Choosing the right grade of steam

There is growing recognition that plant steam is simply not clean enough for some applications. Spirax Sarco reviews the different steam grades commonly used and the guidance available for different sectors and applications

Figure 1: Schematic of clean steam generator

Standard plant steam is a convenient and cost-efficient way for moving heat around a variety of processes and premises, from petrochemical sites to pharmaceutical plants. Yet there is a growing recognition that plant steam is simply not clean enough for some applications. While this has long been understood in the pharma, healthcare and electronic sectors, demand for clean steam is now on the rise in other industries.

Food and drink is the industry where the trend towards clean steam is undergoing the biggest shift. This is partly because manufacturers want to avoid quality issues, expensive product wastage and even product recalls with the associated damage to their reputations. It is also partly because of pressure from customers such as the major supermarkets, who have their own responsibility to ensure the safety and quality of the products they sell.

There is no specific legislation governing the quality of steam in food and drink applications even though concerns over issues relating to taste and taint are becoming more prominent. However, manufacturers are legally bound to ensure the quality of the final product by identifying potential hazards and controlling them – typically by using a Hazard Analysis and Critical Control Point (HACCP) approach.

The US FDA exerts strict control over the manufacture and commercial distribution of food and drink, cosmetics and pharmaceutical products manufactured or for sale in the US. It mandates that manufacturers should ensure that suitable controls for all possible hazards are established and adhered to. Within a HACCP context, steam quality and safety could be described as a HACCP prerequisite or, if steam is added directly into the product, as a step in the food production process.

In the pharmaceutical and healthcare industries, operators should be working to current Good Manufacturing Practice (cGMP) guidelines

The situation is more clear-cut in the pharmaceutical and healthcare industries, where pharmaceutical operators should be working to current Good Manufacturing Practice (cGMP) guidelines. In addition, the Bioprocess Equipment (BPE) group of the American Society of Mechanical Engineers (ASME) provides a measurable way to design, manufacture, specify and purchase equipment for the biotech, pharmaceutical and personal care industries. ASME-BPE guidance applies to:

  • Any production system in contact with the product, raw materials, buffers for pH modification or product intermediates.
  • Systems that are a critical part of product manufacture, e.g. fermentation, separation, purification, intermediate product storage, fill/finish equipment.
  • Utilities that come into contact with manufacturing equipment for cleaning and sanitisation, e.g. Water For Injection (WFI) used for rinsing, caustic fluids for cleaning in place, clean/pure steam for sterilisation in place, and gases used for blanketing or product movement.

Further guidance comes from the International Society for Pharmaceutical Engineering (ISPE), which publishes a number of best practice and framework books, including its Good Automated Manufacturing Practice (GAMP) guide.

While there is no consumer or patient health imperative to use clean steam in the electronics industry, the tiny scale and extreme fragility of electronic components demand the highest standards of hygiene during the production process to maintain product quality.

The tiny scale and extreme fragility of electronic components demand the highest standards of hygiene during the production process to maintain product quality

There are four grades of steam commonly used in industry today, from basic plant steam, through filtered (culinary) steam, clean steam and pure steam. While plant steam is great for heat transfer applications in industries such as petrochemicals or pulp and paper, many food companies use filtered steam, or even the clean steam favoured by hospital sterilisation departments and life science labs. Pure steam is the highest grade option and is required in pharmaceutical and biotech applications.

It is worth noting that steam purity and steam quality mean different things. While steam purity is a measure of the dissolved solids, volatiles or particles in the steam, steam quality refers only to the amount of water in the steam. A more correct term is dryness fraction.

Dryness fraction = Mass of steam/(Mass of steam + entrained water)

Steam grades and applications

Plant steam, the starting point for most steam users, can be used anywhere it does not come into direct contact with the process or product. If it is used directly, users should consider whether the quality and purity of the steam are fit for purpose or whether any possible contaminants could present a problem.

Contaminants can be chemical, physical or microbiological. The most common source of chemical contamination in plant steam arises from treating the feed water as it enters the system. Boilers generate steam from water and that water is typically treated with chemicals to prevent a range of problems such as corrosion or the build-up of scaly deposits. Traces of those treatment chemicals can end up in the steam supply, especially if users do not follow best practice, which is available in standards such as BS 2486: 1997 and BS EN 12953–10: 2003.

If chemicals do end up in direct contact with the process, they have the potential to taint products with an unwanted taste or smell. In the case of food and drink applications, there may also be safety concerns or consumer perception issues associated with their presence.

If chemicals do end up in direct contact with the process, they have the potential to taint products with an unwanted taste or smell

Carryover is another potential source of contamination. It may result from priming (where the distribution system draws off a large quantity of steam quickly and boiler water is entrained in the steam line) or foaming. Carryover can contain potentially high levels of water treatment chemicals. Cross contamination is also a possibility, since most manufacturers will recover condensate from around the factory to save water and energy. If there are any pinholes or other leaks in the system, the returning condensate may be contaminated by process media or by the chemicals used for cleaning-in-place.

