Mechanical parts may suffer failure in many different modes. Mechanical, structural, and material engineers are familiar with failure modes like creep, fracture, corrosion, wear, fatigue, and thermal stress. But how are these terms applied in a practical setting? Here are three failure modes that pipes and pipelines may experience.
Everyone who has seen rust has seen corrosion. The conversion of iron into iron oxide, or rust, is an example of galvanic corrosion.
In galvanic corrosion, different metals connected through an electrolyte exchange electrons to convert to a more stable form. In the case of rusting, iron or steel acts as an electrode and water acts as an electrolyte. Specifically, iron loses electrons in the presence of oxygen and water. Through a series of chemical reactions, the ionized iron combines with oxygen ions to form iron oxide or rust.
Another form of corrosion is called microbial corrosion. Microbial corrosion occurs when a microbe-friendly environment, such as a damp, dark crevice, allows bacteria to grow. Bacteria produce a variety of byproducts through their digestion process, including acids, sulfides, and oxides. These chemicals react with metals to convert those metals into a different form, such as converting iron into iron oxide or rust.
While pipe saddles transfer loads to the tube wall and reduce heat transfer, the interface between pipes and pipe saddles create the perfect conditions for corrosion. External rust on pipes due to galvanic corrosion can be reduced by electrically insulating the pipe from other metals or by eliminating the water that acts as an electrolyte. External rust on pipes due to microbial corrosion can be reduced by eliminating the moisture layer necessary for bacterial growth. In either case, pipe pads can counter the causes of pipe erosion by preventing moisture penetration into the space between pipes and pipe supports.
Wear is a failure mode that occurs due to materials rubbing against each other. Again, this form of material failure is familiar to everyone once we recognize that it could happen to everything from door hinges to engine pistons if not for lubricants.
Failure by wear occurs due to the gradual removal and deformation of material surfaces. This process weakens the surfaces and can create pits and grooves that become the site of cracks or fractures. Abrasion is a separate, but related concept. Any grit, dirt, or wear particles between two surfaces can accelerate the removal of surface material.
For example, a pipe resting on a pipe support rubs against the support every time the pipe moves. Even minor movement caused by hydraulic shock or the shifting weight of the fluid when flow in the pipe starts or stops could, over time, cause wear on the exterior of a pipe. The causes of pipe erosion due to wear can be reduced or eliminated by inserting a pipe wear pad between the pipe and pipe support. Since pipe wear pads are softer than the metal of the pipe, the pipe pad wears rather than the pipe. In other words, the causes of pipe erosion due to wear are not eliminated by eliminating the relative movement of the pipe and its support. Rather, the causes of pipe erosion due to wear are eliminated by using a sacrificial pipe pad material to wear, thereby protecting the pipe material from wear.
Fatigue is a failure mode that is caused by cracks that develop due to repeated stresses. For example, the vibration of a pipe caused by a pump, impeller, or other machinery may cause cracks to develop and propagate, eventually causing the pipe to fail. This is a familiar failure mode for anyone who has ever bent a piece of metal or plastic, such as a plastic credit card, repeatedly until it was weak enough to break.
Some effects of fatigue can be reduced or eliminated by providing a wear pad between a pipe and pipe support. The wear pad acts as a shock absorber to reduce the vibration of the pipe. Less vibration means less crack formation and propagation.
In sum, the failure modes for pipes and pipelines include corrosion, wear, and fatigue. The causes of pipe erosion due to corrosion, wear, and fatigue may be reduced or eliminated through the use of pipe pads.