- Published on Tuesday, 06 August 2013 08:57
- Written by Mark Tilley and Frederic Blanquet
Liquefied Natural Gas (LNG) is exactly what it says: the liquid form of natural gas. The process of liquefying is performed to reduce the volume for purposes of transporting the fuel: LNG reduces volume by 600 times, making it much more economical to transport by sea aboard LNG carrier ships to destinations all over the world.
The temperature required to liquefy natural gas is -260°F (-162°C). As a result, most pipe, valves and fittings will be specified to meet cryogenic temperatures of -270°F (-168°C) to 325°F (-198°C). The development, transportation and receiving of LNG require a step-by-step process that has grown very rapidly over the last several decades. In the past, each step was treated as a different market. However, with the global economy and market for LNG expanding rapidly, the challenge of many manufacturers of pipe, valves and fittings is to expand their product ranges to cover all three areas. Valves are a critical component at each stage, and they need to be constructed of quality cast and forged materials. Valve performance and reliability are vital to the whole process, and these strengths can only be achieved through critical design, manufacturing and material selection.
LNG facilities are constructed in various places in the world where an overabundance of natural gas can be converted to its liquefied state for use elsewhere. A few different facility designs have been developed for reaching the cryogenic temperatures needed. Valve design and specification may be slightly different from facility to facility, but the end result is the same: The volume ratio of 600 to 1 (gas to liquid) is loaded onto specially equipped carriers to be shipped to receiving terminals.
Some of the valves used in the facilities include:
Butterfly valves. Most butterfly valves for this use are 4 inches and larger. Some control valves will be this type, but the majority of butterfly valves are manual, on/off automated and emergency shutdown. While many of these valves are used throughout the facility, most types will be cryogenic to meet the extreme cold requirements. Most of these valves are in the 150-pound, 300-pound and 600-pound classes with flanged ends. The valves will require live loading on the packing to ensure a constant load on the packing chamber. They also will have stem extensions that are calculated in length to maintain a vapor barrier at the upper end of the extension. There are also some higher pressure valves needed that are in the 900-pound class and are cryogenic butterfly valves.
Ball valves. The majority of ball valves for LNG are 6 inches and smaller with pressure classes typically the same as butterfly valves. Like butterfly valves, ball valves are used for many applications and most are cryogenic. The body is typically a butt weld, three-piece design with a cryogenic extension and live loading on the packing chamber. There are many applications where a cryogenic top entry, one-piece body is preferred because of thermal cycling conditions in the process.
Gate, globe and check valves. These valves are also used in many areas of the facility in both cryogenic and ambient applications. The majority of cryogenic valves are going to be butterfly and ball valves, however, depending on the process design.
These vessels are specially designed to transport LNG from the liquefaction facility to the receiving or regasification terminal. The world’s need for natural gas means the LNG carrier market is booming. Because of the gas to liquid ratio, these carriers can transport enormous amounts of LNG to those areas of the world that have the demand. There also are many new innovations for floating LNG regasification terminals and liquefaction facilities, including carriers with regasification facilities on board. This means that in areas where the construction of a receiving terminal does not make sense, such as small ports, natural gas can go right from the ship to feed power plants.
Butterfly valves. As with facilities, the majority of butterfly valves on carriers are 4 inches and larger; some control valves are butterfly, but the majority will be manual, on/off automated and emergency shutdown. While there are many butterfly valves designed for use by utilities or designed for ambient gas, most are cryogenic to meet temperature requirements. Most of these valves are in the 150-pound class with butt weld ends and a side entry port for maintenance. Minimizing flanged connections is important in most areas of the vessel, but they cannot be avoided completely because of space concerns on ships. These valves also will have stem extensions that are calculated in length to maintain a vapor barrier at the upper end of the extension.
Ball valves. As with facilities, most ball valves on carriers are 6 inches and smaller with pressure classes typically the same as for butterfly valves. Also, most will be cryogenic and the body is typically a butt weld top entry design with a cryogenic extension with live loading on the packing chamber.
Gate, globe and check valves. These are used in many areas on the vessel in both cryogenic and ambient applications, though most of the cryogenic valves on ships are butterfly or ball valves.
RECEIVING OR REGASIFICATION TERMINALS
These facilities receive LNG from the carriers and are usually constructed in populated areas of the world where there is a great need for natural gas. The LNG is off loaded into massive insulated tanks on shore before the process of converting LNG to a gas begins. The facilities then introduce the natural gas into area pipelines for use.
Butterfly valves. Again, most butterfly valves for these terminals are 4 inches and larger; some control valves are butterfly valves, but most control valves are manual, on/off automated and emergency shutdown. While many butterfly valves are used throughout the terminals, most are cryogenic, in the 150-pound class with butt weld end and a side entry port for maintenance.
