Clean steam is a general term that applies to steam that is used in the pharmaceutical industry and medical community in processes where the steam comes into contact with the product and thereby may cause contamination. This is primarily in the areas of steam sterilization, but may also apply where steam is used for humidification in clean rooms or critical care areas in a hospital.
The term “clean steam” is used in the food industry to describe steam that comes into direct contact with food or milk products. This steam quality differs from that in the pharmaceutical industry. It is often called “culinary” or “food grade” steam to highlight its distinction from “clean steam” although the terms are often used indiscriminately. Food grade steam specifications are sometimes used to define steam that is suitable for use in sterilizers and direct humidification systems.
High purity steam is required in utility applications where the steam is used to drive turbines. Turbine manufacturers publish guidelines for steam purity and quality that are intended to insure trouble-free turbine operation.
The terms used to describe the required steam purity for a particular application or process quickly become confusing. Clean steam is loosely used to describe “pure steam”, “pharmaceutical steam”, “technical steam”, “process steam”, “culinary steam”, “filtered steam”, “pyrogen-free steam” etc. For the purposes of this article, we will make the distinction between the various grades of steam as follows:
Utility steam for turbine drives is produced in a conventional high pressure boiler. The steam quality and purity limits are set by the turbine manufacturer in conjunction with their warranty.
Culinary steam is produced in a conventional boiler. Steam quality and purity is largely determined by the FDA/USDA, but may be more stringent if required by good manufacturing practices. Culinary steam is also known as “food grade steam” and “filtered steam.”
Clean steam is produced in a dedicated boiler constructed of 316L stainless steel. The steam purity is defined by the quality of the condensate produced by the generator, which is referenced to the US Pharmacopoeia standard for Purified Water (PW).
Pure steam is produced in a dedicated boiler similar to that required for clean steam, but the quality standard for the condensate produced by this system is referenced to the US Pharmacopoeia standard for Water for Injection (WFI). Water for Injection is a higher standard of purity than Purified Water.
STEAM QUALITY AND PURITY
Steam produced by a conventional boiler or high purity generator contains low levels of contaminates. As the steam (gas phase) separates from the liquid phase, droplets of boiler water are carried out of the generator along with the steam, thereby producing a contaminate. Boiler manufacturers guarantee a maximum percent carryover limit of 0.05% to 0.10%.
The level of steam contamination is measured as percent moisture and parts per million (ppm) solids. Steam quality is expressed as percent moisture or conversely as percent dryness. Most boilers are capable of producing steam that is 99% dry (1% moisture) at a minimum.
The boiler water that carries over with the steam contains impurities that are either dissolved in the boiler water or suspended as a filterable solid. These impurities manifest as ppm solids and are expressed as steam purity. A typical boiler is capable of producing steam purities of 1 ppm solids with a 0.1 ppm level very achievable.
In addition, silica that is dissolved in the boiler water can vaporize with the steam. The maximum silica level is limited to 20 parts per billion (ppb) by turbine manufacturers. Other impurity contaminates include sodium salts, organic acids and non-condensible gases like carbon dioxide, ammonia, and amines.
Steam purity specifications for clean steam and pure steam are defined by the quality of the condensate that the steam produces. For clean steam, the condensate quality must be that of Purified Water (PW). The specification indicates a maximum specific conductance and total organic carbon (TOC) level. Purified Water must also comply with a microbiological action limit to insure that the condensate does not contain unacceptable levels of bacteria that could contaminate the product.
Pure steam has the same standard for specific conductance and TOC as clean steam. The microbiological action level is set lower and includes a limitation on the level of endotoxins. Pure steam must also be produced from a specific feedwater source.
Utility steam is used for a wide variety of applications including power generation, comfort heating and room humidification. The utility plant generally sets its own quality and purity standards for its specific requirements. But when the steam is used for turbine drives the manufacturer steps in to impose steam purity limits that are intended to maintain and enforce the warranty on its equipment. These standards limit sodium to 20 ppb, silica to 10 – 20 ppb, chloride and sulfate to 3 to 15 ppb. In addition, cation conductivity is set within a 0.1 to 0.3 uS/cm window. (Cation conductivity “conditions” the specific conductivity by passing the condensate through a hydrogen-form cation exchange column. This helps minimize background conductivity caused by neutralizing amines). A typical baseline cation conductivity is 0.2 to 2.0 uS/cm as compared to a typical specific conductivity of 3 to 5 uS/cm.
No formal restriction or limitation in imposed on the quality of feedwater used to produce turbine steam. In general, however, high purity feedwater of demineralized quality is required in order to meet the limitations for sodium, silica and specific conductance. Likewise, various chemical additives are used to control corrosion, minimize carryover and curtail foaming, which would otherwise contaminate the steam.
The American Society of Mechanical Engineers (ASME), Feedwater Quality Task Group, publishes a “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers (1994). This publication includes tables that indicate suggested water chemistry limits for industrial watertube boilers with and without superheaters and turbine drives. These are not enforceable, but serve as a useful starting point in determining the optimum boiler and feedwater quality for these applications.
Culinary steam quality is required where steam comes into contact with food or dairy products. The steam is produced in a conventional watertube or firetube boiler at various pressures depending on the food processing requirement.
