Closed Recirculating Systems

A closed recirculating system is one in which the water is circulated in a closed

loop with negligible evaporation or exposure to the atmosphere or other influences

that would affect the chemistry of the water in the system. These systems

usually require high chemical treatment levels, and, since water losses are negligible,

these levels are economical. High-quality makeup water is generally used

for best system operation. These systems are frequently employed for critical cooling

applications, such as continuous casters in the steel industry where the slightest

deposit from any source could cause equipment failure.

.

Heat is transferred

to the closed cooling water loop by typical heat exchange equipment and is

removed from the closed system loop by a second exchange of heat from the

closed loop to a secondary cooling water cycle. The secondary loop could use

either evaporative or once-through water cooling, or air cooling.

Velocity of water in closed systems is generally in the 3 to 5 ft/s (0.9 to 1.5 m/

s) range. Temperature rise usually averages 10 to 15

systems can exceed this substantially. Generally closed systems require little or

no makeup water except for pump seal leaks, expansion tank overflows, and surface

evaporation from system vents. This periodic makeup requires regular analysis

for control of correct treatment chemical residuals.

Closed systems usually contain a combination of different metals which provide

a high potential for galvanic corrosion. The potential for dissolved oxygen

attack is generally quite low in closed systems because of the small amount of

makeup water—the main oxygen source. However, in systems that require substantial

makeup because of loss of water from leaks, oxygen is continually supplied

and oxygen corrosion presents a serious problem. Oxygen can, at elevated

temperatures or at points of high heat transfer, cause severe pitting corrosion.

Since relatively little makeup is added to most closed recirculating systems, it

is practical and desirable to maintain the system in a corrosion-free condition.

This is normally achieved by applying chromates, nitrite/nitrate-based inhibitors,

or soluble oil-type treatments at rather high concentrations.

Theoretically, scale should be a minor problem in a closed system since the

water is not concentrated by evaporation. In a tightly closed system, none of the

common scale-forming constituents deposit on metal surfaces to interfere with

heat transfer or encourage corrosion.

With high makeup rates, however, additional scale forms with each new increment

of water added so that in time, scale becomes significant. In addition, there

is opportunity for sludge, rust, and suspended solids to drop out at low flow points

and bake on heat transfer surfaces to form a hard deposit. Therefore, scale retardants

and dispersants are usually included as part of a closed system treatment

program where makeup rates are high. Often soft water or condensate is used for

makeup to closed systems depending on the characteristics of the system being

protected.

Because water circulating through a closed system is not exposed to the atmosphere,

fouling by airborne silt and sand is rare. However, fouling by microbial

masses may occur in closed systems where makeup rate is significant or process

leaks encourage bacterial growths. These are controlled with biological control

agents formulated to be compatible with the chemical treatments and operating

conditions found in closed systems.

It is desirable as a part of routine maintenance to flush closed water systems

with high-pressure high-velocity water to remove accumulated debris if makeup
0F (6 to 90C), although some

BOILER WATER TREATMENT

Of the many uses for energy in the United States today—in industry, in transportation,

in homes and commercial buildings—the largest portion of total use is

directed toward producing steam through the combustion of fossil fuels. Utilities

account for the greatest share of this, but industrial plants also produce enormous

quantities of steam for process uses, often generating electric power through turbines

as a by-product (Cogeneration).

The treatment of water for steam generation is one of the most sophisticated

branches of water chemistry. An understanding of the fundamentals of boiler

water chemistry is essential to the power engineer who continually strives to

increase the efficiency of the boilers and steam-using equipment.

The pressure and design of a boiler determine the quality of water it requires

for steam generation. Municipal or plant water of good quality for domestic use

is seldom good enough for boiler feed water. These sources of makeup are nearly

always treated to reduce contaminants to acceptable levels; in addition, corrective

chemicals are added to the treated water to counteract any adverse effects of the

remaining trace contaminants. The sequence of treatment depends on the type

and concentration of contaminants found in the water supply and the desired

quality of the finished water to avoid the three major boiler system problems—

deposits, corrosion, and carryover.

DEPOSITS

Deposits, particularly scale, can form on any water-washed equipment surface—

especially on boiler tubes—as the equilibrium conditions in the water contacting

these surfaces are upset by an external force, such as heat. Each contaminant has

an established solubility in water and will precipitate when it has been exceeded.

