The Complete Steam Guide

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Grades of Steam

This article elaborates on different grades of steam and characteristics of each of them.

1. Plant Steam

Plant steam is the normal steam which is used for general indirect heating purposes in process plants. Plant steam may have different contaminants present in it. Plant steam can be used to applications which involve indirect heating. But, it is not advisable to use plant steam for applications such as direct heating etc. where impurities in the steam can affect the quality of the product. 

2. Filtered steam

(Also referred to as clean steam, culinary steam, food grade steam, potable steam and 3A's steam) is produced by passing plant steam through a high efficiency filter.  A typical specification would call for the removal of all particles greater than 5 microns in size, including solids and liquid droplets.

3. Clean steam

(Also referred to as hospital steam, HTM 2031 steam, EN 285 steam and simple clean steam) - This middle band of "clean steam" encompasses a wide range of purity and quality requirements, ranging from:

- Very basic clean steam, where user requirements may be for "Boiler feed water chemical" free steam, for perhaps simple building humidification, to:

- Regulated clean  steam,  where user requirements  have very  specific purity and  quality characteristics, so  as to  not contaminate the  end product or process. Sterilization is a good example of this where standards such as HTM 2031 and EN 285 are in place.

Non-critical humidification within the biopharmaceutical industry is another example, where pharmacopeia grade "purified water " criteria are often adhered.

All types of clean steam will be generated in an independent generator but may use either softened potable, De-Ionized (DI) or Reverse Osmosis (RO) as the feedwater, depending on the end user requirements. The type of feedwater used will have a significant impact on the materials of construction and the design of the generator. 

For example; Mild steel or copper may be used where the feedwater is potable, but 316L stainless steel must be used where the feedwater is aggressive or ion hungry in nature, such as DI or RO.

4. Pure steam

(also referred to as clean steam, WFI steam, high purity steam, sanitary clean steam, pharmaceutical clean steam, GMP clean steam and pyrogen free steam) is similar to clean steam; however, the resulting condensate must meet the standards of USP grade water for injection (WFI) and contain no bacteria or pyrogens. Pure steam must be produced by a pure steam generator; which is classed as sanitary in design, or from a multiple effect still.