Waterside surfaces of boilers that are out of service are particularly susceptible to corrosion. Depending on the length of time the boiler will be out of service and how quickly the boiler may need to be ready for service will determine how the boiler should be layed up. There are a few common methods for storing a boiler properly to prevent corrosion while it is off line. This week’s blog will feature Dry Storage and Alternative Dry Storage. Stay tuned for Part 2 next week featuring Wet Storage and Alternative Wet Storage.
The dry storage method is recommended whenever a boiler will be out of service for an extended period of time, but must also be available for unit operation in a moderately short period of time.
The following steps must be taken to ensure proper storage using this dry storage method:
- Bottom blow the boiler at least twice daily for a few days before the boiler is to be shut down. Once the boiler is turned off. Bottom blow the boiler every 10-20 psi drop in pressure.
- Begin to drain the boiler once the pressure is at zero and the temperature is under 140oF. Opening the boiler when it is at this temperature helps dry internal surfaces.
- The boiler should be thoroughly cleaned and inspected. If possible, all horizontal and non-drainable boiler tubes should be blown with compressed air to remove all water and moisture.
- Heat should be applied to dry the boiler walls, insulation, tube internals and refractory. This can be done by firing the boiler at a minimal rate or using an external source of heat. Fuel fired heaters should not be used inside the boiler because of the moisture that is generated during the heating process.
- Immediately after the surfaces are dried, one of the recommended drying agents should be spread on water-tight and corrosion-resistant trays:
- Quick Lime- 6lb/100ft
- Silica Gel- 8lb/100ft
- Activated Alumina- 8lb/100ft3
- These trays should go into each drum of the boiler or on the top tubes of a fire tube boiler.
- NOTE: Specialty drying agents made specifically for dry layup of a boiler can be very effective and convenient and can in many instances remain in the boiler for startup because they volatilize upon heating. Contact your Watertech representative for information on these specialty boiler layup products.
- Manholes, vents and connections should be tightly blanked and closed. The drain between the non-return and main steam stops valves should be left open. The waterside of the boiler should be as air tight as possible.
- The boiler should be opened every 3 or 4 months for inspection of the drying agent. It should be replaced as needed.
Alternative Dry Storage:
For extended periods of storage time for package fire tube and water tube boilers that do not have superheaters and/or economizers, the alternative dry storage method is an option. Note that other boilers must be operating in the boiler room to ensure low humidity in the air since the boiler will remain open.
- First of all, the boiler should be thoroughly cleaned and inspected. If possible, all horizontal and non-drainable boiler tubes should be blown with compressed air to remove water and moisture. Particular care in water tube units must be taken to get rid of water from long horizontal tubes that may be bowed slightly.
- Check the feedwater inlet and the steam outlet to be sure no dampness remains in the boiler. The manhole covers and several handhole covers should remain open, allowing dry boiler room air to enter the units. Forcing dry air through the waterside of the boiler using a fan is always a good method of ensuring moisture does not build up on internal surfaces. The boilers should be inspected every 2 weeks to ensure against any moisture or surface rust.
Stay tuned for next week’s tips on wet storage options. Please contact your Watertech representative for further assistance on how to properly layup your standby boilers.