Routine and "one-shot" purging operations using inert gases play an important part in the oil and chemical industry. Displacing combustible gas mixtures from storage tanks or pipelines requires close consideration of a) flammability limits of mixtures of inert and combustible gases in air, b) instrument selection for gas analysis, C) sampling procedure, d) choice of inert gas, e) purging technique and last but not least, f) environmental constraints. Based on experimental data and hands-on experience, each of these factors is discussed from the perspective of avoiding excessive inert gas consumption while maintaining a safe operation. A computerized evaluation of gas flammability incorporating mixtures of 26 different combustible gases or vapours with either N2 or C02 as diluent, has been developed to monitor the safety hazards throughout the inerting procedure.
Inerting an enclosed areas, whether a storage tank, a reactor or a pipeline, is a job not different than any other industrial activity: to do it right, it requires good planning and execution. In this case, "right" means safely, timely and without excessive use of inert gases. These three factors are closely interrelated: If one tries unwisely to save time and inert gas, the safety of the operation could be jeopardized.
This paper examines how this interrelation is rationalized based on practical experience and scientific data, in such manner that the "end point" of inerting can be attained with minimal inert gas consumption while ensuring complete safety throughout the operation. Coordinating the efforts of the plant or equipment operator and the gas supplier is essential for each project. While the plant operator looks after most of the planning activity, the industrial gas supplier becomes instrumental in monitoring the progress of the inerting job.
Inerting pipelines, storage tanks or equipment involves basically two district operations:
First during routine or emergency shutdown, injecting an inert gas in order to lower the combustible gas content, so than when air is allowed into the vessel, no flammable mixture can be formed.
Second, when placing equipment into service, the reverse action calls for diluting the enclosed air by injecting an inert gas to bring the oxygen concentration to a level which wil1 not cause flammable mixtures to form with the incoming combustible gases or vapours.
Common principles govern both operations. These principles are dlscussed later in view of a purging job where nitrogen gas has been used to displace a combustible gas in order to lake a storage facility out of service.
Trans Mountain Pipe Line Company Limited made a decision to purge the Facilities at their Westridge propane export terminal in Vancouver, preparatory to changing service. The facilities, schematically represented in Fig. I, consist of two low pressure refrigerated liquid storage tanks, two high pressure storage tanks and attendant piping to service rail car unloading and ship loading docks. Also included was piping related to the refrigeration and desiccant systems.