How does it work: Butterfly Valves

A butterfly valve is a flow regulating disc valve. According to the BVAA, a butterfly valve is “a valve in which the obturator rotates about an axis at right angles to the direction of flow and, in the open position flow passes around the obturator.”

In other words, a butterfly valve consists of a circular disc or plate built with a stem through the middle or attached offset. When opened, the disc pivots 90 degrees in the valve bore, aligning with the flow, creating a nearly unrestricted flow path. Butterfly valves operate similar to ball valves in their 90 degree rotation and allow for quick shutoff.

Butterfly valves have gained popularity over the years due to their thin profile between flanges, making them lighter in weight and creating a smaller install footprint. This compact design also usually makes the initial cost for these valves lower when compared to other valve designs.

There are different types of butterfly valves and each type has unique design features and applications. Each type is defined by the orientation of the stem to the disc and the seating angle that the disc closes on.

The most basic butterfly valve design is commonly called a concentric or resilient-seated butterfly valve. In this type of valve, the stem is centered in the middle of the valve disc, which is centered in the pipe bore. This valve typically has a rubber (or resilient) seat and relies on the disc having a high level of contact with the seat to effect a seal. The disc will contact the seat the earliest (85+/- degrees) during the 90 degree rotation.

DEMCO butterfly valves are durable resilient-seated valves that feature a one-piece body for reduced weight and increased strength. The unique stem hole design in the disc ensures a dry stem journal, and the hard-backed seat allows ease of installation, reliable operation, and is field-replaceable without special tools. DEMCO butterfly valves are field-proven to be an excellent choice for industrial and commercial applications.

In a single-offset butterfly valve, the stem is located behind the disc. There are few, if any, valves of this type left on the market today due to the development of the double-offset, or high-performance, valve. The single-offset of the stem causes the disc to contact the seat with three to four degrees left to travel with the idea that less seat contact will enable the valve to last longer.

The double-offset or high-performance butterfly valve has a disc with two offsets and can be rated up to 1480 psi. Similar to a single-offset design, the stem is located behind the disc. With the high-performance butterfly valve, the second offset’s stem is moved once more off the center of the disc to one side. This geometry of offsets enables the 90 degree disc rotation to “rub” over the seat for only one to three degrees of the 90 degree rotation.

The double-offset butterfly valve is used in systems requiring higher pressure resistance. The disc is positioned in the center of the pipe bore and arranged to increase sealing ability and decrease wear to the valve. WKM high-performance butterfly valve is ideal for handling water, oil, steam, and gas in a cost effective, light weight design. The WKM butterfly valve’s corrosion-resistant, single-component thrust bearing/disc spacer reduces body wear and helps ensure positive centering of disc in the valve bore.

Both resilient and high-performance butterfly valves can be operated by handles, gears, or actuators. These devices move the valve disc to the optimal position for complete shutoff or fully open the valve. Resilient and high-performance butterfly valves are used mainly in the water, chemical and petrochemical industries, but can also be used in fuel handling systems, power, and many other applications.

The last type of butterfly valve is the triple-offset butterfly valve (a.k.a. TOV or TOBV). This type of butterfly valve is applied in difficult services where reliable performance under harsh conditions is needed. Exactly like a double-offset valve, the stem is located behind the disc and offset to one side (double-offset). The third offset is the geometry of the seating surface. This seating surface creates a sort of cone-shape of the disc and seat, which wedges into the seat with very minimal contact between the sealing surfaces until fully closed. By reducing the amount of contact that the seat has with the disc, sealing becomes more efficient and the life of the valve is extended.

TOVs are typically metal–seated, unlike single and double-offset butterfly valves. The Cameron WKM TOV has a metal seat design, which allows it to achieve zero leakage* in either direction for extreme services. The true triple-offset geometry of the WKM TOV allows for bubble-tight* sealing to create a fully bi-directional zero-leakage* shut-off valve.

Triple offset butterfly valves are commonly used in steam, chemical, refining, and offshore oil and gas applications up to 1400° F (760° C). The rugged metal seats can also withstand a high degree of abrasion and shock and, with appropriate material selection, can also be utilized in cryogenic services such as LNG and NPG facilities.

*Per API 598

 

Sourced by ekomeri.com

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