Butterfly valves From FISHER HANDBOOK

For butterfly valves where the seat is mounted in the valve body, four basic types of seat exist:

1. Boot seats are the traditional seat style. This type of seat is a separate component which typically has a “U” shaped cross section and completely lines the ID of the valve body. They are retained in the valve body by interlocking “dovetails” at either face of the valve body. The primary advantage of this type of seat is its replaceability but they are generally also available in a wider variety of materials.

2. Cartridge seats also completely line the ID of the valve body but are generally rigid seats by virtue of a “hoop” of rigid material encapsulated by a elastomer jacket. The resulting assembly forms a rigid cylindrical shaped seat. These type of seats are also replaceable and can offer advantages in vacuum applications.

3. Molded in seats are physically bonded to the valve body during the rubber molding process. This style of valve offers good performance characteristics but is generally more limited in material options and seat wear or damage is not repairable. The primary advantage of this type of seat is its pressure rating and its ability to be used in dead-end or end of line applications where no end flange is supporting the valve.

4. Mechanically retained seats are most common in high performance valves and in the water works industry where large diameter valves make the other seat styles described above impractical to manufacture. Seats are retained by a number of methods including mechanical metal fasteners and retention keys formed with advanced adhesives. A few valve designs in the industry mount the elastomeric seat on the disc edge. In this style of valve, mechanical seat retention is used almost exclusively. The boot, cartridge, and molded in seats are most common in the smaller valve sizes (through roughly 24”) and define the range of seat configurations for “resilient seated” butterfly valves . The mechanically retained seats are typically only used on larger diameter (24” and larger) AWWA style valves and in high performance valves. Seat materials for butterfly valves vary but the predominant elastomers utilized would be EPDM and NBR. Additionally, other elastomers in common use include CR (trade name Neoprene), SBR, and occasionally more exotic materials such as fluoroelastomers, silicones, or others. When high performance valves are utilized, thermoplastic seats are also frequently used. PTFE and RTFE are the most common materials but other material types are used for some applications.

4 In general terms, any of these materials described here are suitable in water service. The conditions which lead to a preference of one material over another are just temperature or other chemical constituents in the water. For example, EPDM gives excellent service in high temperature (up to approximately 300 degrees F) water and low grade steam. In this environment its performance is usually significantly superior to NBR which is generally restricted to somewhere in the 200 degree F neighborhood. EPDM is not, however, a good performer in environments where hydrocarbons are found while NBR is excellent in environments where there are hydrocarbons. Generalizations such as this about these material families can be made but the actual performance of the seat material varies widely even within the material family due to differences in the compounding of the material. Because of this, it is extremely important that application data be provided to the manufacturer in order for the best seat material recommendation to be made. The primary information that is needed to make the material selection is as follows: temperature (min. / max. and typical), operating cycle, and chemical makeup of the water including concentrations. From this information, a recommendation can usually be made or detailed questions can be generated to help finalize the selection. Minimum

Pressure Drop A common misconception in sizing large butterfly valves is the need for a minimum pressure drop or large flow coefficient at full open. Most high performance (low pressure loss) butterfly valves have flow coefficients large enough that the need or selection for minimum pressure drop and maximum flow coefficient is irrelevant. The pressure loss of a full open butterfly valve is so small in comparison to the overall loss of the flow system, that it has very little effect. The larger the flow coefficient at full open, the smaller the range of openings for flow control. The concern for minimum pressure loss is often misguided because of the uncertainty of the pressure losses for rest of the flow system. The minimum pressure loss of a butterfly valve is usually less than the variation or accuracy of determining the pipe friction loss for the system. Free Discharge Free discharge from a butterfly valve is similar in nature to the condition of choking or flashing cavitation. Both conditions choke the flow passages and separation zones of the valve with vapor or air. The result of choking is then a limit of flow for a given upstream pressure and a reduction in the efficiency or flow coefficient of the valve. The free discharge flow and dynamic flow torques are less than those calculated for the same pressure drop of a non-cavitating or non-free discharge condition. Knapp (1950) has classified cavitation as being either a vaporous or gaseous type. Free discharge is similar in nature to gaseous cavitation in that free air is drawn into the separation zones from downstream of the valve instead of from the liquid flow itself.

5 ∆H K V g = 2 2 C Q P Sg V GPM = ∆ / Choking

Cavitation Choking or flashing cavitation in a valve is a maximum design limit that results in a level of extreme cavitation. Free discharge at lower pipe velocities and flows will not produce undesirable effects. Dynamic flow and actuator torques are lower than expected, and any cavitation will be significantly reduced. The presence of free air will cushion the cavitation collapse and in many cases will prevent cavitation vapor from even forming. The major disadvantage of free discharge is the reduction in flow and flow coefficient. However, it is important to note that free discharge conditions can produce much larger pressure differentials and velocities than from similar conditions in which downstream piping will limit flow and pressure drop. Care must be taken in designing for free discharge from a butterfly valve to avoid pipe velocities that exceed the maximum design velocities (ie 12 to 16 fps for some classes of AWWA valves). Choking cavitation occurs when the local pressure inside a valve decreases to the vapor pressure of the liquid, and the contracted flow through the valve flashes to vapor.

At choking cavitation, the maximum flow for a given upstream pressure (regardless of downstream pressure) is reached. Typically, the flows to produce large vapor pockets downstream of the valve are very large, and often exceed the maximum design flow and design structural stresses for the valve and actuator. Flashing cavitation can also produce precipitous effects to downstream piping and other flow components. It is not recommended to operate butterfly valves at or near any limit of choking or flashing cavitation

Tags: , ball valve, butterfly valve, control valve, globe valve, positioner, Valve

This entry was posted on Tuesday, June 16th, 2009 at 10:29 pm and is filed under Valve. You can follow any responses to this entry through the RSS 2.0 feed.