Many tube suppliers specify heated indoor storage for replacement tubes. In most power plants there is not enough indoor heated space for this purpose so that tubes are stored outdoors However, tubes can be stored outside for extended periods if proper precautions are taken to prevent rusting.
The first requirement for good storage is that all replacement tubes should be capped. This protects the tubes from accumulating dirt, debris or water. Most vendors use inferior plastic caps for this purpose. When stored outdoors these caps become brittle as a result to the elements and crack letting in rain water and moisture. Severe rusting is a result, especially if the tubes has previously been acid cleaned. If tube are to be stored outside for an extended amount of time, more preferable are UV - treated polyethylene plugs which will not become brittle and are less susceptible to mechanical damage. These offer excellent protection for little cost. For tubes with weld end preparation, metal caps can be used instead of plastic plugs for extra protection against the nicks and dings that happen to some tubes in every bundle.
Even with capping, some internal rusting can occur because of condensation. It is simply not possible to maintain an air tight seal in every tube. Thus some form of corrosion protection is needed for outdoor storage regardless of how well the tubes appear to be capped. Cold drawn tubes will be coated with a film of oil on the inside when they are received from the fabricator or supplier. This oil will prevent rusting of the tubes for a short term (up to approximately three months) and should not be removed until the tube is ready to go into the boiler. The oil can be easily removed by steam cleaning or by washing with hot water and detergent solution. If oil film is removed, the tubes should be used immediately to protect against corrosion as discussed below.
Chemically or mechanically cleaned tubes are more susceptible to rusting during storage than oiled tubes. Freshly cleaned tubes can be oiled which will provide short term protection. Silica gel can be used to prevent condensation but experience indicates that it is of limited benefit because its dehydration capacity is quickly exhausted. A better solution is to use a vapor phase corrosion inhibitor (VPCIs).
VPCI's are a relatively new development in corrosion protection. They have been used for protection of tubes for export and are also applicable for the protection of tubes in outdoor storage at power plants. VPCI's consist of tablets or powder containing an amine-based corrosion inhibitor. The material sublimes in confined space of the capped tube and releases the amine which condenses on the wall and provides corrosion protection. Any residual inhibitor can be removed from the tube end by hand before installation. Even if not removed small amounts of he inhibitor are not harmful to the boiler.
VPCI's have been acceptable by all the boiler manufacturers and many utilities. Installation procedures consist of manually placing or air blowing a measured amount of inhibitor into each end of the tube.
The exterior surfaces of the tubes in outdoor storage can be protected with an oil-based protective coating or paint. Recommended is a translucent coating in order not to obscure the identification markings on the tubes to allow for easier inspection of surface details, such as material specifications, sizing and welds. These coating should be applied at a thickness of 3-5 mils. Manufacturers recommend the use of a coating that dries to a soft waxy film instead of a paint because a soft material is self-healing after it is scratched.
Finally, all tubes should be inspected during storage on a regular basis for missing or damaged caps, which should be promptly replaced. In tubes where air seal is broken, the VPCI must be replaced.
Note: Poor storage practices at the plant can result in a rapid accumulation of rust, dirt and debris in the tubes regardless of the steps taken by the vendor or the fabricator to clean them. Unlike cleaning, storage is the unquestionably the responsibility of the purchaser. If excessive build up if rust and or corrosion are present on the tubes, the tubes may need to be replaced. The local Jurisdictional Authorized Inspector could make this determination.