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Navigating an Energy Audit

June 1, 2011
There are two types of energy audits: simple system and complete facility. This article explains the differences and provides an overview of the processes involved.

Maintaining an optimized boiler system extends the life of equipment and significantly reduces facility operating costs. In many cases, the path to an optimized boiler system begins with an energy assessment.

There are two types of energy assessments: a simple system audit and a complete facility audit. In a simple steam-system audit, a boiler room, boiler, and accessory equipment are evaluated. In a complete facility audit, a boiler system, as well as components located throughout the building, including steam traps, supply and return piping, valves, and supplies to steam users, are evaluated. This article further explains the differences and provides an overview of the processes involved.

The Basics

A simple boiler-system audit costs about $2,000, while a complete site audit, depending on the number of steam traps, the amount of other equipment, and the size of the plant, can cost several thousand dollars. For both, the essential steps are:

  • Data acquisition—examination of how a system or facility uses energy, including costs and utility issues affecting energy consumption.
  • Data analysis—identification of energy-conservation measures.
  • Recommendations—suggestion of solutions.

Steam audits can take several days to complete, depending on the type of audit and the size of the facility. Conducting an audit during normal operations makes abnormalities easier to spot.

During a boiler-room audit, a boiler, a burner, controls, energy efficiency, blowdown procedures, and feedwater conditioning are checked to identify:

  • Energy-saving opportunities.
  • Opportunities for implementing better operating practices.
  • Sustainability opportunities.
  • Health and safety concerns.

In a complete facility audit, an inspector not only checks key boiler-room equipment, he or she focuses on potential improvements throughout the building. The auditor inspects the distribution and return systems, including steam and condensate piping; pressures; temperatures; steam trapping; piping ancillaries, including valving and insulation; heat recovery; etc.

Energy savings are achieved by:

  • Locating and eliminating steam leaks.
  • Correcting bad trap applications.
  • Identifying heat-recovery opportunities.
  • Conserving flash steam.
  • Returning condensate.

Beginning With the Boiler

Regardless of the extent of an audit, one of an auditor's first tasks is determining the condition of a boiler system. The decision to replace a boiler should not be based solely on age. There are 70-year-old boilers that remain in good condition. If, however, a boiler is leaking, is heavily scaled, or has outdated burners and controls, it may be in need of replacement.

If a pressure vessel is in good condition, it may need only efficiency retrofits, such as an updated burner or an advanced control system, which costs significantly less than a new boiler. Often, resulting fuel savings pay for equipment upgrades within a year.

An auditor will check to see if a boiler is prone to overheating, an indication of a problem with insulation or gasketing, which can damage equipment and compromise safety.

Additionally, an auditor will perform a combustion test using stack-gas-analysis equipment to quantify boiler efficiency. This could lead to the recommendation of an oxygen-trim system mounted in the exhaust-gas stream. An oxygen-trim system continuously senses oxygen content, providing a signal to the controller, which adjusts the air damper and gas valve, maintaining a consistent oxygen concentration in the flue. This minimizes excess air while optimizing the air-to-fuel ratio, putting the burner on the proper side of the combustion curve during varying (uncontrollable) atmospheric conditions. Oxygen-trim systems typically increase efficiency by 1 percent to 2 percent, given the proper conditions.

Steam Function

The next step is to review the heating process and piping layout to determine how steam is applied and if decreasing operating pressure to reduce the sensible heat required to produce a pound of steam is possible. If the pressure can be reduced, fewer British thermal units will be required per hour, saving the facility additional energy dollars.

Stack Temperature

A boiler should operate 50°F to 100°F above saturated-steam temperature at full input. If it operates at a greater temperature differential, maintenance and cleaning should bring it back into the optimal range. From an efficiency standpoint, every 40°F decrease in temperature based on operating pressure, firing rate, and ambient temperature results in 1-percent fuel savings.

Sometimes, stack temperature rises because of soot on fire-side surfaces caused by improper air/fuel-ratio control. This means the burner is operating with excess fuel, causing fine black particles, chiefly composed of carbon, to form on heat-transfer surfaces. This slows heat transfer, causing considerable energy loss. Soot can be removed with a flue brush. Once the soot is gone, a professional should be called in to recalibrate the burner.

Another cause of elevated stack temperature is scale formation on water-side surfaces caused by improper water treatment. The remedy typically is acid cleaning or tube replacement, depending on the severity of the scaling. In either case, the cost often is recouped within a year.

