Boiler and Cooling Water Treatment: Choosing the Right Chemistry

Most boiler and cooling water problems in East African plants are not exotic. They are one of three failures, and treating the wrong one is expensive: you buy chemistry that does nothing while the actual mechanism keeps working on your tubes.

This is a guide to telling them apart before you order.

The three failures

Scale

Dissolved calcium and magnesium come out of solution on the hottest surface available — which in a boiler is the tube you most need to conduct heat. A scale layer is an insulator. The burner compensates by running hotter, fuel consumption climbs, and the tube metal runs above its design temperature.

Scale is a feedwater hardness problem. The fix is upstream: soften or demineralise the make-up, then use a phosphate or polymer programme to keep whatever hardness gets through in suspension rather than on the metal.

Kenyan borehole supply is frequently hard, so a plant that moves from municipal to borehole make-up without re-testing its feedwater can find its existing treatment programme is no longer sized for the hardness arriving.

Corrosion

Dissolved oxygen and carbon dioxide attack the metal directly. Oxygen pitting puts holes through tubes; carbonic acid thins the condensate return lines from the inside, which is why the return system usually fails before the boiler does.

Corrosion is a dissolved gas problem. Mechanical deaeration removes most of it; an oxygen scavenger removes the remainder. A neutralising or filming amine handles the condensate side.

Biological growth

Cooling towers are open, warm, and aerated — near-ideal for biofilm and algae. Biofilm is not only a fouling problem: it insulates like scale, it shelters the organisms underneath it from your biocide, and it drives under-deposit corrosion. In a tower it is also a legionella control question, which makes it a safety issue and not only an efficiency one.

Growth is a biocide programme problem, usually alternating an oxidising and a non-oxidising product so the population cannot adapt to either.

Test before you dose

A treatment programme written without water analysis is guesswork. Before specifying anything, get:

• Total and calcium hardness — sets the scaling risk and whether softening

is needed at all

• Total alkalinity and pH — drives both scaling tendency and the

neutralising amine demand

• Conductivity / TDS — sets your blowdown or bleed rate
• Chlorides and sulphates — corrosion aggressiveness, and relevant to

stainless steel selection

• Iron and copper — evidence of corrosion already happening upstream
• Silica — matters at higher boiler pressures, where it carries over onto

turbine blades

For a cooling tower, add a dip slide or plate count to establish the microbiological baseline. You cannot tell whether a biocide programme is working without a number to compare against.

Retest on a schedule, not only when something goes wrong. Trend data is what tells you a programme is drifting; a single sample only tells you where you are today.

Control limits are set by pressure

There is no universal "correct" boiler water chemistry. The limits tighten as operating pressure rises, because the margin for carryover shrinks. Low-pressure shell boilers tolerate far higher TDS and alkalinity than a high-pressure water tube boiler will.

Work to the limits your boiler manufacturer specifies for your unit and pressure, cross-referenced against a recognised standard such as BS 2486 or the ABMA guidelines.

Where your insurer or an OSH inspection sets its own limits, those govern.

Blowdown is part of the treatment

Chemistry controls what is dissolved in the water. Blowdown controls how much is dissolved, by physically removing concentrated water and replacing it with make-up.

Neither works alone. A plant that doses correctly but under-blows will concentrate solids until it scales anyway. A plant that over-blows is throwing away treated, heated water — a direct and continuous fuel cost.

Set the blowdown rate from measured conductivity against your limit, and check it when the make-up water source changes.

A practical specification checklist

When you come to order, have these ready. They are what determines the product and the dose rate, and a supplier who quotes without them is guessing:

1. 1.Boiler type and operating pressure
2. 2.Steam output and typical load pattern
3. 3.Make-up water source and a current analysis
4. 4.Percentage of condensate returned
5. 5.Existing pre-treatment (softener, RO, deaerator) and its condition
6. 6.For cooling: system volume, cycles of concentration, tower type
7. 7.Any food, pharmaceutical or potable contact requiring a specific grade

That last point decides more than people expect. A plant with steam in direct food contact needs treatment chemicals cleared for that duty, and the documentation to prove it — which is a grade and paperwork question, not a chemistry question.

Getting the documentation right

Whatever you buy, insist on the certificate of analysis and the safety data sheet with the consignment, not on request afterwards. The COA is what your QA function checks the delivered batch against. The SDS is what your safety file and your customs clearance need.

For products going anywhere near food or pharmaceutical production, the grade must be stated on the documentation, not assumed from the product name.

Where to start

If you are unsure which of the three failures you have, the honest answer is usually visible: open the system and look. Scale is white and hard. Oxygen pitting is localised and deep. Biofilm is soft and slimy. The deposit tells you which programme you need far more reliably than a catalogue does.

Send us the water analysis and the boiler details above and we will size the programme against them.

Browse water treatment chemicals or request a quotation with your analysis attached.