Butterfly Valve Design Features

This post is initiated to give some basic information regarding butterfly valve design features, it uses, standards, types, advantages, disadvantages etc.

Butterfly Valves

A butterfly valve is a rotary valve that uses a disc as the closure member. The disc shaped closure member is rotated through 90⁰ or there about to open or close the passage. The disc closes against a ring seal to close off the flow. The other main parts are the valve body, the stem/ shaft and the valve actuator.

Different types of valves are used in flow control, for a variety of reasons, such as phase (liquid or gases), pressure, piping restrictions and solids content. Other Valves are chosen for their capability to open and close in a quarter turn. Of all the valve types, the butterfly valve is used as a control device for many reasons including some or all of the above. They evolved from the shutter like damper that was initially in use in applications where no tight shutoff was required, but rather served as a flow restriction.

Butterfly valves are the most specified valve type for moderate-duty conditions in pipe sizes of two inches and larger. Their reduced weight, reliable bubble-tight shut-off, ease of operation and automation allowed them to displace gate valves in traditional applications. They can be installed where there are space limitations and in very large diameters. They are widely used for the control and shutoff of gases and liquids

Butterfly valves are normally of wafer design, fitting directly between piping flanges. Butterfly valves can be either symmetric or eccentric (i.e., the stem is offset from the center of the disc). The flanged valve has a very short body and is flanged at both ends. The lug type has a shortened body with protruding lugs whose bolt circle matches the adjoining flanges. The wafer has no lugs.

Today, butterfly valves have proven their endurance and dependability in a wide range of industries and applications. They are available in line sizes from 2″ to over 72″, temperature ratings from cryogenic to 1500^o F, and pressures to 1440 psi. Butterfly valves are used in isolation and control services, and with media such as slurries, steam, gases and liquids. Continuing development of the butterfly valve is making it possible to use them in aggressive applications that previously were the exclusive domain of gate and globe valves.

Some of the notable applications include pulp stock, corn processing slurries, tertiary petroleum recovery, high pressure water, high cycle air separation services (both at ambient and cryogenic temperatures), LNG and commercial HVAC to name only a few.

With few exceptions, high-performance butterfly valves provide tight shutoff, a long life cycle, and life cycle costs that are lower than any other alternative. Large numbers of butterfly valves are being used today to replace both globe and gate valves, which were first specified decades before the development of the high-performance butterfly valve. Butterfly valves also may be a more cost-effective replacement for ball valves. The total presence of butterfly valves will continue to grow in comparison to other valve types as they make their way into more industrial shut-off and control applications.

Initially, the limited capabilities of elastomers restricted the applications for butterfly valves. However, developments in synthetic elastomer technology vastly increased the scope of applications. For example, new compounds and formulations feature increased cycle life, expanded chemical compatibility and extended temperature limitations at both extremes. PTFE and metal-seated butterfly valves are available for those applications that are limited by the elastomer seating.

Best Suited Control: 

Linear, Equal percentage

Recommended Uses:

• Fully open/closed or throttling services
• Frequent operation
• Minimal fluid trapping in line

Applications:

• Liquids, gases, slurries, liquids with suspended solids

Advantages:

• Low cost and maintenance
• High capacity
• Low pressure drop

Disadvantages:

• High torque required for control
• Prone to cavitation at lower flows
• Good flow control

Installing the valve

The method of mating with the pipeline characterizes the butterfly valve body. Valves that are trapped between mating flanges and long studs are called wafer valves. Neither the wafer valve nor the mating flanges can be loosened before draining the system. This can be a problem if the valve is intended to isolate a piece of equipment that requires servicing.

Lug valves have bosses with tapped holes. Bolts are inserted through the flanges and threaded into the bosses. Either mating flange of a lug valve can be removed independently. Lug valves are preferred for connection to flanged equipment.

Grooved valves also allow equipment to be removed for servicing. The primary caveat when specifying valves for attachment to equipment that will need servicing is that the valve must be able to retain its liner when a mating flange is disconnected. Grooved valves, by definition, provide a bi-directional installation capability. Valves with slip-in liners need to have an uncomplicated method for liner retention.

