Sealing Sense

Thermal growth rate differences of head flanges, tube-sheets, bolts and gaskets create an upset of the gasket-flange interface and a loss of seating stress on the gasket that can cause flanges to leak. As the equipment heats, the exchanger flanges’ faces grow axially and radially. Axial growth causes compressive stresses on the gasket beyond what was provided by the bolt pre-load. Because the flanges and gasket are made of different material, the rates of thermal expansion are different.

Radial growth causes the flange surfaces to wipe across the gasket, upsetting the gasket-flange interface and potentially causing leak paths. Some gaskets, such as corrugated metal with flexible graphite laminates, handle this service better than others. A test developed by the now-defunct Tightness Testing and Research Laboratory of the Polytechnique Montréal included an evaluation to address this called the Radial Shear Test (RAST). Another technology (flange springs) could have been used to give more travel during this time that could have kept a larger seating stress on the gasket.

The 2010 explosion at Anacortes was not directly caused by poor gasket application. The NHT heat exchanger vessel ruptured and caused the fire because of the vessel material's high-temperature hydrogen attack, which is metal deterioration from high-temperature hydrogen contact. Because of safety complacency that accepted heat exchanger leaks during startup, additional personnel was required to manage those leaks, which led to the tragic loss of life. Technology and know-how are available to assure safe sealing that can make sure everyone safely ends the workday.

References

"OSHA Update," presentation given to the Monroe Professional Engineers Society on Sept. 27, 2016, by Gordon DeLeys, OSHA Compliance Assistance Specialist.

Report can be downloaded at the U.S. Chemical Safety Board website: http://www.csb.gov/tesoro-refinery-fatal-explosion-and-fire/

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