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Streamline Your Sampling System

April 3, 2009
Selecting the right stream selection assembly can improve performance.
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. EvolutionIn 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. 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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Figure 3. Cascading DDB valve manifold:
This approach provides a longer flow path
and turns, causing lower flow rates for
streams farther upstream.
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. Assembly AdvancementsToday, 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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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 ConsiderationsWhen 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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Figure 5. Integrated flow loop: This modular valve
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
designs put DDB functionality in a single unit;
some integrate a vented air gap to prevent
mixing of actuator and sample fluids.
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 ChoiceSample 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 [email protected], [email protected] and [email protected].

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