Purging is the use of an inert gas to repetitively dilute a hazardous gas to a level that’s safe for venting to scrubbers or release on disconnection of system components. Nitrogen typically serves as purge gas, but argon is preferred for applications in which nitrogen potentially could enter into undesirable reactions in the process stream. For cylinder connections, there will be the need to disconnect and reconnect every time a cylinder is changed. It’s best to isolate the connection and repeatedly purge with an inert gas using a deep purge assembly. This restricts the area being vented and purged solely to the cylinder connection itself and allows for safe venting of a diluted gas stream to scrubbing systems.
Purging toxic, corrosive or reactive gas systems offers added benefits above safety and environmental compliance — including increased system service life and improved gas purity. This results from eliminating corrosion or reactions that occur when the system or the reactive gas it contains is exposed to ambient air that contains moisture and oxygen.
First diluting, purging and venting the area exposed to ambient air or, if needed, the entire system, can reduce the need to replace regulators or system components. In corrosive or reactive systems that don’t employ a purge mechanism, it’s not uncommon for regulators to fail after three months of use. If a deep purge assembly is employed, the pressure regulator should be able to remain in service for six months or more, provided proper purging is done before cylinder connection and removal from the system.
It’s crucial that purge gas not come from a bulk tank or pipeline but instead from a dedicated purge source that supplies only that specific process gas. This is necessary to prevent any cross-contamination or hazard that can result from the process gas entering the purge source supply. For inert or nontoxic gas systems, using a purge can eliminate ambient air impurities from the process stream, which can yield improved process control or results.
Manifolding is the connection of multiple cylinders or containers of the same gas to a common supply line that goes to a pressure control device or process. This can be as simple as two cylinders connected to a single pressure regulator using flexible hose assemblies as in a protocol switchover station, or as complex as a fully automatic switchover that can interface with remote alarms or process monitoring programs (Figure 2).
Manifolding isn’t as simple as it might seem — the specific process and gas involved dictate design and functions of needed equipment. Where possible the system should allow future expansion to accommodate more cylinders without the need to shut down the process. Features to look for in the manifold design are diaphragm isolation valves for leak integrity and positive closure, metal-to-metal seals with a modular design that allows for system expansion, high-quality materials of construction, preferably Type 316L stainless steel, and high-quality, appropriately rated flexible hose assemblies with integral check valves that prevent backfilling of cylinders and reduce system exposure to ambient air.
Manifolding offers the benefits of reduced risk of hazardous gas release and improved process control. Because more material is available to run the process, cylinder change-outs will be less frequent and can be planned better. The resulting consistency of supply to the process should yield a better outcome by eliminating the introduction of impurities and the variation that comes from changing cylinders in mid-process. Allowing one system to supply multiple processes can result in improved process yields as well as reduced overall cost and number of cylinders or containers onsite.
Locking out equipment
Security and safety is of critical importance to any user of potentially toxic, hazardous or flammable substances, especially since 9/11. Control of access to the materials is crucial. This control must involve more than just limiting who can enter certain areas and must extend to what can be done to the process once access is gained.
Manifolding cylinders so that a common source supplies various processes can ease control of delivery. Toxic or acutely hazardous gases are typically already in areas with restricted access, monitoring, and valve lockouts that prevent dispensing material without appropriate oversight and safety. Often overlooked, however, is the security and safety of what are deemed to be less hazardous inert or flammable gases such as nitrogen, argon and hydrogen. Yet, for instance, the absence of nitrogen during a blanketing process or while purging a line during a system turnover can be as catastrophic as a toxic release if it results in exposure of reactive compounds to air.
Make sure valve lockouts are appropriate and monitored; restrict access to gas storage and use areas; properly train plant personnel in system function and design; and select the right equipment and maintain it correctly.
Keep acutely hazardous gases in a restricted access area or enclosure with online gases in a locked gas cabinet or appropriately constructed bunker. There should be emergency shutoff valves that actuate by manual, remote or process controlled functions when either an area gas monitor detects a leak in the storage or process area, an excess flow condition occurs or when process or security alarms or monitoring devices trip. Pressure control settings must be field-adjustable to specific safeguard limits to ensure that downstream relief devices or system components are not actuated or compromised accidentally or intentionally.