Strive for steam quality

Figure 1. The criticality of steam quality varies by application, but the best way to alleviate steam-quality issues is to maintain the quality of the water. (Source: Cleaver-Brooks)

Moisture in the steam can cause severe damage to the piping, valves, and other control devices, explains Nand Bogman, vice president of applications for Clayton Industries (www.claytonindustries.com). “The drier the steam, the more energy is contained in the steam delivered to the process,” says Bogman.

Although steam quality standards vary by application, common challenges are faced and the steps to achieving sufficient quality are shared:

• measure steam usage and costs
• manage the root issue (water quality)
• optimize the boiler system design
• employ maintenance best practices.

Understand the application

The criticality of steam quality varies by application. “It depends on where the steam is used,” says M. Vasudevan, vice president of operations for Cleaver-Brooks (www.cleaverbrooks.com). “If it is used to drive a turbine, it’s a big concern because there’s an acceptable limit to how much silica/solids can be carried over. Once a turbine is coated with solids, it is less efficient and carries more weight. Similarly, in food applications, steam quality is a big issue. But for heating buildings, steam quality is less of a concern, and in oil sands, where steam is pumped in the ground to heat up and release oil, steam quality is not a concern at all.” (Figure 1)

Pharmaceutical companies need particularly clean steam, adds Pace. “They ensure quality through sampling and by making sure the boiler operates at the right temperature and pressure,” he says. “On the other hand, a school department can tolerate a 30-year-old boiler that has accumulated a lot of rust, but it can still increase its efficiency with the right technology.”

Recognize the challenges

Figure 2. Inherent steam quality is based on conservative design of the boiler and the piping. (Source: Fulton Companies)

The greatest threats to industrial steam quality are maintenance and lack of knowledge of how the system is supposed to be running correctly, explains Horizon’s Pace. “A big problem is the graying of the industry, where older, experienced personnel are retiring and there is not a lot of young talent to fill their shoes,” he says.

“All too often, the new boiler designer does not have the decades of industrial steam experience required for a conservatively designed product that will deliver the required performance in a variety of challenging markets,” adds Melissa Wadkinson, chief engineer for Fulton (www.fulton.com) (Figure 2).

Not only is the workforce aging, but the assets are, as well. “The most pressing risk to boiler steam quality revolves around aging equipment, especially with regard to instrumentation and control,” says Ralph Porfilio, director of technology and application engineering for ABB's Power Generation business (www.abb.com). “So much of the equipment in industrial and utility power plants, not to mention the power grid in general, has outlived its life expectancy and is in need of either repair or replacement.”

Carryover, which results from incomplete separation of steam from the steam-water mixture in a boiler, is considered one of the greatest challenges in industrial boiler steam quality by D. Neil Bradwell, principal engineer for Indeck Keystone Energy (www.indeck-keystone.com). “Many factors, both mechanical and chemical, contribute to incomplete separation,” he says. “Mechanical factors include boiler design, inadequate or leaking separating equipment, high water levels, method of firing, and load characteristics. Chemical causes are high total solids concentrations, excessive alkalinity, and the presence of oily materials and other organic contaminants.” (Figure 3)

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Figure 3. The primary consideration in the selection of a boiler and its operating conditions is the amount of carryover that can be tolerated. (Source: Indeck Power Equipment)

Boiler water carryover not only compromises steam quality, but it is also one of the biggest threats to steam system safety, explains Andy Butcher, product manager for Spirax Sarco (www.spiraxsarco.com/us). “Water carryover is droplets of water sucked up with the steam into the boiler outlet pipe,” he says. “These droplets of water not only reduce the dryness of the steam, reducing the heat content, but also travel at the high steam velocity, causing erosive damage to piping and equipment. When combined, they form high-velocity slugs that can destroy anything in their way, sometimes leading to fatal injury.”

Other risks to boiler steam quality include control system tuning to adapt for typical component usage and wear, and duty cycle changes due to renewable energies entering the grid, adds Porfilio.

Begin with benchmarking

Proper boiler system maintenance begins with benchmarking your steam usage. “It is important to understand the amount of steam used versus how much it costs,” says Horizon’s Pace. “Calculate the cost per pound of steam and benchmark that against others in your market segment. Then, as you perform routine maintenance of your steam traps, monitor how much steam you use, how you use it and why, and determine if there is a better way.”

