8.2
Analyzer Sampling: Process Samples
AR
W. M. BARROWS (1972) P. FOUNDOS
B. G. LIPTÁK (1995, 2003)
AT
(1982) (K. A. Perrotta and R. Strauss on filters)
Sampling
Flow Sheet Symbol
1170
© 2003 by Béla Lipták
Costs:
Compressed air filters cost about $60; stainless steel bypass filters, $300 to $600;
continuous-flow ultrasonic homogenizers, $3000; sludge centrifuge, $600; interval
sampling pump only, $750; automatic liquid sampler system, $1500 and up; complete
sampling system for single-process gas stream, $3000 to $6000; complete sampling
system for single-process liquid stream, $3500 to $7000. The per-stream cost in a
multistream sampling system drops as the number of streams increase.
Partial List of Suppliers:
ABB Automation Analytical Div. (www.abb.com/usa)
Advantech Automation (www.advantech.com)
Air Dimensions Inc. (corrosion-resistant sampling pumps) (www.airdimensions.com)
Air Instruments & Measurements (www.aimanalysis.com)
Ametek Process Instruments (www.ametekpi.com)
Applied Chemometrics (www.chemometrics.com)
Bristol Equipment Co. (liquid samplers) (www.bristolequipment.com)
Cole-Parmer Instrument Co. (homogenizers, composite samplers) (www.coleparmer.com)
Collins Instrument Co. (sampling systems, bypass filters) (www.collinsinst.com)
Draeger Safety (www.draeger.com)
Fluid Metering Inc. (www.fmipump.com)
GLI International (www.gliint.com)
Hach Co. (www.hach.com)
Horiba Instruments (www.horiba.com)
Kahn Instruments (www.kahn.com)
King Engineering Corp. (filters) (www.king-gage.com)
KNF Neuberber (www.knf.com)
Kurz Instruments Inc. (stack sampling) (www.kurz-instruments.com)
Markland Specialty Engineering (www.sludgecontrols.com)
MIE Inc. (www.mieinc.com)
Neutronics Inc. (www.neutronicsinc.com)
OnTrak Control Systems (www.ontrak.com)
Orbital Sciences (www.orbital-ait.com)
Orena ControlNet (www.orena.com)
Parker Filtration (www.parker.com/balston.com)
Perma Pure (www.permapure.com)
PGI International (www.pgiint.com)
Roseomunt Analytical Process Analytic (www.processanalytic.com)
Sensidyne (www.sensidyne.com)
Servomex Applied Automation (www.servomex.com)
Siemens Applied Automation (www.aai-us.com)
Siemens Energy & Automation, Process & Analytical Div. (process samplers)
(www.sea.siemens.com)
TBI-Bailey Controls (probe protector from coating) (www.tbi-bailey.com)
Teledyne Analytical Instruments (process sampling systems) (www.teledyne-aicom)
8.2 Analyzer Sampling: Process Samples
1171
Thermo Onix (www.thermoonix.com)
Thermo Ramsey (www.thermoramsey.com)
Tyco Valves & Controls (www.tycovalves.com)
Y2 Systems (www.y2systems.com)
Zellweger Analytics (www.zelana.com)
INTRODUCTION
This section starts with a discussion of the general considerations that the designer needs to evaluate before starting the
actual hardware selection for a sampling system. Next, the
various sample system components are described. The section is concluded by a detailed evaluation of the various
sampling applications, considering gas, liquid, solids, stack,
reactor, slurry, trace, and other types of process samples.
Sampling is also discussed in the first section of this
chapter and under the individual analyzer categories. The
subject of sampling stack particulate concentration is separately discussed in Section 8.3.
GENERAL CONSIDERATIONS
The sampling system is an integral and key part of an analyzer
system and should be designed to obtain a representative
sample; transport the sample to the analyzer; condition the
sample; accomplish sample stream switching, if necessary;
provide facilities for return and disposal of the sample; and
provide not only calibration facilities, but also preventive
maintenance features, and alarm functions for on-line reliability and operator alerts.
If a sample has to be brought to an analyzer, a sample
transportation delay and a potential for interference with the
integrity of the sample will be introduced. If the measured
composition is to be controlled, the transportation lag can
seriously deteriorate the closed-loop control stability of the
system.
Even more serious is the potential for interference with
the integrity of the sample, due to the effects of filtration,
condensation, leakage, evaporation, and so on, because these
operations can not only delay, but also change information
that is to be measured: the composition of the process fluid.
Consequently, the best solution is to eliminate all sampling systems and place the analyzer directly into the process.
The “in-pipe” analyzer designs are becoming more and more
available. Particularly well suited for this design are the various radiant energy and probe-type sensors.
Feasibility Evaluation
Sample systems are rarely duplicated, and thus each system
must be debugged as a new entity. In deciding the design of
a sampling system, the following questions should be raised:
© 2003 by Béla Lipták
1. Will the sample be adversely affected by sample
transport and conditioning?
2. From what stage in the process will the sample be
taken?
3. How can the sample be transported to the analyzer?
4. Is the sample a solid, liquid, gas, or mixture?
5. In what phase must the sample be for analysis?
6. Must the sample be altered (filtered)?
7. Is sufficient sample available?
8. What will be the time lag introduced by transporting
the sample?
9. Where is the excess sample returned?
10. Will the analyzer be shared by one or more samples?
These considerations are applicable both to the continuously flowing sample systems and to the less frequently used
grab samples. Grab sampling is limited by variations in
cleanliness of the sample containers, changes in the method
of collecting the sample, delays in transporting the sample,
and deviations in withdrawing the sample from the sample
container.
The piped-in sample system usually includes the hardware for calibrating the analysis system. It also provides a
tap for obtaining samples for laboratory testing. Sample conditioning systems are usually costly to maintain.
The installation price for the sampling system frequently
exceeds the cost of the analyzer, but its importance overrides
this economic consideration because a second-class analyzer
can still furnish usable data if it operates with an efficient
sample system, whereas a poor sample invalidates the entire
measurement.
Therefore, the most important criterion is to keep the
samples representative, both in time (short sample lines
guarantee minimum transportation delays) and in composition. Whenever possible, the sample should not be tampered
with because the steps of sample preparation (drying, vaporizing, condensing, filtering, and diluting) always degrade the
sample.
Sample Data Requirements
A comprehensive listing of the characteristics of each sample
stream, including any abnormal conditions, should be prepared before an analyzer sample system is designed. Data
are usually available from process conditions and laboratory
analysis in existing plants and from design data for new
plants. Table 8.2a is a form to be used in summarizing the
characteristics of a sample.
