TL;DR
Scale is calcium and magnesium carbonate or sulfate deposited from untreated or undertreated feedwater. It insulates tubes and causes overheating. Chemical descaling removes it; fixing the water treatment program prevents recurrence.
What you might see
- white or grey deposits visible on waterside surfaces
- flue gas exit temperature rising above baseline
- localized overheating or tube discoloration (bulging)
- reduced heat transfer and steam output per unit of fuel
Likely causes
Feedwater hardness above the softening system capacity, depositing calcium and magnesium salts on heated surfaces
Failed or bypassed water softener allowing hard water into the feedwater system
Inadequate phosphate or scale inhibitor dosing allowing deposit formation
Insufficient blowdown not removing concentrated dissolved solids from the boiler drum
Required tools
- LOTO kit
- Boiler water test kit (hardness, alkalinity, phosphate)
- Scale thickness probe or scraper and calipers
- Personal protective equipment for confined space entry
- Chemical descaling agent (water treatment provider supplied)
Safety first
- Descaling chemicals are corrosive acids or chelates. Wear chemical splash goggles, face shield, and chemical-resistant gloves.
- Confirm zero steam pressure before opening any boiler handhole or man-way.
- Confined space entry requires a permit, atmospheric gas testing, and a trained standby person.
Procedure
- 1
Lock out and cool the boiler. Drain the pressure vessel for internal inspection.
Warning: Confirm zero pressure and temperature below 50 degrees C before entering the boiler or opening any handhole. - 2
Inspect the waterside surface through handholes or man-way. Measure scale thickness with a scale probe or by scraping a sample and measuring with calipers.[1]
- 3
Sample the boiler water and feedwater for total hardness (calcium and magnesium). Feedwater hardness should be less than 1 ppm as CaCO3 for a steam boiler.[1]
- 4
Check the water softener operation: measure the hardness of the softener outlet. If hardness is above 1 ppm, the softener resin is exhausted or bypassed.
- 5
For scale above 1 mm thickness, schedule a chemical descaling treatment. Contact your water treatment provider for the correct inhibited acid or chelate descaling agent for the scale composition.
- 6
After descaling, verify that no tube damage or wall thinning occurred in the areas with heaviest scale. Thick scale can mask tube wastage.
- 7
Increase blowdown frequency and confirm the phosphate or scale inhibitor dosing is correct before returning the boiler to service.
Sources
Cleaver-Brooks CB Firetube Boiler Operation, Service, and Parts Manual, Cleaver-Brooks
Cleaver-Brooks CB Boiler Operation, Maintenance and Parts Manual, waterside scale inspection and water treatment requirements (general)
View source
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