The Use of Asbestos-free Materials in Static Sealing on Pumps

Asbestos-free gaskets have been used to seal flanged connections on various applications. This paper discusses the use of a different style of gasket and details the advantages and benefits resulting from such a change. The discussion includes an analysis, supported by laboratory testing and successful field experiences. The paper is sectioned into three case studies, each topic relating to a type of product. • The development of high strength asbestos-free sheet materials for usage on split case pumps using modern manmade fibers, including a successful study using the material on a natural gas pumping station on activated methyldiethanolamine (MDEA). The MDEA removes carbon dioxide in natural gas process plants. • The successful implementation in the usage of serrated metal core technology with various facing materials, in pump case gaskets, used on hydrocarbon service in oil refineries within the United Kingdom. • The development and usage of structurally modified polytetrafluoroethylene (PTFE) filled products in pumping aggressive chemicals on sealless pumps. The paper details the materials, the application areas, and the problems resolved. Also included are design considerations and an introduction to installation guidance, with discussions on previous failures. INTRODUCTION The problem is moving away from established materials based on asbestos products onto asbestos-free products. The substitute materials available, along with new styles of gaskets, have resulted in many new challenges and opportunities. The transition from an asbestos-based sheet material onto other nonasbestos products has resulted in a variety of high performance products. The technology employed to manufacture the calendered sheet product is utilized using asbestos-free combinations and new developments and manufacturing techniques. There is not one ideal product to replace asbestos and have the same degree of flexibility. High performance sealing materials such as exfoliated graphite, exfoliated vermiculite, along with polytetrafluoroethylene (PTFE) products have entered the marketplace and provided the level of sealing. The material used for the high performance split case pump involved the usage of high strength aramid with rubber binders. The introduction of material combinations of metal and nonmetallic components in new combinations has seen growth in new areas. The development and increased usage of modified materials based on PTFE provide effective solutions. CASE STUDY 1 When the first generation of asbestos-free sealing materials was developed, the reinforcing fibers were either aramid or glass. The usage of alternative materials, cellulose, carbon, and mineral wool, has provided a wider range of products. The high performance requirement in the split case pump has progressed with the aramid and glass combination. The main disadvantage of aramid is its relatively high cost compared to that of asbestos, and the embrittlement of the fibers at elevated temperatures of 200° to 250°C (392° to 482°F). The usage of the monofilament glass provides secondary reinforcement at elevated temperatures. The glass used is manufactured using careful tolerance to provide fiber diameters of 6 to 10 micron. This provides the most benefit when combined with aramid. The glass also has a surface dressing applied to promote compatibility to the matrix. Included in the mix are coupling agents to promote chemical cross links with the rubber binder. When the blend of aramid and glass fibers entangle, the monofilament glass is trapped within the matrix and reduces the tensile strength ratio to around three. This is required with the high performance product. The split case pump gasket has traditionally involved a thin sheet product that has to maintain high seating stress due to the pressure containment envelope. Surfaces would be traditionally smooth, usually without any machine marks, for the gasket to flow and grip onto. Figure 1 and Figure 2 show a typical pump arrangement. Asbestos-based products contained a high percentage of fibrillating fiber that provides strength. The replacement of the fiber by aramid provides a material with superior strength at lower fiber levels (Table 1). The case to consider is a lean amine circulation pump in a gas distribution plant in the northwest of England. Flange rating for connectors were 900 lb pressure class with typical operating pressure of 8 MPa at a temperature of 50°C (122°F). The approximate area of sealing face was 525,000 mm2 (813.75 in2). Bolting on the pump is 52 off, 1.3/4 grade B7 bolting. The use of the pumps was in the removal of CO2 in natural gas. Two pumps were showing leakage on the casing joint. The leaks had occurred after a short period of service time. Several failures had occurred following change of material from an asbestos-based product to a nonasbestos aramid-glass mixture. Previous to the change from asbestos, a run time greater than 12 months was expected. Two different suppliers had been used, and on each occasion failure had occurred after a short period of time. 17 THE USE OF ASBESTOS-FREE MATERIALS IN STATIC SEALING ON PUMPS by Russ Currie Technical Manager Flexitallic Limited West Yorkshire, United Kingdom