When butterfly valves are required to throttle to lower angles, just a small amount of disc movement causes a large increase in flow. At the higher angles of opening, a large amount of disc movement is needed to cause a small change in flow. Another shortcoming is a rapid torque reversal can occur due to the sudden pressure change that occurs on the downstream surface of the disc at about 60-75 degrees of disc angle. To address these issues, Zwick Valve, with the help of Yeary Controls, has taken its triple eccentric butterfly valves to the next level of control by adding Yeary's proprietary Sharktooth Control Cartridges to the product design.
In 2005, Yeary Controls initiated an R&D program to develop and test a device, now called a Butterfly Valve Control Cartridge, that could be added to the downstream side of triple eccentric butterfly valves to turn them into true control valves with flow characteristics equal to the best general-purpose industrial control valves on the market. The resultant device has no moving parts and does not affect any of the "built-in" features of the valve. The initial R&D for the technology was sponsored by Yeary and successfully completed by students at the University of Illinois Department of General Engineering in Urbana- Champaign, using a triple eccentric butterfly valve as the platform for the device.
Then, after obtaining the consulting services of a control valve expert, the design was further refined by adding flow slots, or teeth, in the cartridge. Then the device was subjected to a series of flow tests at the Utah State University Water Research Laboratory on 6-foot and 12-foot valves, which verified its ability to produce an equal percentage flow characteristic using an off-the-shelf triple-offset butterfly valve (TOV) and, additionally, that the device could reduce noise up to 15 A-weighted decibels (dBA) and reduce the effects of cavitation by up to 20 percent. It also dramatically reduced the valve's running torques.
Today, Sharktooth control valves are providing satisfactory performance in sizes of 3 inches to 36 inches on liquids, gases and steam by converting 150, 300, 600 and 900 ASME TOVs into excellent control valves.
In 2011, Zwick Valves began offering an extensive line of control valves called TriShark that incorporate the Yeary Controls Sharktooth technology. These valves have an equal percentage flow characteristic and excellent rangeability. They are available in sizes from 3 inches up to 72 inches and 150 pounds through 900 pounds ANSI pressure classes, with API 609 Lug, ISO 5752 Flanged and ANSI B16.10 Gate valve face-to-face body configurations, and they have even been built with buttweld ends. Their superior metal-to-metal/graphite seating gives these valves an operating temperature range from cryogenic through 1,400 F, while providing zero leakage per API 598 Resilient Seat Test. They are also fire safe per ISO 10497 and API 607 fifth edition.
Since Zwick's triple-offset valves are seated via their stem torque, they do not have the interference-type seating typical of most rotary-style control valves. Such valves that utilize a segmented ball require a higher seating force than the force needed to turn them.
Unlike friction-seated valves, the torque seating of a TriShark allows for the disc to freely move from the seat immediately as the stem torque force that is seating the valve is reduced. This allows for stable control down to about 3 degrees of the opening angle. This zero-friction feature also offers longer seal ring and seat life, thus reducing the need for regular maintenance required by other rotary- style control valves.
For more information, visit www.zwick-valves.com, call (281) 478-4701, or email davebuse@zwick-valves.com.
