Metallic Gaskets for Pipe Flanges: ASME B16.20
1- Metal Ring Joint Gasket
2- Spiral-Wound Metal Gasket
3- Metal Jacketed Gasket
1- Graphitization:
Graphitization is a change in the microstructure of certain carbon steels and 0.5Mo steels after long-term operation in the 427°C to 593°C range that may cause a loss in strength, ductility, and/or creep resistance.
Graphitization has been found in low alloy C-Mo steels with up to 1% Mo. The addition of about
0.7% chromium has been found to eliminate graphitization.
Inspection and Monitoring: removal of full thickness samples for examination using metallographic techniques.
2- Softening (Spheroidization):
Spheroidization is a change in the microstructure of steels after exposure in the 440°C to 760°C range, where the carbide phases in carbon steels are unstable and may agglomerate from their normal plate-like form to a spheroidal form, or from small, finely dispersed carbides in low alloy steels like 1Cr-0.5Mo to large agglomerated carbides. Spheroidization may cause a loss in strength and/or creep resistance.
Inspection and Monitoring: Spheroidization can only be found through field metallography.
3- Temper Embrittlement
Temper embrittlement is the reduction in toughness due to a metallurgical change that can occur in some low alloy steels as a result of long term exposure in the temperature range of about 343°C to 577°C. This change causes an upward shift in the ductile-to-brittle transition temperature as measured by Charpy impact testing. Although the loss of toughness is not evident at operating temperature, equipment that is temper embrittled may be susceptible to brittle fracture during start-up and shutdown.
Prevention / Mitigation
Existing Materials
1) To minimize the possibility of brittle fracture during startup and shutdown, many refiners use a pressurization sequence to limit system pressure to about 25 percent of the maximum design pressure for temperatures below a Minimum Pressurization Temperature (MPT). Note that MPT is not a single point but rather a pressure-temperature envelope which defines safe operating conditions to minimize the likelihood of brittle fracture.
2) PWHT of repaired welds.
New Materials
1) The best way to minimize the likelihood and extent of temper embrittlement is to limit the acceptance levels of manganese, silicon, phosphorus, tin, antimony, and arsenic in the base metal and welding consumables. In addition, strength levels and PWHT procedures should be specified and carefully controlled.
2) A common way to minimize temper embrittlement is to limit the "J*" Factor for base metal and the "X" Factor for weld metal, based on material composition as follows:
J* = (Si + Mn) x (P + Sn) x 104 {elements in wt%}
X = (10P + 5Sb + 4Sn + As)/100 {elements in ppm}
Inspection and Monitoring
a) A common method of monitoring is to install blocks of original heats of the alloy steel material inside the reactor. Samples are periodically removed from these blocks for impact testing to monitor/establish the ductile-brittle transition temperature. The test blocks should be strategically located near the top and bottom of the reactor to make sure that the test material is exposed to both inlet and outlet conditions.
b) Process conditions should be monitored to ensure that a proper pressurization sequence is followed to help prevent brittle fracture due to temper embrittlement.
4- Strain Aging
Strain aging is a form of damage found mostly in older vintage carbon steels and C-0.5 Mo low alloy steels under the combined effects of deformation and aging at an intermediate temperature. This results in an increase in hardness and strength with a reduction in ductility and toughness.
Affected Materials: Mostly older (pre-1980’s) carbon steels with a large grain size and C-0.5 Mo low alloy steel.
Critical Factors
a) Steel composition and manufacturing process determine steel susceptibility.
b) Steels manufactured by the Bessemer or open hearth process contain higher levels of critical impurity elements than newer steels manufactured by the Basic Oxygen Furnace (BOF) process.
c) In general, steels made by BOF and fully killed with aluminum will not be susceptible. The effect is found in rimmed and capped steels with higher levels of nitrogen and carbon, but not in the modern fully killed carbon steels manufactured to a fine grain practice.
d) Strain aging effects are observed in materials that have been cold worked and placed into service at intermediate temperatures without stress relieving.
e) Strain aging is a major concern for equipment that contains cracks. If susceptible materials are plastically deformed and exposed to intermediate temperatures, the zone of deformed material may become hardened and less ductile. This phenomenon has been associated with several vessels that have failed by brittle fracture.
f) The pressurization sequence versus temperature is a critical issue to prevent brittle fracture of susceptible materials.
g) Strain aging can also occur when welding in the vicinity of cracks and notches in a susceptible material.
