After the hectic activity of the 1980s and 1990s, especially on the international level through the International Electrotechnical Commission (IEC), efforts to either
refine or create new standards have slowed considerably. Past activities led to a spate of standards yielding more accurate sizing, due to a better understanding of fluid dynamics, as related to valves, and even equations to predict fluid-induced noise from both liquids and gases. However, current efforts both in ISA and IEC-TC 65B, WG 9 committees primarily revolve around maintenance of existing standards.
Although such inactivity and lack of basic research may have saved companies money, it points to the sad fact that we do not see any more scientific papers based on laboratory experiments by major valve companies. This, in turn, reflects the lack of basic new valve types coming on the market. Of course, this may just be a sign that the control valve industry is maturing, which is also apparent from the fact that more than 85 percent of all control
New control valve alternatives
Over the past 40 years, the market has adapted to using rotary types of control valves to replace globe-style valves, especially in larger sizes (typically above three inches) and for more moderate service conditions. The least expensive types of rotary valves are butterfly valves. However, until recently, a number of flaws limited their acceptance as final control elements:
- They do not offer an equal percentage flow characteristic (preferred by more than 80 percent of users).
- Except for swing-through types, most butterfly valves have a high “breakaway” friction, which tends to cause the travel to overshoot, leading to instability. This also limits the rangeability, or turndown, of such valves, because one should not control below five degrees of opening, in order to avoid this pitfall.
- Conventional butterfly valves have high dynamic torque due to the airplane wing effect of the flat vane surfaces. Again, this can be a cause for instability and for the need for larger, more expensive actuators.
In an effort to overcome these negative effects, several new butterfly valve innovations have recently come on the market. One of these, called a Control-Disk, is manufactured by Fisher Controls, a division of Emerson, whose engineers added controlling profiles on a basically metal-seated, double-eccentric butterfly valve, which then gives the valve an equal percentage inherent flow characteristic. Another patented idea, centered around rubber-lined valves, is presented by Lilly Engineering Company, Inc., of Itasca Ill., with their Z-Disk. It claims to overcome all the detriments listed above, making this valve also a serious competitor for globe-style control valves and other more expensive rotary valves.