outlined inSection 3.3.5. The cylinder was then tested a numberof times using the standard permeability testomitting steps 1 and 2 except for the last two runs.

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3.3.2.2 Sample Temperature

1) A cylinder was tested using the standard perme-ability test of Section 3.3.1-

2) The assembled rig was then placed in an oven at150°C. After allowing the assembly to heat up to150°C ( ~1 hour), the test cylinder was repressurizedto 1.4 MPa. The time was recorded for every 10%(approximate) change in pressure reading.

3) The assembled rig was then placed in a water bathat 2°C (using dry ice to cool the water). Afterallowing the assembly to cool to temperature (~ 1hour), the test cylinder was repressurized to 1.4 MPaand the pressure versus time data recorded.

This test was performed using CCP and MY3K testcylinders.

3.3.2.3 Test Pressure

1) A CCP cylinder was tested five times using thestandard permeability test of Section 3.3.1. Variousinitial test pressures from 0.69 to 2.76 MPa wereused .

2) Step 1 was repeated using a CY10 cylinder.

3) Step 1 was repeated using an MY3B cylinder.

3.3.3 Permeability Variation Within Each Cylinder

The following procedure was used:1) Each cylinder was washed and dried as outlined insteps (1) and (2) of the general procedure Section3.3.1.

2) The 0-ring sealed collection tubes were clamped tothe outer surface of the test cylinder. The gascollection cylinders were filled with water and hunginverted on the side of a water tank. The gas collectiontubes were inserted into the submerged mouths of thecollection cylinders.

4) The test cylinder was internally pressurized to thetest pressure (0.35 to 1.4 MPa - refer to Table 8 ) .

-5-

5) The amount of gas collected from each of thesampling locations was measured.

Where available, three samples of each of the followinggrades were tested: CCP, CCP72, P-49, P-5733, P-19,CY10, MY3K and MY3B. For the less permeable grades(P-19, MY3K, and MY3B) no gas could be collected by thewater displacement technique. Helium gas. wassubstituted for nitrogen gas and a helium leak detector(capable of detecting trace amounts of helium) was used.

3.3.4 External Pressurization Permeability Test

Several samples previously tested were selected to seethe effect if any of reversing the direction of gas flowon the calculated permeability. The standard permeabil-ity test procedure (steps 1-6 of Section 3.3.1) wasfollowed using external pressurization in step 4.

3.3.5 Impregnation procedure

The following procedure was used:

1) Steps 1 and 2 of Section 3.3.1 were followed forcleaning and drying each cylinder.

2) The test cylinder was placed in an impregnationvessel. An absolute vacuum of ~1 kPa was drawn andmaintained for one hour.

3) The vacuum pump was isolated and sufficient impreg—nant was drawn into the vessel to completely cover thetest cylinder. (Refer to Table 3 for properties ofimpregnants used).

4) The vessel was pressurized to ~7 MPa with nitrogengas and left overnight (minimum 16 hours).

5) The gas pressure was used to expel the excessimpregnant from the vessel into a storage container.

6) The test cylinder was removed from the vessel andcured according to the instructions for the impregnantused (specific curing cycles are included in Table 3).

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3.3.6 Test: for Effect of Impregnant

1) The CCP-1* cylinder was impregnated following Section3.3.5 using General Electric G9555 varnish.

2) The CCP-2 cylinder was impregnated using Aremco-Seal529.

3) The CCP-3 cylinder was impregnated using Dow CorningGP-77 varnish.

4) A small cut ( ~25 ym) was machined off the OD and IDof the impregnated cylinders to remove excess impregnant,and the standard permeability test was applied.

3.3.7 Test for Effect of Impregnant Viscosity

1) The CCP-4 cylinder was impregnated followingSection 3.3.5 using G9555 varnish with a viscosity of~1.0 Pa.s @ 25°C.

2) The CCP-5 cylinder was impregnated with diluted G9555varnish (viscosity was ~0.2 Pa.s @ 25°C).

3) The standard permeability test was carried out oneach of the impregnated cylinders.

4) Steps 1 through 3 were repeated three times resultingin a total of four impregnations for each cylinder.

3.3.8 Test for Penetration of Impregnant

The following procedure was followed:

1) A cut (25-250 ym) was machined off the OD and ID ofthe test cylinder.

2) A standard permeability test (Section 3.3.1) wascarried out.

3) Steps 1 and 2 were repeated a number of times.

* NOTE: CCP-1: CCP designates material grade; 1 designatessample identification number.