Environmental factors and selection of valves

External corrosion can be controlled through selection of corrosion resistant body materials or coatings. Another external factor is the pipeline insulation. The neck on the valve needs to be long enough for the operator to clear the insulation on the mating flanges.

The shaft or stem in a butterfly is the structural element that unites the disc and body. It also provides the means for applying torque to position the disc. The connections between the stem and the operator and between the stem and the disc are crucial elements in the selection of a valve. A build-up of tolerances in these connections causes the disc movement to lag the operator movement. This prevents precise positioning of the disc or it may allow the disc to “flutter” into premature failure. Precisely machined fits between the stem and disc are important.

Valve operators run the gamut from simple levers for manually operating the valve to powered and remotely controlled actuators used for flow modulation. General design considerations need to focus on the application, the potential for future upgrades, and the external environment. Selection of a valve manufacturer that provides solutions for the needs of today and tomorrow is an important part of the selection process.

Usage

Butterfly valves are generally used in low pressure system applications where leakage is relatively unimportant. They are normally used in large diameter pipelines.

Advantages of Butterfly Valves

Butterfly valves are the fastest growing segment of the total valve market today because of their numerous advantages, which include:

1. Tight Shutoff – Offset shaft and eccentric disc arrangements combined with modern single piece, flexible-lip, polymeric seats provide bubble-tight shutoff over a wide range of operating conditions.
2. Reduced Torque Requirements – The rotary design of butterfly valves and minimal wear surfaces dramatically reduces torque requirements. A quarter turn is sufficient. Therefore, they can be operated with smaller, less expensive actuators.
3. Reduced pressure drop – These valves have a very low pressure drop and are of relatively light weight construction. The diameter of the valve can be of the same size as the pipe.
4. Lightweight/Compact – Because the butterfly valve has a significantly narrower face-to-face dimension and a shorter center line to the top of the valve profile, it uses less metal than a gate valve. The result is significant weight and size reduction for the same or higher rating. These factors also have a positive impact on piping and plant design. Significantly lower weight, particularly in the larger sizes, means lower piping stress and reduces the number and/or size of the pipe supports. It also results in less load transmitted to the structural steel. Finally, smaller size and lower weight can translate into significantly fewer man-hours to install valves, especially for larger sizes.
5. Extended Cycle Life – The double eccentric disc and shaft, in combination with polymeric seats, reduces seat wear and dramatically increases leak-free life cycles.
6. Ease of Automation – Butterfly valves are easily automated. Every valve is drilled and tapped to accept linkage 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. And because butterfly valves are quarter-turn, the actuated valve profile is much smaller than linear valves.
7. Wide Temperature Range – High-performance, flexible-lip seals extend the operating temperature range of butterfly valves.
8. Ease of Maintenance – Compared to many valve types, butterfly valves are easy to maintain. In one design, the insert can be removed with a screw driver and the self-aligning seat replaced. All this can be done on site and the valve can be put back into the line immediately. No machining is required as with gate valves.
9. Reduced Emissions – Stem leakage is a routine 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. The packing gland must be frequently tightened, and in some cases grease injection is performed, but this rarely results in completely eliminating stem leakage. This is a major concern today as clean air emission requirements must be met worldwide. In contrast, the butterfly valve shaft rotates only 90⁰ 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 applications where more stringent emissions control is required, butterfly valves can be equipped with a spring-loaded packing arrangement.
10. Lower Costs – Butterfly valves are less expensive than most other valve types. With actuation, the total cost ratio of the valve package is reduced even further, especially in larger sizes. Ease of maintenance, actuation and installation can also dramatically reduce total life cycle costs.
11. Control Capabilities – The butterfly valve offers many advantages that include openness for less plugging and good control capabilities.

Disadvantages

The primary drawback of a butterfly valve is that the disc and shaft are situated in the waterway. Therefore, butterfly valves are inappropriate designs when full flow is required, or when a device (i.e., a pig) will be periodically used to clean the lines. The high velocity flow also damages the seals.

