A COMPLETE GUIDE TO BUTTERFLY VALVES
Introduction
The butterfly valve, or rather the various types of valves that are collectively known as butterfly valves, will be discussed here as members of the family of throttling valves. Butterfly-type valves are indeed fully capable of flow control, and that ability will also be discussed. However, it will be apparent from even a casual survey of valve installations that the majority of butterfly valves are used as block valves only. It is also true that some butterfly valves are more suited for throttling than others are, just as some are more suited for severe services than others are. It happens to be that the less expensive valves are among the ones that are not that suitable for throttling services.
Table of Contents:
Main types of butterfly valves
There are two broad families of butterfly valves. Briefly, the difference is whether the stem is concentric with the centerline of the valve body or eccentric or slightly off-centre in two or three planes. This geometric difference is responsible for the differences in the way that various butterfly valves seat and shut off. These two groups overlap somewhat in rating and performance, but in the operational characteristics that really matter, such as pressure rating and shut-off ability, these two types of valve are quite different. Needless to say, because the two valves look quite alike (that is, they both are often very short face-to-face and often do not have separate flanges of their own, and they both have a disc that turns inside the pipe) and they share the butterfly valve name, they are often confused.
Both valves have certain characteristics that make them worthy of choice over, for instance, a gate valve. But both have certain very definite shortcomings, and not the same shortcomings for each type. To illustrate the point that confusion between these two has been rampant, a company that was one of the pioneers in the high-performance butterfly valve market spent 10 years and many advertising dollars trying to get their valve referred to as something other than a "butterfly" valve. They tried referring to it as a "trunnion" valve, which is an apt name but not very distinctive. They tried referring to it as an "eccentric disc" valve, which is more descriptive but longer. Other manufacturers put forth their own nomenclature, as well as a sizeable amount of effort to demonstrate the difference between the two valves, and sometimes the similarities, depending on their strategy at the time. At the time of this writing, it appears that the name "high-performance butterfly valve" is almost universally used for these valves. The other branch of the family is not "low-performance" but is generally referred to as "rubber lined" or "concentric" butterfly valves. Some of these, especially the ones that are not actually rubber lined, are quite special valves in their own right.
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Rubber-lined butterfly valves
To start with the low end first, the typical rubber-lined valve is made of cast iron or other fairly low-cost body material, with a bronze disc and steel or stainless steel stem, and a rubber lining. The lining is the key to the entire valve. The lining is one piece, and seals against the disc and against the mating flanges. The concept employed here is unique. In effect, the flanges that the valve mates against are used to hold the lining in place so the disc can seal against it more firmly. As a payoff for this assistance, the liner seals against the end flanges without requiring the normal gaskets that every other flange set requires. Surprisingly, if you try to use separate gaskets on a rubber lined butterfly valve, the flanges are more likely to leak than if no gaskets were used. This unusual situation has been proved time and time again, much to the surprise of individuals who had not encountered it before. The reason why gaskets do not work as well against the rubber lining is presumably because the liner is designed with ribs that seal with a high local seating force in a narrow area, and the addition of another soft gasket spreads the seating force over a broader area.
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Design and function
As for the design concepts of the rubber-lined valve, the liner also normally has a small moulded rib around the stem at top and bottom, or a tight-fitting area of greater compression to seal off leakage that would go up and down the stem surface, and sometimes an extra tight fit on the disc just next to its top and bottom surfaces next to the stem. Because the stem and disc are normally concentric to the body and the liner centre lines, the disc rotates around without any translational motion in the area at top and bottom. These areas of the liner are subject to wear from the rubbing of the disc, and closing the valve does not force the disc into the seat any tighter here than when it is open. Therefore, not much sealing takes place in these areas. If you examine a leaking rubber-lined valve, the areas at top and bottom are where you would normally look first to find evidence of wear or erosion. In contrast, the remaining 150 or so degrees of arc on either side of the stem penetration seal quite effectively because the liner is designed to be compressed radially outward as the disc closes and pushes into the seat.
