Most industrial water treatment systems are dynamic. They constantly undergo changes
because of seasonal variations in water chemistry, varying plant operating conditions,
new environmental laws, and other factors. Because of this, proper monitoring
is essential to ensure that the water treatment program applied to a boiler, cooling,
wastewater or other industrial water system is satisfactorily controlled so that
the desired results are achieved.
Some of the value added benefits obtained through proper monitoring of a water
treatment program include:
- reduced risks associated, with chemical underfeed or overfeed
- continuing compliance with environmental regulations
- improved quality of plant operation
- increased water and energy savings
- improved plant productivity
Industrial water treatment systems may be monitored by manual methods
or by continuous systems employing automatic instrumentation.
Manual monitoring typically involves plant operators or technicians conducting
chemical tests and comparing the results to specified chemical control limits.
The testing frequency can vary from once per day to once per hour, depending
on the plant resources available. The tests run can include pH, conductivity,
suspended solids, alkalinity, hardness, and others. Using the test results,
the plant operator manually adjusts a chemical feed pump or blowdown valve,
making an estimate of the degree of change necessary.
Manual monitoring is satisfactory for noncritical water systems or systems
in which water and plant operating conditions change slowly. Many systems operate
with manual monitoring, using the many tests contained in Chapters 39-71 Typical
applications include the following:
- closed cooling water treatment systems
- open cooling water systems with consistent makeup water characteristics
and steady load conditions
- low to medium pressure boilers
CONTINUOUS, ON LINE
Because of the dynamic nature of many water treatment systems and the worldwide
need for improved reliability and quality, a higher degree of precision is required
in the monitoring and control of water treatment programs than that obtained
through manual monitoring. To achieve the degree of precision needed, continuous
on-line monitoring with automatic instrumentation is required.
Because of the many technological develop" meets in electronics and microprocessor
technology over the last decade, there is a wide range of instrumentation available
to monitor water treatment systems. The following sections address the systems
available to monitor conductivity. pH, corrosion rate, turbidity, dissolved
oxygen. sodium, fouling, biological activity, and halogens.
Dissolved solids provide a fundamental measure of water quality. In water, the
specific conductance, or ability to carry an electric current, is directly related
to the quantity and mobility of the dissolved solids. As such, specific conductance
is widely used to monitor boiler makeup and condensate purity and to control
boiler and cooling system blowdown.
Recent technological advances have improved the reliability and sophistication
of conductivity controllers. Microprocessor based instruments provide extremely
reliable and accurate measurement of temperature compensated conductivity combined
with sophisticated control modes. The Betz Accutrak® (Figure 36-1) is an
example of a conductivity controller. It offers programmable control modes,
such as proportional; on/off; and proportional, integral, derivative (PID) control.
It also includes self-diagnostics and a display of sensor or electronic fault
conditions. Analog and standard communication protocol signals (RS-485) are
provided for computer interface, facilitating data acquisition and communication.
In most conductivity measurement systems, two metallic electrodes are immersed
in a liquid to complete an electrical circuit. These electrode-type sensors
work well in relatively clean water, but lose accuracy if coated by dirt or
fouling contaminants, which interfere with the flow of current. To reduce interference
from small amounts of contamination in the water, the probe and mounting may
be designed to increase the velocity of the flowing sample, minimizing dirt
buildup on the electrodes. Electrode-type probes usually show good accuracy
in the specific conductance range of 50-8,000 µmho (microsiemens).
Special electrode-type probes are used for contaminant detection in high-purity
water, such as steam condensate, demineralized water, and clean water rinses
in metal finishing. Measurement over a range of 1-2,000 µmho is practical
with these probes.
For heavy fouling conditions (found in some industrial cooling towers and boilers,
waste treatment plants, and some processes, such as metal treatment baths) an
electrodeless (toroidal) conductivity probe (Figure 36-1) must be used.
