A globe control valve is a type of valve designed to regulate the flow of process fluids in a pipeline. It operates using a linear motion mechanism, where a closure member (such as a plug or disc) moves into and out of a seating surface to control the flow. The valve body is characterized by a globular-shaped cavity around the port region, which gives it its name. Globe control valves are widely used in industrial applications to manage flow rates, pressure, and temperature in various processes. They are often paired with an actuator assembly to automate their operation.
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One of the key features of globe control valves is their versatility. Many single-seated valve bodies incorporate a cage or retainer-style construction. This design serves multiple purposes: it retains the seat ring, guides the valve plug, and allows for the customization of flow characteristics. By changing the trim parts, the valve's flow characteristics can be modified to achieve reduced-capacity flow, noise attenuation, or the reduction or elimination of cavitation. This adaptability makes globe control valves suitable for a wide range of industrial applications.
A single-seat globe control valve is a type of globe valve that features a single plug and seat arrangement. The trim (the internal components that control flow) is interchangeable within the valve body, allowing for customization to suit different process requirements. By replacing the trim components, the valve can be adapted to handle various types of liquids and process conditions.
Single-seat globe control valves are among the most commonly used control valves due to their simple structure and reliable performance. They are particularly well-suited for applications requiring tight shut-off and precise flow control. These valves are capable of handling a wide range of fluid services, including liquids, gases, and steam. The maximum flow rate can be controlled by adjusting the size of the plug and seat ring, while the fluid characteristics can be fine-tuned by modifying the shape of the plug's curved surface.
Single-seat globe control valves are ideal for applications where leakage must be minimized, such as in high-pressure or high-temperature systems. However, they are less suitable for applications with high-pressure drops due to the unbalanced forces acting on the plug.
Single-seat globe control valves have a straightforward design, making them easy to maintain and repair. The valve body typically features a single plug and seat arrangement, which simplifies the internal structure. However, this design also results in significant unbalanced forces when the valve is exposed to high-pressure fluids. The fluid pressure acts on the entire area of the plug, creating a large unbalance force that must be counteracted by the actuator. As a result, single-seat valves have lower allowable pressure drops compared to valves with balanced trim designs.
Despite this limitation, single-seat globe control valves are widely used in applications where tight shut-off is critical. Their simple structure and ease of maintenance make them a popular choice for many industrial processes.
A cage-guided globe control valve is a type of straight-stroke control valve that features a cage structure to guide the plug and control flow. Like single-seat valves, the trim in cage-guided valves is interchangeable, allowing for customization to meet specific process requirements. The cage-guided design is particularly effective in high-pressure drop applications, where it helps reduce the effects of cavitation and noise.
The cage structure also provides excellent interchangeability, enabling users to achieve different flow characteristics by replacing the cage. This flexibility makes cage-guided valves highly versatile and capable of delivering precise control in demanding industrial processes.
Cage-guided control valves are commonly used in sizes ranging from 1 to 12 inches and are typically paired with a standard diaphragm-type actuator. They are suitable for a wide range of industrial applications, including those involving high-pressure drops, cavitation, and noise. The valve's metal seat allows it to operate in temperatures ranging from -196°C to +538°C, while a soft seat extends the temperature range to -45°C to +200°C.
These valves are ideal for processes that require precise flow control, noise reduction, and cavitation mitigation. Their ability to handle high-pressure drops makes them a preferred choice for applications in industries such as oil and gas, power generation, and chemical processing.
The cage-guided control valve features a balanced design, which sets it apart from single-seat valves. The cage structure includes balance ports that help reduce the unbalance force acting on the plug. This design allows the valve to be paired with a smaller actuator, making it more efficient and cost-effective.
One of the simplest cage-guided valve designs is the twin-seat structure, which is suitable for applications where leakage is not a critical concern. However, for applications requiring tight shut-off, the cage-guided single-seat control valve is the preferred choice. This design incorporates a sliding seal between the plug and the cage, preventing upstream fluid from leaking into the downstream system. As a result, cage-guided single-seat valves offer both tight shut-off capabilities and balanced performance.