Minimizing flanged connections is also important in most areas of these terminals. The valves must have “live loading” on the packing to ensure a constant load on the packing chamber. They also will have stem extensions calculated in length to maintain a vapor barrier at the upper end of the extension. This is another area where higher pressure, 900-pound cryogenic butterfly valves used with butt weld side entry design are used. Very few manufacturers currently make this type and pressure class of valves.
Ball valves. Most are 6 inches and smaller with pressure classes that are the same as butterfly valves. Again, many types are used, but most are cryogenic. The body is typically a butt weld three-piece design with a cryogenic extension and live loading on the packing chamber. Also, there are many applications where a cryogenic top entry one-piece body is preferred because of thermal cycling conditions in the process.
Gate, globe and check valves. These are used throughout the facility in both cryogenic and ambient applications, though most of the cryogenic valves are butterfly and ball valves depending on the process design used. There may be check valves of a special design used for loading and unloading LNG. These valves can be operated in reverse flow in which case the disc is mechanically overriden to lock open for reverse flow.
For LNG service, the body material is typically austenitic 316 stainless steel to maintain body strength at cryogenic temperatures. Non-cryogenic valves can be WCB-grade, stainless or high nickel alloys that are suitable for that particular service as well as for the offshore environment.
TESTING, CERTIFICATIONS AND APPROVALS
All valves will typically need to meet API 598 or equivalent leakage testing as a minimum. Cryogenic valves will have to meet BS 6364, which is tested with helium at -270°F (-168°C) to -325°F (198°C). While this test is an industry standard for liquefaction, receiving or regasification terminal applications, many receiving terminals will require much better leakage rates during cryogenic testing. This testing is usually random on 10% of valves or at least one of every size and class. LNG carriers are usually more critical in regards to seat leakage rates.
More specifically, testing includes:
- For BS 6364, liquefaction facility requirement is a leakage rate maximum 150 cubic centimeters (cm3) per minute per inch size of valve (cm/minute/inch).
- With the MW Kellogg spec test, most receiving or regasification terminals have a leakage rate maximum of 15 cm3/minute/inch. This test can be very challenging for metal-seated valves, and triple offset valves typically cannot meet this test because of the torque-seated design.
- LNG carrier leakage testing requirements typically have to meet a maximum of 10 cm3/minute/ inch.
- Process valves typically have to meet fire-safe specifications BS 6755, API607 and API6FA.
- Most specifications will require valve body x-ray testing and die-penetrate testing for random valve body and parts.
- LNG carrier valves also typically have to meet one or more of many ship or carrier approval certifications such as the Bureau Veritas, DNV or ABS.
While many of the valve requirements in the receiving terminals, liquefaction facilities and LNG carriers are redundant, there are important differences in pressure class, end connections, certifications and testing.
With the LNG market growing, other markets for the product are poised to grow very quickly. For example, the transportation industry has discovered the benefits of the LNG volume ratio to natural gas: tanker trucks, service vehicles and automobiles can go much greater distances. LNG filling stations also benefit from the volume-saving advantage.
The challenge for pipe, valves and fitting manufacturers as well as other equipment designers and makers, will be public safety and perception. While LNG is not explosive in its liquid state, once it reaches a vapor form and gets between 5-15% of natural gas in air, it can ignite. (Below 5%, there is not enough natural gas to become flammable; above 15%, there is not enough oxygen in the air to be flammable.)
LNG carriers as well as loading docks around water have other risks as well. If LNG is exposed to water at a very fast rate, for example, a rapid phase transition (RTP) can occur. RTP can result in a physical explosion that can release a great amount of energy. While this is not a combustion type of explosion, the energy released can be hazardous. Because of this, valve manufacturers, as well as makers of other products for the LNG market, conduct critical research, development and production of their products to meet safety concerns and industry requirements.
What many valve manufacturers are considering is that there has been an overabundance of LNG projects in North America and around the world. In the late 1990s to mid-2000s, North America had many permit applications for receiving terminals. However, there were only a few regasification terminals actually constructed, several of which never moved much LNG into the pipelines.
Still, because of the plentiful supply of natural gas from the shale gas finds in the U.S. and Canada, North America has many projects planned for liquefaction facilities. In fact, many of the receiving terminals are now building liquefaction facilities on the same site.
At the same time, there already are too many new valve manufacturers in the market producing cryogenic valves and others with plans for developing similar products on the horizon. A major challenge current valve manufacturers, as well as foundries and forge masters, face is to remain competitive in this growing market, while maintaining and improving quality and performance.