If the water is in contact with a hot surface and the solubility of the contaminant

is lower at higher temperatures, the precipitate will form on the surface, causing

scale. The most common components of boiler deposits are calcium phosphate,

calcium carbonate (in low-pressure boilers), magnesium hydroxide, magnesium

silicate, various forms of iron oxide, silica adsorbed on the previously mentioned

precipitates, and alumina (see Table 39.1). If phosphate salts are used to treat the

boiler water, calcium will preferentially precipitate as the phosphate before precipitating

as the carbonate, and calcium phosphate becomes the most prominent

feature of the deposit.

At the high temperatures found in a boiler, deposits are a serious problem,

causing poor heat transfer and a potential for boiler tube failure. In low-pressure

boilers with low heat transfer rates, deposits may build up to a point where they

completely occlude the boiler tube.

In modern intermediate and higher pressure boilers with heat transfer rates in

excess of 200,000 Btu/ft

deposits will cause a serious elevation in the temperature of tube metal. The

deposit coating retards the flow of heat from the furnace gases into the boiler

water. This heat resistance results in a rapid rise in metal temperature to the point

at which failure can occur. The action that takes place in the blistering of a tube

by deposit buildup2/n (5000 cal/m2/hr), the presence of even extremely thin

ture drops through gas or water films have been shown. Section A shows a cross

section of the tube metal with a completely deposit-free heating surface. There is

a temperature drop across the tube metal from the outside metal

in contact with boiler water

development of a heat-insulating deposit layer. In addition to the temperature

drop from

deposit layer from

boiler water temperature

operating pressure, and operating conditions require that the same boiler water

temperature be maintained as before the development of the deposit layer. Section

C illustrates the condition that actually develops. Starting at the base boiler

water temperature of T

line from

by the line from T

higher than the temperature

prior to the formation of deposit on the tube surfaces. If continued deposition

takes place, increasing the thickness of the heat-insulating deposits, further

increases will take place in the tube metal temperature until the safe maximum

temperature of the tube metal is exceeded. Usually this maximum temperature is

900 to 100O

boilers, the problem is more severe: at temperatures in the 900 to 135O

732

structure of carbon steel boiler tubes, and Figure 39.3 illustrates the spheroidization

of carbon and successive changes in structure, which begin to take place

above 80O

are considerably above this critical temperature range. Water circulating(T2) to the metal(T1). Section B illustrates this same tube after theT2 to T1, there would be an additional temperature drop through theT1 to T0. This condition would, of course, result in a lowerT0. However, boiler water temperature is fixed by the0, the increase through the scale layer is represented by theT0 to T3. The further temperature increase through the tube wall is represented3 to T4. The outside metal temperature T4 is now considerablyT2, which was the outside metal temperature0F (480 to 54O0C). At higher heat transfer rates, and in high-pressure0F (482 to0C) range, carbon steel begins to deteriorate. Figure 39.2 shows the normal0F (4270C), weakening the metal. Temperatures within the boiler furnace

Deposits may be scale, precipitated in situ on a heated surface, or previously

precipitated chemicals, often in the form of sludge. These drop out of water in

low-velocity areas, compacting to form a dense agglomerate similar to scale, but

retaining the features of the original precipitates. In the operation of most industrial

boilers, it is seldom possible to avoid formation of some type of precipitate

at some time. There are almost always some particulates in the circulating boiler

water which can deposit in low-velocity sections, such as the mud drum. The

exception would be high-purity systems, such as utility boilers, which remain relatively

free of particulates except under conditions where the system may become

temporarily upse

The second major water-related boiler problem is corrosion, the most common

example being the attack of steel by oxygen. This occurs in water supply systems,

preboiler systems, boilers, condensate return lines, and in virtually any portion

of the steam cycle where oxygen is present. Oxygen attack is accelerated by high

temperature and by low pH. A less prevalent type of corrosion is alkali attack,

which may occur in high-pressure boilers where caustic can concentrate in a local

area of steam bubble formation because of the presence of porous deposits.

Some feed water treatment chemicals, such as chelants, if not properly applied,

can corrode feed water piping, control valves, and even the boiler internals.