Highly abrasive media also present a problem for butterfly valves because they erode the disc. The operation and closure of the disc in a butterfly valve may also be impeded in very thick media such as slurries. In these instances, a ball or knife gate valve may provide more cost-effective performance.

For control applications, butterfly valves may not always provide an appropriate flow characterization for the control scheme. In these instances, other types of valves, although more expensive, may be used for improved control repeatability.

The leakage in a butterfly valve is fairly high unless special seals are used.

These valves usually require high actuation forces and are therefore usually used in low pressure lines.

Applications

Their lightweight, positive shut-off and ease of operation make them the valve of choice for applications that involves low- to medium-pressure and temperature.

Fire protection systems have thoroughly embraced the use of wafer, lug and grooved-end butterfly valves. The main advantages are bubble-tight shut-off for system testing and the ability to incorporate electrically monitored tamper switches to ensure the integrity of the system. Butterfly valves are not favored in the underground portion of the system or if wall or upright indicator posts are required.

Larger, in-water distribution systems frequently use butterfly valves in the hot and cold water distribution lines. CF8M stainless steel discs provide the greatest assurance of long, trouble-free service. Aluminum-bronze may also provide excellent service if the alloy has sufficient nickel to prevent corrosion. Rubber coated discs, used in grooved valves, generally have a sufficient thickness of rubber coating to isolate the disc from the fluid, thereby preventing galvanic corrosion.

Remote control of butterfly valves is also possible. A mechanical linkage that opens one valve and closes another produces a three-way valve assembly. Specify lug or grooved-end valves for equipment isolation.

Butterfly valves can also handle “dirty” water applications. EPDM liners are satisfactory if the waste stream contains no petroleum products. Otherwise, nitrile is an excellent choice.

Processes involving flammable fluids benefit from the use of “fire-safe” valves designed to retain their internal and external integrity. Valves that meet the requirements of API Standard 607 4th Edition–Fire Test for Soft-Seated Quarter-Turn Valves–provide a good initial basis for selection.

Design Elements

The butterfly valve design elements consist of only four main components: body, disk, stem/ shaft and seat. Actuator selection is also an important factor. Within each of these elements are the variations of material and designs that adapt the butterfly valve to the intended service. Understanding the importance and features of the design elements is key to matching the butterfly valve to the intended service.

Body

Butterfly valves generally have bodies that fit between two pipe flanges. The most common body designs are lug and wafer. The lug body has protruding lugs that provide boltholes matching those in the pipe flange. A wafer body does not have protruding lugs. The wafer valve is sandwiched between the pipe flanges, and the flange bolts surround the body.

Each type of body has advantages, some of which are listed:

• The wafer style is less expensive than a lug style.
• Wafer designs do not transfer the weight of the piping system directly through the valve body.
• A lug body allows dead-end service or removal of downstream piping.

There are many advantages offered by butterfly valves compared to other types of valves.

• Inherently simple
• Economic design that consists of fewer parts, which makes butterfly valves easy to repair and maintain.
• The wafer-shaped body and relatively light weight offer a savings in the initial cost of the valve and installation costs–in person-hours, equipment and piping support.

Rotating disk

The flow closure member of a butterfly valve is the disk. A butterfly valve is a flow control device that incorporates a rotational disk to control the flowing media in a process. The disk is always in the passageway, but because it is relatively thin, it offers little resistance to flow. The disk is the equivalent of a plug in a plug valve, gate in a gate valve or a ball in a ball valve. Rotating the disk one-quarter turn or 90° opens and closes the butterfly valve.

Valve discs are made of ductile iron, stainless steel, or aluminum-bronze. Ductile iron discs are nickel plated to provide a corrosion resistant edge. Care must be taken to match the disc to the fluid and the piping. Galvanic corrosion can become a problem if the disc is less noble than the pipe. The proper selection of disc material or disc coating mitigates the problem of galvanic corrosion.

Discs for installation in grooved piping systems have the rubber seat molded into the disc to increase the flow area when the disc is open. In addition, the rubber covering provides excellent abrasion resistance. A rubber-coated disc in a rubber-seated butterfly valve achieves the same degree of abrasion resistance.

