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3 Torque from the drive SHAFT to the disk PROBLEM fell out of its keyway, disconnecting the drive SHAFT from the disk. System pressure was high enough to eject the C. ertain types of CHECK and butterfly valves can undergo SHAFT -disk unrestrained drive SHAFT from the valve, s e p a r a t i o n , a n d f a i l cata carrying with it the external strophically or BLOW-OUT , causing toxic counterweight assembly, weighing over and/or flammable gas releases, fires, and 200 lbs., a distance of 43 feet away. vapor cloud explosions. Such valve failures can occur even when the valves The absence of the drive SHAFT left a hole are operated within their design limits in the valve body the diameter of the of pressure and temperature.
4 SHAFT ( inches) directly to atmosphere, and initiated a high- pressure light hydrocarbon leak. The ACCIDENT HISTORY leak continued for approximately 2 to 3. minutes, forming a large cloud of flammable light hydrocarbon vapor. The I. n a 1997 accident, several workers vapor cloud ignited, resulting in an sustained minor injuries and millions explosion felt and heard over 10 miles of dollars of equipment damage away. The explosion and ensuing fire occurred when a pneumatically assisted caused extensive damage to the facility, Clow stub- SHAFT Model GMZ CHECK (non . ALERT. completely or partially destroying many return) valve in a 300 psig flammable gas major components, piping systems, line underwent SHAFT BLOW-OUT .
5 The instruments, and electrical systems, and valve's failure caused the rapid release requiring the complete shut-down of the of large amounts of light hydrocarbon affected unit for cleanup and repair. gases which subsequently ignited, Minor damage occurred to nearby CHEMICAL S AFETY. resulting in a large vapor cloud residences and automobiles (mostly explosion and fire. broken glass and minor structural damage due to the blast wave). Nearby The CHECK valve was designed with a highways were closed for several hours. drive SHAFT that connects the internal Damage cost to the facility alone is valve disk to an external pneumatic estimated at approximately 90 million cylinder (see diagram on next page).
6 The dollars. Fortunately, no fatalities and valve failed when a dowel pin designed only minor injuries to workers resulted to fasten the drive SHAFT to the disk from the accident. sheared and a key designed to transfer EPA and OSHA. 2 SHAFT BLOW-OUT HAZARD of CHECK and butterfly valves September 1997. Previous malfunctions involving CHECK valves In the 1991 incident, the malfunction was of the same or similar design occurred at manifested by the erratic operation of the valve, facilities in 1980, 1991, and 1994. In each case, which was observed to operate independently the affected CHECK valve was located in a large from its external drive mechanism.
7 System diameter (36-inch or greater) pipe in a pressure was low enough (70 psig) that the hydrocarbon gas compression system. Also in failure was detected before the SHAFT was each previous case, a dowel pin fastening the expelled out of the valve body. (At the time the valve's drive SHAFT to its disk sheared (in the 1980 malfunctioning valve was identified, the valve case the pin was possibly never installed) and a SHAFT was protruding about inches out of rectangular key fell out of its keyway, the valve body.) In the 1980 and 1994 cases, the disconnecting the drive SHAFT from the disk.
8 Malfunction was identified when workers noted Although SHAFT -disk separation occurred in each that the external piston rod connecting the air- previous case, it did not result in SHAFT BLOW-OUT assist cylinder to the drive SHAFT had broken due or catastrophic failure. This may be because the to axial movement of the drive SHAFT . valves in these instances were installed in lower- pressure service, or because the malfunctioning valves were identified before SHAFT BLOW-OUT occurred. systems containing chemicals leading to HAZARD AWARENESS hydrogen embrittlement. C. heck and butterfly valves are used in valves subject to this HAZARD may be designed many industries, including refineries, with a two-piece valve stem (sometimes referred petrochemical plants, chemical plants, to as a stub- SHAFT design).
9 In each of the cases power generation facilities, and others. Most described above, the malfunctioning component modern valve designs incorporate features that was a Clow stub- SHAFT Model GMZ. reduce or eliminate the possibility of SHAFT blow pneumatically assisted swing CHECK valve (see out. However, older design CHECK and butterfly diagram below). In these CHECK valves , one stem valves with external appendages such as piece functions as a drive SHAFT that connects the pneumatic-cylinders, counterweights, manual internal valve disk to an external air-assist operators, or dashpots may be subject to this cylinder and counterweight assembly.
10 The drive HAZARD . SHAFT BLOW-OUT may be of particular SHAFT penetrates the pressure boundary through concern wherever these valves are installed in a stuffing box. The exterior portion of the drive 3 SHAFT BLOW-OUT HAZARD of CHECK and butterfly valves September 1997. SHAFT is connected to a pneumatic piston and Operational Factors counterweight, and the interior portion of the SHAFT is coupled directly to the valve disk using The valve is subject to high cyclic loads. In a cylindrical hardened steel dowel pin and a all of the above incidents, the valve repeat . rectangular bar key.