Selecting A Butterfly Valve For An Application

Butterfly Valve

 

Used as a primary control device in fluid transfer applications, butterfly valves are preferred over most other types of valves for different reasons. Firstly, they perform a quarter-turn open/close operation, which is frequent, quick, and at less fluid resistance. Second, they display good flow control for a wide range of liquids, gases, suspended solids, and slurries. Lastly, they are compact, lightweight, and available in a wide range of materials, too.

The wastewater treatment, pharmaceutical, power generation, paper processing, food & beverage, and fine chemical industries utilize butterfly valves extensively. PTFE lined (FEP / PFA) valves are used in high corrosive chemical environments, whereas hygienic valves made from special types of elastomers are used in the food & beverage industry. Butterfly valves also come in fire safe designs to meet the safety requirements of the petrochemical industry.

Typically, butterfly valves come in sizes ranging from 1 to over 200. Their pressure capabilities range from 150 psi to 750 psi. Available in manual and automatically actuated modes, they are used as an on/off as well as controlling/modulating device. The seat material, body style (wafer or lug), and stem- design play crucial roles in selecting a butterfly valve.

Selecting the Right Style:
The most common types of body designs or mounting styles in butterfly valves are the wafer, lug, semi-lugged, and double flanged styles. While the wafer design is cheaper and safer, the lug style helps in removing downstream piping. Wafer styles attribute to the lightweight structure of butterfly valves, which ensures ease of installation.

Selecting Stem Designs:
The stem or shaft of a butterfly valve comes in one-piece or two-piece designs. While selecting a stem design for a high performance application, one must be aware of the fact that they have to be in contact with the fluid media and hence media compatible.

Selecting the Right Seat Material:
Being critical elements, seals, or seat materials, also determine the quality of a butterfly valve. A variety of factors, including chemical compatibility, pressure, temperature, cost, and wear is considered prior to selecting seats. PTFE (Polytetrafluroethylene) and RTFE (Reinforced Polytetrafluroethylene) are two commonly used seat materials known for their abrasion resistant, electrical insulation, and thermal properties. UHMWPE (Ultra-high Molecular Weight Polyethylene) is another popular material with excellent corrosion and chemical resistance used in the tobacco industry. The material is also ideal for low-radiation environments.

If the application involves a highly abrasive environment, inflatable seats that offer high wear resistance are used. Today, many applications require butterfly valves with FDA approved seat materials. Fire-safe, metal-seated butterfly valves are also popular choices today.

Selecting the Right Type of Butterfly Valve:
Butterfly valves are classified into three types based on their construction:

1. Resilient
2. High Performance
3. Tricentric

The resilient type of butterfly valves uses the rubber flexibility and hence used widely in applications that require lowest pressure ratings. They handle temperatures ranging from 25F to 300F. High-pressure applications use high performance types of butterfly valves as these valves provides a wear-free sealing. Temperature ratings for these valves range from 400F to 450F. Tricentric designs feature a metal seat, which can withstand tremendous amount of pressure and temperatures up to 1000F.

Selecting the Right Valve Size:
We can determine the right valve size by calculating the Flow Coefficient (Cv) of the butterfly valve. You can use the below formula to calculate the Cv:

Cv = GPM / √ΔP

Here, GPM is the max flow requirement and P is the maximum pressure drop. The valve size that is equal to the calculated Cv value is the correct size. If the needed Cv value is between two valve sizes, the larger valve size is selected.

 

About the Author:
Ron Bargman is a regular contributor of manufacturing-theme articles drawing on his decades long experiences first as an owner of an auto-parts supplier and manufacturer, to his present status as the founder and president of Zycon, an internationally recognized search engine for manufacturers. Ron's rich industry history provides insight into manufacturing and engineering news and events that are timely, poignant and relevant.

 

 

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