Perspectives: Energy Saver
Control Steam System Energy Costs, Part II
Don’t accept steam and condensate leaks as low-priority housekeeping issues.
In Part I of this column, I addressed steam cost optimization opportunities, mostly regarding steam generation, as well as the need for maintaining a reliable condensate return system. More opportunities exist in other parts of the system, in particular, the large network of pipelines, valves and other fittings that are possible sources of heat energy loss. In addition, the steam distribution system requires devices to collect condensate, keep steam dry and control its flow and required pressure level. If these devices aren’t designed and maintained properly, the energy loss could be substantial.
A steam distribution system collects steam from boilers, waste heat boilers and steam turbine exhausts. In multiple-pressure-header steam systems, the lower-pressure-level headers automatically collect steam from the higher-pressure headers through letdown valves. As steam travels through various pressure-level pipelines to the point of use, it loses some of its heat and energy content, resulting in condensate formation.
For plants with sections of steam distribution piping outdoors, energy and process engineers can monitor steam demand change when it rains to quickly assess losses due to poor pipe insulation. One chemical processing complex in West Virginia with widely distributed steam distribution piping reported a 5,000-lb/hr steam surge whenever it rained. Losses likely occur even when it doesn’t rain and go up during the winter months. Hence, it’s worth conducting an insulation survey at least once every three years and fixing any damaged or exposed hot surfaces. Providing insulation blankets is preferable to pipe sections and fittings, especially those located outdoors, that require periodic removal for maintenance.
Steam’s thermodynamic properties offer some design challenges in transporting heat to multiple locations. Because heat loss can quickly transform steam into a bi-phase fluid, it’s important to take extra care when designing the steam distribution piping. To ensure dry steam supply and steam flow free from water hammer, the condensate formed in steam lines should be removed at appropriate sections of the steam distribution piping. Piping should slope downward in the flow direction and include drip legs at sufficient distances and before each rising section of pipe. Each drip leg should include a steam trap to drain out the collected condensate, ensuring dry steam delivery. Typically, these requirements should be addressed during the design stage. However, I find missing drip legs, inadequately sized drip legs and drip legs without steam traps in more than 90% of the plants I visit. Both wet steam supply and water hammer — resulting from an absence of steam traps or cold-plugged steam traps — lead to condensate accumulation that can slow down the heating of the process and cause plant stoppages. Hence, plant engineers shouldn’t think only leaking steam traps cause energy losses. An annual steam trap survey and fixing failed steam traps is an essential requirement that shouldn’t be compromised when management trims budgets.
Steam leaks and condensate drains are visible profit drainers in a steam distribution system. Instead of accepting them as low-priority housekeeping issues, fix them as soon as they are noticed. High-pressure superheated steam leaks generally aren’t visible and pose a safety risk to personnel. They might be worth fixing, even if ‘on-line’ leak repair is the only option. If plant engineers have the option to review the design of new or extended steam distribution systems, they should consider providing enough isolation valves, by identifying and classifying critical maintenance-prone sections.
Periodic steam system audits should be a routine part of the plant engineer’s cost optimization plan. Audits typically focus on finding any steam, condensate or heat losses and verifying the correct operation of the steam-heated equipment. Because of the higher temperature of steam and condensate, steam distribution systems are ideal subjects for inspection with infrared (IR) test instruments. Thermal imaging IR cameras are now available at affordable prices and provide temperature information across a wide field of view. Even if a steam leak occurs inside an enclosed object, such as a steam trap, it can be detected easily by an IR camera. Thermal imagers also can be used to identify hot spots on steam handling equipment with broken or damaged insulation. (For more on thermal imagers, see: “Use Thermal Imagery for Process Problems.")
According to a U.S. Department of Energy survey, steam accounts for one-third of all the energy used in process plants. Monitoring and optimizing the cost of your steam system can yield big rewards. Ignoring inefficient operation easily could drain profits.
VEN V. VENKATESAN is Chemical Processing's Energy Columnist. You can e-mail him at firstname.lastname@example.org
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