1172
Analytical Instrumentation
TABLE 8.2a
Sample Analyzer Data Form
Stream name or identification _________________________________________________________________________
STREAM COMPOSITION DATA
COMPONENT
CONCENTRATION
RANGE OF COMPONENT
IN MOL%, WT%, PPM
TO BE MEASURED
OPERATING PROCESS DATA
Temperature _____________________________ Pressure __________________________________________________
Phase: Liquid _____________________________ Vapor ___________________________________________________
Corrosive components/solids __________________________________________________________________________
Stability (polymerize, decompose, etc.) _________________________________________________________________
Sample bubble point___________________________ Dew point ____________________________________________
SAMPLE CONDITIONS
Maximum distance: Tap to analyzer _____________________ Analyzer to return ______________________________
Speed loop required: Yes _______________________________ No __________________________________________
Sample return pressure point _________________________________________________________________________
Sample probe requirements: Connection size ______________________ Orientation ____________________________
Materials of construction: Stainless steel ________ Teflon ________ Viton ________ Glass ________ Other _________
Electrical areas classification _________________________________________________________________________
Power supply ______________________________________________________________________________________
Output signal ______________________________________________________________________________________
Utilities available: Stream __________________ Air ___________________ Cooling water_______________________
Process Vessel
or Pipe Wall
Gate Valve
Pipe-to-Tubing
Connection
To
Sampling
System
Tubing
FIG. 8.2b
Sample probe assembly with process shutoff value.
Sample Takeoff Point
Sample conditioning begins with the location of a suitable
sample takeoff point. To obtain a representative sample, the
takeoff is usually located at the side of a process line, especially in the case of liquid samples where there is the possibility of vapor on the top of a horizontal line and dirt or
solids on the bottom of the line. For sampling vapors, the
connection may be located in the side or top of the process
line, but in both cases with due consideration to accessibility
for maintenance.
Ideally, the sample at the appropriately selected takeoff
point will require little or no conditioning; however, it is good
© 2003 by Béla Lipták
practice to install a sampling probe (Figure 8.2b) for most
applications as a precautionary measure to prevent particulates from entering the sample transport system. Sampling
processes that are still reacting chemically or pyrolysis gases
may require reaction quenching, or fractionation, at the sample takeoff. This is done by cooling or back-flushing with an
inert gas or liquid to keep the sample takeoff clean and
reliably active (Figure 8.2c), while drawing off a reproducible sample for analysis.
With the advent of in situ analyzer detectors, the sample
takeoff becomes the point of analysis and, for locating
in situ analyzers, the above considerations must be carefully
evaluated.
8.2 Analyzer Sampling: Process Samples
Field Run
Sample to
Analyzer
Conditioning
At Analyzer Location
Analyzer
System
Coalescer
1173
Vent
or
Drain
Single-Line Transport
Vortex
Cooling
Tube
Temperature
Controller
Analyzer
System
Cooling
Section
Vent
or
Drain
By-Pass Stream
Instrument
Warm
Air Supply
Coolant
Exhaust
Reflux
Action
Sample
Condensibles
Process Line
FIG. 8.2c
Pyrolysis gas sample fractionation and conditioning unit.
Sample Transport
A representative sample extract from a process line must be
continuously conditioned while in transport to avoid compromising the sample integrity. Thus, provision must be made
to heat or cool the line as necessary for the specific condition.
The sample is normally transported in one of three ways
(Figure 8.2d).
Single-line transport is the most direct approach and is
used when the sample line volume is small in relation to the
analyzer sample consumption so that the transport time lag
is reasonably short. It is usually used when the analyzer is
field-mounted close to the sample point and sample exhaust
facilities are available.
Bypass-stream transport is a commonly used method for
maintaining a high sample transport velocity that provides
minimum transportation lag. This method is used when samples are vaporized at the tap and no facilities exist for returning the vapor to the process, or in similar situations. Consideration must be given to the fact that the sample bypass must
be piped to a drain or vent. This procedure may have a
negative impact on the environment, despite the fact that the
cost of the sample being disposed of may economically justify a sample recovery system.
© 2003 by Béla Lipták
Analyzer
System
Filter
Reflux
Section
Vent, Drain
Sample Return
Point or Sample
Recovery System
By-Pass Return Fast Loop
FIG. 8.2d
Sample transport methods.
Bypass-return fast loop is the most commonly used transport loop for an analyzer mounted away from the sample
takeoff point. By circulating a continuous high velocity
across a device, to create a differential pressure, and drawing
off a bypass stream to the analyzer, a fast loop is obtained,
which is adjustable with no waste in the product. In most
cases, such devices as pumps, control valves, and process
equipment exist in the process for this purpose. Otherwise,
it may be necessary to install an orifice in the process line
or a circulating sample pump. With a circulation sample
pump, care must be taken to prevent cavitation by locating
the pump close to the sample takeoff.
Transport Lag
After selecting the appropriate sample transport method for
each analyzer system, a calculation of the sample time lag
should be made, using conventional flow equations based on
the following:
1. Available differential pressures.
2. Total length of the fast loop from the sample takeoff
point to the analyzer location and back to the sample
return point. Any restrictions on this loop should be
excluded from available differential pressures.
3. Line sizes used.
4. Viscosity of the sample.
Table 8.2e should be used as an aid in establishing volumes and pressure drops for some typical tubing and pipe
used in sample systems.
1174
Analytical Instrumentation
TABLE 8.2e
Dimensions and Volumes of Tubing and Pipe Used in Sample Systems
Nominal
Volume per
Diameter, in.
316 stainless steel tubing
3 /8
in.
/8
1
/4
.0787
0.1850
.0048
0.0268
.9571
5.3035
3
/8
/2
0.0253
0.4055
0.0684
0.1290
13.4417
25.2984
/4
/8
1
/2
3
/4
0.3642
0.4921
0.6220
0.8268
0.1040
0.1891
0.3038
0.5363
20.4521
37.4904
59.7408
106.6800
1
1
Schedule 40 pipe
Internal Area
Inner
Diameter, in.
1
3
Sample Disposal
Sample systems condition a sample suitable for introduction
into an analyzer while maintaining the integrity of the sample.
As has been emphasized throughout this section, once a sample is conditioned, it must be preserved in the conditioned
state; therefore, provisions for heating, or in some cases cooling, the sample lines and system must be furnished for the
integrity of the sample. Thus, the entire system must be protected from varying ambient temperatures, which could condense or flash the sample.
Furthermore, as a general rule, the sample should be
located in enclosures that provide limited access to unauthorized personnel and also protect the equipment from any
corrosive environment. It is accepted practice that systems
be completely preassembled and tested in conjunction with
analyzers prior to installation in the field.
Test and Calibration
The reliability of each analyzer is measured by its ability to
check the analyzer calibration as recommended by the man-
© 2003 by Béla Lipták
cc
Quick
Disconnect
Sample disposal is a critical area, both from the economic
point of view that would preserve any quantity of sample not
used and from the environmental side that would prevent the
emission of most hydrocarbons into the air.
When there is an economic justification for saving the
sample, for example, liquids in boiling point and viscometer
analyzers, a sample collection and return systems must be
furnished to collect the sample at atmospheric pressures and
pump it back at high pressure into the process. For gases with
no sample return point, the sample can be pressurized back
into the process, or as is most frequently done, the sample
can be vented into the flare system. However, except in rare
cases, venting is done directly into the atmosphere. When
this is not possible, extreme caution should be taken to control the back-pressure when venting the sample into a flare
or other collection system with varying pressures.
