Boiler Water Treatment and Dosing: Oxygen, Phosphate and Blowdown

Most boiler house damage comes from three mechanisms running at once rather than from a single missing chemical. Calcium and magnesium hardness carried in with the feedwater deposits as scale on heating surfaces, dissolved oxygen pits the economiser and feed line, and bicarbonate alkalinity breaks down in the boiler into carbon dioxide that travels with the steam and forms carbonic acid in the condensate system. Each needs a different chemical injected at a different point.

The cost of scale is measurable. Calcium carbonate scale conducts heat roughly 20 to 50 times worse than steel, and a 1 mm layer typically raises fuel consumption by around 2 percent. More critical is metal temperature: plate that cannot shed heat through the deposit overheats locally, which turns the issue from efficiency into pressure vessel safety.

A treatment programme therefore attacks all three separately: water preparation (softening, and reverse osmosis where required), thermal deaeration plus an oxygen scavenger, phosphate and alkalinity control, a neutralising amine for the condensate, and blowdown to keep everything inside its band. The logic mirrors the cycle control described in our cooling tower treatment programme; the difference is that boiler temperature and pressure leave a far narrower tolerance band.

A phosphate programme manages hardness; it does not remove it. The target at the softener outlet is below 0.3 mg/L as CaCO₃, and many boiler makers write 1 mg/L as an absolute ceiling for feedwater. A duplex softener with continuous hardness monitoring on the outlet cuts the risk of breakthrough during regeneration far more effectively than a single-column arrangement.

The first sign of hardness breakthrough shows up in the boiler water: the phosphate residual falls unexpectedly, because free phosphate reacts with calcium and turns into sludge. Daily phosphate testing is, in effect, an audit of the softener. If the residual halves between two readings, the correct response is to inspect the resin bed, the brine tank and the regeneration schedule rather than to raise the chemical dose.

The second big driver of chemical consumption is the make-up ratio. Lifting condensate return from 60 to 80 percent cuts softener load, oxygen scavenger demand and phosphate consumption together, and because returning condensate sits at 80 to 90 °C it saves fuel as well. On most sites the cheapest way to shrink the treatment budget is repairing a leaking condensate return line, not buying a new chemical.

Dissolved oxygen falls rapidly with temperature: saturated water holds about 9 mg/L at 20 °C, around 3 mg/L at 80 °C and, in theory, none at boiling point. A thermal deaerator exploits that physics. Running at 0.2 to 0.3 bar gauge, which puts it close to 105 °C, it typically delivers 0.02 mg/L at the outlet, and around 0.007 mg/L where the unit is well designed and correctly operated.

The chemical scavenger supplements the deaerator rather than substituting for it. Sulphite stoichiometry is straightforward: roughly 8 mg/L of sulphite binds 1 mg/L of oxygen, catalysed products are dosed at about 10 mg/L in practice, and a 20-40 mg/L residual is held in the boiler water. Sulphite is cheap and easy to test, but it adds dissolved solids and therefore blowdown, and it is not recommended above roughly 60 bar because it decomposes thermally.

Volatile scavengers are chosen for different reasons: they add no solids, which suits higher pressure boilers, and they carry over with the steam to passivate the condensate system as well. Where steam contacts food, the chemical must appear on the permitted substance list of the applicable regulation. Dosing rate follows from consumption, and the method in our guide to dosing pump flow and pressure calculation applies here unchanged.

Free phosphate in the boiler water converts hardness that escapes the softener into a soft sludge instead of hard scale, and that sludge leaves through the bottom blowdown. The typical residual in low pressure boilers is 30 to 60 mg/L as PO₄. The band drops to 10 to 30 mg/L as pressure rises, because the phosphate-caustic equilibrium narrows at higher temperature and excess phosphate hides out on the hottest tube surfaces.

The pH band is squeezed between two limits. Boiler water pH is normally held between 10.5 and 11.5: below that the passive layer on carbon steel weakens, above it caustic attack and embrittlement risk rises at welds and rolled tube ends. P-alkalinity in low pressure boilers is tracked in the 300 to 800 mg/L as CaCO₃ range, which also indicates how much buffer the phosphate programme has.

Silica sets a separate limit. Above 150 mg/L in the boiler water it forms extremely hard scale and risks carry-over with the steam; systems feeding turbines keep steam silica below 0.02 mg/L. The bands below are common operating values, but the binding figures are always the boiler maker's limits and the water quality table of the applicable standard such as EN 12953-10.

