2000 Hook-up Book

Clean Steam

Overall Requirements of a “Clean” Steam System The overall requirements of a “clean” steam system, irrespec tive of the means of generation of production used, can be very sim ply stated: It is essential that the steam delivered to the point of use is of the correct quality and purity for the process. In order to achieve this end goal, there are three key areas of design which must be considered once the requirement for clean steam has been identified. Design and operation of equipment, piping, components, etc. in all these three areas will influence the quality of the final process or products. It is essen tial for the needs of the user process to be the first concern. Must the steam be pyrogen free? Are any boiler additives allowed? Are products of corrosion going to harm the process or product? Must the risk of biological conta mination be totally prevented? It is by answering these questions, and perhaps others, which will indicate the required type of pro duction, design of the distribution system, and the operation modes of the user equipment, including aspects such as steam trapping. Specific Requirements of “Clean” Steam Systems Clean or pure steam produced from water of very high purity is highly corrosive or “ion hungry”. The corrosive nature becomes more pronounced as the concen tration of dissolved ions decreases with the resistivity approaching the theoretical maximum of 18.25 megohm/cm at 25°C. In order to recover a more natural ionic bal ance, it will attack many of the materials commonly used in pipework systems. To combat this, pipework, fittings, valves and associated equipment such as traps, must be constructed from • Point of Use • Distribution • Production

corrosion resistant materials. Typically, a “clean” steam system of this type will have resistivity val ues of the condensate in the 2-15 megohm/cm range, resulting in very rapid attack of inferior quality components. Even in some filtered plant steam applications, such as in the food, dairy and pharmaceuticals industries, certain corrosion inhibit ing chemicals may be prohibited from the boiler and steam generat ing system. Again, condensate is then likely to be very aggressive and so careful consideration must be given to material selection. A common problem encoun tered on clean and pure steam systems in the pharmaceutical industry is that of “rouging”, which is a fine rusting of pipes and sys tem components. This is encountered most frequently when low grade stainless steels are used, and further corrosion due to galvanic effects can take place where dissimilar alloys are present in the same system. Unless care is taken with materi al selection throughout the system, corrosion can become a major problem in terms of: a) Contaminating the system with products of corrosion, which are undesirable or even potentially dangerous to the process or product. b) Severely reduce life of sys tem components, increasing maintenance time, material replacement costs, and sys tem downtime. In order to prevent these problems, austenitic stainless steel should be used throughout, never of lower grade than AISI 304. For severe duties, the rec ommended material is AISI 316 or 3161L (alternatively 316Ti) or better, passivated to further enhance corrosion resistance. In summary, 316 or 316L stainless steel is essential in pure steam systems from its pro duction at the generator right through to the steam traps. Not only will inferior materials corrode and fail prematurely, they will also

lead to contamination of the sys tem as a whole. Note that although filtered plant steam will not necessarily be so aggressive by nature, the exclusion of many of the corrosion inhibiting feed chemicals for end product purity reasons will still demand the use of austenitic stainless steel, never of lower grade than 304/304L, but preferable 316/316L. Clean Steam and Condensate System Design The proper and effective drainage of condensate from any steam system is good engineer ing practice, as it reduces corrosion, erosion, and water hammer, and increases heat transfer. This becomes even more important in “clean” steam system, where poor condensate drainage in the distribution sys tem or at the user equipment can result in rapid corrosion and also, under certain conditions, the risk of biological contamination. The following points should be care fully considered: • Pipework should have a fall in the direction of flow of at least 1.0 inch in 10 ft., and should be properly supported to prevent sagging. • Adequate mains and service pipe steam trapping should be provided, for example at all ver tical risers, upstream of control valves, and at convenient points along any extended pipe length. Trapped drain points should be provided at intervals of at least every 100 ft. • Undrained collecting points should not be used, as dirt should not be present and they provide an ideal location for bacterial growth where systems are shut down. • Condensate should be allowed to discharge freely from steam traps using gravity and an air break. This air break should be provided at the manifold outlet or the closest convenient location (Fig. 57). Where the air break would otherwise be in a clean room, the potentially harmful

SYSTEM DESIGN

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