Many variations of the disk design have evolved relative to the orientation of the disk and stem in an attempt to improve flow, sealing and/or operating torque.

Modern, high-performance butterfly valves frequently have a double-eccentric design. First, the sealing plane of the disc is offset from the axis of rotation. This provides an uninterrupted circular sealing surface on the disc that makes it possible for a circular sealing element to be placed in the valve. It can be easily removed from the valve without disassembly of the shaft/disc closure elements.

Second, the axis of rotation of the disc is laterally displaced from the true center of the disc so that it will “cam” away from the seat to eliminate jamming or squeezing as the valve is opened and closed. This design eliminates wear points around the disc at the top and bottom of the seat. When closing, the disc cams tightly into its seat to create a bubble-tight seal with consistent torque. This eccentric rotation has a tremendous impact of extending the duration of the valve’s leak-free performance.

The stem axis on discs for use with solid PTFE and metal seats is shifted away from the seat so the stem axis does not penetrate the disc and body seat. This configuration is called an offset disc. Hydrodynamic forces and the forces that offsetting the stem axis introduce increase the torque needed during seating and unseating.

Butterfly valve technology has evolved dramatically over the past half century, as has its industry popularity. This popularity can be partly attributed to the quarter-turn operation, tight shutoff and its availability in a variety of materials of construction.

Early use of butterfly valves focused on water applications, but new designs and component materials have allowed them to be utilized in growing industrial fluid applications. Presently, butterfly valves can be found in almost every chemical plant handling a variety of diverse fluids.

Butterfly valves range in size from 2 in to more than 200 in and most have a pressure capability of 150-psi to 740-psi cold working pressure. The general temperature rating for a resilient seated valve is 25°F to 300°F and 400°F to 450°F for a high-performance butterfly valve.

Stem/ Shaft

The stem of the butterfly valve may be a one-piece shaft or a two-piece (split-stem) design.

The stem in most resilient seated designs is protected from the media, thus allowing an efficient selection of material with respect to cost and mechanical properties.

In high-performance designs, the stems are in contact with the media and, therefore, must be compatible, as well as provide the required strength for seating and unseating the disk from the seat.

The stem axis on discs for use with solid PTFE and metal seats is shifted away from the seat so the stem axis does not penetrate the disc and body seat. This configuration is called an offset disc. Hydrodynamic forces and the forces that offsetting the stem axis introduce increase the torque needed during seating and unseating.

Seat

Since the disc in a butterfly valve is fully supported by the shaft and its bearings, the seat is required only to perform a sealing function, not a strong supporting function, as is common with most ball valve designs. There are many seat designs.

Metal seats, which are more popular in Europe, provide consistent, long-lasting shutoff, but they are not considered bubble-tight. These metal seats allow a butterfly valve to be used in even higher temperatures to 1,000°F. Fire-safe designs are offered that provide the shutoff of a polymer seat valve before a fire, and the metal seal backup provides shutoff during and after a fire. Services which require 100% tight shutoff must rely on soft seats.

Conventional “jam” seats are non-flexing designs that use mechanical devices, such as O-rings, braided cable reinforcements or metal springs within to deform the material into contact with the disc. These types of seats do not compensate well for wear or thermal differences within the valve. They also tend to lose their sealing performance as line pressures increase.

The seat of a resilient-seat butterfly valve utilizes an interference fit between the disk edge and the seat to provide shutoff. The material of the seat can be made from many different elastomers or polymers. The seat may be bonded to the body or it may be pressed or locked in.

In high-performance butterfly valves, the shutoff may be provided by an interference-fit seat design or a line-energized seat design, where the pressure in the pipeline is used to increase the interference between the seat and disk edge. The most common seat material is poly-tetra-fluoro-ethylene (PTFE) or reinforced PTFE (RTFE) because of the wider range of compatibility and temperature range.

One-piece, flexible-lip, polymeric seats (typically PTFE) do not rely on metal back-up springs or O-rings for flexing. Therefore, they can be exposed to a wide range of temperatures and corrosive media. This design consists of a flexible lip that is pressure energized to move against the outer edge of the disc, which is a spherical segment, to create a bi-directional seal. The body and insert hold the seat in position and shield it from flow, which protects it from abrasion and erosion as well as fold-over in high velocity applications.