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The above test was performed on the following samples:

a) CCP-4: impregnated 4 times at CRNL using G9555impregnant with viscosity "1.0 Pa.s @25°C (as per Section3.3.5).

b) CCP-5: impregnated 4 times at CRNL using dilutedG9555 impregnant with viscosity ~0.2 Pa's @ 25°C (as petSection 3.3.5) .

c) CCP-7: in unimpregnated state to see effects of"ichining process (a control sample).

d) CY10C: to see effects of manufacturer's impregnationprocess.

e) P-658RC: to see effects of phenolic resin "coating"by manufacturer.

3.3.9 Testing of Full-Size Seal Rings

After washing and drying the seal rings in the standardway (steps 1 and 2 of Section 3.3.1) the followingprocedure was used:

1) The permeability test apparatus was assembled andchecked for leaks when pressurized. A steel ring wasused in place of the carbon-graphite test ring. A pistonpressure of ~6•9 kPa was used. Any drop in test pressureduring a two hour period was unacceptable.

2) After confirmation of "no leaks", the steel ring wasreplaced by a carbon-graphite test ring and the testapparatus reassembled. After reapplying the pistonpressure, the test ring was externally pressurized to1.4 MPa and the rig isolated from the gas bottle.

3) The time was recorded for every 10% (approximate)change in pressure reading, or the pressure reading afterone hour, whichever was appropriate.

Because of the more complex shape of the seal ringspermeability values were not calculated. Instead, acomparison was made of the pressure loss characteristicsas the gas flowed radially through each of the carbon-graphite rings for a given time period (usually 1 hour).

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4.

4.1

Grade

TEST RESULTS

Standard Permeability Tests

Permeability coefficient values for each of the 12materials tested are given in Table 1. They arereproduced here in order of decreasing magnitude.

CY10 P-5733 CCP P-49 CCP72 MY3K

Bo(cm )

xlO' 1 1

8.6-9.3 1-3 1.4-1.8

XIO"12 xlO'13

1 0.5-8.0

-13xlO

Grade P-69 P-19 MY3B CY10C CNFJ P-658RC

go(cm ) 3.7-4.7 1.5-3.5

ln-15 in-15xlO xlO

1-2 0.5-1.0

xlO

0.8-4.1 0.6-1.0

xlO-16

xlO-16

4.2

The ranges shown were obtained in test.

Typical test data, in this case for MY3K, and a plot ofIn (mean pressure/differential pressure) versus time forthis data are given in Figure 5.

So Variation Due to Procedure

4.2.1 Sample Moisture Content

The calculated values of go for the different conditionsof moisture content are given in Table 4. The presenceof moisture in the carbon-graphite cylinder significantlyaffects the flow of nitrogen through the cylinder walls.

4.2.2 Sample Temperature

It was seen that in the range of temperatures used, thevalue of go was not greatly affected by temperature. Theresults for this test are given in Table 5. The generaltrend was for a slight increase in go as the testtemperature increased*

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4.2.3 Test Pressure

There was a significant difference in the value of godepending on the test pressure used. The general trendindicates a decrease in g o with an increase in testpressure. The results for CCP-6, CY10-1 and MY3K-1 aregiven graphically in Figure 6.

4 .3 Effect of Impregnant

Of the three impregnants tried, the G9555 varnish wasselected for further impregnations. It gave betterresults than the GP-77, and was easier to use than theAremco-Seal 529. The test results for this section areg iven in Table 6.

4.4 Effect of Impregnant Viscosity

The results of this test are given in Table 7. Reducingthe viscosity by a factor of five altered the rate ofchange of permeability initially, but showed nosignificant difference in Bo after four impregnations.

4.5 Impregnant Penetration

Values for go were calculated for each successive cut offthe OD and ID of test cylinders. A decrease in go wastaken to indicate a lower amount of impregnant present.The results of this test are given In graphical form inFigure 7.

4.6 Permeability Variation Within Each Cylinder

The results for this test are given in Table 8. Thewater displacement method was too insensitive to indicatevariation..: in gas flow for permeabilities less than10~^3cn,2# Attempts at using a helium leakdetector in this case were unsuccessful. For theremaining materials, ranking in permeability variation isas follows (based on results in Table 8 ) :

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decreasing variation

grade

variation(%>

3o(cm2)

MY3K*

0-145

4.5

xl0"15

CCP

19-1

1.2

xlO~

72

23

13

CCP

25-65

0.6-1.0

xlO"12

P-49

24-66

1.4-1.8

xlO"13

P-5733

5-16

8.6-9.3

xin"12

CY10

9

1.8-

xlO~

1.9

11

4 • 7

4.8

5.

