CHAPTER 45CHEMICAL FEED SYSTEMS

Understanding the chemistry of water treatment processes is the first step towardmastering the ability to develop a chemical treatment program that produceswater of acceptable quality—meeting predetermined specifications for hardness,dissolved solids, silica, and other controlled impurities—at reasonable cost. Thenext important step is selection of a reliable chemical feeding system—sensorsand instruments, controls, and feeders—to consistently and accurately apply thechemicals needed by each treatment process.

There are many varieties of chemical feeders available for this purpose. Sys-tems may be required to handle dry products like lime or liquid products likealum or caustic soda. A 1000 ton/day (908 t/day) kraft pulp/paper mill using 26mgd (98,400 m3/day) of water would require 25,000 Ib/day (11,300 kg/day) oflime if the water is softened with lime at 120 mg/L. The same plant may generate5,000,000 Ib/day (2,270,000 kg/day) of high-pressure steam, requiring the appli-cation of hydrazine at 20 ppb, or only 0.1 Ib/day (0.04 kg/day). The design engi-neer must exercise the same care in designing the system for feeding 12.5 tons oflime per day as the one for feeding 0.1 Ib/day of hydrazine. The problemsof inventory, of course, are also greatly different in the storage and handling ofheavy chemicals shipped in bulk from those with specialty chemicals shippedin pails, drums, or bags. The storage properties of common water treatment chem-icals are listed in Table 45.1. This chapter summarizes the selection processfor the most common types of feed systems for liquid and dry chemicals usedin water treatment, for feed rates ranging from a few pounds per day to many tonsper day.

LIQUID FEED SYSTEMS

There are feeders for application of liquid chemicals by drip feed ranging from aconstant head device used for feeding tiny amounts of liquid (as in intravenousfeeding) to large flows of acid from bulk storage tanks. However, for the most part,the need for accuracy in chemical treatment is such that this chapter will deal onlywith systems using (1) metering or flow-controlled pumps, of which there is a widechoice based on delivery rate and application specifications (chemical to be fed,concentration, pressure, temperature, etc.); or (2) motor-controlled decantingdevices combined with receivers and nonmetering delivery pumps.

TABLE 45.1 Common Chemicals Used to Treat Water

Solubility

4.2 Ib/gal at 6O0F

InsolubleInsolubleInsoluble3% at 6O0F

Slake at 10-20%Insoluble0.07 Ib/gal at 6O0F2 Ib/gal at 6O0FInsoluble45% at 6O0F

30% at 6O0F1 Ib/gal at 6O0F35% at 6O0F

40% at 6O0F

2.6 Ib/gal at 6O0F

1.5 Ib/gal at 6O0F70% at 6O0F

20% at 6O0F1 Ib/gal at 6O0F

Infinite

Approx.pH 1%

solution

3.4

9126-8

125-6

5-612 A3-4

3-43-41-2

11-12

6-8

1112.8

95-6

1-2

Bulkdensity,Ib/ft orIb/gal

601160

804055

6055gas

7540701370709.6

501360

6065125547

15

Equiv.weight

100*

5040*

103

30*86*35.5

121*67f91*

51.5*139*120*

100*

58.5

53

4047.334

50*

Typical specs

Lump— 17% A I2O3

Liquid— 8.5% A I2O3

96% CaCO3

96% Ca(OH)2

70% Cl2

96% CaO98% GypsumGas— 99.8% Cl2

98% Pure36-40% MgOLump— 20% FeLiquid— 20% Fe18.5% Fe29% Fe30% HCl20° BaumeFlake— 46% Al2O3

Liquid— 26% Al2O3

98% Pure98% Pure58% Pure Na2OFlake— 99% NaOHLiquid— 50-70%49% P2O5

66% P2O5

94-96%66° Baume

Common name

Alum

BentoniteLimestoneHydrated lime, slaked limeHTH

Burned lime, quicklimeGypsumChlorineBlue vitriolDolomitic limeIron chloride

Iron sulfateCopperasMuriatic acid

Aluminate

Rock salt, saltSoda ash

Caustic,LyeDisodium phsophateHexametaphosphateOil of vitriol

Chemical

Aluminum sulfate[A I2(SO4J3 -14H2O]

Bentonitic clayCalcium carbonate (CaCO3)Calcium hydroxide [Ca(OH)2]Calcium hypochlorite

