The Future ofSolenoid Valve

TechnologySignificant changes are

on the horizon as the chemical processing industry

adapts to new demands

See Page 20

February 2006

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SSSPECIAL REPORT www.chemicalequipment.com12

Curbing Contaminationand Enhancing Efficiencies

Sample stream contamination is aserious concern in process analyzersystems. Contamination can invali-date analyses and create headachesfor analyzer engineers and techni-cians. Advancements in analytical in-struments have substantially reducedthe incidence of contamination, leav-ing the process analytics specialists toconcentrate on improving samplehandling systems.

Few sample systems are calledupon to handle only a single fluidstream. In addition to the sample, asample stream must handle calibra-tion, purge and carrier fluids. Theymay also need to handle multiplesample streams. Sample handlingsystems generally require stream se-lection assemblies, which consist of aseries of valves connected to a com-mon passage that leads to an ana-lyzer. As a new stream travelsthrough this passage, it purges previ-ous fluid that could otherwise con-taminate the new fluid and yield in-correct analysis. Purge times must be set for a sufficient length of timeto ensure that a pure sample reachesthe analyzer.

Cross-stream contamination is alsoa concern. It is usually a result of in-ternal leakage or cross-port leakage

in valves. In addition, deadlegs, ortrapped volumes of sample mate-

rial between the valve and com-

lected samples from process lines andtransported them to a lab to conductoff-line analysis. Analyzers wereadded later in the field to accommo-date at-line analysis. Most valves em-ployed for stream selection in thesesystems were general-purpose valvesdesigned for pneumatic and hydraulicoperation. The system was made upof single valves in a single line. Thestreams then connected to a sharedpassage line that led to the collectiondevice or analyzer.

Sample systems built in this man-ner were easily contaminated by leak-ing valves and deadlegs. Incorrectanalysis was fairly common.System designers addressedthese inadequacies usingdouble block-and-bleedconfigurations in two

basic, yet different,designs — traditional and cascading.The flow path of sample materialthrough the assembly on its way tothe common analyzer passage is theprimary difference between the two.

For each stream in traditional dou-ble block-and-bleed systems, twovalves in a series block sample flowto the common analyzer passage. Theroute from the process line to the an-alyzer passage is direct. With blockvalves closed, a bleed valve opens tovent any material trapped betweenthe two block valves. If the firstblock valve leaks, material flows tothe vent instead of cross-contaminat-

Sample stream selection assemblies must address contamination, deadlegs, and system size concerns aswell as issues involving maintenance and performance to ensure efficient analytical system operation.

By John Wawrowski

John Wawrowski, market managerfor analytical instrumentation atSwagelok Co., 31500 Aurora Rd.,Solon, OH 44139, has 20 years ofexperience in plant operations andproduct engineering. He is responsi-ble for identifying market trends,growth opportunities, new productneeds, and applications. His bache-lor’s degree in mechanical engineer-ing is from Cleveland StateUniversity. Questions about this arti-cle can be addressed to Wawrowskiat 440-349-5934. Additional infor-mation is available atwww.swagelok.com.

Valve designers condenseddouble block-and-bleedfunctionalities into minia-ture valve assemblies andmade them modular forstreamlined assembly andmaintenance.

mon analyzer passage, may con-tribute to contamination.

Beginning in the 1980s, samplesystem designers started to deal sig-nificantly with the problem of con-tamination. The use of double block-

and-bleed valve arrangementsreduced problems attributed to leak-prone single valve designs anddeadlegs, reducing contamination tomuch less of a problem.

Today, stream selection assembliesincorporate double block-and-bleedfunctionality in the form of miniatur-ized, modular stream selection valves.These valves reduce the volume ofthe sample stream going to the ana-lyzer and provide consistent sampleflow, fast purge times, low valve ac-tuation pressures and enhanced safetycharacteristics. The valves also offeruser-friendly characteristics such asvisual actuation and flow path indica-tors, ANSI-ISA 76.00.02 compatibil-ity and easy maintenance and trou-bleshooting capacities.

Reducing ContaminationEach stream selection assembly im-provement has provided significantbenefits over previous designs.Designers have addressed contamina-tion, size and sample flow, leading upto today’s more efficient systems.

