Boiler Feed Water Treatment Demineralization Reverse Osmosis Plant Deaerator

Boiler feed water treatment strips dissolved salts, silica, suspended solids and dissolved gases from make-up so tubes do not scale or corrode. Natural sources all carry some of those loads; how much depends on the catchment.
Why it is necessary to treat the raw water?
Untreated raw water will deposit hardness salts on the boiler.
Because-

1. Scale on the inside of heaters and tubes blocks heat flow. The metal then overheats, builds extra thermal stress and can rupture.
2. Dissolved salts and oxygen also attack boiler steel as the water travels through the circuit.
3. Carry-over and corrosion products can also pit turbine blades.

That is why boiler feed water treatment sits upstream of the drum: dissolved and suspended matter has to come out before the water is fed.

Arrangements for Boiler Feed Water Treatment

After filtration, plants commonly use two make-up trains:

1. Demineralization plant (D M plant)
2. Reverse Osmosis plant (R O plant)

A Demineralization plant uses ion-exchange resins to take dissolved ions out of the water. A reverse osmosis plant uses a pressure-driven membrane. Sand or multimedia filters usually sit ahead of both.

This layout also has two deaerators. They strip dissolved oxygen so it cannot pit boiler tubes.
The trains and the hardware inside them are below.

Demineralization Plant

The demineralization plant exchanges dissolved cations and anions for H+ and OH- and so yields high-purity make-up.

Hardness and related salts are mainly chlorides, carbonates, bicarbonates, silicates and phosphates, with sodium, potassium, iron, calcium plus magnesium as the usual cations.
In a D M plant three resin duties cover the boiler feed water treatment process –

1. Cation exchange resin
2. Anion exchange resin
3. Mixed Bed resin

1. Resins are high-molecular-weight polymers that swap their mobile ions for the dissolved salts and so take those salts out of solution.
2. Cation resin swaps dissolved cations. Anion resin swaps dissolved anions.

Cation Exchange Resin

The cations leave as acids, so H₂SO₄ and H₂CO₃ appear.
Na⁺ is gone, and the water is now acidic.

Anion Exchange Resin

Anion resin takes Cl^– and the mineral acidity with it.
H₂SO₄ is removed the same way.

Mixed Bed Resins

mixed bed resins polish what the separate cation and anion units miss, including leftover Na⁺ and SO₃^2-, so the last traces of conductivity come down.

Degasser

The degasser tower strips carbonic acid as carbon dioxide. Water falls from the top. Air is blown up from the bottom. In that air stream H₂CO₃ splits into H₂O and CO₂.

The CO₂ leaves with the air.
Benefits of using degasser are:

1. It strips carbonic acid and other dissolved gases without chemicals, so less CO2 remains to corrode downstream metal.
2. Less CO2 reaching the anion unit means less resin work and lower chemical regeneration cost.

H₂CO₃ is now largely gone. The sump holds that water and a pump sends it to the anion unit.

Reverse Osmosis Plant (RO Plant)

A reverse osmosis plant is the other common make-up train. An RO plant forces water through a salt-rejecting membrane. The result is low-salinity permeate, not ion-free water. The physical idea is below:-

In osmosis, solvent crosses a semi-permeable membrane toward the side with more solute (from the dilute solution toward the concentrated one).
Osmotic pressure:- The extra pressure that must sit on the concentrated side to stop that net flow is the osmotic pressure (π).
π = iCRT
Where,
C is concentration of solution,
R is universal gas constant,
T is temperature in Kelvin scale,
i is van’t Hoff’s factor, different for different solutions. i = 1 for an ideal nonelectrolyte at infinite dilution. π therefore rises with temperature.

Reverse Osmosis

A pressure above π on the concentrated side drives solvent from the salty side toward the dilute side. Plants use that reverse-osmosis flow as a membrane stage in boiler make-up treatment.

Reverse Osmosis Plant

In an RO plant filtered raw water is pumped against the membrane. Permeate salinity tracks the membrane, recovery and feed TDS. The product is low-salt water, not a laboratory blank.

A typical Reverse Osmosis Plant train is below

Auxiliary steam, including a steam air pre-heater, also cuts net output of the power plant.
The procedure is described below:

1. Sodium hypochlorite (NaOCl) is dosed to limit algae and bacteria that would slime a multi grade-filter (MGF).
2. The multi-grade filter is a stacked bed of sand and graded stone that takes out the coarser suspended solids.
3. A mesh or net filter then catches mid-size particles as the water passes through the fine openings.
4. Ultra-filtration then takes the fines. After a long run the UF unit is back-washed with water plus HCl, NaOH and NaOCl (Sodium Hypochlorite). HCl dissolves iron and lifts alkaline deposits left on the UFU.
   NaOH ← It helps to remove acidic salt.
   NaOCl ← To kill algae and bacteria inside the UFU.
5. UF filtrate sits in the RO feed tank, then the RO feed pump of the Reverse Osmosis Plant lifts it. HCl is dosed for pH (RO permeate is often near pH 6 because carbon dioxide passes the membrane) and SMBS (sodium metabisulphite) [Na₂S₂O₅] knocks down leftover chlorine from the hypochlorite dose. Free chlorine would oxidise the membrane. An anti-scalant (AS) is added so hardness salts do not plate the feed channel.
6. A micro-cartridge filter (MCF) then catches leftover solids and any precipitate from the anti-scalant.
7. A high-pressure pump then feeds the RO racks. After the 1^st and 2^nd stage RO passes, permeate goes to a degasser.
8. After the degasser the water is polished on the DM mixed-bed (MB) resin and stored in the DM tank.

Deaerator

A Deaerator is a direct-contact open heater that strips dissolved oxygen from feed water. Air is about 21% oxygen by volume, so blowing air through cold water cannot pull oxygen out; it would add it. Steam stripping after heating is the usual mechanical route.

In the deaerator LP or VB steam heats the feed (pressure: 2.5 – 3.5kg/cm², saturation near 140^oC, not 1400). Oxygen solubility falls as the water warms, and the steam sweep carries the gas out the vent. That oxygen strip is the last gas-removal step on this feed train.

The packing or trays look like a degasser, except LP steam rises and feed rains down.
The storage section also gives net positive suction head (NPSH) to the boiler feed pumps (BFP) so those pumps do not vapour-lock or cavitate.