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Installing the lining
It should be kept in mind that the rubber lining is normally held in place by the force from the mating flanges. There are two ways of installing the lining. One is by machining a lip, or dovetail, into the body below each seating surface and mechanically locking the seat in. This type of seat is forced into place during assembly by flexing the rubber. In some designs, the body is actually made in top and bottom halves and bolted together around the seat. Sometimes the seat is adhesively bonded to the metal body. The second basic assembly method uses a cartridge (usually phenolic or some other material that is rigid but somewhat less rigid than cast iron) that the rubber seat is bonded to, and then the whole affair is slipped into the iron body. This design is easier to repair, and because there is more resiliency in the seat-cartridge system, the seat is not as vulnerable to gross distortion or as likely to be pulled entirely out of the body, which of course would make the seat and the valve useless.
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Body styles
The butterfly valve is normally designed as a "through-bolting" body, either a wafer or lug (single- flange) design. These body styles do not actually have true flanges on the ends, and they both depend on bolting to pipe flanges on each end. In the wafer type, stud bolts or machine bolts are used, inserted through the bolt holes in one flange, past the exterior of the valve, and into the bolt holes of the opposite flange. The bolt length required is thus the length required for a standard flange-to-flange joint, plus the thickness of the valve. The lug type, which is also called single-flange because the valve body behaves somewhat like a flange, is attached to both flanges by cap screws that go through the bolt holes on one flange into tapped holes in the valve body. The same hole in the valve body receives two cap screws coming in from opposite ends. The cap screw length required is the thickness of one flange plus slightly less than half the thickness of the valve body, since the two cap screws must not interfere with each other. A different rarely used technique is to thread a stud bolt completely through the tapped holes in the body, long enough to extend through the bolt holes in each end flange, with a nut behind each flange.
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Wafer and lug styles - Differences
The essential difference between wafer valves and lug style valves is that the wafer type requires the presence of both flanges at all times in order to retain the valve in the line. The lug type, on the other hand, can have the piping on one side removed while the valve remains attached to the piping on the other side. This is valuable in locations where disassembly is required, such as cooling water piping to heat exchangers where the piping must be disassembled in order to remove the tube bundle.
Note that even in cases where the valve can be attached to piping on one side only, the valve is not necessarily capable of holding full line pressure when only one flange is present. Sometimes this is possible, sometimes not, depending on the valve design, size, and rating. If there is any doubt, the user should specify "dead-end shutoff" so the manufacturer can assure that the seat is capable of being adequately retained with only one flange in place. In larger size valves, the number of bolt holes and the size of the stem make it so that a few of the bolts cannot pass outside the valve body in the positions next to the top and (sometimes) bottom of the valve. In these cases, some quantity of cap screws will be required even though the valve body is basically a wafer type and through bolting is used elsewhere around the periphery of the body. Sometimes, for purposes of helping to orient the valve with respect to the flanges, a few of the bolt holes are drilled through the body or are indented into the side of the body. These designs still require through bolts and not cap screws.
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Double-flanged designs
There are several double-flanged designs also. Some larger size valves, from about 24 inches (DN 600) and up, are built as double flanged (with a separate true flange on each end, with the ability to use stud bolts on each joint) just for convenience and ease of bolt-up. They still have the same seat design that wraps around to form the end seal, at least up to moderately large sizes. In some cases, it is still possible to use through bolting where the valve body flanges do not really do anything but guide the bolts, but it is unusual to want to do this. The main reason for it is when there is insufficient room to install an extra set of nuts on the two sets of short studs that would be used for double flange bolting. Valves built to AWWA C504 standard for municipal water service are designed with end flanges even down to 3 inches in size (DN 75). These valves have flanged or mechanical joint or push-on ends just like gate valves, rather than the wafer or single-flange configuration. The resilient seat is retained by internal metal seat retainers, rather than being retained by the body seating surfaces.
