Document X7GyOYz0wDvweLBq9JBnz1KDG

C. F. O'Knfe Asbestos can cause a health hazard ifinhaled in sufficient quantities. The last 20 years has seen increasing awareness of this hazard in industrialized countries; this has been recognized by stringent restrictions to the levels ofexposure allowable and a move to use other products as substitutes. At the Kinieith mill of NZFP Pulp and Paper Limited there has been an active program to reduce asbestos expo sure by strictly controlling its use (eg, using a dedicated controlled atmosphere workshop forwork on brake linings) and removing asbestos completely where satisfactory sub stitutes exist (eg, removing asbestos-containing insulation). The Kinieith Site Asbestos Safety Committee composed of management and workforce representatives was set up in the early 1980s to ensure a unified and coordinated approach on this work. Gaskets In 1985 attention was directed to compressed asbestos fibre (CAF) gaskets which were universally used across the site. While installed within flanges in the plant, there is no prospect ofrelease ofasbestos from such gaskets. However, during cuttingand removal ofCAFgaskets, tests at Kinieith showed that asbestos concentrations greater than the per mitted threshold limit values may be generated. In the late 1970s the use of asbestos packing materials following the successful application in the plant of substi tute materials -- informal mill trials had shown that the high technology teflon/kevlar based packings were suitable substitutes. Hence it was expected that a suitable gasket substitute would be found. From 1981 similar informal mill trials ofasbestos-substi tute gasket materials had been made and by 1985 18 months Table 1 Air test results from cutting compressed asbestos fibre gaskets Catting method Asbestos fibres released (WofTLV)* Hand snips Electric nibbler Grinder Scraping ofTold gasket Punch 500 270 too high to measure 600 too Threshold limit values. March 1989 trouble-free use ofa kevlar-based gasket had been achieved in our `worst use* pulp mill test location. Accordingly in December 1985, CAF gaskets ceased to be a general issue item and the kevlar-based sheet material was substituted alongwith increased stocking ofspiral wound metal gaskets. The substitute chosen was regarded as being overspecified for many duties (such as water, air lines) but by choosing a single product the possibility ofincorrect application in cri tical use was removed. ' On air, water, oil and stock lines, the substitute sheet material has performed well. However, within the pulp mill, on weak black liquor lines a number of blow-out failures were experienced in circumstances indicating the material had definite pressure/temperature limitations. Accordingly, on such high pressure/temperature liquor applications, the use of CAF gaskets was reverted to while fbrther investigations were undertaken. The New Zealand research company. Process Develop ments Limited then surveyed the literature on gaskets and gasket testing methods on our behalf. During this time the company supplying the substitute gasket material were also very active and forthright in addressing the problem and providing technical information. Most of the recently published information on gasket materials focuses on the relative `leakage rate' of the assembledjoints for which the permeability ofthe gasket is the major factor. A German research paper about work in chemical factories showed that substitute gaskets out-perform CAF gaskets in this property. No research or applica tions covering the use ofsubstitute gasket materials within the pulp and paper industry was identified. Examination of the failed substitute gaskets at Kinieith typically revealed that gasket material had become brittle and hard; surfaces exposed to black liquor were eaten away and creep around the bolt holes was evident All ofwhich explained the blow-out type nature of the failures experi enced. Within the plant it has also been noted that should a minor gasket weep develop tightening the bolts was not successful and that such weeps progressed to blow-outs extremely quickly--the forgiving nature of CAF gaskets was not being duplicated. GASKET TEST PROGRAM In conjunction with Process Developments Limited, NZFP Pulp and Paper Limited developed a screening gas ket testing program aimed at evaluating the gasket mater ials relative to CAF gaskets in hot weak black liquor solutions. Gasket suppliers in New Zealand were contacted and asked to participate in a gasket testing program com missioned by NZFP Pulp and Paper Limited and Tas man Pulp and Paper Limited in mid 1986. The testing program was controlledJmProcess Developments Lim- ET-00233 141 ET-83 uea witn me tests oetng maae at tne HUtt valley laborator ies of the Department of Scientific and Industrial Research. Fourteen gaskets were tested: 2 asbestos (as control sam ples); 5 kevlar based substitutes; 5 graphite based substi tutes; 1 PTFE based substitute; and 1 glass fibre based sub stitute. The test fluid was weak black liquor (12-20% con centration) from the Kinleith Kamyr continuous digester. Gasket tests for use requirements The gaskets were subjected to three series of tests: Autoclave tests: Ten days exposure to weak liquor at 175*C and 2000 kPa. Visual assessments to check chemical compatibility. Fluid resistance assessment by mass and thickness meas urements. Strength tests: Tensile strength measurements to establish crushing, blow-out, durability factors. Creep Relaxation: To assess the retained compressive stress for sealing. Visual Assessment test results Only CAF gaskets