J USTIFICACIÓN

Offstream cleaning requires that at least a part of the plant be shutdown during the operation.

Since downtime costs money, there is a lot of incentive to perform as much cleaning as is possible without shutdowns. Several onstream techniques have been developed as follows:

Acid Slugging with HCl

This method is effective to remove scales soluble in HCl. The treatment consists of injecting inhibited acid into the cooling water at relatively low concentration for a short time period while the cooler remains in service.

If the fouling is predominantly scale the cooling water flow is reduced and the acid

concentration is kept about 0.5% acid in the effluent for hardness deposits or about 3% for iron oxides. If the fouling contains appreciable quantities of mud and weed, the cooling flow is maximized and acid concentration reduced. Acid slugging is a rapid method of cleaning, and most bundles can be cleaned in 20 minutes to 1 hour. Preparation work is minimum and often consists of providing only 1½" valve connection at a cooler inlet. Sampling connections and temperature monitors are required for good control of the cleaning.

In general, acid slugging cleans once-through cooling systems, particularly where salt water is used as the cooling medium. Onstream cleaning has been extended to include recirculated systems, but the effluent acid must be diverted to the sewer or neutralized to avoid upsetting the cooling tower operation.

In all cases where chemical injection is used, precautions must be taken to ensure that the injected cleaning agent and dissolved fouling are not allowed to remain in the system in any form which will be incompatible with the water treating chemicals, etc. The pH for instance, must be readjusted to a normal level as soon as possible after acid injection. Inhibitor dosage should be increased to passivate metal surfaces. The materials of construction must be checked before adding any cleaning agent. Inhibited hydrochloric acid has little or no action on carbon steel or the majority of copper alloys at temperatures up to 80°C but can lead to stress cracking on 300 series stainless steels. The metal surface temperatures may be considerably higher than the temperature of the circulating water and that materials such as the cast iron in compressor jackets are liable to be attacked more readily than carbon steel.

An alternative to acid slugging for the onstream cleaning of coolers is a modification of the circulation procedure used offstream. This procedure requires care to ensure that temperature guidelines are not exceeded but has the advantage that the effluent cleaning solvents are discharged to the sewer and, therefore, will not interfere with closed circuit cooling water corrosion control. Procedure requires the installation of tees in the cooling water inlet and return lines that can be connected via hoses to conventional pump and chemical cleaning tank facilities. In addition, connections from the firewater system must be made with sewer

order to provide as large a heat sink as possible because the idea is to circulate the cleaning solution during normal cooler operation.

Firewater is used to backflush the system to the sewer and to make up the solutions because it is higher pressure and cold. Unit is circulated from bottom to top until unit is clean or solution temperature rises to 80°C. If temperature reaches 80°C, stop the chemical pump and use cooling water to displace the acid back to the tank with additional flushing to the sewer.

Ensure that process side temperatures and pressures will not exceed allowable. Units with high process temperatures will permit short contact times before solution temperatures reach the allowable maximum and should be cleaned only with extreme caution.

Sulfuric Acid for Scale Removal

Sulfuric acid, is much cheaper and more readily available, to periodically reduce the pH of the system or segment of a system to about 1.5 to 2.0 for a period of 6 to 8 hours or until the acid is no longer being consumed. At this pH the calcium carbonate hardness deposits, which are the primary foulants in the salt water system, are readily dissolved. If substantial iron oxide were present, a lower pH would be required for adequate removal.

Chemical cleaners for the offstream removal of calcium hardness deposits have not used sulfuric acid because the resultant calcium sulfate is not soluble in the acid solution and the reaction quickly stifles itself. However, calcium sulfate is soluble up to about 1500 mg/L in fresh water at 60°C. In the onstream once through treatment with dilute acid the concentration of calcium sulfate is so low that the solubility is not exceeded. However, it has been

established that calcium sulfate is more soluble in high ionic strength seawater than it is in fresh water. Therefore, the scheme is more applicable to salt water systems than to fresh water systems.

