Valve-Selection Best Practices

Ray Herrera, Valin || 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.

Fig 1

Valve-Selection Process

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, fire-safe, 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.

Temperature and Pressure

Moving forward with the process, critical considerations are the temperatures and pressures to which the valve will be exposed. A few important questions to ask at this point include the following:

What is the maximum pressure of the process?
Where does the fluid go between the source and the valve (upstream)?
Where does the process media go after it leaves the valve (downstream)?
What is the maximum fluid temperature?

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.

Materials of Construction

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:

What fluid is going through the valve?
What is the source of the fluid?
What is the fluid temperature?
Is the fluid clean?
Is the fluid being treated before it reaches the valve?

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.

Seals and Ends

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 with 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:

What is the pressure/temperature rating?
What is the installation/maintenance cost?
What industry and plant standards need to be met?
What type of seal welding will be used?

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.

Operating Mode

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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:

Does the valve need to be manual or automated?
Will the valve operator be lockout type, oval, or Deadman?
Will the actuator be pneumatic, electric, or hydraulic?

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.

Valve Accessories

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 commonly 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. They 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.

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