[C(OCl)2 -4H2O]Calcium oxide (CaO)Calcium sulfate (CaSO4 -2H2O)Chlorine (Cl2)Copper sulfate (CuSO4- 5H2O)Dolomitic lime [Ca(OH)2-MgO]Ferric chloride (FeCl3 -6H2O)

Ferric sulfate [Fe2(SO4)3 • 3H2O]Ferrous sulfate (FeSO4 -7H2O)Hydrochloric acid (HCl)

Sodium aluminate (NaAlO2)

Sodium chloride (NaCl)Sodium carbonate (NaCo3)

Sodium hydroxide (NaOH)

Sodium phosphate (Na2HPO4)Sodium metaphosphate (NaPO3)Sulfuric acid (H2SO4)

* Effective equivalent weight of commercial product.•*• Effective equivalent weight based on Ca(OH)2 content.

Positive Displacement Pumps

In a positive displacement pump, the liquid is first drawn into a cavity, thenforced through an outlet port into the discharge line. The discharge volume isrelatively independent of the discharge head, and the discharge is never deliber-ately throttled. Pressure relief valves must be installed to protect these pumpsagainst accidental shut-oft0 of the discharge line. A separate pump should be pro-vided for each product and for each feed point. A common error is to attempt touse a single pump to supply a chemical to several feed points (e.g., a chelate solu-tion to three boiler drums in the same steam plant). It is impossible to control thesplit flow by throttling valves in the three feed lines; separate pumps are neededfor each point of feed. Spare pumpsshould be provided to allow for main-tenance without interruption of chem-ical feeding.

The reciprocating pump, Figure45.1, has a piston that moves in a cyl-inder, and each discharge stroke deliv-ers a volume close to the product of thecross-sectional area of the piston andthe stroke length. Both suction and dis-charge pass through check valves, sothere is little backflow or slippage;therefore, these pumps are also called"metering pumps." Discharge pressurevaries from typical plant water mainpressure in the range of 50 to 100 lb/in2

(3.4 to 6.9 bars), to boiler pressure of3000 lb/in2 (207 bars). It is essential tohave some back pressure on the dis-charge to ensure that the check valvesclose quickly and tightly. If the systempressure is too low to do this, it is common practice to pump through a pressurerelief valve set for the minimum effective pressure to cause seating. The dischargeflow has a pulsating characteristic, which must sometimes be compensated for toensure even chemical application.

Some examples of reciprocating pump applications are:

1. Heavy chemical usage: Chemical fed directly from bulk liquid storage tanks,such as:Caustic soda, 50% NaOH: Most commonly used for demineralizer regener-ation, where the average flow may be from 2 gal/h to 10 gal/min (7.6 L/h to37.9 L/min). The discharge is diluted to about 5% in such a way that fluctua-tion in delivery rate is modulated, reaching the resin bed at a relatively uni-form concentration.Alum, 50% liquid: Commonly used for coagulation, where the average deliv-ery rate may be in the range of 2 gal/h to 2 gal/min (7.6 L/h to 7.6 L/min).Fluctuating flow is no problem when alum is applied to a clarifier.Sulfuric acid, 94% H2SO4: Commonly used for demineralizer regeneration inthe range of 2 gal/h to 20 gal/min (7.6 L/h to 76 L/min). Pulsations are mod-ulated in the final dilution system. Also commonly used for cooling tower alka-linity and pH control, in the range of 2 gal/h to 20 gal/min.

FIG. 45.1 Simple reciprocating pump usedfor relatively small flow chemical applications,for discharge against normal water supply pres-sure up to boiler pressure. (Courtesy MiltonRoy Company.)

SUCTION CHECKBALL VALVE

PLUNGERPACKING

DISCHARGEBALL CHECK

GREASE FITTING

Note that in these examples, the cost of a piston-type reciprocating pumpfor use in the gallon per minute (or liter per minute) flow range is so high thatcentrifugal pumps or gear pumps are generally more practical and economical.

2. Specialty chemical usage: In these applications, total usage is often less than2 gal/h (7.6 L/h). Where usage is extremely small, e.g., less than 5 gal/day (18.9L/day), the product is commonly diluted to 5 to 10% of product strength beforefeeding to improve the precision of feed rate control.

