Boiler recommendations, specifications

Croce

Scruby

How can engineers right-size—not oversize—a boiler while also providing for flexibility for a client whose needs might grow in the future?

Scruby: There are two ways. Option A: Design an installation that is modular, allowing easy addition of boilers, without the first cost of installing now. Design the steam or hydronic headers to support the ultimate load or to be looped or paired as the plant expands.

Option B: Design an installation using a boiler or boilers that can operate efficiently at very high turndown, so that the full ultimate capacity can be initially installed.

Both options require a careful study of minimum and maximum initial and ultimate loads. Don't forget the part load operation.

Croce: I think clients can use multiple modular units and allocate equipment room space for future modules. For example, use four-plus smaller modular boilers in lieu of two or three large boilers to meet system requirements. In this way, one needs to only add 25% excess capacity instead of 50% to 100%. When the system expands, it's easier to add one or two more small units in parallel depending on the size of the expansion.

To a point, a boiler with a very high range of modulation (turndown) offers a greater range of operation with no penalty except first cost.

What is being done to keep steam from becoming a lost art for applications in buildings?

Scruby: The major vendors of steam boilers and specialties continue to produce excellent engineering manuals and training materials, both in paper and on the web, and the ASHRAE Handbooks still contain a lot of useful information if studied. Spirax-Sarco, Armstrong and Babcock & Wilcox, are just a few of the many vendors with readily available information.

In your experience, how can design engineers improve specifications for boilers controls?

Croce: Design engineers should focus on performance and seasonal efficiency expectations. To that end, they should try to be more specific as to how boiler features and performance are expected to relate to system requirements.

Scruby: With today's high fuel costs and increased concern for energy conservation, specifying engineers need to stay abreast of the technologies available. I encourage engineers doing boiler design and specification to learn about parallel positioning controls and oxygen trim options, and compare them with the standard mechanical-linkage based combustion controls on even relatively small units.

In what ways can engineers improve their control sequences for boilers and boiler controls?

Croce: At minimum, engineers should always follow the boiler manufacturer's recommendations for proper control sequencing or preferably have the boiler manufacturer supply the multiple boiler control panel.

At our company, where a controls engineer assumes responsibility for developing a multiple-unit control program using modulating equipment, our customers will benefit if the program emulates AERCO's BMS strategy. Rather than bring a single unit up to full capacity before bringing the next unit online, AERCO fires multiple units simultaneously—each at its lowest possible firing rate— to meet demand. This approach virtually eliminates cycling and is more energy efficient than conventional one-at-a-time sequencing approaches.

Scruby: Instead of writing a performance spec based on a single vendor, the engineer should understand how combustion control and burner management systems work, as well as the codes governing them. NFPA 85 Series, ASME CSD-1, as well as publications from UL, FM and other underwriters giving requirements for boiler and fuel controls are helpful in learning the safety aspects and considerations. Educational materials and training programs from ISA and vendors is also of great help in understanding the practice.

What are the recommendations for improving energy efficiency for the design for new boiler systems?

Scruby: I can think of several recommendations. 1) Evaluate advances in burners and control systems to maximum the fuel burning efficiency.

2) Design systems for maximum efficiency and at part and minimum load by developing a design and operating strategy that considers all of these conditions and the relative time spent at those conditions.

3) Apply variable speed drives properly. They can save a lot of energy in moving combustion air and feedwater or circulating water, but must be applied in consideration of their environmental limitations—temperature/moisture—and of safety.

4) Take a look at the life cycle of more heat exchange surface on the load side to allow utilization at lower pressure/temperature. Condensing boilers catalog impressive efficiencies, but be aware that they have to be operated at a low temperature in order to achieve them.

Croce: I can think of a few more recommendations: Always use condensing boilers with the highest available turndown and design around a 40°F or 60°F temperature differential. Design the system to deliver low return water temperatures and the lowest possible max header temperature to ensure condensing. Always incorporate some indoor/outdoor reset schedule in the control strategy.

What kinds of system add-ons do you recommend for improving boiler efficiency? Heat recovery devices, extra controls, etc.

