Chemical cleaning

I.            SCOPE

Chemical cleaning removes rust, scale, weld slag and grease from metal surfaces of equipment such as towers, exchangers, furnaces, boilers, tanks, transfer piping etc.

Equipment and piping can be chemically cleaned provided:

Ø      The material to be removed will react chemically with the solvent, or the solvent will disperse the deposit or scale;

Ø      The solvent is not excessively corrosive to the materials of construction;

Ø      The equipment being cleaned is not plugged to the point the circulation cannot be established.

Solvent used to chemically clean carbon steel surfaces will vary from inhibited mineral or organic acids to caustic detergents. Choice of solvent will depend on the type of deposit or scale which must be removed.

Chemical cleaning should be undertaken only by competent personnel with proven experience. Reliable chemical cleaning contractors are engaged for this purpose.

II            PRELIMINARY OPERATIONS

Prior to commencing chemical cleaning, a number of preliminary operations should be undertaken. These include the following:

Ø      Study optimal arrangement for circulation and control of the system and piping loops to be cleaned, including any temporary piping, jumpovers, and bypasses required. Pumps, instrumentation, and relief valves are normally removed before chemical cleaning;

Ø      Install temporary bypasses, jumpovers, special chemical cleaning fittings, adaptors and flanges as required;

Ø      Move chemical cleaning equipment into position: skid mounted pumps, circulation tanks, flexible chemical hosed, filter, etc,

Ø      Ensure that all work areas are safety fenced off;

Ø      Ensure that all personnel are acquainted with safety measures to protect human life and prevent environmental pollution.

III.          CHEMICAL CLEANING OPERATIONS:

              Chemical cleaning is carried out in the following order:

v     Flushing:

v     Degreasing:

v     Acid cleaning

v     Neutralization and passivation

v     Conservation

1)     Flushing:

All sections are flushed with water to remove loose dirt, debris and other foreign matter, and to locate leaks or areas of poor circulation.

Clean fresh water must be used. Recommended circulation velocity is 1 to 4 m/sec inside piping. Inside columns and vessels water will be sprayed in countercurrent against nitrogen of air.

Flushing is considered complete when water exiting from the system contains no suspended solids.

              2) Degreasing:

An alkaline detergent solution is circulated vigorously throughout the system, then heated to near 90⁰C to remove rolling or drawing oils, greases, varnishes and other products. This step is necessary to remove the physical barrier of organic material and to allow subsequent acid steps full and complete contact with internal surface and therefore promote solubilization of metal oxides present.

              Degreasing includes the following steps:

»       Fill system with clean fresh water;

»       Circulating water at recommended velocity of 1 to 1.2m/sec.

»       Heat circulating water to 80 to 90⁰C.;

»       Vent at all high point vents to remove air from system;

»       Check system regularly for leaks;

»       Gradually does circulating water with degreasing solution until following chemical strengths are reached:

§         Caustic soda                     0.3% by weight

§         Oil dispersant                   0.5% by weight

§         Trisodium phosphate       0.2% by weight

§         Wetting agent                   0.1% by weight

»       Maintain solution in circulation at 80 to 90⁰C for about 12hours;

»       During circulation, walk through the system and attentively check that all loops are circulating. If a loop lacks correct flow and has lower temperature, readjust valves accordingly;

»       Measure flow rates and percentage of alkalinity every hour and record values in job log book;

»       After system is degreased, prepared to drain;

»       Having verified that conditions are safe, drain waste chemical solution to a suitable sewer system or to approved receptacles and start rising system with clean hot water:

»       Repeat rising until water pH at outlet is approximately same as inlet water.

2)     Acid Cleaning:

A suitable chemical solution, chosen from one of several minerals or organic acids, is blended and inhibited to retard corrosion of the base metal, while allowing dissolution of metal oxides present ( rust, scale, weld slag, etc). The concentration (%) of dissolved iron is monitored to determine when dissolution is complete.

Whenever mineral acid is used, all acid cleaning operations should be carried out under nitrogen blanket to avoid metal surface oxidation upon contact with air.

Acid cleaning includes the following steps;

·         Fill system with clean fresh water;

·         Circulating water at recommended velocity of 1 to 1.2m/sec.

·         Heat circulating water to 60 to 70⁰C.;

·         Vent system and check for leaks;

·         Maintain hot water in circulating for one hour;

·         Carefully and gradually does inhibited hydrochloric acid in circulating water until 4% to 6% inhibited acid strength is achieved;

·         Periodically check iron content and free acid contents in the solution and record all readings in log book;

·         When iron contents and free acid contents have stabilized, empty system and neutralize the discharge solution with caustic soda to prevent risk of accident or pollution;

·         Start rising system with clean water until pH 7 of outlet water s reached.