Ambient Considerations
2
Pressure
Gauge
PI
Liquid
Cylinder
Pressurized
Nitrogen
or Other
Inert Gases
Flexible
Diaphragm
Shut-Off
Valve
To Analyzer
Liquid Calibration Sample
To
Analyzer
Shut-Off Valves
Purge
Connection
Purge
Connection
PI
PI
PI
PI
Shut-Off
Valve
Two-Stage
Pressure
Regulator
Two-Stage
Pressure
Regulator
Pressurized
Cylinders
Gas Calibration Sample Manifold
FIG. 8.2f
Calibration provisions for liquid.
ufacturer. Therefore, each system must include provisions for
testing and calibration. As is shown in Figure 8.2f, a means
of isolating the inlet is needed in order to allow a calibration
sample to be introduced manually or automatically.
Also necessary is a suitable calibration manifold with gas
or liquid to furnish a reliable calibration sample to the analyzer.
8.2 Analyzer Sampling: Process Samples
Storage of the test sample may be a consideration, especially
for unstable liquids or gases with low dew points. Treatment
of the containers is very important for trace analysis samples.
In all cases, the calibration provisions must be incorporated
into the systems and a sample provided that is compatible with
the desired stream composition and suitable for analysis by the
analyzer used for the system. It is desirable, but not essential,
that the calibration sample be introduced automatically from
a remote location so that the instrument can be periodically
checked; however, in most systems the introduction of the
sample is done by manual switching at the analyzer.
COMPONENTS OF SAMPLING SYSTEMS
A number of devices are discussed here that are components
of analyzer sampling systems. These devices include gas and
liquid filters, bypass filters, liquid homogenizers, liquid grab
sample collectors, chemical reactor sampling systems, and
solids samplers. Later, under Applications, the various sampling probe packages and single-stream and multistream process sampling systems are described.
The more components there are in a sampling system,
the less reliable it is likely to be. The mean time between
failure and maintenance needs of the overall system will
improve as the number of pumps, ejectors, regulating valves,
coolers, heaters, filters, coalescers, dryers, knockout traps,
manifolds, timers, and other components that comprise the
system are reduced.
1175
The successful design of sample systems requires careful
analysis of the physical and chemical conditions of the
stream, as well as serious consideration of the ambient and
transport conditions, to ensure integrity of the sample. Therefore, care should be taken in evaluating the above considerations with respect to a given stream and in applying the
correct analyzer to provide the desired measurement.
Sample system design is based on experience, and whenever possible, previous experience should be given prime
consideration in the selection and application of components.
A successful sample system normally results in a successful
analyzer system. Therefore, no effort must be spared in
proper sample systems design, which requires a careful selection of the system components. These are discussed below,
starting with filters.
Filter Designs
Most analyzer sampling systems will include a filter with at
least one wire mesh strainer (100 mesh or finer) to remove
larger particles that might cause plugging. Available filter
materials include cellulose, which should only be considered
in applications where it does not absorb components of interest. Sintered metallic filters can remove particles as fine as
2 microns; cellulose filters can remove down to 3 microns;
and ceramic or porous metallic elements can trap particles
of 13 microns or larger. When the solids content is high, two
filters can be installed in parallel, with isolation valves on
each. Motorized self-cleaning filters are also available for
such services.
Selecting the System Components
Sampling systems require certain components, which are
commercially available. One source is the analyzer’s manufacturer, who through the years has developed systems compatible with its analyzer, such as filter coalescers, condensers,
and washing and treating systems.
A second source is the analyzer systems vendor, who had
designed special components, such as kinetic separators, filter probes, and the like, for use with analyzers for applications
in rather hard service.
A third source is the specialty vendor, who has developed
unique sampling components, such as pyrolysis gas sample
conditioners; permeation devices for water removal systems;
high-efficiency, self-cleaning filters; and so on. It is desirable
to check whether specialty items are available before trying
to design new components. Most of the specialty components
have taken years of field testing to develop and modify for
successful application.
When a large number of analyzers are used, the components selected must be of the same type and manufacture for
interchangeability and stocking of spare parts. Documenting
a sample system with complete flow schematics, part identification, and manufacturing of various components is an
essential part of being able to properly start up a system and
maintain it successfully over a long period.
© 2003 by Béla Lipták
Separating Liquids from Gases Glass microfiber filter tubes
efficiently separate suspended liquids from gases. The filter
tubes capture the fine droplets suspended in the gas and cause
the droplets to run together to form large drops within the
depth of the filter tube. The large droplets are then forced by
the gas flow to the downstream surface of the filter tube, from
which the liquid drains by gravity. This process is called
1
coalescing.
The coalesced liquid drains from the tubes at the same
rate as liquid droplets enter the tubes. Therefore, the tubes
have an unlimited life when coalescing liquids from relatively
clean gases. The filters operate at their initial retention efficiency even when wet with liquid. The flow direction is inside
to outside to permit the liquid to drip from the outside of the
filter to the housing drain (Figure 8.2g).
The filter tube grade should be selected for maximum
liquid drainage rate, rather than maximum filtration efficiency
rating, because a liquid drainage rate decreases with increasing filtration efficiency rating. If liquid is carried into the
filter in slugs rather than dispersed as droplets in the gas, a
filter that is properly sized for steady-state conditions can be
flooded and permit liquid carryover. If slugging of liquid is
expected, a filter with a relatively large bowl should be selected
to provide adequate liquid-holding capacity, and provisions
1176
Analytical Instrumentation
Out
Gas Out
Gas Out
In
Inner Foam Sleeve
Filtering Media
Beads
Outer Foam Sleeve
Metal
Perforated Cylinder
Gas In
Sparger
Liquid Drain
FIG. 8.2g
Coalescing gas filter serves to remove liquid entrainment. (Courtesy
of Parker Filtration, formerly Balston Inc.)
Gas Out
Gas In
Glass Wool
or Other
Packing
Ball Float
Trap
Drain
FIG. 8.2h
2
Entrained liquid separator with ball float trap.
should be made to drain the liquid automatically from the
bowl of the housing as it accumulates.
An automatic float drain can be used if the pressure is in
the 10- to 400-psig (28 bars) range. Above 400 psig, which
is the upper limit for commercially available float drains
(Figure 8.2h), the possibilities are 1) a constant bleed drain,
2) a valve with an automatic timed actuator, or 3) an external
reservoir with manual valves. The external reservoir can be
constructed of pipe or tubing with sufficient volume to hold
© 2003 by Béla Lipták
Gas In
Packed Tower
Scrubber
FIG. 8.2i
2
Removal of corrosive gases or of condensable vapors.
all the liquid that is expected to be collected during any period
of unattended operation. To drain liquid while the filter is
operating at pressure or vacuum conditions, the reservoir inlet
valve is closed and the outlet valve is opened.
If the filter is under vacuum, the external reservoir is a
practical method of collecting coalesced liquid for periodic
manual draining. Alternatively, if an external vacuum source
is available, such as an aspirator, the liquid may be drained
continuously from the housing drain port.
Spargers, Packed Towers, and Strippers In some analyzer
applications it is necessary to remove corrosive gases or
condensable vapors from the sample. This removal can be
done by bubbling the sample stream through a liquid solution.