*MY3K sample had noticeable variation in Impregnantdistribution.

Except for P-49 being out of place, the materials areranked in order of increasing permeability withdecreasing permeability variation.

External Pressurization Test

The results for this test are given in Table 9. Asexpected there was no significant difference inpermeability with gas flow direction.

Testing of Full-Size Seal Rings

The results for this test are given in Figure 8. As canbe seen, the data can be easily separated into twogroups: i) those rings with a pressure loss less than14 kPa/h, and ii) all the rest. The rings appear to beeither very good, or very bad with few in the grey areain between. Screening out highly permeable rings shouldbe easy because of this.

DISCUSSION

Calculated values of go fortested (P-19, P-49. P-69, Prange 10"11 to lO"*6 cm2,the same range of values as(MY3K, MY3B, CY10, CY10C).CNFJ also reaches this low(0.8-4.1 x 10"16cm2). Thispermeability over the gradefactor of ^

the Pure Carbon materials-658RC, P-5733) cover theThis is approximatelyfor the Morganite materialsUnion Carbide's new gradelimit for permeabilityis an improvement init is replacing (CCP72) by a

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Of the three factors tested, moisture content contributedthe most to variation in Bo. This was by a factor of twoor three on the average. A review of Table 4 revealsthat the permeability of the sample after water impreg-nation did not return to its original value, even afterdrying the sample in an oven a second time. Accordingly,a second CCP sample was subjected to the same sort oftest to see if this behaviour was duplicated. Theresults (see Table 10) were similar in that the sampledid not regain its initial permeability. This discrep-ancy has been attributed to scale formation on the porewalls from lithium hydroxide used to control pH levels inthe test loop from which the water was taken. The othervariations in 3o due to temperature and test pressurewere of a smaller nature.

Of the three impregnants tried, G9555 varnish wasselected for further testing. The GP-77 was lesseffective, and the Aremco-Seal 529 produced more of a"coating", rather than penetrating into the sample. Theeffect of impregnant viscosity is shown in Table 7. Moreimpregnations are required with the reduced "iscosityvarnish to achieve a given permeability. The effect ofimpregnant viscosity on penetration is discussed later.

The impregnant penetration test revealed several thingsof interest. We were told that P-658RC was not impreg-nated, but that material was deposited in the pores bypyrolysis, and that the depth of penetration was shallow.The test data supports the claim for shallow penetration.As shown in Figure 7, after machining 0.3 cm from thewall, the rate of change of permeability with furtherwall reduction increased substantially. In contrast,permeability of impregnated cylinders CCP-4 and CY10C-2increased steadily as the specimen wall decreased. Thisindicates that the amount of impregnant is uniformlydecreasing, and that a steady-state condition has notbeen reached. From this we see that phenolic resin canpenetrate carbon-graphite to depths of at least 1.3 cm.The results for CCP-4 (impregnated at CRNL) include morescatter than for CY10C-2 (impregnated by the manufac-turer). The cause of this difference is unknown (but isprobably due to differences in the procedure and impreg-nants used). There was a significant difference betweenthe plots for CCP-4 and CCP-5. The higher viscosityimpregnant sample, CCP-4, had a substantially lower per-meability after the final test than did CCP-5, althoughboth started at similar values. The unimpregnated CCP-7sample was run as a control to see the effects of themachining process. As expected, as shown in Figure 7,the permeability of CCP-7 has remained relatively stablewith decreasing wall thickness.

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The permeability variation test indicates decreasedvariability with increased permeability. This seemslogical. For a very permeable sample, the number ofthrough-pores (connected porosity extending through thewall) might be very large. A very permeable sample wouldhave many of these in any given region. A few pores,more or less, would not greatly alter the flow in thisregion. Alternatively, for a very impermeable sample,the presence or absence of one through-pore could makethe difference between flow or no-flow in any particularregion. This is essentially what happened with the MY3Ksample (Table 8 ) . In the first test, there was nomeasurable gas flow. During the second test, gas waocollected from two areas, and nothing from the others.It is interesting to note that CCP72, one of the gradeswith the most variation was until recently specified asthe stator material for primary heat transport circu-lating pumps in certain Canadian nuclear generatingstations. Union Carbide has discontinued this gradereplacing it with grade CNFJ. This new grade has a muchlower permeability. Accordingly the corresponding vari-ation could not be measured due to limitations of the gascollection technique.

Flow direction had little effect on permeability, as isshown in Table 9. This was as expected, but now we havemore confidence in saying it.

