Many fluid method elements are utilised in the oil and gas market, including tubing, tube fittings, ball valves, double-block-and-bleed valves, manifolds, strain gauges, and so on. These factors are created from distinct components of design in get to meet demands of business expectations and to supply protected, dependable support. The situations underneath which oil and gasoline are introduced from a reservoir to the surface area can be outright hostile to many common components. In maritime environments, numerous threats exist that could guide to the degradation and even failure of a part. Possibly harmful mechanisms contain localized corrosion, pressure corrosion cracking (SCC), and sulfide stress cracking (SSC). SSC is progressively dominant as a lot more sour reservoirs are becoming produced. For instance, those in the northern component of the Caspian Sea contain up to twenty% of hydrogen sulfide (H2S). Growing older reservoirs can also switch bitter as abiotic and biotic reactions take place, the latter involving microbes in the response of sulfur compounds to H2S. Deciding on the best resources of building, getting into thought the effect of producing procedures, implies your fluid system components will complete reliably for a lot of many years in the demanding bitter environments of oil and gasoline exploration and generation.
Localized corrosion
Pitting corrosion or crevice corrosion occurs when the chromium-rich passive oxide film on an alloy area breaks down in a chloride-abundant setting. Higher chloride concentrations, much more acidic environments, and elevated temperatures all boost the chance for breakdown of this passive film. The greater an alloy’s Pitting Resistance Equivalent Amount (PREN), the greater its resistance to localized corrosion. The most regularly used connection for calculating PREN is:
PREN = %Cr + 3.three(%Mo + .5%W) + 16%N
Tension corrosion cracking
In the existence of chloride ions, e.g., in a marine surroundings, certain alloys are inclined to SCC, or chloride-ion induced SCC. The chloride ion interacts chemically with the materials at the quite idea of a crack the place tensile stresses are greatest, making it less complicated for the crack to propagate. This failure method is hazardous simply because it can demolish a part at stress stages below the generate energy of an alloy. While in progress, this failure manner can be challenging to detect, and final failure can take place suddenly.
Cross-segment of 316 stainless steel tubing exhibiting deep corrosion pits on the outer surface area.
In purchase for SCC to arise, 3 situations must be met concurrently: the content have to be susceptible to SCC the fluid need to be capable of inducing SCC and a tensile tension need to be present that is better than a crucial tensile pressure. The prevailing tensile stress is the sum of the used tensile stress and any residual tensile stresses that could be current as a end result of cold forming, welding, warmth remedy, machining, or grinding.
Some alloys are substantially a lot more vulnerable to SCC than others. Nickel content of the alloy plays a major function in alloy resistance to SCC. Austenitic stainless steels like 304 with 8-10% nickel and 316 with ten-fourteen% nickel are specifically susceptible. Three groups of materials are hugely resistant to SCC: carbon steels, nickel-dependent alloys, and duplex stainless steels.
Sulfide tension cracking
Raw oil can be contaminated with undesirable compounds. When H2S and huge portions of carbon dioxide (CO2) are present, the unrefined fuels are said to contain “acid gas” simply because these gases sort acids when mixed with water. The time period “sour gas” is employed for unrefined fuels that contains H2S, a really corrosive, harmful, and flammable fuel. That's why, fluid method components for transportation of bitter gas should be very reputable and have to be produced from corrosion-resistant resources.
Photomicrograph of chloride-induced tension corrosion cracking in 316 stainless metal (100X magnification).
The specifications for SSC to occur are not in contrast to individuals for SCC. In a very similar method, SSC requires a prone materials a sufficiently bitter fluid, in other words a concentration of H2S earlier mentioned a distinct threshold and a tensile stress above a vital level.
Notwithstanding the similarities in the specifications for SSC and SCC to occur, a specific alloy might be extremely inclined to SSC and extremely resistant to SCC, or vice versa.
When failures of oil production and drilling equipment have been first described fifty percent a century back throughout the improvement of sour reserves in western Canada and West Texas, many observations ended up created which had been hard to comprehend at that time. Failures happened at stresses under a material’s yield tension, or even beneath the mechanical style stresses of gear. Failure could take place in times, or could take months to materialize in the very same kind of part. Failure analysis frequently showed brittle fracture surfaces. And if the very same quality of an alloy was employed, failure was typically linked with the stronger and tougher alloys while areas created from the annealed softer alloys carried out well.
Because the early days of observing component failures in bitter gasoline, significantly has been discovered about the nature of this failure system. Variables that influence SSC have been discovered and quantified. An enhance in the following parameters can contribute to the price at which SSC happens: material homes this kind of as tensile power and hardness hydrogen ion focus in the fluid (i.e., pH-worth) H2S partial strain complete tensile stress (used and residual) temperature and exposure time.