There are three principal techniques used to control the delivery rate ofreciprocating pumps:

1. Stroke adjustment: A crank is imposed between the rotating drive shaft andthe reciprocating piston. The crank arm pivot can be positioned at the end ofthe arm, providing maximum stroke, or at the center of the drive shaft, pro-ducing zero reciprocating motion (Figure 45.2). Stroke adjustment is oftendone manually with the drive motor operating. However, special devices areavailable for making the stroke adjustment from a distant location in a centralcontrol room, either manually or instrumentally, such as by a pH controller.

FIG. 45.2 Adjustment of stroke length of reciprocating pump is used to adjust deliveryrate.

2. Pump speed: The drive can be a dc motor adjustable in speed by a rheostator a variable-speed gear reducer, thus providing a simple means for control ofdelivery rate. Some pumps are controlled by a solenoid rather than a motor.The frequency of actuating the solenoid is varied to change pump delivery rate.

3. Stroke interruption: A device is built into the pump cylinder to interrupt thepiston displacement even though the crosshead makes a full stroke.

There are several modifications to the simple reciprocating pump that keep thepiston from contacting the fluid being pumped, usually to avoid corrosion or toavoid the nuisance of fluid leakage on the floor from packing glands. The dia-phragm pump is commonly used for this purpose. The piston is sometimes

Full stroke

Half stroke

Zero stroke(pump shaft)

Movablecrank pin

Cross-head

• Piston

Head

FIG. 45.3 Diaphragm pump actuated by oil pressuredeveloped by the pump piston moving in an oil reser-voir. (Courtesy Milton Roy Company.)

out of the pump head is 3.14 times the average flow. Piping must be sized to takethis into account. Furthermore, the acceleration of the fluid from O to 3.14 timesaverage flow subtracts a substantial head from atmospheric pressure at the suc-tion, which is often the only pressure available to fill the pump cavity. Therefore,it is essential that these pumps have a flooded suction—despite the temptation tosimplify the system by mounting on a chemical tank. For similar reasons, pump-ing viscous products causes problems; handling viscous chemicals should bereferred to the pump specialist. Table 45.2 shows the viscosities of certain com-mon fluids; and Figure 45.4 shows the relationship between several methods ofexpressing kinematic viscosity.

The second major design category of positive displacement pumps is the rotarypump, which delivers a continuous rather than a pulsating flow. This type is avail-able in two styles, the progressing cavity design (Figure 45.5) and the gear pump(Figure 45.6). This style of pump is affected somewhat more by throttling orchange in discharge pressure than the reciprocating pump, but this is not a pro-cedure used deliberately to control feed rate, because it will either burn out themotor drive or rupture the pump casing. Unlike the reciprocating pump, therotary pump works best on viscous fluids—in fact, it is seldom recommended forhandling water or solutions below a viscosity of about 50 SSU, whether aqueous

directly connected to the diaphragm, but the diaphragm is more commonly posi-tioned by oil sealed in the cylinder head (Figure 45.3).

The reciprocating pump follows the laws of simple harmonic motion, so themaximum rate of displacement occurs when the crank arm is at right angles tothe linear movement of the crosshead. At this point, the fluid velocity into and

DISCHARGEVALVEBALL CHECK

DIAPHRAGM

•SUCTION BALLCHECK VALVE

PLUNGER

AUTOMATIC-BLEED VALVE

P RELIEFVALVE

or organic. The following are examples of rotary positive displacement pumpapplications.

1. Heavy chemical usage directly from bulk liquid storage tanks, such as:Caustic soda, 50% NaOH: Where the delivery rate exceeds 1 gal/min (3.8 L/min), as in demineralizer regeneration.Alum, 50% liquid: Where the flow exceeds 1 gal/min (3.8 L/min), as in largecoagulation-flocculation systems.Sulfuric acid, 94% H2SO4: Where the flow rate exceeds 1 gal/min (3.8 L/min),as in demineralizer regeneration and application of acid to cooling towersystems.

Lime slurries have been fed with this type of pump, but because of the ten-dency for settling, the pump should be used at full flow rate.

2. Specialty chemical usage, such as:High molecular weight polymer solutions used for flocculation. The pump

is used to prepare the stock solution (0.5 to 2.0%) held in a day tank and some-times to feed the stock solution to a dilution line enroute to the point of use,if the rate of feed exceeds 1 gal/min (3.8 L/in).

Emulsion-type high molecular weight polymers as supplied. This typepump is used to deliver product from inventory to a special dilution-feedingsystem.

