Re: Commissioning Activities

Rakesh:

You were interested in commissioning cooling water systems in refineries. Thought you might be interested in our White Paper on cooling water commissioning......it may provide useful information. If you would like specific details (chemistry, duration, etc), please feel free to contact me.

PASSIVATION THE KEY TO EFFECTIVE CORROSION PROTECTION OF COOLING WATER SYSTEMS Corrosion of metal can be prevented through the use of an effective protective barrier. Protective barriers may be such things as protective coatings, corrosion inhibitor films and metallic coatings such as galvanizing. The application of the protective barrier is always made after careful preparation of the surface. If the surface is not carefully prepared, the barrier may not bond properly to the surface resulting in poor corrosion protection. It is therefore extremely important to properly prepare a surface to receive protection, then quickly and completely apply the protecting media to effectively protect that surface. Preparing the surface may be called pretreatment or passivation.

When a new recirculating cooling water systems is constructed, the piping, exchangers, and tower basin may be flushed with water to remove any dirt, loose rust, etc., resulting from fabrication. The water flushing is an attempt to remove these extraneous materials from the system. No other step is usually employed to prepare the metal surfaces prior to applying the protective film. Acid cleaning of new equipment has been done to remove mill scale and rust, however, this method does not remove oils, greases and other miscellaneous materials. Usually, after water flushing or acid cleaning operations, very little is done in the way of protecting the metal surfaces until the entire system is put into operation. Since protection is not available during this time, considerable corrosion may occur due to either stagnant wet conditions or circulation of untreated water on the metal components. Often corrosion is started due to residual acid left from acid cleaning. This condition may exist for weeks before application of a protective corrosive inhibiting film and produces a corroded surface with considerable corrosion products on the metal even before the inhibitor is applied.

During the start-up operation of new cooling systems, the use of corrosion inhibitors is employed at either maintenance or high level dosages for corrosion inhibition. It has been shown by previous work that high level treatment is effective in establishing protection to metal surfaces. All the metal surfaces will benefit from pretreatment, but they must be clean for optimum formation of a protective film. Normal maintenance level dosages will not effectively film even clean surfaces due to initial demand for inhibitor to fill the metal surfaces.

If no attempt is made to prepare the metal surfaces for effective film formation, the corrosion inhibitor cannot properly react with the metal surfaces to form a protective film. The presence of rust, oils, organic films, etc. resulting from manufacture or storage or the metal prior to use prevent effective film formation. Corrosion progresses unchecked under these foreign materials due to the formation of localized corrosion cells similar in effect to metal surfaces prior to painting or applying protecting coatings. We cannot emphasize too strongly the importance of surface preparation prior to applying the protective films or in the use of any barrier type corrosion prevention program.

During the first few months of operation of a new cooling water system, considerable tuberculation and corrosion in the system may occur. This may be due to the presence of dirt and other extraneous materials on the metals surfaces limiting the corrosion inhibitor from forming an effective protecting barrier on the metal. Frequently, mechanical or acid cleaning is required to restore operational efficiency and permit establishment of protective films. Subsequent operation of the system with the same type of inhibitor programs can then be effective in minimizing corrosion because of effective film formation on the cleaned surfaces.

Inhibitors for cooling systems are designed to form and maintain protective films, but are not designed to prepare the metal surface to accept films. Thus, the need of a separate preparation step is a prerequisite for establishing corrosion protection for metal surfaces in cooling water systems.

Pretreatment programs help protect new equipment in cooling systems from corrosion and fouling by:

· Removing interfering substances from the metal surface due to manufacture, storage, fabrication and assembly. The treatment to be used for removal of these materials must be effective on organic (e.g. oils, grease) and inorganic (e.g. silt, mud) materials.

· Filming metal surfaces immediately after the preparatory or cleaning step and prior to the use of the regular inhibitor program.

At 11:08 AM 3/19/2002 +0530, you wrote:
>Hi All,
>
>This is regarding the Commissioning of Cooling Water System in refineries.
>Generally following steps are followed during this process -
>
>1. Pre cleaning
>2. Passivation
>3. Regular Treatment.
>
>What should be the the procedure and duration for the Steps 1 and 2
>(mentioned above).
>
>Why can't we directly start dosing chemicals (like H2So4 Acid, Biocide
>chemicals and Chlorine Gas) inside the system after conducting the
>Flushing of lines?
>
>waiting for your replies.
>
>regards,
>rakesh
>
>
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Received on Sat Mar 23 10:57:00 2002