SPECIAL TOPIC : OIL & GAS
Cryogenic Piping and Valve Selection in LNG Plants
Piping codes and standards for natural gas applications have changed over the years to address challenges , especially in material selection and application . The industry and advising committees have provided insight for improved codes , standards and methods to engineers involved in the LNG plant piping industry , and further improvements are expected .
By Gobind Khiani , Valve Engineering Consultant , GAPV Inc
The natural gas market has grown at its fastest rate in almost a decade . In 2019 , the gas market expanded by 5 percent , while LNG grew by 10 percent . Natural gas / LNG are expected to remain the fastest-growing fossil fuel beyond 2037 due to its relatively low CO 2 footprint compared to coal .
This implies that investments in LNG plants ( gasification , liquefaction ) and piping are needed to cater to this growing demand . LNG plants contain a significant amount of piping that is characterized by one or more of the following : large diameters , high design pressures , cryogenic temperatures , stainless steel , high velocity gas flow , large diameter-to-thickness ( D / t ) ratios ( sometimes exceeding 100 ) and load cases not explicitly addressed by design codes .
These have presented challenges and exposed some of the limitations in existing piping design codes and standards . ASME B31.3 Process Piping was originally developed to serve refineries where the piping is generally carbon steel , low pressure , hot and handles liquids .
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LNG plant piping often has high design pressures , cryogenic temperatures , gas flows , significant use of austenitic stainless steel and larger diameters . Conventional use of carbon steels in the oil and gas industry for piping spool components is specified to ASME / ASTM requirements .
These are components such as A105N flanges , A234 Grades WPA , WPB and WPC , seamless carbon steel fittings , A106N pipes ( all grades ) and A53 seamless pipe are considered by ASME VIII Div I and ASME B31.3 Code as being inherently ductile , and brittle fracture resistant without any Charpy impact testing requirements .
A105 carbon steel flanges below 300 # rating and A106 Gr B carbon steel pipe , less than ½ inch ( 13 mm ) thick are generally exempted from impact testing , for temperatures above -20 ° F ( -29 ° C ). Investigations have shown that the Charpy V-notch impact toughness of CS pipe flanges - operating at the internal pressures listed in the ANSI B16.5 pressure-temperature tables , are adequate only for a MDMT at or above + 20 ° C .
Selection of the piping materials and its ductility is an important underlying assumption for many design rules . It allows the loads in a piping system to shift if the stresses in a certain part reach the yield point . The low design temperatures associated with many lines in LNG plants requires the use of austenitic stainless steel to ensure adequate ductility .
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While providing ductility at low temperatures , austenitic stainless steels present several challenges compared to carbon steels . Stainless steel is more susceptible to microbiologically induced corrosion ( MIC ) and chloride stress corrosion cracking ( CLSCC ). Often the problem is the water from hydrotesting and the subsequent difficulties in drying piping after a hydrotest . MIC is one of the driving factors behind consideration of non-metallic materials for potable water lines .
From a pipe stress perspective , the greater thermal expansion coefficient of stainless steels ( 15.3 vs 11.5 μm / m /° C at 20 ° C for carbon steels ) means dealing with larger thermal loads on equipment , increased thermal stresses and larger expansion loops .
Due to stainless steel being expensive ( about four times more than carbon steel ), there is an incentive to limit its thickness to the minimum code required value . This affects both smallbore and large diameter pipes .
Smallbore ( DN100 and smaller ) Sch10S ( or even 5S ) may be specified if it satisfies code thickness calculations for pressure .
This thinner pipe , together with the greater expansion coefficient , distorts more during welding than a thicker section of carbon steel pipe with the same diameter . One compromise is to specify a minimum of Sch40S for piping runs which will have branch fittings or supports welded to it and to use Sch10S only for sections where there are only end-to-end butt welds .
Larger diameters calculated ( as opposed to standard ) thicknesses are often specified , and therefore , there is no safety margin that is usually present when the selected wall thickness exceeds the minimum required thickness . This ultimately reduces the margin for error in the piping design .
Stainless steel also requires back purging during welding to avoid chromium depletion . Many non-pressure tested closure (“ golden ”) welds are used when constructing LNG plants using modular fabrication . These require purge ports ( typically consisting of a DN150 integrally reinforced branch connection , pipe section and end cap ) on either side of the weld to enable the insertion and removal of inflatable plugs .
Aside from the time and material costs , purge ports can also create their own problem . For example in a line with acoustic induced vibration ( AIV ), a purge port would be a likely failure point . Some new developments in welding ( for example Surface Tension Transfer ) welding may eliminate the need for back-purging but these methods have not yet been widely adopted .
Fitness for Service Levels 1 and 2 are applicable only to carbon steels . This means that future defects and out-of-code issues on stainless steel piping may require Level 3 fitness for service ( FFS ) assessments . Level 3 FFS assessments are more complicated and require special finite element analysis software . They generally take
longer to perform and there are fewer engineers capable of carrying out such assessments . This has the potential to create schedule delays if a Level 3 FFS assessment is required at very short notice .
Pneumatic testing is common in LNG plants as it avoids the issues caused by residual water from hydrotesting . However , the compressibility of the gas creates a stored energy hazard which is normally managed using exclusion zones based on ASME PCC-2 . There have been some notable changes to ASME PCC-2 recently : first , since the 2015 edition exclusion zone calculations must now consider both blast wave and fragment throw ( previously only blast wave was considered ) and second , prior to the 2018 edition stored energy was based on total volume being tested . From the 2018 edition , eight times diameter can be used for piping .
The above changes recognize the hazard posed by fragment throw while providing a more realistic calculation of the potential stored energy that can be instantaneously released by a piping system .
Cryogenic Valves
A recent challenge for cryogenic piping was the lack of a definitive single standard for cryogenic valves . Project specifications for cryogenic valves were sometimes a merger of BS 6364 and API 598 . Prior to the 2017 edition the word “ cryogenic ” did not appear in B16.34 . MSS SP-134 Valves for Cryogenic Service , including Requirements for Body / bonnet Extensions was first issued in 2005 , but it was only in the 2017 edition of ASME B16.34 that MSS SP- 134 was added as a requirement for valves in cryogenic service . The incorporation of MSS SP-134 into B16.34 will benefit both buyers and sellers of cryogenic valves . Buyers do not need to create their own specifications and manufacturers do not need to perform different tests for different customers .
Ball and butterfly valves are commonly used in LNG plants as isolation valves . Ball valves have more variables than gate valves which should be specified by the purchaser , e . g . floating or trunnion mount , single versus 3-piece , side versus top entry , cavity relief for trunnion mount and port size .
Figure 1 : Identification of Vented End on a Ball Valve
Cavity venting is particularly important in LNG service as the ambient heating of trapped cryogenic liquid can result in overpressure . In floating ball valves this is often achieved by drilling a hole in one side of the ball . For trunnion mounted valves , single piston effect ( SPE ) seats provide cavity relief . Providing cavity relief renders the valve unidirectional and it is important that this be communicated on design documents as well as on the physical valve - refer Figure 1 .
12 Valve World Americas | February 2025 • www . valve-world-americas . net