Steam

Steam distribution is critical to deliver required quantities of specified temperature and pressure steam to the end user. Distribution lines spread from power plant or boiler room to final use by main headers at some predetermined pressure, with numerous take off lines, close to consumption points. Boiler room location inside the industry, relative to users, is a first efficiency option, since a centrally located steam plant reduces average steam header lengths, propitiates smaller pressure drops and higher efficiency on deliverance. Final users generally operate at different steam pressures, coherent with their requirements and these are obtained by pressure regulating valves or back pressure turbines, sending reduced pressure steam into smaller local pipe networks and branch pipes, where steam can be conveyed to individual pieces of equipment.

A well-designed and efficient steam distribution piping is adequately sized, laid out and configured, presenting proper pressure balance and regulation. Diameter selection has to consider acceptable steam speed range to avoid either too slow flow that promotes high heat losses to ambient, and excessive condensate formation and very high speed that provokes erosion and tube wear. A particular parameter for the choice of small diameters is attention to avoid speed noise. For safety and efficiency reasons the option for the slightly larger pipe diameters may be more expensive, but reduces pressure drop at a given flow rate and the noise associated with steam flow. Also large diameters introduce flexibility to peak loads and allow for some increase in demand over time. Layout and alignments must be coherent with project flow directions.

Another important issue, related to overall plant design, is avoiding production and use of very low steam pressures, below 3.5 kgfcm-2 (340kPa) or 50psig. Pressures below this level have a saturation temperature under 180 °C, offering a reduced temperature differential for average process heat demands. Specific volumes at these low pressures increase, implying bigger pipe diameters and costs for a relative low energy content flow. Also, there is a tendency for higher condensate formation, demanding more drip legs and traps with more corrosion, hence potentially more leaks.

Configuration aspects like flexibility and condensate drainage also have to be taken in account. For flexibility, piping and especially connections must accommodate expansion and contraction during start-ups and shut-downs, this prevents pipe wear and leaks. Condensate drainage must be addressed through appropriately sized drip legs, placed at regular length intervals in straight pipes and before significant equipment like turbines or valves and elevation changes. Pipe alignment should be designed with a proper pitch to promote condensate flow to drip legs, where steam traps discharge it to the return system.

Many opportunities for energy efficiency are in steam distribution; actually poor design and careless operation of this system can waste more than 10% of overall site energy demand. Even a set of efficient boilers and process plants may have its performance ruined by a badly kept steam distribution system. Installing, maintaining and improving thermal insulation is a first priority. A significant amount of heat energy may be lost for lack of insulation or for improper installation or inefficiency. Its adequate presence delivers fuel savings, better process control by maintaining process temperatures at expected levels, and safer working conditions. For safety reasons, any exposed heated surface must not surpass 60 °C to avoid skin burns and economical insulation thickness usually guarantees temperatures lower than that. Uninsulated points, damaged or wet insulation should be listed and regularly repaired to avoid increasing energy losses.

Monitoring and repairing steam leaks and steam traps also avoids considerable losses. Leaks represent energy and treated water losses allied to hazardous conditions, because high pressure steam burns can cause serious injuries, can damage nearby equipment and introduce high pitch noises, unaffordable in a healthy work environment. The lengthier and older the steam piping is, the more significant the number of leaks. They can be very costly, depending on opening size, steam pressure and period of leakage. Chasing and repairing steam leaks is a continuous and quite rewarding task. Steam traps are fundamental for steam quality and condensate recovery. Also, condensate accumulation in steam pipes is hazardous, because if a considerable amount of liquid begins to flow at steam velocity, water hammer phenomena may occur, potentially causing accidents. This subject is addressed in detail on the following section.

But steam traps can also be major contributors to energy losses in faulty operation. It is very easy for a large industry to have thousands of steam traps installed. Despite the energy loss, there is a good chance that a malfunctioning steam trap is feeding steam directly to a condensate line, pressurizing it and impairing condensate recovery, resulting in a much bigger loss in adjacent systems. Needless to say, that a blocked steam trap induces a bad heating process control, provoking extra and unnecessary energy consumption to compensate it. Regular assessment of steam trap performance is another continuous assignment that can bring about big results in energy savings, productivity and safety. Consequently, the next point to address has to be the condensate return system.

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