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Single Seat Globe Control Valve: Features a simple design with a single plug and seat arrangement. The trim is interchangeable, allowing for customization of flow characteristics. However, the single-seat design results in significant unbalanced forces, limiting its suitability for high-pressure drop applications.
Cage Guided Globe Control Valve: Incorporates a cage structure to guide the plug and balance the forces acting on it. The cage design reduces unbalance forces, allowing for the use of a smaller actuator. The trim is also interchangeable, providing flexibility in flow characteristics and performance.
Single Seat Globe Control Valve: Offers tight shut-off capabilities, making it ideal for applications where leakage must be minimized. However, its performance is limited in high-pressure drop scenarios due to the unbalanced forces acting on the plug.
Cage Guided Globe Control Valve: Excels in high-pressure drop applications, where it effectively reduces cavitation and noise. The balanced design allows for precise control and efficient operation, even in demanding conditions.
Single Seat Globe Control Valve: Best suited for applications requiring tight shut-off and precise flow control in low to moderate pressure drop scenarios. Commonly used in industries such as water treatment, HVAC, and general process control.
Cage Guided Globe Control Valve: Ideal for high-pressure drop applications, such as those found in the oil and gas, power generation, and chemical processing industries. Its ability to handle cavitation and noise makes it a versatile choice for demanding processes.
Single Seat Globe Control Valve: Easy to maintain and repair due to its simple design. The interchangeable trim allows for customization, but the valve's performance is limited by its single-seat structure.
Cage Guided Globe Control Valve: Offers excellent interchangeability, with the ability to achieve different flow characteristics by replacing the cage. The balanced design reduces wear and tear, resulting in longer service life and lower maintenance costs.
Both single-seat and cage-guided globe control valves have their unique advantages and limitations. Single-seat valves are ideal for applications requiring tight shut-off and precise flow control in low to moderate pressure drop scenarios. Their simple design and ease of maintenance make them a popular choice for many industrial processes.
On the other hand, cage-guided valves are better suited for high-pressure drop applications, where they excel in reducing cavitation and noise. Their balanced design allows for precise control and efficient operation, making them a preferred choice for demanding industrial processes.
Ultimately, the choice between single-seat and cage-guided globe control valves depends on the specific requirements of the application. By understanding the strengths and limitations of each type, engineers and operators can select the right valve to optimize their processes and achieve reliable performance.
When control valve professionals talk about "control valve sizing," they really mean the entire process of selecting the valve that will do the best job of controlling the process. Selecting the right size valve is an important part of the process, but there are other equally important considerations as well.
The control valve's size should be selected so that it will operate somewhere between 60 and 80% open at the maximum required flow rate and whenever possible, not much less than 20% open at the minimum required flow rate. The idea is to use as much of the valve's control range as possible while maintaining a reasonable, but not excessive, safety factor.
If the valve is too small, it will be obvious immediately, that it will not be able to pass the required flow. In actual practice, undersized valves are fairly uncommon. Commonly, the valve is too large. An oversized control valve will cost more than is necessary, but more importantly, an oversized valve will be very sensitive. Small changes in valve position will cause large changes in flow. This will make it difficult or even impossible for it to adjust exactly to the required flow. Any stickiness caused by friction will be amplified by the overly sensitive oversized valve, reducing the precision to which the flow can be controlled.
In the illustration above, assuming that both valves are capable of positioning within 1%, the properly sized 3-inch valve will be able to control flow within 8 gpm, while the oversized 6-inch valve will only be capable of controlling flow to within 20 gpm.
Cavitation
Liquid applications must always be evaluated for cavitation. Not only does cavitation cause high noise and vibration levels, it can result in very rapid damage to the valve's internals and/or the downstream piping. Especially with rotary valves, the prediction of damaging levels of cavitation is more complex than simply calculating the choked flow pressure drop. As a result of flow separation and the formation of eddies within the valve, localized areas of pressure reduction and recovery can cause damaging cavitation at pressure drops well below that which results in fully choked flow. One proven method for predicting cavitation damage in rotary control valves is based on a correlation between calculated sound pressure level and the potential for damage.
