Streamline Your Sampling System

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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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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Figure 2. Traditional DBB valve manifold: Here, Stream 2 is shown going to the analyzer passage while bleed valves vent residual samples in other lines.

As a new stream passes through the common analyzer passage, it must remain intact and free of any residue from previous samples. It must run through the system for a while to purge old sample material that otherwise could contaminate the new sample and yield incorrect analysis. In addition, cross-stream contamination may occur; this usually stems from internal leakage or cross-port leakage in valves. Deadlegs (trapped volumes of sample material between the valve and common analyzer passage) also cause contamination; they commonly result from the arrangement of the flow path in a device or a portion of a system.

Sample contamination — and, therefore, incorrect analysis — was common in single-ball-valve system designs due to deadlegs and leaking valves. To overcome these inadequacies, system manufacturers turned to two designs based on DBB configurations — traditional and cascading. The primary difference between them lies in the flow path of sample material through the assembly on its way to the common analyzer passage.

In a traditional DBB system (Figure 2), each stream has two valves in series to block sample flow to the common analyzer passage. The streams take a direct route from the process line to the analyzer passage. When the block valves are closed, a bleed valve is opened to vent the volume between the block valves to the atmosphere or a collection device. If the first block valve leaks, the sample will flow to the vent rather than cross-contaminate other streams in the assembly. Deadlegs still could be a potential problem if users don’t allow for adequate system purging.