Figure 1. Ball valves can be used in a wide range of applications, from general purpose to critical-service applications. They are useful for reliable, leak-tight shut-off and have a low overall cost of ownership.
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Choosing the proper components for your industrial fluid systems is critical to ensuring they function as intended. One of the most important components to consider are the valves, which give operators the ability to control system fluid flow in ways that are most appropriate for the application&#;s needs.
It is important to consider the factors that affect complex fluid system design, including:
You should ask all these questions about your fluid system before finalizing your valve selection. In this article, we will provide examples of different industrial valves to help you make the right choice.
Fluid systems sometimes operate at high pressures and temperatures, and occasionally they carry hazardous materials that could pose a threat to the operators if leaks occur. To ensure fluid systems do not pose unnecessary hazards, best practices for installation and operation should be followed.
Valves play critical roles in enabling safely functioning fluid systems. For example, a safety shut-off valve or pressure relief valve can prevent your system from reaching overpressure, a potentially dangerous scenario that may lead to a blowout. That is why it is so critical to select the right valve for the function you are trying to achieve, which starts with understanding how flow operates within your system.
Ultimately, valves are designed to control flow, which is defined as a substance&#;s movement in a steady and continuous way through your system from higher pressures to lower pressures. Using a flowmeter, the pace of flow is recorded as a ratio of distance or volume per unit of time. For example, flow might be measured as meters per second, liters per minute, gallons per day, or other similar measurement criteria.
The diameter of a valve&#;s end connection and its flow path determine how well valves allow flow to occur. Manufacturers often include a flow coefficient, or Cv, with their valves, which gives operators a better understanding of how much flow a valve will control. The higher a Cv, the higher the flow rate &#; although, higher is not always better. The Cv you choose is highly dependent on the valve type and application. In some situations, that may mean the Cv will be close to zero.
While this may seem complicated, manufacturers can often help you determine the right valve selection for your application. To determine Cv or flow based on pressures, flow rates, temperatures, and media within your system, ask your manufacturer if they have a Cv calculator to offer the guidance you need.
Choosing the right valve for your application may initially seem overwhelming. After all, valves come in many sizes, configurations, materials of construction, and actuation modes. To make the best choice, it is always good practice to ask the first question in valve selection: What do I want the valve to do? Once you have answered that question, you can more easily decide what specific valves you need, which generally fall into one of five categories.
1. On/Off is the most basic type of valve function. On/off valves start or stop the flow of fluid, and there are many different types of valves to choose from, including:On/Off is the most basic type of valve function.start or stop the flow of fluid, and there are many different types of valves to choose from, including:
Figure 1)
Figure 2
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Gate valves are designed primarily for blocking flow rather than regulating it. They contain a plate-like barrier (gate) that can be inserted into the stream of a fluid to block its flow.
Figure 3
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2. Flow control valves are designed to regulate the flow of a fluid through the system. The amount of regulation depends on what type of valve is selected and can range from simple regulation to fine metering. The most common flow control valves are needle valves (Figure 4), which can provide on/off functionality if necessary. Orifice size, stem type, and stem position, which are controlled by the turning of the valve handle, will determine the pace of flow.
3. Directional flow valves guide the flow in the proper manner and are used to change the direction of the flow, if desired. The most common valves used for directional control are check valves (Figure 5) or multiport ball valves. It is important to note that directional flow valves do not throttle flow; instead, they operate in either the on or off position.
4. Much like the overpressure valves, excess control valves are designed to prevent problems if the flow levels at the valve rise to unsustainable levels. If excess flow happens downstream, the valve&#;s poppet is activated to the fully forward position, which prevents an uncontrolled release of system media.
5. In the case of overpressure protection valves, the goal is to prevent pressure buildup beyond a preset limit. For this application, systems typically rely on relief valves (Figure 6) or rupture discs. Relief valves are essential to any system that operates under pressure to avoid blowouts and can be calibrated to open after the pressure reaches predetermined levels. They are generally considered the last line of defense when pressures rise too high and can protect plants by allowing production to continue by relieving overpressure when it occurs.
The true cost of a valve is not its purchase price &#; it is the purchase price plus the cost of owning and maintaining or replacing that valve over time. To calculate the cost of ownership, you must know how long a valve will operate in your system between maintenance checks.
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Maintenance costs must not only be calculated on the cost of replacement parts, but also in labor and downtime. Note that some valves are much easier to service than others. Some can be serviced in place; others must be removed from the process line.
As you design your fluid system to be as optimized as possible, selecting the right valves to meet your application needs is important. The better your understanding of each valve and its proper function, the better able you will be to make the decision that most fully meets your specific needs.
Work with a supplier who offers training on valve selection, identification, and troubleshooting so your team can remain up to speed and knowledgeable about the latest in valve innovations.
An original version of this article appeared on the Swagelok Reference Point blog here: swagelok.com.
