The tricky act of balancing safety, performance, and economics
The boiler was a keystone of America’s Industrial Revolution, helping to propel the country to the status of world superpower. For the better part of a century, however, the boiler posed great risk. By some estimates, from the 1840s to the 1920s, 50,000 people died from boiler explosions every year. Thanks to the work of the American Society of Mechanical Engineers, boiler safety was greatly improved.
Once boilers were made safe, attention turned to performance, as the rating and true capacity of boilers often did not match. Mechanical Contractors Association Chicago, then known as the Chicago Master Steam Fitters Association, helped to remedy the situation, employing engineers who published their own empirical ratings of boilers.
While the boilers of today are nothing like those from earlier times, we still seek to balance safety and performance. Today, our business climate demands we add economics to the equation, viewing the boiler as an investment in efficiency and sustainability. When should I replace my boiler? How long will my boiler last? How can I minimize life-cycle costs? All are important questions for a facility manager or owner.
To Repair or to Replace?
Whether a boiler should be repaired or replaced is largely dependent on age and condition.
Whether a boiler is 25 years old or 50, efforts to maintain its physical condition are key. Has maintenance been regular? Has the boiler been inspected internally? Has the chemical program been executed properly? Also, you will want to compare the costs of repair and replacement with savings from improved operational efficiencies.
Consider the example of a large sectional boiler that is 50 years old and leaking profusely. Should it be replaced? At that age, it probably should, as it is at the end of its useful life anyway. Still, there are questions to ask:
Now, consider the example of a smaller boiler that is 25 years old. Newer, smaller boilers are designed to last only 25 to 30 years. The same maintenance and cost considerations apply. One probably could go either way: repair or replace.
By the Numbers
The easiest way to determine whether to replace a boiler is to perform a simple-payback calculation.
Let’s say you have a boiler with an estimated 10 years of life left in it. Repairing the boiler would cost $2,000; replacing the boiler would cost $9,000. The existing boiler is 65-percent efficient; the new boiler is 85-percent efficient. Annual fuel cost with the existing boiler is $3,000. To determine annual fuel cost with the new boiler, divide the efficiency of the existing boiler (0.65) by the efficiency of the new boiler (0.85), and multiply by the current annual fuel cost ($3,000):
0.65 ÷ 0.85 × $3,000 = $2,294
Fuel-cost savings with the new boiler would be $706 a year ($3,000 − $2,294), or about $7,000 over the 10 years the existing boiler is estimated to have left. If you subtract the 10-year savings from the cost of replacing the boiler ($9,000 − $7,000), you get $2,000, the cost of repairing the existing boiler.
Additional savings from upgrading or replacing the control system are best looked at separately.
Because you eventually will have to replace the old boiler anyway, you might as well replace it now.
Certainly, there are more complex ways to do this (e.g., present value, hours of operation), but this author believes strongly in “keeping it simple.” If interest rates were to jump to 18 percent or we were talking millions of dollars, the time value of money would be worth taking into account. With interest rates at their current levels, however, the author recommends simple-payback calculations. Remember, you are making assumptions (future required repairs, future cost of fuel, future changes in hours of operation) here, so your payback calculations are more of an educated guess than a hard number.
Getting the Most out of a Boiler
Regular maintenance is important to prolonging the life of a boiler. Boiler maintenance is a specialized field, thus, we are not going to go into great detail. Generally, though, daily, monthly, and annual maintenance is required for any boiler.
What are some key aspects of boiler maintenance? Cleaning and servicing a burner to ensure its fuel-air ratio is correct and that it is burning in a safe manner is one. This affects both safety and performance. Oil burners require cleaning and servicing much more often than gas-fired burners.
Water Quality
Especially with steam boilers, regular blowdown is necessary to prevent the buildup of solids.
As water is heated and solids come out of solution, “surface blowdown” is required to bleed off some of the water in the boiler, removing impurities that rise to the water-level line. Blowdown also is done on the bottom of the boiler to keep sludge from forming. The buildup of solids and sludge reduces the heat-transfer capabilities, efficiency, and life of a boiler.
Surface blowdown typically is performed with an automatic device. Bottom blowdown normally is a manual operation, one that requires adherence to a strict schedule. Heat recovery, by which heat is recaptured from water before it goes down the drain, can be added.
With hot-water boilers, chemicals usually are introduced with a “shot feeder” installed around one of the circulation pumps. Once it is treated properly, a hot-water system requires very little maintenance.
