Mar. 07, 2024
Mechanical Parts & Fabrication Services
Valve selection is an important part of proper design and maintenance practices for industrial, piping, and instrumentation systems. Without the right valve for a specific application, operators could face improper or poor fluid system performance, increased downtime, and avoidable safety risks.
Valves are typically selected during initial fluid system design. Throughout the system’s life, maintenance technicians will usually replace a valve, and most other components, with the same type already found in the system, as per the specification. This makes it all the more important to select the right valve from the beginning, and can help operators avoid the need to prematurely replace the valve later on.
How do you make the right choice?
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For proper valve selection, designers and technicians can follow the STAMPED method, which accounts for Size, Temperature, Application, Media, Pressure, Ends or fittings, and Delivery. Thorough consideration of each of these operational conditions can guide industrial fluid and analytical sampling system professionals to the right valve choice.
Here is how to apply STAMPED to your fluid system design:
The size of your valve dictates its flow capacity and will need to correspond to the desired (or required) flow rate of your system. Manufacturers will provide a flow coefficient (Cv), which indicates the relationship between the pressure drop across a valve and the corresponding flow rate.
Flow coefficient, or Cv, is the amount of water in U.S. gallons per minute that will flow through a valve with a 1 psi pressure drop across the valve and at a temperature of 60°F. With compressible fluids, like gases, calculating and using the Cv parameter to predict flow is more complex, but still provides an effective means to size valves for a given application.
Valve design factors influencing Cv include the size and geometry of the flow path; the orifice size of a valve affects the flow of fluid through it. The larger the orifice, the greater the potential flow capacity. Orifices in different types of valves can vary considerably; for example, a ball valve will offer minimal flow resistance, but a needle valve can restrict or slow down flow. These should be considerations in your selection process.
When in doubt, consult your manufacturer—a good one will help in sizing the valve you need. For this purpose, Swagelok offers a handy Cv calculator that can be used as a starting point in selecting the right valve for your application.
Keep in mind temperatures where your valve will be in operation. This includes both the temperature of the system media your valve will help control the ambient operating temperature of the surrounding environment. Ask yourself: “Will these temperatures be constant, or will they change frequently?” These conditions may influence your valve selection or how frequently you will need to perform preventive maintenance.
Consider temperature fluctuations that may cause sealing materials to expand and contract. Also, metallic components can lose strength at higher temperature, reducing pressure ratings. It is important to check with your manufacturer to confirm a valve has been fully tested at the extremes.
Consider what your valve needs to do in your system. Do you need to start or stop flow? Regulate flow level? Control the direction of flow? Protect the system from overpressure?
Your answers to these questions will guide the type of valve you will select for your design. Again, consider a simple two-way ball valve as an example. Although some ball valves from other manufacturers may offer a throttling function, most ball valves should not be used for either throttling or regulating flow and are meant to be used in the fully open or fully closed position. If your goal is to throttle or regulate flow, a better option might be a needle or metering valve. You can learn more about valve function and types in this article.
The process fluid inside your system should also be carefully considered as you seek to select the correct valve with the correct material composition. Make sure that your system media is compatible with the materials that make up your valve bodies, seats, stem tips, and other softer materials. Incompatibility can lead to corrosion issues, embrittlement, or stress corrosion cracking—all of which can pose safety risks, as well as costly production issues.
As with temperature, you should also consider where the valve will be placed into service. Will it operate in a climate-controlled environment, such as the inside of a plant or heated instrument enclosure? Will it be outdoors, exposed to elements such as direct sunlight, rain, snow, frost, ice, and temperature fluctuation? A marine environment with significant exposure to chlorides? Valves and their components come in a variety of materials. Choose those that are best suited to these factors to maximize the longevity and functionality of the valves.
Pressure is another important consideration in your valve selection. Note the two different contexts in which the term is used:
A fluid system’s pressure limitation is based on its lowest-rated component—remember this when selecting your valve. Pressure and temperature of the process fluid have considerable impacts on component performance. The valve you select needs to hold pressure and operate when needed and under a wide range of temperatures and pressures. The design, material selection, and validation are all critical aspects of valve performance. Also remember that pressure and temperature impact one another considerably. Generally, as the temperature of the process fluid increases, the working pressure rating will decrease.
