99 Diseases of Pressure Equipment: Creep and Creep Cracking

Metals will slowly deform under stress and higher temperatures by the mechanism known as creep. The amount of creep deformation that will be experienced is highly dependent upon the level of stress, level of temperature and material properties. It is vital that any component operating in the creep range have Integrity Operating Windows (IOW’s) established where upon operators are required to make adjustments if certain temperatures are reached. For example, a standard IOW may be set for a furnace tube at 850F, which would require the operator to take steps to adjust the burners to move back under the 850F limit. Then, at 900F, a critical IOW may be set, that would require the operator to take steps to shut-down the furnace, if s/he cannot control the temperature. In certain temperature ranges, the theoretical service life of a furnace tube can be cut in half by continuous operation exceeding the design limit by just 25 degrees F. While creep failures can occur at design conditions in just 100,000 hours of operation, rapid stress rupture (highly accelerated creep - covered in a separate article) can occur in as little as 100-1000 hours if design temperature limits are exceeded by significant amounts, e.g. hot spots on furnace tubes, or hot spots on refractory lined equipment. Below the creep range for each material, service life (from a creep standpoint) becomes nearly infinite, and therefore if creep is not an issue in design, it should not be an issue in operation, if temperature IOW’s are not exceeded.

For the carbon and low alloy steels commonly used in the petrochemical industry, creep rates start slowly in the 700- 800F range, and increase gradually with steady loading as temperatures increase. For greater creep resistance, the industry typically turns to austenitic stainless steels, especially the “H” grades. In some higher strength materials/ welds, low creep ductility (sometimes called creep embrittlement) may be experienced, which leads to failure with very little detectable deformation.

Nozzles and other components with high tri-axial loading on some catalytic reformers have been susceptible to creep cracking and low creep ductility. While furnace components, e.g. tubes, supports, hangers, etc. most commonly experience creep damage, “cold-shell” designed equipment that is normally protected by refractory can suffer “surprise” creep damage when the refractory protection deteriorates. Dissimilar metal welds (DMW) are also susceptible to creep damage (e.g. ferritic to austenitic welds) because of the high localized stresses generated by differential thermal expansion.

Inspection for creep damage is not as straight-forward as for many other of the “99 Diseases”, and may require a number of techniques to be used in combination. Perhaps the most widely known inspection method is the use of ring (go-no go) gauges for bulges in furnace tubes, commonly sized for 1- 3% bulging, depending upon the material and design conditions. However, as mentioned above, some materials, e.g. HK-40 tubes and other cast austenitic alloys show low creep ductility and therefore may not bulge significantly before failure. In such cases, a combination of surface and volumetric NDE may be required, along with metallographic samples (destructive or non-destructive) to verify the presence of creep voids, creep fissures, and creep cracking (the three physically detectible stages of creep failure).

Have you identified all your equipment that does (or might) operate in the creep range and specified the appropriate inspection program for that equipment; and have you set the appropriate IOW's for that equipment so that operators know what to do when standard and critical limits are reached and why they must not be exceeded?