CLEANING METHODS/طرق التنظيف الكيميائى لازالة القشور والاملاح والرواسب

REMEDIAL CLEANING PROCEDURES.

Cleaning procedures presented herein are general in nature and must be modified to fit specific applications.

Because contractors perform most cleanings, these procedures are provided only for general information purposes only.
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2.1 CLEANING METHODS

2.1.1

MECHANICAL METHODS.

Mechanical methods are the oldest techniques used for removing deposits.

To perform an adequate mechanical-type cleaning, the equipment to be cleaned may need to be partially or entirely dismantled.

Even when equipment is dismantled, some areas may be extremely difficult to reach and clean.

Chemical cleaning has largely replaced mechanical process equipment cleaning as the most satisfactory method of removing deposits; however, mechanical methods such as wire brushing, tumbling, scraping, and abrasive blasting with sand and grit are still employed in special applications.

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2.1.2

CLEANING AGENTS.

Cleaning agents may be broadly classified as being acid, alkaline, organic, or solvent cleaners.

There is no general or universal cleaner that removes all deposits.

The selection of a solvent or cleaning agent is based on the material's ability to remove or dissolve the deposit, as well as on cost considerations, safety hazards, and the effect of the cleaning material on the metals involved.
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2.1.3

GENERAL GUIDANCE AND PROCEDURES FOR PREPARING CLEANING SOLUTIONS.

General guidance and procedures for preparing cleaning solutions of inhibited hydrochloric (muriatic) acid and inhibited sulfamic acid are provided elsewhere.

Inhibited acid contains special chemical inhibitors that prevent the acid cleaner from attacking the base metal while allowing the acid to remove the unwanted corrosion product or scale deposit.

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2.2

HYDROCHLORIC (MURIATIC) ACID.

Inhibited hydrochloric (muriatic) acid in strengths of 5 to 20% is very effective for removing calcium scale and iron oxide; however, for most applications, a 10% solution is adequate.

The following formulation is for a 10% hydrochloric acid solution.

It can be used for removing scale consisting primarily of carbonates with lesser amounts of phosphates, sulfates, and silicates.

This type of scale is typically found in a steam boiler system containing copper alloys that has been treated with a phosphate-based program.

Depending on the specific descaling application, some of these ingredients can be omitted from the formulation.

For example, diethylthiourea is not needed if there is no copper in the system.

It should be noted that if diethylthiourea is used, the waste material should be treated as a hazardous waste.

Where there is only carbonate scale to be removed, ammonium bifluoride, which is used to remove silica-based scales, may be omitted. The addition of a wetting agent is preferable but not absolutely necessary.
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2.2.1

EXAMPLE PROCEDURE FOR 10% SOLUTION.

The following is an example procedure that can be used to make 3785 liters (1000 gallons) of a 10% solution:

1.

Add 1079 liters (285 gallons) concentrated (36% strength) hydrochloric acid, ,

Specification for Muriatic Acid (Technical Grade Hydrochloric Acid), to approximately 2271 liters (600 gallons) of water.

2.

Add the proper amount of a corrosion inhibitor, Specification MIL-I-17433, Inhibitor, Hydrochloric Acid, Descaling and Pickling, recommended by the manufacturer to the diluted acid solution.

The inhibitor must be compatible with hydrochloric acid and must not precipitate under any condition during the cleaning operation.

3.

In a separate tank containing about 284 liters (75 gallons) of water:

a) Add 39 kilograms (85 pounds) of the chemical (1,3) diethylthiourea to complex any copper and keep it from depositing.

Do not use the diethylthiourea as the corrosion inhibitor required in paragraph 9-2.2.1(step 2) above.

b) Add 55 kilograms (120 pounds) of ammonium bifluoride, technical grade, to help dissolve certain iron and silica scales.

c) Add 3.79 liters (1 gallon) of wetting agent, Specification MIL-D-16791, Detergents, General Purpose (Liquid, Nonionic).

d) Add the dissolved diethylthiourea, ammonium bifluoride, and wetting agent to the diluted acid solution. Add sufficient water to obtain 3785 liters (1000 gallons).
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2.2.2

CARBONATE DEPOSITS.

Carbonate deposits dissolve rapidly in hydrochloric acid, with evolution of free carbon dioxide.

