Short Term Overheating Stress rupture is the phenomenon caused by localized overheating well above the design temperatures. It can cause permanent deformation in process equipment. The equipment may result in bulging and eventual failure by stress rupture. If the temperature is high enough, failure will occur very quickly by tensile overload, since tensile strengths drop off dramatically at very high temperatures.
In the Hydrotreating or Hydrocracking Units, feed or fractionation heaters can be affected due to flame impingement or any other cause of local overheating. Heater fireboxes can suffer localized overheating due to loss of protective refractory or insulation. Further, hydrocracking reactors may be vulnerable to localized overheating of reactor beds due to inadequate hydrogen quench or flow maldistribution.
Temperature, time, and stress are the critical factors for overheating rupture. The higher the internal pressure the shorter will be the time to failure. However, bulging and distortion can be significant even at low pressures and stresses if the temperature is high enough and is well above the design temperature. Previous loss in thickness due to corrosion will also reduce time to failure.
Control Measures to Avoid Overheating Stress Rupture
- Avoid temperature excursions or temperature runaways in hydroprocessing reactors.
- Proper burner maintenance and management to minimize hot spots and localized overheating in fired heaters.
- Installing and maintaining bed thermocouples in Hydrotreating reactors will help minimize the likelihood of hot spots. Proper system design and operation are also important.
- Refractory in-process heaters should be maintained in serviceable condition.
- Firing control in fired heaters is also critical.
Reference
- API 571 Third Edition, March 2020.
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