Dehydration is an important process of LNG plants in which the feed gas is dried before being introduced in the main cryogenic heat exchanger (MCHE). This process is required to avoid any icing formation during the liquefaction process and to meet LNG specifications.
Molecular sieve particles for adsorption
Molecular sieves are used to remove water vapors from feed gas. A molecular sieve is a material with pores of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be absorbed, while smaller molecules can.
Dehydration (water vapor adsorption) utilizes hard, granular adsorbents manufactured in several types of materials including aluminosilicates such as Zeolite, ceramic balls, activated carbon and silica gels.
Zeolite (Type 4A & 5A) are mainly used for drying the natural gas in LNG plants.
Drying cycle is severe cyclic service
Once the Mol-sieve particles have entrapped the water particles, it becomes necessary to remove the trapped water particles so that adsorbents are ready for drying the next batch of natural gas.
For this purpose, hot regeneration gas is passed in the opposite direction which carries the water particles to the regeneration gas cooler and regeneration gas separator. So, the dryer vessels undergo drying cycle and regeneration cycle alternatively.
Sometimes additional cycles like standby are accompanied with the drying and regeneration cycle.
Finally, this drying operation requires a lot of cyclic operation (typically 3-4 cycles/day) and hence this drying cycle is called severe cyclic service.
Switching valves control the molecular sieve sequencing: one vessel in drying mode, and the opposite one in regeneration mode (and vice versa).
"Similarly, gate valves are not suitable for Mol-sieve service since there is considerable contact between gate and seats during the final opening and closing stage of the valve.
Required features of switching valves
Based on the above discussions, we can conclude that following features are required for valve in Mol-sieve service which is often categorized as severe cyclic service.
Since the valve is required to open and close frequently (3-4 times a day), it becomes mandatory that the closure member should have minimum contact with the seats during the opening and closing operation. If we consider this requirement, we can understand that why conventional ball valves & gate valves are not used for Mol-sieve service. Conventional ball valves have continuous line contact between closure member (ball) and seats. If conventional ball valves are used in Mol-sieve service then the seats will wear out very soon and hence a tight shut off can’t be provided. Also, due to continuous contact, the frictional resistance is high and hence the operational force requirements for the actuator will be very high. Similarly, gate valves are not suitable for Mol-sieve service since there is considerable contact between gate and seats during the final opening and closing stage of the valve.
Furthermore, it is necessary that valves should be cavity free because if the valve has a cavity then there is a high possibility that the Mol-sieve particles will be trapped inside the valve cavity rendering the valve inoperable after some time.
Also, valves should have metallic hard faced seats. Since we can’t eliminate the contact between the closure member and seats completely during the shut off time, it is required that metallic hard faced seats are used to avoid any abrasion between closure member and seats.
The temperature of regeneration gas used for regeneration ranges from 200 Degree C to 250 degree C. So, the switching valves have to undergo a lot of thermal cycles as well. Generally the material of valve body is CS/LTCS and the material of closure member (disc/ball) is SS. So, consideration of valve material and gaps / tolerances calculations between closure member and seats should take into consideration the material expansion within the thermal transient.
"When rising stem valves are selected the end user should consider fugitive emission requirements.
Hard faced bushings should be used to avoid any abrasion by Mol-sieve particles. Also, it is recommended to use bushing protectors to avoid entry of Mol-sieve particles between the stem and valve body.
The valve castings and welds are recommended to be 100% radiographed to avoid any notches or cracks. Since the valve undergoes high cycling, fatigue failure is a big concern for these valves.
When rising stem valves are selected the end user should consider fugitive emission requirements since the quarter turn valves have better fugitive emission requirements in comparison to rising stem valves.
However, with the advent of PTFE infused packings and braided packings, even the rising stem valves can offer good fugitive emission performance.
"Three valves are commonly used for Mol-sieve service: Rising stem ball valves, triple offset butterfly valves, and eccentric ball valves.
Actuation needs to have smooth and slow opening. A hydraulic speed control which minimizes wear and maximizes the cycle life can be used with the actuator. It is an important factor in minimizing wear. This allows total control of the ball coming out of the seat to eliminate circumferential acceleration and corresponding frictional wear on the bearings and packing.
If valves are used for FLNG, then the valves should be light and compact.
Rising stem ball valve
Although this requirement is solely based on licensor recommendations and end user experience, the following three valves are commonly used in the Mol-sieve service.
Rising stem ball valve: Rising stem ball valves (RSBV) have been used primarily for/in molecular sieve service. This application requires a specific valve design offering high performance. The RSBV sealing mechanism is based upon axial movement only compared to a standard ball, gate, plug and butterfly valves having their sealing mechanism based upon shear force, friction and tearing. The RSBV sealing is not liable to friction or tearing as there is no contact between ball and seat during rotation.
The friction free quarter turn, mechanical energized sealing means no abrasion on the ball or valve body seat compared with other types of valves, giving exceptionally long service life. The RSBV helix stem design has linear stem movement also 100% metal to metal sealing performance.
Triple off-set butterfly valves
Triple offset butterfly valves: Triple offset butterfly valves are becoming popular as switching valves in Mol-sieve service due to their compact size, light weight and non-rubbing design.
The non-rubbing design produces a single, instantaneous point of contact between sealing elements when the closed position is reached. Also, the metal seal ring, located on the disc, is resilient by design - its resiliency allows for a uniform distribution of the contact force to seat across its circumference, ensuring excellent tightness, and compensates slight differences of expansion between body and disc.
However, when triple offset butterfly valves are used, some special considerations like solid seal ring instead of laminated seal ring, hard facing of seat area, use of solid stellited bearings and use of bearing protectors is recommended.
When triple offset butterfly valves are selected the end user should consider the pressure drop through the valves since the disc is always in the middle of the flow stream.
Also for these valves a horizontal or angular stem installation is preferred to avoid accumulation of Mol-sieve particles in the seating area. The maintenance concept difference between a top entry valve and side entry butterfly valve should be properly studied during the design phase.
Eccentric ball valves
Eccentric ball valves: Eccentric ball valves have made a new entry to the field of valves in Mol-sieve service. The cam action of the valve provides a non-rubbing action which is a pre-requisite for valves in severe cyclic service.
Rotary double eccentric C-ball “no contact type” is designed to meet the most stringent services. The two vectors movement (rotation/translation) provided by the double eccentricity design allows the valve to open and close without friction or wearing at the seat & “C” contact.
About the Author
Neeraj Batra is a Piping Materials Engineer) at Chiyoda Corporation. Chiyoda is a global leader in the LNG business and has built various plants and executed numerous projects in over forty countries around the world since its foundation in 1948.
“It is neither author’s interest nor capability to endorse any valve for the Mol-sieve service. The valve selection is purely driven based on Licensor’s recommendations and end user experience. However, these valves undergo severe thermal cycling and operate in a service with particles. So, the internal selections and maintainability are prime factors that should be considered in the design phase.”