The escaping carbon dioxide tends to create some circulation or agitation of the acid, which ensures the continual contact of fresh acid with the scale.

Once the carbonate has been dissolved from a mixed deposit, a loose, porous structure may be left behind. This residual material can be effectively removed from the equipment either mechanically or by washing with high-pressure water.
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2.2.3

PHOSPHATE DEPOSITS.

The removal of phosphate deposits can usually be accomplished by using hydrochloric acid; however, phosphate deposits have a tendency to dissolve rather slowly.

To minimize the total cleaning time, a temperature of 49 to 60 °C (120 to 140 °F) is usually necessary to remove a predominantly phosphate scale.

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2.2.4

METALLIC OXIDES.

Most metallic oxides found in deposits can be removed with hydrochloric acid.

The rate of dissolution is a function of temperature and solution velocity.

If copper oxides are present on steel surfaces, special precautions are needed to prevent copper metal plate-out on the steel.
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2.2.5

SILICA AND SULFATE SCALE.

Heavy silica and sulfate scale is almost impossible to remove with hydrochloric acid. Special chemicals and procedures are required to remove this scale.

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2.2.6

HYDROCHLORIC ACID LIMITATIONS.

Hydrochloric acid is not used to clean stainless steel because the chloride ion in the acid solution may cause pitting or stress corrosion cracking.

Hydrochloric acid is not used for removing scale from galvanized steel surfaces since the galvanizing will corrode. Aluminum is not cleaned using hydrochloric acid.

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2.3

SULFAMIC ACID.

Sulfamic acid is an odorless, white, crystalline solid organic acid that is readily soluble in water.

An inhibited sulfamic acid compound, in a dry powder form, is available under Specification MIL-B-24155, Boiler Scale Removing Compound.

A 5 to 20% solution (2 to 9 kilograms to approximately 38 liters of water [5 to 20 pounds to approximately 10 gallons of water]) is used for removing scale from metal surfaces.

The following information pertaining to sulfamic acid should be considered:

• Carbonate deposits are dissolved in sulfamic acid in a similar manner as in hydrochloric acid. All the common sulfamate salts (including calcium) are very soluble in water.

• The dry powder form of sulfamic acid is safer to handle than a liquid solution of hydrochloric acid; however, aqueous solutions of sulfamic acid are much slower in action and require heating to remove scale.

The sulfamic acid solution is heated to a temperature in the range of 54 to 71 °C (130 to 160 °F) to obtain the same fast cleaning time that is achieved by using hydrochloric acid at room temperature. Sulfamic acid is more effective on sulfate scale than hydrochloric acid.

• Inhibited sulfamic acid, used at temperatures up to 43 °C (110 °F), will not corrode galvanized steel.

Its use is recommended for removing scale in cooling towers, evaporative condensers, and other equipment containing galvanized steel.

In general, sulfamic acid can be applied to equipment while it is operating but should be drained from the system after a few hours, and the concentration of the normally used corrosion inhibitor should be increased several-fold to protect the metal surfaces.

• Commercially prepared descaling compounds consisting of concentrated or diluted inhibited acid (containing 7 to 28% of the acid and inhibitor) may be purchased under various trade names at prices 4 to 30 times the cost of the ingredients themselves if purchased as generic chemicals.

• Advertisements of some of these products may contain claims that cotton clothing and skin are not attacked by the acid.

These claims are usually based on a very dilute solution of the acid that causes a minimal attack on clothes and skin; however, the cost of the cleaning process may be increased because a higher quantity of dilute product may be needed. Be aware that handling acid in any strength must be performed with considerable care, caution, and adherence to safety procedures.

• The cost of diluted acid is expensive; therefore, concentrated acid of government specifications should be purchased and diluted to usable strengths.

The necessary corrosion inhibitors can be added to the dilute acid solution.

Users of small quantities of acid cleaners (possibly less than 38 liters [10 gallons] of diluted acid per year) may not be able to justify purchasing undiluted acid and spending the time, cost, and effort to prepare the cleaning solution; therefore, consider the specific requirements before ordering.

2.4
CLEANING PREPARATION.

The unit to be cleaned must be isolated from other parts of the system.

