Inspectioneering Journal

99 Diseases of Pressure Equipment: Short Term Overheating

By John Reynolds, Principal Consultant at Intertek. This article appears in the July/August 2003 issue of Inspectioneering Journal.

This failure mechanism is, unfortunately, all too common in our industry. It's also known as stress rupture, and it is usually entirely preventable by proper maintenance and operating procedures. It occurs when equipment, piping or furnace tubes that are designed to operate safely and reliably in one temperature range are suddenly (and sometimes not so suddenly) exposed to higher temperatures. At the higher temperatures the metal is weaker and can only resist operating pressures and stresses for a shorter period of time, sometimes only hours or minutes, if the design temperature is significantly exceeded. Furnace tubes are the most common types of equipment to suffer stress rupture from short term overheating. Flame impingement or internal tube fouling are common causes. Failure of the refractory in refractory-lined equipment is another fairly common way that "cold shell" equipment can fail from stress rupture. Localized hot spots on reactors containing exothermic catalysts is one of the most serious sources of stress rupture. "You haven't lived" so to speak, until you've seen a "pregnant reactor" that fortunately was shut down in the nick of time after a hot spot developed and resulted in a very localized, rapid overheating of a small portion of the shell of the vessel.

Since this type of affliction is fairly well known and not uncommon, there are many ways to avoid it, most of which involve careful operation, instrumentation, inspection and maintenance activities. These include routine IR monitoring of susceptible equipment, heat sensitive paint on cold shell vessels, furnace tube and vessel skin thermocouples, careful burner management of fired heaters, and routine inspection and maintenance of refractory-lined equipment.

Do you consider the potential for short term overheating as part of your RBI and PHA (process hazards analysis)?

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