The key to successful PTFE seat design is to overcome the material’s tendency to cold flow and lose its shape under a compressive load. With the proper seat geometry, PTFE actually has a broad elastic range and resists cold flowing at compressive load levels up to 10 times the sealing stress required for ANSI class 150 and 300 applications. The single-piece seat geometry provides for thick cross sections throughout the seat, pre-compression of the seat for low pressure sealing and clearances surrounding the seat to allow flexibility.

For many severe process applications, butterfly valves are offered with composite polymeric/metal seats. These designs employ the pressure-energized flexible lip seating in combination with a metal carrier that offers secondary sealing in the event of a fire, compensation for thermal cycling in cryogenic applications, or serves as a protector with media that tends to plate or cake on the seat.

Butterfly valves are frequently supplied with specially designed seats to solve problems in a wide range of industries and applications — from coal gasification where the combination of very low differential pressure, solid particles and high pressures are present, to fluid catalytic cracking (FCC) applications where solids, abrasion, fines and high temperatures are all problems, to extremely critical delayed coker switching device applications where media build up in the valves can cause a process to shut down.

The most common way to form a resilient seat is by lining the interior of the valve body with an elastomer. The lining serves three purposes:

• It forms the actual sealing surface.
• It isolates the body material from the fluid in the pipeline.
• It forms a seal against the face of the mating flange. Alternatively, the elastomeric seat can be molded and vulcanized directly into the body. These valves are typically called rubber-lined valves. This type of seat cannot be repaired; one replaces the entire valve body. Typical liner elastomers are EPDM (ethylene-propylene-diene monomer) and nitrile. Other less commonly available liners are fluorocarbon for use in chemically aggressive environments, and food grade versions of nitrile and EPDM. For more demanding applications, the elastomeric liner is, in turn, lined with a PTFE layer.
• The butterfly disc provides the structural integrity to resist the hydrostatic shut-off loads and operating torque. The disc also acts as the moveable sealing member. In the elastomer-lined butterfly valve, the axis of the valve stem passes through the center of the disc and the axis lies in the plane of the seating edge. This means that the stem passes through the seating edge of the disc. Consequently, special contours machined into the disc provide a sealing surface. The care given to machining and dressing this area is one of the determining factors in the longevity of a valve.
• The lining may be removable and repairable, in which case it is integrally molded to a phenolic backing ring that imparts structural integrity to the elastomer. This molded unit is then assembled into the valve body.

Seat Material Options

Over the years, polymeric seat materials have evolved. New materials are evaluated routinely. Today, many alternative materials are readily available to satisfy even the most demanding process requirements.

1. PTFE – The basic butterfly valve seat made of virgin PTFE provides both a wide operational temperature range (-100^o F to 400^o F) and chemical compatibility to fill the widest possible range of service applications.
2. Filled PTFE – The most frequently used material is a “modified” or “filled” version of PTFE. This material has less corrosion resistance than PTFE but can operate at higher temperatures and pressures. The mixture also provides better abrasion resistance. It has, therefore, become a standard against which other types of seat materials are compared. All other material alternatives are measured by their improvements – or lack thereof – over the performance of modified PTFE.
3. Composite Metal/Polymer-Fire-Tite® – 316 Stainless, Alloy 20 or Monel /PTFE or other polymer composite seats are used for fire safe or applications where coking or abrasive media is present. The Fire-Tite design provides sealing during and after a fire and meets API and BS standards. Monel and Alloy 20 are particularly used for corrosion resistance in many petroleum process applications.
4. UHMW Polyethylene – Used for higher radioactive applications where PTFE is not acceptable. This material also meets the requirements of the tobacco industry where PTFE is not acceptable and is especially well suited for handling highly abrasive media.
5. KEL-F® – This material, PCTFE, is used extensively in cryogenic services handling industrial gases and liquefied propane(LPG) and natural gas (LNG).

