Pipes and tubular conduits – Distinct layers – With intermediate insulation layer
Reexamination Certificate
2001-01-03
2003-01-14
Hook, James (Department: 3752)
Pipes and tubular conduits
Distinct layers
With intermediate insulation layer
C138S146000
Reexamination Certificate
active
06505650
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for inhibiting corrosion under insulation on the exterior of a structure by positioning concentrated charges of alkaline material sufficient to raise the pH in water in the insulation to a value from about 8 to about 12. The method of the present invention is particularly effective with insulated pipelines to produce a corrosion inhibited pipeline. The method of the present invention is especially effective to inhibit corrosion in the existing pipeline installations.
BACKGROUND OF THE INVENTION
Corrosion under insulation can occur wherever piping, vessels, or tanks are thermally insulated and exposed to the weather. The problem begins when there is a breach in the outer jacketing protecting the insulation. Rain and melting snow can then penetrate the jacketing and wet the insulation. Liquid water will eventually contact the external surface of the pipe, vessel, or tank beneath the insulation. If the pipe, vessel, or tank is made of non-corrosion-resistant metal and no coating was applied to protect its exterior surface, (a common cost-savings measure), corrosion will occur. Depending on the amount of water present, availability of oxygen, and temperature of the metal surface, corrosion under insulation can be mildly aggressive: up to 60 mils per year (0.060 inches per year) wall loss has been observed. Often there are no visible signs that such corrosion is occurring; the first indication of corrosion under insulation is often failure of the pipe, vessel, or tank. Failures due to corrosion under insulation are usually sudden and catastrophic because the corrosion typically occurs over a sizeable surface area. This is in contrast to internal corrosion which is usually highly localized (pit) and typically just results in a small leak. For this reason, external corrosion is a more serious safety and environmental problem than internal corrosion.
Corrosion under insulation is a significant problem in the oilfields on the North Slope of Alaska, due to the advanced ages of their infrastructures. These fields' production gathering systems consist of above-grade, thermally insulated, bare (no coating), carbon steel pipe and are thus susceptible to corrosion under insulation. Many hundreds of miles of such piping are in place.
The most commonly used insulation system in Alaskan oilfields is known as “spiral-wrap” insulation and is still the preferred insulation for new construction. This insulation consists of urethane foam insulation between the pipe and an outer metal sheathing of corrugated, galvanized steel. The corrugations are in a spiral pattern. This insulation system is applied in a factory (shop-applied). Bare, carbon-steel pipe (usually a 40-ft joint) is first blasted with grit and cleaned with a solvent. The bare pipe is then placed inside a “tube” of corrugated galvanized steel, leaving an annulus between the pipe and the sheathing of either 2 or 3 inches depending on the pipe size. A two-part, liquid, polyurethane foam is injected between the sheathing and pipe where it expands, filling the annulus. The insulated pipe is then shipped to the field for use in constructing pipelines. The insulation may comprise foam insulation, closed cell foam insulation, fibrous insulation and the like.
In the field, the pre-insulated joints of pipe are welded together to form a pipeline. To allow for welding, a short length of pipe at either end of each pipe joint is left uninsulated by the insulation shop. Thus, when two pipe joints are joined, there is a gap in the shop-applied insulation at each field weld, (one gap about every 40 feet). Additional insulation is applied in the field to fill these gaps in the shop-applied insulation. Typically, a piece of flat, galvanized sheet steel wider than the insulation gap is wrapped around the pipe, bridging the gap in the shop-applied insulation (see FIG.
1
). This forms a confined annular space. A two-part liquid polyurethane foam is injected into the annular space through an access hole in the sheet metal. The foam expands, filling the annular space, and making the pipe insulation continuous. The thin gap between the corrugated metal jacket of the shop-applied insulation and the sheet metal surrounding the field-applied insulation is sealed with a sealant (i.e. silicone caulk) to make the installation weather-proof. This field applied insulation is commonly referred to as a “weld pack”.
Due to weathering of the sealant, thermal expansion of the pipe, and wind-induced vibrations, the seal between the field-applied jacketing and the shop-applied jacketing eventually fails. Blowing snow and/or rain then makes its way into the urethane foam insulation within. Although the insulation is closed-cell foam and will not absorb liquid water, it is permeable to water vapor. Water vapor successively penetrates cell walls and then condenses inside the cells. In this manner, water migrates through the insulation, eventually coming into contact with the bare, carbon-steel pipe inside. The process is very slow; it takes years for liquid water to migrate through the few inches of foam covering the pipe. Once the liquid water reaches the pipe, the water, oxygen, and heat (hot fluids inside the pipe) combine to form a corrosive environment.
When there is corrosion at a weld pack, often there are two patches of corrosion on either side of the girth weld along the bottom half of the pipe and centered around the joints between the shop-applied and field applied insulation (see FIG.
1
). This is where the water usually first contacts the pipe.
Corrosion under insulation at weld packs has caused catastrophic failures of pipelines. When such failures occur before the corrosion is discovered and thus before measures can be taken to repair the damage the consequences of the failure can be explosive if the contents of the pipe are under pressure.
The failure of the seals at the weld packs, the progressive wetting of the insulation, and the resulting corrosion under insulation are all slow processes and produce no visible indications that they are occurring. Corrosion under insulation was not recognized until after the first failures occurred.
When corrosion is found, the wet insulation is removed, the corrosion product cleaned off the pipe, and the damage is measured and evaluated. If the damage is not too severe, the pipe is covered with a protective wrapping (such as a plastic tape designed for buried pipeline applications) and then reinsulated. If the damage is too severe, a reinforcing sleeve is installed over the damaged area before applying the protective wrap. Reconditioning weld packs is expensive.
This does not address the issue of wet weld packs which have not yet started to corrode. Even if additional water could be kept from entering the weld pack, the water already in the wet weld packs will continue to migrate through the insulation due to the gradient in water vapor pressure. When the water reaches the pipe surface, corrosion will begin. Because the foam is closed-cell, there is no known practical way of getting the water out of the insulation once there. Drilling holes does no good as the water will not drain out as from a sponge. The only known way to eliminate the water is to remove the wet insulation itself, which is an expensive option as noted above.
Even though corrosion under insulation is a recognized problem, the spiral-wrap insulation with weld packs is still the preferred insulation system and has been used as recently as 1999-2000.
Past approaches to eliminating corrosion under insulation have typically focused on developing a better weld pack design that will remain weather proof. This approach has several drawbacks: The new designs are expensive, and the basic objective may be unobtainable considering the cyclic loading the weld packs are subjected to over their long operational lifetimes. Furthermore, this approach does nothing to address the problem of corrosion under insulation for existing construction.
In new construction, a simple way of eliminating corrosion under insu
Bohon William M.
Ruschau Gregory R.
Hook James
Phillips Petroleum Company
Scott F. Lindsey
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