Bad valve siting leads to accidental opening
A Worcester ball valve usually has a lever for a handle. These are excellent on/off valves, and are often used in process plant. One such valve was installed in a vertical pipe. The valve had been placed so that the fully open position was with the lever in the 6 o'clock position, and the fully off in the 9 o'clock position. The valve was turned off by the operator. After a short while, the weight of the handle, along with some local vibration, caused the handle to descend and open the valve.
A proper Hazop (Hazard and Operability Study) should have identified that there might be a problem with a valve sited in this way. The cure is simple, simply reverse the handle so that fully open is at 12 o'clock, any tendency for the valve position to alter is then to close. In addition some valves have the facility to alter the load required to operate, this could then have been increased. Alternatively a different type of valve could have been employed.