manifold offers a direct route to the analyzer,
ensuring the same flow from all streams.
Specification Compatibility. A primary factor in size reduction of stream selection assemblies is the advent of the ANSI/ISA 76.00.02 specification for miniature and modular analytical systems. This specification, which grew out of the New Sampling/Sensor Initiative (NeSSI), calls for these systems to be surface-mounted onto a substrate featuring inlet and outlet connections contained within a 1.5-inch-square footprint. Compatible stream selection valves save installation and maintenance time because they can be quickly mounted directly to substrates. Systems that require additional tubing and connections to fit into ANSI/ISA 76.00.02 substrates may increase the system’s overall cost in materials, labor and maintenance, especially when reconfiguring an analytical system.
Fail-safe seal between actuator air and system fluid. It’s critical that a stream selector valve have a fail-safe means of ensuring that air from the actuator doesn’t leak into the system fluid, which is under pressure. Oxygen mixed with process fluid under pressure could result in combustion. A fail-safe design ensures that if one seal fails, another still performs the job or, if both fail, the air or fluid will vent to atmosphere or a collection area rather than mixing.
Visual actuation indicators. These make it easy to identify which stream selection valve is pneumatically actuated at a given time in the analytical process. They provide visual confirmation of the sample system’s operation and speed troubleshooting. Large, brightly colored indicators enhance the user’s ability to know that a valve is open.
Easy maintenance. By design, modular stream selection valve assemblies offer ease of installation and maintenance. Multiple valve modules and base blocks are connected to create the sampling system — each can be replaced without removing fluid connections. In addition, vertical disassembly of valve modules from base blocks simplifies maintenance and prevents accidental disassembly of a whole unit. Even small points in the mechanics of assembly, such as independent insert bolts that are captured within the base block, contribute to a system’s ease of use.
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Figure 6. Combined unit: Modern valve
High-pressure valve modules. Pressure requirements in some analytical systems may fall in the 250-psig to 500-psig range, necessitating high-pressure valve modules. System needs will dictate how these modules should be used within the stream selection valve assembly.
Atmospheric reference vents. Such vents are positioned between the analyzer and the stream selector system and equalize the sample loop pressure to the atmospheric pressure. Pressure equalization typically is performed just prior to sample injection to ensure a constant sample pressure for repetitive analyses.
Product cycle life. Because modular stream selection systems are frequently actuated, it’s important to know typical product cycle life results or mean time to failure for preventative maintenance programs.
Range of materials. An analytical system may handle a wide range of fluids. So, assess seal compatibility and the availability of alternative seal materials to cope with more-corrosive sample streams.
Make the Right Choice
Sample stream selection assemblies have moved from bulky maintenance-intensive systems to miniature modular designs that offer easy maintenance and improved performance. However, that hasn’t reduced the need for careful analysis when selecting an assembly. A variety of factors enter into the choice of the most appropriate assembly to ensure efficient operation of a particular system.
John Wawrowski is a market manager, analytical instrumentation, Doug Nordstrom is a market manager, analytical instrumentation, and Joel Feldman is a design engineer for Swagelok Co., Solon, Ohio. E-mail them at firstname.lastname@example.org, email@example.com and firstname.lastname@example.org.