The boiler comes off line Friday and stays off for eight months, or three years, or until the market turns, and the maintenance engineer has to keep the tube banks, headers and downstream piping from rusting in the meantime. The usual answers are dry layup with dehumidifiers, wet layup with treated water, or a nitrogen blanket. The nitrogen blanket in particular fails quietly: a blind contracts on a cold night, the purge leaks, and nobody knows until the restart blows rust out of the stack.
The fourth option is a vapor-phase fogging fluid. Cortec VpCI-337 is fogged into the sealed void at one fluid ounce per cubic foot, the vapor condenses as a thin self-healing film on every steel surface, and the film breaks down to harmless gases within a day of restart. We already have a general boiler layup post on this site; this one is the companion on the dosing itself, drawn from five Cortec case histories where the procedure is spelled out.
A 53 MW unit fogged for a three-year shutdown
Cortec Case History #500 (2015) is the cleanest description of the method. A member-owned utility in Alaska was taking a steam turbine, its boiler and the associated piping out of service for up to three years: about 1,000 square feet of carbon steel piping and vessels indoors and 15,000 square feet of ducting and pipeline outside.
The crew closed and sealed every access port except two, one at the top of each vessel and one at the bottom, and put a utility blower on the far opening to pull a draft through the void. Fogging ran at one ounce of VpCI-337 per cubic foot of void space (0.9 liter per cubic meter) until mist appeared at the second opening, with the blower cycled on and off so the vapor was not pulled through too fast. When the calculated dose was in, the ports were sealed and VpCI Protected labels went on the openings. The case notes that VpCI-337 vapor travels about 100 feet through piping, including bends, so the blower is optional on most systems.
Fourteen years of rust out of the chimney, then none
Cortec Case History #649 (2007) answers the question of whether a fog can reach a whole boiler. A power station in the east of England shut its boilers down for outages once or twice a year, and every restart blew rust off the cooled tube walls and out of the stacks onto neighboring businesses. Some restarts were aborted until the wind changed. That went on for 14 years. A Cortec applicator sprayed VpCI-337 into the top and bottom of each boiler and onto the walls and tube banks across all seven levels. Three weeks later the boilers started with no rust from the chimneys for the first time in 14 years, and the treatment became part of every outage.
HRSGs, witness panels and the case against nitrogen
Cortec Case History #750 (2021) covers two of five heat recovery steam generators at a UK combined-cycle station, offline for several cold months. The applicator fogged VpCI-337 at several levels inside each HRSG at the volume required for up to 12 months of protection, alongside dehumidification, and hung carbon steel witness panels inside and outside the treated areas so the plant could check the result without opening anything. The case states plainly that this replaces nitrogen blanketing, which is expensive to install and maintain and a confined-space hazard for anyone entering. The station’s mechanical engineer noted that the same technique had eliminated corrosion dust at startup on an earlier layup.
Cortec Case History #823 (2023) is the same lesson from a Houston air-cooler manufacturer. Its bundles were nitrogen-purged and sealed with steel blinds and gaskets. Temperature swings contracted the steel, the blinds leaked, and the internals were exposed. The fix was to fog VpCI-337 into the bundles and close them with a PVC bumper gasket, which keeps the inhibitor in and the weather out but does not have to be gas-tight. Flange faces were de-rusted, degreased with VpCI-414 and sprayed with CorShield VpCI-368. Nobody had to walk the yard checking nitrogen any more.
Dosing a long pipe run without guessing
Cortec Case History #684 (2015) adds the technique for a system you cannot see into. An LNG project in Malaysia wanted an alternative to nitrogen for pipe internals. The dose was calculated from the volume of every tube section plus the inlet and outlet header boxes. Half the dose was fogged in through a loose enclosure at the inlet until visible fog appeared at the outlet or a VpCI vapor detection indicator strip changed color; the remaining half went in afterward and the system was sealed. Result: 24 months of protection and an accepted nitrogen substitute.
| Case | Equipment | Dose and method | Term |
|---|---|---|---|
| #500 (2015) | Steam turbine, boiler, 16,000 sq ft of piping | 1 oz/ft³, blower draft, fog until mist exits | Up to 3 years |
| #649 (2007) | Utility boilers, seven levels | Sprayed top, bottom, walls and tube banks | Each outage |
| #750 (2021) | Two HRSGs | Fogged at several levels, witness panels | Up to 12 months |
| #684 (2015) | LNG pipe internals | Half dose to indicator change, then the rest | 24 months |
| #823 (2023) | Air-cooler bundles | Fog, seal with PVC gasket, coat flanges | Ongoing |
How to do this in your plant
- Measure the void: drum, tubes, headers and any connected piping. VpCI-337 is dosed at 1 fluid ounce per cubic foot, so a 5-gallon pail of VpCI-337 covers about 640 cubic feet; a 55-gallon drum covers about 7,000.
- Get the steel clean and dry first. Fogging fluid protects clean metal; it does not remove scale or standing water.
- Seal all but two openings, one low inlet and one high exhaust or the reverse, with pipe caps, blinds or shrink film. The seal does not need to be gas-tight, which is the whole advantage over nitrogen.
- Fog to the exhaust, then to the dose. Run the fog in until mist shows at the exhaust or an indicator strip changes, cap the exhaust, continue to the full 1 oz/ft³, then seal the inlet. On long piping a fan at the exhaust helps; on most boilers the vapor gets there on its own.
- Hang a carbon steel witness coupon at the entry port and stick VpCI Protected labels on every sealed opening with the date. For layups past two years, inspect the coupons every one to two years and re-dose as needed.
- For small voids and touch-ups, a control cabinet, a gearbox casing or a hatch opened for inspection, use a can of EcoAir VpCI-337, about 13 cubic feet per can, no spray equipment.
- Restart without a flush. The film decomposes to carbon dioxide and nitrogen oxides within about 24 hours of return to service. Our VpCI product selection guide lists the water-side powders and Boiler Lizard bags to pair with it.
Which Cortec products
- VpCI-337, 5-gallon pail: the fogging fluid in all five cases; one boiler drum, a heat exchanger shell or a bank of pipe spools.
- VpCI-337, 55-gallon drum and 275-gallon tote: for a full unit layup or an outage program that repeats every year.
- EcoAir VpCI-337 aerosol: the same chemistry in a compressed-air can with a delayed-release actuator, for cabinets, casings and touch-ups.
- VpCI Protected labels: mark every sealed port so the protection survives the next shift.
We stock VpCI-337 in pails and drums, EcoAir 337 and the labels in Elmhurst, Illinois. If you have an outage coming, contact us; we can work out the dose for your drum and header volumes, send a sample and help you trial one unit before you commit the plant.
Sources: Cortec Case History #500 (2015), Cortec Case History #649 (2007), Cortec Case History #750 (2021), Cortec Case History #823 (2023), Cortec Case History #684 (2015); Cortec product data sheets for VpCI-337 and EcoAir VpCI-337.