The toroidal probe uses inductance to sense conductivity changes in a process
pH measurement reveals the hydrogen ion concentration in water. It is used
to determine both the deposition and corrosion tendency of a water. The most
widely used type of pH measurement is the electrode method. The
assembly in Figure 36-2 shows the necessary elements that make up a typical
pH sensor: a glass pH electrode, a reference cell, a temperature compensation
element, a preamplifier, and a sensor body. Because of the difficulty of maintaining
good pH control, manual systems are being replaced by continuous monitoring
and automatic control of pH in many water treatment applications. In cooling
tower systems, pH has been particularly difficult to control manually because
the response curve of pH to acid addition is not linear. Figure
36-3 shows the variation of pH in a cooling tower system with manually adjusted
feed of sulfuric acid. Results of random plant tests were plotted to show
the number of occurrences of each test value.
pH controllers use much of the same technology as the conductivity controllers
discussed above. The Betz Accutrak
pH controller (Figure 36-4) uses microprocessor-based electronics, programmable
alarm and control modes (such as time proportional control, PID control, self-diagnostics,
and a display of electronic or sensor fault conditions), and analog and RS-485
signals for computer interface for data acquisition and communication. pH sensor
technology has advanced significantly to overcome many of the problems encountered
in the past, such as rapid fouling and chemical attack of the pH electrodes,
contamination and rapid depletion of reference cells and electrolytes, and electrical
noise and environmental interference with the low-level pH signal.
Several variations of pH sensor assemblies are available for different applications.
For relatively clean water, where extensive fouling is not a problem (such as
in most cooling towers), a combination pH sensor assembly is normally used.
The single, molded body sensor
assembly shown in Figure 36-2 combines all of the elements.
Glass electrode deterioration, reference junction plugging, and electrolyte
depletion (which occurs in all pH sensor applications) proceed at approximately
the same rate. This progression is slow enough in clean water to provide an
acceptable economic life. When the combination sensor is worn out, it is discarded.
A rugged, modular pH assembly
(Figure 36-5) is used in processes such as metal treatment baths and waste systems,
where fouling or chemical attack of glass electrodes, reference junctions, and
other elements is a problem. The modular assembly allows periodic maintenance
and replacement of individual components.
Corrosion rate instruments are used in many different applications to provide
instantaneous corrosion rate values in mils per year. A
typical package consists of an analyzer and probes, as shown in Figure 36-6.
Corrosion rate instruments are used for critical cooling systems, steam condensate
systems, mill water supply streams, and other applications.
Analyzers are available for portable use or continuous operation. Portable
units are generally used when several probes are installed in remote locations.
The operator connects the analyzer to the probe and takes a reading, and then
moves to the next probe. Continuous analyzers are used when a probe is located
in a critical area that warrants continuous evaluation. They include recorder
and control outputs that can be interfaced with other components such as process
controllers and pumps.
Analyzers usually have internal meters and a means of checking calibrations
against a standard.
The probe houses the electrodes and exposes them to the test stream. Probes
are manufactured in many different configurations. Common configurations include
two or three electrodes. Mild steel, stainless steel, and polyvinyl chloride
(PVC) are common probe materials. Probes are available as standard and retractable
assemblies and are usually provided with standard pipe connections.
Electrodes are made from many different metals, such as stainless steel, mild
steel, Admiralty brass, and 90-10 copper-nickel. Electrodes fasten onto the
probe, and the probe and electrode assembly are inserted into the test stream.
Turbidity is caused by suspended matter and can be defined as a lack of clarity
in water. Turbidity measuring instruments are used to monitor and control clarifiers
and lime softeners and to detect corrosion products in steam condensate.
There are presently two methods used for continuous measurement of turbidity-the
nephelometric method and surface scatter technique.
Nephelometric method. In the nephelometric method, the sample
flows through a cell. Near the midpoint of the cell, a light source sends a
beam of light into the moving fluid. Light receivers are located at various
positions in the cell. The receivers measure the amount of light scattered 90°
from the incident light. The amount of light scattered increases as the turbidity
in the sample increases. The instrument measures the scattered light and develops
a signal that is related to nephelometer turbidity units (NTU).
Surface Scatter Technique. The surface scatter technique is
similar to the nephelometric method in theory of operation. However, in this
method a light source emits a beam toward the surface of a constant level reservoir.
The reflected and refracted portions of the beam are discarded and the scattered
portion is sensed by a photocell. The amount scattered is in direct relationship
to the turbidity of the sample. Because the light transmitter and photocell
are not in contact with the sample, this method eliminates fouling.
