A well engineered feed system is an integral part of an effective water treatment
program. If a feed system is not designed properly, chemical control will not
meet specifications, program results may be inadequate, and operating costs
will probably be excessive. Some of the costly problems associated with poor
chemical control include:
- high chemical costs due to overfeed problems
- inconsistent product quality, reduced throughput, and higher steam and
electrical costs due to waterside fouling
- high corrosion rates and resultant equipment maintenance and replacement
(i.e., plugging or replacing corroded heat exchanger tubes or bundles)
- high labor costs due to an excessive requirement for operator attention
- risk of severe and widespread damage to process equipment due to poor control
or spillage of acid into cooling towers
A significant investment in a chemical feed system can often be justified when
compared with the high cost of these control problems. When a chemical feed
system is not properly engineered, chemical levels are often above
or below program specifications. The use of a proper
feed system can prevent this situation.
Chemical feed systems can be classified according to the components
used, the type of material to be fed (powder or liquid), the control
scheme employed, and the application.
FEED SYSTEM COMPONENTS
Treatment chemicals are usually delivered and stored in one of three ways:
bulk, semibulk, and drums. The choice among these three depends on a number
of factors, including usage rate, safety requirements, shipping regulations,
available space, and inventory needs.
Bulk Storage. Large users often find it advantageous to handle
their liquid chemical delivery and storage in bulk. Liquid treatments are delivered
by vendor tank truck or common carrier. A
large tank, often supplied by the water treatment company for storing the liquid
treatment, is placed on the property of the user near the point of feed (Figure
35-3). Service representatives often handle all inventory management functions.
Treatment can be drawn from these storage vessels and injected directly into
the water system or added to a smaller, secondary feed tank, which serves as
a day tank. Day tanks are used as a safeguard to prevent all of the material
in the main storage tank from accidentally being emptied into the system. They
also provide a convenient way to measure daily product usage rates.
Semibulk Storage. Where
chemical feed rates are not large enough to justify bulk delivery and storage,
chemicals can be supplied in reusable shuttle tanks (Figure 35-4). Usually,
these tanks are designed in such a way that they can be stacked or placed on
top of a permanent base tank for easy gravity filling of the base tank.
Drum Storage. Although 40 and 55 gallon drums were
widely used for chemical delivery only a few years ago, increasing environmental
concerns have sharply reduced drum usage. The restrictions on drum disposal
and drum reclamation have reduced the popularity of this delivery and
storage method in favor of reusable or returnable containers.
Delivery systems are the heart of a chemical feed system. The delivery system
most often used is the chemical metering pump. Nearly 95% of all feed systems
use metering pumps. However, gravity feed is gaining popularity in cooling water
systems. Eductors are also used occasionally.
The most frequently used metering pumps for water treatment applications are
the packed plunger, piston, and diaphragm pumps. Rotary gear and progressive
cavity pumps are also used occasionally. These all fall under the general heading
of "positive displacement pumps."
Design and selection of a metering pump and piping circuit are critical to
ensure that pump output will match specifications. Parameters that must be considered
include suction side static head, net positive suction head (NPSH), pump turndown,
potential syphoning, pressure relief, and materials compatibility.
In order to ensure accurate pumping, operating conditions must be close to
design specifications. For example, with a plunger pump, an increase in discharge
line pressure can significantly reduce pump output. Because many factors affect
pump performance, output should be checked frequently with a calibration cylinder.
Some computerized chemical feed systems automatically verify metering pump output
and make adjustments as necessary.
Packed Plunger Pumps.
Because plunger pumps can be designed for high discharge pressures, they are
often used for chemical treatment in boiler systems. Pumping action is produced
by a direct-acting piston or plunger that moves back and forth in a reciprocating
fashion and directly contacts the process fluid within an enclosed chamber.
Motor speed and/or stroke length may be used to adjust this type of pump. The
useful working range for packed plunger pumps is approximately 10-100% of rated
Packed plunger pumps use packing rings to form a seal between the plunger and
the plunger bore. In some circumstances, this can necessitate periodic adjustment
or replacement of the rings.
Diaphragm Pumps. Diaphragm pumps are becoming increasingly
popular in water treatment applications. The diaphragm design uses the reciprocating
action of a piston or plunger to transmit pressure through a hydraulic fluid
to a flexible diaphragm. The diaphragm isolates and displaces the pumped fluid
and is activated either mechanically or hydraulically.
Figure 35-6 shows a diaphragm
pump that uses an electronic pulsing circuit to drive a solenoid, which provides
the diaphragm stroke. Both stroke length and stroke frequency can be adjusted
to provide a usable control range of 10-100% of capacity. Diaphragm pumps can
be set up for automatic adjustment of stroke frequency based on an external
signal. This capability is commonly used to control the ratio of chemical feed
to water flow rate.
The diaphragm pump illustrated
in Figure 35-7 uses an internal hydraulic system to operate the diaphragm in
contact with the treatment solution. The pump is available in models operating
at discharge pressures exceeding 1500 psig. The delivery rate of the pump is
manually adjustable while the pump is running and can also be adjusted automatically
by a pneumatic or electric control signal. The internal hydraulic system has
a built-in valve to protect against overpressure.
Some diaphragm pumps can be used to feed heavy or viscous materials, such as
slurries and polymers. Figure
35-8 shows a tubular diaphragm pump that is often used in these applications.
