Butterfly valves in control valves applications

Butterfly valves are commonly used as control valves in applications where the pressure drops required of the valves are relatively low. Butterfly valves can be used in applications as either shutoff valves (on/off service) or as throttling valves (for flow or pressure control). As shutoff valves, butterfly valves offer excellent performance within the range of their pressure rating. Typical uses would include isolation of equipment, fill/drain systems, bypass systems, and other like applications where the only criteria for control of the flow/pressure is that it be on or off. Although butterfly valves have only a limited ability to control pressure or flow, they have been widely used as control valves because of the economics involved.

The control capabilities of a butterfly valve can also be significantly improved by coupling it with an operator and electronic control package. Butterfly valves allow high flows with relatively low pressure loss from the valves, and are typically used for flow control for valve openings from 30 to 70 degrees of full open. At valve openings greater than 70 degrees, the pressure loss of a butterfly valve is too low to produce any significant effect on flow or the energy loss of a flow system. Two special applications for a butterfly valve include the use of a valve for free discharge and the use of a butterfly valve for flashing or choking cavitation service. Free discharge typically produces high pipe velocities at moderate pressure drops, and choking cavitation typically produces high velocities with large pressure drops.

The following paper also includes a review of flow coefficients and the application of non-line sized valves, or valves installed in larger diameter pipes with the aid of pipe reducers. Butterfly Valves Types The butterfly valves used in water works applications vary from traditional “resilient seated” butterfly valves to the more specialized AWWA valves as detailed in AWWA’s C504 standard. Regulations do not generally require the use of AWWA valves in water works service but most operators will specify AWWA valves for the majority of applications based on their experience with AWWA valves and their reputation for long service life. Basic resilient seated valves are designed in accordance with industry standards such as MSS SP-67 and most frequently come in lugged and wafer configurations.

They are considered by most to be a “general service” type of valve and as such, are usually restricted to use in the ancillary systems within water treatment and distribution systems. They are usually limited in size to around 48 inches in diameter. and pressure ratings for these valves are typically 75, 150, or 300 psi. The design of the valves with different ratings are very similar. The stem diameter, disc thickness, and disc to seat interference are the main features which differ as the rating changes. Flow characteristics and limits are not generally defined in the standards/specifications nor are they defined by manufacturers in most cases. 2 AWWA valves are considered by most users to be a more robust valve design and in most cases have greater body wall thicknesses, larger shaft diameters, and thicker discs than resilient seated valves with equivalent pressure ratings. In addition, AWWA valves would most frequently be supplied in a double flanged configuration rather than the lugged or wafer styles although all of these configurations are available in the marketplace. Currently the AWWA C504 standard defines three shutoff pressure ratings and two flow velocity sub-ratings. Pressure rating breaks are made at 25, 75, and 150 psi. Flow velocities are defined for the AWWA valves as specific breaks are made in the line velocities at 8 and 16 feet per second. An increasing number of applications however, are pushing beyond these traditional pressure/flow velocity limits with applications at 250 and 300 psi becoming common. A third category of butterfly valve, the high performance butterfly valve, is occasionally utilized in water treatment and distribution applications. These valves are characterized by higher pressure and/or temperature ratings, different disc seating/sealing technology, and premium materials. These valves frequently follow the design guidelines outlined in MSS SP- 68. The primary distinguishing design feature is a stem which is offset from the centerline of the pipe. This offset design generates a camming action which allows for different types of seating technologies when compared to resilient seated valves.

These alternate seating technologies frequently incorporate advanced seal materials, such as PTFE, RTFE, and other advanced polymers which help the valve achieve higher pressure and temperature ratings. A high performance butterfly valve is therefore a butterfly valve which has extended the performance boundaries (in terms of pressure, temperature, and/or corrosion resistance) of traditional butterfly valves.

This entry was posted on Tuesday, June 16th, 2009 at 10:25 pm and is filed under Valve. You can follow any responses to this entry through the RSS 2.0 feed.