Perhaps the greatest increase in safety comes from removing to the maximum extent possible compressed gases from process or quality control areas. This allows for better system design, control and security than if compressed gases are dispersed throughout the plant.
Matching gas handling hardware
This can only be accomplished by employing appropriate total-gas-management design principles and specifying systems based on compatibility of materials of construction, necessary flow and delivery pressure, as well as the desired degree of monitoring and control. For reactive or corrosive gases, the materials of construction generally should be Type 316L stainless steel or electroless nickel-plated brass for acid-forming gases, with either Type 316L stainless steel or Hastelloy internals. With inert or non-reactive gases, the properties of the gases themselves determine whether to use barstock brass either bare or chrome-plated.
Additionally, consider the exterior environment to which the gas control equipment is exposed. In many chemical plants, the surrounding ambient environment may be as corrosive to the exterior of systems as if they were in corrosive service. Even for inert gas applications it’s common for the gas handling equipment of choice to be stainless steel or at least chrome-plated. Just because the regulator or manifold is made of the right material doesn’t mean it will perform the function and requirements of the end use.
Many applications require high flow capabilities for process gases that can only be met by regulators with unique features that provide the capacity without sacrificing accurate pressure control. For instance, a balanced valve-and-seat assembly allows for flow rates exceeding 6,000 std. ft3/hour with accurate pressure control, which is achieved by equalizing forces that act on the internal valve assembly such that it is balanced across a wide range of flow conditions (Figure 3).
When a process requires high pressures either to actuate valves pneumatically or inject process gases, a regulator that uses a Type 316L stainless steel piston as the pressure control and sensing element, not a flexing diaphragm, is a good choice. It not only can withstand pressure up to 6,000 psi but also can deliver that pressure many thousands of times without failure.
A key component of process control is the ability to monitor the process in real time using a valve that delivers an accurate sample to gas chromatographs or other analyzers. When the reaction or sample is at elevated temperature and pressure, you must use a specifically designed pressure-control device to maintain the sample integrity or in some cases vaporize the sample prior to injection into the analyzer. These regulators generally feature all Type 316L stainless steel construction and high-temperature seat material, and can be either electrically or steam heated. Efficient heat exchange between the heating element and the body of the regulator is critical. Electrically heated units must incorporate explosion-proof housings and be fully approved by UL, CSA and ATEX.
Gases used in sufficient volume to warrant it should be dispensed to use points from a central gas management system or manifold incorporating a primary supply source of an adequate number of cylinders or containers and a reserve or secondary supply source of an equal number of cylinders that automatically goes online to supply the process without interruption or change in process line pressure. Where possible, a complete system designed to provide the correct material of construction and the required flow and pressure with the desired degree of monitoring and alarm function yields the best result for both safety and efficiency.
Larry Gallagher is specialty gas products manager for CONCOA, Virginia Beach, Va.; e-mail him at firstname.lastname@example.org.