The tubular diaphragm design also uses a reciprocating plunger, but a tube-shaped
diaphragm expands or contracts with pressure from the hydraulic fluid. Adjustable
pumps with flow rates of up to 60 gal/hr at 100 psig are available.
An air-driven diaphragm pump operates from 1 to 200 gpm. This design is usually
used for viscous products, and because of its high capacity is generally used
to transfer chemical from a storage tank to a day tank. It can be used for feeding
shear-sensitive polymer solutions.
The air-driven pump tolerates abrasive materials and is also used to pump sand
and sludges. Discharge pressure is limited to approximately 100 psig.
Rotary Pumps. Rotary pumps have one or two rotating members
to provide positive or semipositive displacement. The pump may consist of two
meshing gears or a single rotating member in an eccentric housing. In the full
positive displacement type, delivery rate is fixed by speed of rotation. Semipositive
displacement pumps have internal slippage, which affects delivery rate and discharge
pressure. Rotary pumps generally depend on the fluid being pumped for lubrication.
Most designs will not tolerate abrasive material in the fluid. They can pump
highly viscous fluids and are particularly useful for polymer applications,
in which low shear is desirable.
Figure 35-9 shows a rotary
pump with an idler gear moving inside a rotor gear. Pumping action is achieved
by the meshing of rotor and idler gear teeth and by the use of close running
tolerances. With every revolution of the pump shaft, a fixed amount of liquid
is drawn into the pump through the suction port. This volume of liquid fills
the spaces between the teeth of the rotor, progresses through the pump, and
is forced out through the discharge port.
Another commonly used delivery method, the gravity feed design uses the height
difference between chemical in the tank and the point of application as the
driving force. The primary advantages of gravity feed delivery are simplicity
and reliability. This pumpless design eliminates moving parts and associated
maintenance requirements. The elimination of check valves and their periodic
failures greatly improves reliability. When feed verification methods are employed,
gravity feed can provide precise chemical control.
There are several types of gravity feed systems. A shot feeder is an example
of a simple but effective way to dispense premeasured chemical "shots."
The shot feeder uses a measuring pot of known volume, which is filled from the
bulk stor-age tank. A valve at the bottom of the measuring pot is opened and
the product is allowed to flow by gravity into the system.
Feed verification can be achieved by measurement of the velocity of product
flow, or volume per time. This permits accurate feed and measurement of product
to a system without the traditional maintenance problems associated with metering
pumps. The most sophisticated gravity feed systems combine feed verification
with computerized controls to provide optimum chemical control and eliminate
the need for metering pumps.
Proper size is important. An oversized system will cause spikes in chemical
treatment (periodic overfeeds). If the system is undersized, it may not be able
to feed enough chemical treatment. Key variables that must be considered in
sizing a gravity feed system include product viscosity, available static head,
effect of fluctuating tank levels, and system friction losses.
The water-jet eductor harnesses the kinetic energy of a moving liquid under
pressure. An eductor entrains
another liquid, gas, or gas-solid mixture, mixes it with the liquid under pressure,
and discharges the mixture against a counterpressure, as shown in Figure 35-10.
Application of water-jet eductors is limited by the amount of lift or suction
necessary, available motive pressure, and discharge pressure. Generally, a motive-to-discharge
pressure ratio of at least 3.5:1 is necessary.
Operated in conjunction with a valve, a water-jet eductor can be used for continuous
injection of chemical into a water stream. It is usually used in these applications
for mixing rather than proportioning. The water-jet eductor is an important
component of vacuum-type chlorinators and sulfonators and is also used for transporting
Eductors have many important advantages, including low cost, no moving parts,
and the ability to self-prime. Moveover, because electrical power is not needed
for operation, eductors are extremely well suited for use in hazardous locations
where expensive explosion-proof equipment is required. Eductors can also be
adapted to operate with automated control systems.
The accumulation of particulate matter in and around the eductor nozzle can
cause a loss of suction. Filters and strainers can assist in reducing this problem.
Eductors should be inspected and cleaned periodically in installations where
deposition is likely to occur.
Pump/Tank Packages. In most applications, a pump alone is
not sufficient for chemical feed. Usually, a chemical feed system combines pump,
tank, valves, gauges, mixer, strainer and relief valves (to allow chemical solution
preparation), mixing (if required), and pumping.
Mixers. A vertically mounted, shaft-driven impeller is the
most common type of mixer used for chemical feed systems. If the chemical is
a diluted, high molecular weight polymer, a speed reducer may be required. With
certain chemicals, it is desirable to minimize air entrainment. In these cases,
an electrical or air-driven recirculation pump should be used for mixing.
Timers. Timers find numerous applications in feed systems-most
notably, the control of mixers and batch feeding of chemicals (particularly
Alarms. Alarm systems are becoming more and more sophisticated.
It is now possible to monitor and alarm as necessary based on pump status, chemical
use rates, high and low tank levels, and unusual operating conditions.
Injection Nozzles. Specialized nozzles are needed to inject
chemicals into pipelines. Figures
35-11 and 35-12 show
typical low- and high-pressure nozzles. Low-pressure nozzles are used for injection
into a liquid stream. High-pressure quill nozzles are used in vapor systems.
The quill atomizes the chemical into fine droplets that are carried with the
vapor stream. Care should be taken to prevent injection of liquid into steam
lines immediately upstream of pipe bends, where high-velocity liquid droplets
can impinge upon and erode pipe walls.