Onstream Removal of Mud, Silt and Microbiological Fouling

Onstream cleaning of an entire recirculating system has been attempted in areas where a shutdown was intolerable by reducing the pH with acid while adding polyelectrolytes.

The technique can be successful if carefully executed but is employed as a last resort. It probably will not be very effective on deposits high in iron oxide, but can be effective if iron oxide is bound by mud, silt, etc. For best results, the cleaning encompasses several days as is illustrated in the following typical procedure:

1^st day Chlorinate and /or use nonoxidizing biocide.

2^nd day

1. Shot feed polyelectrolyte to obtain desired concentration in system (usually about 25 mg/L)

2. Add dispersant continuously to compensate for that lost in blowdown 3. Decrease pH to range of 5.0 to 5.5

4. Increase blowdown to lower cycles of concentration to 2.0 5. Air bump or backflush critical heat exchangers every 2 hours 3^rd day Continue

4^th day

1. Maintain dispersant concentration at recommended level

2. Decrease pH to 4.0 for eight consecutive hours; otherwise maintain range of 5.0 to 5.5.

3. Maintain cycles of concentration to 2.0 4. Air bump or backflush, as above 5^th day

1. Discontinue feed of dispersant and raise pH to normal range 2. Decrease blowdown to normal level

6^th day Triple the feed rate of corrosion inhibitor to passivate system metals Onstream Removal of Oil from a Cooling Water Circuit

The procedure below has been found to work well in several locations in minimizing the adverse effects of process contaminants that have just leaked into the cooling water system.

The procedure is not effective for removing deposits from old or persistent leaks.

1. Locate and shut off leaking equipment immediately after leak are observed. This is a crucial step. The following techniques may be useful to locate leaks:

- Sample water from outlet of equipment, headers, gas traps, etc.

- Check loss of chlorine residual across suspect exchanger or other areas - Measure oil-in-water content daily

- Use GC analysis to identify possible sources of oil

2. If visible fouling exists clean wooden members at top distribution deck of tower with steam lances. Clean structural and demister chevrons from basin level using water hoses.

3. Skim oil at top distribution deck and at cooling tower basin. During this time:

- Change to non-chromate treatment.

- Maintain normal dosage of phosphate or other non-reducible inhibitor.

- Ensure 2 mg/L free tolyltriazole copper inhibitor if brass bundles are present.

- Add an initial 100 mg/L (as product) shock dosage of non-ionic surfactant (polysiloxane and polyoxyalkylene, fatty acid alkylamines, or polyoxylated ethylene and alkoxylated phenol).

- Maintain normal blow down rate.

- Chlorinate continuously to 0.2-0.5 mg/L free residual (if attainable). Evaluate impact of the quantity chlorine fed on carbon steel and brass corrosion

monitoring devices and on recirculating water pH depression.

- Add biocide (chloromethylsulphone or equivalent) for sulfate reducing bacteria twice per week.

- Perform oil-in-water and chlorine residual daily.

4. Blowdown system for 3 - 4 days at highest rate possible and at as many different points as possible (cooling tower hot return, basin, and at pump cooling jackets, etc.):

- Operate spare pump at cooling tower to increase velocity.

- Operate oil skimmer at cooling tower pump suction bay (if available) - Do not add surfactant during this period.

- Increase dosage rate of corrosion inhibitor to compensate for higher blowdown rate (maintain same concentration).

- Perform oil-in-water and chlorine residual daily.

5. Reduce blowdown to normal level and add surfactant:

- Maintain 100 mg/L of surfactant product for 4 days.

- Operate spare pump at cooling tower for 4 days.

- Chlorinate continuously to 0.2 to 0.5 mg/L free chlorine residual if attainable.

- Examine corrosion coupons (preferably located in an equipment bypass) and test equipment twice per day for oil, and continue to add dispersant until coupons clear up.

- Perform oil-in-water analysis daily.

