Boiler retrofits extend service life
When a boiler starts getting up in years, it is natural to start thinking of such questions as: How long is it going to last? Is it still serving my purposes? What can I do to upgrade it?
In general, a boiler can be expected to last 20-25 yr. Longevity, however, varies widely, de- pending on the care and maintenance the boiler receives. According to data from the American Boiler Manufacturers Association (ABMA), more than 125,000 commercial and industrial boilers have been placed in operation in the last two decades. Does every boiler last 20 yr? Definitely not. Some have a life span of 40 yr or more. Others last far fewer years.
At any given time, more than 100,000 commercial and industrial boilers are in service in North America. ABMA data also show more than 65,000 units are 10 yr or older. None of these boilers was built after 1989, an important consideration because of the number of advances that have been made in boiler technology since the mid-1980s.
The advances can be categorized into three major areas: system controls, burners, and emissions control. Let’s analyze these three areas to see what has changed and what could and should be modified to update and upgrade an aging boiler.
In many cases, certification and approval codes (UL, FM, IRI, and ASME) dictate the controls required on a boiler. These standards change periodically as governing bodies strive to improve equipment operation and operating practices. Today’s controls enhance boiler safety and, in some cases, deter unsafe practices. For example, more advanced controls now prevent the disabling of a safety switch to mask a problem in another part of the system. Modern technology also is flexible, allowing controls to perform functions that were difficult or impossible 10-yr ago.
One area greatly affected by microprocessor technology is the flame safeguard control or programmer. By the mid-1980s, computer-based controls had begun to replace electromechanical devices. Today, even the simplest flame safeguard control is microprocessor based.
Technology has expanded maintenance and troubleshooting capabilities and enhanced the safety of operating the equipment. The operator no longer has to know all the inner workings of the flame safeguard to troubleshoot a problem. A modern flame safeguard diagnoses problems, in some cases as precisely as to a particular switch in a circuit. It displays what the problem is, and when it happened in the operation sequence.
In most cases, information is stored in the control for retrieval later. Technology has advanced to the point where controls can be tied into and communicate with entire building management systems. These systems allow remote annunciation of failures and boiler monitoring from another part of the plant or from a distant location.
Upgrading some flame safeguards can be relatively simple, depending on the age of the control. In most cases, the old control is simply removed and the new one wired to the appropriate terminals. In some instances, slight wiring modifications must be made to the circuits.
The decision to upgrade a flame safeguard is probably best made in conjunction with your insurance carrier. A number of upgrades to flame safeguard controls are related to changes in codes and insurance requirements. Older controls may no longer meet today’s regulations.
Another point to consider is the availability of an older control if a replacement is necessary. Many older controls are no longer being made or rebuilt by the manufacturer. In many cases, upgrading the flame safeguard is the only option.
Insurance and code requirements also dictate improvements in boiler fuel trains. The added safety of a second fuel shut-off valve and a proof-of-closure switch in at least one of those valves is now very common. These devices help ensure that fuel is not entering the burner any time when it would be unsafe. The components in a fuel train are dictated by boiler capacity and by the insurance carrier. It is generally a good idea to talk with your boiler insurance inspector about recommended upgrades to the fuel train.
Combustion controls for commercial and industrial packaged boilers are normally simple devices composed of single point positioning with mechanical linkages. Combustion controls have been refined to more tightly control fuel and air ratios by using more substantial linkages and infinitely adjustable fuel cams.
Oxygen trim systems evolved concurrently. An oxygen trim system maintains the burner’s fuel/air ratio at the optimum point for efficiency and performance. Advances in flue gas sampling methods mean trim systems are more reliable and require less maintenance.
Upgrading combustion controls or adding an oxygen trim system is usually a matter of payback. Can the equipment increase efficiency to save enough fuel dollars to pay for itself? In many cases the answer is yes. Depending on boiler loading and annual fuel use, a typical oxygen trim system application can cut annual fuel costs up to 2%.
Burner design modifications now allow for increased operating efficiencies. The ability to achieve higher turndown ratios on larger boilers is a major factor driving those higher efficiencies. Turndown is the ratio of maximum to minimum output of the burner.
Traditionally, boilers in the commercial and industrial market have had turndown ratios in the area of 4:1. This means a 700-hp boiler can put out approximately 175 hp without cycling on and off. Revisions to burner design now allow turndowns in the area of 10:1 on natural gas burners and 8:1 on burners firing #2 oil.
The 700-hp boiler with a high-turndown burner now has a minimum output of approximately 70 hp. A lower output at minimum firing rate reduces on/off cycling of the burner at low loads. In turn, the reduced cycling increases operating efficiency because it eliminates purge and downtime losses and decreases normal wear and tear that results from cycling a piece of equipment.
Upgrading a burner to a higher turndown ratio normally consists of changing the burner housing, air damper, and a few other burner components. If an attempt has been made to achieve higher turndowns using the original burners, it is possible that the burner housing and air diffuser have been damaged. In those cases, burner replacement is a necessity.
Other burner upgrades relate to the fuels being fired. Emissions issues, fuel availability, and fuel costs have made natural gas the choice for many commercial and industrial boilers. Many burner upgrades include a conversion from oil to natural gas or the addition of gas as a secondary fuel. Secondary fuel capability adds flexibility to the system by providing backup and eligibility for interruptible fuel rates.