Level Gauges. The need to monitor on-site and remote tank
levels has led to the development of many different types of level monitors.
Among the more prominent methods are pressure transducers, ultrasonic monitors,
capacitance sensors, pressure-sensitive linear potentiometers, and bubble tubes.
Care must be used to ensure the following:
- compatibility with the materials of construction
- adequate temperature compensation
- isolation from damaging pressure shocks
CHEMICAL FEED SYSTEMS
Chemicals may be fed on a "shot" (batch) basis or on a continuous
basis. The choice between these two methods depends upon the degree of control
required, the application, and the system design.
Shot Feed. Chemical may be shot-fed by on-off control of a
chemical feed pump or by discharge from a calibrated vessel or measuring chamber.
Shot feeding may be used in cooling systems, bio-oxidation basins, and other
places where the system volume to blowdown ratio is large. In these systems,
the shot simply replenishes lost or consumed material. Shot feeding is also
used in applications that only require periodic feed. Antimicrobials for cooling
water systems are usually fed on a shot basis. Shot feed cannot be used in once-through
systems, where a uniform concentration of chemical is needed.
Continuous Feed. Continuous feed systems meter chemical to
the water constantly. The better systems proportion the feed according to the
volume being treated and the chemical demand requirements. Continuous feed is
suitable for many systems that can also be shot-fed. It is absolutely necessary
in applications such as domestic water chlorination and deposit control in once-through
systems, where each gallon of water must be treated and no retention vessel
exists. It is also necessary when water clarification chemicals are fed to ensure
that all turbidity particles encounter polymer molecules before entering the
Continuous feed may be provided by a simple, gravity drip feed method, in which
feed rate is regulated by a needle valve. Metering
pumps or rotometers and control valves feeding from a recirculating pressurized
line (Figure 35-13) can also be used.
Large amounts of alum, lime, and soda ash are commonly applied in wastewater
treatment plants and large industrial water conditioning plants. Because of
the large quantities involved, these additives are usually stored and fed as
dry materials. The primary advantage of dry feed is the lower shipping and storage
costs. Disadvantages include dust, lack of control, high operational and maintenance
costs, and increased operator attention.
Dry feed systems commonly used for water treatment applications include volumetric
feeders, gravimetric feeders, and dissolving feeders.
Volumetric Feeders. Volumetric feeders accurately dispense
powdered material. The material may be applied directly or used to produce a
slurry that is applied to the process. Volumetric feeders are used for lime
feed and lime slaking, dry polymer and clay feed, and the feed of fire-side
additives to boiler furnaces.
The performance and accuracy of volumetric dry feeders depend largely on the
characteristics of the powder being metered. Key characteristics that affect
powder feed are particle size distribution, loose and packed bulk densities,
moisture content, and abrasiveness.
A typical volumetric feed system includes a bulk storage bin or silo, a feed
hopper, and a metering device. The most common metering device is a helical
screw or auger. The rotational speed of the screw determines the feed rate.
Some powders tend to bridge, or "rathole," causing uneven feed. To
ensure even flow of powder to the helix area, auxiliary devices may be required.
Among the more common are flexing hopper walls, bin vibrators, and oversized
auxiliary augers positioned above the feed helix.
Gravimetric Feeders. Gravimetric feeders proportion chemicals
by weight rather than by volume, and are accurate to within 1-2%. A gravimetric
feeder is a scale, balanced to ensure delivery of a desired weight of chemical
to the system. The chemical discharged by a gravimetric feeder is usually put
into solution or suspension.
Because gravimetric feeders are considerably more expensive than volumetric
feeders, they are used only with large systems needing accurate feed or for
chemicals whose flow properties prohibit the use of volumetric feeders.
Dissolving Feeders. Dissolving feeders regulate the rate at
which a dry chemical is dissolved. A dissolving tank is charged with dry chemical,
and a regulated flow of water is fed into the vessel. The concentration of discharged
product is governed by the contact area between the dry material and water,
and the rate of dissolution. Examples of dissolving feeders include solid halogen
feeders and polyelectrolyte dissolving systems.
In some dissolving feeders, extra energy is required to ensure adequate dissolution
(wetting) and thorough mixing. For example, in the solid halogen feeder, spray
nozzles direct a high-velocity stream of water into the bed of product. In other
dissolving feeders, an eductor is used for wetting and mixing.
Automatic and semiautomatic systems have been built to deliver, wet,
age, transfer, and feed dry polyelectrolytes (polymers). The delivery
and wetting portion of these systems is like that of a dissolving feeder.
Either spray curtains (wetting chambers) or eductor devices are used
to "wet" the polymer. Various tanks, controls, and pumps are
used to agitate, age, transfer, and feed the made-down polymer solution
(see Figure 35-14). These feed systems commonly have volumetric
screw feeders for precise metering of dry polyelectrolytes. The only
manual labor involved is the loading of the bin associated with the
volumetric screw feeder.
CHEMICAL CONTROL SYSTEMS
Another important component of a well engineered chemical feed system is the
control scheme-the method by which chemical feed rate adjustments take place.
The control scheme can have a dramatic effect on chemical residual control,
manpower requirements, and ultimate treatment program results. Key variables
that must be considered in the selection of a control scheme include required
control range, system half life, dead time, manpower availability, and economics.