6. Reduce blowdown to minimum possible and passivate exchanger surfaces by increasing corrosion inhibitor concentration 2.5 - 3X for 5 days.

- Maintain necessary pH to avoid any precipitation of inhibitor or waterborne salts.

- Maintain 100 mg/L surfactant product for one week.

- Continue to add biocide twice a week.

7. Return to normal operation:

- Shutdown spare pump.

- For two days per cell, shut off one cooling tower cell in sequence so that flow is maximized through other cells (to flush oil off structures).

- Maintain normal concentration of corrosion inhibitor.

- Chlorinate continuously (0.1 - 0.3 mg/L residual) or once per day to 0.5 to 1.0 mg/L free chlorine residual.

Appendix 7 – Cleaning of Twisted Tube Heat Exchangers

Due to the congested configuration of the twisted tube heat exchanger bundles, normal hydrojetting does not have the capability to reaching the external surface of the inner tubes.

Therefore, a special cleaning technique shall be used that will remove scales from the tube bundles safely and effectively. The new technique utilizes a combination of chemical and mechanical cleaning procedures.

Scope:

This procedure outlines the cleaning method used to clean bundles of twisted tube heat exchangers. The technique utilizes a combination of chemical (vessel) bath and high pressure jetting. The use of high pressure water jetting is covered in Appendix 5.

Advice is available from Materials Engineering & Corrosion Operations Division/Consulting Services Department (ME&COD/CSD) to review cleaning procedures and evaluate alternative cleaning procedures.

Preparations for Chemical Cleaning:

The general preparations for cleaning is covered and controlled by Section 6.

1. Pull the bundle out from the heat exchanger housing.

2. Use high pressure water jetting to clean as much of the loose scale as possible, see Appendix 5.

3. Prepare a chemical cleaning bath. The cleaning bath should be big enough for the heat exchanger bundle to be submerged completely into the chemical cleaning solution.

Chemicals used should be chosen based on the metallurgy of the heat exchanger tubes and the type of scale that needs to be removed. For chemical selection, see Section 8.

4. Section 9 controls the cleaning parameters of the selected cleaning chemical.

5. Heat the solution to the desired temperature, and maintain this temperature throughout the cleaning operation using an external heating source.

6. Start circulating the solution, from one end of the vessel to the other end, using external pumps. Note: use as much return nuzzles as possible and locate them at different location in the cleaning bath to enhance the cleaning action.

7. Using a crane, lift the bundle in a slight slop, and lower it into the cleaning vessel that contains the cleaning solution.

8. Monitor corrosion rate, if required, and make sure to comply with conditions mentioned in Section 10.

9. Continue submerging the bundle into the solution and circulating the cleaning solution for two (2) hours.

10. Lift the bundle with the crane and place it outside the cleaning bath and let it cool down.

11. Repeat steps 2 to 9 until satisfactory cleaning level is attained.

Note: During circulation and after lifting the bundle from the bath, gently hummer the tube bundle at different location using a small plastic or copper hammer, to help dropping any loose scale. Also, shock the system occasionally, by interrupting the flow, by shut down the pump completely and wait for five minutes then restart the pump and restore full flushing velocity of about 3 m/s

Evaluating Cleaning Performance by Inspection:

Plant Inspection, Operations, Operations Engineering and the cleaning contractor are to inspect tube bundles after the cleaning to determine that the job has been successfully completed.

Visual and video boroscope inspections are carried out to determine the effectiveness of cleaning. No visible traces of water, loose or adherent deposits inside the equipment are acceptable.

Equipment Lay-Up:

Saudi Aramco Mothball Manual, SAER-2365, should be used to protect tube bundle from corrosion right after cleaning. In general, the protection should include:

- Removal of free water - Draying with air

- Treating with corrosion inhibitor Safety Hazards:

Be aware of the safety hazards (see Appendix 10) associated with chemical cleaning and take adequate safety measures.