Tube Failure

There are various mechanisms which cause heat exchanger tube failures, namely:

   - high stress due to differential thermal expansion, tensile or compressive,

   - corrosion, chemical or galvanic,

   - flow induced erosion,

   - flow induced vibration,

   - fatigue and/or creep,

   - brittle fracture,

   - baffle chafing, and

   - material defects "as delivered" from the tube mill. (Unfortunately this more common than logic would suggest.)

These mechanisms are the result of poor design and/or material selection, and quality control.

ASME Section VIII, Division1, UG-133(d) states "Heat exchangers and similar vessels shall be protected with a pressure relief device of sufficient capacity to avoid overpressure in case of an internal failure". Unfortunately ASME does not provide specific guidelines for heat exchanger pressure relief.

API Standard 521 (Pressure-relieving and Depressuring Systems), 6th Edition, Section Shell-and-tube Heat Exchangers discusses minor versus major failures and provides recommended good practices for pressure relief. Pressure Considerations state that "Minor leakage can seldom overpressure an exchanger during operation, however such leakage occurring where the low-pressure side is closed in can result in overpressure." A bayonet heater inserted into a non-vented low pressure API tank is an example of a "closed" system.

For relief guidance Table 1 - Guidance for Required Relieving Rates Under Selected Conditions, Item No. 15 considers a leak to be major when the rupture flow area equals the flow area of one tube. Video

Overpressure Protection of Shell and Tube Heat Exchangers - Energy Institute Seminar

Evaluate Heat Exchanger Tube Rupture Scenarios Using Dynamic Simulation - Soumitro Nagpal, Fluor Daniel