Clean Steam
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What standards exist for clean steam systems? |
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There are, as at October 2000, no direct standards for clean steam systems. However, specifications suitable for clean steam environments are covered within the following industry standards and guidelines: USA FDA CFR21 LVP GMP (FDA) ANSI/AAMI/ST25 - 1987 3 - AS ASME BPE 1997 EUROPE Various BS:EN incl. EN285 & EN554 HTM2010 & 2031 Publ: HEAT PRESERVED FOODS FDA/ISPE Baseline Guide |
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Steam Tracing
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What factors need to be considered to determine the correct number and size of tracer lines? |
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The rate of heat loss must be determined from the insulated pipeline under design conditions. This will depend mainly upon the difference between the highest expected product temperature and lowest expected ambient temperature. The number and size of tracer lines will depend on their material (often steel or copper), on the tracer steam pressure, the product temperature, and whether heat transfer cement is used. Tables are available from Spirax Sarco to ease calculation. |
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What is steam tracing? |
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Steam tracing is a simple method of allowing a normally viscous product to flow through a pipe. Small diameter tracer line fed with saturated steam is wrapped around the product pipe and encased in insulation along its length. Heat transfer from the tracer maintains product mobility, while heat lost to surroundings is minimised by the insulation. There are three main applications for steam tracing; heat maintenance, frost protection, and heat addition. Each can be controlled easily and cost-effectively with self-acting control valves. |
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Steam Trapping
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I have heard that fixed orifice devices are more efficient than automatic steam traps. Can this be true? |
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A fixed orifice device fitted to any heat exchanger will hold back condensate so that it sub-cools below saturation temperature for much of the time. If the heat exchanger and/or control valve are oversized (and most are), there may not be any difference in thermal performance as there may still be enough heating surface to cope. Sub-cooled condensate will also reduce flash steam from condensate receiver vents. These two observations can be seen as a way to save energy, and it is sometimes deduced that orifice devices and thermostatic traps are more efficient than mechanical traps. However, this is not as straightforward as it seems, as waterlogging with sub-cooled condensate causes unseen corrosion with costly results. The service life of the heat exchanger is reduced, and lifetime costs are increased. Sub-cooling on some applications can bring short-term catastrophe too. Sub-cooled condensate in a frost coil with cold air passing can easily freeze with sudden and dire results. |
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How do I size steam traps? |
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A steam trap has to be sized to pass a worst-case condensate load with the differential pressure existing at that time. Differential pressure is the difference between upstream and downstream pressures, and the worst-case condensate load will differ depending on the application. For example, for mains drainage, it will occur at start-up, whereas for any controlled heat exchanger it may occur at a stall condition. It is not uncommon for a correctly sized trap to be a different size to the connected pipes. |
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