For systems that cannot be isolated by the closing of valves, isolation may be accomplished using rubber blankets, wooden bulkheads with seals, inflatable nylon or rubber bags, rubber sponge-covered plugs, or blind flanges and steel plates with rubber seals.

Long lines may require auxiliary connections for chemical cleaning.

The following information should be considered before the cleaning process is started:

• Decide whether to clean using a soaking process or by circulating the cleaning.

In either case, temporary piping or hose lines will be required to connect the cleaning solution mixing tanks or trucks to the unit, with return lines to tanks or drains.

Proper precautions and adequate provisions must be made to protect equipment, isolate control lines, replace liquid level sight glasses with expendable materials, and provide suitable points for checking temperatures.

It may be necessary to remove selected system components if the cleaning process might damage them.

• The entire cleaning procedure/process must be developed in detail before starting chemical cleaning operations.

Factors to be considered include: the methods for controlling temperatures; the means of mixing, heating, and circulating the chemical solution; proper venting of dangerous gases from equipment to a safe area; and means for draining, filling, and flushing under inert atmospheres. Sampling points, test procedures, and control limits should also be established.

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2.5

METHODS FOR REMOVING SCALE.

Removing scale may be accomplished by circulating the inhibited acid solution through the equipment or by soaking the equipment in a tank of inhibited acid.

Before starting any descaling process, check the acid to make sure it is properly inhibited.

You may check the acid by placing a mild steel coupon into a beaker containing the prepared, diluted acid. You should notice no reaction around the coupon.

If you observe a reaction generating hydrogen gas bubbles around the coupon, add more inhibitor.
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2.5.1

RECIRCULATING CLEANING PROCESS FOR BOILERS.

The following example is an appropriate procedure for cleaning small boilers or other systems using a hot recirculating inhibited acid solution:

1. Fill the boiler or system with preheated (71 to 77 °C [160 to 170 °F]) dilute inhibited acid solution.

2. Allow the dilute inhibited acid solution to remain in place for 8 hours. Circulate the acid solution for approximately 15 minutes each hour at a rate of about 3.15 liters per second (50 gallons per minute) to ensure good mixing.

3. Keep the temperature of the acid solution preheated at 71 to 77 °C (160 to 170 °F). Measure and record the temperature at least once every 30 minutes.

4. Check and record the acid strength at least every hour (see paragraph 9-2.6).

5. Drain the system by forcing the acid solution out using 276 to 345 kilopascals (40 to 50 pounds per square inch gauge) nitrogen; follow Federal Specification A-A-59503, Nitrogen, Technical, Class 1. If leaks develop when the system is under nitrogen pressure, you must use an alternate method for removing the acid, such as pumping.

6. Fill the boiler with preheated (65 to 71 °C [150 to 160 °F]) water and soak at this temperature for 15 minutes.

7. Drain under nitrogen pressure of 276 to 345 kilopascals (40 to 50 pounds per square inch gauge).

8. Prepare this mild, acid-rinse solution: Add 7.57 liters (2 gallons) of hydrochloric acid (ASTM E 1146) for each 3785 liters (1000 gallons) of water. Also add corrosion inhibitor, Specification MIL-1-17433, in the amount recommended by the manufacturer.

9. Fill the boiler with the preheated (71 to 77 °C [160 to 170 °F]) mild acid-rinse solution and soak for 30 minutes.

10. Drain the mild acid-rinse solution under nitrogen pressure at 276 to 345 kilopascals (40 to 50 pounds per square inch gauge). Maintain a positive pressure of nitrogen in the boiler to prevent outside air from leaking inside.

11. Prepare this passivating solution: To each 3785 liters (1000 gallons) of distilled water (or other water with less than 50 ppm total hardness [as CaCO3]), add 36 kilograms (80 pounds) of passivation compound 0.5% by weight sodium nitrite and 0.25% by weight monosodium phosphate.

12. Fill the boiler with the passivating solution preheated to 65 to 71 °C (150 to 160 °F), circulate for 10 minutes, and hold in the boiler at 65 to 71 °C for an additional 30 minutes.

13. Drain and rinse boiler until the pH of the rinse water is pH 8 to 10.

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2.5.2

CIRCULATING METHOD WITHOUT HEAT.

