To monitor or maintain your piping system, shutoff valves can be used to route flows or isolate parts of them. The type of valves you choose for your stainless steel piping system will significantly impact its performance and service life.
This guide compares 3 types of valves that are commonly used for on and off operations in modern processing systems.
By understanding each valve type’s strengths and weaknesses, you’ll be able to identify options applicable to your needs with greater certainty and will gain a solid foundational understanding of the important characteristics of each valve type.
THROTTLING VALVES VERSUS ON-OFF VALVES
All of these flow control valves have one characteristic in common: they are mostly on-off valves. Before we dive into the comparisons, let’s establish this fact. You can temporarily throttle flows with them, but long-term use as throttling valves could cause premature valve failure.
They also won’t provide the greatest level of precision for applications requiring fine control over mixtures or flow rates. Rather, these valves are all excellent at providing high flow rates, minimizing pressure drops, and reducing friction and turbulence.
Our comparison between needle valves and ball valves provides more information about the considerations surrounding throttling valves.
BUTTERFLY VALVES EXPLORED
In butterfly valves, a flat disc attached to a stem opens or closes the flow path. They are easy to actuate, requiring only 90 degrees of movement between full-open and full-close.
By using the valve handle, you can easily verify whether the valve is open or closed, unlike some valves that require handwheels or screw taps to operate. When matched properly to the connecting pipe sizes, butterfly valves create minimal pressure drop and can allow nearly unhindered flow.
Despite this, a minor pressure drop and some flow turbulence are inevitable when the disc is open since it sits perpendicular to the flow when it is open. Furthermore, butterfly valves are typically not suitable for processes involving suspended solids, grit, or stringy substances because they can accumulate on the disc and prevent a tight seal.
There is also a risk of fluid hammering due to the lack of flow restriction and quick actuation time. To minimize damage risks and ensure the safe operation of butterfly valves in your piping system, you need to design the system with this risk in mind.
Even though butterfly valves can be operated in a partially opened or throttling state, long-term flow throttling is not recommended because the precision offered is not as good as needle valves.
GATE VALVES EXPLORED
A gate valve consists of a flat- or wedge-shaped gate on a threaded stem. The design creates a strong seal by moving the gate into the flow path and seating it in a recess opposite the stem.
When it is open, it is completely out of the flow path, which means that there is no obstruction or pressure drop. Knife gate valves specializ gate valves that feature bevel edges that can handle slurries, suspend solids, or grit.
To fully close the gate, this shape cuts through any buildup that might accumulate in the recess and reduces the torque required. A valve with linear actuation and threaded stem takes longer to move from full-open to full-close than a valve with 90-degree actuation, such as a wafer type butterfly valve or plug valve. However, this virtually eliminates the risk of fluid hammering.
Additionally, rising stem models of gate valves can provide a quick visual indication of the valve’s condition. The rising stem, however, requires more space above the valve to accommodate. Non-rising stem designs are likely to be the better option when space is at a premium.
The majority of gate valve designs are not intended for use in throttled conditions. It is possible to damage valve internals, seals, gaskets, and other critical components by prolonged throttling.
High-pressure systems also benefit from gate valves. A low-pressure operation may result in seeping at the valve while closed in low-pressure systems where a tight seal is critical.
PLUG VALVES EXPLORED
As its name implies, a plug valve controls flow using a plug in the flow path. Flow channels that vary in size and shape are located in the center of the plug, which determines the valve’s pressure drop and throughput.
Since the plug attaches to a stem and the valve’s casing sits closely around the plug, suspend solids, grit, and other materials won’t accumulate inside the valve. The versatility of plug valves enables them to use in a wide variety of industries.
Cleaning the plug and accessing the internals of the valve is also made easier by the design. Additionally, since the stem is not in the flow path, there is less chance of corrosion or wear than with gate valves.
The valve is easy to operate. The flow channel can rotate to align with the pipe to increase flow. The solid plug wall can rotate to align with the pipe to stop the flow.
In addition, plug valves are quick to operate, requiring only a 90-degree throw, and their handles make it easy to check their status visually. When designing your system, keep in mind that handles can take up space, so be sure to account for the full range of motion required.
There are two drawbacks to consider when considering plug valves: pressure and surface area. Due to the plug’s location across the entire flow path in the closed state, moving the plug under higher pressure can require considerable effort and torque stress on the valve.
These are mitigating lubricant plug valves, which continuously lubricate the metal plug, which reduces the operating torque required and provides excellent temperature resistance. In addition, there are elastomeric coatings available for lower temperature processes, which optimize the seal and torque requirements.
KEY TAKEAWAYS
A shutoff or on-off valve will help you route flows or isolate parts of your piping system for monitoring or maintenance. It is important to consider temperature, pressure, substances that you wish to pipe, pricing, maintenance, actuation methods, and forces required when choosing a valve.
Plug valves and butterfly valves both have a 90-degree throw and are fast to actuate. If you wish to use rising stem models for visual valve state verification, gate valves provide excellent sealing performance for substances that contain suspended solids and grit.
It takes longer to travel between full-on and full-off states with rotation-based linear actuation of gate valves, but it provides an excellent seal with minimal torque stress. Although some shutoff valves can operate in throttled states, doing so for extended periods can cause damage to valves, gaskets, fittings, and other piping components.
Also, read “Basic Valves and their Types“
