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.
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.”
Once you have good water chemistry and an adequately-sized steam drum, properly designed drum internals are the last line of defense, continues Jacobi. “Most manufacturers utilize primary and secondary separation systems within the drum to ensure pure and dry steam leaves the boiler on its way to process. Primary separators, such as belly pans, cover the boiler’s steam generating riser tubes and serve to discharge wet steam into the drum volume above the waterline. Secondary separators, such as chevrons, essentially filter the wet steam by forcing it through a labyrinth of small passages which capture the smallest of water droplets allowing only clean and dry steam to escape. Most boiler manufacturers offer steam quality or purity guarantees, so it is in their best interest to ensure the unit is designed for the worst-case scenario.”
Jacobi cautions his explanation applies to packaged industrial watertube boilers (natural circulation), which are most often selected for industrial steam applications. It doesn’t address commercial firetube boilers or the unique needs of once-through boilers (forced-circulation), which are used primarily for heavy oil recovery. “These must be discussed on a case-by-case basis,” says Jacobi.
Existing steam systems
While it’s important to determine when steam systems have aged to the point where complete system replacement is indicated, most years the majority of managers will be constrained to achieve savings by addressing losses incurred on the existing system. “A typical modern steam system features four distinct elements: steam generation, steam distribution, steam usage, and condensate recovery,” says Kaman’s Clayton. “Steam generation involves the heating of the water to its boiling point and will typically involve elements that include a fuel supply, combustion burner area, boiler, and combustion exhaust stack. Steam distribution carries the steam to its point of use and involves piping, valves, regulators, steam separators and accumulators, steam traps, and flow meters. Steam usage can involve any number of application-specific functions, but some of the more common elements include heat exchangers, condensers, turbines, fractionating columns, chemical reaction vessels, dryers, and evaporators. Condensate recovery involves transferring the cooled condensate back to the boiler for re-boiling and typically includes steam traps, piping, tanks, pumps, and condensate treatment equipment.”
Steam generation and distribution are often referred to as the supply-side functions. Steam usage and condensate return are usually grouped as the demand side of the system.
Stack loss can divert as much as 25% of the energy from burned fuel by sending it up the stack as hot exhaust. There are a wide variety of heat recovery systems most of which preheat the water or condensate on its way to the boilers. Most of the stack losses can be captured by recovery systems, depending upon the application, says Clayton.
Supply side losses
Figure 2. If use is intermittent, boilers and the structures around them can radiate a great deal of heat to the environment. Thermal analysis and insulation are the primary tools for reducing standing losses. (Source: Clayton Industries)
Standing losses are another possible financial drain on the supply side. Boilers and the structures around them can radiate a great deal of heat to the environment. This is particularly true if boiler use is at all intermittent. Thermal analysis and insulation are the primary tools for reducing standing losses (Figure 2).
Blowdown loss refers to steam, heat, and condensate that is removed from the boiler to reduce the concentration of solids that have been left behind by the water that has been converted to steam. The main solution to blowdown losses is to improve the chemistry of the water entering the system so that there are fewer impurities to remove from the boiler. Returning condensate to the boiler will also help with blowdown losses, as condensate has already shed most of its impurities in the process of being converted to steam.
Demand side losses
Steam trap loss is an important cost, as the traps consume steam to do their job of expelling condensate from the system. It is also important to use the right kind of steam traps for your system — mechanical, thermostatic, thermodynamic, or venturi, to minimize losses.
Condensate flash loss occurs when hot condensate is expelled from the system, rather than returned to the boiler. The answer here is to improve condensate return percentage. If the old system doesn’t have a return, it presents a great opportunity, especially if the condensate can be returned to the boiler without major heat loss. Condensate is often 200 °F, making it far less costly to re-vaporize than fresh water, and as mentioned previously, recirculation can reduce water conditioning cost by reusing previously conditioned water.
Pipe insulation loss occurs when piping is inadequately insulated. Thermal imaging should be used aggressively to identify system hot spots so that they can be insulated.
|J. Stanton McGroarty, CMfgE, CMRP, is senior technical editor of Plant Services. He was formerly consulting manager for Strategic Asset Management International (SAMI), where he focused on project management and training for manufacturing, maintenance and reliability engineering. He has more than 30 years of manufacturing and maintenance experience in the automotive, defense, consumer products and process manufacturing industries. He holds a bachelor of science degree in mechanical engineering from the Detroit Institute of Technology and a master’s degree in management from Central Michigan University. He can be reached at firstname.lastname@example.org or check out his Google+ profile.|
Leakage losses develop over time in any piping system. Thermal imaging and ultrasonic inspection are important here, as well. Regular pipe inspection pays off financially and in safety results.
“Enabling cost reduction does not automatically make savings happen,” says Kaman’s Clayton. “It is still necessary to capture the cost reduction. For example, steam trap repairs might pave the way for 10% energy cost reduction, but the steam supply must be throttled back to actually capture the savings. Sometimes a pressure change may even be required to fully realize the benefit of the changes. Similarly, if steam quality is improved and condensate recirculation is increased, blowdown loss can be reduced, but only if blowdowns are made less frequent.”
Thermal oxidizers burn byproducts and use the heat to help produce steam, adds Mike Pace, energy engineer, Horizon Energy Solutions. “The result is two kinds of improvement; a reduction in fuel purchases and an environmental improvement in plant emissions,” says Pace.
“Aside from ecological and financial sustainability, let us not forget that good steam quality is of paramount concern to plant safety,” says Cleaver-Brooks’ Jacobi. “Carryover of liquid water can be a very serious issue. Too much water in a steam line may sound harmless but can actually be very dangerous when one considers the velocities within a steam line. Small amounts of carryover can cause water-hammer, which at first may just seem like a nuisance, but may be a warning sign of a much bigger problem if large amounts of water are allowed to accumulate within the steam piping. A good slug of water can travel very fast, essentially acting as a bullet with devastating inertia that can lead to catastrophic failure and potential loss of life. Water carryover was cited as a possible cause of the 2007 New York City underground steam explosion in Midtown Manhattan, which resulted in two fatalities and dozens of serious injuries. Routine preventive maintenance of equipment like steam traps is essential to ensuring that safety is always top priority.”