Streamline Your Sampling System

Selecting the right stream selection assembly can improve performance.

By John Wawrowski, Doug Nordstrom and Joel Feldman, Swagelok Co.

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Assembly Advancements
Today, modular valve assemblies accommodate multiple process streams in a limited amount of space. Valve modules control each stream, operating as both shutoff and stream-selector valves (Figure 4). End users now can have DBB and actuation functions within a single module instead of having to utilize several instrument ball valves. Combining the DBB functions in a compact module minimizes the total space needed to perform sample stream selection and reduces overall installation time. As system requirements change, modules can be added or removed, which also saves time.

With modular stream selection assemblies, system designers still have multiple choices: traditional or cascading DBB configurations or an integrated flow loop arrangement.

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Stream Selector System
Figure 4. Miniature modular assembly: Such an
assembly can accommodate multiple streams in
a limited amount of space and reduce contamination
and deadlegs.

Modular cascading designs — like their non-modular counterparts — cause inconsistent flow rates from stream to stream. The modular integrated-flow-loop design eliminates this problem. The modules’ base blocks incorporate a flow loop that provides a direct route to the analyzer. DBB valves open directly to the flow loop (Figure 5). This streamlines sampling and purging, and ensures consistent flow rates for all streams. Uniform stream flow rates allow designers to set the same purge and analysis times for all streams, enabling faster detection and correction of a problem.

Additional Design Considerations
When choosing a modular device for an analytical instrumentation operation it’s also wise to take compatibility, safety issues and the assembly’s user-friendliness into account. Factors to consider include:

Low actuation pressures. Built-in pneumatic actuators in automated stream selection assemblies provide repetitive shutoff with fewer potential leak points than conventional systems. Common industry practices dictate air system actuation pressures of 40 psig. Therefore, designers should choose stream selection assemblies with this rated pressure to avoid the need for additional higher pressure air lines to accommodate actuation pressure requirements that differ from the rest of the system.

Compact size. Today’s small valve modules save significant cabinet space. However, side-by-side comparisons of modular assemblies show that not all are alike. So, system designers should compare the footprint of a complete assembly for the same number of streams to determine which design best fits their size constraints. Also some designs include DBB functionality as well as an integral vented air gap to prevent the mixing of pneumatic actuator supply fluid and system fluid under pressure (Figure 6).

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DBB with Flow Loop
Figure 5. Integrated flow loop: This modular valve

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