May. 13, 2024
If you need to adjust the flow rate of media anywhere in your piping system, you’ll need a throttling valve. In addition to stopping and starting the flow of liquid or gas media, throttling valves allow system operators to reduce the volume and pressure of the media flowing through the system. In a piping and instrumentation diagram (P&ID), throttling valves are identified using the following symbol:
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Throttling valve symbol in a P&ID.
Unlike a control valve, which works by partially opening or closing to change the flow rate of media, a throttling valve does not necessarily need to change position at all, since its sole purpose is to reduce media pressure and velocity in a piping system. For example, a globe valve that remains partially open throughout its entire lifespan to slow the flow of media could be considered a throttling valve.
However, since environmental factors like temperature can alter media flow rate, position control is often required to maintain consistent flow rates as required by the piping system. Throttling valves are often called control valves, but the terms are not always interchangeable. Control valves allow for the control of flow rate through a positioning actuator, enabling precise valve positioning. Throttling valves, on the other hand, are specifically designed to reduce the velocity or pressure of media within a piping system.
Essentially, almost all control valves can function as throttling valves, but not all throttling valves qualify as control valves due to the lack of dynamic valve position control.
Pipeline systems are not complete without industrial valves. They come in different sizes and styles to cater to varied needs.
Industrial valves can be classified according to their function. There are valves that stop or start the flow of media, those that control the direction of fluid flow, and others that can vary the flow rate of media.
Choosing the right kind of valve is crucial to an industrial operation. The wrong type of valve can lead to system shutdowns or underperformance.
A throttling valve can open, close, and regulate media flow. Throttling valves are essentially regulator valves. While some people use "control valves" interchangeably with throttling valves, there is a distinct difference between the two. Throttling valves have discs that not only stop or start media flow but also regulate the amount, pressure, and temperature of the media passing through, based on their position.
Throttling valves exhibit a higher pressure on one end and a lower pressure on the other, which determines the degree of closure of the valve. A diaphragm valve is an example of this mechanism. In contrast, control valves adjust media flow using an actuator and cannot function effectively without one. They manage the disruptions in flow caused by pressure and temperature changes, often altering flow or pressure to meet the system's requirements.
In this context, control valves are specialized throttling valves. Although control valves can throttle, not all throttling valves function as control valves.
When a throttling valve is used in a pipeline, the media flow rate changes. By partially opening or closing the valve, a restriction is created, slowing the flow of fluid and compacting the media within the valve. This leads to friction between the media molecules, which further reduces flow velocity.
To illustrate, consider a garden hose: when unobstructed, water flows out with minimal resistance. Using a thumb to partially cover the hose's opening increases the water's velocity and pressure due to the restriction—this is basic throttling.
Applying this concept in a pipeline system, if cooler gas needs to be heated to a certain temperature, a throttling valve can increase the gas temperature by forcing molecules to rub against each other as they pass through the restricted opening.
Source: https://www.quora.com/What-is-the-throttling-process
There is a broad range of uses for throttling valves. Often, one can find throttling valves in the following industrial applications:
Not all valves are suitable for throttling. The design of a valve is a key factor in determining its appropriateness for throttling applications.
Globe valves are among the most popular types of valves, primarily used as throttling valves. Belonging to the linear motion valve family, the globe valve's disk moves up or down in relation to the stationary ring seat, regulating the amount of media that passes through.
The space between the seat and the ring makes the globe valve an effective throttling valve with minimal wear to the seat and disk.
In high-pressure applications, globe valves require an automatic or powered actuator to move the stem and open the valve. Considerations for efficient throttling capabilities include pressure drop and flow control range.
There is a risk of leakage due to seat damage, as well as susceptibility to vibration effects, especially when handling gas media.
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Butterfly valves, part of the quarter-turn valve family, differ from gate valves in this aspect. An actuator, influenced by an external force, attaches to the stem linked to the disc. The butterfly valve is commonly used for throttling, as a full quarter turn can open or close the valve, while a partial opening allows media to pass.
The disc of butterfly valves remains in the media flow path, making it prone to erosion and difficult to clean internally. Accurate calculations are required to determine max flow and pressure requirements for effective use.
Gate valves, another linear motion valve type, have discs that move up and down to open or close the valve. Though primarily used for on-off services, they have throttling limitations.
Throttling occurs in a nearly shut position, increasing media velocity upon exit. However, gate valves are not suitable for long-term throttling due to potential seat damage and vibration effects.
Gate valves should only be used for throttling when nearly closed, as a 50% closure does not achieve desired throttling capabilities. Pressure can damage the gate seat, and vibration can further affect the valve's integrity.
Pinch valves, with a simple design and soft elastomer liner, close by pinching the liner using fluid pressure. They are lightweight, easy to maintain, and effective in sanitary conditions.
The valve closes when the compressor pinches the liner, offering throttling capabilities from 10% to 95% flow rate, with optimal efficiency at 50%.
Pinch valves are ineffective with media containing sharp particles or for gas media, high-pressure, or temperature applications.
Similar to pinch valves, diaphragm valves use an elastomer diaphragm as the throttling element. They allow larger particles to pass and are better suited for throttling in systems requiring sterile conditions.
The weir-type diaphragm valve is particularly effective for throttling purposes.
While providing a non-leak design, diaphragm valves can only handle moderate pressure and temperature ranges and are unsuitable for multi-turn operations.
Needle valves resemble globe valves but feature a needle-like disk for precise regulation, ideal for applications requiring exact flow control.
Fluid flow changes direction at a 90° angle, which may involve partial disk movement through the seat before complete closure.
Needle valves are unsuitable for viscous media due to their small openings, which can trap particles in slurries.
Understanding the purpose of the throttling valve and recognizing each type's advantages and limitations will narrow down the choices for the right kind.
Appropriate valve size will prevent future issues. An oversized valve may offer limited throttling capacity, leading to vibration and erosion. Additionally, oversized valves require costly fitting adjustments to match existing pipes.
The valve material must be compatible with the media type. For instance, chemical-based media require non-corrosive materials, while high-temperature or high-pressure media need strong alloys with internal coatings.
Actuation is crucial, especially in high-pressure applications where a manual actuator may be insufficient for opening or closing the valve.
Valves should adapt to the existing pipe connections to minimize costs and complications. For example, flanged valves should match flanged pipe ends.
Adhering to industry standards ensures safety by specifying appropriate materials, end connections, and valve construction for specific applications.
While most valves have limited throttling capabilities, selecting the suitable valve for specific throttling applications ensures longevity and efficiency.
Reference valve manufacturer resource: The Ultimate Guide: Best Valve Manufacturers in China
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