Turbidity measuring instruments usually include an analog or digital readout
and a signal output that can be interfaced with a computer or chart recorder.
An example of a surface scatter
unit is shown in Figure 36-7, and the
technique is illustrated in Figure 36-8.
Dissolved Oxygen Instrumentation
The ability to measure dissolved oxygen is very important, especially in boiler
systems, where oxygen corrosion can be very damaging.
A typical dissolved oxygen measuring
instrument consists of a sensor, a sensor cell, and an analyzer, as shown in
Figure 36-9. The sensor measures the dissolved oxygen concentration and
transmits a signal, proportional to the oxygen concentration, to the analyzer.
The analyzer provides a readout in parts per billion or parts per million and
an output that can be connected to a recorder or data logging device.
Dissolved oxygen is commonly measured by a membrane-isolated electrochemical
cell. This cell contains a cathode, an anode, and an electrolyte solution. A
gas-permeable membrane admits the dissolved oxygen from the sample to the electrodes.
There, an electrochemical reaction generates an electric current with a magnitude
proportional to the dissolved oxygen concentration. The reaction can be summarized
by the following equation:
For dissolved oxygen analyzer calibration, the sensor is exposed to humid air.
The concentration of dissolved oxygen in the moisture is between 8 and 10 ppm,
depending on the ambient pressure and temperature. The analyzer reading is adjusted
to the correct value for the pressure and temperature. Some analyzers have an
automatic calibration feature that measures the temperature and pressure at
the push of a button.
Sodium instrumentation has become very important as a means of determining
steam purity. To determine the total dissolved solids concentration of the steam,
the sodium level in a cooled steam sample is compared to the ratio of total
solids to sodium in the boiler water.
The most common technique used to measure sodium is the specific ion electrode.
The sodium specific ion electrode responds logarithmically to changes in sodium
concentration. The only other factors affecting the readings are temperature
and pH. Temperature is measured by an internal thermistor. A reference electrode
provides the primary potential signal required for the measurement. Before the
sample contacts the electrodes, the sample is circulated through a diffusion
tube that is immersed in ammonia; this procedure eliminates hydrogen ion interference.
For calibration of the specific ion analyzer, both electrodes are immersed
in a known standard solution. The electrodes are also immersed in another standard
with a tenfold higher concentration of sodium ions for determination of the
electrode slope. Modern microprocessor technology has provided advanced calibration
techniques that verify electrode stability during calibration.
A typical sodium instrument
is shown in Figure 36-10.
There are several specialty systems designed to monitor the rate of fouling
and corrosion in industrial equipment, including those discussed in the following
MonitAll®. The Betz MonitAll®
(Figure 36-11) is a portable assembly used primarily to measure the fouling
and corrosion potential of cooling water streams on heated tube surfaces.
The MonitAll contains a clear flow-through assembly. Sample water flows in
at the bottom and out of the top of a tube. A heat probe is inserted into the
flow assembly along the axis of the tube. The heat probe generates an adjustable
heat flux across a tubular metal test section. Fouling or corrosion can be accelerated
if the heat flux is raised above design levels.
The test section is removable and interchangeable for other metals, which include
mild steel, Admiralty brass, 304 stainless steel, 316 stainless steel, and 90-10
The heat probe includes two temperature sensors that measure probe surface
temperature and bulk water temperature. The temperatures are monitored by a
meter with a light-emitting diode (LED) display. The temperature meter has an
analog output for a recorder or data logging device.
As the test section fouls, less heat is dissipated into the bulk water and
the tube skin temperature decreases. The result is an increase in the temperature
difference (DT) which can be related to fouling factor Rf by the following equation:
|Heat Flux (Btu/hr/ft2)
The MonitAll is equipped with flow control valves to maintain a constant flow
rate and insertion tubes to increase velocity in the clear flow cell.
