Reducing the Risk of High Temperature Hydrogen Attack (HTHA) Failures
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LESSONS LEARNED
Reducing the Risk of High Temperature Hydrogen Attack (HTHA) Failures Daniel J. Benac • Paul McAndrew
Submitted: 23 July 2012 / Published online: 14 August 2012 Ó ASM International 2012
Abstract The objective of this article is to provide lessons learned from materials, structure, and equipment failures so that costly failures can be prevented through good design, maintenance, and inspection practices, thus increasing safety, equipment reliability, and integrity of designs. Keywords Hydrogen damage High temperature Failure mechanism Ferrous metals HTHA
Introduction Has equipment been deteriorated by elevated temperature exposure and hydrogen? This question is frequently asked by those in ammonia, refinery, and chemical plants, who use piping, heat exchangers, and pressure vessels containing hydrogen at elevated temperatures. Beginning with research performed in the 1940s [1], equipment exposed to hydrogen at elevated temperatures is known to potentially degrade over time in a phenomenon called high-temperature hydrogen attack (HTHA). Failures of hydrogencontaining equipment can result in fires, fatal accidents, loss of production, and leaking of hydrocarbon products that can ignite, resulting in an explosion. This article discusses some of the necessary safety considerations and controls used by plant designers and operators to reduce the risk of failure of such equipment.
D. J. Benac (&) P. McAndrew Baker Engineering and Risk Consultants, Inc., 3330 Oakwell Court, Suite 100, San Antonio, TX 78218, USA e-mail: [email protected]
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HTHA Phenomenon High-temperature exposure of the carbon and low-alloy steels used for piping and pressure vessels (Fig. 1) used in high-pressure hydrogen service leads to a special form of degradation known as HTHA, sometimes called hydrogen attack. Note that this is not the same as hydrogen embrittlement which degrades toughness at low temperatures. HTHA leads to degradation of material properties at elevated operating temperatures, but like hydrogen embrittlement, HTHA can result in sudden and catastrophic brittle failure. Some equipment involves the use of, or production of, hydrogen at pressures greater than 0.8 MPa (100 psig) and at temperatures of 230 °C (450 °F) or above. These service conditions can lead to deterioration of carbon steel components and result in equipment failure, notably of pressure vessels and piping. Under the influence of certain temperature conditions and hydrogen partial pressure, atomic hydrogen permeates the steel and reduces iron carbide (Fe3C) in the steel to form methane (CH4). Note that the methane does not diffuse from the metal, and its pressure may exceed the cohesive strength of the metal, causing fissuring between grains (Fig. 2). When fissuring occurs, the ductility of the metal is significantly and permanently lowered. The severity of hydrogen attack increases with increasing temperature and hydrogen partial pressure. Usually, hydrogen attack occurs in three stages: 1. Atomic hydrogen diffuses into the metal, 2.
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