There are countless types of valves for use across a variety of industries and applications. When it comes to flow control valves, valve types range from simple to sophisticated; some valves are complex enough to adjust automatically to pressure and temperature variations. No matter their construction, flow control valves are designed to regulate the flow or pressure of fluids, and they typically react to signals generated by flow meters or temperature gauges.
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Flow control valves can serve a number of different functions within a hydraulic flow system depending on the specific type that is used. One of the most common uses of a flow control valve is to regulate the speed of motors or cylinders within the system. This function is possible due to the capability of a flow control valve to affect the rate of energy transfer at any given point in a system by impacting the flow rate.
The ability to reduce or increase pressure in a system has a number of benefits. System operators can use a flow control valve to rapidly depressurize a serviceable hose and change fittings quickly. They are also used in many consumer applications such as showers, faucets, and lawn watering systems to easily reduce the amount of water consumed without impacting the overall system performance. Flow control valves are also known for their reliability and typically have a long operating lifetime as they are not prone to clogging due to their design.
Due to these flexible performance parameters, flow control valves have found wide use in applications across materials handling, food processing, and automated factory and warehouse equipment.
The most common valve types in flow control industries include:
Continue reading to learn more about each of these types of flow control valves and their functions.
Gate valves are general service valves primarily used for on/off, non-throttling service. Specifically, gate valves are used in applications requiring a straight-line flow of fluid with minimum restriction is desired. Gate valves operate when the user rotates the stem in a clockwise to close (CTC) motion or a clockwise to open (CTO) motion. The gate moves up or down on the threaded step when an operator moves the stem, which is why it is a multi-turn valve; the valve must turn several times for it to go from open to closed, and it is the slow operation that prevents water hammer effects. Engineers also utilize gate valves when minimum pressure loss and a free bore are required. Typical gate valves have no obstruction in the flow path, which results in a minimal loss of pressure.
Gate valves may be used for several fluids. Generally, gate valves are applicable for potable water, wastewater, and neutral liquids; in temperatures between -20 and 70 degrees Celsius; maximum 5 meter/second flow velocity; and up to 16 bar differential pressure. Gate valves also are applicable for gases with temperatures between -20 and 60 degrees Celsius; maximum 20 meter/second flow velocity; and up to 16 bar differential pressure.
There are two types of gate valves: parallel and wedge-shaped. Parallel gate valves feature a flat gate between two parallel seats. Wedge-shaped gate valves are comprised of two inclined seats and an inclined gate that is just a bit mismatched.
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A linear motion valve, globe valves stop, start, and regulate flow. Globe valves initiate closure via a plug featuring a flat or convex bottom that is lowered onto a horizontal seat situated in the center of the valve. When a user opens the valve, the plug raises to allow fluid to flow. Globe valves are used for on/off and throttling applications because the disk of the valve can be removed from the flow path completely or it can completely close the flow path. While this type of flow control valve does produce slightly higher pressure drops than straight-through valves like gate, plug, and ball valves, they are applicable in situations where the pressure drop through the valve is not a controlling factor.
The practical size limit for globe valves is NPS 12 (DN 300) because the entire system pressure exerted on the disc transfers to the valve stem. It is possible, however, to have globe valves larger than NPS 12 (DN 300), and manufacturers and engineers have created and used globe valves up to NPS 48 (DN ).
A cost-effective flow control valve, pinch valves are ideal for applications of slurries or liquids containing significant amounts of suspended solids. Pinch valves seal using one or more flexible elements like rubber tubes that become pinched to turn off the flow. These rubber sleeves are the valve&#;s only wetted part, and their flexibility allows pinch valves to close tightly around entrapped solids. Air or hydraulic pressure is placed directly on the elastomer sleeve to actuate pinch valves. A pinch valve&#;s body acts as a built-in actuator, which eliminates expensive hydraulic, pneumatic, or electric operators and results in the cost-effectiveness of this type of flow control valve.
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Diaphragm valves are characterized by a flexible disc that contacts a seat at the top of the valve body and forms a seal. The diaphragm is flexible and pressure-responsive; it transmits force to open, close, or control a valve. While diaphragm valves are related to pinch valves, they use an elastomeric diaphragm rather than an elastomeric liner in the valve body. The elastomeric diaphragm is attached to a compressor and separates the flow stream from the closure element. Diaphragm valves are ideal for handling corrosive, erosive, and dirty services.
There are many advantages to using diaphragm valves: they are extremely clean, feature a leak-proof seal, have a tight shut-off, are easy to maintain, and reduce leakage to the environment. Diaphragm valves also may be repaired without interrupting a pipeline. On the other hand, the disadvantages of using diaphragm valves include only being able to use them in moderate temperatures of -60 to 450 degrees Fahrenheit and in moderate pressures of approximately 300psi. Diaphragm valves cannot be used in multi-turn operations and do not have industry standard face-to-face dimensions. Also, the body of a diaphragm valve must be made of corrosive-resistant materials.
