Replaceable pressurized cylinders, gas generators or membrane permeation systems commonly supply inert gases, such as nitrogen, argon, helium and carbon dioxide. Nitrogen gas is the most widely accepted inert purge gas for refinery applications. Argon and helium are commercially available in gaseous or liquid form, but are not as economical to produce. Carbon dioxide is commercially available in liquid form only and being slightly acidic, it can potentially corrode separator components.
Steam is an acceptable purge gas, provided the enclosure’s temperature remains above the steam’s condensing temperature. The steam’s condensing temperature should not exceed the design temperature limits of the enclosure or separator equipment.
Fuel gas, such as methane or natural gas, may be appropriate for vapor control systems that use a flare or incinerator. Such systems require a greater volume of gas than comparable systems that use inert gas purging. To avoid serious safety problems, the fuel gas volume must adequately dilute the vapor stream and reduce variations in vapor volume and heating value.
To feed the control device on OCR purge systems, the atmosphere within the enclosure must remain above the upper flammable limit and at 500 Pa to 1500 Pa (2” w.c. to 6” w.c.). If gas pressure falls below this range, refineries may install a downstream gas booster, such as a blower. Whenever possible, the control of the purge gas to the enclosure should exclude electrical devices that have the potential to fail during power outages, becoming an ignition source.
Pressure can significantly affect the structural integrity of the oil/water separator tank and covers. Sensing and maintaining a preset pressure within the enclosure can accommodate a variable rate of purge gas and also completely prevent air from entering the vapor space. Temperature changes, leaks, rapid atmospheric changes and separator start-up affect the maximum purge gas demand. Determining peak demand is important to ensure sufficient purge gas is available and to prevent a vacuum from developing.
Cover Material
Vent Devices
Vapor Control Device
Enclosed Combustion Devices
Vapor Recovery Devices
Flare Devices
Ignition Sources