Page 265 - Boiler_Operators_Handbook,_Second_Edition

250 Boiler Operator’s Handbook

ity and the hardness in the makeup water. Since resin ment and piping. Usually good ventilation in the room
eventually degrades (chlorine is rough on it), some of containing the equipment is adequate but sometimes
it breaks up and is washed out, and the hardness of special coatings are required to act as insulators. Check
makeup can vary, you have to check operation by testing the backwash water after any system maintenance to
the water. ensure the resin isn’t washing out and when water
A condensate polisher is almost identical to a wa- temperatures drop. Colder water is more dense and can
ter softener. The differences are mainly due to the high carry out resin that warmer water couldn’t.
temperature of the condensate. The resin beads and Another important thing to remember is the ion ex-
mechanical parts of a polisher are designed to take the change process isn’t perfect. A few ions manage to sneak
higher temperatures. The resin also has an affinity for through depending on the equipment design, loads,
++
iron (FE ) in addition to calcium and magnesium to and how they are operated. Demineralizers are almost
remove iron from the condensate. So, products of corro- perfect ion exchange devices. Softeners reduce hardness
sion, dissolved iron oxides, get removed by condensate to 2 to 5 ppm and dealkalizers are about 80% to 90% ef-
polishers. Operation of a polisher is very similar to a fective. All ion exchange devices have limited turndown
softener, using brine to regenerate. and tend to “channel” at low flow rates where the low
Dealkalizers are also similar to softeners and are flow of water takes the easiest route through the resin to
regenerated with salt. The principal difference is deal- consume the ions there and allow leakage of untreated
kalizers contain anion exchange resin, accumulating a water. Know the limitations of your equipment.
concentration of chlorine ions on the resin beads instead A good rule of thumb is to maintain a velocity
of sodium. Their principal purpose is exchanging the of less than 2.5 gpm per square foot of resin surface to
chlorine ions to replace the bicarbonate ions in makeup prevent channeling. When water demand is low you’re
water. Now you would think that salt water isn’t the better off shutting off water flow through one or more
best thing to put in a boiler but we just explained that a of several ion exchangers to keep the flow rate up. To
combination of softener and dealkalizer do exactly that. better understand this, imagine that small creek you
The reason is that salt, unlike many other chemicals, will played in as a child. When the water flow was filling
stay dissolved in water as the water is heated up. The the stream all the rocks were wet; when flow was down
calcium, magnesium, and iron will not; they’ll drop out many of the rocks stuck out of the water and were dry
of solution as the water is heated to form scale. Some on top; the water flowed by in little channels. When the
dealkalizers are also regenerated with a little caustic flow is low in an ion exchanger some of the resin never
soda added to add hydroxyl ions for exchange instead of sees water flow while the resin in the channels sees it all.
sodium. That helps to remove other anions while raising The result is the resin in the channels is exhausted (all
the pH of the water. the ions it had to exchange are used up) while the rest
Demineralizers are combination ion exchange isn’t used much at all. Since we control ion exchange
units that incorporate both cation and anion exchange units by measuring the throughput of the water there’s
resins. They can consist of trains of two tanks (one cation a good chance untreated water will be passing through
one anion) in series or a “mixed bed” that contains both before the quantity of water that could be treated by the
resins in one tank. Demineralizers differ from other ion exchanger passes through.
exchangers because they actually remove dissolved ma- An important part of an ion exchange operation is
terials from the water. The cation resins are regenerated cleaning and replacement of the resin bed. The normal
with an acid to build up a concentration of hydrogen backwash doesn’t remove all the sediment and particles
ions on the beads. The anion resins are regenerated with that get imbedded in the resin beads during operation.
caustic soda to build up a concentration of hydroxyl ions Chemical cleaning with a resin cleaner that’s pumped
on their beads. As the makeup water flows through the into the idle exchanger then rinsed out is a normal func-
demineralizer all the dissolved material is replace with tion in many plants. A complete replacement of the resin
hydrogen and hydroxyl ions which combine to form every five years is common where the chlorine in the
water. The result is an output that is pure water, better makeup is high.
than distilled. Reverse osmosis (RO) is becoming more com-
One of the most important things an operator can mon as the cost of the membranes decreases. Rather
do to maintain ion exchange equipment is to prevent than absorbing all the theory of osmosis, treat them as
condensation on them. The constant formation of mois- filters that will let water through but won’t let the ions
ture with access to air accelerates corrosion of the equip- dissolved in the water get through. The pressure drop