7.1 Introduction and Application
Butterfly valves are high pressure recovery valves (also called high capacity and high area ratio valves) with relatively small overall pressure drop in the fully open position as compared to globe valves. Butterfly valves are used for both isolation and throttling service. This section provides general discussions of butterfly valves in typical nuclear power plant applications. Special considerations related to butterfly valves in
modulating/throttling service are given in the next section. The Butterfly MOV Application Guide [1.6] provides detailed discussions for the design, installation, operation, and torque requirements for butterfly valves in nuclear power plants.
Reference 1.6 should be consulted for additional details not covered here.
Figure 7-1 shows an overall assembly of a motor-operated butterfly valve in which the following principal components are identified:
• Butterfly valve
• Limitorque HBC gear operator for quarter-turn operations
• Limitorque SMB actuator
• Motor
• Switch compartment
Figure 7-1
Typical Motor-Operated Butterfly Valve
Butterfly valves offer several advantages over other types of valves, especially in applications where soft seats are acceptable. Their advantages include:
• Reduced initial installation cost, weight, and space requirements, particularly in large sizes
• Reduced operating energy cost because of high flow capacity (Cv) and low pressure drop in the full open position
• Reduced maintenance cost even when handling dirty fluids and fluids with suspended solids (for example, in service water applications)
• Improved sealing capability with seat tightness up to Class VI [6.12], particularly with high performance valve designs
• Versatility in material selection, which extends butterfly valve applications to higher operating pressures (typically up to Class 600) and temperatures (typically up to 400°F; 200°C) and lower leakage (typically up to Class VI) requirements
• Generally self-closing hydrodynamic torque characteristics that make some butterfly valves a good choice for fail-close operation
• Flow characteristics that make butterfly valves well suited for throttling service Proper sizing, selection, and installation techniques result in years of trouble-free butterfly valve service. Most problems with butterfly valves in nuclear power plant systems result from misapplication and improper sizing rather than deficiencies in valve or actuator designs.
Butterfly valves use circular, flat discs that can be rotated approximately 90° from fully closed to fully open positions. The disc rotation of some valves is limited to 60° to 70°.
Some “angle seated” butterfly valve designs close at angles other than 0°. The disc is attached to a shaft that extends outside the body and can be rotated by an actuator.
In nuclear power plants, butterfly valves are most commonly used in low pressure and low temperature water systems and in containment purge and venting systems.
Although they are most often used in pressure service of ANSI Class 300 or less, higher pressure designs are available up to ANSI Class 1500 in smaller sizes. Sealing these valves is accomplished by rotating the valve’s flat disc into the flow stream until it is approximately perpendicular to the flow axis of the connecting pipe, thus effectively blocking the flow area. Butterfly valves are available in a variety of materials and end connections, but are generally limited to 400°F (200°C) because of the soft seats
commonly used to achieve an effective seal.
Some butterfly valve designs accomplish a metal-to-metal seat along a tapered seating surface, making these valves suitable for high temperature service. Butterfly valves are compact, lightweight, and relatively inexpensive, and they are available in sizes
exceeding 72 inches (1,800 mm).
The pressure drop across butterfly valves is small, but not as small as in gate valves or ball valves which have no obstruction in the flow stream when in the wide-open position. Because the butterfly disc is always in the flow stream, erosion of the disc must be considered.
7.2 Design
7.2.1 General
Butterfly valves are typically installed as line size valves where valves and inlet/outlet pipes have the same nominal diameters. Alternatively, butterfly valves may be
installed in larger diameter pipes using inlet reducers and outlet increasers in order to enhance the low-flow/throttling characteristics and to reduce the cost of the MOV and its installation.
The overall population of butterfly valves in a U.S. nuclear power plant are divided into two broad categories:
AWWA Design Butterfly Valves. A large population of ASME Class 2 and 3 nuclear safety-related as well as nonsafety-related valves in U.S. nuclear power plants are limited to maximum shutoff differential pressure of 200 psi or less (1,379 kPa), a maximum normal service temperature of 300°F (150°C), and a one-time faulted temperature capability of 350°F (175°C). Many valves for these service conditions are basically designed in accordance with requirements of ANSI/AWWA Standard for
Rubber-Seated Butterfly Valves [6.36]. Henry Pratt, Fisher, Allis-Chalmers, and BIF are the major suppliers of this type of design.
