Some common sources of leaks

Small cocks have often been knocked open or have vibrated open. They should never be used as the sole isolation valve (and preferably not at all) on lines carrying hazardous materials, particularly flammable or toxic liquids, at pressures above their atmospheric boiling points (for example, liquefied flammable gases or most heat transfer oils v./hen hot). These liquids turn to vapor and spray when they leak and can spread long distances. 

Many leaks have occurred because workers left drain valves open while draining water from storage tanks or process equipment and then returned to find that oil was running out instead of water. 

In one incident, a man was draining water, through a 2-in.-diameter line, from a small distillation column rundown tank containing benzene. He left the water running for a few minutes to attend to other jobs. Either there was less water than usual or he was away longer than expected. He returned to find benzene running out of the drain line. Before he could close it, the benzene was ignited by the furnace which heated the distillation column. The operator was badly burned and died from his injuries. 

The furnace was too near the drain point (it was about 10 m away), and the slope of the ground allowed the benzene to spread toward the 

Spring-loaded ball valves should be used for drain valves. They have to be held open, and they close automatically if released. The size of drain valves should be kept as small as practicable. With liquefied flammable gases and other flashing liquids, % in. should be the maximum allowed. 

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