Clean steam is defined as steam which is free of all kinds of contaminants. There are many different substances, which may be considered as contaminants; in general, they can be classified into four groups:
1. Chemical contaminants:
Include chemical substances that are found in the boiler feedwater. For example, heavy metals such as mercury, cadmium and lead which affect human health, as well as substances which may damage the steam system, such as chlorides, which cause stainless steel to corrode. In addition, various chemicals are added to the boiler feedwater, and can be carried over in the steam. These include filming amines that are used to prevent corrosion in the condensate return lines and feedwater pre-treatment additives.
2. Biological contaminants:
Most water supplies contain microorganisms such as bacteria, viruses, fungi and yeasts. The local water authority controls their levels, ensuring that the water is potable (of drinking quality) however, this may not be sufficient for a number of applications.
This is of particular concern in the production of medical devices or medicinal products. Since these products may bypass the human body''''s natural defence mechanisms, such as the skin and the mucous membrane s, they can introduce contaminants directly into parts of the body that would otherwise be protected. For example, water that is safe to drink may not be considered safe if it is directly injected into the blood stream.
The high temperatures associated with steam are generally sufficient to destroy most living pathogens (disease-producing agents such as bacteria and viruses) present in boiler feedwater. However, living bacteria can produce harmful substances known as ''''pyrogens'''', which are thermally stable, and can withstand a temperature of up to 180°C for several hours. This means that heating in a boiler will not necessarily destroy them and they must be removed from the feedwater.
3. Non-condensable gases (NCGs):
Oxygen, ammonia, carbon dioxide and other gases dissolved in the feedwater or introduced by other means may produce undesirable effects in clean steam systems. For example, cold spots in steam sterilizers and reduced heat transfer efficiency.
4. Pipe scale rust and debris:
Pipe scale results from the presence of carbonates and sulphates of calcium and magnesium, which are naturally, present in the public water supply. As plant steam systems are typically of iron or steel construction rust will often be present.