Control methods may be classified according to the manner in which the final
control element is regulated and the degree of sophistication of the control
logic. Typical control schemes used in water treatment applications include:
manual, on-off, feedforward, ratio, feedback, and feedforward/feedback combinations.
The most sophisticated methods of control require the use of programmable logic
controllers or computers.
In a manually controlled system, chemicals are added continuously and at a
constant rate. Adjustments are made by plant operators at fixed time intervals-generally
once per shift or once per day. These adjustments include pump stroke length
or frequency, strength of chemical solution, and valve position.
Manual control is most suitable for applications in which chemical control
is not critical, established control ranges are wide, and system retention time
is long. In these situations, manual control maintains average chemical balances
within acceptable limits.
The disadvantages of manual operation include possible lapses in control, high
chemical treatment costs, increased manpower requirements, and the possibility
of unacceptable results. With today's emphasis on improved quality control,
there is a trend away from manual control and toward the use of more sophisticated
On-Off Constant Rate Mode
In an on-off control method, a chemical feed pump (or other constant rate feed
device) is automatically cycled on and off by a control signal. This method
is applicable to systems (e.g., cooling towers) that do not require continuous
feed of chemical and have large volume to blowdown ratios.
An example of on-off control is an acid feed pump that turns on at a high pH
setpoint and off at a low pH setpoint.
The meter-counter-timer is another on-off control system employed in cooling
water systems. In this control method, a chemical pump is turned on for a fixed
period of time after a preset volume of makeup water or blowdown has accumulated.
Feedforward control systems are designed to detect changes in chemical demand
and compensate for them to keep the system under control. In contrast, feedback
control systems react only after a system error is detected. Feedforward control
is typically used to adjust corrosion inhibitor feed rate (based on changes
in water temperature), chelant feed rate (based on a hardness tests), and coagulant
feed rate (based on influent turbidity readings).
Ratio Control. Ratio control is a form of feedforward control in which output
of the chemical pump or other metering device is automatically adjusted in proportion
to a variable, such as water flow rate. Ratio control is most frequently used
to maintain a fixed concentration of chemical in a water stream where the flow
rate fluctuates. A common example is the feed of corrosion inhibitor in proportion
to mill water supply flow rate.
The primary disadvantage of this control scheme is lack of on-line feed verification.
Although the controller sends a signal to the pump, there is no guarantee that
the metering pump output matches the controller signal or even that the metering
pump is working. Recent advances in feedback control technology have provided
a solution to this problem.
Feedback Control. With feedback control, the actual value of the controlled
variable is continuously compared with the desired value. When the detected
value varies from a predetermined setpoint, the controller produces a signal
indicating the degree of deviation. In many water treatment applications, this
signal is sent to a metering pump and the pump's output is changed.
One of the most common examples of feedback control is the feed of acid to
a cooling tower based on pH. When the controller detects a difference between
the setpoint and the actual pH, it changes the pump speed or valve position
to adjust pH back to the setpoint.
Manual adjustment of a chemical feed pump, based on a boiler water phosphate
residual test, is a simple form of feedback control. The accuracy of this method
is limited only by the frequency of testing, the time required to affect a change,
and the reliability of the monitoring technique.
The main disadvantage of feedback control is the fact that control
action does not occur until an error develops. Another key problem with
feedback control is that it is highly dependent upon the analyzer signal.
In many systems, analyzer accuracy and reliability are questionable.
COOLING SYSTEM TREATMENT
Most open recirculating cooling systems require the addition of four classes
of chemicals to minimize corrosion, scaling, and fouling: corrosion inhibitors
- corrosion inhibitors
- deposit control agents or dispersants
- pH adjustment chemicals
Blowdown control is also an integral part of cooling water chemistry management.
Corrosion inhibitors and deposit control agents are often fed neat (undiluted)
to a cooling tower basin. Common methods for chemical delivery include metering
pumps and programmed gravity feed systems. Gravity feed systems may employ water-jet
eductors to carry chemicals to the basin. Acids or alkalies used for pH control
and some antimicrobials require dilution prior to injection into the bulk cooling
Feed pumps can provide accurate continuous feeding, provided that the pump
rates are modulated to reflect system changes. Because of the large ratio of
cooling water system volume to blowdown rate, periodic shot feed of chemical
can often provide satisfactory chemical control.
Inhibitors and Dispersants. Inhibitors and dispersants may be fed neat to the
cooling tower basin, where dilution in the recirculating water can easily take
place. Feed systems vary from a simple continuous pump or periodic shot feed
to computerized automatic control.
The two simplest feed systems in use are continuously operating chemical feed
pumps and timed periodic shot feed systems. These methods provide adequate control
in some cases, but are inexact if cooling system operating conditions or chemistry
vary. When conditions vary, the plant operator must become an integral part
of the control loop, testing chemical residuals and adjusting chemical feed
rates in order to maintain proper inhibitor and dispersant levels in the water.
