Boiler Layup Procedures
It is generally agreed upon that wet layup is a best practice for idle periods, while dry layup is recommended for long-term shutdown, but is this justified?
Boiler maintenance procedures and water treatment chemicals are critical in the prevention against internal scale formation, corrosion, and pitting inside of a steam boiler. While the boiler is in normal operation, feedwater containing inhibitors and oxygen scavengers flow throughout the boiler and spread a vital layer of protection over the vessel’s exposed metal surfaces. When the boiler is out of operation, or off, the boiler stops the flow of chemically treated water that inhibits corrosion. The primary purpose of laying-up a boiler is to extend its life. Outside of time, the recommended method of boiler layup will be dictated by a boiler’s type and size, and by economic and safety-oriented advantages achieved performing the layup. To control corrosion, the boiler should be protected by either keeping the internal surfaces completely dry or actively treating the water.
If the shutdown will only be for a few days to a month, typically the most economical and feasible method for the idle period is to utilize a wet layup approach. A wet layup is defined as filling an idle boiler with chemically treated water so the boiler can remain a “flooded” and sealed environment until it is ready to return to service again. When using this method for terms of storage, the objective is to prevent oxygen and acidic corrosion while maintaining adequate operational internal chemistry when called back into service.
WET LAYUP PROCEDURE
Prior to taking the boiler offline, it is typically recommended adding enough chemical to be present in the boiler for the prevention of internal corrosion. In this case the chemical treatment for layup is injected into the boiler just before it comes offline. Occasionally a bypass feeder installed directly to the boiler can be employed for chemical addition although the feedwater tank can be utilized and a small recirculating pump installed to proactively circulate water. As the main objective when using a wet layup is to prevent oxygen
and acidic corrosion, it is generally accepted to maintain 200 ppm of catalyzed sodium sulfite as SO3 and 1000 ppm total alkalinity as CaCO3. Although these recommended control thresholds are higher compared to operational conditions, the boiler may experience change in pressure/temperature and supplemental chemical additions may pose a challenge once the boiler has been sealed. It is most important that the chemicals be well distributed in the boiler water.
Photo 1: Oxygen pitting corrosion on boiler tubes.
Fill the boiler to the top of the drum, blanking off outlets as necessary to avoid flooding the steam line or the
superheater. The most likely place for corrosion problems to develop would be at any air-liquid interfaces. An additional option uses steam blanketing. This is accomplished by connecting an operating boiler to an idle boiler via the steam header. By doing so, slight pressure is maintained in the off-line boiler by siphoning steam from the on-line boiler. Isolation valves can be used to ensure this method works properly. Also, a lower sulfite dosage is required (80-100 ppm) since the boiler is pressurized.
It is important that the boiler water be periodically tested to ensure chemical residuals stay within their recommended ranges. If the boiler system is allowed to drop to ambient pressure and temperature, the requirement to chemically scavenge dissolved oxygen will increase. As depicted in the photo attached, oxygen pitting can occur if residual sulfite is not sufficiently maintained.
If a shutdown is planned and likely to be long, e.g., a few weeks or longer the oxide coating will start to fail, and oxygen may enter the system. To maintain that protection, the system should be stored totally dry if possible. Dry layup, as the name suggests, is achieved when the boiler is drained. However, it is more complicated than just draining the boiler and leaving it. If the boiler is left exposed to air, the variable temperatures and humidity will allow for significant deterioration to the boiler’s interior unless it is protected.
DRY LAYUP PROCEDURE
Flush thoroughly and inspect the interior of the boiler for any remaining sludge deposits. If cleaning is needed, this should be done before the boiler is laid up. Otherwise, wash the waterside surfaces with high-pressure water while the boiler is still wet. First, wash the steam drum, and then the tubes and, lastly, the mud drums and blowdown lines. For firetube boilers, be sure to clean all water sides paying specific attention to any remaining sludge and scale chips between tubes or in the lower shell. If necessary, use a rigid brush to remove all deposits.
Photo 2: Boiler Inspection during dry layup period.
Dry thoroughly, giving special attention to drying out superheaters if any. If there are puddles, rust will form quickly at the air-water interface. Dry layup offers a great opportunity to inspect the internal waterside for scale/corrosion formation as a justification of the water treatment program. For high humidity atmospheric conditions: quicklime (CaO), commercial-grade silica gel, or a VCI is recommended to be distributed in the boiler drums. When using a desiccant (quicklime or silica gel), the trays should be placed on thin blocks so that air can circulate under and above them. Seal the boiler very carefully, blanking off all openings through which air or steam might enter. Replace quicklime every two months or when needed; for silica gel, replace every two months or when needed, or regenerate by heating at 325 - 350°F until moisture has dissipated.
For low humidity atmospheric conditions: Leave all manholes, handholes, furnaces and stack dampers open
for air circulation. Check the boiler every few months to ensure the atmosphere remains at a consistently low
humidity. If humidity has increased, the boiler may need to be switched to closed dry layup, above. There is a soft line drawn in the sand when it comes to generalizing the best layup procedure for off-line boilers. More often is it the case that a layup procedure is chosen conditionally rather than the preferred method on paper. In a hospital setting, it is unlikely that a boiler can be drained for a long period of time for backup purposes. It can also be difficult to circulate and treat some boilers during wet layup. I would agree with the generalized statement that wet layup is a best practice for idle periods, while dry layup is recommended for long term shutdown but as always, consult a water treatment professional prior to making that decision.