Testing of full-size grade CNFJ seal rings proved to bebeneficial in establishing guidelines as to what toexpect from Union Carbide under existing material stand-ards. Permeability of rings tested, as depicted by rateof pressure loss in Figure 8, falls into two distinctgroups:

1) a narrow band about an average value2) values much greater than the previously calculated

average.

This suggests it should be relatively easy to establishacceptance/rejection limits fcr seal rings on the basisof a simple pressure test. This in fact has now beenincorporated into a new materials standard for carbon-graphite . ̂ .

A second batch of 21 CNFJ seal rings was ordered (not toany particular standard) and was subsequently tested.This time permeability of all rings was low (pressureloss rate of less than 1.4 kPa/h).

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Only a few rings of the different grades from eachmanufacturer were tested in this program. Accordingly, acomparison of suppliers based on material quality wouldbe unreasonable. Experience in the duration of thisprogram, however, leads to the following observations:

(1) Pure Carbon prefers to sell rings in the finishedsize. This is almost a necessity in the case ofP-658RC which has such a low depth of penetration ofthe material used to reduce permeability. UnionCarbide and Morganite on the other hand will supplyblanks from which finished rings can be machined.

(2) Two samples supplied by Morganite (MY3K-antiroonyimpregnated) had noticeable variation in impregnantdistribution.

(3) Quality control in the carbon-graphite industry isat a very early stage of development. They haveresponded to customer requirements, but untilrecently, those requirements have been minimal.

6. CONCLUSIONS

Based on the test results reported here, the followingconclusions have been mads:

1. go values for the full range of carbon-graphitematerials tested range from 10"11 to 10~ 1 6

cm . (Specific values for each material aregiven in Table 1.)

2. Phenolic resin can penetrate at least 1.3 cm ofcarbon-graphite. When impregnated from all sides (asis usually done) this corresponds to a component 2.6cm thick. The amount of impregnant varies inverselyas the distance from the as-impregnated surface.This effect is dependent to some extent on theviscosity of the impregnant used.

3. The permeability of the P-658RC sample increased sub-stantially beyond 0.3 cm below the finished surface.One should consider this "skin-effect" if makingfinished rings from discs of this material, orremachining finished rings.

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4. Abnormal permeability in seal rings is readilydetectable. This fact can be used as the basis ofacceptance/rejection criteria to ensure betterquality and uniformity.

5. f£> is slightly affected by test pressure, sample tem-perature and moisture content, the most significantbeing moisture content-

7. RECOMMENDATIONS

Impregnant concentration varies inversely with distancefrom the as-impregnated surface. Because of this Irecommend that:

1) finished seal rings be bought from the manufacturer(and not fabricated from blanks);

2) prior to "finish" machining, seal rings bere-impregnated by the manufacturer to ensure lowpermeability.

NITROGENGAS

PRESSURE

GAUGE

GRAPHITE SAMPLE

RUBBER GASKETI

SAMPLE SIZE: 8.9 cm OD x 5.1 ID x 3.8 cm Long

FIGURE 1: APPARATUS FOR PERMEABILITY TESTS

FIGURE 2: PERMEABILITY VARIATION TEST FACILITY(Inset shows details of sampling feature)

-17-

END LOAD

TESTPRESSURE

FIGURE 3: EXTERNAL PRESSURIZATION PERMEABILITY TEST RIG.The test ring is clamped hetween steel platesand sealed by rubber gaskets. Gas pressure isapplied externally.

-18-

RUBBER GASKETTEST RING

TESTPRESSURE

PERMEABILITY TEST JIG

FIGURE 4: PERMEABILITY TEST JIG FOR FULL-SIZE SEAL RINGS.This fixture is used to test seal rings used inBruce and Point Lepreau Nuclear Generating Stationsprimary circulating pumps.

- 19 -

uPL,

Q

CCO0)

a

o

CM

O1

O1

o1

PERMEABILITY TEST

MY3K-1 and Nitrogen GasInitial Test Pressure 1Equation of Plot: Y =

-14 2- go ~ 6 x 10 cm

I 1 i i

.38 MPa0.009745X - 0.5559

i i i i

4. 8.Time (Minutes)

12. 16.

FT)

Ap =

pl =

go =

P2 + Pl2

P2 " Pl .

atmosphere

T-i U n IT

Time(min)

0

.51

23

45

1015

P2(MPa)

1.38

1.381.28

1.201.12

1.050.99

0.770.62

(MPa)

0.791

0.7660.741

0.7010.661

0.6260.596

0.4860.411

In (Pjn/AP)

-0.556

-0.551-0.546

-0.537-0.526

-0.517-0.507

-0.458-0.410

FIGURE 5: TYPICAL PERMEABILITY TEST DATA FOR MY3K.The plot In (mean pressure/differentialpressure) versus time is a straight line.This means Darcy's Law is obeyed and So isproportional to the slope of the line CseeAppendix for a sample calculation).