Another exception is very large butterfly valves, ranging from 24 inches (DN 600) up to 84 inches (DN 2100), 108 inches (DN 2800), 144 inches (DN 3600) or even larger, depending on the capacity of the manufacturer. These valves are so large that the seat retainers are usually assembled in segments, although the resilient seat itself is normally in one piece, usually not moulded but made from an elastomer extrusion and fused into a continuous piece. Other valves where the seat is retained internally and not as part of the end connection are some very small butterfly valves (down to 1 inch in size (DN 25)) with bronze body and threaded ends, and a number of sanitary service butterfly valves with special clamp ends for easy disassembly. Here, the seat is retained in the body joint of a two-piece body that is itself capable of being taken apart for cleaning. Butterfly valves are also made with grooved coupling ends and sanitary connector ends, and are even available in oval body models for duct work. The primary markets for resilient-seated valves involve applications related to water and air, meaning utility water services, HVAC, and other low pressure, low-severity services where there are lots of valves and where, if the valves leak a little, it is not too serious. Although this is a blunt assessment of the market, it is the one that is heard frequently from valve users. It is true that rubber-lined valves will shut off tightly, but they will not shut off tightly over a period of many years or under severe conditions (sand in the lines, higher pressures), unless great care is exercised. Great care involves installing the valves exactly according to the manufacturer's instructions so that the rubber lining is not under undue strains and that the seating surfaces are properly loaded, and generally that the valves are not operated too often or left to sit in one position for too long.
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Materials
The most common body material is cast iron since by design the body itself is not under a lot of force from the adjacent piping and the liner theoretically protects the body from the line fluid. Other body materials are occasionally available, including ductile iron and steel. Aluminium bronze is often used where the external corrosion problem is severe, such as in marine environments or brackish water services. It is also common to apply a highly resistant paint system to the body, such as a multipart epoxy coating, or to use a chrome or nickel plating on the exterior of the body, also for corrosion resistance or for cleanliness.
The disc is most commonly aluminium bronze for water services, but it can be ductile iron with a plated or electroless metal-coated surface, or occasionally stainless steel. The stem is steel if no corrosion resistance is required, such as in air service in HVAC lines, or more commonly a 300 series or 400 series stainless steel. The top of the stem is normally machined with two or four flats, and the operator is generally a lever handle where torque permits. In larger sizes, a worm gear drive with a hand wheel is usually supplied and of course, any rotary actuator that can be set for 90-degree rotation can be used. Normally the stem extends through the disc, and the disc is pinned to the stem with taper pins going through the stem or with cap screws going through the stem and into the disc on the other side, or with cap screws and nuts going through the stem and out the other side, or occasionally with cap screws going on to a flat in the stem. It is also possible to build butterfly valves in an all-plastic design, usually in PVC, with a plastic body and disc, standard rubber lining, and coated-steel stem.
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Concentric disc valves for corrosive services
Some concentric disc valves are not rubber lined at all but are designed for more corrosive services with a polymer lining material such as PTFE or PFA or polypropylene. These valves generally have to have an elastomer backup behind the more rigid plastic lining, for resiliency in order to get good sealing. Many of these valves have either a stainless disc or a disc coated with the same material as the lining. These valves normally have to have a two-piece body, since the lining is not resilient enough to be assembled into a one-piece body. Since the body has to be assembled around the disc and stem and lining anyway, it is then feasible to make the disc and stem one piece, which eliminates the area between disc and stem as a site for corrosive attacks. The two-piece body also permits the disc to be made thinner, since the shaft does not have to pass through the centre of the disc. This makes this valve even more attractive in the smaller sizes. The split in the body is at the horizontal centerline, usually with one cap screw on each side to hold the top and bottom halves of the body together.
