Creep and Stress-Rupture

Only a small number of pressure vessels in process plants operate at temperatures high enough for creep to occur during normal operation. Creep is the continuous plastic deformation of a material under a constant stress and at high temperatures (approximately 700°F for carbon steels, 850°F for low-alloy Cr – Mo steels, and 900°F for austenitic stainless steels) and ultimately results in stress-rupture crack nucleation. It is a time-dependent process that implies a finite life for vessels operating in the creep range (i.e., at temperatures high enough for creep to occur at the maximum allowable design stress). The ASME Code maximum allowable design stress for vessels with design temperatures in the creep range is based on the 100,000-hour stress-rupture life of the material used for construction of the vessel. Normal experience shows, however, that vessels give satisfactory service for considerably longer times.

FCC reactors and Rheniformer reactors are examples of vessels that can be designed for operation in the creep range. Some other vessels (including “cold wall” FCC and Rheniformer reactors) are refractory-lined to keep the vessel shell temperature below the creep range. However, deterioration of the refractory lining can result in “hot spots” on the vessel shell, which can subject those overheated locations to creep and the formation of stress-rupture cracks.

Stress-rupture cracks in pressure vessels operating in the creep range usually develop first at locations of relatively high stress, such as nozzles. Weldments (weld metals and heat affected zones) also tend to have somewhat shorter stress-rupture life than plates and forgings. Stress-rupture cracks generally originate near the surface of the vessel shell, but it is not unusual for them to initiate internally with no detectable indication on the surface until the latter stages of crack propagation. Internal initiation of stress-rupture cracks is more common for relatively thick (1-inch) vessel shell components.

Creep must cause some plastic deformation of the shell of a pressure vessel before stress-rupture cracks develop and failure occurs. The plastic deformation, however, can be very difficult to detect before the stress-rupture cracks cause failure and should not be relied on to give sufficient warning of approaching failure. Because the plastic deformation can be highly localized in regions of high stress where the stress-rupture cracks develop, it may not cause any noticeable distortion of the vessel. Observable “bulging” of overheated areas may be evident, however, where hot spots created by the deterioration of the refractory lining cause creep.

Stress-rupture cracks can usually be observed on the surface of a pressure vessel before failure occurs, but visual examination (VT) should not always be relied on to give an adequate indication of deterioration.

Ultrasonic examination (UT) using shear wave procedures can be used to detect and size stress-rupture cracks. Stress-rupture cracks propagate relatively slowly and can usually be detected at an early stage in their development, in time to allow repair or replacement of deteriorated components of the vessel without unnecessarily interrupting production. UT examinations should concentrate on areas of relatively high stress, such as nozzle welds, head-to-shell welds, and longitudinal seam welds. Any hot spots that develop should receive intensive examination.

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