Designing Stream Selection Assemblies for Analyzers

Process engineers rely heavily on analytical instrumentation to ensure product quality. Properly designed analytical systems help prevent contaminated fluids and gases from being delivered to consumers or from reaching the next stage of production, yielding significant savings in reduced product loss and system maintenance.

07/01/2007


Process engineers rely heavily on analytical instrumentation to ensure product quality. Properly designed analytical systems help prevent contaminated fluids and gases from being delivered to consumers or from reaching the next stage of production, yielding significant savings in reduced product loss and system maintenance. Sample analysis has moved from lab to field, bringing efficiencies to analytical operations. To minimize costs, many facilities use one automated process analyzer to evaluate multiple streams. These systems often use a stream selection assembly to direct multiple sample streams sequentially to a shared passage line that leads to the analyzer.

Stream selection assemblies must deliver a representative, uncontaminated sample from a process line to the analyzer. System designers should select assemblies carefully that:

  • Use minimal space to select a given stream automatically;

  • 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 have choices among various stream selection assemblies that are based on double block-and-bleed (DBB) valve configurations. These choices grew out of earlier designs with more pronounced drawbacks.

Sample stream technology evolves

In the early days of analytical instrumentation, engineers retrieved samples from process lines and brought them to the lab, but eventually analyzers were moved to the field. In these early systems, each individual process line had a single ball valve. All streams then shared a common passage to the collection device or analyzer.

As a new stream passed through the common analyzer passage, it had to remain intact and free of residue from previous samples. The new stream had to run through the system for a period of time to purge old sample material. In addition, cross-stream contamination could occur due to internal or cross-port leakage in valves. Deadlegs, or trapped volumes of sample material between the valve and common analyzer passage, also caused contamination.

process control

A modular DBB valve manages all necessary functions with only o ne moving part.

For these reasons, sample contamination—and therefore incorrect analysis—was common in single ball valve systems. System manufacturers then turned to two designs based on double block-and-bleed configurations—traditional and cascading.

In a traditional DBB system, 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. If the first block valve leaks, the sample will flow to the vent, rather than cross contaminate other streams in the assembly. Deadlegs can still be a potential problem if users do not allow for adequate system purging.

A cascading DBB configuration, in which one stream flows through the bottom bleed valve of an adjacent stream or streams, avoids deadlegs by purging the system through the flow path. When stream 2 is running to the analyzer, it 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.

Both traditional and cascading DBB designs rely on instrument ball valves for their high flow, ease of actuation, and low maintenance. However, ball valve assemblies are bulky requiring significant space due to the fittings, tubing, and valves needed.

Advances in assemblies

Recent advancements have led to modular valve assemblies that accommodate multiple process streams in a limited space. Valve modules with DBB functionality control each stream, operating as both shutoff valves and stream selector valves. See cutaway graphic.

Based upon modular technology, these valves house multiple functions in one unit. End users can now have double block, bleed, and actuation functions within one module versus using several instrument ball valves. Combining DBB functions in a compact module minimizes the total space needed, reduces overall installation time, and is easy to reconfigure.

With modular stream selection assemblies, system designers have more choices. In addition to traditional and cascading DBB configurations, there is also an integrated flow loop design. Designers should pay close attention to the efficiency of the assembly in terms of sample flow and integrity.

Modular cascading designs—like their non-modular counterparts—move sample material through the bleed valves of downstream lines on the way to the analyzer passage. Unfortunately, this method causes inconsistent flow rates from stream to stream. The primary stream has direct access to the outlet, but as the streams get farther away, the flow path becomes more tortuous, diminishing flow and increasing purge times.

The modular integrated flow loop design eliminates the problem of inconsistent flow. A flow loop is integrated in the base blocks of the modules. Double block-and-bleed valves open directly to the flow loop, which provides a direct route to the analyzer. Sampling and purging are streamlined. Regardless of which stream is running, the flow rate will be consistent.

Consistent stream flow rates allow designers to set a consistently short purge and analysis time for all streams when varying flow rates can be eliminated. 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.

process control

As sampling systems evolved, the general objective was to minimize dead legs and create consistent paths to the analyzer.

Additional design considerations

Other points of consideration when choosing the best modular device for an analytical instrumentation operation include system compatibility, safety issues, and the assembly’s user-friendly characteristics. Designers may look for:

Low actuation pressures. Automated stream selection assemblies with built-in pneumatic actuators operating at 40 psi (2.7 bar) provide repetitive shut off with fewer potential leak points than conventional systems. Valve types that require higher air pressures can complicate air delivery.

Vented air gaps. Many sample stream selection valves are used in applications where the process fluid should not be combined with air. An integral vented air gap can prevent such mixing. The area remains void when empty, and allows actuator air or fluid media to vent if one or both of the seals should be compromised. In addition, the vented air gap keeps actuator air from reaching the analyzer.