High Performance Butterfly Valves. In the 1960s, a new class of butterfly valves emerged with a higher pressure/temperature envelope and shutoff capability
conforming to the full pressure ratings of ANSI B16.34 Class 150, 300, and 600 [6.24].
This class of valves is now commonly referred to as “high performance butterfly
valves.” Posi-Seal, Rockwell (McCanna), and Jamesbury are the major suppliers of high performance butterfly valves to U.S. nuclear power plants.1
Butterfly valve bodies are generally very stiff in comparison to the adjacent piping, making them virtually immune to line loads (axial, bending moment, or torsion). They are also insensitive to thermal gradients through the body due to their symmetric axis and stiff construction.
The torque required to fully seat the disc can be minimized by using
pressure-energized seats (see Section 7.2.9). Butterfly valves have no body cavities that can trap solids or contaminants. Servicing any of the major components of the valve requires removal of the valve from the line. Because the shaft rotates without axial motion, the butterfly valve shaft cannot be backseated. However, some designs can be furnished
1 It should be noted that some manufacturers provide both American Water Works Association
with secondary shaft seals inboard of the shaft bearing to protect the bearing from contamination.
The most common butterfly valve disc shapes used in U.S. nuclear power plants are shown in Figure 7-2, and can be divided into two basic disc designs: conventional symmetric (concentric) disc and nonsymmetric disc designs (Figure 7-3).
Figure 7-2
Most Common Butterfly Valve Disc Shapes Used in Nuclear Power Plants
Figure 7-3
Typical Variations in Butterfly Disc Designs
7.2.2 Symmetric (Lens Type) Disc with Concentric Shaft
The symmetric disc type design (Figure 7-3a) is generally referred to as the standard disc, conventional disc, or lenticular disc. Flow and torque characteristics of a
symmetric disc valve do not depend on the flow direction, and the valve has no preferred flow direction. Symmetric disc design is typically furnished with a rubber-lined body to provide a seal in the fully closed position, as shown in Figure 7-4. In this design, the shaft penetrates the rubber liner, and an enlarged hub area around the shaft is provided with interference against the rubber liner to prevent leakage around the shaft in the fully closed position. The disc hub area maintains a continuous contact against the body liner throughout the disc rotation, which has a tendency to cause higher wear in this region.
The main advantages and the disadvantage of the use of symmetric disc butterfly valves are summarized below.
Advantages:
• Simple and compact construction compared to nonsymmetric disc butterfly valves
• Suitable for bi-directional service due to the symmetric disc shape.
• Requires smaller dynamic torque in the closing direction than in the opening direction because the hydrodynamic torque is typically self-closing [1.6]. This is particularly beneficial in applications where the isolation valve is required to close.
Disadvantage:
• Sliding action under interference between the seat and disc causes higher seat wear than in nonsymmetric disc valves.
Figure 7-4
Typical Symmetric Disc Design with Elastomer Lined Body
7.2.3 Nonsymmetric Disc with Single Offset Shaft
In the single offset nonsymmetric disc design (Figure 7-3b), the shaft centerline (and the center of disc rotation) is offset axially from the plane of the valve seat along the pipe centerline. In this shaft/disc design, the valve seat is continuous, and the shaft does not penetrate the seat as shown in Figure 7-5. This design is available in resilient as well as metal-to-metal seat. The disc face away from the shaft is typically flat or has a small curvature and is commonly referred to as the flat face. The other disc face is generally convex and contoured to accommodate the shaft. This face is generally referred to as the curved face.
Flow and torque characteristics of the valve depend on the flow direction with respect to the disc. When the shaft is on the downstream side (or the flat face of the disc is on the upstream side) of the flow direction, the installation is commonly referred to as shaft downstream or flat face forward (Figure 7-6). Similarly, when the shaft is on the upstream side (or the curved face of the disc faces the upstream side), the installation is referred to as shaft upstream or curved face forward.
Figure 7-5
Cross-Section of a Typical Nonsymmetric Butterfly Valve