In general, the materials used in oil and gas facilities are classified into 3 basic groups: metals, ceramics, and polymers. This scheme is primarily based on the chemical makeup and atomic structure. However, metallic materials are the most widely used group.
Metal for example, carbon steel, Alloy (stainless steel etc), aluminum, copper, titanium, and nickel. Atoms in metals and alloys are arranged in a very regular and relatively dense manner. With regard to mechanical characteristics, these materials are relatively stiff and strong, but ductile (ie, capable of large deformations without fracture), and are resistant to fracture. Material properties of metal that usually considered :
- yield strength
- ultimate strength
- allowable stress at elevated temperature
- strain
- modulus of elasticity
- hardness
- toughness
- poison ratio
- coefficient of expansion
stress – strain correlation of iron. at higher temperature, the strength of material (stress) is decrease.
Performance of material depends on its properties, which in turn is a function of its structure; Furthermore, the structure is determined by how the material has been processed. The facility engineer used the properties to make design calculation for equipment. Allowable stress at elevated temperature is used to determine the equipment thickness. Below is example of pipeline thickness requirement as per ASME B31.3. The lower the allowable stress (S), the higher thickness required (t).
if the facilty engineer design a pipe as per ASME B31.3 and use pipeline material API 5L grade A ( yield stress 30,000 psi), then he needs to see the Allowable stress according to metal operation temperature.
- at metal temperature < 500 F, the allowable stress is 16,000 Psi.
- at metal temperature = 750 f, the allowable stress is 10,700 Psi ( decrease 33%)