Digital Valve Positioner Can Increase Efficiency

When thousands, if not millions, of people are relying on the reliability of a power plant to allow them to carry on their daily lives, plant efficiency and speed of response are crucial. A large power producer in the Northeast United States has a four-unit power generation plant that operates on-demand, based on bid price to the area independent system operators. Inside of Unit No. 3 at this plant, the operations department fills the drum with the controls in manual mode. Startup procedures call for the operator to provide an output to the feedwater valve at a prescribed level. Repeatability and accuracy of the valve position in response to the controller output in manual mode are critical for the drum-fill procedure.

The Problem

A problem arose when the existing analog I/P (input) and pneumatic input analog positioners being used on the feedwater and startup valves did not provide repeatable valve positioning with the controller in manual. The operations fill procedures called for an output to the valve of 50 percent. The plant experienced a slow fill that delayed tie-in to the grid. Following evaluation of the issue, it was revealed that the valve was only at approximately 25 percent of travel. The analog devices were providing input that “drifted” with changes in temperature and vibration. While recalibration was attempted, it did not solve the control problem.

The Chosen Solution

Eventually, the power producer made the decision to replace both the analog I/P and the pneumatic positioners with digital valve positioners. The digital valve positioners were selected because they could simplify the control circuit and eliminate the components that were drifting. The selected positioner was chosen because:

• digital positioners have a history of sucessful implementation in other difficult plant applications, including burner-mounted air damper drives and attemperator valves
• on-site technical support is available for replacement mounting hardware design, installation and configuration
• of the availability of a position feedback transmitter integral to the positioner

By using intelligent, digital devices, operators are able to give full control back to the positioner, where deviations and errors can be detected faster and often addressed without manual interventions. Consider a valve assembly with a pneumatic leak. A leak can occur in the fittings, actuator, boosters, gauges, or anywhere else in the assembly. In this situation, there will be regular, steady drifting of position due to the loss of air. To make up for this leak, the valve must constantly move to get back to the setpoint, causing premature wear and more frequent maintenance. With a digital positioner, leak compensation can be introduced. When this type of drifting is detected, the positioner will regularly deliver small doses of air to maintain position—before the valve is forced to move and compensate. When this compensation exceeds the acceptable limit, operators are then notified and can perform the necessary maintenance before there is a situation that cannot be solved by the positioner alone. With a digital positioner, features like deadband monitoring detect deviations between the setpoint and the actual position of the valve. If the deadband is too wide, precise control is not possible and the valve position may not be where it is expected. If the deadband is too narrow, that degree of precision may not be physically possible, and there will be continuous hunting or oscillation around the setpoint position. A digital positioner can automatically and continuously adapt the deadband to suit the requirements of the control circuit and the real-world operating conditions of the valve assembly. The newly installed devices included a two-wire valve position feedback transmitter to confirm the valve position to the operator in the control room. This feedback transmitter provides a signal for monitoring the physical position of the valve, not just the setpoint from the control system. Monitoring the true position allows anomalies like drift, hunting for position, and other deviations from setpoint to not only trigger alarms, but to be quantified and addressed in a maintenance plan. Normal air loss typical to conventional position controllers can be costly. In its balanced condition, the replacement positioner consumes virtually no air. This feature can minimize the consumption of air and reduce electricity costs associated with compressed air generation. The design of this digital positioner has saved this Northeast power producer more than $120 per valve, per year.