For improved chemical control, chemical may be fed in proportion to the flow
signal from a blowdown or makeup water flowmeter. This can be done on a continuous
basis with a flow signal directly controlling the pumping rate. It can also
be done on a semicontinuous basis by a flow counter, which triggers a shot feed
Additional improvement in control is possible with computerized controllers
that use measured parameters to calculate cycles of concentration and combine
that information with real-time flow data to calculate and feed the proper amounts
of inhibitor and dispersant.
pH Control. Control of cooling water pH and alkalinity within a specified range
is usually required for proper performance of the treatment program. Good pH
control has become more important because chromate inhibitor treatment programs
are being replaced and higher cycles are being used in cooling towers to minimize
Commercial concentrated sulfuric acid (66° Baume) is usually used for cooling
water pH control. When fed neat, it is nearly twice as dense as water and drops
to the bottom of the cooling tower basin. This can damage basin concrete and
cause poor pH control. For this reason, the acid should be well mixed with water
prior to entering the basin. A dilution trough is used to feed acid to the cooling
tower basin, using makeup water as the diluent.
Mild steel tanks are usually used to store concentrated sulfuric acid. Proper
ventilation is required to prevent the buildup of explosive hydrogen gas in
the storage tank. Strainers upstream of acid pumps are advisable to remove any
residual corrosion products or other solids that may be present in the storage
Feedback control is almost always used to control acid feed to a cooling system.
The control schemes most often used are on-off and proportional-integral-derivative
(PID). Metering pumps or control valves are normally used to regulate the feed
of acid. The location of the pH probe is critical; in most applications, the
probe should be placed close to the circulating water pumps.
Proper design is important in acid feed line installation. The lines should
be installed so that slow filling and draining, which would cause excessive
lag time in the control loop, are prevented. Horizontal sections should slant
slightly upward in the direction of flow. Installation of an elevated loop at
the discharge end of the line, higher than the acid pump, ensures a continuously
filled line. In installations where the acid storage tank is higher than the
feed point, an anti-siphon device can be used to provide extra protection against
acid overfeed. Concentrated acid feed lines generally need to be no larger than
in. and are usually made of 304 or 316 stainless steel tubing. Polyethylene
or schedule 80 rigid PVC pipe may be used if protected from physical damage.
Other important considerations include pump/valve size, acid quality, maintenance
procedures, and calibration frequency.
Sulfamic acid, hydrochloric acid, nitric acid (liquids), and sodium bisulfate
(solid) may also be used for pH reduction. Sometimes, pH control is linked to
chlorine gas feed, because chlorine gas combines with water to form hydrochloric
acid along with the antimicrobial hypochlorous acid. This practice is not recommended,
because overchlorination can result.
If increased alkalinity is needed, soda ash or caustic soda is normally used.
Cooling water dissolved solids (measured by conductivity) are maintained at
a specified level by a continuous or intermittent purge (blowdown) of the recirculating
water. In certain cases, it is sufficient to blow down periodically by opening
a valve until the conductivity of the water in the tower reaches a certain specified
level. Improved control can be obtained with an automatic blowdown controller,
which opens and closes the blowdown valve based on conductivity limits or modulates
a blowdown control valve to maintain a conductivity setpoint.
Even more accurate dissolved solids control can be attained when computerized
control systems are used. The measured conductivity of the recirculating water
divided by that of the makeup water provides an estimate of the cycles of concentration.
The recirculating water conductivity setpoint is then adjusted by the on-line
computer to maintain the desired number of cycles.
Computerized Chemical Feed and Control Systems
Computerized cooling water chemistry control systems can incorporate some or
all of the control functions already discussed in this section, including inhibitor
and dispersant feed, pH control, blowdown and cycles control, and nonoxidizing
antimicrobial feed. Figure
35-15 is a schematic of a computerized cooling water chemical feed, monitoring,
and control system setup.
Computerized control systems can usually be programmed to feed chemicals or
adjust operating parameters according to complex customized algorithms. This
allows the feed system to compensate automatically for changing operating conditions
that are often highly plant-specific. For example, in some cases the makeup
water may contain varying amounts of corrosion inhibitor. The corrosion inhibitor
feed rate to the recirculating water must be adjusted to compensate for the
inhibitor entering the system with the makeup. In other instances, the dispersant
feed rate setpoint may need to be adjusted according to system water chemistry
(e.g., pH, conductivity, or calcium levels). In each of these cases, a computer
can be used to perform the necessary calculations and implement the adjustments
Some computerized systems provide verification of chemical feed amounts. Combined
with on-line chemistry monitoring and customized control algorithms, feed verification
permits the most precise treatment control. The measurement system identifies
the water chemistry. The computer then calculates needed chemical dosages, and
the feed system verifies the quantity of chemical feed. A
commonly used system is shown in Figure 35-16.
Remote computers are used to monitor and store cooling system status and program
results. Parameters of interest usually include recirculating and makeup water
pH and conductivity, chemical feed rates, corrosion rates, and fouling data.
Following data collection, statistical techniques are used to analyze treatment
Modems are incorporated into some computerized feed systems so that
alarm conditions trigger an automatic telephone call to the proper operating
personnel and advise them of the problem. This prevents minor problems
from becoming serious. For example, if a valve is inadvertently left
open and the contents of the acid tank begin to drain into the cooling
tower basin, a low-pH alarm is activated, and a call is placed automatically
to the system operator, who returns to the area and shuts off the valve.
Modems are also used to allow operating personnel to make adjustments
to system operating parameters and chemical feed rates from a remote
BOILER SYSTEM CHEMICAL
For best results, all chemicals for internal treatment of a steam generating
facility must be fed continuously and at proper injection points. Chemicals
may be fed directly from the storage tank (neat) or may be diluted in a day
tank with high-purity water. Certain chemicals may be mixed together and fed
from the same day tank.