TABLE 45.2 Viscosity of Some Common Fluids*

[Based on measurements at 2O0C (680F)]

* Viscosity relationships:Metric:

. , absolute viscosity (cP)centistokes = ^—-

density (g/cm3)English: ft2/s = cSt X 1.076 X 1(T5

Saybolt seconds universal: the time required for a measured volume of fluid to pass through astandardized orifice at a controlled temperature. Because the test is conducted under fluid flowconditions, SSU is related to kinematic viscosity rather than absolute viscosity.

Note: For effect of temperature on water viscosity, see Figure 1.6.

Liquid

WaterEthanolSaturated brineHCl, 31.5%NaOH, 30%H2SO4, 66° BePropylene

glycolMotor oils

1OW3OW

Glycerine

Sp. gr.

1.000.791.191.051.331.84

1.04

0.900.901.26

ViscositycP

1.01.23.02.0

13.326.7

54

90315781

Kinematicviscosity,

cSt

1.01.522.51.9

10.014.5

52

100350620

ViscosityEnglish

units, ft2/s(X IQ-5)

1.0761.642.692.04

10.7615.6

56

108377667

ViscocitySaybolt,

SSU

31.031.734.033.05975

245

47016502950

FIG. 45.5 A screwlike shaft rotating in a resilient stator of matched form produces positive dis-placement of progressing cavities in this pump, delivering a continuous flow at the dischargeend. (Courtesy Fluids Handling Division ofRobbins & Myers, Inc.)

Viscosity, SUSFIG. 45.4 Relationship between kinematic viscosity and the empirical Saybolt viscositymeasurement at 2O0C (680F).

Kine

matic

visc

osity

, cen

tistok

es

FIG. 45.7 In the peristaltic pump, the fluid issqueezed through a flow tube by external roll-ers. (Courtesy Waukesha/Bredel PeristalticHosepump, WaukeshDiv.,Abex.)

FIG. 45.6 In the gear pump, cavities created betweengear teeth and housing are moved from suction to dis-charge, and these are sealed from each other by the mesh-ing of one gear with another. One gear is driven, and theother is an idler. (Courtesy Liquiflo Equipment Co.)

Two techniques are used to control the delivery rate from this type of rotarypump, only one of which actually controls the pump itself. The pump dischargerate may be controlled by speed variation, or the flow can be intermittently inter-rupted by a timer-controlled diversion valve, shunting the pump discharge backto the pump supply tank.

Another version of the progressive cavity pump is the peristaltic pump (Figure45.7). Small pumps like this are used in a variety of instruments for sampling,automatic titration, and colorimetric measurements. They are used in many med-ical applications. They develop only limited head pressure, and the peristalticaction applied to the flexible tube requires close attention so that the tubing canbe replaced before it fails. They can handle low-viscosity fluids. Flow rate is con-trolled by change of speed of the rollers that "milk" the tubing or by substitutionof different sized tubing.

Rotary positive displacement pumps require large suction piping of shortlength, with limited valving and fittings, plus ample static head, because of theviscosity of the fluids being handled. The fluid acts as a lubricant for the movingparts, so continuous flow is essential. This type of pump must never be kept inoperation with the discharge shut off or the suction line empty.

Centrifugal Pumps

In this type of pump, an impeller (a disk having radial vanes) rotates at highspeed—usually 1800 or 3600 r/min—in a housing. Liquid enters at the center ofthe impeller, and centrifugal force throws the liquid at an accelerating velocity tothe periphery of the housing, where the velocity is converted to head pressure.Liquid discharges tangentially from the peripheral passage of the housing.

FIG. 45.8 Volute-type centrifugal pump with open impeller.

The volute-type centrifugal pump (Figure 45.8) is the most common design. Ithas impeller vanes with spiral passages, the vanes beginning at a point near thecenter and advancing to the edge of the impeller. In the open impeller design, thevanes are visible when the pump casing is removed; with the closed impeller, amore efficient design, the vanes are encased like a sandwich between the impellerdisk and the thin-walled cover, having an opening at the eye for entrance of the

Discharge

Directionof rotation

Volute

Impeller'

Suction

Packing

Coupling

Driveshaft

FIG. 45.10 Closed impeller centrifugal pumpused for pumping clear liquids. It provideshigher efficiency than the open impellerdesign. (Courtesy Ingersoil-Rand.)

method of controlling delivery rate. Examples of the use of the volute-type cen-trifugal pump are as follows:

