Why Bangladeshi groundwater destroys boiler tubes

Pull a tube from a boiler that has run two years on untreated groundwater in Gazipur, Savar and Narayanganj and the failure is always the same story: a hard grey-white layer on the water side, metal discoloured underneath, and a bulge where the tube could no longer pass its heat into the water.

The chemistry behind it is simple and entirely preventable.

What the water carries

Groundwater across the delta typically shows:

Two of those cause most of the damage.

Hardness becomes scale

Spirax Sarco’s reference on water for the boiler and the US Department of Energy’s steam system resources both set out the limits and the fuel penalty behind the numbers below.

Calcium and magnesium stay dissolved in cold water. Heat them and they drop out onto the hottest surface available, which is the fire side of your tubes. Scale conducts heat at roughly one fortieth the rate of steel, so the tube metal runs hotter and hotter for the same firing rate.

The fuel penalty is the first symptom - around two per cent per millimetre. The second symptom is the expensive one: metal that spends months above its design temperature creeps, bulges and eventually splits (see our guide on preventing boiler explosions).

Oxygen becomes pitting

Dissolved oxygen attacks steel at the water line and in the finned-tube economizer, producing deep, narrow pits rather than general wastage. A pit does not announce itself on a fuel bill. It shows up as a leak, usually during an annual OCIoB hydraulic test, sometimes in service.

The treatment train that works here

1. Softening

A strong acid cation exchange resin exchanges calcium and magnesium for sodium. Sizing is a function of raw hardness and daily make-up volume, and the regeneration schedule follows from that. Two rules keep a softener honest:

• Test outlet hardness daily. A softener that has quietly exhausted looks exactly like one that is working.
• Regenerate on volume, not on the calendar. Production volume moves, and a fixed schedule over-regenerates in a slow week and under-regenerates in a busy one.

2. Deaeration and oxygen scavenging

Heat drives most oxygen out. A deaerator running at 0.2 bar and about 105 °C takes feedwater to roughly 0.02 ppm; a well-run hot well gets partway there. The remainder is chemically scavenged - sulphite for lower pressure plants, with the residual tested daily.

Returned condensate helps twice over: it is hot, and it is already low in oxygen and hardness.

3. Internal treatment and pH

Phosphate dosing precipitates any hardness that slips through as a soft sludge that blowdown removes, rather than as bonded scale. Boiler water pH is held between about 10.5 and 11.5 so the protective magnetite layer stays intact.

4. Blowdown

Softening does not reduce dissolved solids, it swaps one ion for another. As steam leaves, solids concentrate. Blowdown is the only route out.

• Continuous (surface) blowdown holds TDS at the working limit, and its heat is worth recovering into the feed tank.
• Intermittent (bottom) blowdown clears the sludge that phosphate dosing creates, in short sharp bursts once per shift.

Too little blowdown gives foaming, carryover and wet steam. Too much throws treated hot water away.

The daily log that prevents all of it

Every factory that avoids tube failures keeps the same short log:

1. Softener outlet hardness
2. Feedwater temperature and pH
3. Boiler water TDS, pH and phosphate residual
4. Sulphite residual
5. Blowdown carried out, and by whom

Five lines, ten minutes a shift. It is the cheapest maintenance in the plant, and it is the record an inspector reads first when the internal examination looks good.

When the scale is already there

If the tubes are scaled, chemical descaling with an inhibited acid, followed by neutralising and passivation, restores heat transfer. It is a controlled job - the wrong strength or an exhausted inhibitor attacks the parent metal instead of the deposit. After descaling, fix the water treatment. Otherwise the same layer is back inside a year.

Our water treatment team sizes softeners and dosing systems from your own water analysis, provides complete boiler water treatment services and boiler retubing and refractory repair, commissions the daily log with your operators, and carries out descaling during a planned shutdown—contact our water engineering desk.

Related reading: Boiler blowdown: TDS control and heat recovery that pays, Condensate recovery: what it saves and what it costs to install and Pure steam vs utility steam for pharma and food plants.