"A two minute lesson in water chemistry"
The maintenance engineer looked at Ol' Festus and shrugged. "It
didn't force us off-line, so the water chemistry couldn't have
been that big of a problem. Besides, if you ignore the cation
conductivity readings, most of the chemistry meets EPRI guidelines
anyway."
Fuel oil had gotten into the water side of the unit while it had
been on-line. They had been ordered to stay on-line (despite a
cation conductivity between 1.0-2.0 µS/cm and a 25% combustible
reading on the vacuum pump discharge) because the grid needed
electricity due to a severe winter storm. After the cold spell
passed, they went off-line and conducted a brief overhaul. Although
tube samples and turbine blade inspections showed no problems
that could be attributed to the event, Ol' Festus, as plant chemist,
had instructed that the entire system be drained anyway. Now,
he listened to the young engineer tell him why management had
decided not to do it.
"I'm sorry your people had problems with the start-up
chemistry, but we didn't have time to drain the entire system,
just the boiler. Besides, I don't see how it could have helped.
"
Ol' Festus sighed at such short term thinking. He had only a few
minutes before his meeting with the plant manager, but he decided
to educate his inexperienced colleague anyway.
"The purpose of controlin' water chemistry is to create
a condition that'll have the most positive economic impact on
plant performance," he drawled. "Ya do it by controlin'
the negative effects caused by impurities in the system. Poor
water chemistry lowers the reliability an' life of the unit while
increasin' yer maintenance an' operating costs. Here's how it
works:
"A low pH creates corrosive conditions on metal surfaces.
That ain't too good for yer feedwater train, yer boiler, or yer
turbine. The reaction rate of most chemicals increase with temperature
with most of 'em doublin' in speed every 18o F. Corrosion's greater
in yer economizer inlet, drum, steam an' turbine than in yer feedwater.
Ya know about the corrosion that takes place around sea water,
don'tcha? Well, the pH of sea water's around 8.5 an' while it
ain't an acid condition, there's still a whole mess a' salts an'
corrosion goin' on. Now, yer lake or well water has salts in it
too--just not as much as sea water. Them salts can deposit on
the boiler tubes an' create overheatin' or accelerated corrosion
in certain places. Salts can also weaken them special alloys used
to make turbine blades an' create stress cracks on the blades
ta boot. At normal pH ranges, most of yer salts ain't carried
over in the steam, but stay concentrated in the boiler. But once
them salts get into yer system, boiler blowdown's the only practical
way to get rid of 'em.
"Another contributor to corrosion is air inleakage. Oxygen
in air dissolves in water to form iron oxide (that's rust to pipefitters
like you). Once iron reacts with oxygen, impurities in the water
react with the iron oxide to form corrosion.
"Carbon dioxide is another gas from air inleakage that dissolves
in the water. CO2 can also be a by-product from the decomposition
of organics, like fuel oil. Now, CO2 can form carbonic acid in
water which is real nasty stuff. If CO2 is present, it takes more
chemicals to maintain proper pH, but ya may not know it 'cause
CO2 can mask pH measurements as much as half a pH. The whole thing
becomes one big acid/base reaction, sort've like neutralizing
pH in the basin after a demineralizer regeneration.
"Now, all these acid/base reactions yield salts and them
salts contribute to corrosion 'cause they remove iron oxide from
metal surfaces. By controlling pH you can reduce the rate of corrosion
but the only way to stop corrosion is to reduce the concentration
of salts. So taking the extra time to clean the entire system
reduces overall corrosion and makes it worthwhile for everyone.
"Or, in other words, quit treatin' the symptoms and start
treatin' the disease!"
And with that, Ol' Festus turned and started for the plant manager's
office.