In the case of a sparger (Figure 8.2i), the gas enters below
the scrubbing medium through a sintered disc, which breaks
it into small bubbles. This increases the liquid–gas contact
surface area and thereby provides improved scrubbing efficiency. At the same time, the small bubbles also increase the
tendency for foaming and liquid carryover, and therefore the
sample flow velocities should be kept low.
Packed towers (Figure 8.2i) can also be used as scrubbers. In these designs the gas sample bubbles up through a
nonreactive packing of beads that are wetted by a liquid
solution. Foaming and liquid carryover are problems in
packed towers, as in a sparger.
When the impurities in the process gas stream are both
solids and liquids, such as in particulate matter and mist—
carryover problems in chlorine plants—the fiber mist elim3
inator (Figure 8.2j) should be considered. The liquid particles
form a film on the fiber surface, and the drag of the gas moves
this film and the dissolved solids radially, while gravity
causes them to move downward, resulting in a self-cleaning
action.
8.2 Analyzer Sampling: Process Samples
Clean
Chlorene Gas
High Pressure
Polymerization
Product in
Liquid Phase
Fiber
1177
PI PCV
To Analyzer
Sampling
System
Dirty
Chlorene
Gas
Draining Liquid
Screens
Seal
Pot
Liquid
Overflow
FIG. 8.2l
Flash chamber makes the analysis of unreacted monomers possible.
Clean gas to
Analyzer
or Condenser
Fiberglass
Screen
Spray Nozzle
Steam
Jacket
FIG. 8.2k
When dissolved inorganic solids or polymer-forming compounds
are present, stripping the liquid sample may be the answer.
Dissolved solids or polymer-forming compounds in a
process stream would leave a residue and eventually plug the
liquid sample valve. The logical solution is to force the residue formation to take place in a controlled area, such as in
the fiberglass filter of the spray-stripping chamber shown in
Figure 8.2k.
If polymers represent a substantial portion of the process
stream, the need for filter replacements becomes excessive
and therefore impractical. A better technique is to vaporize
the unreacted monomers through pressure reduction while
keeping the polymers in a molten state through heating. This
technique (Figure 8.2l) not only discharges polymers continuously, but also provides a usable vapor sample.
When it is necessary to clean the windows on the various
photometers operating on gas samples, a warm air purge can
be used, keeping the window compartment isolated from the
sample.
© 2003 by Béla Lipták
LC
Molten
Polymer
FIG. 8.2j
Fiber mist eliminator.
Crystal Clear
High-Boiling
Sample, Filtered
to 0.5 Micron
By Bypass Filter
Upstream
Steam
Jacket
Separating Two Liquid Phases Theoretically, glass microfiber filter tubes can separate suspended droplets of a liquid
that are immiscible in another liquid using the same process
by which they separate droplets of liquid from a gas. The
liquid droplets suspended in the continuous liquid phase are
trapped on the fibers and run together to form large drops,
which are then forced through the filter to the downstream
surface. The large drops separate from the continuous liquid
phase by gravity difference, settling if heavier than the continuous phase and rising if lighter. The coalescing action of
glass microfiber filters is effective with aqueous droplets
suspended in oil or other hydrocarbons, and also with oil-inwater suspensions.
In practice, however, liquid–liquid separations are much
more difficult to achieve than are liquid–gas separations. The
specific gravity difference between two liquids is always less
than that between a liquid and a gas and, therefore, a longer
phase separation time is needed. Either the filter housing must
be oversized or the flow rate greatly reduced to avoid carryover of the coalesced phase.
As a rule of thumb, flow rate for liquid–liquid separation
should be no more than one fifth the flow rate for solid–liquid
separation. Even at low flow rates, if the specific gravity
difference between the two liquids is less than 0.1 U (for
example, if an oil suspended in water has a specific gravity
between 0.9 and 1.1), the separation time for the coalesced
phase may be too long to be practical. In that case, if there
is only a small quantity of suspended liquid, the filter tube
can be used until saturated with suspended liquid and then
changed.
Another practical problem with liquid–liquid separation
is that small quantities of impurities can act as surface-active
agents and interfere with the coalescing action. For this reason, it is not possible to predict accurately the performance
of a liquid–liquid coalescing filter, and each system must be
tested on-site. Testing can be started with 25-micron filter
tubes and with inside-out flows at very low flow rates. If the
1178
Analytical Instrumentation
Automatic
Gas Vent
Filter
High Flowrate
Inlet
Liquid Out
Low
Flowrate
Pressure
Reducing
Valve
Process
Stream
Analyzer
Bypass
Glass Wool
or Other
Packing
High Flowrate
Liquid In
FIG. 8.2n
Slipstream or bypass filtration.
Slipstream
Filter
FIG. 8.2m
2
Entrained gas separator.
Coalescing
Filter
Inlet
High
Flowrate
Pressure
Analyzer
Reducing
Valve
Coalesced Liquid
suspended liquid is lighter than the continuous phase, the
housing should be oriented so that the drain port is up.
Removing Gas Bubbles from Liquids Glass microfiber filter tubes readily remove suspended gas bubbles from liquid,
eliminating the need for deaeration tanks, baffles, or other
separation devices. Flow direction through the filter is outside
to inside, and the separated gas bubbles rise to the top of the
housing and are vented through the drain port. If slipstream
sampling is used, the separated bubbles are swept out of the
housing with the bypassed liquid. Filter tubes rated at 25
microns are a good choice for gas bubble separation.
Columns with glass wool packing can also be used to
remove entrained bubbles (Figure 8.2m). Here the gas bubbles collect and agglomerate on the packing and form large
bubbles. These bubbles break away and rise to the top, where
they are separated from the liquid and vented.
Slipstream and Bypass Filters In order to minimize the
transportation lag, a relatively large slipstream is usually
taken from the process and brought near the analyzer. As the
actual sample flow to the analyzer is small, only a small
portion of this slipstream is sent to the analyzer; the bulk is
returned to the process (Figure 8.2n). This arrangement permits the main volume of the filter to be swept continuously
by the high-flow-rate system, thus minimizing lag time; at
the same time, only the low-flow stream to the analyzer is
filtered, thus maximizing filter life.
A slipstream filter requires that its inlet-to-outlet ports be
located at opposite ends of the filter element to allow the
high flow rate of the bypassed material to sweep the surface
of the filter element. A third port connecting the low-flowrate lines to the analyzer allows filtered samples to be withdrawn from the filter reservoir.
© 2003 by Béla Lipták
Low
Flowrate
Bypass
Drain
High Flowrate
FIG. 8.2o
Slipstream plus coalescing filtration.
If bubble removal from a liquid is a requirement, this
function may be combined with slipstream filtration, since
the recommended flow direction for bubble removal is outside to inside, and the separated bubbles will be swept out
of the housing by the bypass stream. In this case, the liquid
feed should enter at the bottom of the housing and the bypass
liquid should exit at the top of the housing.
A special problem arises in slipstream sampling if the
filter is to coalesce and continuously drain suspended liquid
from a gas stream or to coalesce liquid droplets from a liquid
stream. The coalescing filter requires two outlet ports, one
for the dry gas and one for the liquid drain. To combine
coalescing and slipstream filtration, a filter housing would
need four ports—two for inlet and bypass and two for filtered
gas and coalesced liquid—which is not a practical design.
