By Gobind Khiani – Consulting Fellow-Piping/Pipelines
When natural gas is removed from the ground, it contains water, sand, H2S, mercury, and other gases and liquids. This raw gas goes through processing by a series of filters, strainers, pipes, vessels, and associated equipment where gravity helps separate the gas from heavier liquids.
Once the impurities are stripped down, the natural gas passes through water or a solvent in order to absorb carbon dioxide, hydrogen sulphide and associated mixes. The removal of CO2 and H2S is important to avoid freezing when the gas is cooled, and to avoid blockage in the piping system.
The gas is stage cooled by using refrigerants with successively lower boiling points. After these steps, the remaining water is removed to avoid freezing completely and finally, the remaining lighter liquids such as propane and butane are extracted and can be used as refrigerant later in the cooling of gas. At this stage any mercury in the gas is also removed.
Gas Supply Chain
Once the gas is pure, likely methane with some ethane is left, then it is considered be ready for liquefaction. This process takes place in heat exchangers. A coolant which is chilled by refrigeration absorbs heat from the gas, and cools it by -162°C, shrinking its volume by approximately 700 times.
At this stage the gas turns into a clear liquid that is water-like, colorless, and non-toxic, called liquified natural gas, also known as LNG. The LNG is pumped into insulated storage tanks through an insulated piping system, making it much easier to store and transport. The liquified natural gas is kept inside these insulated tanks to maintain its temperature. Once the LNG carrier is docked, the LNG is transported through insulated pipelines into the ship. The ships have refrigeration systems and dome shaped tanks, which are safe and easy to manage. These systems circulate controlled temperature uniformly across the cargo. When the ship arrives at its cargo unloading point, the LNG is shipped into a regasification plant where it is heated to turn back into its gas state. Now it can be transported through pipeline, trucking, or cylinders into customers homes for use as energy fuel.
Cryogenic valves are used in LNG plants to handle control, on-off, shut down, double block and bleed and double isolation and bleed functions, in manual, actuated ON-OFFs to ESDs. Although cryogenic buttweld side entry valves are often specified during FEED phase, engineers often opt for top entry valves, as they are easy to operate and maintain, without the replacement of spares towards their high operability and safety features. Manufactured with a top entry buttweld body style, the valve delivers its standard top-level performance, combined with the benefits of easy access to the trim components.
The choice of a buttweld body in LNG plants is preferred due to reduction of risks of gas leakage, which is becoming common worldwide. Further top entry design allows inline maintenance capabilities. Having stated this, a side entry valve has a hatchlike feature on one side in order to allow operators to access the valve trim by hand or, in case of large diameter valve sizes, by physically getting inside the pipe.
Various Freezing Points
– Propane at -33°C
– Ethylene at -90°C
– Methane at -150°C (at this temperature methane can turn into liquid in pressurized environment only)
– LNG at -162°C (Liquid methane is flashed or fed into a low pressure vessel that causes some of the liquids to vaporize)
While an inspection is considered practical, maintenance and/or repair work becomes extremely difficult or impossible for maintenance personnel. In addition, there are serious hazards, such as plant HSE requirements and corporate safety policy. Plant operation is a key factor in determining valve designs used in LNG.
Cryogenic valves provide further benefits that are derived from using a robust triple, offset design with Stellite seat overlays and solid seal rings, and are considered to be non-rubbing seats for this application. These features will ensure valves have a long lifecycle and reduce maintenance needs, enabling plants to operate without maintenance interruption for years.
Inline access requirement is provided through a top entry design, should any unpredictable and extraordinary valve repair needs arise. Manufacturers do research to create developments on all trim components in order to improve operability or tightness. This ensures safety as a priority, thus avoiding exposure to health risks for operators working in direct contact with the pipes.
Throughout decades of testing, manufacturers have developed compliance to meet best industry practices for cryogenic valve manufacturing. Initially available only as double flanged, cryogenic valves have evolved in a range of different body styles to respond to modular construction, complex locations and types of installations across the LNG plants and floating (offshore) LNG technologies.
With the widespread concern on safety these days, code committees (consisting of valve manufacturers and end users) have responded by creating appropriate tests. These include 100% NDE, liquid penetrant, magnetic particle, fugitive emissions, extended hydro test, and cryogenic test, to name a few.
These tests ensure valve safety, integrity and high performance especially for critical flow control applications. Many of these extreme tests were specially requested in specifications and purchase orders as additional requirements years ago, but today, they are adapted by the codes as the minimum industry standard.
Critical severe service valve applications require further stringent specifications, innovative manufacturing methods and IIOT designs. Undoubtedly, there will be newer and tougher extreme valve tests to ensure product performance in challenging environments.
Current Cryogenic Testing Standards
– BS6364 British Standard Organization, UK
– MSS SP-134 Manufacturers Standards Society, USA
– ISO 21011, International Standards Organization, France
– Linde EN12266-1 Rate C
– Shell MESC SPE 77/200
About the Author
Gobind N Khiani, a UCalgary alumnus of Masters in Mechanical Engineering is a seasoned change-maker. He has a proven track record in technical and value engineering and holds a Fellowship in Engineering and an MBA. He is the Chairman of the End User Group at API and Vice Chairman of the Standards Council of Canada. He has done peer review on Emissions Management regulatory documents for ECCA and participated in research and development initiatives. Further, his experience is in the energy sector in the improvement of standards, technical compliance, strategy, governance, digital innovation, engineering management, technology, sustainable development, and operations. He is also skilled in Asset Integrity and Maintenance Management. As a volunteer, he is involved in technical standards (energy, tech, public safety) and has been a mentor/judge at First Robotics Canada. He is also the past chair of the CBEC of APEGA.