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Using a battery of advanced analytical techniques, Structural Integrity Associates, Inc. Associates, Inc. (SI), of San Jose, CA, has developed a promising new method of analyzing process equipment under extremes of heat, pressure or chemical attack to find stress corrosion damage. Depending on the application, the analysis can be done on the fly, without shutting down the process or disassembling the equipment. And it is sensitive enough (finding flaws as small as 0.125 in. deep) to locate these flaws well before the equipment must be replaced, making this technique a preventive-maintenance measure that will save process operators from process upsets.

This nondestructive testing (NDT) technique has been successfully applied at an operating refinery where a group of ring-joint flanges (Type RTJ) are in use. These flanges operate under 2,500-psi pressure and a process-fluid temperature of over 600°F. They are bolted together, with hexagonal-profile grooves on their faces to ensure the joints' integrity. Because of the presence of chloride (whose suspected source is a washdown procedure between process cycles), the operating pressure and the mechanical stress from the bolts securing the flanges, stress corrosion has been observed.

The SI test showed where cracking was occurring, and allowed the refinery operator to make an assessment of which flanges could continue in operation, and which should be repaired or replaced. "The innovation here is not so much the analytical technique," says Harold Queen, an associate at the firm, "but that significant results can be obtained while the plant is in operation. Previous testing of this sort has been only marginally precise, and forced the plant management to break down the flange joints during turnarounds to examine their integrity. The process of disassembling and reassembling the joints is itself stressful on the flanges." This is especially true of the conventional test, which involves disassembly, application of a liquid penetrant, which only confirms the presence of cracks without providing depth information for determining flange integrity.

SI uses a specialized electromagnetic-acoustic transducer (EMAT), which has the advantage of not requiring a "coupling medium" such as water or gel to transmit the acoustic signal between the transducer and the flange. (At extreme temperature—very hot or cryogenic, this coupling medium is ineffective.) After a 20-min setup, the field test is conducted by moving the EMAT around the circumference of the flange taking readings at 0.100-in. intervals. The transducer emits a signal, and wherever a flaw is encountered, the signal is reflected back and recorded.

A uniaxial encoding device, hooked to a computer, records the reflected signal, and allows the data to be presented in a two-dimensional chart, with color coding to indicate the intensity of the signal (see illustration). A flange of the type for which the first field tests were done can be analyzed in about 40 min. "If the testing has gone well, our instrumentation allows us to present results to plant managers before we leave the facility," says Queen. "They like that."

Depending on the application, further testing and analysis may be desirable. SI has developed finite-element analysis (FEA) and fracture mechanics software that allows engineers to determine the extent of cracking in the process equipment, and to predict how the crack will propagate. Plant managers therefore have a tool that allows them to assess the margin of safety of the equipment, or to plan a change-out.

Queen says that there is a very simple rule of thumb for where the SI test may be applicable—wherever there is a combination of stress (either process-generated, or mechanical stress from, say, bolt tensioning) and an environmental source of corrosion, especially chloride. Just about any process equipment—including pipes, flanges, reactor or storage vessels and heat exchangers—is a candidate. The testing can be done online or offline. Stress corrosion cracking, as process engineers know, is an especially insidious flaw. The cracks can propagate from an extremely small surface defect through even thick, strong metal parts, and when the part fails, it fails catastrophically.

SI, which was recently acquired by the HSB Group, Inc., is headquartered in California and has regional offices in eight states, has been performing integrity testing, inspection and consulting services to the utility and process industries for over 15 years.

By Nick Basta

For more information:
Charlie Roberts
Structural Integrity Associates, Inc.
3315 Almaden Expressway, Suite 24
San Jose, CA 95118
Tel: 408-978-8200
Fax: 408-978-8964.