Therefore, slipstreaming plus coalescing requires two
stages of filtration (Figure 8.2o). The second (coalescing)
stage must be located in the sample line into the analyzer
and should be as small as possible to minimize lag time. If
the quantity of suspended liquid is not large, a miniature
inline disposable filter unit may be used as a trap for the
suspended liquid, to be replaced when saturated.
8.2 Analyzer Sampling: Process Samples
Filtered
Filtering Element
(Can be Porous
Metal with 400
Mesh Size)
Process
Stream
to be
Analyzed
1179
Fluid
Quick
Disconnect
Filtered Fluid
Cleaned
Sample
Particles
Sample to
Analyzer
No Cake Buildup
Dirty
Sample
Sample to
Analyzer
By-Pass
Filter
Restriction
Orifice
or Slide
Gate Valve
FIG. 8.2r
Rotary disc filter.
Nitrogen
or Steam
Blow-Back
Another good filter design is the rotary disc filter (Figure
8.2r). Here the filtered liquid enters through the small pores
in the self-cleaning disc surfaces. The sample liquid is drawn
by the sample pump through the hollow shaft and is transported to the analyzer.
For the removal of small amounts of polymer dust in vapor
samples, there are melt filters with removable, heated metallic
surfaces that melt and collect the polymer dust from the sample.
Liquid analyzer sample streams usually have a high solids content. In addition, the analyzers are often located in
remote areas of the plant and are infrequently serviced.
Therefore, the sample filter system must have a long life
between filter tube changes even when solid loading is high.
The recommendation for this type of application is a twostage filter system, a 75-micron prefilter followed by a 25micron final filter. The filters should be oversized as much
as possible without causing excessive lag time. Plastic filter
housings are usually a good choice.
FIG. 8.2p
Self-cleaning bypass filter and its installation.
Cleaned Liquid
or Vapor Sample
to Analyzer
O-Ring Seal
Body
Cap
Base
Support Pad
Keeping Probes Clean
Liquid or Vapor
Bypass Stream
Containing Solid
Particles
In
Out
Filter
Element
Support
Screen
FIG. 8.2q
Bypass filter with its cleaning action amplified by the swirling of
the tangentially entering sample.
Self-Cleaning and Rotary Disc Filters If the material to be
removed is dust, the self-cleaning bypass filter (Figure 8.2p)
with automatic blow-back constitutes a potential solution. In
some instances, cyclone separators should be considered. In
the latter device (Figure 8.2q), the process stream enters tangentially to provide a swirling action, and the cleaned sample
is taken near the center. Transportation lag can be kept to less
than 1 min, and the unit is applicable to both gas and liquid
samples. This type of centrifuge can also separate streams by
gravity into their aqueous and organic constituents.
© 2003 by Béla Lipták
Figure 8.1p illustrates one of the probe cleaner designs, and
Table 8.1q lists the features and capabilities of a number of
probe cleaners. There is yet another approach to keeping
probes clean, which grew out of the mining industry, where
gypsum buildup on pH and oxidation–reduction potential
(ORP) probes was blinding these sensors.
The Filterate Master (Figure 8.2s) consists of a filter cup
at the tip of a 2-in.-(50-mm)-diameter standpipe, which is
inserted into the process. The pH or other electrode is inside
this cup, which is made out of porous sintered stainless steel.
A pump housed outside the process draws a vacuum inside
the cup, thereby drawing in the filtrate of the process fluid.
The excess filtrate can be continuously returned by the pump,
while the filtered-out solids accumulate on the outside surface
of the filter cup.
Operating experience shows that solids can build up to
over an inch (25 mm) thickness without blocking the filtrate
flow. The timer is set to periodically reverse the filtrate pump.
1180
Analytical Instrumentation
Backflush
Filtrate
Signal
Wire
Vibration
Horn
ContinuousFlow Cell
Overflow
2"
(50 mm)
Orifice Plate
Inlet
Orifice
Outlet
FIG. 8.2t
Ultrasonic homogenizer. (Courtesy of Cole-Parmer Instrument Co.)
Standpipe
Inserted
Into Process
Homogenizers
Sample Conditioning
A frequent problem of sampling systems is plugging. There are
two ways to eliminate it. The older, more traditional approach
is filtering. Unfortunately, as the filters remove materials that
The extracted sample begins conditioning at the takeoff point,
continues through the transport, and finishes conditioning at the
analyzer location prior to entering the analyzer (Figure 8.2u).
10µ Porosity
Sintered
Metal Filter
Cup
Electrode
When the flow direction is reversed, the filtrate flows back
into the filter cup and out through the porous filter. During
back-flush, the buildup is quickly removed and another filtering cycle is initiated automatically.
might otherwise plug the system, they also remove process
constituents and make the sample less representative.
The newer approach is to eliminate the potential for plugging by reducing the size of solid particles (homogenization)
while maintaining the integrity of the sample. Thus, when a
pulverizer is used to replace the filter, the sample becomes
representative.
Homogenizers serve to disperse, disintegrate, and reduce
the particle size of solids and thereby reduce agglomerates
and liquefy the sample. Homogenizers can be mechanical,
using rotor-stator-type disintegrator heads. In this design, the
rotor acts as a centrifugal pump, which is recirculating the
slurry while the shear, impact collision, and cavitation at the
disintegrator head provide homogenization.
In ultrasonic homogenizers, high-frequency mechanical
vibration is introduced into the probe (horn), which creates
pressure waves as it vibrates in front of an orifice (Figure 8.2t).
As the horn moves away, it creates large numbers of microscopic bubbles (cavities), and when it moves forward, these
bubbles implode, producing powerful shearing action and agitation due to cavitation.
Such homogenizers are available with continuous-flow
cells for flow rates up to 4 g/h (16 l/h) and can homogenize
liquids to less than 0.1-micron particles sizes. The flow cell
is made of stainless steel and can operate at sample pressures
of up to 100 psig (7 bars).
Filtrate
in Cup
1"
(25 mm)
FIG. 8.2s
Electrode is protected from material buildup by back-flushed porous
filter cup. (Courtesy of TBI-Bailey Controls.)
© 2003 by Béla Lipták
8.2 Analyzer Sampling: Process Samples
Analyzer Rack or House
Field
Sample Conditioning
To
Bypass
HDR
Shut-Off
Valve
250’
Lab
Sample
Open-Air
Cooler
Shut-Off
Valve
Steam In
Y-Strainer
T = 1510°F
P = 460 PSIG
0−100
PI
Out
30
PSIG
In
Shut-Off
Valve
Kinetic
Separator
Liquid
Seal Trap
Pressure Reducing
Station With Gauge
and Relief Valve
Set at
Light Steam ~100#
Trace for Cold
Weather Protection
Steam Out
1181
Pressure
Gauge
0-100
Dew Point
at
45 PSIG
18°F
FI
PI
FI
3-Way
Solenoid
F
Fine
Filter
Shut-Off
Valve
1/4 T
316
Analyzer
Flow Indicator
With Adjusting
Valve Set at
~7100 cc/Min
Flow Indicator
With Adjusting
Valve Set at
400 cc/Min
1/4 T
316
Zero
2-Way Solenoid
Span
Conditioning System
Cooler and Field Pressure
Reduction Station
FIG. 8.2u
Typical sample conditioning system with remote preconditioning unit.