-20-

sa

*<ni—t

oi—t

Xo

CQ

Coe

i

OJ

Pi

CCP-6

*6oxl0u for CYiO-1 Plot - To Facilitate Comparisonof Plots

I1.0 2.0

Initial Test Pressure (MPa)

3,0

FIGURE 6: So VARIATION WITH PRESSURE. Permeability pressure decay-tests were run for three carbon-graphites at variousstarting pressures. The effect on 3o was mostpronounced for the CCP sample.

- 2 1 -

1000 i -

750

500

to

X

O

mLU

OfLUa.

E 100

75 -

50 ~

25

CCP-7

(UNIMPREGNATED) COr>O Q

1 ( 0 . 6 2 ,

\

P-658RC-1 A

(AS RECEIVED)

CCP-4

(FULL STRENGTH \ ^- VARNISH) tr*O-^

CYIOC-2 * ~ * " ~ " ^ * ^

(AS RECEIVED)1 1

363)

\

I

CCP-5

' (THINNED

i

VARNISH)

\

0 . 5 0 0 . 7 5 1 . 0 0 1 . 2 5 1 . 5 0 1 . 7 5 2 . 0 0

RADIAL WALL THICKNESS OF SAMPLE (cm) « -

FIGURE-7: IMPREGNANT PENETRATION TEST RESULTS. go wascalculated after each, successive cut off the ODand ID of the samples. CCP-4 and CCP-5 wereimpregnated at CRNL with G9555 varnish.

-22-

Q_

oa.:en

CO

enor

112

98

84

70

56

42

28

ACCEPTANCE LIMIT

PERMEABILITY CHECK OF 22STATORS

CARBON-GRAPHITE

FIGURE 8 TEST RESULTS FOR PERMEABILITY OF CNFJ SEAL RINGS.The acceptance limit is arbitrarily set at 14kPa/h as defined in CRNL Standard MECH-8.

TABLE 1: AVERAGE PHYSICAL PROPERTIES OF CARBON-GRAPHITES TESTED

Manufacturer

IndividualMaterial Grades

Bulk Density(g/cm3)

Hardness Scleroscope

Compresslve Strength (MPa)

Flexural Strength (MPa)

Elastic Modulus (GPa)

Coefficient of ThermalExpansion (x 10-6/°C)

Thermal Conductivity

(W/(m.'C))

Maximum ServiceTemperature (°C)

Porosity % Volume

Impregnant

6o(cmz) T e s t R e s u l t s

Union Carbide

CCP

1.77

78-85

200

59

27

2.3

20.8

400

7

None

1" 3 -12x 10

CCP 7 2

1.83

75-85

230

61

28

2.3

20.8

260

1

Phenolic

Resin

-ixio'13

CNFJ

1.82

84

207

64

24.0

5.2

13.8

260

1

PhenolicResin

0.8-4.1x 10" 1 6

Morganite

MY3K

2.2

80

270

69

17

4.4

17

350

0.7

Antimony

0.5-8.0x 10-14

MY3B

2.25

75

210

69

15

4.7

16

120

0.6

Babbit

1-2x 10-15

CY10

1.55

70

150

51

9.7

3.6

12

350

12

None

~2xl0-U

CY10C

1.8

84

210

66

12

4.3

11

300

4.3

PhenolicResin

0.5-1.0x 10-15

Pure Carbon

P-19

2.35

70

193

66

22

4.7

12

204

4

Babbit

1.5-3.5x 10-15

P-49

2.30

70

241

86

25

5.2

12

288

notavailable

Antimony

1.4-1.8

x 10-13

P-69

1.80

70

155

62

21

4.9

9

260

8

PhenolicResin

3.7-4.7x 10-15

P-5733

1.75

65

145

52

14.5

4.3

12

288

notavailable

None

8.6-9.3

x 10-12

P-658RC

1.80

90

259

76

21

4.0

not available

260

2

resincoating

0.6-1.0

x 10" 1 6

N3w1

-24-

TABLE 2: TEST CYLINDER ROUGH BLANK SIZE

Supplier

Union Carbide

Morganite

Pure Carbon

Rough BlankSize (mm)

101.6 O.D. x 50.8 I.D.x 79.4 long

96.6 O.D. x 50.3 I.D.x 102.5 long

finish machined by themanufacturer

TABLE 3: TYPICAL TECHNICAL DATA OF IMPREGNANTS USED

Specific Gravityat 25°C

Viscosity (Pa.s)