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Other types of butterfly valves
Some rubber-lined valves are not truly concentric disc valves. Some types have the disc somewhat offset, so there is some extra force applied to the liner in the areas near the top and bottom to get better sealing in those areas. Many butterfly valves are built for duct work in HVAC services. Some are just standard rubber-lined butterfly valves such as would be used in process water services. In fact, chilled-water services in a building have many of the same requirements as cooling water in a process plant. However, many valves that are marketed purely for building or commercial services, since the governing codes and standards are less strict, are a simplified or lightened version of the valve used in process service. Often the pressure rating of the valve is reduced by using a lighter disc and stem. The seat compound used is likely to be nitrile rubber, such as Buna-N since it is low cost and no particular chemical resistance is required.
Other valves used in duct work generally look quite different from rubber-lined butterfly valves, often being fabricated of light steel or cast iron or aluminium, and built to the contour of the duct. Most of these valves are rectangular or square, and many do not have the ability to shut off tightly. Some valves used in duct work that passes through fire walls are required to shut off sufficiently to prevent the spread of a fire. Most of these valves are actuated and designed to close down the air supply system in a building when a fire is detected, in the same manner as emergency shutdown valves in a process plant.
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Pressure ratings
Most concentric disc or rubber-lined valves have a design pressure rating somewhat lower than that of the adjacent piping. Because most of these valves do not have true flanges, the rating is determined by the ability of the liner to shut off pressure. Many valves are rated at 150 lb, but this is not ANSI class 150 (PN 16) rating. Normally in a butterfly valve, this means exactly 150 psig at I 100°F (7 bar at 38°C). Various valves made by the same manufacturer will be rated at anywhere from 100 to 250 psi (7 to 17 bar), and there are some (not many) that are rated at 285 psi (20 bar), which is full ANSI class 150. There is not as much margin for overpressure in a rubber-lined butterfly valve as there is in some other valves. While most metal-seated valves can be expected to take 150 percent of rated pressure in the closed position, the most that the rubber-lined butterfly valve is required to be tested to is 110 percent of rated pressure. This is reflective of the actual performance of these valves.
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The other half of the butterfly valve family is the high-performance valve. Here, again, there are several variations, but the characteristic features of the standard high-performance butterfly valve are a stem offset from the body centerline, a relatively thick disc with a seating surface contoured like a spherical section, and a body with removable end piece to hold a seat that contains a circular spring.
A high-performance butterfly valve closes by pushing the disc into the seat, in the same way as the rubber-lined butterfly valve does, but instead of compressing the seat by line contact, the high- performance valve wedges into the seat and spreads it uniformly 360 degrees around. The spherical seating surface is centred at a point somewhat behind the disc centerline so that in the open position no part of the disc is in contact with the seat. This is the principal difference between the two valve types. In the rubber-lined type, the pressure force does not contribute much to the sealing quality; in fact, it is more likely to work against good sealing. But in the high-performance type, the pressure force is employed to push the seat more tightly into the disc, at least in one direction. By geometry, the best sealing force is obtained when pressure is coming from the side with the seat retainer toward the disc, opposite the side of the disc that the shaft is on. The pressure force does act on the disc also, to push it away from the seat, but that movement in a well-designed valve is minute compared to the movement of the seat.
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Seat & Seal
The seat in a standard high-performance valve is a semirigid fluorocarbon polymer such as PTFE or PFA, and since a fair degree of deformation is required during seating (as opposed to a ball valve, where the ball is forced directly onto the seat), some means of providing resiliency is required. This is normally provided by an elastomer backup, enclosed by the polymer so as to protect it from direct contact with the line medium, or by a spring enclosed by the seat material. The spring can be a flat-type or a coiled toroidal spring. In virtually any valve intended for manual block service, some capacity for bidirectional sealing must be provided for. In a butterfly valve, because the same seat must seal in both directions, the concept of forcing the seat against a solid backup the way a ball valve seals is not practical and the spring action is one of the two mechanisms involved in the sealing act. The other is the geometry of the seat itself, in relation to the pressure force. By careful design, it is possible to configure the seat such that a pressure force operating on it in either direction pushes it into the disc surface. Of course, sealing is almost always better in one direction than the other. Very few valves are really 100 percent bidirectional, in that the leak rate is just slightly less in one direction than the other. But in many cases, the leak rate with the pressure force coming from the stem side can be almost as low as the leak rate with the pressure force coming from the disc side.