While the elimination of oxygen from boiler feed water is the major step in

controlling boiler corrosion, corrosion can still occur. An example is the direct

attack by steam of the boiler steel surface at elevated temperatures, according to

the following reaction:

4H

This attack can occur at steam-blanketed boiler surfaces where restricted boiler

water flow causes overheating. It may also occur in superheater tubes subjected2O + 3Fe - Fe3O4 + 4H2t (1)

water salts, such as silica and sodium compounds; or it may be caused by foaming.

Carryover is most often a mechanical problem, and the chemicals found in

the steam are those originally present in the boiler water, plus the volatile components

that distill from the boiler even in the absence of spray.

There are three basic means for keeping these major problems under control.

1.

before it enters the boiler, to reduce or eliminate chemicals (such as hardness

or silica), gases or solids.

2.

or condensate with corrective chemicals.

3.

bleeding off a portion of the water from the boiler.External treatment: Treatment of water—makeup, condensate, or both,Internal treatment: Treatment of the boiler feed water, boiler water, steam,Slowdown: Control of the concentration of chemicals in the boiler water by

CORROSION AND SCALE CONTROL,FOULING CONTROL,MICROBIAL CONTROL

Corrosion in recirculating cooling water systems is controlled by employing either

inorganic or organic inhibitors. The four major inorganic inhibitors are chromate,

zinc, orthophosphate, and polyphosphate. Minor supplements include molybdate,

nitrite, nitrate, various organic nitrogen compounds, silicate, and natural

organics.

The earliest chemicals for treating recirculating cooling waters were inorganic

polyphosphates and natural organic materials. The concept was to add a small

amount of acid to control the stability index to a slightly scale-forming value.

Organic corrosion inhibitors include organic phosphorus compounds, specific

synthetic polymers, organic nitrogen compounds, and long-chain carboxylic

acids.

Polyphosphate and natural organic materials were added to the program to

provide both corrosion protection and scale inhibition. The scale inhibition

stemmed from the use of the polyphosphate as a threshold treatment. In addition,

the polyphosphate combined with calcium to form a cathodic inhibitor that

reduced the corrosion rate. The natural organic material tended to keep the metal

surface relatively clean and aid the inhibitor in establishing a protective film. It

also dispersed suspended solids, and modified calcium carbonate and tricalcium

phosphate precipitates if they tended to develop on hot surfaces.

The greatest disadvantage of this treatment approach is the reversion of polyphosphate

to orthosphosphate, which can combine with calcium to form calcium

phosphate scale. For this reason, this type of program has evolved into the stabilized

phosphate program. In this treatment, both ortho- and polyphosphate are

used as corrosion inhibitors. To prevent calcium phosphate deposition, the pH i

generally controlled at 7.0 and specific synthetic polymers are added to disperse

and stabilize calcium phosphate.

The next cooling water treatment was chromate, an exceptionally reliable corrosion

inhibitor. Initially, chromate was applied at very high dosages, frequently

in the range of 200 to 300 mg/L as CrO

the pH to between 6 and 7, preventing calcium carbonate from precipitating. This

treatment was quite effective in both scale inhibition and corrosion protection,

but one shortcoming was that pitting attack tended to occur if the chromate residual

became low. It was found that if chromate were combined with other inhibitors,

particularly cathodic types (e.g., zinc and polyphosphate), the chromate level

could be reduced to 20 to 30 mg/L CrO

to 300 mg/L CrO

acid, frequently controlling the pH to 6 to 7. An additional advantage of synergized

chromate was the margin of safety provided against pitting attack should

the chromate be momentarily underfed.

These synergized chromate formulations are still considered among the best

corrosion inhibitors in use today. However, increasing environmental pressures

are forcing the development of innovative synergized chromate formulations that

permit carrying chromate levels in a recirculating system substantially below 10

mg/L CrO

achieve results with this approach the system pH must be controlled precisely,

and dispersants and biocides used to keep the system clean. An obvious limitation

to this approach is that the reservoir of protection available with the higher CrO

