How to Select Valves for Industrial Fluid Systems

How to Select Valves by Applying the STAMPED method

Flow coefficient, or C_v, 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 C_v parameter to predict flow is more complex, but still provides an effective means to size valves for a given application.

Valve design factors influencing C_v 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. Along with guide to help you with valve sizing, Swagelok offers a handy C_v calculator that can be used as a starting point in selecting the right valve for your application.

 

T: How Does Temperature Affect Valve Selection?

Keep in mind the temperatures where your valve will be in operation. This includes both the temperature of the system media your valve will help control and 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.

A: What Valve Design Factors Should You Consider for the Application?

Understanding what your valve needs to do in your system is critically important. Do you need to start or stop flow? Regulate flow level? Control the direction of flow? Protect the system from overpressure? What types of valves does Swagelok offer?

Your answers to these questions will guide the type of valve you will select for your system 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.

M: Why is Valve Material Selection Important?

Proper valve material selection will make sure that the process fluid inside your system is compatible with the valve’s materials composition. Make sure that your valve bodies, seats, stem tips, and other softer materials are all compatible with system media. 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.

P: How Does Pressure Impact Valve Selection Criteria?

Pressure is another important consideration in your valve selection criteria. Note the two different contexts in which the term is used:

• Working pressure: The normal operating pressure in your system.
• Design pressure: The maximum pressure allowed by the valve manufacturer; never exceed this limit except in controlled tests.

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 valve design, valve 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.

E: How Do You Choose the Right End Connections?

Valves come with a variety of different end connections, such as integral tube fittings, pipe threads, pipe flanges, welded ends, etc. Although not traditionally associated with the valve’s design, 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.  

D: Delivery

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?”

Valve selection is integral to designing safe, efficient fluid systems. 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?