Flexible Lip Seat Design

The WAFER-SPHERE® single-piece, polymeric seat is designed so the seat is actually a percentage smaller than the sealing edge of the disc. As the disc cycles into the seat, the seat flexes up and outwards to provide the initial stress for low-pressure sealing. The elastic memory of the seat allows it to compensate for any wear that may occur during cycling.

As line pressure is applied with the disc downstream of the seat, the full cross section of the seat is pressurized, which causes the seat to follow the natural deflections of the disc under pressure. Pressure activation of the seat enhances sealing with increasing line pressure, even though the disc is moving away from the seat due to the same pressure.

With the flow in the opposite direction and the seat downstream of the disc, the seat is supported by the seat insert. The disc is deflected by pressure into the seat, again enhancing the sealing as pressure increases. In order to limit the compressive load of the seat, the clearances around the seat come into play again to allow the seat to flex slightly and limit the compressive load on the seat.

Line pressures are not the only source of disc and seat deflections. Temperature changes or natural differential temperatures that exist between the cooler body and hotter disc can also cause increased seat loads. Again, the clearances are designed into the seat to provide essential relief.

Flow Characteristic

The flow characteristic is a curve that compares the percentage of flow to the percentage of valve travel (i.e., butterfly disc rotation or linear movement of a globe valve). Inherent flow characteristic applies to situations when constant pressure drop is maintained across the valve. Installed flow characteristic takes into account the variations in the pressure drop caused by conditions in the system where the valve is installed.

Common inherent flow characteristic for various valve types include quick-opening, linear and equal percentage. With the quick opening flow characteristic, the valve achieves most of its flow before the valve has been opened more than 50%. The inherent flow characteristic is said to be linear when the amount of valve opening is proportional to the rate of flow. With an equal percentage flow characteristic, the amount of valve opening and the amount of flow increases by a fixed percentage.

The quick opening flow characteristic, common to globe valves, inhibits precise control of media particularly at low flow rates. For most applications, high-performance butterfly valves make good control valves because of their modified equal percentage flow characteristic. This curve approximates the linear flow for greater throttling precision and control stability, resulting in decreased process variability.

Lined butterfly valves

The most common way to form a resilient seat is by lining the interior of the valve body with an elastomer. The lining serves three purposes:

• It forms the actual sealing surface.
• It isolates the body material from the fluid in the pipeline.
• It forms a seal against the face of the mating flange. Alternatively, the elastomeric seat can be molded and vulcanized directly into the body. These valves are typically called rubber-lined valves. This type of seat cannot be repaired; one replaces the entire valve body. Typical liner elastomers are EPDM (ethylene-propylene-diene monomer) and nitrile. EPDM is widely used in HVAC applications. It has a operating range of -20 to +250 degrees Fahrenheit. It resists attack by water, glycols, chlorinated water, 20 percent sodium hypochlorite (bleach), phosphate esters, alcohols and acids. EPDM is not resistant to hydrocarbon solvents and oils, gasoline, chlorinated hydrocarbons, turpentine or petroleum-based oils. For more demanding applications, the elastomeric liner is, in turn, lined with a PTFE layer. This layer isolates the elastomeric seat from the medium flowing in the pipeline. The PTFE liner also seals against the faces of the mating flanges. An adhesive attaches the liner to the elastomer backing. For the most demanding services, the seat is made of a pressure responsive contour. The seat material can be solid PTFE, solid PTFE with a secondary metal seat, or a pressure responsive all-metal seat. The secondary metal seat provides a fire-safe seal if a fire damages the soft PTFE. A metal seat is used for high temperature or abrasive applications. An all-metal seat allows the valve to meet the pressure temperature ratings of ANSI B16.5; a general design referred to as a high-performance butterfly.
• The butterfly disc provides the structural integrity to resist the hydrostatic shut-off loads and operating torque. The disc also acts as the moveable sealing member. In the elastomer-lined butterfly valve, the axis of the valve stem passes through the center of the disc and the axis lies in the plane of the seating edge. This means that the stem passes through the seating edge of the disc. Consequently, special contours machined into the disc provide a sealing surface. The care given to machining and dressing this area is one of the determining factors in the longevity of a valve.
• The thickness of the PTFE liner is important. If it is too thick, the PTFE unduly increases the torque required to close the valve. If it is too thin, the fluid medium may be able to permeate the liner and attack the underlying elastomer.
• Nitrile is another commonly used elastomer. It has a temperature range of 0 to 180 degrees Fahrenheit and is well suited for use with most petroleum oils and greases, most automotive-grade gasoline, fuel oils, glycol, alcohol and other petroleum products. Other less commonly available liners are fluorocarbon for use in chemically aggressive environments, and food grade versions of nitrile and EPDM.
• The lining may be removable and repairable, in which case it is integrally molded to a phenolic backing ring that imparts structural integrity to the elastomer. This molded unit is then assembled into the valve body.