Pace recommends the U.S. DOE’s Steam System Assessment Tool (SSAT) software suite as a great first step in understanding how to use steam, its cost, and opportunities to improve. “It allows you to model different scenarios to make efficiency improvements and calculate the financial impact,” he says. “For example, you don’t want to add a $100,000 control before determining whether it is financially worthwhile. The DOE offers one-day and five-day SSAT training sessions.”

Spirax Sarco’s Butcher attributes the rise of measurement to the recent economic climate, which has companies searching for ways to save money and improve efficiency. “We’ve offered flow meters for measuring steam, condensate, and other fluids for many years, but modern technology, such as digital communications, PLCs, and software, allows us to bring that information together and give it to the user in a format that he needs: percentage efficiency and cost per pound of steam,” adds Butcher. “Not only does this allow the user to see the results of any improvements he has made, but it also flags maintenance issues. For instance, the failure of a condensate return pump will be seen as a drop in efficiency from the trend line.”

Address the root cause

The best way to alleviate steam quality issues is to maintain the quality of the water, says Cleaver-Brooks’ Vasudevan. “Water has some impurities, like silica, dissolved salts, and organics,” he says. “To prevent impurities from water particles from being carried over, you need to use better quality water, monitor it on a regular basis, and maintain it within the industry standards for recommended water quality based on the steam pressures used.”

The primary mechanism for impurities in steam is water droplets getting carried over with the steam from the boiler. “For scrubbing steam coming out of a boiler, there are a number of separation/mechanical scrubbing strategies to achieve 99.0% to 99.95% steam purity,” explains Vasudevan.

Boiler system design is crucial

Inherent steam quality is based on conservative design of the boiler and the piping, says Fulton’s Wadkinson. “The technology is not new, but it is often overlooked in today’s cost-driven market where newer boilers entering the market are designed and constructed to reduce manufacturing costs,” she says.

The primary consideration in the selection of a boiler and its operating conditions is the amount of carryover that can be tolerated, says Indeck’s Bradwell. “Once the right equipment is in place, then steam lines should be insulated and a proper steam line drip leg steam trap station should be established,” he says.

Maintenance best practices

Proper maintenance and controls can alleviate boiler steam quality threats. ABB’s Porfilio suggests the following five steps.

• Perform instrumentation and control equipment audits to help discover potential weak points and make recommendations for improvements.
• Leverage on-site support services to re-tune process control loops.
• Consider advanced control techniques such as model predictive control (MPC) and advanced coordinated control.
• Improve automation systems.
• Offer simulation systems to help train operations staff.

MPC applications are the most promising development in steam quality management, says Porfilio. “MPC applications are even more important for industrial plants — usually smaller boilers with larger swings, multiple units, and multiple steam load and pressure demands,” he says. “Excellent problem solving capabilities are possible with advanced controls.”

Sheila Kennedy is a professional freelance writer specializing in industrial and technical topics. She established Additive Communications in 2003 to serve software, technology, and service providers in industries such as manufacturing and utilities, and became a contributing editor and Technology Toolbox columnist for Plant Services in 2004. Prior to Additive Communications, she had 11 years of experience implementing industrial information systems. Kennedy earned her B.S. at Purdue University and her MBA at the University of Phoenix. She can be reached at [email protected].

Outside expertise fills the void when internal skills are lacking. “When given the opportunity, we will review the project application, piping, controls and system, and make recommendations accordingly,” says Fulton’s Wadkinson. “This expertise in the entire system design and not just the boiler product has been gained through our many decades of experience in manufacturing factory-packaged boiler systems with all of the applicable components.”

Proper attention to boiler system steam quality provides industrial plants and utilities greater control over heat transfer efficiency and boiler performance, and it lowers their total cost of ownership.

“Steam sometimes gets a bad rap for being outdated, inefficient, and difficult to maintain, but it remains the most efficient way of transporting heat energy around a plant or building,” says Spirax Sarco’s Butcher. “Steam systems are so robust that they continue to work despite neglect and lack of maintenance. The problem is they then become costly to run and potentially dangerous in the same way an electrical system can. A correctly designed and maintained steam system is clean, safe, and extremely efficient.”