Condensate corrosion originates not in the boiler itself but in the alkalinity of the feedwater. Bicarbonate and carbonate break down at boiler temperature to release carbon dioxide; complete breakdown yields roughly 0.8 mg/L of CO₂ per 1 mg/L of M-alkalinity, while field calculations generally use a factor of 0.4 to 0.6. The gas travels with the steam and dissolves in the condensate as carbonic acid.

Carbonic acid pulls condensate pH down to 5 or 6 and grooves carbon steel pipe evenly along the bottom of the line, so the symptom is usually a pinhole leak rather than general rusting. Neutralising amines bind that acid and lift pH into the 8.3 to 9.0 range. Where copper alloys are present in the system the upper limit is kept below 9.0, since high pH and ammonia attack copper.

Amine selection follows the distribution ratio: cyclohexylamine sits at roughly 4, diethylaminoethanol at 1.7 and morpholine at 0.4. A high-ratio amine protects the far end of the system while a low-ratio amine protects the first condensing zone, which is why products for long, branched networks are formulated as blends. Whether the chemistry suits the hardware in the line belongs to the same decision as valve selection for steam lines.

At steady state, blowdown flow equals feedwater flow multiplied by the ratio of feedwater conductivity to boiler water conductivity. With feedwater at 200 µS/cm and a boiler water target of 4,000 µS/cm, blowdown is roughly 5 percent of feedwater. Energy matters too: at 10 bar, 5 percent blowdown sends about 1 percent of the heat input to drain, and a flash vessel with a heat recovery exchanger returns much of it.

The two blowdowns do different jobs. Surface (continuous) blowdown holds dissolved solids at the limit, while bottom blowdown removes settled phosphate sludge, typically once per shift in pulses of a few seconds, for the duration the boiler maker specifies. Automatic surface blowdown combines a conductivity controller, a measuring cell and a modulating blowdown valve; for the isolating duty, a motorised stainless steel ball valve outlasts a solenoid valve in hot water carrying solids.

Conductivity readings mislead unless temperature is handled: each 1 °C of difference shifts the reading by about 2 percent, so the sample is passed through a sample cooler to a 25 °C reference or a temperature-compensated probe is specified. Because boiler water alkalinity is high, laboratories often report neutralised conductivity; if the basis of the setpoint is not recorded, a systematic gap of 10 to 20 percent appears between two sets of readings.

The oxygen scavenger goes into the storage section of the deaerator, below water level, through a dip pipe. Dosing into the feed pump suction leaves no time for reaction and puts the pump at unnecessary risk. The practical aim is 3 to 5 minutes of contact time in the storage volume before the chemical reaches the boiler; since reaction rate falls with temperature, catalysed products are preferred on feed tanks below 60 °C.

Phosphate is injected into the boiler shell, or into the feed line as close to the boiler inlet as possible, so that it cannot deposit in the economiser; a distribution pipe into the steam drum gives the most even mixing. The neutralising amine is dosed at the feed tank outlet or directly into the steam line. Every injection point needs an isolating valve, an injection quill reaching the pipe centreline and a spring-loaded check valve, with a two-stage arrangement treated as standard where dosing is against boiler pressure.

Without a back pressure valve in the dosing line, siphoning occurs whenever pressure drops and the real dose goes unrecorded. A calibration column, a tank level switch and a dry-running alarm are the cheapest equipment that actually proves dosing took place. Colour coding and different coupling sizes between chemical tanks prevent wrong filling, and with it the reaction risk created by mixing incompatible products.

Flow sizing works backwards from consumption: divide the daily litres of chemical by the daily operating hours, then choose a pump that will run between 20 and 80 percent of its nominal capacity. A pump running at 10 percent of nominal cannot hold an accurate rate and loses repeatability, while a pump specified twice too large damages dosing quality rather than just the capital cost.

Back pressure is not the boiler operating pressure but the highest pressure the injection point can see: the peak close to the safety valve setting, plus quill and line losses. The practical rule is to take the first standard class at least 20 percent above that figure, so a 10 bar boiler typically calls for a 16 bar pump and a 16 bar boiler for a 20 to 25 bar class. Maximum flow from a given pump head falls as pressure class rises, so the two parameters are chosen together.

Technology and materials follow from those two figures: solenoid diaphragm pumps cover roughly 1 to 15 L/h at 10 to 20 bar, while higher pressure or sustained high flow calls for a motor-driven diaphragm head. PVDF wetted parts with a PTFE-faced diaphragm are the common choice for phosphate and amine solutions. Vanera sizes the dosing pump and the injection hardware together from your boiler data and water analysis, and turns capacity, pressure class and material selection into a single quoted list.