The steps below describe a typical process for descaling smaller equipment, such as enclosed vessels or hot water heater coils, without heating the inhibited acid solution:

1. Note that an acid cleaning assembly may consist of a small cart on which is mounted a pump and an 18.9- to 189-liter (5- to 50-gallon) steel or polyethylene tank with a bottom outlet to the pump.

2. Install sill cocks at the bottom of the water inlet of the heat exchanger and the top of the water outlet so that a return line can be connected directly from the acid pump and from the heat exchanger to the acid tank.

3. Prepare an inhibited acid cleaning solution.

4. Pump the acid solution into the heat exchanger through the hose connection. Continue circulation until the reaction is complete, as indicated by foam subsidence or acid depletion.

5. If the scale is not completely removed, check the acid strength in the system. If the acid strength is less than 3%, add fresh acid solution and continue circulation until the remaining scale is removed. Usually an hour of circulation is adequate.

6. Drain the heat exchanger.

7. Neutralize remaining acid by circulating a 1% sodium carbonate (soda ash) solution (about 3.6 kilograms per 38 liters [8 pounds per 100 gallons]) for about 10 minutes.

8. Rinse thoroughly with water until the pH of the rinse water is pH 8 to 10.
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2.5.3

FILL AND SOAK METHOD

1. Prepare an inhibited dilute acid solution (see paragraphs 9-2.2 and 9-2.3) in a container of suitable size.

2. Depending on the item to be cleaned and the types of scale involved, you may want to place an agitator (mixer) in the tank or install a pump outside the tank to circulate the acid solution. A method to heat the acid may be required, such as a steam coil.

All equipment must be explosion-proof and acid-resistant.

3. Immerse the item to be cleaned in the dilute acid solution. Continue soaking until the reaction is complete as indicated by foam subsidence or acid depletion.

4. If the scale is not completely removed, check the acid strength (see paragraph 9-2.6). If it is less than 3%, add additional acid and continue soaking the items until the remaining scale is dissolved. Usually 1 to 2 hours of soaking is adequate.

5. Remove item from tank.

6. To neutralize remaining acid, immerse the item in a 1% sodium carbonate (soda ash) solution (about 3.6 kilograms per 38 liters [8 pounds per 100 gallons]) for 2 to 3 minutes.

7. Rinse the item thoroughly with water.

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2.6

CHECKING ACID SOLUTION STRENGTH.

The initial strength of the dilute inhibited acid will vary from 5 to 20%, although 10% is typical.

As the acid is consumed by dissolving the scale, the strength of the acid decreases.

The strength of the acid solution should be measured periodically during a cleaning operation.

When the acid strength falls below 3%, the solution may be discarded since most of its scale-dissolving capability will have been used.

Use the following procedure to check the acid strength:
Apparatus:

Burette, 25 milliliters (0.8 ounce) automatic (for sodium hydroxide solution)
Stirring rod

Bottle, with dropper, 50 milliliters (2 ounces) (for phenolphthalein indicator solution)
Graduated cylinder, 10 milliliters (0.3 ounce
)
Casserole, porcelain, heavy duty, 210-milliliter (7.1-ounce) capacity

Reagents:

Sodium hydroxide solution, 1.0 normality (N)

Phenolphthalein indicator solution, 0.5%

Method:

1. Measure 10 milliliters of acid solution accurately in the graduated cylinder.

2. Pour into the casserole.

3. Add 2 to 4 drops of phenolphthalein indicator solution to the casserole and stir.

4. Fill the automatic burette with the 1.0 N sodium hydroxide solution; allow the excess to drain back into the bottle.

5. While stirring the acid solution constantly, add sodium hydroxide solution from the burette to the casserole until color changes to a permanent faint pink. This is the endpoint. Read the burette to the nearest 0.1 milliliter (0.003 ounce).

Results:

1. For hydrochloric acid:

Percent hydrochloric acid = milliliter of 1.0 N sodium hydroxide x 0.36

2. For sulfamic acid:

Percent sulfamic acid = milliliter of 1.0 N sodium hydroxide x 0.97

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CLEANING METHODS/طرق التنظيف الكيميائى لازالة القشور والاملاح والرواسب Intro112

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CLEANING METHODS/طرق التنظيف الكيميائى لازالة القشور والاملاح والرواسب Uoo134

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