Chemical Cleaning and Descaling

The build up of hard water scale in boilers, heat exchangers, cooling towers and pipework reduces the efficiency of heating equipment. Acid washing is the only satisfactory method of removing scale deposits. Heat exchangers that use river or sea water for cooling can also build up deposits due to biological or suspended matter fouling. These types of deposits can be removed by specially formulated alkaline cleaners. Our wide experience of formulating chemicals will ensure the correct chemical for the problem is presented.

Why Acid Clean?

Hardwater scale in boilers, heat exchangers, cooling towers and pipework reduces their efficiency, in a case such as steam boilers, can cause the boiler to be dangerous to operate. Acid cleaning is the only satisfactory method of restoring the heat transfer efficiency by complete removal of the deposits.

Scale, which is mostly calcium and magnesium, acts as an insulator in between the metal surface of a boiler and the water, and will result in an increase in the temperature of the boiler metal. With enough scale, the metal overheats, causing blisters and eventually a ruptured pressure part. The effect on the boiler efficiency can be seen from the following table:

When Should A System Be Acid Cleaned?

Boilers

Usually, with boilers the insurance inspector will fail a boiler if it is considered to be in a dangerous condition, and will not allow the boiler to operate until cleaned satisfactorily. However, if it is required to restore heat transfer efficiency, then the decision to clean can be taken by maintenance personnel.

Heat exchangers

These will suffer from blockage, restricted flow and increased pipework pressure drop, as well as a reduction in heat transfer efficiency. Acid cleaning should be carried out before the pipework is blocked completely, otherwise the acid solution cannot be circulated.

Cooling towers

Scale build up in a cooling tower can harbour Legionella bacteria and, since it is a requirement of the HSE that systems are clean, acid cleaning will be required if scale is present. Any heat exchanger associated with the cooling system may also require an acid clean.

Evaporative Condensers

The tubes of an evaporative condenser are simply a heat exchanger that is cooling refrigeration gases. Not only will there be a reduction in cooling efficiency, but the refrigeration plant will also be inefficient if the gas is not cooled correctly. Scale present is also subject to the requirements of the HSE in a similar manner to a cooling tower.

What Is Involved In Acid Cleaning?

Acid cleaning involves the circulation of acid at low concentrations around the equipment that requires scale removal. The acid used will depend on the type of deposit and material of manufacture of the equipment being descaled. The acids used typically are Hydrochloric, Sulphamic, Citric, Formic, Hydrofluoric and Phosphoric. Specific corrosion inhibitors can be used with these acids where necessary in order to ensure that the integrity of the equipment being acid cleaned is maintained

The procedure utilises specialised equipment designed for chemical cleaning and is as follows:

• A sample of the deposit is analysed to establish the most suitable acid to be used
• The equipment to be cleaned is isolated to ensure that the cleaning solution cannot escape and contaminate other equipment, pipes or water courses
• Pumps are connected for recirculation
• The equipment is filled with acid to the required strength
• Corrosion inhibitor is added to prevent acid attack on any exposed metal
• The descale is monitored for acid strength and further acid added if required
• The procedure is stopped when the acid strength remains constant, which indicates that no more scale removal is taking place, or by visual examination if this is possible.
• The acid cleaning solution is removed to drain and neutralised at the same time
• The equipment is refilled with water with a small amount of alkaline solution to neutralise any residual acid

What Can Be Done To Prevent Scale Formation?

Scale deposition can usually be prevented, and in most cases is caused by the lack of water treatment regime, or poor control if there is one. Processes such as steam generation, cooling towers and evaporative condensers will normally have some form of external water treatment equipment, such as a water softener, deioniser, reverse osmosis etc that will remove the hardness forming salts of calcium and magnesium from the water before it enters the equipment. This is the only long term satisfactory method of ensuring scale free conditions, and will be successful as long as the equipment is maintained and monitored correctly. See our plant and equipment section for further information on this type of plant.

If external water treatment equipment is not used, then scale formation can be expected, but the amount will depend on the quantity of water being used and the temperature the water is heated to. Some examples of equipment that cause scale problems might be calorifiers, hot water cylinders, heat exchangers, process tanks, water stills, plastic moulding machines, extruders, dishwashers, humidifers and water cooled machinery.

For more information on B & V Chemical Cleaning and Descaling, please contact our offices on 0844 372 7344 or e-mail us on enquiries@bvwater.co.uk