Picking out an actuator for your valve system can be a daunting task. This is because a wrong-sized actuator can lead to accidents that may not be easy to amend. The process of choosing an actuator includes calculating the torque required for your valves- and in this blog post, we’ll guide you through all you need to know about torque!
What is valve torque? The power we use to ‘open’ or ‘close’ a valve is called ‘torque.’ Fluid flow velocity and pressure drop dictate the torque required for a valve to rotate. Torque is a performance indicator that aids in determining the type and size of actuator required for your valve system.
There are different types of torque, and to better understand them- keep reading!
What Is an Actuator?
To understand what an actuator is, all you need is a flashback to your science learning days!
Repeat after me- input to output. Now, the actuator is the device that converts the input into output. This input is the energy which in this case is the ‘torque,’ and the output is the ‘movements’ of the valves.
Simply put, the actuator is a device installed on the valve responsible for moving the valve- opening, closing, and maintaining movement.
The actuator can be either manual- for example, a gearbox operator- or automatic.
What Is an Actuator Torque for Valves?
As established previously, the force required to turn a valve is known as ‘Torque.’ This leads to the concept of an ‘Actuator Torque.’
An ‘Actuator Torque’ is the amount of torque an actuator can produce. Remember: every type of valve requires a different amount of torque (force) to rotate fully opened and closed.
The quantity of torque an actuator can generate is a critical element in determining an actuator’s ‘rating specifications.’ Varying types of actuators can be used to accomplish different functions depending on their torque ratings.
Moreover, when trying to select the correct size of actuator required by your industry, it is essential to consider the different torque measurements required by your valve to complete a function.
Different Torque Measurements To Consider for Varying Applications
The following are the four main torque measurements that must be considered when searching for an actuator to carry out certain functions within your industry. Different valves and applications require different torque measurements.
Breaking Torque
The breaking torque is also known as the ‘Opening Torque.’ Breaking torque refers to the amount of torque that is required to unseat an initially closed valve.
Running Torque
Running torque is also called ‘Mid-stroke Torque.’ Running torque refers to the amount of torque required to control the frictional forces that accompany the application’s load.
Hence running torque is the torque required to maintain the valve at a given position when it is neither at a closed position nor an open position.
Closing Torque
The closing torque is otherwise known as the ‘Seating Torque.’ The closing torque refers to the amount of torque that will lead to a valve being fully closed.
Maximum Allowable Stem Torque
The maximum allowable stem torque (MAST) or ‘Maximum Shaft Torque’ is the maximum amount of torque that can be implemented on a valve train before it malfunctions or is damaged.
A valve train comprises all the parts of a valve drive- from the operator to the closure member.
However, it is essential to note that the operator, such as the gearbox, is not part of the valve drive. On the other hand- the closure member is included in the valve drive.
The critical takeaway is that an actuator’s maximum torque must not exceed the valve’s maximum allowable stem torque (MAST) at any point of energy travel.
However, if your company uses a manual actuator, the torque generated by the gearbox is usually not that large. Therefore, the manual actuator’s maximum torque surpassing the valves MAST is not a significant concern.
Safety Precautions for an Actuator’s Torque
Of course, for many industrial firms, the maximum amount of torque an actuator can enforce is a cause for concern.
This is because there may be imperfections when calculating the valve’s maximum torque value. This allows room for accidents within the industry that may be costly and may come with a lot of damage.
We firmly believe safety must always come first, so we advise you to maintain a 25% safety margin. For example, if a valve takes 125 in-lb torque to close, you’ll consider the valve to take 150 in-Ib torque to close- hence maintaining a 25% safety margin.
What Are a Modulating Service and On-Off Service?
The modulating service is a control valve known as the ‘Throttling Service.’
This specific control valve is responsible for controlling the output in the valve. This means it is usually never fully opened or closed and faces everchanging movement- constantly increasing, decreasing, or stopping mid-stroke.
Therefore, the control valve requires a greater torque than valves with simple functions such as opening or closing.
An on-off service is the opposite of a modulating service. An on-off service is a valve used to open or shut. Since an on-off valve will either be fully opened or closed and will not stop mid-movement- it does not require much torque.
Do Different Types of Valves Have Different Torque Measurements?
As indicated previously by the two different services- yes, they do! The torque that different valves require depends on their functions and the structure and type of the valve itself.
For instance, a resilient seated butterfly valve takes a large amount of torque for the on-off service but only a little for modulating services.
On the other hand- a metal seated ball valve needs a large amount of torque for both- on-off and modulating services.
What Happens if the Torque Required for a Given Application Is Inadequate?
The lack of required torque for a specific application can result in grave issues taking place.
For example, if the actuator’s torque is insufficient to unseat a valve, it will not open and might get stuck mid-stroke. On the other hand, if the actuator’s torque is more significant than required, it may inflict damage on the valve and system. Moreover, an actuator may have enough torque to unseat a valve but not close it.
Furthermore, a thesis by Trevor N. Price further signifies the importance of obtaining the correct size of an actuator. According to this thesis, if the valve is closed too quickly (for example, due to too much torque)- considerable transient pressures are created and travel in the form of waves through the pipelines.
These transient waves raise the pressure difference across the valve, increasing the dynamic torque applied to the valve. Considering the dynamic torque now exceeds the MAST of the valve, it could lead to the actuator or valve failure.
Conclusion
Everyone knows now- different types of valves have different torque measurements! However, it is rare for valve manufacturers to publish valves’ torque requirements- therefore, it will be essential to contact them to obtain accurate torque measurements for your respective valves.
Contact us– if you want to acquire the accurate torque measurements of our different valves- butterfly, gate, and check valves!