Noise
In addition to the fact that a noisy valve in liquid service will most likely suffer unacceptable rates of cavitation damage, high noise levels usually cause vibration that can damage piping, instruments, and other equipment. Control valves in steam and gas service can generate noise levels well in excess of plant standards, even at moderate pressure drops, especially in sizes above 3 or 4 inches. As a result, the valve sizing and selection process must always include noise calculations.
Installed Flow Characteristic
In nearly all applications, a control valve should have a linear installed flow characteristic (the relationship between controller output and flow in the system). The control valve's inherent (published) flow characteristic interacts with the system's flow vs. pressure loss characteristic to yield the installed flow characteristic. If the installed characteristic deviates significantly from linear, it will be difficult or impossible to tune the loop for both accurate and stable control throughout the entire flow range. A computerized analysis of the installed characteristics should be part of the control valve sizing and selection process.
Actuator Sizing
Sizing actuators for on-off service is fairly straightforward, requiring only that an actuator be selected with a torque output slightly higher than the seating and unseating torque of the valve. The situation is more complex with control valves. The torque output of most rotary actuator's changes with the angle of opening. At the same time, the valve's torque requirement depends both on the opening angle and the throttling pressure drop at that particular angle. To ensure adequate spare torque to guarantee smooth, accurate control, a computerized analysis is recommended.
Selecting Control Valve Style
The choice of control valve style (globe, ball, butterfly, etc.) is often based on tradition or plant preference. For example, a majority of the control valves in pulp and paper mills are usually ball or segmented ball valves. Petroleum refineries traditionally use a high percentage of globe valves, although the concern for fugitive emissions has caused users to look to rotary valves because it is often easier to obtain a long-lasting stem seal with rotary valves.
Globe valves offer the widest range of options for flow characteristics, pressure, temperature, and noise and cavitation reduction. Globe valves also tend to be the most expensive. Segment ball valves tend to have a higher rangeability and size for size, nearly twice the flow capacity of globe valves, while they are typically less expensive than globe valves. On the other hand, segment ball valves are limited in availability for extremes of temperature and pressure and are more prone to noise and cavitation problems than globe valves.
High performance butterfly valves are even less expensive than ball valves, especially in larger sizes (eight inches and larger). They also have less rangeability than the ball valves and are more prone to cavitation.
The eccentric rotary plug valve combines the features of rotary valves, such as high cycle life stem seals and compact construction, with the rugged construction of globe valves. Unlike the other rotary valves whose flow capacity is approximately double that of globe valves, the flow capacity of eccentric rotary plug valves is on par with globe valves.
While the selection of a valve style is highly subjective, in the absence of a specified valve or plant preference, the following approach can be used to select a control valve style for applications where the valve will be six inches or smaller:
- Considering pressure, pressure differential, temperature, required flow characteristic, cavitation, and noise, will a segment ball valve work?
- If a segment ball valve is not suitable, select a globe valve. Keep in mind that cage guided globe valves are not suitable for dirty service.
- For applications where the valve will be 8 inches or larger, first investigate the applicability of a high-performance butterfly valve because of the potential for significant savings on cost and weight.
Ensuring Accuracy
Today control valve sizing calculations are usually performed using a computer program. Most manufacturers of control valves offer control valve sizing software at no cost, though most are specific to that manufacturer's valves only. Metso's Nelprof, however, includes a number of generic valves, such as globe valves, ball valves, plug valves, and butterfly valves, to choose from. These generic selections permit the user to investigate the applicability of different valve styles and sizes to a particular application, without showing a preference to a particular valve manufacturer.
Selecting a properly sized control valve is essential to achieving the highest degree of process control for the liquid, gas, or multi-phase fluid. To ensure accuracy, use the following information for control valve sizing:
- If a set of loop tuning parameters only works at one end of the control range and not the other, the valve's flow characteristic is most likely the wrong one.
- If a system has a lot of pipes, use an equal percentage valve.
- If a system has very little pipe, use a linear valve.
- A control valve that is sized to operate around 60% to 80% open at the maximum required flow and not much less than 20% open at the minimum required flow will give the best control.
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