Chemical feed points are usually selected as far upstream in the boiler water
circuit as possible. For chemical feed beyond the feedwater pump or into the
steam drum, the pump must be matched to the boiler pressure. For high-pressure
boilers, proper pump selection is critical.
Product Feed Considerations
As shown in Figure 35-17, a
steam generating system includes three major components for which treatment
is required: the deaerator, the boiler, and the condensate system. Oxygen
scavengers are usually fed to the storage section of the deaerator. The boiler
internal treatment is fed to the feedwater pump suction or discharge, or to
the steam drum. Condensate system feed points also vary, according to the chemical
and the objective of treatment. Typical feed points include the steam header
or other remote steam lines. Chemical feed may also be fed directly in combination
with internal treatment chemicals or oxygen scavengers.
Oxygen Scavengers. Oxygen scavengers are most commonly fed
from a day tank to the storage section of the deaerator. Some oxygen scavengers
have also been applied in steam headers or condensate piping to reduce oxygen-related
corrosion in condensate systems. In utility systems, it is common to feed oxygen
scavengers into the surface condenser hotwell. Oxygen scavenger feed rates are
based on the level of oxygen in the system plus the amount of chemical addi-tives
in the system.
Sulfite. Uncatalyzed sodium sulfite may be mixed with other chemicals. The
preferred location for sulfite injection is a point in the storage section of
the deaerating heater where the sulfite will mix with the discharge from the
If sulfite is fed alone, the following equipment is needed: 304 stainless steel
- stainless steel agitator
- stainless steel relief valve
- iron piping, valves, and fittings
- a pump with machined steel or cast iron liquid end and stainless steel trim
In all cases, an injection quill should be used.
Sulfite shipped as liquid concentrate is usually acidic and, when fed neat,
corrodes stainless steel tanks at the liquid level. Tanks must be polyester,
Fiberglas, or polyethylene. Lines may be PVC or 316 stainless steel.
Catalyzed Sulfite. Catalyzed sulfite must be fed alone and
continuously. Mixing of catalyzed sulfite with any other chemical impairs the
catalyst. For the same reason, catalyzed sulfite must be diluted with only condensate
or demineralized water. To protect the entire preboiler system, including any
economizers, catalyzed sulfite should be fed to the storage section of the deaerating
Caustic soda may be used to adjust the pH of the day tank solution; therefore,
a mild steel tank cannot be used. Materials of construction for feed equipment
are the same as those required for regular sulfite.
Hydrazine. Hydrazine is compatible with all boiler water treatment
chemicals except organics, amines, and nitrates. However, it is good engineering
practice to feed hydrazine alone. It is usually fed continuously into the storage
section of the deaerating heater. Because of handling and exposure concerns
associated with hydrazine, closed storage and feed systems have become standard.
Materials of construction are the same as those specified for sulfite.
Organic Oxygen Scavengers. Many organic compounds are available,
including hydroquinone and ascorbic acid. Some are catalyzed. Most should be
fed alone. Like sulfite, organic oxygen scavengers are usually fed continuously
into the storage section of the deaerating heater. Materials of construction
are the same as those specified for sulfite.
Internal Treatment Chemicals
There are three major classifications of chemicals used in internal treatment:
phosphates, chelants, and polymers. These chemicals may be fed either separately
or in combination; in most balanced treatment programs, two or three chemicals
are fed together. The preferred feed point varies with the chemical specified.
For example, when caustic soda is used to maintain boiler water alkalinity,
it is fed directly to the boiler drum. When caustic is used to adjust the feedwater
pH, it is normally injected into the storage section of the deaerating heater.
Phosphates. Phosphates are usually fed directly into the steam
drum of the boiler, although they may be fed to the feedwater line under certain
conditions. Treatments containing orthophosphate may produce calcium phosphate
feed line deposits; therefore, they should not be fed through the boiler feed
line. Orthophosphate should be fed directly to the boiler steam drum through
a chemical feed line. Polyphosphates must not be fed to the boiler feedwater
line when economizers, heat exchangers, or stage heaters are part of the preboiler
system. If the preboiler system does not include such equipment, polyphosphates
may be fed to the feedwater piping provided that total hardness does not exceed
In all cases, feed rates are based on feedwater hardness levels. Phosphates
should be fed neat or diluted with condensate or high-purity water. Mild steel
tanks, fittings, and feed lines are appropriate. If acidic phosphate solutions
are fed, stainless steel and polyolefin materials are recommended.
Chelants. All chelant treatments must be fed to the boiler
feedwater line by means of an injection nozzle at a point beyond the discharge
of the boiler feed pumps. If heat exchangers or stage heaters are present in
the boiler feed line, the injection point should be at their discharge. Care
should be exercised in the selection of metals for high-temperature injection
At feed solution strength and elevated temperatures, chelating agents can corrode
mild steel and copper alloys; therefore, 304 or 306 stainless steel is recommended
for all feed equipment. Chelant products may be fed neat or diluted with condensate.
Chelant feed rates must be carefully controlled based on feedwater hardness,
because misapplication can have serious consequences.
Chelants should never be fed directly into a boiler. Stainless steel chemical
lines would be required, and chloride or caustic stress corrosion could cause
the chelant feed line to fail inside the boiler. Localized attack of boiler
metal would then occur. Chelants should not be fed if the feedwater contains
a significant level of oxygen.