Affected Units or Equipment : Strain aging is most likely to occur in thick wall vessels manufactured from susceptible materials that have not been stress relieved.
Appearance or Morphology of Damage: Strain aging can result in the formation of brittle cracks that are revealed through detailed metallurgical analyses, but damage most likely will not be identified as strain aging until fracture has already occurred.
Prevention / Mitigation:
a) Strain aging is not an issue for newer steels that contain low levels of interstitial impurity elements and sufficient aluminum (>0.015 wt%) to fully deoxidize the steel.
b) For older equipment, extra care should be taken to avoid the potentially damaging effects of strain aging by avoiding stressing or pressurizing equipment until the metal temperature reaches an acceptable level where the risk of brittle fracture is low. Refer to curve “A” in UCS 66 of the ASME Code Section VIII, Division I for pressurization temperatures of vessels susceptible to strain aging effects.
c) Applying PWHT to weld repairs.
Inspection and monitoring are not used to control strain aging.
Related Mechanisms: When deformation occurs at the intermediate temperature, the mechanism is referred to as dynamic strain aging. Blue brittleness is another form of strain aging.
Wet H2S Damage (Blistering/HIC/SOHIC/SSC) according to API 571
Affected Materials: Carbon steel and low alloy steels.
Critical Factors: environmental conditions (pH, H2S level, contaminants, temperature), material properties (hardness, microstructure, strength) and tensile stress level (applied or residual).
pH: it is minimum at pH 7 and increase at both higher and lower pH.
H2S: 50 wppm
Temperature
Blistering, HIC, and SOHIC damage: between ambient and 150C higher.
SSC generally occurs below about 82C
Hardness: Blistering, HIC and SOHIC damage are not related to steel hardness. Hardness is primarily an issue with SSC.
Steelmaking
Blistering and HIC damage are strongly affected by the presence of inclusions and laminations which provide sites for diffusing hydrogen to accumulate.
Steel chemistry and manufacturing methods also affect susceptibility and can be tailored to produce the HIC resistant steels outlined in NACE Publication 8X194.
Improving steel cleanliness and processing to minimize blistering and HIC damage may still leave the steel susceptible to SOHIC.
The disadvantage is that an absence of visual blistering may leave a false sense of security that H2S damage is not active yet subsurface SOHIC damage may be present.
HIC is often found in so-called “dirty” steels with high levels of inclusions or other internal discontinuities from the steel-making process.
PWHT
Blistering and HIC damage: PWHT will not prevent them from occurring.
SSC: PWHT is highly effective in preventing or eliminating SSC by reduction of both hardness and residual stress.
SOHIC: SOHIC is driven by localized stresses so that PWHT is also somewhat effective in reducing SOHIC damage.
Refer note under UW2, it shall be the responsibility of the user and/or his designated agent to determine if it is lethal. If determined as lethal, the user and/or his designated agent [see U-2(a)] shall so advise the designer and/or Manufacturer. It shall be the responsibility of the Manufacturer to comply with the applicable Code provisions.
Sour serive & lethal service are totally different terms. A sour service vessel can be of non-lethal.
Please refer below definitions extracted from Specification.
Wet Sour Service: Following process streams containing water and hydrogen sulfide:
i) Sour water with a hydrogen sulfide (H2S) concentration above 2 mg/L (2 ppm) and a total pressure of 400 kPa absolute (65 psia) or greater.
ii) Hydrocarbon services meeting the definition of sour environments in SAES-A-301, where the H2S concentration of 2 mg/L (2 ppm) or more in the water phase is equivalent to H2S partial pressure of 0.05 psia. Sour crude systems upstream of a stabilization facility and sour gas upstream of a sweetening or dehydration plant are examples of such environments.
iii) Hydrocarbon systems exposed to an environment with a H2S concentration above 50 mg/L (50 ppm) in the water phase, regardless of H2S partial pressure. (As per Aramco Specification)
Lethal Service are meant use of poisonous gases or liquids of such a nature that a very small amount of the gas or of the vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled ( As per UW-2, ASME SecVIII, DIv.1)