Hydrated lime, 5 to 10% slurry: In many plants, lime is fed as a slurry froma day tank in which the slurry is prepared. In the example given earlier, a plantfeeding 25,000 Ib/day (11,000 kg/day) of lime would pump about 250,000 Ib/day (110,000 kg/day) of slurry, requiring an average flow of about 20 gal/min(75 L/min) of lime slurry. To prevent deposition in the lime feed lines, a recir-culation system is recommended, especially for such large volumes of limefeed. The recirculation pump may feed 50 gal/min to the circuit, with inter-mittent "blips" of 40 gal/min (150 L/min) to the point of feed, 50% of the on-time (Figure 45.11).

fluid. The open impeller (Figure 45.9) is generally used for pumping slurries; theclosed impeller (Figure 45.10) is used for clear liquids, especially for large watersupply pump stations. Close clearances must be maintained to prevent internalrecirculation and loss of efficiency. Throttling the pump discharge is the common

FIG. 45.9 Open impeller centrifugal pumpused for pumping slurries such as lime orclay. (Courtesy Ingersoll-Rand.)

FIG. 45.11 Scheme for proportional feeding of lime with meter-actuated timerdosage control for large volume requirements.

Brine (saturated NaCl solution): In many municipal zeolite softening plants,salt is delivered in bulk to a wet salt storage basin (see Chapter 12). Duringregeneration, the 26% brine is usually delivered directly from the saturationbasin to the zeolite unit at rates up to 50 gal/min (190 L/min), usually througha brine meter. Centrifugal pumps are ideal for this service.

Because delivery rate changes with discharge pressure, as shown by a typicalcentrifugal pump curve, Figure 45.12, control of flow is important because line

Discharge head *

FIG. 45.12 Typical performance curve for anopen impeller centrifugal pump. The designpoint allows for control of flow over a reasonablerange by throttling discharge pressure. Maxi-mum power is required at zero discharge head.

pressure may change and upset the initial flow setting. Because of this, a rate con-troller actuated by a flow meter is often used.

One of the major limitations of volute-type pumps is viscosity. Highly viscousfluids are not easily handled by the volute-type pump. Another limitation is the

Constant rate

Lime slurrytank preparedat presetconcentration

On-off lime and water feed flowat constant rate initiated bytank level

Water

Lime fromdry feeder Excess return To point

of use

"Blip"valve

Adjustable timeractuated by f lowmeter

Flow

, ga

l/min

Designpoint

Power

Pow

er,

hp

Delivery

handling of abrasive slurries, especially lime. This requires the use of lanternglands on the pump shaft to flush the packing into the pump housing. The flushingwater must be nonreactive to the lime slurry—even zeolite-softened water is notacceptable. Any dilution water that reacts with the lime will scale the pump andpiping.

The pump housing and the impeller are usually of cast construction, usingiron, steel, bronze, or brass. Some alloys are not easily or economically fabricatedby casting, so where these alloys are needed, the volute pump design may be ruledout.

A much less common type of centrifugal pump is the turbine design (Figure45.13). In this pump, pressure builds up progressively as fluid enters vanesmachined on the circumference of the impeller disk, and recirculates in stages asthe vanes move through 360° from inlet to outlet openings. This type of pump ismore like a positive displacement pump in its characteristics than a volute pump;

FIG. 45.13 Although it operates as a centrifugal pump, theturbine pump is in many respects a cross between a positivedisplacement pump and a volute pump. (Courtesy BurksPumps, Decatur Pump Co.)

flow should not be controlled by throttling the discharge. Since the pump casingand impeller can be machined, this pump can be made of a variety of alloys asrequired by the chemical to be fed. It is not recommended for handling viscousfluids.

As mentioned earlier, meter-actuated flow controllers either of the loss-of-headtype or the variable area (rotameter) design can be used to control the delivery

rate of centrifugal pumps where large volumes of liquid [over 10 gal/min (38 L/min)] are fed from bulk storage tanks. A much more common liquid feed device,used for applying 5 to 10% chemical solutions or slurries to clarifiers or lime soft-eners, where the chemical feed application rate is usually less than about 500 Ib/day (225 kg/day), is the decanter, Figure 45.14. This comprises a chemical solu-tion tank holding either a slurry (e.g. lime or clay) or a solution (e.g., alum, causticsoda, or soda ash) fitted with a motor-operated decanting pipe that usually lowersthe pipe into the liquid at a fixed rate in the range of 0.1 to 0.3 in/min (0.25 to0.75 cm/min). The lowering motor is operated through a timer. The decantedfluid flows into a receiver, where it is diluted, and the final solution is then deliv-ered by centrifugal pump to the point of use. Change of dosage is easily handledby changing the timer setting, supplemented by change of solution strength wherepractical. In this system, the centrifugal pump is the device that delivers the solu-tion to the point of use, but its operation does not play a part in dosage rate orchanges in feed rate.

