What is the future of control valve technology?

Transcription

1 What is the future of control valve technology? Dave Busse, President, Zwick Valves North America 2552 South Battleground, Deer Park, Texas, , After many years of performing successfully in a wide range of isolation services, Triple Eccentric Butterfly Valves are now universally recognized and accepted by industry for their compactness, low weight, zero friction seating, low emissions and excellent shut-off abilities. However, when used to modulate flow, the triple-offset valve (TOV) suffers from the same inadequacies as those found in all butterfly valves when they are called upon to throttle flow, i.e. instability at disc angles below approximately 25, or greater than approximately 65. When butterfly valves are required to throttle at lower angles, just a small amount of disc movement causes a large increase in flow; and at the higher angles of opening, a large amount of disc movement is needed to cause a small change in flow. Another shortcoming of butterfly valves in throttling services is that a rapid torque reversal can occur due to the sudden pressure change that occurs on the downstream surface of the disc at about 60 to 75 of disc angle. To address these issues, Zwick Valve, with the help of Yeary Controls, has taken their Triple Eccentric Butterfly Valves TOV to the next level in control by adding Yeary s proprietary Sharktooth Control Cartridges to their Tripe Eccentric Butterfly Valves. In 2005, Yeary Controls initiated a research and development (R&D) program to develop and test a device, they now call a Butterfly Valve Control Cartridge, that could be simply added-on to the downstream side of triple eccentric butterfly valves to turn them into true control valves with flow characteristics that would be equal to the best general purpose industrial control valves on the market. The results created a device that 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, using a triple eccentric butterfly valve as the platform for the device. After obtaining the consulting services of control valve expert Dr. Hans D. Baumann, the design was further refined by adding flow slots, or teeth, in the cartridge. The device was then subjected to a series of flow tests at the Utah State University Water Research Laboratory on 6 and 12 valves. The tests verified the device s ability to produce an equal percentage flow View of Zwick Valves TriShark control valve.

2 characteristic using an off-the-shelf TOV, as well as reduce noise up to 15 dba (FIG. 1), decrease the effects of cavitation by up to 20% (FIG. 2), and reduce the valve s running torques (FIG. 3). FIG. 1. Reduces aerodynamic noise up to 15 dba. As the vane turns with the Sharktooth Throttling Trim Cartridge, multiple, optimized, throttling slots are gradually exposed to flow. Then, at higher flows, when the vane has cleared the contoured slotted portion of the control cartridge, the resultant lower pressure drop will alleviate any further noise problems. FIG. 2. Anti-cavitation properties. Tri-Sharks 25% higher incipient cavitation index allows for higher pressure drops than ordinary butterfly valves, while exhibiting much less noise, cavitation, and damage. Even if cavitation does occur in higher pressure drop situations, the shorter vapor jets produced by the slots will avoid the damaging effects caused by large vapor jets, referred to as super cavitation.

3 FIG. 3. More control with less torque. Tri-Shark s dynamic torque requirements are 60% less than ordinary butterfly valves, even at high-end flow capacity, thereby providing exceptional stability throughout its control range. Tri-Shark s Triple Eccentric Metal-to-Metal Seating requires lower breakaway torques than either butterfly or rotary plug control valves, and its extended torque reversal point allows for an expanded range of control. Yeary Controls has summed it up by highlighting that the control valves design: Has shown a predictable design for extrapolating to larger sizes; a great flow curve; predictable noise and cavitation levels and reductions; a smoother operating valve that would put less wear and tear on its actuator. Presently, Sharktooth control valves are providing satisfactory performance in sizes 3 thru 36, on liquids, gases, and steam by converting 150, 300, 600, and 900 ASME TOV s into excellent control valves. In 2011, Zwick Valves, USA, began offering an extensive line of control valves called TriShark that incorporates the Yeary Controls Sharktooth technology. A detailed look at the TriShark throttling trim cartridge and operational positions are shown in FIG. 4 and FIG. 5. These valves have an equal percentage flow characteristic, and an excellent range. They are available in sizes from 3 up to 72 in 150 lbs. through 900 lbs. ANSI Pressure Classes, with API 609 Lug, ISO 5752 Flanged, and ANSI B16.10 Gate Valve Face-to-Face body configurations, and have even been built with Butt-Weld-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. Zwick is also fire safe per ISO and API th Edition.

4 FIG. 4. TriShark throttling trim cartridge. The Sharktooth throttling trim cartridge is used to turn the Triple-Off-Set-Butterfly Valves into excellent throttling control valves. One style of valve can be used for both on-off, and control applications. FIG. 5. TriShark, controlling at every angle. Since Zwick s triple offset valves are seated via their stem torque, they do not have the interface 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 once they are more than roughly

5 5 to 7 open. As a consequence, when pneumatically actuated, the actuator must build-up the force needed to open the valve, but once the ball segment has disengaged its seat, the actuator will have built-up more force than is needed to throttle the now open valve. The sudden reduction in the required torque will cause a rapid over-travel that must be dealt with by the control system and actuator positioner. These high seating and breakaway torque issues are mitigated by the TriShark s triple eccentric seating. 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 approximately 3 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. TriShark control valves have an effective control range that extends from 1% at 5 of travel to 100% at 90 of travel (FIG. 6): FIG. 6. TriShark control valves demonstrate excellent control range. The Sharktooth throttling trim cartridge eliminates the low angle instability inherent in most quarterturn control valves. For example, ordinary butterfly valves exhibit poor control at angles of opening that are less than 30 (TABLE 1). Sharktooth technology greatly extends the control range of quarter-turn valves to as low as 3 of valve opening while also providing noise attenuation and anti-cavitation benefits.

6 TABLE 1. Zwick flow capacity (Cv) comparison with & without Sharktooth Standard product range is denoted below:

7 Resistance plates. When the control valve exit velocity (UV) for gases exceeds 250 ft/sec, the installation of a resistance plate to reduce the aerodynamic noise is recommended. Similar use of such plates is advised for high pressure liquids to reduce excessive hydrodynamic noise, or cavitation. The inclusion of a resistance plate is shown below: Applications. By combining the Sharktooth throttling control cartridge with Zwick s triple eccentric butterfly valves, just one style of valve can be used for both on-off and control applications. These control valves are ideal for most applications. This includes: Oil/gas production Refining storage/transmission Electric power generation Chemicals & petrochemicals Textiles Pulp & paper Mining & metals Micro-electronics Pharmaceutical & biotech Water pumping & transport Office buildings & hotels Water treatment Aircraft, aerospace, military Ship building & marine

8 Typical standard services include: Pump by-pass control Pressure reducing Back pressure control Compressor surge control Flow control & balancing Ballast water flow control Cooling tower bypass Steam condenser inlet Brine well interjection Overboard discharge Fire water ring control Steam Brine blowdown Jetty loading control Natural gas pressure control Sea-water re-circulation & drain Typical severe and special services include: Cavitating liquids Noise reduction in gas, steam, and vapor Two phase fluids Cryogenic liquids & gases Fire safe shut-off plus control Metal-to-metal tight shut-off plus control Some typical installations: