API RP 941-2008 pdf free download

API RP 941-2008 pdf free download.Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants.
6 Inspection for HTHA The selection of optimum inspection methods and frequencies for HTHA in specific equipment or applications is the responsibility of the user. The information below and Table E.1 and Table E.2 are intended to assist the user in making such decisions. The user is also referred to Annex C, of the technical basis document, “Estimating Damage Rates for Life Assessment” [39]. This damage rate model may assist in determining inspection needs and prioritization. Experience with steels operated below their respective Figure A.1 Nelson curves has been good. Consequently, most users do not inspect equipment for HTHA damage unless it has been operated near or above its curve. An HTHA inspection program should also consider equipment that operates infrequently above its curve (e.g. operations such as “hot hydrogen stripping” in hydroprocessing reactors and associated piping and equipment). Only a small number of documented instances of HTHA occurring at conditions below the curves have been reported to the API (see Annex A, Annex B, and Annex C). Most of these have involved C-0.5Mo steel [33]. Periodic inspection of C-0.5Mo steel equipment and piping should be considered if operated above the carbon steel curve, based on factors such as relative position of the operating parameters versus the carbon steel curve, consequence of failure, presence of cladding, prior heat treatment, etc. Because is time dependent, existing C-0.5Mo steel equipment and piping may continue to deteriorate with time, if susceptible. As this equipment and piping age, the owner should consider increasing the inspection frequency (also see Annex A). HTHA damage may occur in welds, weld HAZs, or base metal. Even within these specific areas, the degree of damage may vary widely. Consequently, if damage is suspected, then a thorough inspection means that representative samples of these areas be examined.
HTHA is a difficult inspection challenge. The early stages of attack with fissures, or even small cracks, can be difficult to detect. The advanced stage of attack with significant cracking is much easier to detect, but at that point there is already a higher likelihood of equipment failure. In addition to general attack of the base metal, HTHA has been known to occur as a very narrow band of intense attack and cracking, running alongside and parallel to welds. This highly localized form of attack requires special nondestructive testing (NDT) techniques for detection, such as high frequency shear wave and angle-beam spectrum analysis [36], [37]. For base metal examination, ultrasonic testing (UT) methods have the best chance of detecting HTHA damage in the fissuring stage. Most effective is the use of a frequency dependent backscatter method in combination with the velocity ratio and spectral analysis techniques. Backscatter can be used as a first step of inspection and can be used to quantify the depth of damage. Velocity ratio and spectral analysis are useful for confirmation of backscatter indications. Other methods are capable of detecting HTHA only after discrete cracks have formed and there is significant degradation of mechanical properties. For weldment examination, only two UT methods of examination are considered effective. High frequency shear wave and angle-beam spectrum analysis techniques should be used to detect HTHA damage in the fissuring stage. Conventional shear wave UT and time of flight diffraction (TOFD) techniques can be used to detect HTHA in the advanced stages, when there is significant cracking.