Boqin Gu, Ye Chen and Jianfeng Zhou
Nanjing University of Technology,
China
1. Introduction
Asbestos was once considered to be a "miracle mineral". This naturally occurring silicate has many desirable characteristics, including resistance to fire, heat, and corrosion. It is strong, durable and flexible. Asbestos is inexpensive because it is available in abundant quantities. Its versatility has led to its use as a component of a variety of products in numerous industries (American Academy of Actuaries, 2007). The development of fiber – reinforced elastomer gaskets began in the 1880s and led to the patent application for Klingerit in 1904 (Piringer & Rustemeyer, 2004). Since more than one hundred years ago, these kinds of non-metallic gaskets, which were made of compressed asbestos fibers (CAF) materials, have been the most widely used sealing elements with a maximal yield.
Up till the 1970s, the health hazards of asbestos were recognized. Several diseases have been linked to asbestos exposure, including mesothelioma, lung cancer, other cancers, asbestosis, and pleural changes (American Academy of Actuaries, 2007). Due to the restriction on the use of asbestos, the pressure was on scientists and engineers to develop non-asbestos gasket material replacements, and asbestos was replaced by alternative fibers and fillers. These alternatives were employed in an attempt to replicate the product properties of the former CAF materials. These substitutes were developed using reinforcing fibers like aramid, glass and carbon fibers to achieve high strength, and additives like inorganic materials (e. g., clays, precipitated silica, graphite etc.). Since the 1990s, a great number of worldwide famous sealing materials manufacturers have put significant efforts onto the development of a variety of novel non-asbestos sealing composites (NASC), such as Garlock in USA, Klinger in Austria, Kempchen in Germany, and Valqua and Pilar in Japan. Collaborating with some international organizations and research institutions, they conducted a series of experimental and application researches, and obtained many valuable results (Payne & Bazergui, 1990). These investigations have laid a foundation for further studies on performance evaluations and industrial applications.
Aramid was used as reinforcing fibers in the earliest non-asbestos gasket materials, because it provided processing advantages, especially during calendaring. However, it is very expensive and has poor thermal stability. Most commonly used fibers and fillers in nonasbestos gasket materials do not achieve the outstanding stability as CAF materials do when the binding elastomers become aged. More recently, novel formulations and manufacturing
processes have been developed based on cheap reinforcing fibers, the hybrid effect of different reinforcing fibers or an adapted material structure. These non-asbestos gasket materials not only are significantly cheaper but also have a better performance at high temperatures. Gu & Chen developed two kinds of sealing composite materials. One was reinforced with aramid and pre-oxidized hybrid fibers and prepared by molding preparation method and the other was reinforced with carbon and glass hybrid fibers and manufactured by using calendar preparation method. The effect of different surface treatment methods of the fibers on the heat resistance of the composite materials was studied. The optimum prescriptions of the composites were obtained by regression design method (Gu & Chen, 2007; Chen & Gu, 2008). Gao & Chen carried out explorative investigations on the preparation method of nanometer calcium carbonate filled modifications of rubber-based sealing composite materials and on the influence of nanometer filled modifications on the mechanical properties of the prepared gasket materials (Gao & Chen, 2009). A novel material concept for compressed fiber materials was proposed to significantly decelerate the ageing of elastomer bound gasket materials by the use of special elastomers and an adapted material structure, i. e., the multi-layer structure. The steam testing, as opposed to the standard gasket testing, has been used to demonstrate this improvement (Piringer & Rustemeyer, 2004).
It is essential that gasket materials possess good mechanical performances and sealability. Mechanical performances include compressibility, resilience and stress relaxation property. They are usually used to evaluate abilities to cause gasket material deformation into flange face irregularities under assembly condition, to hold sealing surfaces of joints contacted tightly under internal pressures, and of stress retention at high temperatures, respectively. Sealability is a comprehensive performance of gaskets and indicates the ability to prevent the sealed fluid from leakage through the joints. Sealability can also be called tightness and can be measured quantitatively by the leakage rate.
