Pressure relief valve technology - products

Railway Age, Feb, 2004

Midland Manufacturing Co., a member of the Dover Corp., is a supplier of pressure relief valves (PRV)--tank car safety devices that routinely handle aggressive liquid chemicals and gases, including corrosive liquid chlorine, flammable jet fuel, and high viscosity food products. One of the benefits of PRV usage is the reduction of internal car pressure, which decreases the likelihood and/or severity of BLEVEs (Boiling Liquid Expansion Vapor Explosions) during accidents involving tire. Another benefit is the protection of people and property. The PRV reduces pressure within a car by venting its contents. When a PRV opens, the pressure build-up release may create an audible warning that's recognized by experienced personnel, who can then initiate an evacuation of the immediate area. In addition, PRVs help maintain container integrity. When a car is inadvertently overfilred and the tank becomes hydrostatically full, internal pressure can increase asymptotically from even small temperature increases or product addition. PRV actuation causes excess product discharge, which can help prevent catastrophic failure of the tank car shell due to overpressure. PRVs also help prevent the inadvertent creation of excess pressure. During loading or unloading, a tank car may become overpressured when using steam of another heat source to liquefy the product. PRVs are among the components that help relieve such pressure.

According to Midland, capacity influences typical PRV performance. Colorado Engineering and Experimental Station, Inc., has tested Midland's larger-and smaller-capacity PRVs: A-37280 and A-1280, respectively. Results indicate that the larger version provides greater flow rates following actuation at an initial discharge pressure of 280 psi. A-37280 exhibited 11 times the exit flow rate of product compared to A-1280. As a result, the higher-capacity valve will "significantly reduce the time required to vent a tank car in an overpressure situation," the manufacturer points out. A-37280 offers a three-inch diameter orifice size, an initial discharge pressure of 280 psi, and a flow rate of 39,600 scfm. A-1280 has a 1-1/8-inch diameter, an initial discharge pressure of 280 psi, and a flow rate of 3,600 scfm. E-mail: KevinC@MidlandMfg.net; Website: www.MidlandMfg.net.