4. Acid was added to the system to lower4 with better results than obtained at 2004 used alone. The synergized chromate approach also employed4 while continuing to provide acceptable corrosion protection. To4

levels does not exist. Therefore, process contamination, uncontrolled microbial

activity, fouling, and deposition will disrupt the system much more quickly than

at the more traditional 20 to 30 mg/L CrO

Although chromate has done an outstanding job for years, increasing environmental

concerns have brought pressure on research into new corrosion inhibitors

with potentially less environmental impact. An early result of such research was

the development of organozinc combinations. Since zinc, a cathodic inhibitor,

has a lower film strength than chromate, the pH of the system for an organozinc

program was increased to between 7 and 8 to make the water less corrosive, allowing

the zinc to form a satisfactory inhibitor barrier. The organic portion of the

treatment was a dispersant to keep the system free of deposits, thereby encouraging

formation of an adequate zinc film. In addition to dispersancy, certain types

of organics increased zinc solubility at the higher pH required for this method of

treatment. These programs were adequate in many industrial plants, but because

the inhibitor film at the operating pH was not as effective as a chromate film, these

programs did not substantially replace traditional chromate-type treatments.

Subsequently, an innovative concept in cooling water chemistry arrived with

the introduction of organophosphorus compounds. Like inorganic polyphosphates,

these prevent scale formation by the threshold effect. However, there the

similarity ends; inorganic polyphosphates easily revert to orthophosphates, with

increasing holding time, temperature, and microbiological attack. Organophosphorus

compounds do not revert under normal cooling tower conditions except

under severe microbiological attack. Further, unlike the inorganic polyphosphates,

the organophosphorus compounds are generally able to inhibit precipitation

of calcium carbonate and other scale-forming species at a higher pH and alkalinity

than tolerated by the inorganic polyphosphates. This development opened

the door to what is now known as the alkaline approach to treating cooling water

systems.

The basic treatment concept is to raise the pH of the operating system to 7.5

to 9.0, thereby substantially reducing the natural corrosivity of the recirculating

water. Experience has shown that although the higher pH provides a less corrosive

water, frequently this reduction is not of sufficient magnitude to protect all mild

steel systems, especially mild steel heat exchangers with high heat flux or low flow

velocities. Thus a specific all-organic inhibitor package is required to control corrosion

and scale. In general, all-organic inhibitors combine organic phosphorus

compounds, synthetic polymers, and aromatic azoles. These combinations provide

corrosion control for steel and copper alloys, scale control, and deposit

control.

Another approach to alkaline treatment involves the use of modern scale and

deposit control agents along with more traditional corrosion inhibitors. Organic

phosphorus compounds and polymers can be supplemented with inorganics like

chromate or zinc. These programs can provide the performance of an all-organic

program at a lower cost, where chromate or zinc can be used.

The significant advantage provided by alkaline operation over earlier treatments

is the buffer capacity provided by the water that reduces the impact of system

upsets on performance. Another particular advantage of the alkaline concept

of treatment is the substantial reduction or occasional elimination of acid feed.

This, of course, depends on the chemistry of the system.

Deposit control in cooling water systems is absolutely essential for maintenance

of heat transfer rates. However, control of deposits is often more difficult in alkaline

systems than in lower pH systems. The makeup water may contain dissolved

solids, organic matter, and suspended solids, any of which can contribute to fouling.

A system may become grossly contaminated with microbes; for example,

makeup water with a high BOD, such as a recycled municipal or industrial

effluent, is particularly susceptible to fouling from slime-forming bacteria.

Table 38.5 shows some sources of foulants in a typical recirculating system.

The raw water and air inoculate a system with colloidal organic matter, silt, soluble

iron, and microbes. Hydrogen sulfide, sulfur dioxide, and ammonia may

enter from the plant atmosphere.

The selection of the proper dispersant for any operating system is based on

actual analysis of a deposit. Synthetic organics, including polymers and surfaceactive

agents, are generally applied for dispersing microbial and organic depos

Synthetic polymers such as polyacrylates or polyacrylamides are dispersants for

silt, sand, iron, and other inorganic deposits. These polymers can be tailor-made

by varying the components and molecular weights to maximize dispersant performance

on specific foulants. Organophosphorus compounds, including polyol

esters and phosphonates, are inhibitors for calcium carbonate and calcium sulfate

precipitates. However, once deposits form, any scale removing action by these

dispersants takes place slowly, so the best approach is to prevent the scale from

forming in the first place.