Model Condenser. The
Betz Model Condenser, Figure 36-12, is a test device used primarily to simulate
surface condenser fouling and corrosion. It consists of a horizontal, cylindrical
stainless steel shell with one, two, or four removable tubes. The tubes run
the length of the shell and terminate at the tube sheets. An electric heater
is located in the bottom of the shell to generate a constant heat flux. Temperature
sensors are located in the shell and tube discharges to monitor temperature
The principle of operation is very similar to that of a standard surface condenser.
The test water flows through the tube(s) and discharges to drain. The shell
is filled with distilled water, which covers the electric heater but is below
the tubes. A vacuum of 27 inches of mercury is applied to the shell to simulate
condenser conditions. Heat is applied to the distilled water with the electric
heater. The distilled water boils and the vapor rises to the tube surfaces.
Cool water flowing through the tubes condenses steam on the tube surfaces. The
condensation falls to the reservoir of distilled water and the cycle repeats.
Condenser operating conditions, such as heat flux and tubeside velocity, are
simulated by the model condenser. Shell temperature and tube discharge temperature
are monitored continuously. As foulant accumulates on the internal tube surfaces,
less heat is transferred through the tube wall. As a result, the shell temperature
increases and the tube discharge temperature decreases. At a constant flow rate,
the increase in the temperature difference can be related to a fouling factor
by the same equation given for the MonitAll. Typically, tubes are removed and
sent to a laboratory for further analysis.
Test Heat Exchanger.
A test heat exchanger (Figure 36-13) is used to monitor the fouling and corrosion
tendencies of a particular cooling water stream. Cooling water passes through
two remov-able tubes contained in a cylindrical shell. The tubes, which are
available in many different materials, can be arranged for two single-pass or
one two-pass operation. Steam or hot condensate flows into the shell and heats
the water flowing through the tubes. The condensate exits the shell through
a flowmeter that is used to monitor heat input.
COSMOS™ Cooling System Monitoring Station. Monitoring and analysis of
key operating parameters are important tools in the development of an effective
cooling water treatment program. The Betz Cooling System Monitoring Station
(COSMOS ) is a versatile tool that can be used for this purpose. It monitors
pH, conductivity, and corrosion rates. In addition, a MonitAll® hot tester
can be included for evaluation of heat flux, water velocity, and fouling factors.
A variety of metals can be evaluated.
The monitoring station consists of two units: a data acquisition cabinet and
a piping and instrumentation cabinet. Figure
36-14 shows the data acquisition cabinet, with the panel door open, connected
to the piping and instrumentation cabinet.
The piping and instrumentation cabinet (wet side) includes the MonitAll hot
tester, a flow sensor, two corrosion probes, a conductivity sensor, a pH sensor,
coupon holders, stainless steel piping, and a drain line. A small electrical
enclosure within the cabinet supplies the electrical power for the MonitAll
and the space heater provided for climate control.
The data acquisition cabinet contains the microprocessor-based controller,
which manages all of the data acquisition, storage, and display. It also controls
a printer, the floppy disk drive, automatic corrosion probe switching, automatic
shutdown safeguards and alarms, and the climate controls. The controller has
a keypad and a display window for operator interface. A personal computer can
be used to generate reports, graphs, and statistical analysis from the acquired
Biological Activity in Cooling Systems
A biofilm fouling monitor (Figure
36-15) is used to determine levels of microorganisms attached to surfaces in
a cooling system. The monitor consists of a holder that is threaded on both
ends. Each half of the holder contains a screen that secures glass beads to
the sampling surfaces.
The biofilm monitor can be attached at any suitable location in the hot water
return where the flow through the monitor is at least 1-2 gpm. The monitor is
normally on line at least 1 week before the sampling starts. The time required
to develop a steady-state biofilm on the beads varies depending on system conditions.
Steady-state is reached when the amount of biological material removed by turbulent
flow is equal to the amount of new biofilm produced by microbial growth. After
steady-state is achieved, changes in levels of biofilm reflect changes in the
system environment; for example, increased nutrient levels lead to greater amounts
of biofilm, while the addition of toxic materials causes a reduction in biofilm
levels. Individual systems must be monitored to determine what level of fixed
microorganisms is acceptable.