Critical acid may also be used successfully on carbon steel.

In this case required acid strength is circulating solution will be 4% to 10% and temperature 90 to 95⁰C.

Citric acid allow considerable savings in time and water use, because neutralization is not required nor it necessary to drain under nitrogen blanket.

              4) Neutralization and passivation:

To neutralize all traces of acid left in the system and in order to confer residual corrosive protection to surfaces, all sections must be flushed with a neutralizing and passivation solution.

Neutralization/passivation includes the following steps:

→       Fill system with clean fresh water;

→       Circulate water at recommended velocity of 0.6 to 0.9 m/Sec.

→       Heat circulating water to 50 to 60⁰C.;

→       Does ammonia and passivating agent in the circulating water until following strength are reached.

§         Ammonia or sodium nitrate            1.0%

§         Passivation agent                            0.8%

→       Monitor solution pH and record readings in log book. Final pH should be about 9: if not add additional alkaline neutralization chemicals ( ammonia or sodium nitrate ) to achieve this pH value;

→       Maintain water solution is circulation for 4 to 6 hours at 50 to 60⁰C  to neutralize traces of acid in system and to confer residual corrosion protection to surfaces;

→       Prepare to drain solution from system;

→       Having verified that conditions are safe, drain waste chemical solution to suitable sewer system or to approved receptacles;

→       Rinse system with clean fresh water to remove residue or particulate remaining in system. This must be done carefully and not too vigorously, as the passive film is only 40-100 angstroms thick. It is also advisable to add a small amount of the passivation agent to rise water.

5) Conservation:

To remove remaining fluids, the system must be flushed with nitrogen, and low points purged.

When drying is over, system re-assembly is carried out. It is advice to light-flush the line with nitrogen during assembly. Risks of water infiltration in treated line must be avoided (condensation due to fog, rain, etc). The system must then be maintained under nitrogen blanketing

IV.          QUALITY CONTROL

To maintain control of solvent solution used in chemical cleaning, and to monitor both their effectiveness and progress of the cleaning process, the following quality control checks are required.

1)     Degreasing Solution

             

              Monitor residence time and temperature.

     This solution operates by physical dissolution of oil and grease, and is not chemically consumed during the course of degreasing. Time required to complete depends on

temperature, circulation velocity, and level of contamination. Generally 8-12 hrs are recommended at 80 to 90⁰C.

             

2)     Water Risen:

Visually inspect for particulates and check pH of solution.

Water flush is performed to remove max amount of chemicals & materials before introduction of next solution.

3)     Acid Cleaning Solution

Analyze acid and dissolved iron content of circulating solution hourly.

Active or free acid content is given indirectly by calculations based upon dissolved iron content, Record residence time.

Monitor temperature, flow rate, percentage of iron and pH.

Acid cleaning is complete when content of iron and free acid is quite constant, ignoring fluctuations caused by circulating and mixing factors. Time required to complete depends upon many variables, but generally 8-10 hours are adequate.

1       Neutralization and passivation Solution

Monitor pH 3 temperature, flow rate and residence time.

This solution is prepared by adding neutralization chemicals and passivation agent to fresh water. PH is monitored during circulation while neutralization chemical is added.

Once a pH of 7.0 is obtained, the solution is chemically neutral.

Further additions of neutralization chemicals are made during circulation, until a pH of 9 to 9.5 is obtained.

2       Documentation

Result of each test or inspection, and time of each result are recorded in log book. At end shift, logs are verified and signed by Inspector.

V.           SAFETY AND ENVIRONMENT CONTROL

During chemical cleaning proper measures for safety and environment control are required. Safety measures will safeguard personnel and prevent environment pollution.

              Measures to be taken:

-               Observe and enforce all local codes & regulations to safeguard personnel and environment:

-               Restrict entry into cleaning area to authorized personnel only;

-               Post notice to forbid smoking, welding, flame-cutting and other hot works inside the cleaning area;

-               Instruct cleaning personnel how to stop cleaning actions immediately when required.

-               Transport all cleaning agents, inhibitors, and other chemicals in sealed containers, drums or tanks;

-               Protective clothing, PVC apron gloves, bolts, helmets, goggles and face shields shall be worn by cleaning personnel during cleaning and when handling cleaning agents;

-               Provide an emergency shower for personnel to wash off splashed hazardous chemicals;

-               Arrange for safe disposal of cleaning agents via a neutralization unit or disposal tank. Direct disposal to safe sewer shall only be done after neutralization;

-               Vent equipment to prevent pressure build-up by store gases;

-               Use gas-detecting instruments to check for presence and concentration of released gases;

-               Before cleaning, pressure-test equipment to be cleaned and cleaning circuit.