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Basics of Steam
Clean & Pure Steam
Have you considered the quality of your steam? In the production of certain goods such as food, electronics, and pharmaceutical products, a higher degree of steam quality is vital. In order to meet these needs, it would be ideal to use steam that is devoid of condensate, debris, and any other impurities (or almost so). The goal in producing filtered, clean, or pure steam is to get as close to the ideal state for each specific application as possible.
Since steam must be boiled from water, it is easy to think that steam produced from distilled or purified water can be considered sufficiently clean; however, in reality, the process is not that simple. Clean steam must be generated in a high-quality state, and that quality must be maintained throughout the distribution and application of the steam. Pure steam can have even more stringent requirements.
Factors that Affect Steam Quality
There are three major factors that can affect steam quality:
- Boiler feed water quality (processing & treatment)
- Steam-generating equipment
- Steam distribution piping & valving
Water intended for use in steam boilers typically undergoes soft water treatment, which reduces magnesium and calcium ions often found in water to acceptable levels. Any visible debris will be removed in this stage.
Even so, small amounts of these ions remain, as well as other ions which are not targeted by standard water treatment. Additionally, boiler feed water may contain gases such as oxygen, carbon dioxide, and even small traces of contamination from protective coatings applied to most standard boilers and piping.
When water changes to steam and rises from the boiler, gases dissolved in the water may also rise and flow together with the steam. Additionally, droplets of boiler water may coat the outside of steam bubbles as they break the surface of the water, carrying all the ions and chemical traces contained in the water up into the steam. As a result, these contaminants, unless removed by separate means, are often able to reach the steam-using equipment where they may compromise product quality/safety. In sensitive applications, it is essential to thoroughly consider what contaminants could be present in the boiler water and take the steps necessary to remove them as needed.
Different Levels of Steam Quality
There are different levels of steam quality depending on the application, and systems should be designed to achieve the level of quality required. Here are a few general examples:
Typical Steam System (Plant Steam)
- Used for indirect heating applications. Small amounts of impurities typically will not cause problems.
- The steam quality level used in some food processes. Steam is filtered immediately before use.
Highest Levels of Steam Quality (Clean & Pure Steam)
- Boiler water has undergone treatment to remove potential contaminants; steam is then generated & transported using stainless steel equipment to prevent contamination during distribution.
Production of Clean/Pure Steam
Clean steam and pure steam must meet very strict requirements which may vary based on the end application. The water used to create both types of high quality steam must be virtually free of impurities. One common process for achieving this is reverse osmosis (RO), which utilizes a fine, semi-permeable membrane to physically remove impurities.
In addition to ensuring water quality, heating surfaces coming into contact with the water and all valves/piping used in distributing the steam should be made of high-grade stainless steel in order to minimize contamination and preserve the steam quality. As a result, many clean/pure steam processes utilize a separate stainless steel steam generator and supply heat to the generator with ordinary plant steam, effectively isolating the clean/pure steam line.
Cleaner than Clean?
In its definition of the highest level of water quality, Japanese Industrial Standards (JIS) includes criteria such as a very low total organic carbon (the amount of carbon contained in organic matter) and an electrical resistance of 18MΩ⋅cm or higher, but does not mention specific limits for all possible impurities.
Water quality can be controlled even further by applying additional treatments, such as active carbon treatment, UV treatment, or ultra-filtration. In order to generate pure steam, often regarded as the highest level of steam quality, thorough removal of chemicals and microorganisms may also be required. Pure steam is commonly used in pharmaceutical applications or Sterilization in Place (SIP) procedures.
The following is a general guide giving some examples of different steam quality levels and their applications:
|Plant steam||Filtered Steam||Clean steam||Pure steam|
|Application||General heating||Food & beverage||Clean room humidification, food & beverage etc.||Injection solvent, transfusion, Sterilization (SIP), pharmaceuticals etc.|
|Boiler feed water quality||Soft water (contains boiler compounds, etc.)||Soft water (may contain some additives safe for consumption)||RO Water or Purified Water||RO Water + additional treatment|
|Steam Production and Distribution Equipment||Primarily carbon steel and cast iron||Stainless steel|
|Methods to ensure steam quality||Not required. Steam may be run through a separator before use||Steam is run through a fine filter directly before use||High-quality water, stainless steel steam generation and distribution equipment. (Filtration not necessary because quality is preserved throughout)|