Compact size. The size of sample stream selection assemblies has been greatly reduced from the days of instrument ball valve systems. However, modular assemblies are not all alike. Compare the footprint of a complete assembly for the same number of streams to determine which design fits best.

ANSI/ISA 76.00.02 compatibility. A primary factor in the size reduction of stream selection assemblies is the use of ANSI/ISA 76.00.02 specifications for miniature and modular analytical systems. The directives call for these systems to be surface mounted on a substrate featuring inlet and outlet connections within a 1.5-inch square footprint. Units made to these specs save installation and maintenance time, as engineers are able to mount valves directly to substrates. Systems that require additional tubing and connections to mate with ANSI/ISA 76.00.02 substrates may increase the overall system cost in materials, labor, and maintenance, especially when reconfiguring an analytical system.

Visual actuation indicators. Field engineers have found visual indicators useful when identifying which stream selection valve is pneumatically actuated at a given time. These indicators provide visual confirmation of operation and simplify troubleshooting.

Stream identification. Color-coded valve caps assist quick identification of the various process streams in a system. For instance, a green cap can identify a sample stream, a blue cap to identify a zero gas stream, and others as needed. Color-coded markings make troubleshooting and valve maintenance easier as engineers can track a process stream quickly.

Easy maintenance. By design, modular valve assemblies are easy to install and maintain. Multiple valve modules and base blocks connect to create the sampling system, and they are individually replaceable without disturbing fluid connections. In addition, vertical disassembly of valve modules from base blocks permits easy maintenance and prevents accidental disassembly of a whole unit. Even small conveniences, such as independent insert bolts that are captured within the base block, contribute to ease of use.

Atmospheric reference vents. An atmospheric reference vent is positioned between the analyzer and stream selector system to equalize the sample loop pressure to atmosphere. This action, typically performed just prior to the sample injection, ensures a constant sample pressure in repetitive analysis situations.

High-pressure valve modules. Pressures in some analytical systems may operate in the 250 to 500 psi (17.2 to 34.4 bar) range. These systems will require high-pressure valve modules. The system needs will dictate how these modules will be used within the stream selection valve assembly.

Product cycle life. Modular stream select systems are actuated frequently. Therefore, when choosing a stream select system, it is important for the manufacturer to provide typical product cycle life results or mean time to failure (MTTF) for preventive maintenance programs.

Range of materials. Analytical systems that have wide material compatibility needs may require alternative seal materials. System designers should look for assemblies that offer optional seals rated to handle corrosive sample streams.

Sample stream selection assemblies have advanced considerably, along with more sophisticated field analytical instrumentation. These assemblies have moved from bulky, maintenance-heavy systems to miniature, modular designs that offer easy maintenance and improved performance. A variety of factors enter into the right system choice for a particular analysis process. System designers should carefully choose the appropriate sample stream selection assemblies to ensure efficient operation of their analytical systems.



ONLINE EXTRA


How inconsistent flow affects sampling systems
The following theoretical example illustrates potential process time savings realized by using a sampling system with consistent flow rates compared to a similar system with inconsistent flow rates.


Assume that it takes three minutes to analyze each process stream in both the modular, integrated flow loop system (A) and modular cascading system (B). In addition, each system requires flush time to purge older sample material from the process line. The System B operator sets consistent stream cycle times to ensure the previous stream is flushed. This is done by adding excess flush times to Streams 1, 2, and 3.

Suppose contamination occurs in Stream 1 just as the Stream 2 flushing begins. It will not be detected until Stream 1 analysis concludes again. In other words the total process cycle time will elapse– 28 minutes for System A and 40 minutes for System B – before the analyzer identifies the contamination. The additional 12 minutes of Stream 1 flow in System B means a larger amount of product may be wasted before the problem can be identified and fixed. Depending on the volume passing throughthe lines, the product loss could be significant. In addition, the excess flush times in System B cause additional product to be wasted even when there is no contamination.

Process
stream

Modular integrated flow loop system (A)

Modular cascading system (B)

Flow
rate
(units)

Flush
time
(min.)

Analysis
time
(min.)

Cycle
time
(min.)

Flow
rate
(units)

Flush
time
(min.)

Analysis
time
(min.)

Excess
flush
time
(min.)

Cycle
time
(min.)

1

0.180

4

3

7

0.180

4

3

3

10

2

0.176

4

3

7

0.155

5

3

2

10

3

0.172

4

3

7

0.130

6

3

1

10

4

0.176

4

3

7

0.105

7

3

0

10

TOTALS

16

12

28

22

12

6

40

 

 


Author Information

Doug Nordstrom is analytical instrumentation market manager, Swagelok. Reach him at marketing@swagelok.com .




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