Steam Piping

In a complete facility audit, the auditor surveys steam piping for energy losses caused by radiation and steam leaks. Often, steam leaks are identifiable to the naked eye.

Cutting down on radiation-induced heat loss is as simple as insulating a distribution system. More than half of process plants lack pipe insulation or have insulation that has deteriorated to the point of uselessness. The larger the pipe diameter and greater the length, the more effective insulation can be in helping to save energy.

Reducing steam leaks caused by piping corrosion and compromised flanging also can save a significant amount of energy.

According to the U.S. Department of Energy, ensuring steam mains are laid out, sized, drained, and vented properly is another way to reduce energy costs. An auditor can check that distribution-system piping is properly sized and adequately supported, guided, and anchored and that suitable allowances for pipe expansion are made.

Feedwater and Condensate

During a complete facility audit, an auditor also will investigate how feedwater supply and condensate return are handled. The auditor will check to see if the water sent directly to a boiler is free of dissolved gases, such as oxygen and carbon dioxide, which can corrode a boiler and condensate lines. If dissolved gases are found, the auditor is likely to recommend installation of a deaerator.

Additionally, an auditor will check to see if a facility has a properly functioning water softener. Without a water softener, scale will build up in a heat exchanger. A quarter of an inch of scale will increase fuel use by as much as 15 percent.

The auditor also will examine the condensate return. For many years, condensate was dumped down drains. Over the last few years, cost pressures and environmental concerns have forced facility engineers, managers, and supervisors to rethink how condensate is handled.

Turning condensate back into steam requires far less heat and fuel than producing steam from an equal quantity of cold water. That is because condensate already has been through a steam system. This reuse can save a facility hundreds of thousands of dollars annually, depending on the amount of condensate, the size of the boiler, and the boiler's operating hours.

Feedwater Temperature

An economizer can reduce the fuel requirements of a steam boiler by transferring heat from flue gas to incoming feedwater before the feedwater enters a boiler.

An auditor is likely to suggest an economizer if a system does not already have one. In determining if an economizer should be used, an auditor considers boiler size (200 hp and up), boiler operating pressure, degree of burner modulation, and feedwater temperature. Payback analysis based on size, pressure, modulation, and temperature conditions, combined with hours of operation and fuel costs, then is performed.

By recovering waste heat, an economizer can reduce fuel requirements by 5 percent to 10 percent. For a facility that spends $1 million to $3 million on energy annually, the savings can amount to $50,000 to $300,000 a year.

Steam Traps

As part of a complete facility audit, an auditor typically will look for steam traps that pressurize a condensate line ("blow through"), causing excessive venting, waterlogging, and inefficient overall process performance. Traps normally are checked using heat-sensitive or ultrasonic instrumentation.

Another critical issue auditors look for is water hammer. In steam systems, water hammer most often occurs when steam condenses into water in a horizontal section of steam piping, usually because of poor condensation drainage. Subsequently, steam picks up the water, forms a slug, and hurls the slug at high velocity into a pipe elbow or other restrictive device, creating a loud hammering noise and greatly stressing the pipe.

Imagine a filled 8-in. pipe with a slug of condensate 8 in. long. The slug weighs about 15 lb, about the weight of a bowling ball. Now put 100 psi of pressure behind the ball and roll it at 60 mph—the level of force of differential water hammer. This can cause pipes to break or even disintegrate, scalding or severely injuring individuals in the vicinity.

An auditor records the number of steam traps in a facility, tests and tags the steam traps, and documents energy-saving measures and payback.

Return on Investment

Financial analysis is a key component of a steam audit. Utilizing software that details annual cost savings from certain energy-efficiency upgrades, retrofits, and replacements aids in maximizing operating-cost savings over the long term.

Boiler-Room Log

A boiler-room log serves as a guide to a comprehensive maintenance program, helping facility management to determine when to clean a boiler, adjust combustion, repair or maintain a refractory or insulation, and adjust water-treatment requirements.

A boiler-room log also serves as a preventive-maintenance guide, covering the replacement of safety devices, fuel filters, gaskets, and other expendables. Each of those items helps to keep a boiler system running as efficiently and cost-effectively as possible year after year.

The director of training and marketing services for Cleaver-Brooks, Steve Connor has more than 40 years of experience in steam generation, including engineering, service training, and field-application sales. He speaks and writes frequently on the topics of boiler design, efficiency, and operation.

Did you find this article useful? Send comments and suggestions to Executive Editor Scott Arnold at [email protected].