Control of leaks

Many fires have been prevented or quickly extinguished by remotely operated emergency isolation valves. We cannot install them in the lines leading to all equipment that might leak. However, we can install them in the lines leading to equipment that, experience shows, is particularly liable to leak (for example, very hot or cold pumps or drain lines, as described in Section 7.1.2) or in lines from which, if a leak did occur, a very large quantity of material, say 50 tons or more, would be spilled (for example, the bottoms pumps or reflux pumps on large distillation columns). 

In all these cases, once the leak starts, particularly if it ignites, it is usually impossible to approach the normal hand-isolation valves to close them. Emergency isolation valves are discussed in detail in Reference 3, and the following incidents show how useful they can be. They can be operated electrically, pneumatically, or in some cases, hydraulically. 

The control valve in the delivery line to the furnace was also closed. Unfortunately, this valve was bypassed by the line through the heat exchanger. In the heat of the moment, no one remembered to close the valve in the bypass line. In addition, the check (nonreturn) valve did not hold. The return flow of oil from the furnace was stopped by closing a hand valve next to the furnace, which was about 30 m from the fire. Afterward, another EIV was installed in the pump delivery line. 

After the fire, the check valves on all three pumps were found to be out of order. On one the seat had become unscrewed. On another the fulcrum pin was badly worn. On the third the pin was worn right through, and the flap was loose. The valves had not been inspected since the plant was built. 

Check valves have a bad name among many plant operators. However, this is because many of these valves are never inspected or tested. No equipment, especially that containing moving parts, can be expected to work correctly forever without inspection and repair. When check valves are relied on for emergency isolation, they should be scheduled for regular inspection. 

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As an example, as far as the plant modifications are concerned, for pressurized plants the following provisions were considered necessary alternative means for cooling of components in extreme emergency situations, containment venting, a refractory liner on the cavity walls below the vessel, improvements in the control of leaks from the containment, if accidentally higher than the specified ones. 

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