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PREVENT CORROSION AT PIPE SUPPORTS SOLUTION

Posted by Antony Thomas at Wednesday, February 15, 2012

Corrosion at pipe supports is one of the leading causes of process piping failures, which can have potentially catastrophic results (see related news). All styles of pipe supports, including beam supports and pipe saddles, create crevices where water is trapped and held in constant contact with the pipe surface. Once corrosion is initiated in these pockets, it can quickly undercut the paint film and move away from the crevice, causing rapid wall loss. If these conditions are not addressed, entire sections of pipe can fail and require replacement.

Related news

Pipe failure cited in Marcus Hook refinery fire.

Fire officials blame a May 2009 fire that destroyed part of Sunoco’s Marcus Hook refinery on a rusty 10-inch pipe that ruptured and caused a catastrophic natural gas leak. The pipe failure was caused by external corrosion on the bottom of the pipe ..

Problems

Not surprisingly, it is the beam supports and the saddle clamps that have historically caused the majority of the problems. They have the following undesirable features in common:

1. Crevice Forming - This is the root of the problem, the formation of a crevice at the pipe surface.
2. Water Trapping - These support types all allow water to be trapped and held in contact with the pipe surface.
3. Poor Inspectability and Maintainability - These support types make it virtually impossible to paint or otherwise maintain some areas of the pipe at the support. Visual inspection is often difficult, and until fairly recently, it was also very difficult to inspect these areas with NDT methods.
4. Galvanic Couple Forming - Some of these support types may develop bi-metallic contact. Even though both the pipe and support are steel, the metallurgical differences can still provide a small potential difference to drive a corrosion cell.

The Corrosion Mechanism

It is a common misconception that metal-to-metal contact coupled with water entrapment is the major cause of corrosion at these points. This is not the case; the sequence of events is as follows:

1. Water is trapped - The very nature of the supports allows water to be held in contact with the painted pipe surface as well as the paint on the support element.
2. The paint system fails - Even if the paint on the pipe and support beam are perfect, the paint system is designed for atmospheric exposure and not immersion service. The longer the paint surface is continuously exposed to water, the more it softens. As the pipe softens it is inevitable that the steel substrate will be directly exposed to the water.
3. Corrosion is initiated - The small area of steel now exposed to oxygenated water (often with high chlorides) starts to corrode.
4. Corrosion undercuts paint film - The initial corrosion soon undercuts and spreads  Soon the whole support area is bare steel.
5. Crevice corrosion starts - From this point on the crevice corrosion driven by differential aeration takes over from the general corrosion mechanism that initiated the corrosion. As corrosion products build they further restrict oxygen diffusion and the oxygen concentration gradient gets steeper. Pitting now becomes the main problem with corrosion rates acceleration by an order of magnitude.

6. Pipe fails - If the inspection program is not set up to detect this mostly concealed wall loss. The pipe will fail.

Historical Solutions

The industry has long been aware of the problem, but has failed to appreciate the true causes; this is evidenced by some of the solutions that have been implemented to stop the problem which have actually accelerated the problem.

Rubber Pads and Liners

As previously stated, it was thought that the metal-to-metal contact was the main problem causing pipe support corrosion. As a result, initial designs incorrectly targeted this aspect of the supports. Some operators still use rubber pads of varying types to solve the problem, despite industry knowledge that they are in fact counter productive.  In fact, rubber pads under pipes do a wonderful job of reducing the life of the pipe. The crevice that was formed without the rubber pad is mild in comparison to the new crevice, which now has the ability to actually suck water in (by capillary action). Not only does the pad invite water in, it is better at holding it trapped against the pipe surface, since air circulation and natural evaporation is eliminated. The situation is further worsened by the length of the crevice which allows an oxygen concentration gradient to go from full natural concentration to anaerobic in a few centimeters.

Fiberglass Pads

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

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. There have been some other solutions adopted; none of which really address the major cause of the problem: water entrapment!

Summary and Conclusions

When designing pipe supports avoid the use of saddle clamps wherever possible. Never use a rubber pad between a pipe and a pipe support if the area is exposed to a corrosive environment. When using U-bolts to stabilize piping, always used polyolefin sheathed bolts. The half round rod solution has proven to be very effective in controlling pipe support corrosion over the last 15 years on thousands of offshore structures.

pipe support corrosion

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