For process control and instrumentation reliability, I have often discussed the strategies of product tiering, advanced diagnostics and best practices. Control valves can also benefit from these strategies. Previous posts have presented considerations for maintaining reliability of both the mechanical and instrument portion of a control valve. There are a number of best practices that can be adopted in order to ensure the process is reliable, available and profitable. I have outlined them here.
Predominately, there are two different kinds of control valve actuators: these are single-acting and double-acting. A single-acting actuator supplies air to one side, while a double-acting actuator supplies air to both sides. As a result, there are two different kinds of valve positioners. There are also positioner modules that modify operation: a single-acting becomes double acting and vice-versa. While this can be done it is not considered a best practice. Frankly, you should use the style of positioner that corresponds to the operation of the valve actuator. While the modules will work, they negatively impact valve performance which affects your process. Because it incorporates more parts, it reduces the valve reliability and ultimately the process reliability.
Control valves are available with different flow characteristics. These are on-off, linear and equal percent. An on-off characterization is used for valves that are either open or closed. A linear characterization increases the flow capacity (Cv) in direct proportion to the signal. An equal percent characterization produces a curve compared to signal, but produces a more linear process. Historically, this was the only way to characterize a valve as pneumatic positioners were only linear. With the advent of digital control valve positioners, you now have the capability to characterize the valve in the positioner. The same plug and seat can be used in multiple applications. The result is a single style of valve with few spare parts.
There are other features within a digital positioner that can result in a more reliable process. Soft limits will not allow the control valve position to operate outside a defined area. If the soft limits of a valve were set at 20% and 80%, a set point of 15% or 95% would keep the valve at the 20% and 80%, respectively. In these events an alert can be sent to the operator. In similar fashion you can program the valve to be open or close above or below a set point. If this limit were set at 10%, a 5% set point would result in the closure of the valve. This prevents the valve from operating close to the seat.
A valve signature should always be captured for a new control valve. This procedure performs a number of test and measures control air pressure, valve position and set point. Performing a valve signature on a scheduled basis can determine if there has been any degradation in performance. If more air is required to move the stem, this could be an indication that packing is too tight (darn maintenance guys) or their is some other mechanical obstruction. An increase in hysteresis could signal linkage that needs to be addressed. This process can be automated with the application of a partial stroke test (PST). This procedure allows you to keep your process online while maintaining good positioner health.
There are many other best practices which address fundamental preventative and predictive maintenance. Using clean dry air and maintaining a clean valve yoke are among them. Calibration should also occur on a scheduled basis. Naturally, this period is longer for premium tier products. Adoption of control valve best practices, proper control valve tier and utilization of advanced diagnostics which help you ensure a reliable process.
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