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Steam Boilers And Their Wild Load Swings
By Barb Checket-Hanks / Service/Maintenance
and Troubleshooting Editor
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here to download PDF
Hospital Solves Its Problems By
Installing New Boilers.
Today's boiler designs offer many advantages over
the older boilers they replace. Today's boilers tend
to be smaller, more efficient, and frequently offer
lower NOx emissions to comply with some local emissions
regulations, such as those in Los Angeles, CA.
Some newer boilers are also able to deal with a particularly
troublesome operation problem, wild load swings.
As the name implies, wild load swings are caused
by sudden changes in the load. "In many industries
and institutions, high steam volume is needed suddenly
at different times of the day," explained Mark
Utzinger, vice president of Miura Boiler's U.S. operation,
Miura Boiler West, Inc. "When that large volume
is needed, it has to be there immediately. In between
demands the boiler doesn't even have to be on, or
it may be running at an idle speed."
According to Utzinger, this cyclical, heavy-demand
steam usage can lead to heavy system wear and unreliable
operation.
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Kaiser Hospital
maintenance manager Wayne Daggett stands between
the two new boilers that were installed at the
hospital. |
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SWING RESULTS
Low water: When the boiler's
load swings back to large volume from low volume,
"often one boiler just cannot produce enough
steam," Utzinger said.
"When a [system] requires a huge volume of
steam, the moment a valve is opened to provide it,
too much steam may be drawn from the boiler. In fact,
it can pull all the steam into the line, causing a
siphoning action that can pull not only steam out
of the boiler, it can suck out the reservoir of water
that's forming the steam."
Such a low-water situation automatically shuts down
the boiler.
"The metal of the tubes
is being forced in and out of the tube sheet over
and over again. In time, it will crack the tubes of
a firetube or coiled-tube boiler."
Slow response: In critical-use
facilities such as hospitals, low-water shutdowns
are intolerable. These facilities generally install
two or more boilers as backups. However, "This
does not always solve the problem satisfactorily,"
Utzinger said.
"The backup boiler, because it's idling and
not up to full steam, cannot produce the needed steam
fast enough. So, the necessary volume of steam is
not available when it is needed, and it could take
as long as 30 minutes to build up."
Cracked tubes: When a steam
valve is opened, steam is abruptly drawn from the
boiler. The boiler's burner ramps up from its low-fire,
idling state to full fire as it tries to meet the
demand of the draw on the steam.
The firetubes quickly expand as the heat within
them increases - and they contract just as quickly
when the steam demand drops off, Utzinger explains.
"In fact, cold air may rush over the tubes when
the boiler cycles from full fire to low fire."
This contraction and expansion creates problems.
"The metal of the tubes is being forced in and
out of the tube sheet over and over again," says
Utzinger. "In time, it will crack the tubes of
a firetube or coiled-tube boiler."
ONE HOSPITAL'S DEMANDS
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| The
new boilers at Kaiser (part of the blue firetube
boiler is visible in the foreground). |
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Kaiser Hospital in Sunnyside, OR, had two 300-BHP
boilers and one 500-BHP Scotch Marine firetube boiler.
The initial problem was, when the facility had a heavy
draw for heat and sterilizers, one boiler could not
produce enough steam fast enough to keep up, so the
other boiler would have to kick in.
The secondary problem: The backup boiler couldn't
kick in fast enough.
Wayne Daggett, maintenance manager at Kaiser, describes
it like this: "In the wintertime, the two 300-horsepower
boilers would not hold the load. So, we'd have to
run the 500-BHP firetube boiler with a 300 BHP in
standby, staying warm. In the summertime, we had one
300 BHP online with the other 300-BHP firetube in
standby."
GVA Northwest, LLC, an HVACR manufacturers' rep firm
in Portland, analyzed the situation for Kaiser. A
report concluded that it wasn't the size of the firetube
boilers that was the problem, it was the suddenness
of the upsurges in the hospital's steam demands and
the boilers' inability to respond in time.
The firetube boilers took as long as half an hour
to kick into high gear, reported GVA's John Hilton;
the hospital needed a boiler that could be up to steam
in minutes. GVA recommended that the hospital install
two 200-BHP Miura LX watertube boilers.
PURCHASING PERSUASION
Making purchasing decisions in hospitals typically
isn't easy. Kaiser and other hospitals have purchasing
programs that include a list of accepted products
and vendors. Miura began North American operations
in 1988, making it relatively new in the American
boiler market. The company was not on Kaiser's list.
(The company does have a significant international
boiler presence, with seven manufacturing plants around
the globe and roots in Japan.)
GVA decided that showing the boilers in operation
would speak volumes. Hilton and a coworker took Daggett
and Dave Kath, the hospital's chief engineer, to the
Miura installation at Portland Brewery (a lunchtime
visit, Hilton pointed out). Hilton reported that Daggett
timed the brewery's boiler and said, "My gosh,
it does go from a cold boiler to a full head of steam
in five minutes!"
Daggett and Kath recommended the boilers. According
to Hilton, they have "the proper ears to talk
to within the Kaiser organization, as well as the
respect."
Daggett, with 22 years at the VA Hospital in addition
to his experience at Kaiser, said, "When we installed
the 200-BHP Miuras, we put the 500-BHP Scotch Marine
in a lay-up condition. Now we're not even using it."
According to Hilton, "The 500-BHP Scotch Marine
will be removed later with future expansion and dual-fuel
Miuras will be put in." He reckons this will
allow the system to supply steam to the expansion
from one boiler plant. "With the big firetubes
or watertubes, this would not be an option,"
he said. "They would have to plan another boiler
plant to supply any expansion."
ADDED EFFICIENCY
"The boilers modulate themselves as the load
dictates," Daggett explained. The resulting efficiency
increase is one more benefit, and a big one. "The
firetube boilers couldn't keep up with the load because
of their inefficiency," he said.
Hilton pointed out, "If you figure that previously,
600 to 800 horsepower was required for peak demand
and now it's just 400, it is not surprising that the
hospital estimates they are saving 25% on their fuel
bill."
Apparently, very little heat radiates from the new
boilers. Chief engineer Kath said, "I have to
wear my coat in the boiler room now." It's a
small price to pay.
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