Although effective water treatment should minimise problems such as scale and corrosion if carried out correctly, plant steam may still carry solid contaminants, such as flakes of rust or residual scale from inside the boiler and steam distribution system.

The temperature and pressure of steam kills common microbial hazards such as Salmonella, Listeria or E.coli. Heat is an effective and convenient physical control agent for destroying microbes, which is why clean steam is used to sterilise medical instruments. Even so, dead microbiological debris (pyrogens) can still induce an adverse reaction if injected, which is why many pharmaceutical applications demand pure steam. In addition, any solid residue deposited by plant steam as it contacts the process or product could potentially provide a home for future microbiological growth once the product has cooled down.

Filtered or culinary steam is plant steam that has passed through a filter, typically 5µm. This removes 95% of all particles larger than 2µm. A pre-filter (typically 25µm) is placed upstream of any 5µm filter to prevent rapid blinding (blocking) of the main culinary filter.

Any solid residue deposited by plant steam as it contacts the process or product could potentially provide a home for future microbiological growth once the product has cooled down

While EU Regulation (EC) No. 852/2004 says: ‘Steam used directly in contact with food is not to contain any substance that presents a hazard to health or is likely to contaminate the food’, it does not specify the acceptable quality or purity of steam.

In practice, many European operators refer to the US 3-A practices for producing culinary steam. Accepted Practices for a Method of Producing Culinary Steam, No. 609–3, is the US standard that establishes requirements for producing culinary steam. It stipulates the materials used, surface finishes, installation and boiler operation with regard to the use of culinary steam.

Note that water treatment, boiler carryover and cross contamination still pose a risk, because the filter may not remove all the potential contaminants. The 3-A Practice specifically stipulates that boilers should be ‘operated in such a manner as to prevent foaming, priming, carryover, and excessive entrainment of boiler water into the steam’.

Clean steam relies on a secondary generator and tightly controlled feed water quality to eliminate many of the potential issues already outlined. It is critical to start with the right water quality. Raw water is not adequate and will require pre-treatment. Reverse osmosis (RO), deionised/demineralised (DI) and continuous electrodeionised (CEDI) water are all good possibilities. They all remove the need for chemical treatment by removing most of the particulates, inorganics and dissolved solids at the pre-treatment stage. The risk of water treatment chemical contamination is therefore eliminated when using clean steam.

In addition to the quality/purity of the clean steam leaving the generator, there are other factors to consider when installing a clean steam system.

Clean steam is very aggressive, so grade 316 or 316L stainless steel is typically used on contact surfaces throughout the system to protect against rouging. In addition, even though the temperature of the steam will keep most bacteria at bay, the surface finish of equipment should minimise any crevices that could encourage microbial growth. Similarly, a clean steam distribution system should be designed to good engineering practices. Guidance can be sought from 3-A Sanitary Standards (www.3-a.org).

Clean steam is used in applications such as sterilisation, not only to eliminate contaminants but also to ensure quality control of critical attributes such as dryness, superheat and production of non- condensable gases, all of which could adversely affect the process and equipment. This has been largely driven by UK and European sterilisation standards HTM 2010/EN285.

Figure 2: Schematic of pure steam generator

Pure steam must be pure, dry and pyrogen-free. When it condenses it should comply with international pharmacopoeia requirements for WFI, i.e. pure enough to be injected into the human body with no ill effects. Again, a supply of highly purified feed water is essential, using the same principles as for clean steam. However, the standard is higher, with the resulting condensate again meeting WFI standards. A dedicated pure steam generator then distils the water either once or multiple times to produce the purity of steam required.

Most pure steam being used is for pharma applications, where all the equipment and processes should meet cGMP, as regulated by national agencies such as the US FDA.

The direct benefits of opting for steam with a higher standard of purity than plant steam vary depending on the industry and the particular application. In industries such as pharma, healthcare and electronics, the patient safety, regulatory and product quality requirements make the decision to use high-purity steam extremely clear-cut – it is essential for a successful operation.

Food retailer influence

On the other hand, some operators in the food and drink industries still view the use of clean steam as discretionary because of the lack of concrete regulatory requirements. However, it can be crucial in helping manufacturers to demonstrate that they are applying an effective food safety regime according to HACCP principles. This is increasingly a requirement from major retailers who, along with manufacturers, are responsible for ensuring the safety and quality of the products they sell.

A recommended approach to applying HACCP involves the following steps:

    1. Determine quality of raw boiler feed water;
    2. Determine levels and types of dosing chemicals;
    3. Identify other potential sources of contamination arising from use of an inappropriate grade of steam;
    4. Assess any risks associated with ‘product’ contamination, e.g. potential health hazards;
    5. Devise and adopt an effective steam quality testing regime;
    6. Adopt best practice in the design, maintenance and testing of the steam system to ensure that the correct quality of steam reaches the process.

While product safety and quality are likely to be prime reasons for choosing clean steam, there can be additional benefits. For example, using reverse osmosis (RO) strips out 99% of dissolved solids before the feed water reaches the boiler. This reduces the need for boiler blowdown, slicing as much as 3% off the fuel bills of typical steam users.

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