I/EC
Instrumentation
Successful Sampling Systems Approach by P. H. Stirling
Simplifies Analyzer-Sample
and Henry Ho, Canadian
ΓΛυΐΌΜΛΤΚ; ANALYZERS used for monitoring or automatic control arc often the final arbiters of quality and economic gain. These demanding applications require rapid response, accuracy, and the utmost in reliability. The weakest link of an automatic analyzing system is often that be tween process and analyzer—i.e., the sample-handling system. It is the most frequently neglected part of the analytical system, and failure of many analyzers to perform pro perly as well as much costly main tenance can be traced to this. False economy often leads to the installation of inadequate sampling systems. The cost of an adequate sampling system can sometimes bring the over-all installation cost to double the cost of the analyzer. Failure to realize that these costs must be borne, if the analyzer is to give trouble-free service, is at the root of many unprofitable installa tions. A systems approach to analytical installation design requires a func tional description of each part of the system and its interrelationships with other parts. The block dia gram shows these, and also high lights the importance of sampling system functions to proper analytical performance. Two other necessary blocks are also shown—enviromental and safety controls—which must also be considered when designing an over-all analytical installation.
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High returns a n d rapid pay-offs show that the precautions n e e d e d for reliable oper ation of analyzers are worth while. This is a g o o d rule to follow: An a n a l y z e r should not be separated from its sampling system
use of the barest minimum of com ponents necessary to provide the particular essential functions yields the most efficient sampling system. As an aid to sampling system design, the generalized functions of the con trols shown in the block diagrain arc further broken down in the out
line of functional order. It must be stressed that the order in which sampling functional operations are performed is important and, al though specific rules arc not possible, it is convenient to follow the func tional order, as given, for most simple system design.
S a m p l i n g System Requirements
Sampling systems must be de signed to meet the specific demands of the particular analytical instru ment they serve. In general the
The analytical loop VOL. 53, NO. 3
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Functional Order for Sampling Systems
INSTRUMENTATION
This outline, which ple system, gives various functional tailed breakdown
Design Procedures
can be helpful in designing a sama suggested order for performing operations, along with a more deof generalized functions
Quality Controls (Composition Phase) to Provide 1.
Removal of entrained material, undesirable condensables, and undesired chemical species.
2.
Appropriate phase form.
3.
Chemical conversion to desired chemical species.
Temperature Controls to Provide 1.
Process fluid and auxiliary fluids within acceptable temperature limits for sample system components and analyzer operating temperature.
2.
Required temperatures for quality control operations.
3.
Suitable environmental temperatures and temperature controls.
Pressure Controls to Provide 1.
Process and auxiliary fluids at suitable operating pressures for scmple system components and analyzer operating pressure (absolute pressure if necessary).
2.
Required pressures, with suitable regulation for quality contro operations and desired flows.
Flow Controls to Provide 1.
Flow velocities with an acceptable delay between process and analyzer.
2.
Suitable velocities of process and auxiliary streams for correct operation of heat exchangers, scrubbers, chemical reactors, or driers, etc.
3.
Sample stream to analyzer at a suitably regulated flow rate.
Stream Selection to Provide 1.
Individual lines with manual closures from each process stream to selector array.
2.
Selection (automatic and manual, or manual) with positive indentification of selected stream (random preferred to sequential types).
3.
Positive closure of unselected lines and protection against interline leakage and dead volumes in selector array.
4.
Vents or return paths to process for continuous flow in sample lines. May need isolating nonreturn valves before headers.
Safety and Environmental Requirements to Provide 1.
Protective shields and enclosures.
2.
Authorized maintenance and operating procedures.
3.
Necessary automatic devices for equipment protection—e.g., pressure reliefs, nonreturn valves, cut-outs, and fire explosion protection.
4.
Correct atmospheric control for personnel and equipment protection—e.g., fans, purges, auxiliary analyzers, vent exhaust location, etc.
5.
Adequate stand-by systems for use during failure or servicing periods.
6.
Neutralization of harmful liquid wastes.