Featured content:Valves come with a variety of different end connections. These might be integral tube fittings, pipe threads, pipe flanges, welded ends, etc. Although not traditionally associated with construction of a valve, end connection selection is critical to the overall makeup of the valve and its ability to maintain a leak-tight system. Make sure your end connections are appropriate for your system pressure and temperature and are sized correctly—the right end connection can simplify installation and avoid additional leak points.
Once you have considered each of these factors and have selected the valve most appropriate for your application, ask yourself: “When do I need my valves? How many do I need?”
On-time delivery and reliable supply are just as important to keeping your fluid system operational and efficient as any other factor. As the final step of the STAMPED method, vet your suppliers. Are they able to get you the parts you need when you need them? Are they accessible? Will they work with you to understand your system needs?
Valve selection is integral to designing safe, efficient fluid systems. Interested in learning more in-depth information about valve selection and fluid system design? Consider attending one of our training courses to enhance your working design knowledge. Learn more about our upcoming virtual courses today:
Learn more about Swagelok EssentialS Training
Joe Bush is Senior Product Manager for Swagelok, and is responsible for setting the long-term vision and strategy for the general industrial valve product line. In his 26 years at Swagelok, he has a wide range of experience including technical service leadership for the high purity service group, Senior Pricing Analyst, and Market Manager for the Oil and Gas market.
Before turning to the details of the fermentation process, it is useful to review the general steps to take when selecting a valve for a chemical processing application.
The first step in every situation is to consider the type of application for which the valve will be used and select the most cost-effective option that fulfills the requirements of that particular application. Common application types for chemical-processing valves include the following: frequent versus infrequent operation, process versus drain, firesafe, normally open (N/O), normally closed (N/C), critical service, safety and environment. All other valve selection decisions will be based on the category and specific requirements of the application.
Once the application data are gathered, engineers can move on to examine the details of the application and determine which valve will work best for the particular requirements at the lowest price.
The most common types of valves used in chemical processing operations include the following: ball, butterfly, check, control, diaphragm, float, gate, globe, needle, plug, relief, solenoid, segmented or V-port, Y-pattern and three-way. Each of these valve types has unique characteristics that make it more suitable for some applications than for others. The details of all the valves will be discussed in further depth when we look at their involvement in the fermentation operation.
The valve-selection process involves a series of questions designed to systematically narrow down the possible valve solutions until one particular valve stands out as the ideal choice. First, consider the size required by the application. Ask the following questions:
What is the pipe size at the inlet and outlet of the valve? What is the flow capacity ( Cv)?
The answers to these questions will immediately limit the options of valves depending on the sizes available from the manufacturer.
Moving forward with the process, critical consideration are the temperates and pressures to which the valve will be exposed. A few important questions to ask at this point include the following:
The process fluid’s combined pressure and temperature must be within the manufacturer’s published rating for a given valve. The rating will be unique to a given body shell, valve body and trim-material combination, as well as seal material and end connections. Select a rating that ensures these combinations are sufficient to handle the maximum possible process conditions for temperature and pressure.
After evaluating the temperature and pressure, narrow down the valve selection based on the materials involved in the process. First, consider the media being processed and ask the following questions:
The answers to these questions will help determine the body materials required for the valve. Select the body and trim materials based on their strength (pressure/temperature rating), the internal/external environment, chemical compatibility and resistance to corrosion and erosion for a given process fluid. Plastic can be used for very low-pressure systems where corrosion is of primary concern. Brass and bronze are very economical choices for valve material and are fairly corrosion-resistant. Iron is a very cost-effective material and can be economically coated or lined for compatibility with corrosive fluids. Select carbon steel for the valve material where strength is needed. Stainless steel has very good strength as well as corrosion resistance.
The material that the valve seals are composed of is equally important in the decision process. Select elastomeric and plastic seals, liners and diaphragms based on their chemical compatibility to the process fluid. Elastomeric elements (natural and synthetic rubbers) have better sealing characteristics, but plastics [for example polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) and so on], are typically chosen for better resistance to harsh chemicals. Chemical-resistance guides, which are offered by most manufacturers, can be a good resource for proper selection of seal materials.