MICROBIAL CONTROL

Microbial deposits present a special case of fouling. Treatment often requires biocides

to kill microbe colonies and dispersants to loosen and wash them away. The

most common biocide employed in all systems is chlorine. In general, chlorine is

the only biocide required in most systems. If applied continuously at a residual

of 0.2 to 0.4 ppm it will provide effective control at all cooling water pH values.

At alkaline pH, the continuous presence of chlorine species in the water will provide

the required microbial killing power because of the infinite contact time

available. In intermittent chlorination, such as utility cooling systems, the chlorine

contacts the microbial organisms for short periods of time. In this case pH

can be more important. Sterilization studies have shown that chlorine kills faster

at pH 7 than above pH 8. This may be due to the greater amount of HOCl present

in the hypochlorite equilibrium at pH 7. Thus slug chlorination may be more

effective at neutral pH because HOCl has a faster killing power than OC1~.

There are problems associated with the use of chlorine. It can react with some

organic materials, particularly phenolic compounds, to form reaction products

that are nonbiodegradable or refractory, presenting potential effluent problems.

Generally speaking, chlorine can be applied to most recirculating systems without

danger of tower lumber delignification if free chlorine residuals do not exceed 1

mg/L. It is seldom necessary to continually carry a free chlorine residual over 0.2

to 0.3 mg/L to control microbial growths in most systems. Bromine is often a

more practical treatment than chlorine because it remains effective at higher pH

values and avoids formation of the kinds of halogenated by-products resulting

from chlorination.

Although chlorine and bromine are excellent killing agents, their performance

can be significantly improved by the use of biodispersants. Biodispersants aid the

toxicant by breaking loose the biofilms and enabling them to contact more microbial

organisms. In cases of gross contamination or loss of toxicant feed, a contingency

nonoxidizing biocide may be required (See Chapter 22).

4 levels.

COOLING WATER TREATMENT

Most of the water employed for industrial purposes is used for cooling a product

or process. The availability of water in most industrialized areas and its high heat

capacity have made water the favored heat transfer medium in industrial and

utility type applications. Direct air cooling is finding increasing use, particularly

in water-short areas but is still far behind water in total numbers of applications

and total heat transfer loading.

During recent years, the use of water for cooling has come under increasing

scrutiny from both environmental and conservational points of view and as a

result, cooling water use patterns are changing and will continue to do so. For

example, many systems pass cooling water through the plant system only once

and return it to the watershed. This creates a high water withdrawal rate and adds

heat to the receiving stream. On the other hand, cooling towers permit reusing

water to such a large extent that most modern evaporative cooling systems reduce

stream withdrawal rates by over 90% compared to once-through cooling. This

substantially reduces the heat input to the stream but not to the environment,

since the heat is transferred to the air.

These changes in cooling water system design and operation have a profound

impact on the chemistry of water as it influences corrosion, deposition, and fouling

potential in the system. This chapter reviews the industrial operations which

use water for cooling purposes, the problems of corrosion, scale, and fouling in

these systems and how these problems affect plant production through loss of heat

transfer, equipment failures, or both. In addition, various cooling water treatment

concepts are examined and the control procedures required for their success are

discussed.

HEATTRANSFER

Heat transfer is simply the movement of heat from one body to another, the hotter

being the source and the cooler the receiver. In cooling water systems, the

product or process being cooled is the source and cooling water the receiver.

Cooling water usually does not contact the source directly; the materials are

usually both fluids, separated by a barrier that is a good conductor of heat, usually

a metal. The barrier that allows heat to pass from the source to the receiver is

called the heat transfer surface, and the assembly of barriers in a containment

vessel is a heat exchanger.

In many industrial heat exchangers both the source and receiver are liquids. If

the source is steam or other vapor that is liquefied, the heat exchanger is called a

condenser; if the receiver is a liquid that is vaporized, the exchanger is called an

evaporator.

The simplest type of heat exchanger consists of a tube or pipe located concentrically

inside another—the shell. This is called a double pipe exchanger (Figure

38.1). In this simple exchanger, process liquid flows through the inner tube

and cooling water through the annulus between the tubes. Heat flows across the

metal wall separating the fluids. Since both fluids pass through the exchanger only

once, the arrangement is called a single-pass heat exchanger. If both liquids flow

in the same direction, the exchanger is parallel or cocurrent flow; if they move in

opposite directions, the exchanger is a countercurrent type.

