CORROSION AT PIPE TO SUPPORT CONTACT AREA

PREPARED BY : AHMED REYAD

Introduction Corrosion at pipe to

support contact area is one of the leading causes of topside process piping failure.

Corrosion at the pipe to support contact area is a common form of crevice corrosion .

Catastrophic failure With time corrosion at this area

can lead to an undesirable condition where accelerated corrosion results in local metal loss which in turn can affect the integrity of the pipe or supporting steel.

It can cause large rupture failure (Ethylene Unit Fire Ethylene Unit Fire - May 17, 2009 May 17, 2009 in Sunoco Marcus Hook Refinery ,USA)

It is so criticalPipe corrosion that went undetected for years led to an explosion and fire that rocked a Sunoco oil refinery in Claymont, Del. in May, authorities said Tuesday.The corrosion, caused by unseen condensation moisture that had collected between the bottom of a 10-inch pipe and a loose steel sleeve on which it rested, resulted in a gas leak that triggered the May 17 fire and explosion, investigators said..

Types of Pipe Supports Standard Beam Support. Saddle Clamp. Welded Support. Others The first two categories account

statistically for better than 95% of support points on a typical offshore structure

Critical factors Crevice Forming - This is the root of the problem, the formation of a crevice between the support and the

pipe surface. Water Trapping ;results in : - the paint failure - Plays as electrolyte in the corrosion cell Contaminants Atmosphere : Marine & Industrial :the aggressiveness of the combination of industrial and marine

environments is usually higher than either would be alone.High levels of chlorides & other salt such as magnesium chloride, (which are hygroscopic). Hygroscopic materials tend to absorb water and tend to release the water only during conditions of very low relative

humidity. Surfaces contaminated with sea salts will remain wet much longer than uncontaminated surfaces. . These chlorides are also particularly aggressive because metal chlorides tend to be soluble. Chloride— containing

corrosion products do not form the protective films of insoluble corrosion products. Temperature: Normally at temperatures above 90°C (194°F) crevice corrosion is not a problem, because moisture simply

evaporates In other cases if the temperature is below the dew point the water will condensate on the metal surface and it will be a source of the water.

Pipe Movement / Vibration: At the point where a pipe comes in contact with the support, abrasion associated with pipe vibration and movement can wear away protective coatings on the pipe to support contact area.

Historical Solutions

Rubber Pads and Liners In fact, rubber pads under pipes do a wonderful job of reducing the life of the pipe

Historical Solutions Fiberglass PadsContoured pads attached to

the pipe at support points. Obviously, another attempt to eliminate metal to metal contact. This is better than the rubber pads as it is higher strength than the rubber, but still allows a crevice to be formed at the pipe surface

Historical Solutions Welded Supports The welded support is a

viable solution. However it adds significant

cost to a typical project both in terms of construction and inspection.

In some situations it would be undesirable to make so many external longitudinal welds to a pressured piping system.