Butterfly valves are quarter-turn flow control devices that utilize a metal disc that rotates around a fixed stem axis.They are quick acting flow control valves that allow 90 degrees of rotation to move from fully open to closed position.
When the disc is perpendicular to the centerline of the pipe, the valve is in the closed position.When the disc is parallel to the centerline of the pipe, the valve will be fully open (allowing maximum fluid flow).The size of the flow control mechanism (disk) is approximately equal to the inner diameter of the adjacent pipe.
These valves come in different sizes and designs that determine their performance in industrial process applications; sanitary valve applications; fire services; heating, ventilation, and air conditioning (HVAC) systems; and slurries.Broadly speaking, butterfly valves are essential for flow regulation and flow isolation.
The movement of the disc starts, slows or stops the flow of fluid.Applications requiring high precision rely on actuated butterfly valves that monitor pipeline conditions, opening or closing the valve as necessary to maintain a uniform flow rate.Butterfly valves used for flow regulation have one of the following flow characteristics:
• Nearly linear – the flow rate is proportional to the angular motion of the disc.For example, when the disc is 40% open, the flow is 40% of maximum.This flow characteristic is common in high performance butterfly valves.
• Fast opening – This flow characteristic is exhibited when using resilient seated butterfly valves.The fluid flow rate is highest when the disc travels from the closed position.As the valve approaches the fully open position, the flow drops steadily with little change.
• Flow Isolation – Butterfly valves can provide on/off fluid service.Flow isolation is required whenever some part of the piping system needs maintenance.
Butterfly valves are suitable for different applications due to their lightweight design and fast operation.Soft-seated butterfly valves are ideal for low temperature, low pressure applications, while metal-seated butterfly valves have good sealing capabilities when handling harsh fluid conditions.This process operates at high temperatures and pressures and conveys viscous or corrosive fluids.The advantages of butterfly valves include:
• Lightweight and compact construction—Butterfly valves use thin metal discs as the flow control mechanism.The discs are small and take up little space, but are strong enough to regulate the flow of fluids.These valves have a compact body making them suitable for use in piping systems in narrow locations.Large-diameter pipes require larger valves using more fabrication material, increasing costs.A butterfly valve will be less expensive than a ball valve of the same size because it consumes less material to manufacture.
• Fast and Efficient Sealing – Butterfly valves provide fast sealing at actuation, making them ideal for high precision flow applications.The sealing characteristics of a butterfly valve depend on the type of disc offset and the nature of the seat material.A zero offset butterfly valve will provide adequate sealing for low pressure applications – up to 250 pounds per square inch (psi).The double offset valve provides excellent sealing for processes up to 1,440 psi.Triple offset valves provide sealing for flow applications over 1,440 psi.
• Low Pressure Drop and High Pressure Recovery – Butterfly valves have low pressure drop despite the fact that the disc is always present in the fluid.Low pressure drop is critical to managing the pumping and energy demands of the system.Butterfly valves are designed to allow fluid to recover energy quickly after it leaves the valve.
• Low maintenance requirements – Butterfly valves have fewer internal components.They have no pockets that could trap liquids or debris, therefore, they require minimal maintenance intervention.Their installation is just as simple as they require clamping between adjacent pipe flanges.No complicated installation process such as welding is required.
• Simple Operation – Due to their compact size and light weight, butterfly valves require relatively low torque to operate.Thin metal discs use a small amount of force to overcome the frictional resistance of the fluid.Butterfly valves are easier to automate because small actuators can provide enough torque for their operation.This translates into lower operating costs – smaller actuators consume less power and cost less to add to the valve.
• Butterfly valves are susceptible to cavitation and blocked flow – in the open position, the valve does not provide a full port.The presence of the disc in the fluid flow path exacerbates the buildup of debris around the valve, increasing the potential for cavitation.Ball valves are an alternative for fluid applications requiring full ports.
• Rapid corrosion in viscous fluid services – fluids flush butterfly valves as they flow through them.Over time, the discs deteriorate and can no longer provide a seal.Corrosion rates will be higher if handling viscous fluid services.Gate and ball valves have better corrosion resistance than butterfly valves.
• Not suitable for high pressure throttling – the valve should only be used for throttling in low pressure applications, limited to 30 degrees to 80 degrees of opening.Globe valves have better throttling capacity than butterfly valves.
The valve flap in the fully open position prevents the cleaning of the system and prevents pigging of the line containing the butterfly valve.
The installation position of butterfly valve is usually between flanges.Butterfly valves should be installed at least four to six pipe diameters from discharge nozzles, elbows, or branches to minimize the effects of turbulence.
Before installation, clean pipes and check flanges for smoothness/flatness.Make sure the pipes are aligned.When installing the valve, keep the disc in the partially open position.Flanges may need to be expanded to avoid damage to the seat surface.Use pilot holes or slings around the valve body when lifting or moving the valve.Avoid lifting the valve at the actuator or its operator.
Align the valve with the insert bolt of the adjacent pipe.Hand tighten the bolts, then use a torque wrench to tighten the bolts slowly and evenly to estimate the clearance between them and the flange.Turn the valve to the fully open position and use a torque wrench to tighten the bolts to check for even tension on the bolts.
Maintenance of valves includes lubrication of mechanical components, inspection and repair of actuators.Valves that require periodic lubrication include greased fittings.Sufficient lithium-based grease should be applied at recommended intervals to minimize rust and corrosion.
It is also important to regularly inspect the actuator to identify any signs of wear or loose electrical, pneumatic or hydraulic connections that may affect valve operation.
In addition, the user should clean all parts of the butterfly valve with a silicone-based lubricant.The seat should be inspected for any signs of wear and replaced if necessary.Butterfly valve discs used in dry applications such as compressed air service require lubrication.Butterfly valves that cycle infrequently should be operated at least once a month.
Valve selection may seem like a selection and mating activity, but there are several technical specifications to consider.The first involves understanding the type of fluid control required and the type of service fluid.Corrosive fluid services require valves made of stainless steel, nichrome, or other corrosion-resistant materials.
Users need to consider the capacity, pressure and temperature changes of the piping system and the level of automation required.While actuated butterfly valves provide precise flow control, they are more expensive than their manually operated counterparts.Butterfly valves are not controllable and do not provide a full port.
If the user is unsure about the chemical compatibility of the process or actuation selection, a qualified valve company can assist in ensuring the correct selection.
Gilbert Welsford Jr. is the founder of ValveMan and a third-generation Valve entrepreneur.For more information, visit Valveman.com.
Post time: Mar-02-2022