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Needle valves are volume control valves that restrict flow in small lines. Fluid moving through the valve turns 90 degrees and flows through an orifice that serves as the seat for a cone-shape-tipped rod. The orifice size changes when the user positions the cone in relation to the seat. Needle valves are similar to globe valves in that they share a few design features and have similar benefits; for example, both needle valves and globe valves empower operators to change flow rate using a threaded rotating stem. The difference between needle valves and globe valves is the precision that needle valves can achieve. In fact, needle valves are an ideal choice for calibration applications because they are capable of being fine-tuned.
Needle valves can provide positive shutoff in order to allow gauges and other measurement instruments to be installed or removed safely. That&#;s also why needle valves may be used in a range of industries, from petrochemicals to biofuels. It is the needle valve&#;s finely-threaded valve stem that gives it a significant mechanical advantage by allowing operators to seal it using only minimal force. One disadvantage of needle valves, however, is that the visual inspection alone is not enough to determine whether a needle valve is open or closed.
Flow control valves are necessary components in a broad range of industries. Determining which flow control valve type is best for your particular situation depends on a host of criteria, but the most commonly used types include gate valves, globe valves, pinch valves, diaphragm valves, and needle valves.
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While the five types of flow control valves described above are some of the most commonly used valve types, there are other types of flow control valves with features that make them suitable for different applications. Here&#;s a look at a few other types of flow control valves.
Butterfly valve. A butterfly valve is operated by rotating a disk within the flow area and, due to this design, it does not have linear flow characteristics. This makes these valves less precise than the more common flow control valve types above. For this reason, it can often be dismissed as a flow control valve choice even though it is useful in some applications that do not require a very high degree of accuracy. They are also a very affordable valve option, which makes it worthwhile to consider them in the right applications.
Plug valve. Plug valves come in a variety of configurations and are operated by rotating a cylindrical or cone-shaped plug within the valve body to regulate the flow through a hollow area of the plug. For flow control applications the most common design is an eccentric plug valve, which uses a half plug to create a higher seating force with minimal friction as it is opened and closed. This has the advantage of greater shut off capability which is ideal for flow control situations.
Ball valve. Ball valves are commonly used in flow systems across numerous industries due to their low cost, durability, and excellent shutoff capability. Similar to butterfly valves, they are not as effective for flow control applications that require a high degree of accuracy and control. One of the reasons for this is that a ball valve requires a high degree of torque to open and close that prevents an operator from making fine adjustments. There is also a certain amount of &#;play&#; between the stem and the ball which can make finding specific flow rates difficult. For flow control applications where a ball valve is possible, such as filling a tank to a reasonable degree of accuracy, a trunnion or v-port ball valve design is usually the best choice.
Flow control valves are used in a variety of applications, such as plumbing, mechanical, and gas dispensing applications. There are many factors to consider when choosing the appropriate flow control valve for an application, such as the characteristics of the fluid, service conditions, how frequently the valve is operated, and maintenance and environmental considerations. With a variety of valve types available, comparing the function and performance of various valves alongside your application specifications will help you identify the most suitable flow control valve for your application.
Selecting the appropriate control valve technology doesn't fall into the one-size-fits-all category. The process to be controlled has an influence on the best choice of valve. So too does the level of control accuracy required to meet the needs of the process.For example, the question should be posed, &#;how accurate does the control need to be?&#; Initially, this question sounds a bit silly. Of course, if we&#;re controlling a flow, it should be accurate, right? Well, it may need to be, but on the other hand, it really may not need to be very accurate. The reason this is so important is the accuracy level has a direct correlation to the price tag.As control packages become more precise and speed of response increases, the cost typically rises as well. There are certainly operations in the field where a more expensive control package is in place than is required based on the level of control needed. Alternatively, there are control packages in operation that don&#;t make the grade based on what's required. How does one know if what they're trying to control dictates a more expensive, more precise approach, or if a less expensive, less accurate option will do a sufficient job? There are several factors that should guide this decision, allowing plant operators to select the best option.A globe control valve is frequently considered the &#;gold standard&#; of control valves (Figure1). They were the first modern control valves, and over the past century have been refined and modified to work in some of the toughest services. Some form of globe valve will get the job done in almost all cases. They're particularly well suited for high-temperature, high-pressure-drop applications where long, constantly bending flow paths cause continuous loss of pressure and reduced pressure recovery, minimizing noise generation and cavitation.Globe valves have more severe service options than other valve types, so there are times when nothing but a globe valve will survive and function properly. They also offer replaceable trim sizes for each line size, allowing an improperly sized valve to be corrected, or letting users start with a reduced-trim valve and change to a larger trim as capacity increases.However, the inherent design of a globe valve can create drawbacks in some applications. They don't tolerate suspended solids well and can jam and quit working. They also tend to be heavy and physically large, which can be a problem in a system requiring tight packaging. Finally, they also tend to be the most expensive type of valve for a given size.More recently, quarter-turn or rotary valves have been used in many