Control valve technology

It's an underappreciated factor in successful process automation initiatives

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By Steve Hagen

Advanced control schemes can't produce optimum results unless the control valves operate properly. Instrument technicians must understand these final control elements as well as their diagnostic software to ensure the valves in the plant operate as the system designers intended.

Renewed interest in the performance of control valves is emerging, partly as a result of numerous plant audits that indicate roughly one-third of installed control valves are operating at substandard levels. Even though properly operating control valves are essential to overall plant efficiency and product quality, maintenance personnel frequently don't recognize the signs of poor performance. The basics of control valve design and operation must be well understood for end-users to reap the benefits of improved valve operation.

 The most common and versatile types of control valves are sliding-stem globe and angle valves (see Figure 1). Their popularity derives from rugged construction and the many options available that make them suitable for a variety of process applications, including severe service. For example, sliding stem valves typically are available with options that satisfy a range of requirements for ANSI Class pressure-temperature ratings, shutoff capability, size, temperature compatibility and flow characteristics.

Figure 1. Sliding-stem globe and angle control valves.

The first distinction among globe and angle valves is whether the valve is post- or cage-guided. In post-guided valves (see Figure 2), the closure member is guided by a bushing surrounding either the valve stem or a portion of the valve plug. In cage-guided valves (see Figure 3), a close-fitting, cylindrical cage guides the plug. Cage guiding provides a comparatively larger guiding surface, which helps minimize vibrations, improve valve plug stability and increase pressure drop ratings.

Figure 2. Post-guided valve.

Rotary-shaft control valves are available in several different types, each characterized by the geometry of the closure member (disk, ball, ball segment or plug). Compared to sliding-stem valves, rotary-shaft valves typically have limited pressure drop ratings, are less expensive, have greater rangeability and provide greater flow capacity for a given valve size. Rotary valves typically are selected to provide unique features, such as high capacity (full-bore ball valve), shear on closing action (ball segment valve), low cost (standard butterfly) or resistance to erosion damage (eccentric rotary plug shown in Figure 4). In general, rotary-shaft valves offer fewer severe service options (trim options to attenuate control valve noise, eliminate cavitation and provide other desirable characteristics).

Figure 3. Cage-guided valve.

Control valve selection is based on the process fluid to be handled and a number of performance objectives. Required sizing parameters include specific gravity, pressures at the valve inlet and outlet, pressure drop across the valve, fluid temperature at the valve inlet, flow rate and vapor pressure. Other vital information includes the desired response time, process gain characteristics and the potential for cavitation or flashing.

Achieving complete valve shutoff is important in many applications to prevent leakage that either could contaminate a process fluid or result in product loss. Tight shutoff also prevents erosion damage that could occur if a high-velocity stream leaked across seating surfaces.

Many control valves are oversized as a result of inaccurate information and safety margins added by each individual or group that participates in the sizing procedure. Oversized valves are a problem for three reasons.

First, the valve operation may become unstable because it never opens very far from the fully closed position. Process gain is generally high when the valve is throttling near its seat. The combined valve and process gains may be too high to maintain stable operation at low lifts. Second, excessive seat wear may result from high velocity flows between the closure member and the seating surface. Third, the design flow characteristic may not be achieved, resulting in controller tuning problems.

Valve manufacturers and vendors usually use specialists in fluid thermodynamics who can provide system designers with state-of-the-art solutions to unusual sizing situations.

Figure 4. Eccentric rotary plug valve.

A properly selected and sized control valve can deliver optimum performance only when the plug, disk, ball or ball segment positions itself properly in response to the control signal. Closure member positioning is a function of actuator performance and the instrumentation that provides loading pressure to the actuator. There are three relevant factors to consider.

Force at the closed position: For globe and angle valves, the actuator must provide sufficient force (or thrust) to achieve the specified ANSI Class shutoff. For rotary-shaft valves, the actuator must provide sufficient torque to move the closure member into and out of the seat.

Actuator stiffness: To maintain valve plug stability, the actuator must offer sufficient resistance to fluid buffeting forces by means of a mechanical spring or air spring effect.