The limitation of this device in feeding solutions of dry chemicals as its needfor manual charging, usually from 50-lb (23-kg) bags. When the application ratedemands too much labor, then the plant may elect to feed the dry chemicalsdirectly from storage silos through dry feeders.

FIG. 45.14 In the decanting feeder, a skimming pipe is lowered at a fixed rate for a controlledperiod of time to withdraw chemical from the tank into a feed reservoir for delivery to the pointof use. (Courtesy Crane Company, Cochrane Division.)

DRAINJACK LEGS

CHEMICAL PUMP

WATER SEAL

VENT

FLOW METER

RESETTIMER

SELECTORSWITCH

CONDUIT

RAW WATERLINE

PUMP DISCHARGETO FEED POINT

RECIRCULATlONLINE /

MOTOR DRIVEN- AGITATOR

DECANTING HEAD!

STILLING BAFFLE

. LEVELINDICATOR

SWING JOINT

PROPELLER

DECANTROL

WATERFILLING LINE

WATER SUPPLY

OUST REMOVER(BY PURCHASER)

CHEMICALCHARGING DOOR'

DECANTING PIPE

SUMP

DRY FEED SYSTEMS

Dry chemical feeders are selected over wet feeders for handling large volumerequirements of chemicals available in dry form and in bulk. In feeding lime as aslurry through a wet feeder, the delivery of 1 ft3 of slurry made up as a 5% con-centration would apply only about 3 Ib (1.4 kg) of lime to the point of use. Onecubic foot (0.028 m3) of lime fed through a dry feeder, on the other hand, woulddeliver about 40 Ib (18 kg) of lime, or more than 10 times that of the liquid feeder.This illustrates the basic advantage of the dry feeder in being more compact, lesscostly, and able to handle materials delivered to the feeder in bulk.

FIG. 45.15 (a) Endless belt with gate that sets the dimensions of the ribbon of chemical. Deliveryrate may be changed by varying speed of belt drive or adjusting the control gate height, (b) Augerdelivers chemical through tube to receiver. Delivery controlled by adjusting auger revolutions perminute, (c) Rotating table with adjustable plow. Delivery volume is changed by speed of tableand pitch of the plow on the table, (d) Revolving lock gate delivers chemical onto conveyor belt.Dosage is controlled by speed of revolution.

Hopper filledwith chemical

Gate to controldepth of cakeon belt

Motordrive

Auger

,Deliverytube

Receiverwith mixer

Deliverypump Delivery

pump

Hopper

Receiverwith mixer

Endless belt onbelt drive

Dilutionwater

Spout of.hopper

Pyramid of chemicalbelow spout

Spout ofhopper

Rotatingtable

Doctorknife

Dilutionwater

Mixer

Doctorblade Rotating table

Receiver

Spillover ofdry chemical

Float- N

controlledvalve

Centrifugalfeed pump

Hopper

Revolving lock gate

DilutionwaterConveyor belt

Reservoirwith mixer

Deliverypump

Dry feeders are widely used in industries other than the water treatment indus-try, handling high-tonnage flows of coal in coal-burning steam plants and ofchemicals in the chemical and mining industries.

The two basic categories of dry chemical feeders are volumetric and gravimet-ric. The volumetric feeder is simpler and less costly, with a volumetric accuracyof about 98 to 99%; but because the bulk density of the dry chemical leaving thesilo or hopper varies, the accuracy on a delivered weight basis is reduced to 94 to95%. The gravimetric feeder is more sophisticated and actually weighs out thedelivered chemical with an accuracy greater than 99%.

