Pros and Cons of Butterfly Valves
Municipal water distribution and wastewater treatment facilities rely on a variety of valves. The different categories of valves contribute to the overall efficiency and safety of pipelines. Butterfly valves are popular across water collection, supply and distribution pipelines. They also offer dependable service in pumping stations for flow isolation purposes.
In wastewater treatment, butterfly valves appear in secondary and tertiary processes. These treatment processes rely on biological and chemical processes to remove organic matter, suspended matter and disinfect wastewater. Actuated butterfly valves are suitable for these wastewater treatment processes as they respond promptly to the pressure conditions of the pipeline and operator-initiated control signals.
Butterfly valves are quarter-turn flow control devices containing a metallic disc that rotates around a fixed stem axis. The valve moves from a closed position to a fully open position when the disc turns through 90 degrees. The flow of the service fluid is maximum when the orientation of the disc is parallel to the centerline of the pipe. To ensure tight shut-off, valve manufacturers use a disc whose circumferential size is approximately equal to the internal diameters of impinging pipes.
Butterfly valves are available in different types and sizes. Design variations affect their performances across water distribution and wastewater collection applications. The roles of butterfly valves are:
• Flow Isolation — They provide an on/off fluid service. Shutting down butterfly valves facilitates the isolation of some sections of the pipeline. Flow isolation is necessary when there is a need to undertake maintenance of pipe sections and auxiliary units like pumps.
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• Flow regulation — In municipal water supply systems, it is necessary to maintain specific pipeline flow rates. The same is true for wastewater applications. The movement of the flow control mechanism (disc) causes the flow rate to reduce or increase according to the pressure conditions of the system. The disc also stops flow past different pipe sections.
There are different classes, sizes and designs of butterfly valves. Each valve category has unique design features which dictate its performance characteristics. The categories include:
Valvle body construction: Wafer type, lug style, rlanged and butt-welded end type butterfly valves.
Disc offset: Zero offsets, double offset and triple offset butterfly valve.
Seat type: soft-seated and metal-seated butterfly valves.
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How are butterfly valves adapted to water distribution and wastewater treatment applications? What advantages do they have over other valve types? Let us look at their advantages and shortcomings in detail.
Pros of butterfly valves
Butterfly valves are versatile flow control devices that utilize simple mechanisms. Their internal structures contain a few mechanical components. Water distribution and treatment processes occur at low to medium temperatures (between 40 degrees to 100 degrees F). The pressure limits are equally low (30-80 psi). It means that most of these valves use soft-seated seals. The simple design and construction of butterfly valves come with several advantages.
1. Lightweight and compact construction
A thin metallic strip acts as a flow-control mechanism for butterfly valves. The disc connects to the actuator through a lightweight stem. Manufacturers optimize the design of the disc to ensure it has sufficient strength to regulate fluid flow. As a result, the space required for housing the disc reduces. The bodies of butterfly valves are compact and occupy a smaller space than other valve types. The valves can fit in pipelines that are in confined spaces.
The cost of manufacturing valves depends on the quantity of materials required. Butterfly valves are cheap since they do not consume a lot of material. Their compact construction means that the valves do not get bulkier as the diameters of the water distribution and treatment systems increase.
2. Low-pressure drop and high-pressure recovery factor
Water distribution systems require continuous pumping to bridge consumer demands. When a pressure drop exists in the system, the energy requirement for pumps spikes. Valves induce pressure drops across pipe sections and can strain pumps. The disc of the butterfly valve is always present in the flow. However, its lightweight construction ensures that the pressure drop across the valve is minimal. The disc has a high-pressure recovery factor and responds fast to changes in pipe pressure conditions.
The ability of butterfly valves to maintain low-pressure drops ensures that the energy consumption for pumping water across the complex municipal water systems is low. That way, the cost of operating the pipelines is low.
3. Ease of operation and actuation
Using a thin metallic strip as the flow regulation mechanism simplifies the operation and actuation of butterfly valves. The disc requires small amounts of force to combat the frictional resistance of the service fluid. The torque required to initiate the quarter-turn of the valve is equally low.
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A tiny actuator can deliver sufficient torque to operate a large butterfly valve. In return, the energy and space requirements for automating butterfly valves are lower.
4. Low maintenance requirements
Butterfly valves have few internal components, and simple constructions ensure that few parts interfere, resulting in fewer breakdowns. The valves do not have pockets that trap debris and dirt in water and wastewater.
With fewer breakdowns and maintenance interventions, the valves can operate through several cycles, a desirable quality for large-scale water distribution and wastewater collection.
5. Quick and efficient sealing
Butterfly valves respond quickly to control signals. They seal efficiently against the service media. Municipal water distribution applications require the highest levels of precision to guarantee the purity and potability of water. In wastewater collection and treatment systems, the valves prevent leakages that may affect the quality of processes.
Cons of butterfly valves
1. Susceptibility to cavitation and choked flow
The disc is always present in the path of flow. It increases the accumulation of debris around the valve, increasing the probability of choked flow and cavitation. To prevent this in wastewater applications, use valves that provide full ports, like ball valves, in primary treatment applications.
2. Prone to corrosion in viscous fluid service
Butterfly valves experience rapid corrosion in wastewater applications conveying viscous fluids. Disc corrosion limits the responsiveness and sealing capabilities of the valve.
3. Butterfly valves are non-piggable
The disc is continuously present in the pipeline, making it impossible to clean using pipeline intervention gadgets.
4. Ineffective for high-pressure throttling
Using butterfly valves for throttling is limited to low-pressure applications only.
Installation and maintenance
Butterfly valves are installed between pipe flanges. Before installation, clean the adjacent pipelines and ensure the flanges are flat and that the pipes align.
Keep the disc in a partially open position during installation to avoid damage. Don’t lift the valve at the actuator or the operator. Use a torque wrench to tighten the valve.
Conduct regular inspections and maintenance of the valves. Where possible, lubricate components using lithium-based grease. Inspect actuators to identify and rectify loose electrical, pneumatic or hydraulic connections.
Butterfly valves are vital for municipal water distribution systems and select wastewater treatment applications. The valves have several desirable features and performance characteristics to facilitate precise flow control. When choosing these valves, pay close attention to the operating conditions of the pipeline, maintenance requirements and the level of automation.