Additionally, take the end-connections on each side of the valve into consideration. The following questions are useful in making a decision:
Valve-body end connections are typically chosen based on initial cost, plant standard, and maintenance preference. Maintenance consideration is the preferred method of selection. Threaded ends (either NPT or screwed) have a low initial cost, but are subject to leak paths and stripping. Use threaded ends where maintenance is not a concern. Welded ends provide for rigid, leak-tight connections. They have a low initial hardware cost, but a high maintenance cost, should they need to be cut out of the line for repair or replacement. Flanged ends have the highest initial cost, but are preferred from an installation and removal standpoint. Wafer bodies give the benefits of a flanged installation with very low initial cost. Use wafer bodies only where the pipe is rigid or fully supported. Three-piece ball valve designs give the benefit of threaded or welded joints with integrally flanged wafer bodies.
After evaluating the materials and connections, the operation and actuation method of the valve should be taken into account (Figure 2). The following are the major considerations that can influence the valve type:
For automated valves using pneumatic, electric or hydraulic actuators, the force output of the actuator must be sufficient to overcome valve static friction and dynamic torque. Static friction is developed in the metal-to-metal surfaces, seats and seals. Dynamic torque is that unbalanced force of the process acting on the plug, disc or ball. Valve torque requirements are supplied by the manufacturer and are based on the pressure drop across the valve. A minimum of 10–20% safety factor should be added to ensure reliable operation. An on/off actuator positions the valve in the open or closed position. Modulating actuators use controllers and positioners to maintain a valve position based on an input signal.
The final consideration in the valve-selection process is choosing the accessories required to complete the process. Accessories are components within a valve-automation system that are required to operate, override and support the actuation assembly. Select accessories based on the valve, actuator and control-system requirements. These requirements can include: solenoids; switches; Indicators; overrides; positioners; and gages.Of these options, the most common considered are the solenoid valve, the limit switch for on-off valves and the positioner for modulating valves.Solenoid valves — simple electronic devices ideal for fluid shutoff and switching in general-service applications — are connected to the actuator either directly or remotely, so compact size and reliability are of concern. Solenoid valves are used on every pneumatically actuated valve and are also used as automated valves for small lines between 1/4 and 2 in. The difference between a solenoid valve and an automated valve is that solenoid valves do not support accessories. Where an automated control valve would be used in a process-control application due to its ability to use an accessory switch to confirm its operation, solenoid valves would fail due to their lack of that additional functionality.Limit switches (valve position indicators) are connected directly to the actuator and must be compact, due to size constraints. The must also be highly visible and have the ability to provide reliable feedback to the control system. An unreliable switch will upset continuous process control and adversely affect quality and safety.Positioners are devices used to position a modulating valve based on a control signal and are also attached to the actuator. Newer digital (smart) positioners are advantageous because they are more reliable and have more installation options than analog positioners. They are microprocessor-based and can also provide valuable fieldbus communications and diagnostic information.Article featured on www.chemengonline.com
In chemical processes requiring automated control, a properly selected valve can make the difference between a mediocre outcome and a top-quality product. Mistakes are often made when selecting the proper equipment for each job function. The decision process of selecting these components requires extensive knowledge and expertise. There are many factors to consider when engineering an automated chemical process, including motion requirements, cost effectiveness and chemical compatibility. In most cases, a precise combination of valves, sensors and other equipment is required to ensure an efficient and successful operation.Using a biopolymer fermentation process as an illustrative example, this article discusses how specific pairings between valves and surrounding equipment, such as sensors, can increase safety and product quality. Each individual step in the process of fermenting the liquid to create this biopolymer requires a different valve/sensor pairing to ensure the success of the overall process. The article analyzes the process from beginning to end and explains how and why the decisions were made for selecting each individual valve involved.The benefits of selecting the correct components for a chemical processing application are numerous. The most obvious is the overall quality of the operation. Another important benefit of proper valve selection is the prevention of system damage and process malfunctions. When a valve is required to fulfill an essential process function, but is not properly designed to do so, the results can be catastrophic to the individual valve, as well as the overall process. Finally, correctly selected valves will enhance the safety, efficiency and reliability of a chemical processing application. Choosing the correct valve will result in the system performing at the peak of its ability for the longest period of time and with the least maintenance requirements (Figure 1). An example process is used to illustrate valve-selection considerations.
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