58 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
Successful sample handling requires careful consideration of detail during both design and installation. This includes details which arc particular to a given installation but rarely specified in the schematic diagram. The deficiencies of schematic diagrams in showing lengths or slopes of lines should not be forgotten—e.g., pressure let-down regulators may be at the analyzer end of several hundred feet of tubing, contributing a healthy distancevelocity time lag to the over-all system; or poor drainage may cause lines to block up or build up reservoirs of embarrassing contaminants. Knowledge of the location, mechanical configuration of the sampling point, and the nature of the stream to be analyzed arc necessary starting information. The nature of the process stream to be analyzed is often ill-defined and the stream may contain unsuspected embarrassing materials not shown on the process flow sheet—e.g., process by-products, compressor lubricants, corrosion products, drill chips, or welding detritus, etc. Where the sampled process stream contains entrained materials or condensable vapors likely to deposit out later in the system, it is good practice to eliminate these first, because they tend to cause blockage in valves and fittings and deposit out on surfaces. The use of shrouded probes which minimizes the collection of entrained materials is much to be preferred as they cut down the loading on system filters. Size of the entrained particles should be considered when selecting a suitable entrainment separator. Large particles substantially heavier than the fluid in which they are entrained are easily removed by dust cyclones or hydrocyclones. Smaller particles are more effectively removed by filters (about 0.05 micron) or electrical precipitators (about 0.01 micron). New filter material developments indicate that nonwetting materials such as terylcne arc effective in removing small mist particles. It is recommended that stainless sintered filters which can be dropped into standard compression fittings be used as isolating
INSTRUMENTATION niters at appropiatc places in the system. Sample streams requiring either condensation or evaporation as a step in sample preparation should be handled to avoid composition changes due to distillation effects. Streams near saturated conditions should not be rapidly reduced in pressure to avoid condensation resulting from Joule-Thompson effects. In instances where chemical removal of undesired chemical species is involved, reactors and dryers should be placed after bypass purge in the system to keep size small. Adequate temperature control must be provided for reactors, vaporizers, condensers, or other parts of the system. It is worth while to note that sudden slugs of condensables may have sufficient latent heat effects to overload temperature controls. Extra precautions are required for outdoor installations subject to fluctuating climatic conditions and account should be taken of seasonal changes in auxiliary fluid temperatures, such as water or air used for cooling. Overheating of rubber diaphragms or valve packings and freezing up of valves or regulators can be avoided by suitable design of temperature control system. Steam tracing, if used, must include side-lines, and care must be taken to see that Bourdon gages and flow regulators do not act as reflux condensers, thus changing sample composition. If steam tracing causes coking of the lines, it may be more advantageous to insert the heated line inside the lagged outer tubing so that coking or deposition is on the outer surface of a tube and fouling is more easily cleaned off during maintenance periods when the traced line is removed. In many instances, the process stream pressure must be either increased or reduced to a level acceptable to the analyzer. Pressure let-down valves, regulators, and relief valves must be provided to avoid dangerous pressure build-up in glass-body filters, dryers, reactors, etc. Pressure fluctuations and atmospheric pressure variations which can give rise to density errors in infrared analyzers and mass injectionrate errors in chromatography can be eliminated by absolute pressure control.
