Why You Should Consider Butterfly Valves Over Gate Valves
Gate valves are used in a wide range of applications, most commonly in water lines treating process water as a utility and in wastewater treatment. There are, however, considerable drawbacks to the use of this valve, mainly due to the slow closing and opening action.
The goal of most continuous processes is to quickly reach its steady state; excessive dynamic state times equate to process inefficiency and financial loss. Any delay in this operation can lead to losses and problems, so gate valves are usually not the most optimal choice.
The reason behind the time-consuming door movement is that the gate valve needs more than 360 degrees of rotation of the handwheel from a fully open position to a fully closed position. Therefore, the most common purpose of using gate valves is restriction of mean flow. However, we also see them frequently used to control pressure and temperature.
Gate valves can also achieve the required flow – the parameter we want to control – even when partially open. In this case, it is important to keep in mind the calculated Cv required to pass the required flow rate. Restricting the valve opening below the required calculated Cv means that the desired flow cannot be exceeded and therefore opening the valve can impact the flow. Changing the size of the valve in question, and if held open enough, will only impact fluid velocity but will maintain an even flow rate.
During the movement of the valve — which is carried out either manually by the operator or using an actuator — the water circulating in the pipeline has no use. It can be considered a total loss because its mass flow rate is not the target.
Compared to the gate valve, the butterfly valve – due to its specific construction – offers a much faster opening/closing action. The butterfly valve uses a fixed disc on a metal stem, which requires a 90 degree rotation between the fully open and fully closed position. Let’s explore some of the other benefits of choosing butterfly valves that are also important when designing your systems.
Gate valves are generally less expensive than butterfly valves for nominal sizes below DN80. However, due to their sturdy structure and the need for additional support, their price increases sharply with diameter. On the other hand, the butterfly valve has a compact design in all sizes: it requires less space for mounting, even in larger pipes, which makes it a more affordable option once the diameter exceeds DN80. Since pipe diameters in sewage treatment or water-as-a-utility applications can be quite large, butterfly valves come are a perfect solution.
flow control capability
In addition to acting as an on/off flow control, butterfly valves are an excellent solution for regulating fluid flow. Due to its specific design, a butterfly valve can easily control fluids with small and excessive flow range requirements. Butterfly valve manufacturers typically provide information on the relationship between valve opening and flow, under constant pressure conditions.
It is essential to note that the valve opening represents the relative position of the valve plug to its closed position against the valve seat. The passage area of the orifice has no influence on the relationship between valve opening and flow. The relationship between flow rate and orifice passage area is directly proportional for all valve types.1
Butterfly valves are divided into two groups, based on the relationship between flow rate and valve opening:
Linear: This type of butterfly valve has a linear relationship between the opening of the valve and the maximum flow. Therefore, a 45 degree disc opening brings fluid flow to 50% of maximum. Linear valves are primarily used in flow and liquid level control loops. In addition, they are compatible with systems where the expected pressure drop changes are minimal.
Equal: Equal butterfly valves are an excellent solution for processes where the required water flow varies considerably. In the case of a butterfly valve with an equal percentage relationship, equal increments of valve travel produce equal percentage changes in flow. For example, if moving the disk from 20 to 40 degrees increases throughput by 50%, moving an additional 40 to 60 degrees will increase throughput by an additional 50%.
This means that the inherent characteristic of an equal butterfly valve is an exponential function – information on this function is provided by the valve manufacturer.1 Equal valves are used in pressure and temperature control loops, and in systems where large changes in pressure drop are expected.
Process automation has become a necessity for successful operations in modern facilities due to the many benefits it offers. Implementing control valves is a wise long-term investment, even for processes with minimal parameter variations. Operating costs of control valves are minimal, while their contribution to achieving and maintaining constant values of controlled variables provides immeasurable process improvements.
For example, if water is used as a cooling system, its required flow rate depends on the temperature of the medium to be heated. The most accurate way to achieve the set temperature is to adjust the water flow using a control valve. The opening of the valve is regulated using a butterfly valve actuator, which can be electric or pneumatic. The butterfly valve actuator rotates the valve disc from 0 to 90 degrees, relative to the desired water flow.
The actuators are connected to processors which send commands according to the required actuator movement, based on the processed information from the temperature sensor. Widely used industrial PID control loops are customized for each application, ensuring that a steady state is reached as quickly as possible, which is the goal of automatic actuation. It is essential to note that a butterfly valve is the optimal choice for this purpose, due to its compatibility with typical cooling water temperatures and pressures found in industry.
- The butterfly valve is designed to withstand speeds of up to 6 m/s, a speed well above the recommended upper speed limit for water flow in industry: 3 m/s.
- The butterfly valves include a polymer seal which guarantees minimal leakage by default, which is not the case with the gate valve.
- In the event of an actuator failure, the actuators are programmed to open or close in the event of a failure, depending on the choice of the engineer and depending on the conditions of a particular process. This failure mechanism makes the operation of the butterfly valve safe, especially in the treatment of wastewater containing toxic contaminants. While gate valves can also use an actuator, closing speeds can make a significant difference in protecting your equipment.
- The end of a pipeline is where engineers often choose to mount the gate valve. The Lugged Butterfly Valve is the perfect replacement as its design is matched to the end application. It is recognizable by the threaded lugs molded into the valve body, used for bilateral screw connections allowing mounting in this position.2 The winged butterfly valve allows the dismantling of one side of the installation while maintaining the other side under pressure; this cannot be done with other butterfly valves, such as the wafer butterfly valve.
There are many applications where a gate valve presents the optimal solution for regulating the medium flow, but with industrial applications and waste water management this is not always the case. First, from a financial perspective, a butterfly valve has lower initial costs and incurs significantly lower operational costs due to its quick opening and closing. Second, the technical features of a butterfly valve allow users to automate and optimize processes efficiently. Finally, the butterfly valves are easily programmable to stay in the desired position in the event of a fault and immediately send a signal to the operators so that they can act quickly. Therefore, replacing a gate valve with a butterfly valve is a wise investment.
Gilbert Welsford Jr is the founder of ValveMan.com and a third generation valve entrepreneur. He learned valves from an early age and brought his entrepreneurial ingenuity to the family business by founding the online valve store in 2011: ValveMan.com. Gilbert is focused on building the legacy that his grandfather started, his father grew, and amplified.
- Zappe RW, Smith Peter, Valve Selection Handbook – Engineering Fundamentals for Selecting the Right Valve Design for Every Industrial Flow Application (5th Edition), 2004
- Driscoll Ryan, Actuated Butterfly Valves 101: Everything You Need to Know About Their Application in Piping Systems, ValveMan, August 10, 2022 https://valveman.com/blog/actuated-butterfly-valves-101-all-you- need-to-know-their-application-in-piping-systems/