Stream 2 is shown going to the analyzer passage
while bleed valves vent residual samples in
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.
In a cascading DBB configuration (Figure 3), one stream flows through the bottom bleed valve of an adjacent stream or streams — this avoids deadlegs by purging the system through the flow path. For instance, as shown in the figure, Stream 2 flows through a set of block-and-bleed valves and then through the bleed valve of Stream 1 before reaching the line to the analyzer. Stream 2 forces out any residual sample material from Stream 1. When Stream 2 is running, its bleed valves are closed, which reduces potential sample contamination from another stream.
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However, the cascading configuration causes inconsistent flow rates from stream to stream. The primary stream (Stream 1 in Figure 3) has direct access to the outlet. The streams farther from the outlet face increasingly tortuous flow paths, which progressively diminishes sample stream flow. The varying flow rates can lead to wasted product because purge times differ for each stream and the system may be set for the period required to purge the slowest stream. This increased overall analysis time also may cause inefficiencies in detecting contaminated process streams.
Both traditional and cascading DBB designs rely on instrument ball valves. While these valves provide high flow, simple actuation and relatively easy use and maintenance, the sample stream selection systems require significant space due to the large amount of fittings, tubing and valves needed. These systems, therefore, are bulky and difficult to maintain.