Process engineers originally col-

‘The sooner a faulty reading is

realized, the sooner a system can be shut down or corrected.’

ce62SpRpt4Swag..qxd 2/2/2006 12:32 PM Page 12

ing other streams. Deadlegs are still apotential problem if users do not al-low for adequate system purging.

Cascading double block-and-bleedconfigurations avoid deadlegs by purg-ing the system through the flow path.In this design, one stream flowsthrough the second block valve of anadjacent stream or streams. The newstream forces out any residual samplematerial from downstream lines. Whena stream is running, its bleed valvesare closed to reduce potential samplecontamination from another stream.

Miniaturizing AssembliesWith contamination and deadlegs resolved, system designers turned tominiaturizing stream selection assem-blies. Traditional and cascading dou-ble block-and-bleed designs rely onbulky, difficult-to-maintain assem-blies that require a large amount offittings, tubing and valves to carryout sample stream selection. Valvedesigners condensed double block-

and-bleed functionalities intominiature valve assemblies

and made them modularfor streamlined assem-

bly and maintenance.The new valveshouse multiplefunctions inone unit, in-cluding double

block-and-bleedand actuation. The

total space needed to per-form sample stream selection is,

therefore, minimized. In addition,modules can be added or removed assystem requirements change.

New modular stream selection as-semblies still offer designers a choicein system constructions — a cascad-ing double block-and-bleed configu-ration and an integrated flow loop de-sign. Designers should pay closeattention to the efficiency of the as-sembly in terms of sample flow and integrity.

Like their non-modular counter-parts, modular cascading designsmove sample material through thedownstream block-and-bleed arrange-ments on the way to the analyzer pas-sage. This flow path causes inconsis-tent flow rates from stream to stream.While the primary stream has direct

access to the outlet, streams fartheraway from the outlet have an increas-ingly tortuous flow path. Therefore,sample stream flow is diminished thefarther the stream is from the outlet.In addition, purge times increase forthose streams.

The modular integrated flow loopdesign provides consistent flow ratesfor all streams. A flow loop is inte-grated in the base blocks of the mod-ules. Double block-and-bleed valves

open directly to the flow loop and pro-vide a direct route to the analyzer.Sampling and purging are streamlined.

Consistent stream flow rates allowdesigners to set a consistent purgetime and analysis time for allstreams. The varying flow rates foundin modular and non-modular cascad-ing double block-and-bleed designscan lead to wasted product, as thesystem may need to be set to purgeeach stream to the length of time it

takes to purge the slowest stream.This increased overall analysis timealso may cause inefficiencies in de-tecting contaminated process streams.The sooner a faulty reading is real-ized, the sooner a system can be shutdown or corrected. A problem maybe detected and corrected severalminutes sooner in a system with con-sistent flow rates, thereby minimizingwasted product.

13

Vented air gaps prevent mixing of pneumatic actuatorsupply fluid and system fluid under pressure.

Double block-and-bleed valves open directly to the flow loop inthis assembly and provide a direct route to the analyzer.

CHEMICAL EQUIPMENT® FEBRUARY ’06

1. Low actuation pressures enable automated stream se-lection assemblies to provide repetitive shut-off whilematching analysis system pressures.2. Vented air gaps prevent mixing of pneumatic actuatorsupply fluid and system fluid under pressure.3. Compact size allows for significantly reduced cabinet space.4. ANSI/ISA 76.00.02 compatibility permits miniature andmodular analytical systems to be surface-mounted onto asubstrate featuring inlet and outlet connections containedwithin a 1.5-inch square footprint.5. It’s important to meet NeSSI requirements for minia-ture, modular systems.6. Visual actuation indicators identify which stream selec-tion valve is pneumatically actuated at a given time in theanalytical process.

7. Color-coded caps provide quick identification of analyt-ical system process streams.8. Easy maintenance comes in the form of individually re-placeable components that don’t require removal of fluidconnections.9. Atmospheric reference vents equalize sample looppressure to the atmospheric pressure to ensure a con-stant sample pressure in repetitive analysis.10. High-pressure valve modules meet pressure require-ments that may fall in the 250 to 500 psig (17.2 to 34.4bar) range.11. Product cycle life determines the number of times avalve is expected to be actuated before failure.12. A range of materials allows for wide material compat-ibility needs including assemblies rated to handle corro-sive sample streams.

12 Design ConsiderationsAnalytical instrumentation system designers should consider the following when specifying stream selection assemblies.

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