The Food and Drug Administration (FDA) publishes in 21 Code of Federal Regulations (CFR) Sec. 173.310 a list of boiler water additives that may be safely used in the production of steam that comes into contact with food. According to these regulations, culinary steam must be generated in boilers that exclusively use FDA approved chemical additives for scale and corrosion control such that the amount of additive is not in excess of that required for its functional purpose.
In days gone by, the United States Department of Agriculture (USDA) published a “List of Proprietary Substances and Nonfood Compounds” that it had tested and approved for use in egg, meat, poultry and rabbit processing plants. However, the work load and expense of evaluating these products became so great that it abandoned this policy in 1999. Now, the chemical supplier must self-certify that its products are formulated from ingredients that appear on the FDA list of approved additives and that the chemical product meets all other requirements for purity and safety.
The FDA list includes neutralizing amines that are approved for use in culinary steam. Amines are often applied to neutralize carbonic acid in the condensate and cause an upward adjustment in pH. The FDA regulation specifies the maximum permissible concentration of amine in the steam and excludes the use of amines in contact with milk and milk products.
A more recent addition to the FDA list permits the use of up to 15 ppm sorbitol anhydride esters as an anti-corrosion agent in steam that contacts food. This compound is treated as a boiler additive and is not a traditional neutralizing or filming amine as defined in paragraph (d) of the regulations.
Culinary steam is further purified by the use of steam filters at or near the point of use. These are 5 micron filters that are capable of removing 95% or more of particulates greater than 2 microns in size. This is intended to improve the steam purity prior to contact with the food product.
In some cases, chemical suppliers and end users have extended the FDA list of approved chemical ingredients to include the treatment of steam that is used for surgical sterilization and humidification. Several incidents have been reported that suggest high concentrations of amines in humidified room air may be irritating to some individuals and may also play a role in “sick building syndrome.” Museums are also cautioned against the use of amines in steam as this practice may produce an undesirable chemical film on the artifacts. As a result, the owner/operator of the steam boiler is advised to consider the potential risk and liability assumed by applying the FDA regulations beyond its intended scope.
Clean steam is produced in a dedicated generator fabricated from corrosion resistant materials; generally 316L stainless steel. The purity of the steam produced by this method is defined by the quality of the condensate it produces. Clean steam must produce condensate that meets the US Pharmacopoeia specifications for Purified Water. This requires a specific conductance less than 2.1 uS/cm and TOC level of less than 500 ppm.
In addition, Purified Water includes an action limit for microbial activity of 100 colony forming units per milliliter (cfu/ml). The steam does not pose a concern with regard to bacteria contamination because no known organisms can survive saturated steam temperatures and pressures. However, bacteria and biofilms may grow in cool, stagnant condensate pools.
Unlike utility steam and culinary steam where various chemicals are used to control scale and minimize corrosion, no chemical additives are permitted in the treatment of water used in clean steam generators. As a result, the generator is fabricated from corrosion resistant materials such as 316L stainless.
No specific feedwater quality is mandated in the production of clean steam, but the best practice is to use water that is softened (zero hardness) and dealkalized (low alkalinity). Zero hardness feedwater prevents excessive scale deposition inside the generator. The low alkalinity provision minimizes the generation of carbon dioxide gas, which would contaminate the steam and make it more difficult to maintain the required < 2.1 uS/cm conductivity. Hence, feedwater that has been demineralized by ion exchange or distillation is preferred. Another option is reverse osmosis followed by mixed bed ion exchange.
Pure steam is produced in a dedicated 316L stainless steel generator. The resultant condensate purity is identical to clean steam with respect to conductivity and TOC requirements. The condensate from a pure steam system must meet the US Pharmacopoeia specifications for Water for Injection (WFI), which carries a more stringent provision for a bacteria action limit of 10 cfu/100 ml as compared to that of clean steam.
Pure steam carries an additional specification for endotoxins of less than 0.25 endotoxin units (EU) per milliliter (EU/ml). Endotoxins are also called pyrogens in that they are known to cause fever and sickness. This is mainly a concern in the manufacture of parenteral products that are intended for injection into the patient. As a result, the most common steam specification in the pharmaceutical industry is pure steam since this meets WFI, which includes limits for conductivity, TOC, bacteria and endotoxin.
Pure steam must be produced from feedwater that is of drinking water quality and is further treated by distillation, demineralization, deionization and/or reverse osmosis to remove dissolved mineral solids that would contaminate the generator with scale and adversely affect the pure steam quality. The feedwater must contain less than 1 ppm silica, less than 1 ppm total hardness and be free of chloride, amines and chlorine. The final feedwater specific conductivity should be less than 5 uS/cm, which is of steam condensate quality.
Unlike culinary steam, filtration is not required in clean and pure steam systems in that no particulates or chemical residuals are entrained in the generated steam.
Steam quality and purity specifications are used as a guide in the production of a continuous supply of steam for a particular application. Utility steam specifications are intended to prolong the useful life of turbine drives and minimize unscheduled repairs. Culinary steam, clean steam and pure steam specifications are intended to protect public health by preventing contamination of our pharmaceuticals, food, humidified air, and surgical instruments. The specifications, guidelines and recommendations discussed in this article are intended as a useful tool in achieving these important goals. Further details and assistance are available from technical documents and competent chemical and engineering consultants.