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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Process engineers heavily rely on analytical instrumentation to ensure product quality. Such instrumentation plays a vital role in preventing contaminated or off-specification material from reaching the next stage of production or going out the plant gates. Catching any problems as early as possible can yield significant savings in reduced product loss and system maintenance. The efficiency and accuracy of spotting these problems principally lies in the delivery method of the sample for analysis.

Sample analysis has moved from the laboratory to the field, enhancing efficiencies. To minimize system costs, many facilities use a single automated process analyzer to evaluate multiple sample streams in succession. These systems often rely on an assembly that selectively directs the sample streams to a shared passage line that leads to the analyzer.

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single block sample analysis
Figure 1. Simplest Approach: Single ball valve
blocks sample streams ahead of a common
analyzer passage.

Stream selection assemblies must deliver a representative, uncontaminated sample from a process line to a detector in the analyzer. So, system designers should ensure that assemblies: use minimal space to automatically select a given stream, maintain the sample’s integrity by avoiding cross-stream contamination and quickly purge old sample material while moving the new stream to the analyzer.

System designers can choose from various assemblies based on double-block-and-bleed (DBB) valve configurations. The most efficient are compact and offer consistent flow characteristics, fast purge times, low valve actuation pressures and enhanced safety characteristics. Other worthwhile features include visual actuation and flow-path indicators, ANSI-ISA 76.00.02 compatibility, easy maintenance and troubleshooting capacities.

Before we discuss assembly characteristics, let’s examine how sample stream technology has progressed.

Evolution
In the early days of analytical instrumentation design, engineers retrieved samples from process lines and brought them to the laboratory to conduct analysis. Later, analyzers were added in the field. In these systems, each process line typically led to an individual ball valve. All streams then shared a common passage to the collection device or analyzer (Figure 1).

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Traditional DBB
Figure 2. Traditional DBB valve manifold: Here,

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