Purpose of chemical cleaning :
The primary reasons for chemical cleaning of boilers/ heat exchangers are to prevent tube failures and improve unit availability. Tube failures in low pressure boilers/heat exchangers are normally the results of creep which occurs when internal deposits produce excessive metal temperature. A relatively smaller quantity of deposit creates difficulties in high pressure boilers. Caustic corrosion and hydrogen damage, which occur only in the presence of deposits, may cause tube failures at temperatures well below the creep limit. Deposits originating both from fabrication and during operation should be considered potential problems.
All the pressure parts of steam generator may be subjected to heat treatment of some sort during fabrication or erection – during forming operations, stress relief, welding or bending. Whenever carbon low alloy steels are subjected to high temperatures in the presence of air, oxidation occurs, the oxide produced is known as mill scale. Mill scale on boiler/ heat exchanger tubing is normally very thin with the exception of areas near welds and bends. Even where mill scale is initially uniform its brittleness upon cooling may produce flaking. The resulting non-uniform surface is undesirable from the standpoint of corrosion susceptibility. During operation mill scale is rapidly eroded from the steam generating surfaces and may subsequently be redeposited in critical areas. Preoperational acid cleaning removes mill scale and serves to remove atmospheric rust which inevitably accumulates to some degree during erection.
After a boiler / heat exchanger placed into service, numerous solid constituents may enter the units with the feed water and some portion of the insolubles can be expected to deposit on surfaces. If not removed these deposits accumulated over a period of time can minimize the quantity of these materials; however, complete freedom from deposition is not possible in a high pressure system. The need for occasional chemical cleaning during the life of the equipment has become a recognized fact and should be accepted as a routine maintenance practice. A frequency of service cleaning of every 3 to 4 months is recommended. This frequency should be increased if individual unit operating history dictates.
Neutraflow is a liquid preparation with an alkali base. The neutraflow treatment has the effect of swallowing up the scales through hydrolysis of the scale components. The initial result of neutraflow helps the subsequent scaleclean-a or scaleflow treatment to become faster and proper. The sticky substances like oil, grease etc. Are removed with the use of this at the initial stage.
The final stage of descaling procedure does not complete without neutralizing the equipment. Here again neutraflow play its vital role. Use of neutraflow at this stage makes the equipment neutral with no acid reactions inside the equipment at the time of first use after the descaling procedure and prevents further corrosion of the parent metal.
Basically, the term pickle is used by jewellers to refer to an acid solution that is used to remove the black firescale that forms on alloys when they are subjected to heat. Metal clay artists generally encounter firescale when sterling silver is in place or soldered on to fire clay pieces.
This is a process which chemically removes scale or oxide from steel to obtain a clean surface. When applied to bars or coils prior to bright drawing, the steel is immersed in a bath of diluted acids and heated to a temperature of around 800C. An inhibitor is added to prevent attack and pitting of the cleaned metal. After pickling, a washing process takes place followed by immersion in a neutralizer bath to neutralise any remaining acid. For environmental reasons shot blasting has largely replaced pickling.
For pickling process, big vessels are required to immerse the parts alongwith the same heating is required. The temperature around 600 C to 800 C is required and that is to be maintained during the entire process. So many prefers descaling than pickling.
There are different acids which can be used for the purpose of pickling. The different methods are used for different acids. But after pickling fresh water flushing and neutralizing is very necessary to avoid further acid attack and pitting on the metals surface. Many opt for passivation after the process of pickling to avoid further damage of metals.
Forming a thin film coat inside the equipment by circulation of different chemicals through the equipment is known as passivation. It forms a film in microns over surface which is Descaled and Neutralized. It prevents further corrosion of the treated surfaces. Why the passivation is necessary.
After descaling and the neutralizing procedure, passivation is essential for any vessel or the equipment. It is essential that the residual acid attack on the surface should be properly neutralised and properly quoted by applying a suitable solution, before applying the equipment back in service. Following cleaning with an acid, rinsing with water and neutralizing, the highly reactive surface tends to flush rust. The formation of this type of ferric acids can be prevented by adding some organic agents to the passivation chemicals. These organised agents form very stable complexes with Ferric Iron and thus prevent the formation of rust without interfering with the development of the Passivating film of Fe2O3.