Analyzer
Cooling
In
Sample
Out
Water
Out
Pressure
Gauge
In
Temperature
Gauge
PI
Self
Cleaning
Filter
Control
Valve
FI
Sample
Cooler
Shut-Off
Valves
FI
Flow
Indicator
Check
Valve
To
Drain
T1
Flow
Indicator
With
Needle
Valve
FI
Flow
Indicator
Coalescer
Pressure
Regulator
Flow
Indicator
with
Needle
Valve
Calibration
Sample
Shut-Off
Valves
Lab Sample
Take Off
Pressure
Relief Valve
Pressure
Gauge
FI
Check
Valve
PI
Shut-Off
Valve
FIG. 8.2v
Typical liquid product sample system for refinery applications.
All samples require some form of conditioning to make them
suitable for the analyzer and to assure reliable on-stream operation. The conditioning is done at the appropriate location in
the sample system loop in order to maintain the integrity of the
sample (Figure 8.2v).
Sample washing is usually limited to dirty, particle-laden
streams whose composition will not be affected by the solubility
© 2003 by Béla Lipták
of the components in the liquid used to wash the sample. The
conditions of flow, temperatures, and pressures must be controlled to maintain a relatively constant predetermined relationship of the composition. When washing, care must be
taken to keep the sample in the vapor phase by providing
heated transport lines or by making provisions for final moisture removal as the analyzer may require.
1182
Analytical Instrumentation
Temperature
Adjusting Screw
1/4 NPT
Vapor Outlet
“O” Ring Seal
AC - IN
Temperature
Sensor
Heater
1/8 NPT
Liquid
Sample Inlet
FIG. 8.2w
Vaporizing regulator assembly (electrically heated).
Vaporizing Samples Vaporizing is frequently necessary for
equilibrium liquids or when the analyzer requires it. The vaporization is usually done by a vaporizer regulator (Figure 8.2w).
In such a regulator the sample is vaporized simply by pressure
reduction across a capillary or, more often, by using a heater
as well. Care must be exercised to avoid partial vaporization
and fractionation by selecting a suitably heated vaporizing
regulator to accommodate the sample.
Entrainment Removal The removal of entrainment from
liquid or gas samples normally starts with locating a sample
tap and providing a sample probe at the takeoff point. Filtration is normally used for both gases and liquids, because it
removes both liquid and particulate entrainment from gases
and particulate matter from liquids.
For gases requiring further conditioning of heavy loading, cyclone filters can be used if the sample has adequate
velocity. For liquids, coalescers are frequently used to remove
both undesirable gases and liquids by gravity. The removal
of free water from a hydrocarbon stream is usually accomplished by passing the liquid through a hydrophilic element,
causing the water droplets to accumulate on the element. The
hydrocarbon stream is then passed through a hydrophobic
element that rejects the water, removing it from the bottom
with a hydrocarbon bypass stream.
Another method of removing moisture from a stream uses
a selective permeation device with a drying medium that creates differential pressures to drive the water through the permeable materials, thus removing it from the flowing stream
(Figure 8.2x).
Selection of Component Materials Adsorption of the sample components of interest on the walls or surface with which
the sample comes in contact will affect the analysis, especially
© 2003 by Béla Lipták
High Pressure
Wet Feed
Inlet
Permeable
Tube
Low Pressure Wet
Purge Gas Outlet
Shell
High Pressure
Dry Product
Outlet
Low Pressure
Dry Purge Gas
Inlet
FIG. 8.2x
Permeable tube sample dryer.
for measurements in the parts per million (ppm) range. Therefore, proper selection of materials and conditioning is essential for establishing an equilibrium that will make analysis
reliable.
Specifically, water vapor samples reach equilibrium more
rapidly in stainless steel lines than in copper or plastic tubing.
Diffusion is another consideration. Samples system design
should assure that gases do not permeate the walls of the
sample system. This is especially important in high-pressure
systems and ppm analysis. Another problem with diffusion
is leakage, which can change the sample composition because
gas molecules will flow in both directions of the leak and
can significantly affect ppm analysis. This is best illustrated
by the fact that in an oxygen ppm analyzer system, the slightest leak will create a full-scale reading on the instrument even
though the leak is from a high-pressure sample to atmospheric pressure.
Some sample streams may contain corrosive gases or
water vapor that influence the accuracy of the measurements
or potentially damage the analyzer. Removal of such undesirable matter or components can be accomplished by passing
8.2 Analyzer Sampling: Process Samples
the sample through a packed bed of soil chemicals or desiccant. Further, a liquid treating agent may be applied with
provisions that gas streams be broken up into small bubbles
to assure proper contact between the liquid and gas phases.
Care should be taken in such systems to avoid alteration of
the sample and to condition the sample to a desired and
reproducible form for analysis.
Capillary
Tubing
Filter
1183
High Pressure
Filter Blowback
Vapor
Sample
Pulled by
Ejector
Process Vessel
Guard
APPLICATIONS
Here a brief discussion will follow of the various sample
probe and sampling system package designs that are used for
gas, liquid, and solids sampling. The discussion will also
cover the more special cases of stack, reactor, condensate,
trace component, and multiple-stream sampling systems.
Gas Sampling Probes
When taking gas samples, the goal is to obtain representative
samples with minimum time lag, using short, small-volume
sampling lines. Whenever possible, it is preferred to draw
dry and clean samples in order to minimize the need for
filters, dryers, knockout traps, or steam tracing.
The sample tap should be located on the side of the
pipeline to minimize liquid or dirt entrainment, and the sample should be taken from the center of the pipe. If it is
necessary to periodically remove the probe for cleaning or
to perform a sampling traverse, it is desirable that the probe
be inserted through a packing gland and block valve, as
shown in Figure 8.2y.
The volume of the sampling system should be kept to a
minimum, while the velocities through the sample lines should
be high to protect against the settling of entrained liquids or
particulates that can cause plugging. Sample line tubing can
be small as 1/8 in. (3 mm) in diameter, and sample flow
velocities should be between 5 and 10 ft/sec (1.5 to 3 m/sec).
When the sample is taken from a combustion zone or other
dirty processes, a filter is usually provided at the tip of the
sample probe, and a high-pressure filter blow-back line is
provided for periodic filter cleaning (Figure 8.2z).
Small Bead Around End of Probe to Prevent
Pulling the Probe Completely Through the
Packing Gland
Probe
Packing Gland
Gate Valve
FIG. 8.2y
Gas sampling probe should be inserted from the side of the pipe to
2
near the center of the pipe.
© 2003 by Béla Lipták
FIG. 8.2z
Gas sampling probes used on combustion processes are usually
provided with probe-tip filters and blow-back lines for filter cleaning.