“Non-wetted” and “wetted”

Lined butterfly valves rely on elastomers (rubber) and/or polymers (PTFE) to completely isolate the valve body and stem journal area from the corrosive and/or erosive effects of the line media. When the body and stem journal area are isolated from the line media, the valve is considered a “non-wetted” design. By isolating the valve body and stem with rubber or PTFE, it is not necessary for the valve body to be made of expensive corrosion-resistant materials such as stainless steel, Alloy-20 and C-276.

When the valve body and journals are exposed to the line media such as in gate valves, globe valves and lubricated plug valves, the valve is considered to have “wetted” parts.

Characteristics and system requirements when used for modulating service

The following are some general control valve terms and characteristics for butterfly valves when used for modulating service. A valve having a stated inherent characteristic may provide a different installed characteristic due to interaction with the system.

Linear

The flow rate is directly proportional to the amount of disk travel. For example, at 50% open, the flow rate is 50% of maximum flow.

Equal percentage

Equal percentage characteristic means that equal increments of valve travel produce equal percentage changes in flow rate as related to the flow rate that existed at the previous travel position.

For example, if a valve travel change from 20% open to 30% open produced a 70% change in flow rate, then a valve travel change from 30% open to 40% open would produce another 70% change in flow rate. If the flow rate at 20% open was 100 gpm, then flow rate at 30% open would be 170 gpm and the flow rate at 40% open would be 70% greater than at 30% travel or 289 gpm. The same would be true for each additional incremental travel position.

Quick opening

A quick-opening valve means exactly that. Flow rate through the valve increases very rapidly for incremental changes in valve travel when valve position is near closed. As valve position becomes more open, flow rate changes diminish with incremental changes in valve travel approaching zero change as the valve position nears full open.

Traditionally, most butterfly valves have exhibited equal percentage inherent characteristics at angles of opening from 20° to 70°. Advances in disk design have allowed the extension of the equal percent characteristic through to the 90°, full-open position.

Designs employed for the extension of the equal percentage characteristic have varied from special contouring with planned flow disturbance to wafer-thin types with almost no flow disturbance. The former causes flow restriction at intermediate travel characteristics.

There are other types of valve disks offered that exhibit inherent flow characteristics approaching linear. This deviation from the traditional characteristic is the result of very heavy disk cross sections. As these valves are used in control applications, the user must ensure suitability of the linear characteristic.

Still another type of disk exhibits a characteristic midway between linear and equal percentage. Typically, this would be a disk design for high-pressure service but with minimal available capacity.

The selection of the appropriate control valve characteristic is dependent on the needs of the system. Because there are several factors to be considered, a complete system analysis is required to determine precisely which is the optimum characteristic. Often, it is not practical to perform a system analysis; therefore, certain rules of thumb are offered:

• If in doubt about the preferred characteristic, choose equal percentage. Such a choice may result in a perfect match. If the match is not perfect, it will not be as detrimental as the selection of a linear characteristic when it is not a perfect match.
• Except for pressure-relief applications, the quick-opening characteristic is seldom employed for control applications.

Other rules of thumb for the selection of characteristic for liquid applications are:

• If greater than 25% of system pressure drop is available to the valve at maximum flow conditions, the use of the linear characteristic provides the best results.
• If less than 25% of system pressure drop is available to the valve at maximum flow conditions, the use of the equal percentage characteristic provides the best results.
• If the system is for control of pressure, the use of equal percentage characteristic is preferred.