Swagelok procedure interface valves and instrumentation manifolds for sour gasoline provider produced from alloys 316, 254SMO, 825, 625, 2205, and 2507.
On an atomic scale, SSC is a particular circumstance of hydrogen embrittlement. When a vulnerable steel surface arrives into make contact with with bitter fuel, the H2S molecules respond to kind metallic sulfide and hydrogen atoms. The latter diffuse into the content at the suggestion of the crack at which tensile stresses are optimum. Hydrogen diffusion and accumulation in the lattice, on interfaces and on grain boundaries, decrease the material’s potential to deform plastically, leading to hydrogen embrittlement that facilitates crack propagation.
Tube fittings and instrumentation valves for sour gas services obtainable in alloys 825, 625, and 2507.
In sour environments this kind of as mixtures of oil + seawater + H2S, SCC and SSC can pose a synergistic risk. Crack propagation caused by the chloride ion conversation with the tensile-loaded crack tip might proceed much more conveniently if the substance ahead of the crack idea has been embrittled by atomic hydrogen. The phrase environmental cracking is utilized to explain the synergistic actions of SCC and SSC.
Choosing components for resistance to SCC and SSC
In 2003, an intercontinental common was revealed that is offered from the Worldwide Organization for Standardization as ISO 15156 and from NACE Global as NACE MR0175/ISO 15156. The two kinds are equivalent, have the same title “Petroleum and organic fuel industries – Components for use in H2S containing environments in oil and gasoline production,” and consist of three areas:
Part one – General concepts for the assortment of cracking resistant resources
Portion 2 – Cracking-resistant carbon and reduced alloy steels, and the use of forged irons
Part three – Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys.
This regular gives requirements and suggestions for the selection and qualification of metallic resources for services in products utilized in oil and gasoline generation and in all-natural gasoline sweetening vegetation in H2S-that contains environments. It addresses all mechanisms of cracking that can be triggered by H2S, including SSC and SCC, and other types of hydrogen-induced cracking.
Pre-experienced materials are detailed in the standard and can be used for the supposed service with out having to carry out any additional laboratory screening. All listed supplies with documented microstructural traits (e.g., annealed or pressure-hardened) and properties (e.g., hardness) have done satisfactorily in area installations or in laboratory checks carried out below outlined environmental circumstances.
In this regular, alloys are determined by substance teams, and inside every team, by resources variety or as personal alloys. Satisfactory metallurgical problems and environmental limits are offered for which alloys are anticipated to resist cracking. These conditions and limits are stricter for components for common use than for individuals in downhole installations. Environmental limits are presented for H2S partial stress, temperature, chloride focus, in situ pH, and elemental sulfur. Crucial points are summarized below for the substance teams of austenitic stainless steels, very alloyed austenitic stainless steels, sound remedy nickel-primarily based alloys, and duplex stainless steels.
Austenitic stainless steels
Austenitic stainless steels comprise one particular substance group that involves typical alloys 304, 316, 317, 321, and 347. In addition, alloys 309, 310, Nitronic 50, and forged alloys such as CF8 and CF8M are component of this content team. Totally free-machining austenitic stainless steels which have elevated levels of sulfur are excluded especially.
There are two sets of environmental limitations. For a greatest use temperature of 60 ºC (a hundred and forty ºF), the highest H2S partial force is 100 kPa (15 psi). When the maximum chloride concentration is limited to fifty mg/L, the content can be utilized with no limits on temperature and H2S partial stress.
The NACE MR0175/ISO 15156 regular gives far more comprehensive guidelines for austenitic stainless steels utilized to produce valve stems, seal rings, gaskets, and as parts in compressors or in subsurface programs. Pressure-hardened 316 stainless steel may be utilised in surface apps for compression fittings, instrument tubing, and control line tubing without having restriction on temperature, H2S partial stress, chloride focus, or in situ pH in production environments. The standard cautions that even though no limits on person parameters have been established, some mixtures of the values of these parameters may possibly not be appropriate.
Hugely alloyed austenitic stainless steels
Very alloyed austenitic stainless steels comprise another components group. If utilized for any equipment and factors, sort 3a and 3b alloys (e.g., 254SMO and AL6XN) are permitted in their resolution-annealed point out. For downhole tubular parts, the alloys must have been remedy annealed and may be in the chilly-labored condition with a greatest hardness of 35 HRC. Variety 3a and 3b alloys may possibly be used in area applications for compression fittings, instrument tubing, and handle line tubing without restriction on temperature, H2S partial strain, chloride focus, or in situ pH in manufacturing environments.