Solids Content %

Type of Base

Dielectric Strength(volts/mil) +

Typical Cure

Typical Usage

G9555

0.915 @ 21°C

0.3 @ 21°C

60

phenolic resin

2200

4-6 h @135°C

impregnate motors,transformers,magnet coils

GP-77

1.08

0.10

49

silicone-organiccopolymer

1700-2000

6-10 h <315O°C

insulate electricalequipment at 200°C

Aremco-Seal 529

1.02

0.175

—

plastic-ceramic

300

*

moisture-proofingelectricalconnectors

* Dilute 529 liquid approximately 25% by volume with toluene, stirring thoroughly.Preheat cylinder to 150°C and impregnate. Remove excess impregnate and air-dry one hour. Heat cure at 121°C for one hour, then at 177°C for another hour.

+ 1 mil = 25.4 urn

-25-

TABLE h: 00 VARIATION WITH MOISTURE

-one CCP sample used throughout (designated CCP-6*)

Test

1.

2.

3.

4 .

5.

Standard permeability test

2 months later

all -standard permeability testwithinthe same "Standard test

day -standard test

Soaked in distilled water for% hour, then tested(not oven-dried)

-repeated test after allowing nitro-gen gas to blow through the sample

Vacuum-pressure impregnated withdistilled water, then tested(not oven-dried)

-repeated test after blowingnitrogen gas through it andlet stand for 24 hours(not oven-dried)

-repeated test after six days(not oven-dried)

Heated in oven 2 hours at 110°C

-standard permeability test

-standard permeability test

PermeabilityCoe ff icient

Bo (cm2)

1

9

9

9

3

5

2

3

5

6

6.

.0

.1

.4

.1

.1

.9

.6

7

5

2

0

X

X

X

X

X

X

X

X

X

X

X

10

10

10

10

10

10

10

10

10

10

10

-12

-13

-13

-13

-13

-13

-14

-13

-13

-13

-13

*CCP-6: CCP designates material grade; 6 designates sampleidentification number.

-26-

TABLE 5: go VARIATION WITH TEMPERATURE

Sample

CCP-7*

MY3K-1

Test Temperatureoc

2

23

150

2

23

150

PermeabilityCoefficientgo (cm2)

6.1 x 10~13

7.3 x 10~13

7.4 x 10~13

4.6 x 10"14

7.2 x 10"14

_ 1 A

8.3 x 10

*CCP-7: CCP designates material grade; 7designates sample identification number

TABLE 6: TEST RESULTS FOR THE IMPREGNANTS USED

Sample

CCP-1

CCP-2

CCP-3

Impregnant

none

G9555

none

Aremco-Seal 529

ii

none

GP-77

Remarks

-standard test

-0D & ID machined*

-standard test

-in as-curedcondition

-0D & ID machined

-standard test

-0D & ID machined

PermeabilityCoefficient

Bo (cm2)

1.0 x 10"12

S.4 x 10"14

8.8 x 10"13

1.7 x 10~15

3.7 x 10~14

8.6 x 10"13

1.4 x 10~13

% Reductionin go

95%

96%

84%

*This means just removing excess impregnant to return specimento original dimension.

-27-

TABLE 7: EFFECT OF IMPREGNANT VISCOSITY ON PERMEABILITY

Sample

CCP-4

CCP-5

Impregnant

Full strength

G9555 varnish

diluted

G9555 varnish

•

ImpregnantViscosityPa.s @ 25OC

-1.0

-0.2

Number ofImpregnations

0

1

2

3

4

0

1

2

3

4

PermeabilityCoefficient

$0(cm2)

6.2 x 10"13

4.5 x 10"14

5.0 x 10-16

4.6 x 10"16

3.9 x 10-16

7.6 x IO-I3

4.4 x 10~13

4.8 x 10"15

8.1 x 10"16

6.7 x 10"16

-28-

TABLE 8: PERMEABILITY VARIATION TEST RESULTS

Sample

CCP-12CCP-13CCP-14

P-49-2P-49-3P-49-4

P-5733-2P-5733-3P-5733-4

CY1O-1

CCP 72-1CCP 72-2

MY3K-1**

TestDuration(min)

103030

151515

255

2

1530

1010

GasPressure(MPa)

0.71.41.4

1.41.41.4

0.70.350.35

0.35

1.41.4

1.41.4

Gas

n433518

383643

494446

44

2819

040

Volume Collected at Position (ml)*

#2

474015

762744

634843

44

190

00

#5

631016

71950

704747

53

2717

00

#4

711241

421753

514839

48

240

00

#5

451557

262569

705053

53

330

010

#6

391155

244634

715154

53

237

070

(s/x)+%

256559

663924

16512

9

19123

0145

MY3K-2MY3BP-19

no measurable gas flow

t s - standard deviation; x - arithmetic mean

* Values given for gas volume collected are averages of two runs

** MY3K-1 sample had noticeable variation an impregnant distribution.Test data are for two runs at different positions on the testcylinder.

