Precious Metals Population is not of concern if there are enough resources to go around. Important resources like water of suitable quality for growing crops, drinking, cooking, and cleanliness, fertile soil for growing food and trees, and fuel for warmth and cooking. Depletion of important resources leads to poverty, disease, malnutrion and often death.
Welded support This type of support involves welding a part to the pipe which is usually free to move at the interface to the support.
There are a number of variations on this theme, and is a common approach for insulated piping systems. Typical welded pipe support. Allows for movement at support interface, but allows corrosion as well.
Others There are a number of other methods used, such as flange bolt supports, various type of pipe hangers and other specialty-type supports. 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: Crevice forming - This is the root of the problem: Water trapping - These support types all allow water to be trapped and held in contact with the pipe surface.
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. 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 create 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: 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.
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. Corrosion is initiated - The small area of steel now exposed to oxygenated water often with high chlorides starts to corrode.
Corrosion undercuts paint film - The initial corrosion soon undercuts and spreads Fig. Soon the whole support area is bare steel.
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 accelerating by an order of magnitude.
Pipe fails - If the inspection program is not set up to detect this mostly concealed wall loss, the pipe will fail. Paint is progressively undercut away from initiation site Figure 7: Advanced crevice corrosion 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 following 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 in an attempt to solve this problem, despite industry knowledge that they are counter-productive.
The crevice that was formed without the rubber pad is mild in comparison to the new crevice, which now has the ability to 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.
Rubber pads accelerate crevice corrosion Fiberglass pads Contoured pads attached to the pipe at support points Fig.The Unity of the Spirit: 1: I therefore, the prisoner of the Lord, beseech you that ye walk worthy of the vocation wherewith ye are called, 2: with all lowliness and meekness, with long-suffering, forbearing one another in love; Col.
, 13 3. References cited. Arrhenius, S. On the influence of carbonic acid in the air upon the temperature on the ground. London, Edinburgh and Dublin Philosophical Magazine and journal of . U.S.: Hold Steady. June 09, Earth Island Journal If we don't stabilize population growth, life as we know it is unlikely to continue.
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