Sound remedy nickel-dependent alloys
The 3rd resources team is made up of strong-answer nickel-primarily based alloys. The NACE MR0175/ISO 15156 normal defines five content kinds: 4a through 4e dependent on the Cr, Ni, Mo, and W content of the alloy, and its metallurgical issue, i.e., remedy annealed or cold-worked. Typically employed alloys 825, 625, and C-276 to satisfy the specifications for types 4a through 4e can be used in the answer-annealed or annealed problem for any tools or component at any mixture of temperature, H2S partial strain, chloride concentration, and in situ pH happening in creation environments. For downhole tubular components, cold-worked alloys 825, 625, and C-276 can be used so extended as their hardness does not exceed 40 HRC and their produce energy is not above specific limitations. Alloy C-276 can be employed at increased temperatures and H2S partial pressures than alloy 625, which in flip is certified for higher functioning situations than alloy 825. The regular offers detailed mixtures of restrictions, temperatures, and H2S partial pressures.
Duplex stainless steels
Duplex stainless steels are divided into material kinds with lower PREN, among 30 and forty, and larger PREN, previously mentioned forty to forty five. If used for any gear, the alloys have to be in the answer-annealed and liquid-quenched point out, and have a ferrite content among 35% and sixty five%. Highest use temperature is 232 ºC (450 ºF) for each content sorts. Maximum H2S partial pressure is ten kPa (1.5 psi) for alloys with PREN between 30 and forty, and twenty kPa (3 psi) for alloys with PREN above 40. If utilized as downhole tubular factors, the materials can be pressure-hardened with a maximum hardness of 36 HRC. The decrease PREN alloys can be utilised only up to a optimum H2S partial force of two kPa (.three psi), and the ones with PREN earlier mentioned forty up to 20 kPa (three psi) and a maximum chloride concentration of one hundred twenty,000 mg/L.
Manufacture of fluid system parts
Making fluid method elements that comply with all facets of the NACE MR0175/ISO 15156 regular demands the use of high-high quality uncooked resources, cautious testing, and prudent choice of production methods. Alloys should have long gone through a controlled solution annealing process, and their microstructural quality have to be assessed with exams this sort of as ASTM A262, which probes austenitic stainless steels for intergranular corrosion, or ASTM A923, which qualifies duplex stainless steels for absence of harmful intermetallic phases. Austenitic stainless steels ought to be cost-free of martensite and have ideally no or at most two% ferrite, simply because these phases are far more susceptible to hydrogen embrittlement than austenite.
A enough number of hardness measurements must be done and an regular hardness worth calculated, which should not exceed the respective authorized highest price. No person hardness reading is permitted that is greater than two units on the Rockwell C hardness scale earlier mentioned the permitted highest hardness. When parts are welded, care need to be taken to execute hardness measurements on the welds following the procedures described in the NACE MR0175/ISO 15156 standard. Where solution-annealed supplies are required, any cold drawing of bar or cold rolling of Selecting fluid program parts for use in bitter oilfields have to be avoided. Cold deformation of surfaces is appropriate only if it is triggered by procedures this sort of as burnishing that do not impart more cold perform than standard machining functions. Identification stamping with lower-tension stamps is acceptable, but the use of sharp V-stamping need to not be performed in large-stress regions.
Substance assortment and production of fluid method elements for service in bitter fuel environments are complicated. The finish consumer must outline the prevailing bitter gas services conditions, like those of continual state procedures and of potentially unintended exposures. Other than substance properties, there are numerous aspects that impact the susceptibility of a content to cracking in bitter fluids: H2S partial pressure, in situ pH, chloride focus, existence of elemental sulfur, temperature, galvanic outcomes, mechanical pressure, and time in get in touch with with an aqueous remedy. The stop user need to be acquainted with the requirements of the NACE MR0175/ISO 15156 normal in order to select the best content of design for a fluid method element.
When a customer orders a specific valve, fitting, or other fluid program part for bitter gasoline services, the maker must complete a merchandise assessment in which all wetted areas are evaluated in opposition to normal needs, e.g., for sort of materials, manufacturing processes, and greatest hardness. This sort of a review guarantees that the items picked for sour gasoline provider fulfill the specifications of the NACE MR0175/ISO 15156 normal, as properly as the customer’s need for sturdiness, performance, and dependable provider.
Reference
one. NACE MR0175/ISO 15156, “Petroleum and all-natural fuel industries—Materials for use in H2S-that contains environments in oil and fuel production” (Houston, TX: NACE International, 2003).
About the author
Gerhard Schiroky, senior scientist of Engineering, joined Swagelok in 2000. Schiroky is dependable for addressing customers’ supplies concerns and identifying possibilities for providing worth-additional solutions. He develops roadmaps for improved and new alloys, from which future fluid program factors could be constructed. He also identifies and evaluates novel materials that offer efficiency or value advantages.
Schiroky gained his Ph.D. in Resources Science and Engineering from the University of Utah. He has authored several specialized publications on diverse topics, which includes fluid dynamics and components science, and is named on in excess of 20 patents.