Macrofouling monitors (Figure
36-16) are used to monitor the growth rate of zebra mussels, Asiatic clams,
and other mollusks. The problems caused by these animals are described in
28. The strategic placement of macrofouling control monitors helps to quantify
growth and settlement cycles in a particular area. They also provide quantification
of kill rates following chemical treatment.
A macrofouling unit contains a set of fouling plates. Water flows upward through
the unit. The mussel or clam larvae attach themselves to the fouling plates.
Their rate of growth is monitored visually by regular examination of the plates.
Continuous on-line measurement devices used to monitor halogen residuals fall
into two categories: amperometric and colorimetric.
Amperometric analyzers, depending on the mode of use, measure free or total
halogen concentrations in water samples. Changing halogen concentrations in
the sample produce a corresponding change in the electrical current that flows
from the cathode to the anode of a sensor. Some amperometric analyzers also
correct for variations in sample temperature and pH.
Figure 36-17 shows a colorimetric
analyzer, which changes color intensity depending on the chlorine concentration
of the sample. Small volumes of sample, an indicator agent, and a buffer
solution are precisely metered and mixed. During a development interval, the
indicator oxidizes and produces a magenta or red compound which is photometrically
measured. The color intensity is compared to a reference and the difference
is used to characterize the chlorine concentration of the sample. Measurement
accuracy can be affected by the presence of chromates, chloramines, nitrite,
iron, manganese, and other strong oxidants in the sample. Careful selection
of the chlorine analyzer and proper installation should help to minimize these
Continuous monitoring is an important part of many chlorine applications:
- to control feed rate in potable water supplies
- to prevent damage to ion exchange demineralizers or reverse osmosis
systems in municipal and industrial water supplies
- as an antimicrobial in cooling tower applications
- verifying regulatory discharge requirements for wastewater or industrial
OTHER MONITORING TECHNIQUES
Visual inspection equipment is often useful for the inspection of internal
surfaces in boiler tubes, condenser tubes, heat exchangers, and turbines. Visual
inspection is used to determine failure potential due to deposit accumulation
Fiber Optics. A
fiber optics device (Figure 36-18) is commonly used for equipment inspection.
A lens on each end of the fiber optics bundle provides a clear, undistorted,
color image. Video equipment and 35 mm cameras may be used with a fiber optics
Video Inspection. Television camera inspection equipment provides
an alternative to fiber optics. The typical package consists of a miniature
camera, lights, a rotating mirror for radial viewing, and a monitor.
Additional Monitoring Technologies
Ion Chromatography. Ion chromatography (IC) is widely used
in laboratories and is finding a place in some continuous process analysis applications.
It is used to detect low-level contamination of otherwise high-purity streams.
The detection capability of IC permits routine analysis in the parts per billion
range and, in some cases, in the parts per trillion range. The advantages of
IC are its selectivity, sensitivity, and speed in the analysis of anions and
An ion chromatograph consists of an anion or cation separation column and an
anion or cation suppressor column. In the separation process, metals are eluted
from a separating resin with a strong acid, such as HCl. The metals are then
exposed to the suppressor column, which is a strong anion exchanger in the hydroxide
form. The chloride (Cl) is removed by the anion resin and the eluted hydroxide
reacts with the acid proton to form H2O. Thus, the metals elute in very dilute
water solution as metal hydroxides, and the conductivity is measured. For alkali
metals and many other metals, the conductivity imparted to pure water is a simple
function of species concentration. Anions are separated in an analogous process.
Flow Injection Analysis. Air-segmented continuous process
analyzers have been the foundation of automated industrial water testing for
over 30 years. The technology has evolved to a point at which such systems are
cost-effective and productive for a wide variety of industrial process monitoring
applications. However, in the 1990's, nonsegmented flow injection analysis (FIA)
was introduced as an alternative method for these applications.
In the FIA method, small quantities of sample are transported through a narrow
bore tube and then mixed with reagents to develop a color that is monitored
by a detector. In this new technique, air bubbles are not used to separate individual
samples. Samples are injected into a flowing, continuous stream of reagent.
To maintain sample integrity, injection intervals must be long enough to prevent
cross-contamination. FIA is highly reproducible due to the elimination of air
bubbles, the use of precise injection techniques, constant flow rates, and exact
timing of analytical reaction from injection to detection.