In natural gas processing, dehydration is necessary to combat corrosion. Molecular sieve dehydration (MSD) is used in many processes that use natural gas as a feedstock, including pipelines, LNG regasification and power plants. MSD units require specialised isolation switching valves to control the flow of gas through the process and to deal with catalyst particles, temperature cycling and frequent cycling of the valves. Zero leakage past the valves and zero friction during the cycling of the valves are imperative.
Valve demands
Valves in MSD units face multiple challenges. One of the biggest comes from the catalyst. MSD beds can generate catalyst carryover. This granular material can abrade the valve seat and sealing surfaces on conventional valves. Temperature is also an issue. During the regeneration process, temperatures can range from 175 to 427°C. The valves will cycle frequently during this time. With conventional valves, this frequent cycling will accelerate wear and reduce the life of the valves. Temperature cycling and cycling of the valves requires high performance features.
The Zwick Tri-Con answer
The Zwick Tri-Con metal-seated valve delivers absolute zero friction during the opening and closing of the valve and more importantly has absolute zero leakage past the valve because of the true 250 inclined cone-seating plane. The Tri-Con seal ring is statically held on the disc with a 250 inclined angle, allowing true cone-in cone seating with no deformation of the seal ring. This ensures there is no wear on the body seat as the catalyst flows through the valve while the valve is closing. Every Tri-Con valve is tested and certified to zero leakage per API 598 resilient seat test standards. The Tri-Con can also be tested and certified to API 6D zero leakage upon request.
Tri-Con valves have a Stellite 21 welded overlay on the body seat, giving long seat life and wear resistance and helping to maintain the efficiency of the drying system. Zwick’s unique self-centring disc addresses valve issues with temperature cycling during regeneration. The Tri-Con disc is not pinned to the shaft, as with all other triple offset valves. Therefore during any temperature cycling the shaft, which features a different material to the disc, will grow or contract. With valve designs that have pinned shaft-to-disc, this will result in either pulling the disc up during the heating process or pulling the disc down during the cooling cycle. With the self-centring disc design, the shaft can grow or contract, but the disc will body seat.
The Tri-Con valve utilises a keyed connection from the shaft to the disc, providing balanced torque transmission without the threat of shearing pins. Also, the valve disc is made of the same material as the valve body, resulting in no temperature limitation due to thermal expansion and contraction up to the limit imposed by ASTM to WCB material.
Another unique feature the Tri-Con offers is the patented seal bearing. A common problem is particles in the process media migrating into the bearing cavities and causing bearing failures. Zwick’s patented sealed bearing design has solved this problem with a 3-piece bearing complete with mechanically energised graphite packing rings that protect the shaft or bearing cavity from any process media, especially any catalyst particles. The sealed bearing feature has proven to be the answer to protecting the shaft or bearing cavity from the migration of sulphur in sulphur tail gas applications as well. With the sealed bearing feature it becomes the primary emission barrier and the packing becomes a secondary emission barrier. The sealed bearing feature can offer a 10 ppm or lower emission system.
Zwick understands that MSD applications are essential to the natural gas processing market and downtime due to valve failure is not acceptable. The Tri-Con answers the challenge with zero friction, zero leakage and zero migration of catalyst into bearing cavities.
For more information contact Fred Venter, Valve & Automation, +27 (0)11 397 2833, [email protected], www.valve.co.za
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