Pushing the Limits, High Performance Butterfly Valves Reconsidered
High-performance butterfly valves work smarter instead of harder. That is why they provide a sound alternative to larger, more expensive linear valves and heavier rotary-type designs in a wide range of industrial and commercial applications.
Quarter-turn butterfly and ball valves were introduced in the 1950s to overcome the problems inherent in multi-turn linear valve designs, particularly gate valves. The first high performance butterfly valves entered the marketplace in the ‘60s.
Since then, high performance butterfly valves have undergone continuous improvement, making them applicable to an increasingly wider range of temperatures, pressures and aggressive media. Within the past few years, the introduction of advanced polymeric seating materials has significantly broadened this range. If you ruled out butterfly valves for some of your unforgiving applications three to five years ago, they now deserve reconsideration.
Part of this reconsideration involves a review of the advantages any butterfly valve (let alone high performance) has over linear and heavy-duty rotary valve configurations.
Reduced size and weight: lightweight/compact design.
Because the butterfly valve has a significantly narrower face-to-face dimension and a shorter centerline to top of valve profile, it uses less metal than a gate valve. The result is significant weight and size reduction for the same or higher pressure rating. Significantly lower weight, particularly in the larger sizes, means lower piping stress and reduces the number and/or size of the pipe supports. Smaller size and lower weight also translate into significantly fewer man-hours to install the valves.
The rotary design of butterfly valves and minimal wear surfaces dramatically reduce torque requirements. Therefore, they can be operated with smaller, less expensive actuators.
While gate valves require almost total disassembly and extensive machining to reseat the valve, butterfly valves are easy to maintain. The insert can be removed with a screwdriver in a wafer style body design, while the self-aligning seat in a lugged style body is replaced without taking the valve out of line.
Every valve is drilled and tapped to accept linkages for a broad range of actuators. Unlike the gate valve, there is no need to purchase a special yoke or other device to modify the gate valve body to accept actuation. In addition, because butterfly valves are quarter-turn, the actuated valve profile is much smaller than the gate valve.
Stem leakage is a nagging, recurrent maintenance problem with most linear valves. A leak path is easily generated from the vertical and/or long multi-turn strike of the stem through the packing. Frequent tightening of the packing gland, and in some cases grease injection, is performed, but this rarely results in completely eliminating stem leakage.
In contrast, the butterfly valve shaft rotates only 90° within the stem packing. This minimizes the potential for leakage. Should leakage occur, it can be eliminated by simply tightening the packing gland. The gland compresses the V-ring packing, spreading the "wings” of the rings and creating a tighter seal. Because the packing is not jammed, the torque remains constant. For those applications where more stringent emissions control is required, butterfly valves can be equipped with a spring-loaded packing arrangement.
The High Performance Difference
While many of the features of standard butterfly valves would be on the wish list for most gate valve applications, this still may not be enough to justify a departure from the traditional gate valve specification in ANSI Class 300 applications and above. The high-performance design, which insures tight shut-off and exceptional long-term durability at higher temperatures and pressures, makes the difference.
Neither gate valves nor conventional butterflies can assure long-term, bubble-tight shut off. Sealing a gate valve involves jamming a metal solid or split wedge into a metal seat or weir cavity. The effectiveness of the seal, therefore, is subject to the condition of the mating metal surfaces and the cleanliness of the weir. In practice, tight sealing of a gate valve is uncertain. Typically, the gate valve experiences leakage from minor erosion of the wedge or seal area, from wire drawing caused by steam leakage, from slurry particles that accumulate in the weir, and from caking and scaling of the weir and seat surfaces.
Conventional butterfly valves also have design limitations that accelerate leaking. These include deterioration of the seat due to excessive friction and exposure to abrasive and corrosive media, along with tearing of the seat that can occur when the valve is operated after a long period without being exercised.
The high performance butterfly valve’s offset shaft and eccentric disc arrangements, combined with modern, flexible lip polymeric seats, provide bubble tight shut-off over a wide range of operating conditions.
Longer cycle life.
In many severe service applications, gate valves and conventional butterfly valves will begin leaking within the first year or two of installation (if not within months). The high performance butterfly valve design, particularly the double eccentric disc and shaft in combination with polymeric seats, reduce seat wear and dramatically increase leak-free life cycles. It is not uncommon for valves of this design to provide bubble-tight shut-off and leak-free operation for one or two decades without being maintained.
Wider applications range.
The flexible lip seat design allows high performance butterfly valves to operate at higher temperatures than conventional butterfly valves. On-going developments of new seating materials have extended high performance butterfly valves into applications involving media that would otherwise be difficult to contain. As a result, high performance butterfly valves continually prove their ruggedness and dependability in a wide range of industries and applications.
Lower total costs.
As pressures increase, critical valves become disproportionately larger to insure long-term safety and leak-free operation. As they increase in size, high performance butterfly valves are less expensive than linear valves and other types of rotary valves. With actuation, the total cost ratio of the valve package is reduced even further. Ease of maintenance, actuation and installation can also dramatically reduce total life cycle costs.
Even Higher High-Performance
In 2001, Metso Automation introduced its Xtreme® sealing technology. Many of the performance advantages of the high performance butterfly valve (e.g., tight-shut-off and longer cycle life) were significantly enhanced, while maintaining excellent chemical resistance characteristics.
