By John Wawrowski Sample stream selection assemblies must address contamination, deadlegs, and system size concerns as well as issues involving maintenance and performance to ensure efficient analytical system operation.
| | Valve designers condensed double block-and-bleed functionalities into miniature valve assemblies and made them modular for streamlined assembly and maintenance.
Sample stream contamination is a serious concern in process analyzer systems. Contamination can invalidate analyses and create headaches for analyzer engineers and technicians. Advancements in analytical instruments have substantially reduced the incidence of contamination, leaving the process analytics specialists to concentrate on improving sample handling systems.Few sample systems are called upon to handle only a single fluid stream. In addition to the sample, a sample stream must handle calibration, purge and carrier fluids. They may also need to handle multiple sample streams. Sample handling systems generally require stream selection assemblies, which consist of a series of valves connected to a common passage that leads to an analyzer. As a new stream travels through this passage, it purges previous fluid that could otherwise contaminate the new fluid and yield incorrect analysis. Purge times must be set for a sufficient length of time to ensure that a pure sample reaches the analyzer. Cross-stream contamination is also a concern. It is usually a result of internal leakage or cross-port leakage in valves. In addition, deadlegs, or trapped volumes of sample material between the valve and common analyzer passage, may contribute to contamination. Beginning in the 1980s, sample system designers started to deal significantly with the problem of contamination. The use of double block-and-bleed valve arrangements reduced problems attributed to leak-prone single valve designs and deadlegs, reducing contamination to much less of a problem.
Today, stream selection assemblies incorporate double block-and-bleed functionality in the form of miniaturized, modular stream selection valves. These valves reduce the volume of the sample stream going to the analyzer and provide consistent sample flow, fast purge times, low valve actuation pressures and enhanced safety characteristics. The valves also offer user-friendly characteristics such as visual actuation and flow path indicators, ANSI-ISA 76.00.02 compatibility and easy maintenance and troubleshooting capacities. Reducing Contamination
Each stream selection assembly improvement has provided significant benefits over previous designs. Designers have addressed contamination, size and sample flow, leading up to today's more efficient systems. Process engineers originally collected samples from process lines and transported them to a lab to conduct off-line analysis. Analyzers were added later in the field to accommodate at-line analysis. Most valves employed for stream selection in these systems were general-purpose valves designed for pneumatic and hydraulic operation. The system was made up of single valves in a single line. The streams then connected to a shared passage line that led to the collection device or analyzer.Sample systems built in this manner were easily contaminated by leaking valves and deadlegs. Incorrect analysis was fairly common. System designers addressed these inadequacies using double block-and-bleed configurations in two basic, yet different, designs - traditional and cascading. The flow path of sample material through the assembly on its way to the common analyzer passage is the primary difference between the two.
| | Vented air gaps prevent mixing of pneumatic actuator supply fluid and system fluid under pressure. click to enlarge
For each stream in traditional double block-and-bleed systems, two valves in a series block sample flow to the common analyzer passage. The route from the process line to the analyzer passage is direct. With block valves closed, a bleed valve opens to vent any material trapped between the two block valves. If the first block valve leaks, material flows to the vent instead of cross-contaminating other streams. Deadlegs are still a potential problem if users do not allow for adequate system purging.Cascading double block-and-bleed configurations avoid deadlegs by purging the system through the flow path. In this design, one stream flows through the second block valve of an adjacent stream or streams. The new stream forces out any residual sample material from downstream lines. When a stream is running, its bleed valves are closed to reduce potential sample contamination from another stream. Miniaturizing Assemblies
With contamination and deadlegs resolved, system designers turned to miniaturizing stream selection assemblies. Traditional and cascading double block-and-bleed designs rely on bulky, difficult-to-maintain assemblies that require a large amount of fittings, tubing and valves to carry out sample stream selection. Valve designers condensed double block-and-bleed functionalities into miniature valve assemblies and made them modular for streamlined assembly and maintenance.
| | Double block-and-bleed valves open directly to the flow loop in this assembly and provide a direct route to the analyzer. click to enlarge
The new valves house multiple functions in one unit, including double block-and-bleed and actuation. The total space needed to perform sample stream selection is, therefore, minimized. In addition, modules can be added or removed as system requirements change. New modular stream selection assemblies still offer designers a choice in system constructions - a cascading double block-and-bleed configuration and an integrated flow loop design. Designers should pay close attention to the efficiency of the assembly in terms of sample flow and integrity. Like their non-modular counterparts, modular cascading designs move sample material through the downstream block-and-bleed arrangements on the way to the analyzer passage. This flow path causes inconsistent flow rates from stream to stream. While the primary stream has direct access to the outlet, streams farther away from the outlet have an increasingly tortuous flow path. Therefore, sample stream flow is diminished the farther the stream is from the outlet. In addition, purge times increase for those streams. The modular integrated flow loop design provides consistent flow rates for all streams. A flow loop is integrated in the base blocks of the modules. Double block-and-bleed valves open directly to the flow loop and provide a direct route to the analyzer. Sampling and purging are streamlined. Consistent stream flow rates allow designers to set a consistent purge time and analysis time for all streams. The varying flow rates found in modular and non-modular cascading double block-and-bleed designs can lead to wasted product, as the system may need to be set to purge each stream to the length of time it takes to purge the slowest stream. This increased overall analysis time also may cause inefficiencies in detecting contaminated process streams. The sooner a faulty reading is realized, the sooner a system can be shut down or corrected. A problem may be detected and corrected several minutes sooner in a system with consistent flow rates, thereby minimizing wasted product. 12 Design Considerations
Analytical instrumentation system designers should consider the following when specifying stream selection assemblies. 1. Low actuation pressures enable automated stream selection assemblies to provide repetitive shut-off while matching analysis system pressures. 2. Vented air gaps prevent mixing of pneumatic actuator supply fluid and system fluid under pressure. 3. Compact size allows for significantly reduced cabinet space. 4. ANSI/ISA 76.00.02 compatibility permits miniature and modular analytical systems to be surface-mounted onto a substrate featuring inlet and outlet connections contained within a 1.5-inch square footprint. 5. It's important to meet NeSSI requirements for miniature, modular systems. 6. Visual actuation indicators identify which stream selection valve is pneumatically actuated at a given time in the analytical process. 7. Color-coded caps provide quick identification of analytical system process streams. 8. Easy maintenance comes in the form of individually replaceable components that don't require removal of fluid connections. 9. Atmospheric reference vents equalize sample loop pressure to the atmospheric pressure to ensure a constant sample pressure in repetitive analysis. 10. High-pressure valve modules meet pressure requirements that may fall in the 250 to 500 psig (17.2 to 34.4 bar) range. 11. Product cycle life determines the number of times a valve is expected to be actuated before failure. 12. A range of materials allows for wide material compatibility needs including assemblies rated to handle corrosive sample streams. About the Author: John Wawrowski, market manager for analytical instrumentation at Swagelok Co., 31500 Aurora Rd., Solon, OH 44139, has 20 years of experience in plant operations and product engineering. He is responsible for identifying market trends, growth opportunities, new product needs, and applications. His bachelor's degree in mechanical engineering is from Cleveland State University. Questions about this article can be addressed to Wawrowski at 440-349-5934. Additional information is available at www.swagelok.com.