As with a ball valve, it is necessary to prevent leakage behind the seat, but generally, this can be accomplished in a high-performance butterfly valve just by extending the seat material back a distance into the body. Like a ball valve, the high-performance butterfly valve can be made fire-safe. The simplest way is to provide a metal lip such that when the seat is melted out, the metal lip will provide some sealing. However, since the disc cannot move farther down into the body, the metal backup seal is also designed as a spring. Normally, in soft-seated valves with fire-safe backup, this metal lip is mounted behind the soft seat so that it is helping to push on it. Butterfly valves can be built for high-temperature or abrasive service with a metal-only seat, using basically the same design. Generally speaking, the all-metal seat is less capable of tight shutoff than a soft seat is, except with very special designs.
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Applications
Although butterfly valves can be and are used for abrasive or erosive services, such applications are rare and require special design. In general, the high-performance valve doesn’t stand up as well to abrasive material in the fluid as the rubber-lined valve does because the rubber tends to deform around small particles caught in the sealing area and to bounce particles off rather than suffer scratches as a metal surface does. However, for both valves, the damage caused by unavoidable abrasion such as sand in water can be minimised. If the valve is oriented with the stem horizontal, rather than the more obvious vertical position, and the particles tend to hug the bottom of the flow stream, then as the valve begins to close the local flow velocity near the seat tends to increase. This washes out particles rather than settle them near the seat. In the vertical position, these particles would tend to congregate near the lower stem and bearing area, where there is little disc movement to dislodge them and where they could drift downward between the stem and its bearing surfaces. The disadvantage to this orientation is that normally there is more room for stem and operator above a group of pipelines than alongside, especially between two lines. However, the savings in life extension of valves are such that it is worthwhile to make room for horizontal orientation wherever the presence of solid particles is known.
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High-performance butterfly valves tend to be available in a wide selection of materials, including stainless steels and high alloys, but the most typical is a steel body (either cast or cut from thick plate), with a stainless disc or a steel disc with electroless metal coating, a stem made of precipitation-hardened steel such as l7-4PH for greater strength in torque, seat springs of Inconel 718 or similar high-strength heat resisting material, and stem bearings top and bottom. Many high-performance valves have either a packing gland at both top and bottom or at least some means of accessing the bottom stem bearing from outside. The disc attachment mechanism is normally either taper pins or bolts. Often the stem runs completely through the disc, and to withstand the seating torque the stem and disc are somewhat larger and heavier than in the rubber-lined valve. It is possible to provide steam jacketing around the exterior of the high-performance valve, and even possible to bore through the stem and pipe steam or purge gas up the stem and through the inside of the disc.
A high-performance butterfly valve does the same job as a gate valve with significantly less metal mass, so in high alloys, there is often a considerable cost saving in using a high-performance butterfly valve. In carbon steel, the difference is less pronounced. Speaking in general terms, the butterfly valve is more expensive than the gate one in sizes up to 20 inches to 24 inches pipe size (DN 500 to DN 600) in carbon steel, while in stainless steel the size break is about 6 inches or 8 inches pipe size (DN 150 or DN 200). If both valves can be considered to do the job equally well, the price break can be used to determine which to use. However, there are also other factors to be considered.