Sampling system f o r gas containing solids a n d steam
Careful selection of component sizes and adequate control pressures are required to ensure sufficient velocity for operation of heat exchangers, reactors, scrubbers, cyclones, etc. The flow system must be designed to provide smooth flow to the analyzer with acceptable distance-velocity lag and to avoid dead pockets which are likely to give false compositions. If multiple streams are analyzed with a single analyzer, requiring a common manifold, steps must be taken to avoid pressure surges and contamination of one stream by another. Solenoid valves do leak and cross contamination by diffusion from nonflowing manifold legs must be avoided. The sample streams should be kept flowing at all times to provide up-to-the-minute analytical information. This can be simply accomplished by providing fixed bypass bleeds just prior to the switching manifold. Three-way solenoids can be used to switch streams from vent to the analysis manifold if series twoway solenoids are used as "blocks" against sample stream leakage to the manifold. By using secondarybypass bleeds to vent after the block valves, the connecting lines to the common manifold are continuously purged by the selected sample stream, and cross contamination can thus be eliminated. Flow control restrictions may be placed immediately before or after the block valve and the use of a back
pressure restrictor and check valve at the analyzer is recommended to eliminate contamination caused by "backing-up" from vent lines. Over-all system design must provide ease of accessibility and maintenance, and materials must be selected to minimize corrosion. Test, maintenance, and operating procedures must be set up during the design stage so that they are available when the system is finally built. Useful precautions here involve adequate cleaning of equipment after assembly, pressure testing, and inspection throughout to ensure that all components are actually in place. .Adequate drying of equipment must be carried out if a hydraulic leakage test is used. If this is difficult, inert-gas testing may be preferred. Up-to-date maintenance and operating procedures arc mandatory to prevent accidents. Modifications made without procedures being checked through can cause hazards to inexperienced personnel. These are but a few of the general rules which are useful in planning a sample handling system. Example
An example of a system for handling a fairly difficult process stream is shown in the schematic diagram. Here the sample gas is a raw gas at 300 to 400 p.s.i. and 400° F., saturated with water vapor, and con(Continued on page 62 A) VOL. 53, NO. 3
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INSTRUMENTATION taining carbon particlesinsuspension. The inlet, gas, is selected through inlet block valves which arc forged steel with asbestos-graphite packings. The gas enters a knockout cooler where it bubbles through water, and passes through a sprayed packed column to remove particles carried by bubbles and to provide further cooling. T h e gas stream emerges at 100° F., the temperature being controlled by varying the amount of water to the knockout cooler. T h e level of the knockout cooler is con trolled by a ball-float trap, while overheating is avoided by the use of a pressure switch which shuts off" the air-operated inlet valves. The cooled gas is then let down in pressure from 400 to 50 p.s.i. A relief valve set at 100 p.s.i. is installed at the 50-p.s.i. line to pro tect the next let-down valve body which is rated at 100 p.s.i. A vent from the 50-p.s.i. line provides in creased flow, thus reducing time lag. The next regulating valve lets down gas pressure to 10 incites of water. The gas passes through a fine filter, drier needle valve, flowmeter, and then on to the analyzer. T h e flowmeter and pressure indicator are kept at constant values during both sampling and calibrating pe riods. The calibrating gas is directed to the upstream side of the lowpressure regulating valve at about 55 p.s.i. T h e calibrating gas then travels the same route to the analyzer as the sample gas. Provisions for purging the lines with nitrogen to remove traces of carbon monoxide are made to reduce hazards before maintenance is carried out.
HIGHPRESSURE FLOWS
BROOKS CAN METER THEM AS H I G H A S 1 0 0 , 0 0 0 P S I ! Brooks rotameters work well under pressure. Right n o w there are several operating around the clock at 40,000 psi, in process lines ranging from Vz to 2 inches. Operation has been very satisfactory. Even on heavily pulsating flows. (The meters have built-in provision for pulsation damping. It is simple. A n d very effective.) • Brooks high-pressure meters can be supplied with either electric or pneumatic trans mitting extensions. Both are compatible with most receiving instruments. Both use the Brooks magnetic position converter, the most reliable transducer of its kind. • If you have a high-pressure metering job, ask us about it. W e c a n probably give you exactly what you need. And at a price somewhat lower than you'd expect to pay. Design BROOKS INSTRUMENT CO., INC. Specification Sheet 3613 will 5603 W. VINE ST. · HATFIELD · PENNSYLVANIA give you more information. Brooks Instrument Canada Limited, Scarborough, Canada · Brooks Instrument Company, S. Α., Fribourg, Switzerland · Brooks Instrument Nederland, N. V., Veenendaal, Netherlands SA 2 3 6 0 Circle No. 58 on Readers' Service Card 62 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
The whole analytical installation, including two analyzers and two sampling systems, is contained in a temperature-controlled hut. T h e installation has given good service and present provisions for main tenance have proved adequate.
Out authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, DC. Letters will be forwarded and answered promptly.