Solid Filter with High
Temperature Capability
Line may be Heated
to Prevent
Analyzer
Condensation
Temperature
80° F or Lower
Sample
Pump
Filter Housing with Glass
Bowl and Coalescing
Filter Tube, Flow Inside to Outside
Valve Normally Closed
Stack Gas Temperature
to 900° F
Condensate Reservoir
(Optional)
Valve Normally Open
FIG. 8.2aa
Stack gas sampling.
Stack Gas Sampling
When sampling hot, wet stack gas, a filter capable of withstanding the gas temperature should be installed in the stack
at the tip of the sample line to prevent solids from entering
the gas sample line. After the sample is cooled, a coalescing
filter is used to remove suspended liquids before the sample
goes to the analyzer (Figure 8.2aa).
The sample flow direction is from inside to outside. Filter
housings with Pyrex glass bowls are often used in this application to permit visual check of the liquid level in the filter
housing. Since there is often a considerable amount of liquid
present in the sample, steps must be taken to drain the housing
to ensure that liquid does not build up and carry downstream
to the analyzer.
The liquid coalescing filter should be located as close to
the analyzer as possible to minimize the chance of condensation between the filter and the analyzer. Additional precautions
that can be taken to avoid downstream condensation are to
cool the sample below ambient temperature upstream of the
coalescing filter and to heat the line gently between the filter
and the analyzer.
Automatic Stack Sampling In these sampling packages, a
microprocessor directs the automatic sampling method,
which can be selected to follow U.S. Environmental Protection
1184
Analytical Instrumentation
Agency (EPA) Methods 1 to 6, 8, 17, and 23 or international
methods specified by VDI, BS, or ISO. The microprocessor
stores all measurements, reviews and diagnoses all inputs,
controls the required parameters, calculates isokinetic conditions, and either reports the results in a printed form or
transfers them to a floppy disc.
Besides the controller, such a package usually consists
of a probe, a filter (hot) box, a cold box, a flexible sample
line, glassware, a node box, and a monorail system. The probe
is usually 3, 5, 7, or 10 ft (0.9, 1.5, 2.1, or 3 m) and made of
stainless steel with a glass liner. Most probes are jacket heated
and are provided with both a liner thermocouple and a stack
temperature thermocouple.
The measured variables include the temperatures of the
stack, probe liner, filter box, condenser outlet, and the dry
gas meter. The pressures are detected by an absolute and a
differential pressure transducer and are used to measure the
pressure of the stack gas, the barometric pressure, and the
velocity pressure of the stack gas. The normal capacity of
the vacuum pump that draws the sample is 0.75 CFM (21
l/min), and the dry gas meter has an operating range of 0.1
to 1.5 CFM (2.8 to 42 l/min).
The node box provides the interface between the filter
box and the cold box by measuring the temperatures in both.
It measures and stores the temperature, pressure, and velocity
in the stack. The monorail eliminates the need for bulky
supports.
Spring Cap Nut
Outer Spring Cap
Outer Spring
Diaphragm
Control Side
Inner Spring
Sample Side
Stem Seal
Sample Outlet
Valve Stem
Adjustable
Fitting
Sample Tube
Sample Inlet
Valve
FIG. 8.2cc
Adjustable automatic load sampler.
Automatic Liquid Samplers
Automatic liquid samplers collect intermittent samples from
pressurized pipelines and deposit them in sample containers.
The sample can be collected on a time-proportional or on a
flow-proportional basis. Figure 8.2bb illustrates a sampler that
withdraws a predetermined volume of sample every time the
actuator piston is stroked. In the time-proportional mode, this
sampling frequency is constant, while in the flow-proportional
mode, this unit would vary the sampling frequency as a function of flow.
In some automatic liquid samplers, the sampling frequency is adjusted by pneumatic pulse relays or by electronic
controls (Figure 8.2cc). Pulse duration is usually adjustable
from 0.25 sec to 1 min, while pulse frequency can be adjusted
from a few seconds to up to an hour. The intermittent in-line
FIG. 8.2bb
Intermittent collection of samples. (Courtesy of Bristol Equipment
Co.)
© 2003 by Béla Lipták
sampler illustrated in Figure 8.2cc can take samples at pressure up to 1500 psig (105 bars).
Sampling of High-Pressure Condensate In high-pressure
boiler systems, measurements of condensate conductivity,
specific ion concentration, and feed-water additive concentrations are often required. In a continuous sampling system,
the high-pressure steam or condensate is cooled below 100°F
(38°C) and then reduced to near atmosphere pressure for
metering to the analyzers. Filtration is required upstream of
the pressure-reducing valves to prevent pitting to the valve
seats by suspended particles and to eliminate variations in
flow rate to the analyzers.
A stainless steel filter housing with the appropriate pressure rating and 25-micron filter tube is recommended. Since
the analyzer system is often located some distance from the
sampling point, slipstream filtration is usually required.
Figure 8.2dd shows a sampling system in operation at a steam
generation facility.
Chemical Reactor Samplers When samples are taken from
chemical reactors by opening manholes, the operators can be
exposed to dangerous fumes while the product can be
degraded or cause explosions. To eliminate these problems,
reactor sampling systems have been designed that allow the
safe taking of samples.
8.2 Analyzer Sampling: Process Samples
1185
Condensate
Pressure
Regulators
Stainless Steel Filter
Housing, 800 psig (56 barg)
Rating, With 25 Micron
Filter Tube
Samples to
Conductivity Cells
Slipstream
FIG. 8.2dd
High-pressure stream sampling.
Diaphragm Pump of Teflon-Frequently
Used for its Ideal Flow Rate, Corrosion
Resistance and Minimal Damage to Crystals
pH Probe
Sample Out
Sample Return
Liquid/Gas Addition
Ball Valve of Glass and Teflon-Also Serves
As Sight Glass for Liquid Monitoring
Sampler
FIG. 8.2ee
4
Continuous or intermittent sampling of chemical reactor.
The design illustrated in Figure 8.2ee requires only one
nozzle (3 in. or 75 mm) and utilizes a Teflon-coated sampler
assembly that can be used up to 150 psig (10.5 bars) and
350°F (177°C). For continuous sampling applications, such
as for closed-loop pH control, a Teflon diaphragm pump is
used to continuously return the analyzed sample.
Duckbill Samplers This sampler should be considered
when liquid or sludge samples are to be collected at remote
locations, from below the level in tanks, sewers, channels,
5
sumps, lakes, or rivers. As shown in Figure 8.2ff, this device
has no moving mechanical components, only a rubber (EPT,
®
Buna-N, or Viton) bucket-shaped Duckbill , which is inside
a housing made of polyvinyl chloride (PVC), aluminum, or
stainless steel. This rubber insert closes around fibers or
particulate matter without jamming. It is operated by compressed air.
The sample enters by gravity through the bottom connected Duckbill inside the sampling chamber, which is
© 2003 by Béla Lipták
installed below the process liquid (or sludge) level and traps
some air at the top of the chamber as it fills with the process
fluid. When a sample is required, compressed air is introduced, which closes the Duckbill inlet and discharges the
sample from the bottom of the chamber. When a new sample
is to be drawn into the sample chamber, the compressed air
is vented and a new fill cycle is initiated.