One factor is that the gate valve, being metal seated, is basically intrinsically fire-safe. Both valves leak, as all valves do to some extent, and if the seat is damaged, generally no amount of force will get either valve to shut off. However, the butterfly valve is more likely to shut off bubble-tight in normal use. The butterfly valve is certainly smaller and it weighs less than the gate valve, which can be very significant in locations like ships and offshore platform installations. The butterfly valve closes faster, which makes it better both for control applications and garden-variety block valves if ease and speed of shutoff are important. The butterfly valve does have the ability to throttle. Generally, throttling butterfly valves have a slightly different design than on-off butterfly valves, but any butterfly valve will throttle better than any gate valve can.
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Installation
Since the two ends of the high-performance valve are not identical, it does naturally seat better in one direction than the other. Early high-performance valves were marketed as one-directional only, and some older designs are still primarily capable of seating in only one direction. Most modern designs will seal equally well in either direction, especially at lower pressures, but where there is a choice the valve should be installed with the stem end downstream, that is, with the blank side of the disc facing the flow. The thick disc does not deform significantly from pressure force, but with the blank side of the disc facing the flow, the fluid force helps to push the seat more tightly against the disc while in the opposite direction the fluid force tries to oppose the sealing force. Metal-seated valves are more decidedly one-directional than are soft seated valves.
Most high-performance valves have a travel stop located just behind the seat on one side to keep the disc from over travelling and lifting back off the seat. The travel stop is not possible with rubber-lined valves. The actuator normally has some type of stop in addition, usually a lever-lock device in the case of a handle operator. Generally, the seat end is clearly marked on the outside of the body to indicate which way the valve should be installed.
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Assembly & Maintenance
As with an end-entry ball valve, access to the seat for assembly and maintenance is through the end of the valve. Unlike those in some ball valves, this end piece almost always comprises the complete gasket seating surface on one end of the valve. This end piece is normally retained by set screws or cap screws driven into its face. When the screws are toward the outside of the seating surface, normally no disruption of gasket seating occurs. Some designs have the screws more toward the inner edge of the seating surface, sometimes directly in conflict with the primary seating area. Spiral-wound gaskets, especially, seal over only a narrow band toward the inner edge of the flange face, and disruptions in the seating surface such as screw holes are very injurious to the ability of spiral-wound gaskets to seal. This feature is being phased out by many manufacturers of high-performance valves, but if there is any possibility of a problem the user should still specify the cases when spiral-wound gaskets will be used.
The end piece also provides an opportunity for leakage to the outside if the design has the end piece extending completely to the outside (which is better for end flange sealing purposes). This requires an additional gasket just as any other valve with a body joint does, although often the seat itself can act to seal off this leak path. For services in which there must be as few leak paths as possible, there are some valve designs in which the end piece joint does not lead to the outside of the valve. These are generally special order designs.
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Offset
An improved version of the high-performance valve has the stem offset not just toward one end of the body, but offset slightly (about 1/10 inches; 1 to 2 mm) to one side also. In addition, the centre of the disc is offset slightly downward from the vertical centre of the stem. This triple-offset design, which goes by various proprietary names, has an even better shutoff ability than the standard high-performance butterfly valve does. This valve is often capable of sealing bubble-tight in very large sizes. As a rule, sealing ability drops as valves get larger, mostly as a result of accumulated manufacturing tolerances, but this valve is the exception. The sealing area in this valve often consists of laminated metal and soft-seat components much like a spiral-wound gasket and is located in the edge of the disc. These valves can also be built with a significantly offset stem with much the same seat geometry as a tilting-disc check valve, and the stem can be arranged to allow the valve to behave like a globe-type stop-check valve. These valves are even available with refractory lining for service at very high temperatures, such as steel mill hot gas lines.
Valves of this type are generally fabricated steel, although cast bodies are becoming more common, and this type is of course somewhat more expensive than a standard high-performance butterfly valve, but it does deliver even better performance, where such performance is needed.
Because this valve often has an even thicker disc and longer seat area than a more conventional high-performance valve, it is often built as a double-flanged design with overall length quite a bit longer than the common butterfly valve dimensions. Other special application butterfly valves are often built as double flanged, generally in the larger sizes. There are also butterfly valves, designed primarily for cryogenic service, that have butt-weld ends and a bonnet for accessing the internals from the top. This is a very rare configuration.