With steam boilers, chemicals normally are introduced in makeup water to help control scale and oxygen. Typically, chemical feed systems are automatic and used in conjunction with boiler blowdown. The trick is to use the right combination to minimize both the amount of chemicals used and the amount of boiler water going down the drain. Boiler water treatment is highly specialized; it is best left to experts.
Safety Devices
Relief valves open automatically to lower pressure in a boiler. They must be checked regularly to ensure they are not blocked or closed and that they are fully functioning. Otherwise, pressure could build until a boiler blows apart. Relief valves on large high-pressure boilers require periodic removal and recertification.
Another important control is the low-water cutoff. Should boiler water drop below a safe operating level, a low-water cutoff will shut off the burner before the boiler “dry fires.” Dry firing can be likened to an empty pot or pan being left on a stove for too long, but the consequences are far more serious. Like relief valves, low-water cutoffs require regular maintenance.
Because they are so critical, safety devices should be maintained per manufacturer instructions. Additionally, they never should be bypassed.
Other Types of Maintenance
About once a year—more or less often depending on the condition of a boiler—a boiler should be dismantled and inspected for leaking tubes or problems with the refractory (insulation). Refractory is used inside of the combustion chamber, firebox, and back doors of boilers. Its purpose is to protect any part of a boiler that does not have water behind it.
There are ways to check tubes and components without dismantling a boiler, but those typically are for very large units or special circumstances. One type of non-destructive testing is eddy-current testing, by which electricity is used to measure the thickness of tubes and pipes inside of a boiler.
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Boiler maintenance is very complicated. It should be performed on a regular schedule by trained and competent personnel, with the intervals and findings logged.
Conclusion
This article answered the big questions concerning when to repair or replace a boiler. A boiler’s age, its operational costs, and the degree of difficulty in repairing or removing a boiler are important considerations. Maintenance is critical, too, as it affects a boiler’s longevity, operating costs, and safety. Read the sidebars to learn how various building owners and managers have handled the repair-vs.-replace dilemma.
Bill Hennessy, HVAC field superintendent for Mechanical Contractors Association Chicago member company Hayes Mechanical, was reviewing the heating and air-conditioning equipment at Local Union 597 (LU 597) Chicago Pipe Fitters Hall when the hall’s cast-iron boiler sprang a leak.
“The boiler system in place was originally installed in the 1960s,” LU 597 Financial Secretary/Treasurer Kevin Morrissey said. “It was time for a major update, as the system had issues for years.”
According to Hennessy, the boiler/burner configuration was firing at a rate of 1.25 MMBtu to maintain system temperature, and replacement parts were becoming difficult to find.
“We took this opportunity to upgrade the boiler equipment and provide a gas savings for the union hall,” Hennessy said.
The new system, Hennessy said, “will provide adequate heat for the building on the coldest Chicago days. And for its capability to provide a 5-to-1 turndown ratio from 750,000 Btu per boiler to as low as 150,000 Btu, this will allow the boilers to reduce their firing rate. It also allows for only one boiler to run if the building load decreases.”
The bottom line: “Now, we’ll have a highly efficient heating system actively monitored by a whole-building computer system,” Morrissey said.
A hotel in downtown Chicago had steel boilers well over 50 years old.
“A consulting firm came in and recommended the removal of three old boilers and installing nine smaller boilers as a replacement,” Frank M. Lacny, president of Mechanical Contractors Association Chicago member company American Combustion Service Inc., said. “We ... offered ... to rebuild the existing boilers in place and install high-efficiency burners on the old boilers.”
In the end, the hotel went with the rebuild, as the costs were nearly 50-percent less. Also, the hotel received a rebate of nearly $80,000 from the local gas utility for installing high-efficiency burners.
“The rebuilding included replacing 444 stay bolts on each of the three boilers, replacing all the boiler tubes, tube sheets, refractory firebox, and circulator tubes,” Lacny explained. “We removed all scale from inside the water walls and re-insulated the rebuilt boilers. We then installed new high-efficiency burners on each boiler and a new master control panel for lead-lag operation and to alternate each boiler’s run time evenly.
“As far as performance goes, the annual fuel savings with the high-efficiency burners and controls will be somewhere in the projected area of 70 to 80,000 therms,” Lacny concluded. “Some people forget that these savings will repeat year after year after year.”
A mechanical engineer with expertise in safety, green building, and construction technologies, Dan Bulley, LEED AP BD+C, O+M, is senior vice president of Mechanical Contractors Association Chicago (http://mca.org). Formerly, he served as an officer and board member for the U.S. Green Building Council and president of the Illinois chapter of ASHRAE.
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