Emerson’s Riyaz Ali presented Digital Valve Controller Used with Final Control Elements Inching Towards Net Zero Emission at the 2024 Emerson Exchange conference in Düsseldorf, Germany. Here is his presentation abstract:
Energy consumption is a big part of process plant operating costs, so its use must be closely monitored and controlled. However, attempting to minimize energy consumption by restricting energy flow to the final control element may compromise the flow control loop performance. To address this issue, Emerson’s Fisher FIELDVUE DVC6200 Series Digital Valve Controllers provides a low bleed option for use on control and on-off valves to minimize the use of instrument air. The minimal amount of bleed air from these controllers can be used to keep a small positive internal pressure to prevent entry of external plant corrosive vapors and dust. In some applications, this bleed air can be fed into actuator enclosures to keep corrosive atmospheres out.
Riyaz opened by highlighting the objectives of many oil and gas companies across the globe:
Challenges in their valve operations include excess air consumption, constantly stroking valves increasing air consumption, and lack of measurements on valve to be unable to spot air leaks in a timely fashion resulting in additional energy loss. This wasted air consumption affects overall energy consumption and the operating costs for the operation. In addition to energy losses, degraded performance can affect quality, production, and waste.
For air-operated control valves, the pneumatic power at constant supply pressure is proportional to the mass flow rate of instrument air. On most control valves used in closed loop control, where the input signal is continuously changing, and the pneumatic instrument is a positioner. When considering the operating costs of candidate positioners, one is tempted to use the only flow data normally available, which is static air consumption.
Zero air consumption is not a legitimate design goal. The final control element must consume energy if it moves in response to controller output changes, which occur after any disturbance to a closed loop control system. However, reducing instrument air use is now possible. In many applications, the instrument air is bled into actuator enclosures to keep corrosive atmospheres out. However, the lower limit of this beneficial bleed has not been established by standards.
S75.13 provides procedures for measuring the static consumption, i.e., with the device at rest. However, S75.13 also gave separate procedures for frequency response (sine wave input) tests, using stem motion to measure performance. Today sensors are available for measuring the dynamic air consumption during a frequency response test. Control valves that are rarely required to move are an exception to this principle and are not covered.
Important criteria in selecting valve controllers include reliability and ruggedness, maintenance views, and low air consumption per ISA 75.13 for energy savings. From a performance perspective, the valve control should provide independent linearity, minimal hysteresis, and dynamic performance of the low bleed relay (which matches with the standard relay.)
A smart digital valve controller can save considerable time, resource, and expenses during a turnaround by taking valve internal mechanical X-ray, without taking the valve from pipeline to workshop.
The benefits of smart digital controllers with low-bleed options include:
Pneumatic leaks (tubing, actuator) are the main contributor of air consumption, and that is why it is important to have a mechanism to diagnose valve air consumption. The approximate cost saving for the company who implemented this project was $800K per year. It improved reliability by using the diagnostic information from these smart positioners to predict any issues in advance, which improved unscheduled shutdowns and environmental wastes.
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