Another fuel gaining in popularity is propane. Because of emissions restrictions on the burning of oil, propane is frequently chosen over oil as a secondary fuel. In some cases, a propane/air mix is delivered to the burner. The ability to burn a secondary fuel is achieved by adding the proper burner components, fuel trains, and controls.
Upgrading a burner to reduce NOx emissions is normally dictated by the rules of the local air quality governing bodies. In some areas, only new installations are affected. Other areas require changes to existing equipment based on capacity and sometimes on the amount of emissions coming from other sources in the area. Regulations from state-to-state are changing rapidly and will eventually affect all boiler owners.
When packaged boilers that normally burn natural gas or some type of fuel oil are involved, the emissions of concern are carbon monoxide (CO), sulfur oxides (SOx), particulates, and nitrogen oxides (NOx). High CO levels are usually the result of damaged burner parts or poor burner setup. These conditions can be corrected by replacing the damaged parts and properly tuning the burner.
SOx results from burning fuels that contain sulfur (fuel oils). Particulates are caused by burning heavy oils and solid fuels. SOx and particulate emissions usually are reduced by burning cleaner fuels.
NOx reduction is the area of most concern today. Thermally produced NOx is the largest contributor to these types of emissions. Thermal NOx is produced during the combustion process when nitrogen and oxygen are present at elevated temperatures. The two elements combine to form NO or NO. NOx is generated by many combustion processes other than boiler operation. It combines with other pollutants in the atmosphere and creates O, a substance known as ground level ozone.
NOx in boiler burners can be reduced with either pre-combustion or post-combustion technology. Post-combustion technology allows NOx to form, then breaks it down in the exhaust gases (a process called catalytic reduction ). This method is normally confined to larger, utility-size equipment.
More common for the packaged boiler is the pre-combustion method, which prevents NOx from forming in the first place. Pre-combustion NOx reduction is accomplished by either staging the combustion process or recirculating flue gases into the combustion process (FGR).
FGR is accomplished by forcing the flue gases with a separate fan back into the combustion zone (forced FGR), or by drawing the flue gases through the combustion air fan (induced FGR). Both methods reduce the bulk flame temperature in the furnace to inhibit the chemical reaction between the nitrogen and oxygen. FGR systems reduce NOx emissions without reducing efficiency. NOx values can drop to less than 20 ppm corrected to 3% O when burning natural gas. Uncontrolled NOx readings are generally in the area of 80-120 ppm.
Which control technology is most appropriate for updating your system depends upon the type of burner and boiler. The more common technology is induced FGR, where flue gases are drawn into the combustion air fan. Induced FGR requires a minimum of components and moving parts. These include:
– A pathway for boiler flue gases to reach the combustion air fan
– A control valve to vary the amount of recirculation with the burner firing rate
– A slightly larger combustion air fan and motor than that used with a non-FGR burner.
If a four-pass boiler is involved, the upgrade is simpler because the gases exit the boiler at the front. The pathway for the recirculated flue gases is from the exit of the fourth pass into the air intake.
This discussion has examined some of the more typical modifications that can be made to upgrade aging boilers. An updated boiler may provide another 15 yr or more of safe, efficient service. One final point to consider when deciding to upgrade your boiler is to select only experienced, qualified people do the work. — Edited by Jeanine Katzel, Senior Editor, 630-320-7142, firstname.lastname@example.org
Modifying and upgrading an aging boiler can extend its service life.
Technological advancements for boiler systems fall into three major areas: system controls, burners, and emissions control.
The most appropriate advancements and technologies for updating your system depend on the type of burner and boiler.
The payback of high-turndown burners
The examples below illustrate the estimated payback that a typical plant can expect from retrofitting a boiler with a high-turndown burner. A high-turndown burner is defined as one that achieves 8:1 when burning #2 fuel oil and 10:1 when firing on natural gas. The purpose of a high-turndown burner is to eliminate excessive boiler cycling and reduce boiler wear and tear.
Original burner turndown, 4:1
Boiler size = 700 hp
Burner is firing natural gas
Boiler operates at 100 psi for 16 hr/day, 365 days/yr
Boiler cycles 6 times/hr
With high-turndown burner, 10:1
Efficiency gain is 1.6%
Total savings (fuel and maintenance) = $4700
Same boiler size
Operating at 125 psi, 24 hr/day, 365 days/yr
Boiler cycles 10 times/hr
With high-turndown burner
Efficiency gain is 2.8%
Total savings = $12,700
The author is available to answer questions about this article. He may be reached at 414-577-2746.
The American Boiler Manufacturers Association is located at 950 N. Glebe Rd., Suite 160, Arlington, VA 22203-1804; 703-522-7350; fax: 703-522-2665; web site: www.abma.com.
Boilerspec, a web CD-ROM tool for boiler specification and selection, is available from Cleaver-Brooks. For information on obtaining a copy, call 414-359-0600 or check out the company web site at www.cleaver-brooks.com.
Also see the “Process and space heating” channel on Plant Engineering Online at www.plantengineering.com for more articles related to this topic.