unaffected by the weak black liquor autoclave treatment Kevlar gaskets became soft and bowed. Graphite gaskets started delaminating. PTFE gasket showed little change. Glass fibre gasket swelled greatly, became soft and rub bery. Gasket volume changes test results Difference before and after autoclave treatment CAF gaskets showed a slight increase. Kevlar gaskets showed a significant increase (40 to 115%). Graphite gaskets essentially impervious. PTFE gasket essentially impervious. Glass fibre gasket swelled grossly (300%). Creep relaxation test results Most samples displayed an increased tendency to creep after autoclave treatment Creep relaxations were: CAF gaskets exhibited 70 to 80%. Kevlar gaskets exhibited 85 to 100%. Graphite gaskets exhibited 40 to 70%. PTFE gasket exhibited 80%. Glass fibre gasket exhibited 100%. Pensile strength test results The actual strengths of the gaskets before and after auto clave treatment were measured in accordance with ASTM . method FI 52. Figure 2 shows the tensile strengths after exposure in the autoclave. Installation assessment The ease and durability ofthe gaskets was assessed with a view to their practical application within a plant. CAF gaskets; robust and easy to handle, tendency to stick reduced by applying gasket compounds. ' Kevlar gaskets; similar to CAFgaskets to handle, became extremely brittle, cannot use gasket compounds. Graphite gaskets; fragile to handle and difficult to install between flanges, tendency to stick, high conductivity can lead to flange face corrosion. PTFE gasket; similar to CAF to handle, easily removed. Glass fibre gaskets; similar to CAF to handle, tendency to stick, cannot use gasket compounds. Cost comparisons Local supply factors can significantly affect the actual cost of each produicf, however the trends do reflect the type of gasket involved and a comparison against a typical CAF gasket shows all substitutes are significantly more expen sive (Fig.-3). Fig. 2--Tensile strength ofgasket materials. 142 Fig. 1 -- Creep relaxation ofgaskets. Fig. 3 -- Compa rarive cost of gaskets. ET-00234 4pprta VoI. 42 No. 2 -------. \ A. \0 Following the test program a comparison was made with i plant experiences at Kinleith and the following conclusions drawn: N'o single universally applicable asbestos-substitute gas * ket is currently available. Glass fibre based gaskets should not be used in weak black liquor applications. Kevlar based gaskets are the closest product to CAF gas kets currently available however their lower tensile strength and tendency to become brittle means they should not be used in high pressure/temperature black liquor duties. PTFE basedgaskets have application in specific duties but require retightening after installation and are very expen sive. Graphite based gaskets provide good sealing characteris tics but are difficult to handle and can cause flange surface corrosion. CAF is a safe gasket choice that performs well throughout the plant The use of CAF gaskets will continue in high temperature/pressure weak black liquor applications. ' FUTURE SUBSTITUTE GASKETS The Kinleith Mills have no intention of becoming the sponsors or test-bed for future gasket material develop ments by manufacturers. Accordingly we will not con sider any future gasket'for pulp mill use that has not been subjected to the screening testing program described above and has been shown to have equal or superior per formance to CAF. The gaskets tested have all been sheet type products of similar form to CAFsheets, othergasket types are used on the Kinleith Site. In particular extensive use has been made of spiral wound metal gaskets on steam and liquor duties (our recent pulp mill modernization project speci fied spiral wound gaskets). On this type ofgasket the filler material is contained oy me metal windingand its strength property requirements are not as high as in a sheet gasket r -- consequently the replacement of asbestos fibres as a filler media with other substitute fibres has not resulted in the same difficulties. Gasket manufacturers are actively working to find suit able asbestos substitute materials especially in the USA where the combined efforts of the Environmental Pro tection Agency, product liability laws and insurance com panies seem likely to eliminate asbestos-containing pro ducts in the near future. Secondary users such as valve manfacturers are also examining the impact ofsubstitutes on their products performance and guarantees (the nuclearindustryhas been veryactive in this regard). Other factors such as the requirement for gaskets to be fire-safe in the Petroleum industry make the asbestos substitute requirements more difficult than is first apparent For pressure vessel use the American Pressure Vessel Research Committee are currently reviewing the ASME Code gasket joint factors and requirements and their interim reports indicate major changes are likely in high pressure applications. Against this-Jjackgroiind the most prudent course of action for the Australasian pulp and paper industry is to proceed to the use of asbestos-substitute gaskets with extreme caution and continue using CAF in critical appli cations in the meantime. ACKNOWLEDGEMENTS The writer acknowledges the help of the following in compiling this paper: NZFP Pulp and Paper Limited; NZFP Technology; Process Developments Limited; Tas man Pulp and Paper Limited; HJ Asmus Limited; AW Chesterton (NZ) Limited; Garlock NZ Limited; Industrial Technology Limited; James Walker (NZ) Limited; Payen NZ Limited; Taylors Limited. Manuscript received for publication 30.8.88. March 1989 ET-00235 143