Volumetric Feeders (Figure 45.15)

Dry chemical stored in a hopper is fed by gravity into the feeder mechanism. Thedevice for displacing the chemical at a controlled rate from the feed bin to thereservoir, or solution tank, may be:

1. A traveling belt, with some type of gate to control the depth and width of theband of chemical leaving the spout

2. A screw or auger, turning on its axis in a tube3. A rotating table, or disk, directly below the hopper spout, with an adjustable

doctor blade to deflect a controlled volume of chemical from the table into areceiver

4. A rotating "paddle wheel" lock valve, similar in some respects to the liquidgear pump, delivering the measured volume contained in each compartmentonto a conveyor belt as the lock valve slowly revolves

Each of these devices can deliver chemical at a rate proportional to the flow ofwater to be treated by using a flow meter signal either directly—to control beltspeed, for example—or indirectly, through a timer.

Gravimetric Feeders (Figure 45.16)

In the gravimetric feeder, chemical is actually weighed out of the hopper onto aconveyor belt supported on a scale. The belt delivers the material to a feed res-ervoir, common to almost all dry feed systems. If the rate of delivery falls off orincreases, material either collects on the belt or is fed too rapidly, upsetting thebalance and initiating corrective action, e.g., adjustment of the control valve orthe speed of the revolving gate rotor.

With either design of dry feeder, there are some common variables to considerto make sure that performance is consistent and reliable:

1. The characteristics of the chemical, includinga. Tendency to flood, requiring a rotary lock in the hopper spoutb. Tendency to cake, requiring protection from moisture, and vibrators or

similar accessory devices to promote material flowc. Angle of repose, for proper design of hopper and chutesd. Particle size distribution, so that dusty materials can be confined and kept

from damaging electric devicese. Method of storage and transfer from inventory

FIG. 45.16 The gravimetric feeder weighs dry chemical delivered by the belt to the chemical slurry orsolution reservoir. (Courtesy of Wallace & Tiernan.)

MECHANICALBEAM

COUNTERWEIGHTS BEAM

IDLER ROLL

STATIONARYDECK

WEIGHDECKS

DRIVE ROLL

TRACTIONROLL

•VERTICALGATE

FLEXURES

WEIGHBELT

TABLE 45.3 Typical Control Programs for Chemical Feeders

1. Method of initiationA. Nonproportional to flow

(1) On-off manual control(2) Clock-type program timer, on-off*

B. Proportional to flow(1) Meter delivers signal from integrator(2) Meter delivers a digital signal

2. Method of dosage controlA. Timer setting, initiated by integrator signal (e.g., timer controls on-time of pump,

delivery valve, etc.)B. Control of motor speed by digital signalC. Gear reducer speed change (usually manual)D. Change of mechanical linkage (e.g., pump stroke, gate setting, etc.)E. Recycle of discharge to chemical tankf

* Nonproportional control is commonly used for such services as (a) ion exchange regerneration,where the dosage and concentration are seldom changed and the dilution water flow rate is constant and(b) polymer feed to a sludge flowing at a constant rate to a dewatering device.

f pH controller can be used alone or with flowmeter to produce a signal for dosage correction.

FIG. 45.17 Adaptation of instruments and controllers to chemical feed equipment serving acooling system. (1) Flow meter (M) controls feed of inhibitor and dispersant proportional to watermakeup rate. Dosage controlled by individual timers or by pump. Biocide timer is clock con-trolled (C). (2) Makeup valve is level controlled (L). (3) Slowdown valve is conductivity con-trolled (A). (4) Acid feed is pH controlled (pH). In some systems the acid feed may be metercontrolled with a pH override.

Flowmeter

Cooling tower Heated waterfrom plant

Acidfeed

Slowdownto sewer

Cooledwaterto plant

Recirculationpump

BiocideDispersantInhibitor

Makeup

FIG. 45.18 A compact control panel with instruments used for control of the system illustratedin Figure 45.17.

2. The design of the silo, chutes, and hoppers to prevent size segregation andvariability in density of the material entering the feed device

3. The operating environment of the feeder, such as ranges of temperature andhumidity, and dust loading in the air

There are many methods of controlling chemical feed devices from simple on-off operation of a pump to meter-proportioned control of a feeder with a pH-controlled override. The more common of these various methods are listed inTable 45.3.

A typical example of a chemical treatment control system used with open,evaporative cooling systems is shown in Figure 45.17. A meter on the makeupwater line controls the application of most treatment chemicals (corrosion or scaleinhibitors, dispersants), while other chemicals (biocides, acid) may be controlledby a program timer or pH controller. The blowdown from the system to holdmineral solids content of the water in an acceptable range is actuated by a con-ductivity meter with high- and low-conductivity contacts. The assembly of theseunits into a single control panel is shown in Figure 45.18.