Take a system-wide approach to energy conservation

Making the business case for steam quality.

By Stanton McGroarty, Senior Technical Editor

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About half the fuel consumed by U.S. industry goes to produce steam, according to the U.S. Department of Energy’s Advanced Manufacturing Office. Virtually all of the electrical energy consumed, including transmission losses, throughout the United States is produced by steam-powered generating equipment. In other words, the vast majority of America’s carbon and nuclear footprints are created by the process of steam generation. This suggests economical steam production should be a national priority at least on a par with vehicular fuel efficiency.

Economics, corporate citizenship, and government incentives have created varying levels of awareness of the importance of steam management throughout the business community. “The U.S. Energy Information Administration estimates that as of 2006 there were 194,733 commercial establishments in the United States with active steam systems. Only 14,107 of these had installed or retrofitted equipment with the primary purpose of improving their steam systems’ energy efficiency. The study further reveals that less than 10% of these establishments perform annual inspection and repair of steam leaks. Less than 7% tested their steam traps on an annual basis, and an amazing 97.3% did not even maintain a steam trap database,” says Ted Clayton, marketing manager for energy and power management services at Kaman Industrial Technologies.

In other words, the job of improving and maintaining the efficiency of U.S. industry’s steam infrastructure is being performed in less than 10% of the places where it’s needed. Moreover, in the vast majority of steam plants, the system components most vulnerable to wear and degradation are not even listed and monitored. Clearly, there is room for almost all steam plant managers to do better.

steam quality1
Figure 1. Where new steam systems are being designed, each entire system can be optimized for the job to be done. (Source: Clayton Industries)

“Modern industrial steam systems often have an overall system thermal efficiency rate as low as 50%, or even less,” says Clayton. “While 100% efficiency may be physically impossible, 95% efficiency can certainly be had, which could effectively reduce fuel consumption to save as much as $45,000 on every $100,000 of fuel purchased.” Given energy prices, this means that significant financial rewards await those who improve their energy management performance.

Two levels of steam management savings are available to manufacturers. Where new steam systems are being designed, each entire system can be optimized for the job to be done (Figure 1). Where improvement of existing systems is the task at hand, many variables are already locked in place. Even here, a program of system-wide improvement can still be designed to capture a surprising level of energy savings.

New steam systems

“It is first important to distinguish the difference between quality and purity,” says Jason Jacobi, sales manager, Cleaver-Brooks’ Engineered Boiler Systems. “Quality is mostly a measure of the amount of moisture present in the steam after exiting the boiler. Most industrial boilers produce greater than 99.5% dry steam quality, or less than 0.5% moisture carryover. Purity, on the other hand, is often a measure of the total dissolved solids entrained in the steam itself. Quality and purity are linked. For instance, low steam quality usually results in poor purity. Another important definition is carryover, which is a phenomenon where water droplets are not completely separated from the steam-water mixture and are allowed to pass into the main steam line alongside normal dry steam. Carryover is a result of poor water treatment, poor boiler design, poor operating practices, or any combination of the above. It is the main cause of poor steam quality.”

The key characteristics that determine steam quality or purity for industrial watertube boilers include:

  • proper water treatment
  • proper boiler design
  • proper operating/maintenance practices.

“From the standpoint of a packaged boiler OEM, the boiler feedwater chemistry is crucial in determining the level of steam quality and/or purity,” says Jacobi. “Put simply, part of what goes in will come out. The cleaner the feedwater, the better the steam. Therefore, proper water treatment is the key to achieving clean steam. Both ABMA and ASME have published tables that the industry uses as guidelines for water treatment practices. The type of upstream water treatment system largely determines the incoming feedwater chemistry.

Demineralization plants provide some of the purest water and are typically used for power applications in which steam is sent to turbines, which can be damaged by poor steam containing carryover with silica/solids. Reverse-osmosis, water softeners, and other filtration methods are also commonly used for less critical steam applications. Chemical injection plays an important role in maintaining water chemistry. Chemicals are introduced into the system, both upstream of the boiler, as well as in-situ, depending on the purpose. The types of chemicals used are based on individual plant operating procedures for monitoring water/steam sampling and boiler blowdown rates, the latter of which affects thermal efficiency.”

While feedwater chemistry is by far the most important parameter affecting steam purity, it’s important to understand that the design of the boiler itself is also key to achieving quality steam to process, explains Jacobi. “For industrial watertube boilers, which are most often chosen for heavy industrial processes and power plant applications, the main design considerations are the steam drum size and the drum internals,” he says. “A large steam drum allows for increased steam volume and disengaging surface, which reduces the potential for water carryover. Excessive water carryover often occurs when the water level is too high. An adequately sized steam drum allows for greater flexibility in water level, which can fluctuate rapidly in some applications.”

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