Rules of thumb for selection of characteristic for gas applications include:

• For small-volume systems, the use of the equal percentage characteristic is preferred.
• For large-volume systems, the use of the linear characteristic is preferred if more than 25% of the system pressure drop is available to the valve.

In the course of selecting the desired control valve characteristic, do not be misled into choosing a linear one on the basis that it will provide an overall linear system.

While it may be desirable to have a linear system, the linear valve characteristic may not provide system linearity. This is due to a difference between the inherent valve characteristic and the installed characteristic.

The inherent characteristics are what the valves provide under constant pressure drops that are typically found in test situations. Installed characteristics occur when the valve is installed in a system where pressure drops vary with changes in valve position.

Economy and reliability

Obviously, butterfly valves cannot perform in every application. Variables dependent on the service, temperature and pressure also determine which valve type to choose. The service conditions must first be determined in order to properly apply any valve.

Technological innovations combined with its simple design have made the butterfly valve a dependable, economical and flexible solution to a variety of industrial flow control needs.

Actuators

The butterfly valve can be used for on off service or modulating service. Actuation is typically achieved either manually (handle, wrench, gear operator) or through an external power source to cycle the valve automatically.

Automatic actuators include electric, pneumatic and hydraulic operators.

Butterfly Valves, 2-1/2 Inch and Larger for domestic service

MSS SP-67; rated at 200 psi; cast-iron body conforming to ASTM A 126, Class B. Provide valves with field replaceable EPDM sleeve, nickel-plated ductile iron disc stainless steel stem, and EPDM O-ring stem seals. Provide lever operators with locks for sizes 2 through 6 inches and gear operators with position indicator for sizes 8 through 24 inches. Provide lug type as indicated. Specify chain wheel operators for valves installed above 8 foot above finished floor.

Wafer Butterfly Valves

Typical Wafer Butterfly Valve Specifications (200/150 PSI)

All butterfly valves are to be rated at 285 PSI minimum bubble tight shutoff and tested for flow retention at 315 PSI shut off.

Body:
Wafer and lug type bodies shall be of Cast Iron or Ductile Iron and shall have positive alignment features for installation between ANSI 125/ 150 flanges. The manufacturer shall offer valves with a neck length that allows for minimum of 2 inch of insulation. The operator-mounting flange shall allow any operator furnished to be field repositioned without modification to three additional positions and without removing the valve from the line.

Disc:
Valve discs shall be of a streamlined design for low-pressure drop and resistance to cavitation. Disc edge to be contoured and polished to minimize seating torque and seat wear. The valve disc shall be attached to the shaft by one or two precision taper pins to assure an absolute movement of the disc when the stem is rotated. Valves whose disc/shaft attachment lends itself to disc flutter will not be acceptable. Primary shaft sealing is to be accomplished by a flat area on the disc hub interfacing under compression with a flat area on the valve seat. A secondary seal consisting of an “O” ring will be used to back up the primary seal.

Shaft:
The valve shaft shall be one piece. Operator rotational forces shall be transmitted to the valve shaft through a key to assure positive movement of the shaft.

Seat:
Seat material shall be compatible with the line fluid with which it will be in contact. Valve seats shall be of the non-collapsing type utilizing a phenolic outer ring. The flange face portion of the seat is to protrude past the body face enough to form a flange seal. Seats, which bunch or distort during opening or closing will not be acceptable. Valve seat design shall allow the disc to be in the fully closed position during installation between flanges.

Bearings:
The valve body shall have a minimum of three (3) reinforced Luberized Bronze sleeve type bearings, which will amply support the valve shaft at the operator connection and at points immediately outboard of the seat. Valves utilizing the body material as a bearing will not be acceptable. .

Actuators:

Valves and Actuators shall be supplied by the valve manufacturer as a single source. The valve manufacturer shall accept full responsibility during the warranty period for the valve, actuator and all accessories they contract to furnish.