-29-

TABLE 9: PERMEABILITY - INTERNAL VS EXTERNAL PRESSURIZATION

Sample

*MY3K4-2

MY3B

*CCP721-1

P-19

P-69

CCP

CYlO

Permeability

External Pressure

3.0

3.45

1.4

3.1

6.1

3.6

2.0

X

X

X

X

X

X

X

10-15

10"l4

10-13

10"15

io-15

10-12

10-11

Coefficient

1

3

1

1

Bo

Internal

0.5

3.8

1.1

.5-3.5

.7-4.7

.9-9.7

.8-1.9

X

X

X

X

X

X

X

(cm2)

Pressure

-1510

10-14

lo"13

10"15

io"15

10-13

10-11

*These samples were tested using both internal and externalpressurization." For the remaining materials one sample wasexternally pressurized for comparison against previouslyobtained ranges of $0 using internal pressurization.

- 3 0 -

(1)

(2)

(3)

(4)

(5)

TABLE 10: Bo VARIATION WITH MOISTURE

Test

Standard permeability test

Soaked in distilled water for ^ hourthen tested (not oven—dried)

Vacuum-pressure impregnated withdistilled water, then tested(not oven-dried)

-repeated test after blowingnitrogen gas through it and letstand for 24 hours (not oven—dried)

Standard permeability test (dried inoven 1 hour at 110°C)

-standard test repeated

Dried in oven 1 hour at 150°C, thennitrogen gas blown through whilestill in the oven for another hour.Then tested after air-cooling forH hour .

- CCP-8 SAMPLE

PermeabilityCoefficient

3o (cm2)

7.8 x 10"13

6.6 x 10

1.9 x 10

2.7 x 10"l4

4.3 x 10"13

5.4 x 10

3.7 x lO"13

-31-

APPENDIX

Al PERMEABILITY THEORY

The flow of gas through a porous medium can be comprisedof turbulent, viscous (Poiseui11e), slip flow andmolecular streaming (Knudsen flow). The combinedgeneralized equation may be written^1' :

= 3op /n + 4/3 KoVm ...[1]

where K = total permeability coefficient (cm2's)Pl = pressure at inlet side of specimen (Pa)P2 = pressure at outlet side of specimen (Pa)Ap = Pi - P2 (pa)Pro = (PI + P2)/ 2 (pa)q = total flow rate (cm3/s)L = length of porous medium in direction of flow

(cm)A = cross-sectional area of porous medium

(cm2)go = permeability coefficient for viscous flow

(cm2)n = gas viscosity (Pa-s)Ko = permeability coefficient for combined slip

and Knudsen flow (cm)v" = mean thermal molecular velocity of gas

. (cm/s)

There are numerous methods of measuring permeability ofcarbon-graphite (e.g. 1,2,3,4). Many, however, involveelaborate equipment for the purpose of research into amore thorough understanding of permeability theory, orfor a systematic testing of the permeability equations.The work described here was intended only as a means ofrating carbon-graphites, one to another, with regard tohow permeability may or may not affect their dimensionalstability. This work was not intended as an exhaustiveresearch project, but rather a practical look at howpermeability may affect seal component performance. Assuch, Jenkins' and Roberts' simple high pressure decaytype of apparatus was chosen for the Investigation usingnitrogen gas as the pressurized fluid. Gas was chosen asthe test fluid for experimental

-32-

convenience and because of Jackson's") statementthat Darcy's Law does not hold for flow of water throughgraphite. It was realized that although permeabilityvalues for water and gas would be different, the relativeorder for the carbon-graphite would be the same. Havingobtained an ordering of the test materials, then a checkwould be made to see if a correlation existed betweenpermeability and dimensional instability (as brought onby the influence of water, pressure and heat actingsingly and in combination on the test materials).

A2 VISCOUS FLOW OF GAS FOR A HOLLOW CYLINDER INTERNALLYPRESSURIZED

Darcy's Law serves as the basis for the work done onpermeability in this program. It states that flow rateis proportional to the pressure gradient:

q = - (go/n)A (dp/dx) . . .[2]

where dp/dx = pressure gradient along the direction offlow.