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Joe Bush is a Senior Product Manager for Swagelok Company.
Tight shut-off refers to the condition where there is no leakage of fluid when a valve is in the closed position. This is vital in flow systems where isolation is necessary between the upstream and the downstream. The isolation could be required as a safety measure or for maintenance activities to be carried out on downstream equipment. Shut-off valves are fitted into the flow systems to achieve this isolation.
There are many shut-off valves with unique characteristics and it's important to select the right valve for the intended tight shut-off application. Below are some of the factors to consider when selecting a valve for a tight shut-off application.
Valve Class
As defined by ANSI/FCI 70-2 , there are six leakage classifications for shut-off valves.
Class I: Valves in this class are also referred to as dust-tight valves and can include both metal and resilient seated valves. No actual tests are carried out to determine the leakage rate. They are similar in construction to class II to class IV valves.
Class II: These valves have metal-to-metal seats with a metallic piston ring seal. They can be constructed as balanced, single port, low seat load, or balanced, double port, high seat load.
Class III: They have a similar construction as class II valves. However, they also include medium-seat loaded valves.
Class IV: These are also referred to as metal-to-metal valves. They basically include balances and unbalanced single-port valves that have extra tight piston seals with metal-to-metal seats.
Class V: They have a similar construction as class IV valves; balanced, unbalanced, single port with low and high seat load.
Class VI: They are also known as soft seat valves designed for resilient sealing. Amongst the various classes, these are the most reliable for tight shut-off as the leakage allowed is much smaller than the other classes. These valves are tested with different test media and under different test pressures to determine the leakage rate. Class VI valves offer the best tight shut-off compared to other valve classes.
Valve Type
Though there are different types of valves (ball, butterfly, ball, gate, plug valve, etc.) for shut-off applications, each has its pros and cons, which must be considered during the selection process.
Butterfly valves are suitable for systems with clean fluids. Due to their disc sealing system, they are not suitable for fluids with slurries or grits. By design, when the disc is in the perpendicular position, the plate sits against the internal seal, creating a tight closure. They are more common in large-diameter pipes.
Gate valves by design can accommodate slurries and grits in the fluid, unlike butterfly valves.
Plug valves are quarter-turn style valves like butterfly valves. Plug valves are designed such that the rubber-encapsulated plug seats and the plug face are offset from the shaft centerline thus, providing a tight seal when closed. Though similar to ball valves, plug valves have no cavities that might trap media/fluid in any position.
Pinch valves, also known as clamp valves, are designed to shut off the flow from abrasive, granular, fibrous, or corrosive media optimally. The rubber sleeve in the clamp valve has the capability to trap the particles around it, resulting in an excellent shut-off.
Type of Actuation
A valve can be actuated by different mechanisms. They are manual, electrical, pneumatic, and hydraulic actuators.
Manual actuation is operated by hand. In simple flow systems, this is economical. However, in high-pressure flow systems, hydraulic actuators are capable of generating high power to shut off the flow.
Pneumatic actuators are generally quicker than hydraulic ones but still provide comparable actuation power much more than electrically energized actuators. Pneumatic actuation may be powered by compact air cylinders.
Shut-off valves should be selected with the intended actuation in mind to ensure optimum compatibility.
Valve Seat Type
Valve seats refer to the closure members that are in contact with each other during the closure of the valve. The seat materials are subject to wear and tear during the valve-closing process. There are two types of valve seats; metal seating and soft seating.
Soft seating uses soft materials such as rubber or plastic on one or both of the closure surfaces. These materials are easy to match and can achieve a high degree of fluid tightness repeatedly without considerable wear.
Metal seats tend to wear off over time with repeated open and close movements of the valve.
Valve Material
There are different materials for the different components in a valve, ranging from the housing to the seating material. Materials commonly used in valves include aluminum, copper, bronze, brass, iron, carbon steel, stainless steel, etc.
Stainless steels offer excellent resistance to corrosion and stress cracking.
Aluminum has extremely high corrosion resistance, but it is reactive to other metals.
Plastics are lightweight but have less strength than metals. It's vital that the ambient condition for application is properly assessed to determine the best material for the valve.
Fluid Type
A flow system could contain different fluids such as liquids, gases, or even slurry. In addition, the type of fluid could be corrosive or non-corrosive. For combustible gaseous systems, gas ball valves are much better than standard ball valves. They offer better shut-off and sealing than conventional ones. Their quick open and close movement makes them ideal for preventing backflows and even contamination.
Shut-off valves with an element directly in the flow path, such as butterfly and globe valves, will wear off in slurry applications. Ball valves remain one of the most reliable shut-off valves in liquid flow systems.
Conclusion
Choosing the right valve does not end in selecting the valve type. It also considers the component materials, class of valve, the fluid type, and the operating conditions. There are different valves for different industrial applications. There is a need to understand the tight shut-off requirements of the flow system before selecting a shut-off valve.
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