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Steam boilers in particular need regular maintenance in order to avoid unscheduled shutdowns, the majority of which are caused by human error, our sources agree.
While seasonal maintenance should include the boiler, burner, controls, etc., this article will focus on the waterside, related controls, and corrosion problems that can occur when water, dirt, and sediment are allowed to remain in the system. Water treatment during startup depends largely on the quality of the system’s layup.
STAGNANT WATER, LAYUPSMark Hodgson, an ASHRAE Member with Clayton Group Services, Edison, NJ, discussed the “Impact of Stagnant Water Conditions” during a seminar on “Protecting Idle Equipment: Critical Water Needs” held at the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Annual Meeting in Honolulu, HI.
Hodgson explained that stagnant water can occur:
“Corrosion, microbial growth, fouling, and deposition are often interrelated,” he pointed out. Loss of sterilant can lead to microbial growth on the equipment’s walls; this in turn secretes mucusoidal slime (biofilm), which in turn creates an oxygen-free environment for anaerobic bacterial growth. These bacteria produce acids, which then attack metals.
This microbially induced corrosion (MIC) “can take 10 to 15 years off your pipework,” Hodgson said. In new systems, oil, grease, and millwork provide the food source.
Howard Benisvy, ASHRAE Member, works with Ondeo Nalco Co., a Naperville, IL-based water treatment company that specializes in boiler treatment technologies among other areas.
At the same seminar, Benisvy presented “Boilers: Idle System Failures and Prevention Through Proper Seasonal Layup and Startup.” He pointed out that with boilers, “The best preventive program can be undermined during idle times.” His talk focused on the waterside, although the fireside is also corrosion prone during idle conditions. “On the fireside,” he said, “ash in combination with sulfur and moisture can form acidic deposits which are very corrosive to the boiler tubes in the furnace. It is very important to minimize moisture to the boiler from the outside environment.”
On the boiler waterside, “Most corrosion is caused by oxygen, which easily gets into the water during idle conditions. It is particularly difficult to control when the boiler is filled with water, and idle for long periods of time,” he said.
Benisvy recommended dry layup for boilers that will not be used for longer than 30 days. Use a wet layup for periods of less than 30 days.
For the waterside dry layup, Benisvy recommended that:
For the fireside, maintenance staff must clean combustion deposits, especially if the boiler is burning No. 6 oil. “If the fireside can’t be sealed, some coatings can form a moisture barrier.”
Dry layup inspections need to be done monthly, he added. Check the internals, humidity, desiccant quality, and RH indicator cards.
For the short-term wet layup, Benisvy noted that “Most corrosion occurs at water-oxygen interfaces.”
Regarding layup chemistry for a boiler 300 PSI and below, he recommended:
Two to seven days before the wet layup, increase the chemistry to these ranges.
“Testing and evaluation are important, during a wet layup,” Benisvy said. “Check the chemistry two times per week, and sample from more than one location at least weekly.”
THE STARTUPHow good was the boiler layup? That can determine what to look for at startup.
“Bring the chemistry into operating ranges, which are much lower than the layup ranges,” Benisvy noted. “You may need to increase blowdown to achieve these ranges.”
Boiler manufacturer Cleaver-Brooks notes that “A rise in flue gas temperature usually indicates dirt on the fireside of the boiler or scale on the waterside. As a rule of thumb, a 40 degree F rise in temperature reduces boiler efficiency 1%.”
The Milwaukee, WI manufacturer also notes that “Waterside and fireside surfaces should be inspected and cleaned annually. A visual inspection provides an early warning that the vessel needs repair or water treatment or that combustion needs adjustment.
“Inspecting and cleaning water-column connections should receive special attention. Soot in the breeching is a fire hazard and can cause severe combustion-related problems.”
Richard Cannon, owner of Cannon Water Technology, Loomis, CA, works regularly on low-pressure (under 300 PSI), “Joe Average Citizen” boilers installed in hospitals and schools. Those in schools, he points out, are the most neglected. Their operation also is very seasonal, so boilers may sit idle the entire summer.
“If these water items are not inspected and/or repaired at startup,” he says, “the system could go down during the first blizzard.”
For him, seasonal steam boiler startup from a cold shutdown includes the following:
1. Check all valves and place in their startup position.
2. Open the sight gauge and water column high- and low-water shut-off valves. Make sure the water level safety controls are blown out. (“Many failures are caused because there was no low-level water alarm,” he states.)
3. Close the bottom blowdown valves, then open the upper drum vent valves.
4. Start water filling with soft water.
5. Manually inject water treatment chemicals including oxygen scavenger chemicals, so that the chemicals are added with the fill water.
6. Open the fuel system and fire the boiler. Carefully bring the pressure up to 15 PSIG, with the vent valve open to expel noncondensable gases.
7. After the pressure reaches 15 PSIG, close the drum vent and slowly bring the boiler up to operating pressure.
8. Collect a boiler water sample and test for the chemical concentrations recommended by your water treatment service company.
“Most water treatment problems take a long time to happen,” Cannon points out. “It’s the level control — boilers thought they have water but they don’t — that can cause immediate failure.”
Sidebar: Boiler Violation Statistics For First-Quarter 2002COLUMBUS, OH — The National Board of Boiler and Pressure Vessel Inspectors announced that it has released its “First-Quarter Report of Violation Findings for 2002.” According to the report, “boiler controls” had the highest percent of total violations, at 2,798.
“Piping and other systems” came in second, at 1,781 violations.
The report identifies the number and type of boiler and pressure vessel inspection violations found among participating member jurisdictions. It also identifies problem areas and trends related to boiler and pressure vessel operation, installation, maintenance, and repair.
For more information, visit www.nationalboard.net (website).
Publication date: 09/23/2002