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Shutoff valves provide an effective means of routing flows or isolating parts of your piping process for monitoring or maintenance.
But much like other stainless steel piping components, the types of valves you choose to use in your piping system will have a major impact on the performance and service life offered.
In this guide, we’re going to compare three of the most common valve types used for on and off operation in modern processing systems.
By looking at the strengths and weaknesses of each design, you’ll have a solid foundational understanding of the important characteristics of each valve type and be able to identify options applicable to your needs with greater certainty.
Before we dive into the comparisons, it’s important to establish one flow control characteristic all of these valves have in common: they’re primarily on-off valves.
While you might be able to use them to temporarily throttle flows, long-term use as throttling valves could lead to premature valve failure.
They also won’t provide the greatest level of precision for applications requiring fine control of mixtures or flow rates.
Instead, these valves all excel at providing high flow rates, minimizing pressure drop, and providing low levels of friction or turbulence in the flow path.
If you’re looking for more information on considerations surrounding throttling valves, be sure to check out our comparison between needle valves and ball valves.
Butterfly valves utilize a flat disc attached to a stem to open or close the flow path.
Requiring only 90-degrees of movement to move from full-open to full-close, these valves are fast and easy to actuate.
The valve handle also provides fast verification of the valve’s open or closed state, unlike some valves which rely on screw taps or handwheels for actuation.
When properly matched to the connecting pipe sizes, butterfly valves create minimal pressure drop and can allow nearly unimpeded flow through the valve.
However, as the disc sits perpendicular to the flow when open, a minor pressure drop and slight flow turbulence are unavoidable.
This also means that butterfly valves are not typically suited for use in processes involving suspended solids, grit, or stringy substances as they might accumulate on the disc and prevent a tight seal when attempting to close the valve.
The lack of flow restriction and quick actuation time also elevate the risk of fluid hammering.
If you plan to use butterfly valves in your piping system, designing with this risk in mind and careful actuation of the valves is critical to safe operation and minimizing damage risks.
While butterfly valves can operate in a partially opened or throttling state, long-term flow-throttling use is not recommended as the precision offered is not as good at alternatives such as needle valves.
Gate valves feature a flat- or wedge-shaped gate on a threaded stem.
The design provides a means of linear actuation in which the gate moves into the flow path and seats into a recess opposite the stem, creating a strong seal.
When opened, it is completely out of the flow path, resulting in zero flow obstruction or pressure drop.
For processes involving slurries, suspended solids, or grit, knife gate valves are a specialized form of gate valve that features a bevelled edge on the gate.
This shape helps the gate cut through any build-up that might accumulate in the recess and reduce the amount of torque needed to fully close the gate.
Because the valve uses a linear actuation and threaded stem, moving from full-open to full-close states takes longer than most 90-degree actuation valve types, such as butterfly valves or plug valves.
However, this virtually eliminates any risk of fluid hammering.
When important, rising stem models of gate valves can also provide a quick visual indicator of the valve’s state.
However, this comes at the cost of requiring more space above the valve to accommodate the rising stem.
When space is at a premium, non-rising stem designs are likely the better choice.
Most gate valve designs are not designed for use in a throttled state.
Extended throttling use can create vibrations and resonance, leading to damage to the valve internals, seals, gaskets, and other critical elements.
Gate valves also work best in high-pressure systems. Use in low-pressure systems where a tight seal is critical might require special accommodations as low-pressure operation might lead to seeping at the valve while closed.
Much like the name implies, a plug valve uses a plug in the flow path as a means of control.
The center of the plug features a flow channel of varying size and shape that ultimately determines the pressure drop and throughput characteristics of the valve.
Because the plug is on a stem and the valve’s casing sits closely around the plug, suspended solids, grit, and other materials won’t accumulate within the valve.
This makes plug valves versatile enough for use in a range of industries.
The design also makes accessing the valve internals and cleaning the plug easy.
Also, since the stem is not in the flow path, there are fewer worries of corrosion or wear as you might encounter with gate valves depending on their design.
Operating the valve is simple.
By rotating the flow channel to align with the pipe, you can increase flow.
Rotating the solid plug wall to align with the pipe stops flow.
Plug valves are also quick actuating, requiring only a 90-degree throw and providing the same easy visual status checking as butterfly valves thanks to their valve handles.
However, these handles can take up space, so be sure to account for the full range of motion required when implementing a plug valve in your system designs.
Pressure and surface area are two drawbacks to consider when considering plug valves.
Because the plug sits across the entire flow path in the closed state, this means it can require considerable effort and place torque stress upon the valve to move the plug while under higher pressures.
Lubricated plug valves work to mitigate these concerns by continually lubricating the metal plug, reducing the operating torque required while also providing excellent temperature resistance.
However, for lower temperature processes, there are also options with elastomeric coatings, optimizing the seal and torque requirements at a lower price point.
[call-out-box]NOTE: Ball valves are an interesting alternative to plug valves when a perfect seal is not required. They operate in a similar manner, providing most of the benefits and eliminating the concern of the valve requiring excessive torque to adjust when under higher pressures.
However, their theoretical seal strength is lower. Their design makes them less ideal for processes involving suspended solids or grit as well. You can find out more about their function and benefits in our comparison between needle valves and ball valves.[/call-out-box]
Shutoff or on-off valves offer a tight seal to help route flows or isolate parts of your piping processes for monitoring or maintenance.
Optimal valve choice will take temperature, pressure, the substances you wish to pipe, pricing, maintenance, actuation methods, and forces required into consideration.
Butterfly valves and plug valves both offer fast actuation with a 90-degree throw and easy visual verification of the valve state.
Gate valves offer excellent sealing performance for substances with suspended solids and grit but may require additional room if you wish to use rising stem models for visual valve state verification.
The rotation-based linear actuation of gate valves provides an excellent seal with minimal torque stress but takes longer to travel between full-on and full-off states.
While some shutoff valves are capable of operating in throttled states, doing so for extended periods is not recommended as it may damage your valves, gaskets, fittings, and other piping components.
As a leading provider of stainless steel pipe, valves, flanges, and other essential piping process components across Canada and North America for more than 40 years, Unified Alloys has the product selection and experience with a range of industries to help you design and maintain virtually any piping process. Contact us today to speak with one of our expert sales analysts and discuss the options available.
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