The 6th Edition of API STD 521 provides information on the design of overpressure protection of heat exchangers for the tube rupture scenario. E2G|The Equity Engineering Group Inc.’s Process Technologies Team can provide consulting services in this complex area.

API STD 521, “Pressure Relieving and Depressuring Systems,” is the industry standard (RAGAGEP) for pressure relief system design. Along with discussing other possible overpressure scenarios, the standard offers guidance regarding the design of overpressure protection of heat exchangers for the tube rupture scenario. Although paragraph UG-133(d) of the ASME Code requires heat exchanger overpressure protection to have sufficient capacity to avoid overpressure in the event of an internal failure, the Code has always left the determination of “credible” overpressure scenarios, as well as the definition of “internal failure,” up to the User. Designing the pressure relief protection to adequately handle a pin-hole leakage type internal failure meets the Code requirement; however, extending the internal failure definition to a full-bore tube rupture scenario has always been considered to be at the discretion of the User since it represents such a low probability event.

The tube rupture scenario should be considered if the MAWP (maximum operating pressure, on a case-by-case basis) of the high-pressure side of the exchanger exceeds the corrected hydrotest pressure of the low-pressure side of the exchanger. Note that API STD 521 allows the User to choose a pressure other than the corrected hydrotest pressure, provided a proper detailed mechanical analysis is performed showing a loss of containment is unlikely.

Mitigation Method 1 – Provide Adequately Sized and Designed Low-Pressure Relief Capacity

API 521 provides two methods to address the tube rupture scenario in applications where the low-pressure side could be exposed to pressures greater than its hydrotest pressure. The first method assumes that a full-bore overpressure scenario is credible and requires the User to ensure that the relief protection on the low-pressure side is adequately sized and can respond quickly enough to limit the overpressure to a pressure below the hydrotest pressure.

API 521 Method 1 would require the User to perform a dynamic/transient analysis of the rupture flow to determine the magnitude of the resulting pressure spike (water hammer). This analysis often results in physical modifications to the relief system.

E2G uses our breakthrough computer program “TBREAK,” from E2G’s Plant ManagerTM platform to dynamically assess the tube rupture scenario. Users can simulate the transient release of the fluid from the high-pressure side of the heat exchanger to assess the impact and requirements for pressure relief protection on the low-pressure side.
Because of the need to perform a dynamic analysis and the potential for atmospheric discharge, many companies make the decision to take advantage of the second method that API 521 allows to mitigate the tube rupture scenario.

Mitigation Method 2 – Perform Detailed Analysis on Heat Exchanger to Designate the Tube Rupture Scenario as Non-Credible

As an alternative to designing the low-pressure relief system to handle a full-bore tube rupture, API STD 521 allows the User to perform a detailed analysis to determine the relief system design basis for scenarios other than a full-bore tube rupture. API STD 521 suggests a detailed mechanical analysis of the exchanger design can be performed to show the tube rupture scenario is sufficiently remote as to be classified as non-credible.

Although API STD 521 does not cite specific methodologies for the above analysis, good engineering judgement would suggest the analysis consist of, at a minimum, the following:

  • Tube vibration analysis of the exchanger bundle
  • Review of shell and bundle entrance velocities to assess erosion potential
  • Assessment of the tube-to-tubesheet joint strength
  • Metallurgical analysis to assess the likelihood for environmental stress corrosion cracking (SCC), brittle fracture, and creep
  • Thermal/mechanical fatigue assessment for those exchangers that are expected to be exposed to frequent variable operating conditions
  • Corrosion analysis to assess the severity of any corrosion mechanisms
  • Review of the inspection programs and techniques used to determine whether they are adequate to assess the onset of cracking problems or to acquire evidence of tube pullout.

E2G has used our proprietary Tube Rupture Credibility Assessment (TRCA) in 60+ applications to resolve our clients’ issues related to heat exchanger tube ruptures.

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