Butterfly valves produce dependable bubble-tight shutoff and are ideally suited for throttling control applications because the flow is near linear over 70% of the flow range. They are quick opening and highly efficient because the approach velocity of the flow stream is not lost as the fluid passes through the valve bore. They can be operated manually, mechanically, or automatically and they can be used in handling a variety of media, including liquids, solids, slurries, gases and vapor (steam).
A butterfly valve is a simple device designed to control or block flow. A single wafer disc pivots in the valve body. From completely closed to the open position, the disc rotates 90 degrees. Torque requirements to make this rotation are determined by static forces, caused by pressure drop across the disc in the closed position, and by dynamic forces, caused by fluid velocity in the pipe and at the edge of the partly closed disc.
When the disc begins to rotate toward the open position, it behaves like the wing of an airplane, and is subject to both the lift and drag forces of the flow stream. These fluid forces tend to close the valve, and reach a maximum value when the disc is approximately 67 degrees open.
Although a butterfly valve is hydraulically balanced when fully open or fully closed, force is required to move the disc from either position. Operating torque of a butterfly valve is in general the result of four partial torques:
- seating torque - torque to overcome the rubber seat friction
- bearing friction torque – torque to overcome the friction between shaft and bearing.
- dynamic torque – torque developed by pressure differences across a partly opened valve as a result of high flow velocities.
- hydrostatic torque – torque caused by the difference in static head of liquid on the valve disc above and below the valve shaft in a horizontal position. (only important for large sizes > Above 20”).
The operating torque of a valve under operating conditions may vary depending on different fluid aspects. When ordering it is important to submit the right fluid data to Max-Seal to avoid valve operating failures due to incorrect actuator sizing. When no information is available Max-Seal takes the following assumptions as a basis for actuator sizing:
- the fluid is water (without solid particles) in the temperature range of +1 up to 110° F.
- the fluid does not include chemicals or contamination that may increase the friction between the seating surfaces.
- at least one operation cycle per month.
- flow velocity in the pipe not more than 13 ft/s,
- max break torque of valve at full pressure rating.
When other field conditions are expected please contact Max-Seal for detailed advice regarding operating torques and actuator selection (examples: dry gas or air, slurries, low temperatures, infrequent cycling, high flow velocities)
Up-to-date design and state of the art materials are tailored to market needs and wishes such as no maintenance, low operating torques and long service life. The Max-Seal design philosophy is based on the principle objectives of achieving high reliability, high efficiency and also be highly cost effective.