Scale, Priming and Carry-Over on a Softened-Water Boiler
A water-tube boiler producing saturated steam at 12 bar has been showing rising stack temperature, falling steam output, and occasional priming. The feed water is softened but has a total hardness of 15 mg/L as CaCO3, and the boiler water chloride is 900 mg/L against a limit of 300 mg/L. The gauge glass shows a cloudy water level, and the steam is wet at the superheater inlet. The attendant proposes to increase the continuous blowdown, add more softening chemicals to the feed tank, and carry on until the next planned shutdown in six weeks.
1. Identify the likely causes of the symptoms and assess the attendant's proposal.
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The symptoms point to scale and carry-over. A hardness of 15 mg/L means the softener is passing hardness, which will deposit scale on the hottest tubes and raise stack temperature and reduce output. The chloride of 900 mg/L against a 300 mg/L limit shows that dissolved solids are concentrated well above the control range, which causes foaming, a cloudy gauge glass, priming, and wet steam.
Increasing continuous blowdown is appropriate, but adding more chemicals to the feed tank is not: softening chemicals belong in the softener or in a controlled internal-treatment programme, and dosing the feed tank can cause sludge to carry over. Carrying on for six weeks risks tube overheating and failure. The softener should be checked and regenerated or repaired, blowdown increased to bring chloride within limit, and the boiler water tested daily.
2. What immediate steps would bring the boiler water within limits, and what tests would you use to confirm?
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Increase continuous blowdown and carry out controlled bottom blowdown to reduce dissolved solids, while checking the softener and restoring its output quality. Test boiler water chloride or conductivity, total hardness, alkalinity, phosphate, and sulphite or oxygen scavenger residual at least daily until stable. Confirm the gauge glass clears and that steam is dry at the superheater inlet.
Targets should be set from the boiler manufacturer's and water-treatment supplier's limits, for example chloride below 300 mg/L, hardness essentially nil, and phosphate and alkalinity within the recommended bands. If the softener cannot be restored quickly, reduce the boiler load or take it off line rather than continue with hardness breakthrough.
3. If the boiler must run for a further two weeks, what risks should be managed and what should be planned?
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The main risks are tube overheating and rupture from scale, and carry-over that contaminates the superheater and steam system with dissolved solids. Manage them by keeping blowdown high enough to hold chloride within limit, monitoring stack temperature and tube-metal temperatures, watching for priming, and reducing load if output falls. Do not allow hardness breakthrough to continue unaddressed.
Plan a shutdown for internal inspection and descaling as soon as practicable, and arrange softener repair before then. Record the water-test results and actions in the boiler log so the inspector can see the trend.