Polymeric Dispersants. In most applications, polymeric dispersants
are provided in a combined product formulation with chelants and/or phosphates.
Dilution and feed recommendations for chelants should be followed for chelant-dispersant
and chelant-phosphate-dispersant programs. Dilution and feed recom-mendations
for phosphates should be followed for phosphate-dispersant programs. These combination
programs typically have the best results with respect to boiler cleanliness.
Filming Amines. All filming amines should be fed into steam
headers at points that permit proper distribution. A single feed point is satisfactory
for some systems. In every case, the steam distribution should be investigated
and feed points established to ensure that all parts of the system receive proper
Filming amines must be mixed with condensate or demineralized water. Water
containing dissolved solids cannot be used, because the solids would contaminate
the steam and could produce unstable amine emulsions.
Steel tanks have been used to feed filming amines, but some corrosion can occur
above the liquid level. The use of stainless steel is recommended. Equipment
specifications are the same as those for regular sulfite, except that a vapor-type
injection nozzle or quill is required.
Neutralizing Amines. Neutralizing amines may be fed to the
storage section of the deaerating heater, directly to the boiler with the internal
treatment chemicals, or into the main steam header. Some steam distribution
systems may require more than one feed point to allow proper distribution. An
injection quill is required for feeding into a steam distribution line.
Neutralizing amines are usually fed based on condensate system pH and measured
corrosion rates. These amines may be fed neat, diluted with condensate or demineralized
water, or mixed in low concentrations with the internal treatment chemicals.
A standard packaged steel pump and tank can be used for feeding.
Computerized Boiler Chemical Feed Systems. Computerized boiler
chemical feed systems are being used to improve program results and cut operating
costs. These systems can be used to feed oxygen scavengers, amines, and internal
A typical system, as shown
in Figure 35-18, incorporates a metering pump, feed verification equipment,
and a microprocessor-based controller. These systems are often linked to
personal computers, which are used to monitor program results, feed rates, system
status, and plant operating conditions. Trend graphs and management reports
can then be produced to provide documentation of program results and help in
In many cases, these systems can be programmed to feed boiler treatment chemicals
according to complex customized algorithms. For example, chelant feed can be
adjusted automatically, based on analyzer or operator hardness test results,
boiler feedwater flow, and minimum/maximum allowable product feed rates. Thus,
chemical feed precisely matches system demand, virtually eliminating the possibility
of underfeed or overfeed.
Feed verification is another important facet of some computerized feed
systems. The actual output of the pump is continuously
measured and compared to a computer-calculated setpoint. If the output
doesn't match the setpoint, the speed or stroke length is automatically
adjusted. The benefits of this technology include the elimination of
time-consuming drawdown measurements, the ability to feed most chemicals
directly from bulk tanks, precise chemical residual control, and minimal
POLYMER FEED SYSTEMS
Polyelectrolytes used in water treatment systems have certain storage, handling,
feeding, and dilution requirements. It is imperative that these materials be
fed accurately to prevent underfeeding and overfeeding, which can result in
wasted chemical treatment and poor system performance.
Polymers are available as powders, liquids, and emulsions. Each form has different
feeding, handling, and storage requirements.
Dry Polymers. Both cationic and anionic high molecular weight
polymers are available in powdered form. These products have the advantage of
being 100% polymer, which can minimize shipping and handling costs. However,
it is absolutely essential that dry polymer materials be handled and diluted
properly to prevent underfeeding and overfeeding.
Solution Polymers. Solution polymers are usually cationic,
low molecular weight, high charge density products, and are usually used for
clarification of raw water. Solution polymers are easier to dilute, handle,
and feed than dry and emulsion polymers. In many cases, predilution of a solution
polymer is unnecessary, and the product can be fed directly from the shipping
container or bulk storage tank. Solution polymers offer the convenience of neat
feed, and they can be diluted to any convenient strength consistent with chemical
feed pump output.
Emulsion Polymers. Both cationic and anionic high molecular
weight polymers are available as emulsions. An emulsion product allows the manufacturer
to provide concentrated liquid polymer formulations that cannot be made in solution
form. It is only after the emulsion polymer has "inverted" with water
that the polymer is available in its active form. Therefore, these products
must be diluted properly prior to use.
Dry Polymers. Dry polymers are susceptible to caking if stored
under highly humid conditions. Caking is undesirable because it interferes with
the polymer make-down and dilution process. Therefore, dry polymers should be
kept in areas of low humidity, and opened containers of dry material should
be sealed prior to restorage. In general, polymer products begin to lose their
activity after 1 year of storage. Although this process is gradual, it ultimately
affects the cost of chemical treatment. It is highly recommended that polymers
be used before their expiration dates.
Solution Polymers. Solution polymers should be stored in an
area of moderate temperature to protect them from freezing. Some solution products
are susceptible to irreversible damage when frozen. Others exhibit excellent
freeze-thaw recovery. In no case should solution polymers be stored at temperatures
above 120°F. As solutions, these polymers do not require periodic mixing
(to prevent separation) before use. However, some solution polymers have a short
shelf life, and inventory should be adjusted accordingly.
Emulsion Polymers. Because emulsion polymers are not true
solutions, they separate if allowed to stand for a prolonged period of time.
