Page 264 - Boiler_Operators_Handbook,_Second_Edition

Water Treatment 249

ment equipment also does a certain amount of filtering. The backwash also flushes out most of the dirt in
The most common piece of pretreating equipment the water that was filtered out by the resin bed. Under
found in a boiler plant is a water softener. Softeners are unusual and upset conditions there can be a lot of dirt
just one of several types of ion exchange equipment. and mud collected by a softener so you should try to
They’re called softeners because they reduce the hard- take a look at the backwash water near the end of the
ness of the water. A water is considered hard when it cycle to ensure it’s clear. Sometimes storms, and at other
is difficult (hard) to make soap foam in the water. The times the water company crews flushing hydrants, can
original tests of a softener involved mixing a sample of stir up mud and dirt to put a concentrated amount in the
the output water with a standard soap solution to see if water for short periods.
it would foam. Water is soft when soap produces a foam After the backwash is complete brine is drawn into
readily. the softener. The brine solution is a high concentration
The softener tanks contain resin that fills the tank of dissolved salt. Since salt is sodium chloride, brine is
one third to half full. The resin just lays in the tank so we a solution of sodium and chlorine ions. The resin beads
call it a resin bed. The resin in a softener has an affinity exchange ions to balance with the high concentration
for specific ions, (ions with a positive charge) principally of brine in the softener, giving up the magnesium and
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(Na ) sodium, (Mg ) magnesium, and (Ca ) calcium. calcium ions collected during the service mode and in-
The beads of resin are selfish little things, always want- creasing the number of sodium ions they hold.
ing what they don’t have. They tend to collect ions until When the brine draw is complete the softener is
they are in balance with the solution surrounding them. rinsed to remove the spent brine and the calcium and
The purpose of the softener is swapping the magnesium magnesium salts removed from the resin. A fast rinse
and calcium ions in the makeup water with sodium flows down through the bed to quickly displace most of
ions, exchanging one for the other. The reason for the it. A slow rinse then follows to completely remove all the
exchange is that calcium and magnesium form scale in brine. A salt elutrition test is run occasionally to ensure
the boiler and sodium doesn’t. The resin traps some of the softener is operating properly, absorbing most of the
the dirt and large particles in the water so it also acts as brine.
a filter. Those previous modes of operation were all part
Where do we get the sodium ions for the softener? of the regeneration cycle which restores the softeners
From salt. Salt is sodium chloride (NaCl) a common and ability to remove calcium and magnesium ions from the
very cheap material. It’s dissolved in water by forming water. They take from a few minutes up to two hours
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sodium (Na ) cations and Chlorine (Cl ) anions. By us- depending on the size and capacity of the softener. Most
ing brine (concentrated salt solution) in the softener to of the time the softener is in the service mode where
remove hardness we reduce the amount of expensive makeup water enters at the top and, as it flows to the
chemicals that we have to use in the boiler. In very small bottom, calcium and magnesium ions are exchanged for
plants with very little makeup water or where city water the sodium ions on the resin beads. Since they’re over-
is fully softened or naturally soft a softener isn’t justified saturated with sodium from the brine draw operation
but there aren’t many situations like that. The smallest the resin beads readily give up those sodium ions when
plant can benefit from a softener if it doesn’t use a more they can grab one of the calcium or magnesium ions
exotic form of ion exchanger or reverse osmosis. from the water.
Operating modes of a softener include backwash, That explains those greedy little resin beads, they
brine draw, fast rinse, slow rinse, and service. Back- always grab what they don’t have. The drop the calcium
washing removes dirt and “fluffs up” the resin. Water and magnesium when they’re loaded up with sodium
flow during a backwash is up through the bed. The then readily toss the calcium and magnesium when the
space in the tank above the resin provides room for the water around them is full of sodium for them to grab.
resin to separate from the backwash water before the wa- An important element of managing a water soft-
ter leaves the tank. If the water flow rate is too high then ener is knowing the hardness of the inlet water. A
resin will be flushed out of the softener so it’s a good idea softener’s capacity is normally listed in kilograins, thou-
to look at the water draining during a backwash to spot sands of grains. It depends on how much resin there is
resin loss. That’s best done with a flashlight pointed into in the softener and how many sodium ions each particle
the water, the resin will cause the light to sparkle. You of resin can exchange. Grains, by the way, are a measure
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might notice an occasional piece of resin leave because of weight equal to one 7,000 of a pound. The amount
small pieces of resin break off occasionally. of water your softener can soften depends on it’s capac-