An automatic controller is provided on which the user
can adjust the frequency at which samples are to be taken
into a composite sample collection bottle.
Solids Sampling
When solids in bins or silos need to be sampled, one of the
various solids feeders (described in Section 2.23) can be considered. When solids are to be sampled while flowing by
gravity or while pneumatically conveyed under the pressure,
the choice of sampling devices becomes more limited. The
screw-type solids sampler can be used on these applications
1186
Analytical Instrumentation
Process
Stream
Motor
Sample
FIG. 8.2gg
Archimedian screw solid sampler.
Analyzer
Sample Return
Actuator #1
Actuator #2
FIG. 8.2hh
The tray-type solids sampler.
Sampling Difficult Processes
FIG. 8.2ff
Duckbill sampler. (Courtesy of Markland Specialty Engineering
Ltd., www.sludgecontrols.com)
(Figure 8.2ff), but it is limited in the amount of pressure or
vacuum it can seal against, and it does not provide an intermixed, representative sample. Screw samplers also introduce
large transportation lags and cannot easily return the sample
into the process.
An improved sampler design is described in Figure 8.2gg.
This sampler can be sealed against small pressures or vacuums. It is operated by two actuators. During the sample collection phase, both actuators are extended into the process
line. When a representative sample has been collected, it is
withdrawn and simultaneously intermixed by pulling back
actuator 1. As the analyzer is right above the withdrawn
sample, the composition can be measured as soon as the
sample has been taken. After analysis, actuator 1 is once again
extended to return the old sample into the process. When it
is time to take a new sample, actuator 2 is also extended and
another sample is collected.
© 2003 by Béla Lipták
An analyzer system enhances process control by providing
specific measurement of physical or compositional data of a
process or ambient condition. With the advent of computer
technology, the need for such analysis has increased substantially to provide on-line data that the computers use to optimize process control. Some of these advances have also
improved the handling of such difficult tasks as sampling for
trace component analysis or sampling multiple streams.
Trace Analysis Sampling Trace analysis sampling systems
necessitate more stringent requirements than normal analyzer
sample systems because of contamination, adsorption, and
desorption (Figure 8.2hh). Care should be taken in the selection of construction materials and proper applications of
design criteria to avoid alteration of the sample. The following is a list of recommended practices for such systems:
1. Stainless steel seamless tubing is the preferred material because it provides inertness, smooth surfaces, and
low porosity.
2. All components should be thoroughly cleaned of oil,
gas, or other contaminating materials.
3. Dense fluorocarbons or other soft inert materials may
be used as diaphragms as required.
8.2 Analyzer Sampling: Process Samples
4. Tubing sizes are critical, especially where low flows are
used, because they limit the amount of increase in the
adsorption–desorption phenomenon. A rule of thumb
is to use the smallest possible tubing to achieve maximum flow to accommodate the sample loop design.
5. Packless shutoff valves with a diaphragm or bellows
seals should be used; however, due to their high cost,
serious consideration must be given to this area, and
conventional valves are suitable for most applications.
6. Filtration of the sample in ppm analysis can create
significant problems unless the filter is totally inert.
Therefore, stainless steel filters using a high flow rate
or dense fluorocarbon inert materials are recommended
for such applications.
7. Conditioning of the lines and system to accommodate
the ambient temperature requirements for preventing
condensation of components of interest in the sample
must be considered; if necessary, heat tracing of the
lines must be furnished.
8. When it is necessary to provide an aid for transporting
the sample from its sample point to the sample system
through the analyzer, as is frequently done in ambient
monitoring systems, a sample pump, ejector, or aspirator
is necessary. In such cases, the pump must be a diaphragm. If practical, an ejector to pressurize the sample
or an aspirator to aspirate it through the measuring device
can be used. Both are more desirable than a sample pump.
analyzer. However, because this system is more complex than
single-stream sample conditioning, the following considerations should be reviewed to determine if multistreaming is
feasible:
1. The potential problem of cross-contamination among
multiple streams
2. The importance of each analysis and frequency of
analysis
3. The loss of information from more than one analysis
in case of analyzer failure
4. The cost of an additional analyzer vs. the cost of
multistreaming
5. Maintenance requirements
Multistream Switching Multistream switching is usually
used when it is practical to analyze several streams using one
After reviewing the above, one can decide if multistreaming is feasible and whether it should be manual or automatic.
Whether manual or automatic, multistreaming requires goodquality valves for stream switching.
A typical multistream sampling system is shown in
Figure 8.2ii. A common and important requirement in all
such systems is that a continuous bypass be provided for each
sampling point to avoid dead-end sample lines. The sample
system should be laid out in such a manner that contamination between streams is avoided. This is best accomplished
by arranging the solenoid valve in a double-block doublebleed configuration, which is rather expensive.
More often, a three-way solenoid valve is used for each
stream, with the venting port always at low pressures to create
a relief in case of a leak. To prevent contamination, dead
Instrument
Air
Supply
PI
Analyzer House
Field
F
TI
Liquid
Sample
200 PSIG
PI
Steam
PI
F
70°F
(1378 kPa, 21°C)
Drain
TC
Steam
FI
Drain
Heated Enclosure
Mounted at
Sample Tap
Saturated
Vapor
Sample
4 PSIG
120°F
(27.6 kpa,
49°C)
Steam
Traced
To
Atmosphere
Vent
FI
Return to Header
A
FI
ATM
Bleed
PI
F
Calibration
Sample
Drain
Steam Heated Enclosure
Bubbler for
Leak Detection
FIG. 8.2ii
Two-stream sampling system for trace analysis with a double-block, double-bleed configuration.
© 2003 by Béla Lipták
1187
1188
Analytical Instrumentation
To Vent or Common Sample Return Point
PI
FI
Stream 1
FI
PI
FI
Stream 2
Pressure
Gauge
FI
Analyzer
Bleed
Orifice
Relief
Valve
Shut-Off
Valves
PI
Stream 3
Flow
Indicator
with
Adjusting
Valve
Calibration Gas
FI
Pressure
Regulator
Flow
Indicator
with
Adjusting
Valve
3-Way
Solenoid
FIG. 8.2jj
Typical multistream automatic sampling system.
volume of the sample system should be considered, as well
as equalization of the pressures upstream of the three-way
valves. The problem is more severe in ppm sampling systems
because of the adsorption–desorption effects, and careful
consideration should be given to the design criteria described
above.
References
1. Baker, G., Erk, G., Hudelson, J., Manka, D., Siebert, K., and Wachel,
L., Automated Stream Analysis for Process Control, Vol. 2, Orlando, FL:
Academic Press, 1984, pp. 97–101, 105–106.
2. American Petroleum Institute, API RP 550, Part II, “Process Stream
Analyzers.”
3. Nichols, J. H. and Brink, J. A., Jr., “Use of Fiber Mist Eliminators in
Chlorine Plants,” Electrochemical Technology, July–August 1964.
4. “System Samples Materials without Opening Manway,” Chemical Processing, mid-November 1986.
5. www.sludgecontrols.com.
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