Croce: Economizers, VFD pump controls, and radiant heating.

Scruby: Also include parallel positioning, oxygen trim, variable speed draft fans, variable speed feed pump control for steam, variable speed distribution pumping for hot water, which must be arranged to maintain minimum flow through the boiler.

What can be done to help owners maintain vigilance with boiler safety and efficiency? What are the most critical safety concerns for boiler systems for commercial buildings?

Scruby: The ASME Boiler and Pressure Vessel Code, Sections VI and VII contain extensive recommendations for safely operating and maintaining boilers. As far as I am concerned, a current copy of this standard should be on the shelf of every person responsible for a commercial or industrial installation and should be used by service organizations responsible for residential installations.

Croce: I agree. Owners should adhere to local codes and standards, and follow the recommended maintenance schedules.

What are some design tips that provide benefits toward long-term operations and maintenance? Such as locating valves and steam traps in easy-to-find locations, proper documentation.

Scruby: Manufacturers should design for safety, operability and maintainability.

Learn piping and valve applications. Specify piping, valves and fittings that are designed to last the design life of the installation, not necessarily those that meet the minimum pressure design. Consider steel vs. iron valves, welded vs. threaded construction, extra wall thickness or alternate material to resist corrosion, alternate valve and pump trim for more severe duty.

Make valves and instruments accessible. Pretend that you will operate and maintain it. Don't put instruments, pumps and frequently operated or critical valves up in the air in the plant where avoidable.

There are some great steam trap designs now that use a cartridge concept and can be completely repaired in line with a standard socket wrench. Pretend it is you who have to go down into the steam tunnel or manhole to repair it when you write the spec and draw the detail.

Croce: I think manufacturers should consider accessibility of components, remote failure alarms, preventative maintenance programs, remote system monitoring for preemptive maintenance.

What are some design errors that you have seen lead to premature wear and failure? Such as, oversizing or undersizing boilers or components, improper safety valves.

Croce: Allowing controls contractors to override manufacturers multiple boiler controls and/or other improper controls strategies that needlessly cycle the boiler's increasing wear. Just as important are changes in personnel without transfer of detailed manuals and procedural steps to follow.

Scruby: Oversizing boilers results in cycling instead of modulation at part load. Undersizing boilers results in the necessity of operating them at maximum firing rates for extended periods—much less common in my experience than oversizing, and usually corrected by an additional unit.

Oversizing feed pumps, failing to consider NPSH and minimum flow requirements all result in premature pump and often level control valve failure on steam boilers.

Pay attention to relief valve discharge to safe locations. On a high-temperature water boiler, remember that the discharge will contain water and a lot of flash steam. Make certain there are no water legs or traps that are undrained in any relief valve discharge due to the near certainty of severe water hammer in the event of a discharge. If you read the ASME recommended rules for operation, they recommend periodic relief valve lever and pop tests. The discharge system must be designed to safely allow this.

What challenges and opportunities await boiler manufacturers, engineers and owners, concerning strict national and regional emissions requirements and CO2 falling under EPA control?

Scruby: Less CO2 means less carbon-based fuel consumption or clean burning of carbon-neutral bio fuels. Efficiency improvement results in lower fuel consumption, both in the boiler and in the plant auxiliaries, with CO2 reductions coming from electrical generation carbon-based fuel consumption.

Hydrogen fuel for space heating? Electric boilers with green or non-carbon—nuclear—power source? Is nuclear power green? That's an open topic for discussion.

Croce: Developing low cost methods to achieve zero emissions for oil, wood and gas.

What new technologies are exciting to boiler owners that engineers should consider in their designs? Such as, new controls, new materials, new burners, heat-recovery systems, emissions controls.

Scruby: Engineers should consider advances in burner designs and controls. Low emission designs being offered in smaller sizes.

Croce: Reliable fiber mesh burners that provide low NOx emissions as standard. User-friendly proprietary controls that allow for expanded 2-way communications, status and fault enunciations both remotely and at the boiler. Controls that support data logging for trend analysis and to capture LEED-related performance information. Condensing oil-fired boilers, ground source heat pumps, geothermal heat pumps, fuel cells.