Therefore, emulsion polymers must be mixed prior to use with a drum mixer, tank
mixer, or tank recirculation package. A bulk tank or bin recirculation package
should be designed to recirculate the tank's contents at least once per day
to prevent separation. Emulsion polymers contained in drums should also be mixed
daily. Neat emulsion polymer must be protected from water contamination, which
causes gelling of the product and can make pumping difficult or impossible.
In areas of high humidity, tank vents should be outfitted with a desiccant in
order to prevent water condensation within the emulsion storage tank. Even small
amounts of condensation can cause significant amounts of product gelling. As
with liquid products, emulsion polymers must be protected from freezing and
should be stored at temperatures below 120°F.
Dilution and Feeding
Dry Polymers. Dry polymers must be diluted with water before
use. Most operations require preparation of polymer dilutions once per shift
or daily. Typically, a plant operator is charged with the responsibility of
measuring a correct amount of dry polymer into a container. The contents of
the container are conveyed to the mixing tank through a polymer eductor. The
eductor is a device that uses water pressure to create a vacuum and is designed
so that dry polymer particles are wetted individually by the water as they pass
through the eductor assembly (Figure 35-19). If dry polymer particles are
not wetted individually before introduction into the dilution tank, "fisheyes"
(undissolved globules of polymer) will form in the solution tank. Fisheyes represent
wasted polymer and cause plugging in chemical feed pumps.
Dry polymer solution strengths must be limited to approximately 0.5-1% or less
by weight, depending on the product used. This is necessary to keep the solution
viscosity to a manageable level. The mixer employed in the solution tank should
not exceed 350 rpm, and mixing should proceed only until all material is dissolved.
Normally, a batch of diluted dry polymer should be used within 24 hr of preparation,
because the diluted product begins to lose activity after this amount of time.
Automatic dry polymer dilution systems can be used to perform the wetting,
diluting, and mixing functions previously described; however, the system must
be manually recharged with dry polymer periodically. Although costly, these
systems can save appreciable time for plant personnel, and operations are usually
more consistent when automatic make-down units are used.
Solution Polymers. Solution polymers may be diluted prior
to use or fed neat from a shipping container, bin, or bulk storage tank. Dilution
of these products becomes necessary if there is insufficient mixing available
to combine the polymer with the water being treated. In-line static mixer dilution
systems are acceptable for solution polymers and are the simplest method of
solution polymer dilution and feed. A solution polymer can be fed through one
of the many commercially available emulsion polymer dilution and make-down systems.
However, in general, the use of these systems for solution polymers is not necessary.
Solution polymers can be pumped most easily with gear pumps. However, many solution
polymers have a viscosity low enough to be pumped by diaphragm chemical metering
Emulsion Polymers. Emulsion polymers must be diluted before
use. Dilution allows the emulsion product to invert and "converts"
the polymer to its active state. Proper inversion of emulsion polymers is rapid
and effective. Improper inversion of the emulsion polymer can result in loss
of activity due to incomplete uncoiling and dissolution of the polymer molecules.
Batch and continuous make-down systems are acceptable for emulsion polymer
use. In batch preparation, a plant operator feeds a premeasured amount of neat
emulsion product into the agitator vortex of a dilution tank. The product is
mixed until it is homogeneous, and then the mixers are shut off. As with dry
polymer products, mixer speed should always be below 350 rpm and the mixer should
be shut off as soon as the product is homogeneous. This prevents excessive shearing
of the polymer molecule and resultant loss of polymer activity.
A batch emulsion polymer make-down system is shown in Figure 35-20.
Several manufacturers market continuous emulsion polymer make-down and feed
systems. These systems pump neat polymer from the storage container into a dilution
chamber, where the polymer is combined with water and fully activated. The polymer-water
solution then flows by water pressure to the point of application. Provision
is made for secondary in-line dilution water to dilute the polymer further prior
to use. These polymer feed systems are by far the easiest and best ways to feed
emulsions continuously. Their manufacturers claim a superior ability to invert
the polymer molecule over batch tank dilution systems. A
commercially available continuous emulsion polymer makedown system is shown
in Figure 35-21.
It is not acceptable to use in-line static mixing alone for dilution of emulsion
polymers. However, in-line static mixing can be employed for blending secondary
dilution water with diluted emulsion product prior to application. Initial dilution
of emulsion polymers should be 1% or 2% by weight. This solution strength ensures
proper particle-to-particle interaction during the inversion step, which aids
in complete inversion.
It is usually desirable to provide secondary dilution water capabilities to
emulsion polymer feed systems, because these products tend to be most effective
when fed at approximately 0.1% solution strength.
In addition to the above, some general guidelines apply to the feeding and
handling of all water treatment polymers. In areas where the temperature routinely
drops below freezing, it is good practice to insulate all polymer feed lines
so that feed line freezing does not occur.
For tank batches of diluted polymers, tank mixer speeds of over 350 rpm should
not be used. In the preparation of diluted batches of polymer, water should
always be added to the tank first. Then, the mixer should be started and the
polymer added on top of the water.
Diaphragm metering pumps can be used to pump most polymer solutions. However,
due to the viscosity of some products, gear pumps may be necessary. Plastic
piping should be used in polymer feed systems; stainless steel is also acceptable.
Most polymers are corrosive to mild steel and brass. Extra precautions should
be taken to prevent spilling of polymers, because wet polymer spills can become
extremely slippery and present a safety hazard. Spills should be covered with
absorbent material, and the mixture should be removed promptly and disposed