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History
The butterfly valve concept is a fairly common one. The choke in a carburettor, for example, is a butterfly valve. Its application for industrial piping, though, began only in the 1930s. For years one of the main applications of butterfly valves was in oil field use. Eventually, their use spread to a number of industries, but as the various types of butterfly valves appeared the attempts that were made at standardisation were not very successful. For a long time the butterfly valve was one of the least standardised of all valves in the fluid-processing industries. There was a standard for flanged waterworks valves, and then later a standard for rubber-lined lug and wafer valves. Finally, in 1982, the high-performance valve joined the world of standardisation. It is still possible to buy valves that are not standard end to end to replace valves built prior to standardisation, but it may not be so for many more years.
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There are two types of actuated high-performance butterfly valves, just as there are major differences between globe valves and globe-style control valves. Many butterfly valves are built for remote operation in the shutoff or block mode, and these are generally standard high-performance butterfly valve types. Almost all high-performance valves are already equipped with bracket mountings for actuators. The actuator sizing needs some consideration because the disc and the flow stream interact aerodynamically in ways that might not be expected. For instance, the greatest dynamic torque occurs when the valve is near full open (actually, at about 80 degrees) because this region is where the cross-sectional flow area is varied the most by disc movement. Seating torque occurs at the other end of travel, at 5 degrees of opening and below.
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Throttling valves
Butterfly valves used purely for throttling can be standard unmodified butterfly valves, but generally, some design differences are found. One common feature is a more secure disc-to-stem attachment such as a spline, or sometimes a one-piece disc stem (which requires a two-piece body), to reduce or eliminate mechanical hysteresis. Since throttling butterfly valves are often not required to shut off, some are built with no true seat at all. The best these valves can do is to approach 5 percent or so of maximum flow in the closed position. Eliminating the seat makes the valve less expensive, and throttling with an extreme turn-down ratio (very small percentages of maximum flow) is erratic at best unless the valve and the actuator are specifically designed to accommodate such requirements. Valves of this type may be intended to operate in the region of 40 percent to 90 percent of maximum flow rate.
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Valves for modulating services
Some valves that are built specifically for modulating service discard entirely any attempt to seal in both directions, since control valves are rarely expected to see reverse flow, and are built with an asymmetrical disc that interacts better with the flow stream. These designs have a wider disc edge on the back side of the disc - the side that opens downstream. The purpose of the wider disc is to eliminate some of the hydrodynamic forces on the trailing edge of the disc, thus reducing the torque required to operate the valve, and in some cases raising the C, enough to permit the use of a smaller valve. In many cases, standard valves no longer change the flow rate effectively after they open to about 60 degrees (two-thirds of the travel from closed to full open). Use of the wider disc edge extends this control closer to 90 degrees, which also makes the actuator capable of more precise regulation since it then can operate over a longer stroke. This design is still capable of sealing against a standard seat. Some other valves can be furnished without any seat, and act purely as a throttling valve with no capability at all of shutoff. This is perfectly adequate in many situations such as flow balancing. Other types that do not completely seat can be built with a reverse-tapered disc edge to help cut through deposits that accumulate near the seat area.
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"Three-way butterfly valves"
It is even possible to build a three-way butterfly valve. Perhaps the term "three-way butterfly" is a misnomer since no one butterfly valve can actually be capable of possessing three ports. This term is usually applied to actuated butterfly valves since the configuration includes two separate valves mounted on two sides of a tee. There is a common linkage that connects the levers of each valve such that as one closes, it opens the other one. This does permit flow splitting or mixing or diverting. Each of the different position combinations is possible, some requiring more complicated linkage than others. The whole point in doing this is allowing one actuator to operate both valves as if they were a single valve.
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