control applications. These can include a butterfly valve, ball valve, segment valve or an eccentric plug valve. While there were some failures when used in processes where they were applied inappropriately, several decades of progress in materials and design enhancements have greatly increased the range of processes in which quarter-turn valves can be safely used.Quarter-turn valves are not just occasionally acceptable substitutes for globe valves (Figure 2). Rather, they actually outperform globe valves in processes with suspended solids, like pulp-stock, sewage, mining slurries and powder conveying. And all but eccentric-plug valves have higher flow capacities (CV) for a given line size than globe valves, allowing a smaller valve to be used, saving weight and cost.Butterfly and segment valves (Figure 3) are physically small and light relative to their CV, making pipe design easier than accommodating and supporting a heavier valve. Not coincidentally, they're also less expensive because they require less metal and machining time than globe valves.What are the areas we need to consider when determining if a valve type is a good fit for a process? Temperature and pressure are two of the most important.When discussing temperature limitations, one must focus on the extremes. If temperature requirements are moderate/mild, many soft-seated rotary valves will do the job. However, once temperatures reach or exceed 400 ºF, soft-seated valves start to have trouble handling the stress, and a metal-seated valve may be required. A metal-seated valve is typically higher in price. If temperatures approach 800 ºF, it may make the most sense to recommend a globe control valve.There are competing factors that can affect the decision-making process regarding pressure. Rotary valves are available with ANSI pressure class ratings of and higher, just as for globe valves. But rotary valves typically generate more noise and are more prone to cavitation if control differential pressures are high. Proper sizing and noise calculations are required to verify a rotary valve will work in a high-pressure system. It's typically easier to find a globe valve for high-pressure and especially high-pressure-drop applications.However, rotary valves also can have an advantage in high-pressure systems. The required actuator size for a globe valve increases quickly with increasing pressure, even with more expensive balanced designs. Many rotary designs have less increase in required actuator size as pressure increases, saving money and reducing assembly size.Similar to temperature, increasing differential pressure requirements reach a point when it becomes difficult to find a rotary valve that will work. Depending on the process fluid and conditions, this can be at 100-30-psi differential pressure. If there are higher pressure drops across the valve, however, it's likely that a properly designed globe control valve is the only option that will work.Steam can be worrisome as it tends to do a great job of eroding materials. The characteristics of basic rotary valves are well-suited for air and water, but can have problems with steam. You may need to use a special severe-service rotary valve for steam applications with a 50-psi pressure drop or more. High-performance butterfly valves can be remarkably effective for low and medium differential pressure applications and be extremely cost effective. High differential pressure applications, such as steam turbine power plants, usually will require a specially designed and very expensive globe valve.Once the owner understands the temperature, pressure and material conditions the process needs to operate under, the next question is &#;how precise does the control need to be?&#;There are some applications when using a rotary valve for control will work perfectly. A good example is a simple water temperature control system for creating a warm output. If the process has a lot of capacity and temperatures don&#;t change very quickly, a ball valve will work well. This can also be done with a three-way globe control valve, and it will do a more accurate job of controlling the outlet. However, this approach will be far more expensive than using a basic ball valve. The question is, how accurately do you need to control the temperature? Does it really matter if the water is exactly 100 °F, or will 98-102 °F work? If it really doesn&#;t matter for a specific application, a three-way globe control valve may be unnecessary and an unwarranted expense.Part of this discussion also lies in rangeability, which is the difference between lowest flow and highest flow. What is that difference? One can control a globe control valve down to 5% of its flow and up to 90% without too much concern. There is a rangeability of 10-20:1 for a valve with linear trim, and as much as 50-100:1 with equal percent trim.With a butterfly valve, due to its design, one should not operate it below 10% and above 70% open, where the gain curve flattens and the valve is effectively all the way open.Fortunately, there are rotary valves that do offer wide rangeability. A segmented ball valve, commonly called a V-Ball, has a deeply characterized v-shaped control element, and can provide the same 50-100:1 rangeability as an equal percent globe valve. A segmented ball valve is more expensive than a butterfly and most other ball valves of the same materials and pressure class, but it will typically be half the price of a globe valve with the same flow capacity.Price is a significant factor at this stage of the decision-making process. In general service, a ball valve will increase in price faster than a butterfly valve as the size increases. At small sizes, typically under 4-in., there may be little difference in the price of a ball valve or a high-performance butterfly control valve. However, once you reach 6-8-in., the ball valve will be more expensive than a butterfly valve, and for large sizes (greater than 12-in.), you'll definitely want to use a butterfly valve if it will handle the process conditions.A globe control valve, on the other hand, may be three to four times the price of the butterfly valve, with similar increases in the total cost of ownership (TCO) over the life of the valve.The decision should always be determined by the application requirements. If the process conditions and rangeability requirements allow the use of a rotary valve, it will likely be much less expensive than a globe valve. If the conditions can only be met by a globe control valve, then that is what should be used.On the other hand, globe control valves are often specified when they simply don&#;t need to be. If a rotary valve can handle the temperature, pressure, nature of the product flowing through the line and meet control requirements, then it should be considered. It will cost the operation far less in TCO over the life of the valve.Article featured in Control Global Magazine
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