Softener Failure, Scale, and Carryover on a Dairy Boiler
A 250 hp, 150 psig firetube boiler at a dairy supplies 12,000 lb/hr of saturated steam; about 20% of feedwater is well-water makeup with 18 grains per gallon total hardness. The softener resin was exhausted during a four-day holiday and went unnoticed. Current readings: feedwater hardness 3 gpg, sulfite residual 0 ppm, pH 10.2, boiler water conductivity 3,200 micromhos/cm, and blowdown about 4% of feedwater. During the past month the boiler has made a popping or rumbling noise, steam lines have carried water, and the waterside of the tubes shows a hard scale layer estimated at 1/16 in. The operator proposes to raise blowdown to 15% and double the sulfite feed, and to postpone opening the boiler until the next scheduled outage in four months.
1. Evaluate the proposed response. Will more blowdown and sulfite fix this?
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No. More blowdown and more sulfite would treat symptoms while the boiler keeps accumulating scale. Blowdown controls dissolved solids, not hardness scale: the hardness has already entered the boiler and deposited on the waterside, and raising blowdown to 15% would waste energy and water, stress the feedwater system, and still leave existing scale in place. Sulfite is an oxygen scavenger, not a scale preventive; correcting a zero residual is appropriate, but doubling it does nothing about 1/16 in of scale, which is a heat-transfer barrier. Scale of that thickness can raise metal temperature enough to cause tube overheating and failure, and it contributes to the carryover and popping. The popping and water hammer indicate steam-space carryover, which will damage the steam system and may contaminate product-contact surfaces in a dairy. Waiting four months is not acceptable if the scale and carryover continue.
2. What is your immediate priority to protect the boiler and steam system?
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Restore the softener and verify effluent hardness before any more makeup goes to the boiler; test feedwater and boiler water for hardness, sulfite, pH, and conductivity; add oxygen scavenger and verify residual; begin controlled blowdown based on conductivity; reduce firing if steam demand allows, and watch for carryover. Investigate the carryover itself: check the water level, steam separator, and steam purity, and verify that the steam drum internals are not damaged. If scale is significant and the boiler cannot be cleaned immediately, arrange an early shutdown rather than waiting four months. Because the dairy uses steam for process, carryover is also a food-safety issue, not just a mechanical one.
3. Lay out the water-treatment and inspection plan.
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Repair or replace the softener resin, install a hardness monitor or alarm on the softener outlet, and establish a daily testing schedule with logged results. Set control limits for conductivity, sulfite, pH, and hardness; use continuous blowdown with manual bottom blowdown on a defined schedule. For the existing scale, plan a mechanical or chemical cleaning by a qualified contractor, followed by an internal inspection to check for under-deposit corrosion and tube thinning. Determine whether the 20% makeup is normal or whether condensate is being lost; recovering condensate reduces treatment load. After cleaning, verify the boiler is clean, restart treatment, and confirm the scale does not return. Document the program and results for the inspector.