We now make the following limitations:

1) we consider only isothermal flow;2) viscosity is independent of pressure.

For the test specimens, the cross-sectional area subjectto flow is given by:

A = 2 TTrH

where H = specimen axial lengthr = specimen radius bounded by r^ (inner) and

r2 (outer)

Equation [2] can be rewritten as:

qp(dr/r) = - (2-irHgo/n) (pdp)

and integrating:

/ qp(dr/r) = - /

r i Pl

(2TrHBo/n) (pdp)

-33-

2 2qp (In r2 - In rL) = - (27rHgo/n) [(p2 - p2)/2 ] ...[3]

If V represents the internal volume of the specimen andall associated tubing, and -dp/dt represents theincremental pressure decay with time then:

q = - (V/p) (dp/dt)

or qiPl = " V dpi/dt ...[4]

Combining Equations [3] and [A] and rearranging yields:

- In(r2/r1) V (dPl/dt) = (27TH6/) [CJand integrating

/*[ In (r2/r!)]dt -..[5]

Using the integration formula

/dx/(x2 - a2) = (l/2a) In [(x-a)/(x + a)]

then Equation [5] reduces to:

-(l/2p2)lnEp1-p2)/(pi+p2)] = TTHgot/Vnln(r2/r1)+C ...[6]

Recalling that - lnx = ln(l/x), Equation [6] can berewritten as

In |[2(Pl+r2)/2]/(Pl-p2)| = 2TTp2H3ot/Vnln(r2/r1)+2Cp2

Using In2x = In2 + lnx and recalling the definitionsfor pm and Ap

p m = instantaneous mean pressure

= (Pi + P2>/2

^P = instantaneous pressure difference across thecylinder wall

= Pi " P2

then:

ln(pm/Ap) =

-34-

where C = 2Cp2 - In 2

or

** In (pm/Ap) = (6oA'p2 / nLV).t + C ** .-.[71

where A' = logarithmic mean area of wall of thicknessL(=r2-r1)

= 2TTH(r2-ri) / In (r 2/ r i)

If a straight line is obtained when In (pm/Ap) isplotted against time, t, then the flow is streamline,'Cnudsen a,nd slip flow are negligible and Darcy's Law isobeyed. If Equation [7] does indeed represent a straightline, then the slope which can be read off the plot isgiven by:

h = Bo A' p2/nLV

and the permeability coefficient for viscous flow isgiven by:

Bo = hVnln (r2/r1)/2TTp2H ...[8]

The dimensions of (So are cm' in metric units (and areexpressed in cm2 in this report) , but values arecommonly given in darcies- For clarification, bydefinition a permeability of 1 darcy corresponds to aflow rate of 1 cm3/s through a 1 cm cube with apressure difference of 1 atmosphere across opposite faceswith a fluid of 10~3 Pa-s (lcP) viscosity. Since 1atmosphere = 10.13 N/cm2 (1.013 x 106 dynes/cm2), 1darcy -0.987 x 10~ 8 cm2.

A3 SAMPLE CALCULATION

Based on the raw pressure decay data for MY3K-1 given inFigure 5, a value of h = 0.009745 was obtained for theslope of the line. This is then plugged into Equation[8] along with the following values for the remainingvariables:

V = 90.32 cm3 p 2 = 0.1014 MPaD = 0.0176 mPa-s H = 3.896 cmr 2 = 4.437 cm r = 2.540 cm

-35-

4.437(0.00P745) (90.32) (0.0176)ln 2.540 • 10

2TT(0.1014) (3.896) 60

= 5.8 x 10"14 cm2

The factor 1/60 is required to correct for h being inunits of (minutes) •

A4 REFERENCES

1. Jones, B.F., Duncan, R.G., "The Gas Permeability ofExtruded Graphite", Atomic Energy of Canada Limited,AECL-3770, 1970.

2. Anderson, W.A., "Permeability Relationships Using DarcyPermeability, Vacuum Decay, Pressure Decay and Pore SizeDistribution. Methods on Graphitic Materials", Carbon vol4, 1966, pp 107-114.

3. Grove, D.M., "Porous Carbon Solids" (edited by R.L.Bond), Academic Press, London and New York, 1967, pp155-201.

4. Jenkins, T.R., Roberts, F., "Gas Permeability Studies onSome Artificial Graphites", in Proceedings of the FifthCarbon Conference, vol II, Pergamon, New York, 1963 pp335-346.

5> Jackson, J.L., "Nuclear Graphite", (edited byNightingale, Yoshikawa and Losty), Academic Press, NewYork and London, 1962, p.181.

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