What Is Scale

A scale is a hard water soluble substance formation or incrustation resulting from the general deposition of dissolved solids in water, over heat transferring metal surfaces. The principal scale forming material, coolant water is calcium carbonate (CaCO3), a hard tenacious adherent baked scale. Other scale forming constituents are magnesium hydroxide, calcium sulphate and silica. Other types of scales which are formed on hot or cold worked metals consist of metallic oxides, sulphide and other components.

When heat is applied to water containing bicarbonate, the CO2 is expelled from Calcium compound. Since the relatively high solubility of Calcium bicarbonate is due to this gas, CaCO3 is precipitated rapidly as soon as the gas is removed. Removal of CO2 by chemical means the same results should be obtained & the precipitation of non-carbonate of calcium follows.

As a result of this phenomenon calcium carbonate is precipitated within the boiler/equipment, forming a scale.

Also calcium sulphate, owing to its insolubility at high temperature, is precipitated rapidly. Unless it is converted into some other form by chemical treatment or removed from water as it is formed, it causes a hard, dense scale which adheres tenaciously to the internal metal surfaces of the boiler/equipment.

The problem of deposition of boiler / equipment scale is primarily one of the solubility at elevated temperature i.e. certain slightly soluble substances become less soluble with increase in temperature. The whole body of a boiler solution constantly tends towards a state of super saturation as steam is removed.

In addition, since scale forming substances decreases in solubility with increase in temperature, the film of boiler water adjacent to a heating surface may be relieved by spontaneous crystallization, possibly promoted by irregularities in the surfaces even when vaporisation is taking place.

The growth of deposits is accelerated by the effect of skin friction at the internal surface of the tubes, since the velocities along the tube wall are much smaller than those near the centre of the tube. Deposition of silicate scale might not otherwise develop in the presence of the minimal amount of siliceous materials in feed water.

The solubility of different salts in water is given below :

Various types of deposits which are found in Boilers, Auxiliaries & miscellaneous plant equipments are given in table 1, Tables 2 & 3 provide analysis of typical scales. (These tables will be found at the back.)

"SCALECLEAN - A" is a liquid preparation with Hydrochloric Acid base, designed to dissolved industrial deposits without affecting metals. "Scaleclean - A" is supplied in fully inhibited and concentrated form and is to be diluted with water for use. In general, any type of equipment on which scale has been deposited, can be cleaned by using "Scaleclean - A".
"SCALFLOW" is specially developed for removal of hard scale like silicate of calcium, magnesium, sulphates, carbonates etc. "Scaleflow" is supplied in fully inhibited and concentrated form and is to be diluted with water for use.
"NEUTRAFLOW" is a liquid preparation with an alkaline base. The "Neutraflow" treatment has the effect of swelling up the scales through hydrolysis of the scale components. The result is that the subsequent "Scaleclean - A" of "Scaleflow" treatment becomes faster and proper.

Some of the operational difficulties caused by scale deposits are :

1. Obstruction of fluid flow

2. Impedance of heat transfer

3. Wear & tear of metal parts

4. Localization of corrosion attack

5. METAL failure at hot spots under scale deposits (plate)

Metal oxide scales (Mill scale, rust etc.) must be removed from metal surfaces before painting, galvanizing, tinplating and other principal metal-finishing processes

Scaleflow, Scaleclean - A and Neutraflow uses HCL as principal agent.

Although many acids namely Sulphuric, Sulfamic, Citric, Hydrochloric can be used to remove scale, Hydrochloric Acid offers the following definite advantages over others :

• Calcium Chloride, Iron Chloride etc. which are formed by reaction of scale are highly soluble with HCL in water.

• The time taken for the solution of rust or scale is minimum, in HCL as compared to that required other acids.

• The solution rate of mill scale is 10% to 27% higher in 10% & 5% HCL respectively than in similar solutions of H2SO4.

• A dark slime or "smut" left on metal surface pickled in other acids is not present when HCL is used.

• During pickling in HCL the scale is directly dissolved by the acid and the "mechanical effect" of removal of scale by hydrogen bubbles (formed by reaction between metal and acid), is not required. Accordingly HCL can be inhibited with the strength inhibitors.

• The degree of Hydrogenation and consequently Hydrogen embrittlement are absent when HCL is used.

• No heating is required during cleaning with HCL as the acid dissolves the scale at normal temperature

• The tolerated iron contents in pickling solution are higher when HCL is used.

• The corrosive action of HCL on clothing and skin milder.

What Equipments Must Be Treated With Scaleflow, Scaleclean-A And Neutraflow ?

In general any type of metallic equipment on which scale has been deposited can be cleaned by Scaleflow, Scaleclean-A and Neutraflow. Typical examples of equipments are Boilers, Economizers, Super Heaters, De-aerators, Vents, Condensors, Stage Heaters, Heat Exchangers, Surface Condensors, Feed water treating equipments, valves, and miscellaneous equipments, piping, water mains, oil wells, equipment used for acidizing of oil wells (to improve yields of oil from oil bearing lime stone formations), air-conditioning equipment, canteen equipment, stabilizers, electric boilers, kettles, surgical instruments, immersion heaters, heating coils, sugar and caustic evaporators, black liquor evaporators in the pump industry, railway carriages and bus bodies, brewery equipments, steel strips and sections requiring pickling, cold drawings etc.

1 st Stage : Take "NEUTRAFLOW" and mix with water in the ratio of 1:6 (One litre of NF and six litres of water). Circulate the solution or immerse the equipment in this solution for 2 hrs. This will be helpful in loosening the rock hard scale. This will also help to remove oil & grease from the surface.

2nd Stage : Flush the entire system with ample fresh water, at least for 15 to 20 minutes.

3rd Stage : "SCALEFLOW" and mix with water in the ratio of 1:6 (One litre of SF and six litres of water). Circulate the solution or immerse the equipment in this solution for 6-8 hrs.

4th Stage : Flush the entire system with ample fresh water, at least for 15 to 20 minutes.

5th Stage : Take "NEUTRAFLOW" and mix with water in the ratio of 1:6 (One litre of NF and six litres of water). Circulate the solution or immerse the equipment in this solution for 2 hrs.

6th Stage : Flush the entire system with ample fresh water, till the ph of inlet & outlet of fresh water matches.

The information contained in this technical bulletin is adequate for common descaling problems but in case further information is required please write to us giving details of equipments to be de - scaled and type of scale encountered.

Scaleflow, Scaleclean - A and Neutraflow is Marketed in 30 Ltrs. Contaliners

TYPES OF DEPOSITS WHICH ARE FOUND IN BOILERS, AUXILIARIES AND MISCELLANEOUS PLANT EQUIPMENT