Reverse flow into service mains

This occurs when the pressure in the service line is lower than usual or when the pressure in the process line is higher than usual. Many plants have experienced incidents sueh as the following  

Liquefied gas leaked into a steam line that had been blown down. Ice then formed on the outside of the steam line. 

The paint was dissolved in a cabinet that was pressurized with nitrogen acetone had leaked into the nitrogen [2]. 

Toxic fumes in a steam system affected a man who was working on the system (see Section 1.1.4). 

A service that is used intermittently should be connected to process equipment by a hose, which is disconnected when not in use, or by double block and bleed valves. If a hose is used, it should be provided with a vent so that it can be depressured before it is disconnected. 

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