Evacuation and Sniff Systems

Besides the liquefaction tail gas discussed in Section 9.1.9, there are several other sources of chlorine-containing vapor, most of them intermittent, which must be handled. Silver [77] gave a summary of these streams and methods for their treatment. Some streams are continuous, such as those from air-based dechlorination. Intermittent streams include 

This is a diverse list. The sources differ greatly in frequency, size, pressure, and quality. They sometimes are collectively referred to as snifP or snift gas and considered together. 

Returned vehicles and shipping containers frequently are emptied before reuse or to allow maintenance and revalving. Once the operators have established that the chlorine is free of dangerous contaminants, they can rework it at a rate determined by the capability of the process. While the chlorine is nearly always dry and often at or above process pressure, it is a common practice to introduce it to the inlet of the chlorine dryers. Its rate of entry into the process must be limited, and Section 11.3.2.3B describes the need for control and protective systems. 

Process connections to the dry evacuation system are at the chlorine drying towers, the compressor system (coolers and compressors), liquefaction (phase separators and liquefiers), and secondary recovery systems. When an emergency spare tank exists, it is held under constant vacuum to be ready to receive chlorine as needed. The evacuation system, as its name implies, is not intended for systems under substantial pressure. These should be vented down through the process for recovery of the chlorine whenever possible. After the pressure has been reduced to a certain level, the evacuation system can take over. 

Critical safety precautions associated with evacuation systems are  

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