A 25,000-gallon tank used to store nitrogen or carbon dioxide at 250 psig needed maintenance that required the removal of a four-foot diameter, 500 lb manhole cover. The tank was evacuated to 0 psig using the manually operated relief valve. Once the workers no longer heard air flowing through the valve, they released the stick they had been using to keep it open.
Three of the bolts on the manhole cover required a welding torch to be removed, and just after one had been loosened the cover popped off, throwing both workers off their elevated positions and fatally injuring them.
This incident illustrates how the area of the cover affects the overall force. Even with what would be considered “low” pressure, the area of the cover multiplied the force greatly. There are several things to consider regarding this incident:
- Once a system is closed, the temperature change will affect the internal pressure. Even a small amount of pressure change (~0.27 psig) was enough to equal the weight of the 500 pound cover. No amount of pressure building up in this system was safe. It was especially problematic because the internal temperature of the tank was lower than the outside temperature after the pressure release due to evaporative cooling.
- Related to this, the workers did depressurize the system before they started work and incorrectly thought it remained depressurized. Just because something is depressurized does not mean it will stay that way depending on how the system works.
- Work arounds, rigging something up, hacks- whatever you want to call them, are putting you and everyone around you in danger. There is a reason LANL policy is to assess every hazard, every time. The system can be more complicated that any one person expects and all the information about it is not necessarily available. It’s often what people don’t know that turns into the problem.
Source: Los Alamos National Laboratory, Pressure Safety Course