Positioner Pneumatic Pressure Bleed in Throttling Control Valves

With the emphasis on improving energy efficiency and reducing greenhouse gas emissions, process manufacturers have turned to pneumatic low-bleed and zero-bleed valve positioners. Emerson’s Riyaz Ali copied me on an email about a recent article that mentioned a zero-bleed valve positioner.

Riyaz noted that positioners mounted on valves used in throttling applications need to be dynamic in operation. He gave an example where the control system will send a command to the positioner to move the valve from 65% open to 65.1%. If the positioner is designed to save on air usage, then the sacrifice will be in overall controllability. Either the positioner is not closely throttling or there is a large deadband built into the positioner.

Riyaz pointed to a 2003 article by Emerson’s Mark Coughran, Dynamic Air Consumption of Control Valves. Mark wrote:

Energy consumption of the control systems is part of the operating cost of a process plant. However, attempting to minimize energy consumption by restricting energy flow to the final control element may compromise the purpose of the control loop. Degrading the performance of the control loops reduces product quality, reduces plant output, or wastes process fluids far more valuable than the instrument air. The operating cost of the control valve must be weighed against the control valve’s effect on the control loop.

The design of the Fisher DVC6000 positioner is such that it continuously bleeds a small positive pressure. This helps ensure that its pneumatic relay is ready for the change in output based a change in the controller signal for changes as small as 1/8%. This level of control helps keep the process closer to setpoint reducing process variability. The small positive pressure in the positioner’s housing also keeps corrosive gases such as hydrogen sulfide or sulfur dioxide from creeping inside the positioner and potentially damaging the components.

Riyaz also highlighted an article he wrote, Performance diagnostics – for peak performance. It describes smart positioner diagnostics related to performance, supply pressure, relay adjustment, travel deviation, I/P and relay integrity, and air mass flow. These diagnostics interpret the feedback from the valve travel position, supply pressure, and actuator pressure to provide early warning of any valve, actuator or positioner issues.

In the article, Riyaz summed up the impact that the early warning diagnostic signals can have on process efficiency and quality, productivity and yield, reliability, maintenance planning, and process stability.

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