Progressing from this exchanger, more sophisticated units are designed to

improve the efficiency of the heat exchange process. Figure 38.2 shows a shelland-

tube exchanger. Process fluid and cooling water could be located on either

side of the barrier.

Another simple heat exchange device is the jacketed vessel, with cooling water

passing through the space between the double walls of a chemical reaction vessel,

removing heat from the process. This design is like a thermos bottle, but in this

case, the double wall is used for heat removal instead of insulation. Plate-type

heat exchangers, somewhat resembling plate-and-frame niters, are used in many

chemical process industries because of their compact design and availability in a

wide range of materials of construction.

Removing Undesirable Heat

Once the water completes its job and cools the source, it contains heat that must

be dissipated. This is accomplished by transferring heat to the environment. In

once-through systems cool water is withdrawn, heated, and returned to a receiving

stream, which subsequently becomes warmer. In this type of system each

pound (0.454 kg) of cooling water is heated I

removed from the source.

In open recirculating systems, water is evaporated; this phase change from liquid

to gas discharges heat to the atmosphere instead of to a stream. Evaporating

water dissipates about 1000 Btu per pound (555 cal/kg) of water converted to

vapor. When evaporation is used in the cooling process, it can dissipate 50 to 100

times more heat to the environment per unit of water than a nonevaporative system.

(This is explained in more detail in a later section of this chapter.)

0F (0.560C) for each Btu (0.252 cal)

Sensible Heat Transfer

The two most common ways heat is transferred from process fluid to cooling

water in the heat exchange process are conduction and convection. Heat flows

from a hot fluid through a heat exchange surface to the other side by conduction.

Heat is then removed from this hot surface by direct contact with cooling water

i.e., by conduction. Subsequently this heated water then mixes with other cooler

water in a heat transfer process called convection.

The five factors controlling conductive heat transfer are:

1. The heat transfer characteristics (thermal conductivity) of the barrier.

2. The thickness of the heat transfer barrier.

3. The surface area of the barrier.

4. The temperature difference between the source and the cooling water (the driving

force).

5. Insulating deposits on either side of the barrier.

Of these five factors, the first three are inherent in the design of the exchanger.

Items 4 and 5 are operational characteristics that change depending on the conditions

of service. Deposits on either side of a metal barrier have a lower thermal

start up and shut down operations for R O system

As the start up and shut down operations may result in some problem when not performed properly, we would like to bring your attention to the recommended procedures as below.

Start-up procedure

Before starting up an RO system, it should be verified that all pretreatment systems ar eworking according to their specifications. It may be necessary to take water samples for analysis. In the case of polyamide (thin film composite) membranes free chlorine must be 0.0 ppm. The Silt Density Index (SDI) should be according to the RO design guidelines(typically < 5.0). On startup, the inlet valve should open prior to the initiation of the high-pressure pump, to completely fill the system with low pressure water (<100 psi [< 7 Bars]). This “soft start” will prevent hydraulic shock at startup. Pre-treatment chemical addition should begin at this time (making sure the chemicals are not over-injected). The high-pressure pump should then be started and the system slowly bought on-line, up to design permeate flow. If starting up after a period of shutdown, flush the permeate to drain for 30 minutes to remove residual preservation chemicals. Produced water permeate can be used when it meets the qualityrequirement of downstream processes.

• Check all valve settings are correct.

• Ensure that all air is flushed out at low feed pressure.

• Check pipe connections are tight.

• Close the feed pressure control valve.

• Reject valve is fully open.

• Start feed or HP pump.

• Slowly open feed control valve. Ensure that max feed flow per vessel is not exceeded.

• Slowly close reject valve.

• Check product flow.

• Fine tune the feed and reject control valves.

• Check the chemical addition, feed pH, LSI and the SDI.

Sayfa / Page
2/5

•

• Take the first reading of all operating parameters. This is important for future normalisation calculations.

• Measure conductivity of each vessel and identify out of spec performance and take necessary action.

• Take water samples and analyse.

• Compare performance with prediction.

• Check operation of safety devices.

• Lock the plant in automatic operation.

• It is recommended that all the operating parameters are measured several times in the first 48 hours.

