Several workers sustained minor injuries and millions of dollars worth of equipment was damaged by an explosion after a shaft blew out of a check valve. The valve failure rapidly released a large vapor cloud of hydrogen and hydrocarbon gases which subsequently ignited.Certain types of check and butterfly valves can undergo shaft-disk separation and fail catastrophically or "blow-out," causing toxic and/or flammable gas releases, fires, and vapor cloud explosions. Such failures can occur even when the valves are operated within their design limits of pressure and temperature. Most modern valve designs incorporate features that reduce or eliminate the possibility of shaft blow-out. However, older design check and butterfly valves, especially those with external appendages such as pneumatic-cylinders, counterweights, manual operators, or dashpots, may be subject to this hazard.A number of design and operational factors may contribute to this hazard. The valve has a shaft or stem piece which penetrates the pressure boundary and ends inside the pressurized portion of the valve. This feature results in an unbalanced axial thrust on the shaft which tends to force it (if unconstrained) out of the valve. The valve contains potential internal failure points, such as shaft dowel-pins, keys, or bolts such that shaft-disk separation can occur inside the valve. The dimensions and manufacturing tolerances of critical internal parts (e.g., keys, keyways, pins, and pin holes) as designed or as fabricated cause these parts to carry abnormally high loads. In this incident, the dowel pin rather than the key transmitted torque from the shaft to the disk. The valve stem or shaft is not blow-out resistant.Two-piece valve stems ("stub shafts") that penetrate the pressure boundary (resulting in a differential pressure and unbalanced axial thrust as described above), single-diameter valve shafts (i.e., shafts with internal diameters smaller than the diameters of their packing glands), or shafts without thrust-retaining devices (such as split-ring annular thrust retainers) are susceptible to blow-out. These valves are subject to high cyclic loads. In many incidents, the valve repeatedly slammed shut with great force during compressor trips and shutdowns. Such repeated high stresses may causes propagation of intergranular cracks in critical internal components, such as dowel pins. The valve is subject to low or unsteady flow conditions, such that disk flutter or chatter occur, resulting in increased wear of keys, dowel pins, or other critical internal components.Valves in high-pressure service lines may be more likely to undergo shaft blow-out. In this incident, system pressure at the failure point was approximately 300 psig. Valves used in hydrogen-rich or hydrogen sulfide-containing environments may be more susceptible to blow-out due to hydrogen embrittlement of critical internal components, particularly if these are made from hardened steel (as was the dowel pin in this incident).
Facilities should review their process systems to determine if they have valves installed that may be subject to this hazard. If so, facilities should conduct a detailed hazard analysis to determine the risk of valve failure. Detailed internal inspections may be necessary in order to identify high-risk valves. Facilities should consider replacing high-risk valves at the earliest opportunity with a blow-out-resistant design. If immediate valve replacement is impossible or impractical, facilities should consider immediately modifying the valves to prevent shaft blow-out. Valve manufacturers should be consulted in order to ensure that any modifications are safe.A web-based resource developed by Sandia National Laboratories to provide data on hydrogen embrittlement of various materials is available at Technical Reference for Hydrogen Compatibility of Materials.