History: |
Problem: United States Steel Corporation's Chemicals Division plant in Haverhill, Ohio, which has a projected capacity of 120 million 1b/yr of Bis- phenol-A (BPA), was experiencing downtime problems due to failure of critical expansion joints on certain processing equipment. BPA is a major ingredient in polycarbonate resins and is also used for the production of epoxy, phenolic, polyester and other resins. The Haverhill plant utilizes a proprietary French process that can produce either the polycarbonate or epoxy grades of BPA, depending on market demand for products such as corrosion and abrasion resistant processing equipment and piping systems, shatterproof glazings, protective coatings, inks, appliances. Equipment for manufacturing BPA must be able to withstand attack by corrosive chemicals such as hydrochloric acid, phenol and acetone at temperatures up to 350'F with pressures ranging from full vacuum to 30 psi. Many of the processing units are made of glass-lined steel and of graphite because these materials exhibit excellent resistance to corrosive attack at elevated temperatures.  These materials of construction, however, cannot tolerate high flange loads so flexible connectors or expansion joints must be used in many cases to protect the equipment from damage due to misalignment and to thermal expansion. A large number of expansion joints in various sizes were installed, made from either corrosion-resistant alloys or fluoropolymers, depending on temperature/ pressure conditions and the nature of the process materials. While most of the connectors could handle the service conditions, there were some situations where conventional units could not meet the requirements. In one case, calculations indicated that a 65' high glass-lined column would grow 1" vertically when heated from ambient temperature to the operating temperature of 350'F. An expansion joint was needed between the 24" diameter column nozzle and a mating elbow of graphite to prevent overstressing and failure of the nozzle and/or elbow during startup. The lateral spring rate had to be kept below 1000 1b/in to keep stresses within the operating limits of glass-lined steel and graphite flanged connections. In another application, a 30" diameter expansion joint was required between a glass-lined steel flange and the flange on a column of graphite construction to prevent overstress damage. There were also some expansion joints on glass-lined steel reactors that had to be replaced due to failure.
Solution: The problems were referred to a company that specializes in the design and fabrication of expansion joints and flexible couplings for corrosive service. The joints are molded from chemically inert virgin PTFE fluorocarbon to form a seamless body with con- volutions of uniform wall thickness to assure a high tensile strength and long flex life. They are available in diameters from % through 36" with up to 12 convolutions to provide the desired spring rate. The coupling designed to mate the 65' column nozzle with the graphite elbow consists of two 24" diameter, 5 convolu- tion PTFE expansion joints intercon- nected by an 8" long, 24"  diameter carbon steel spool piece lined with PTFE. Proprietary vacuum support rings of Hastelloy alloy B were used to meet the pressure conditions of full vacuum to 30 psi at 350'F. U.S.S. Chemicals' engineers witnessed and verified the structural integrity and lateral spring rate below the specified 1000 lb/in before the joint was shipped to the Haverhill plant. Molded PTFE expansion joints were also used to connect the 30" diameter glass-lined flange to the graphite col- umn, and to replace all of the expansion joints that had failed on the glass-lined steel reactor.
Results: The molded PTFE expansion joints have provided satisfactory service since the first units were installed on the BPA processing equipment in January, 1978. They are also being used in a number of other applications where posi- tive protection for stress-sensitive equip- ment, corrosion resistance and high vacuum/pressure capabiIities are required. Flexijoint PTFE expansion joints are detailed with complete engineering data in Bul 2002 Sermatech-Ethylene, 775 Central Ave., Murray Hill, NJ 07974. Glass-lined steel vessels Bul 1122 Pfaulder Co., Div., Sybron Corp., 1000 West Avenue, Rochester, NY 14603.
Reprinted from February 1982 CHEMICAL PROCESSING |