Let the system stabilise for at least 1 hour.

Shut down procedure

Permeate Flush

As salts in the feed water have concentrated up and exceeded their solubility during operation, they should be rinsed out prior to any shutdown (>15 minutes). Rinsing of the membranes with permeate water on shut will also aid the flushing of colloids and bacteria from the membrane surface.Flow rate during flushing should be based on the recommended cleaning instruction flowrates. This is normally 30 – 40 gpm [6.8 – 9.1 m3 /hr] per pressure vessel. Flushing time should be long enough for the conductivity out to equal the conductivity in. This is typically 15 – 20 minutes.If the permeate flush is unavailable, feed water can be used by allowing low-pressure water to replace the water within the system by delaying the inlet valve closing. Scale inhibitors hould be turned OFF during the permeate flush.If the water temperature in the membranes exceeds 115 oF, flush water should becontinuously passed through the system to prevent membrane degradation.

Sayfa / Page 3/5

•

• The feed pump and the inhibitor and acid dosing is shut down.( Keep the acid for CA)

• The feed valve is opened and the system is flushed with feed water for 10 minutes before the pretreatment train is shut down.

• The flushing tank should have been filled with permeate prior to shut down and the RO stack should be flushed with permeate water.

• The RO train can be stopped for up to 24 hours without preservation.

The start-up sequence is reversed by slowly opening the reject valve and closing the feed valve.

In case of extended shutdown

In this case, preservation with sodium-bisulfite (1%).It is recommended to measure the pH regularly. A fresh solution is needed when the pH < 3.A fresh solution is also needed when the liquid becomes turbid or changes color. Regular inspections (weekly) are recommended.

Ensure that the elements do not dry out.

RO Data Collection and Monitoring

Data collection is critical for monitoring the performance of the membrane system. Without it, there will bee no idea if the system is fouling, suffering from scale formation, or if themembranes are deteriorating.When operating data is recorded, it should be compared to previously established alert andalarm levels. These levels should be associated with well-defined response procedurescorresponding to the potential problem.The alert and alarm levels are set for a 15% change from normalized start up data.

Sayfa / Page 4/5

A good record keeping as mentioned on the table below will definitely help a lot along with chemical residual control. The data should then be normalized to extract the information.

Sayfa / Page 5/5

It should be noted that it is essential to clean membranes at an early stage of fouling. It is often difficult to clean excessively fouled membranes and irreversible damage may occur during the cleaning process.

Cleaning is recommended when on or more of the following parameters change by 10 – 15% after data normalization:

• An increase in product water conductivity or salt passage

• An increase in

• An increase in feed pressure

• A decrease in normalized permeate flow (NPF) output or flux.

If any of the above performance parameters deteriorates by more than 30%, it maybe impossible to recover plant performance by routine cleaning practices. Cleaning is not expensive and requires little amount of chemicals. As an example, a 1000m3/day plant would require a total cleaning solution of around 4000 liters. ΔP across the plant

•

• The system is adequately protected or is flushed every 24 hours.

• The system is protected from extreme temperature conditions.

Procedure for boiler cleaning

.Close main isolating valves and completelydrain the  unit.

.Fill with water 90% and add  Ferrolin8623 10%.
.Start heating upto to40
.Solution pH dropsto1–2.
rt circulationand add 0.1% of dispersantin to solution. (P3 Ferrofos8461)
.Circulatethesolutionfor3-6hours minimum and check pH and total iron content frequently. If pH arises drasticly add cleaning chemical until pH reachesto3 –4 again. You may need to repeat this step if pH increase is observed.(oC.P.S. Additiono f chemicals and circulationtime dependson depositin the boiler).If total iron reaches to 8000 ppm, drainthe system and repeat steps2, 3& 4.
.Check the pH, if pH does not increase after adding further fresh solution of  Ferrolin8623, the cleaning process is complete and finished.
.Drain cleaning solutionton eutralizationtank of wastetreatment. Drain cleaning solution to neutralizatio ntank of wastetreatment.
1Rinse the system with warm water
.Drainrinsewater

.Fill with water and prepareneutralization solution (causticsoda)up to pH 8.(Alternatively you may use Na3PO4 as 3000 ppm and heat), recirculate about 2-4 hours.