In the mid 1980s, the American Pressure Vessel Research Committee (PVRC) set up "Room – temperature Mechanical Test Procedure" and "Hot Mechanical Test Procedure" for estimating the probable long-term performance and potential fire survivability of nonasbestos gaskets to guide the qualification and selection of non-asbestos spiral wound, jacketed and sheet gaskets for petroleum and petrochemical plant services in the range of 423-866K. Procedures, test fixtures, and typical test results for several process plant gaskets were discussed. An aged exposure parameter was introduced that correlates cumulative damage with exposure time and temperature for materials that degrade over time (Payne et al., 1989a; Payne et al., 1989b; Payne & Bazergui, 1990). The change in properties of some compressed sheet gasket materials subjected to temperature exposure for periods of up to one year was investigated by Marchand & Derenne. This resulted in a better understanding of the long term effect of thermal degradation on the properties of elastomeric sheet gasket materials. An improved qualification protocol based on the obtainment of the thermal endurance graph of a sheet gasket material was proposed for the extrapolation of the long term service temperature (Marchand & Derenne, 1996). Tsuji et al. researched experimentally the effect of aging time on sealing performance of non-asbestos spiral wound gaskets at elevated temperatures under either the stress controlled condition or the strain controlled condition. The tightness parameter Tp at different elevated temperatures was obtained. The results indicated that the non-asbestos spiral wound gasket had the same sealing performance as the substitute for the asbestos spiral wound gasket between 483 and 693 K (Tsuji et al., 2004). Xie et al. investigated the compressive and resilient performances and relaxation property of the developed compressed non-asbestos fiber reinforced rubber sheet gasket materials, and discussed in detail the effect of the non-asbestos fiber on the properties of the sheet gasket materials (Xie & Cai, 2002; Xie & Xie, 2004).
Systematic researches on gasket performances and their characterizations have been carried out in the Fluid Engineering and Sealing Technology Laboratory at Nanjing University of Technology since the 1980s. The compressive-resilient performance, creep and stress relaxation properties, and leakage behaviour of some types of gaskets were investigated. The representation of gasket performances was put forward, and the formulae for expressing gasket performances were obtained by means of the proposed models and regression analysis of experimental data (Gu et al., 1999; Gu et al., 2000; Gu et al., 2001; Gu, 2002; Zhu et al., 2007; Zhu et al., 2008; Gu et al., 2010). A novel tightness concept was presented, and the tightness analysis and design methods of gasket sealing joints based on the criterion of the maximum allowable leakage rate were developed. Relatively accurate predictions of leakage rates of some gasket sealing connections were also obtained (Gu & Zhu, 1988; Gu & Huang, 1997; Gu et al., 2004; Gu & Chen, 2006; Gu et al., 2007a; Gu et al., 2007b).
Up to now, much work has been fulfilled on the development, performance evaluation and engineering applications of the sealing composites reinforced with non-asbestos fibers, and many results have been achieved. However, relatively comprehensive and systematic reports on design, manufacture and performance evaluation of NASC are still very scarce, especially on the characterization of micro structural parameters, and the mechanical analysis and macro performance prediction of these materials according to the theories of micromechanics and viscoelastic mechanics.
In this chapter, manufacturing technology, surface treatment methods for reinforcing fibers, and formulation design methods of NASC are introduced. Measurements and characterizations of some micro structural parameters of fibers including their aspect ratio, orientation and distribution are investigated. A micromechanical model of single fiber cylindrical cell and a model of compressive type single-fiber cell are established, on the basis of which the methods are proposed for evaluating macro-mechanical performances of NASC, such as tension, compression, and stress relaxation. A leakage model for predicting non-asbestos gasket leakage rates is presented and verified experimentally. Furthermore, the performances of some developed NASC are also evaluated.