Xtreme sealing technology is based on a resilient new fluoropolymer material developed at Metso Automation's Polymer Research Laboratory. The Xtreme valve seat material maintains its excellent performance characteristics in higher temperature environments than filled PTFE and does not increase valve torque requirements. In addition, Xtreme seats match the chemical resistance of filled PTFE while offering longer cycle life.
The extended life of this seat is largely attributable to the Xtreme material’s minimal permanent deflection, which is linear from 100° F to 450° F. Even in the face of severe temperatures and pressures, the seat remembers and returns to its ideal shape after loading. This results in optimal contact between the seat and the valve disc for an ultra-long leak-free cycle life. While the permanent deflection of PTFE and filled PTFE increase more rapidly as temperatures approach 450° F, deflection of Xtreme® seats actually decreases. At 450° F the deflection value for the material is only half that of PTFE (see Figure 1).
Figure 1. Improved seat recovery improves tightness under pressure and thermal cycling.
The Xtreme seat is now a standard offering with Jamesbury® Wafer-Sphere® high-performance butterfly valves. This makes it possible to replace large inventories of expensive special seats with a single, cost-effective alternative.
Significant savings accrue due to the valves’ exceptionally long cycle life, which slashes maintenance and replacement costs. In head-on laboratory tests versus numerous competitors, Xtreme-seated valves have proven to be five to eight times longer-lasting than those with Teflon® seats.
Each Wafer-Sphere® valve model with Xtreme® seats went through a period of selective trials to insure that expectations raised by laboratory performance studies were, indeed, matched by experience.
Ever since high performance butterfly valves were first introduced, the limiting factor for cycle life was seat performance. With the introduction of the Xtreme material, different valve components (particularly the shaft bearing and seals) became the weakest link. By redesigning these and other components for ruggedness that matched the durability of the seat, it was possible to double the lifetime guarantee for Wafer-Sphere valves with Xtreme seats from 500,000 to one million leak free cycles.
The new generation of high performance butterfly valves has been employed in a wide range of high-temperature and high-pressure applications in numerous industries. Because of the long association with Jamesbury® valves and an installed base of the valves, two industries – air separation and corn processing – have been among the enthusiastic early adopters of the product for critical applications.
The air separation industry has numerous high-pressure, high-cycle applications, which severely test the longevity of high performance butterfly valves. It is not uncommon in these applications for on-off valves to cycle every five or six minutes, going from full open to full closed in less than a second. Many of these valves must cycle one million times a year.
Because the media is exceptionally clean, the previous generation of Wafer-Sphere butterfly valves had a high-cycle option, certifying that the valves would last for a year in air separation equipment before a shutdown was required for maintenance and seat replacement. The Xtreme material now allows this high-cycle option to guarantee two million cycles (two years of operation) before any shutdown is required for valve maintenance or replacement.
This is extremely important to the air separation industry customers, since the shut down of captive air separation units, in such applications as computer chip manufacturing, has enormous cost impacts in terms of lost production and unusually high process start-up costs.
During the past two years, Xtreme®-seated valves have been used by the corn processing industry in numerous applications, pushing beyond the range of conventional high performance butterfly valves. Saturated steam is an example. Normally, soft-seated valves would only be used for saturated steam at pressures up to 150 psi. Wafer-Sphere® valve equipped with Xtreme seats have been providing bubble-tight shut-off for saturated steam at 300° F and 450° F for more than two years.
One of the fastest growing segments of corn processing is involved in manufacturing ethanol, an additive used widely to replace MTBE in gasoline. High performance butterfly valves are used extensively because of their long cycle life, smaller relative size and weight, and lower total cost.
One of the challenges sometimes faced by valves in this service is frequent thermal cycling over wide temperature ranges. In one application, a high performance butterfly valve was cycling CO2 at -40° F, followed by super-heated steam, which exited the boiler at 525° F at 220 psi. The valves began leaking shortly after installation. The only viable alternative appeared to be using a metal-seated product at the expense of bubble tight shutoff. However, since the super-heated steam's temperature was below 500° F when it reached the valve, the application was a candidate for an Xtreme-seated butterfly valve. The substitution was made and shut-off has been absolutely tight for more than a year. The need for a more expensive and ultimately less effective replacement valve was avoided.
Xtreme® seats, which are FDA approved in Wafer-Sphere® valves, are also moving into food processing applications. In addition to resisting extreme temperature, pressure and chemically aggressive media, these valves are specially engineered to eliminate cavities in which bacteria can accumulate. For example, a large food processor used a specially designed, Xtreme-seated butterfly valve to replace a large plug valve. The butterfly valve presented a smaller flow cavity and Xtreme material was used to fill small dead spaces to further reduce the opportunity for bacterial accumulation.
During the past three years, more than 50,000 Xtreme-seated valves have been shipped and installed. The vast majority is still in service and performing to high expectations. A significant number of these are being used in ANSI class 300 applications to prolong leak-free cycle life and reduce total costs.In addition, engineers in a wide range of industries (see table) are considering introduction of the product into a new range of ANSI 300 and ANSI 600 applications, where butterfly valves are used infrequently, if at all. Within the next three to five years, these substitutions will redefine the role of high performance butterfly valves in some of the most demanding industrial applications.
Improved seat recovery leads to pressure/temperature ratings beyond that of the competition and only achievable from metal or engineered plastics.