Pneumatic controllers are process control automation devices used widely in the natural gas industry to operate valves that control liquid level, pressure, and other process variables. While they can be powered by natural gas or compressed air, natural gas-powered pneumatic controllers are commonly used. Some pneumatic controllers are designed to bleed (or emit) continuously, while others may emit natural gas to the air only when actuated or if malfunctioning. Pneumatic controllers are one of the largest sources of vented methane emissions from the natural gas industry.
The natural gas industry uses a variety of process control devices to automatically operate valves and control pressure, flow, temperature, liquid level, and other process variables. Such instrumentation and process control equipment typically fall into one of three categories: (1) pneumatic; (2) electrical; or (3) mechanical. In most applications, the industry uses natural gas-powered pneumatic controllers, or simply “pneumatics,” which make use of readily available high-pressure natural gas to provide the required energy and process control signals. In the production segment, pneumatics control and monitor gas and liquid flows and levels in dehydrators and gas/liquid separators, temperature in dehydrator regenerators, and pressure in flash tanks and compressors. Most processing plants, Liquefied Natural Gas facilities, and offshore manned platforms use instrument air-driven devices, but some use gas pneumatics. In the transmission segment, pneumatic controllers regulate liquid level in compressor scrubbers, gas flow, and pressure; they also actuate isolation valves. The distribution segment primarily uses pressure regulators that vent the power gas downstream of the control valve, back into the natural gas stream, and thereby do not vent gas to the atmosphere.
Figure 1 shows a schematic of a gas pneumatic control system. In a liquid level controller, pressurized gas is directed to the valve actuator to push the valve plug open (when the liquid level is too high), or excess gas pressure is bled off the valve actuator allowing a spring to push the valve plug closed (when the liquid level is too low).
Pneumatics are classified as either continuous bleed controllers or intermittent bleed controllers.
- Continuous bleed controllers vent gas to the atmosphere at a steady rate when the valve is stationary, and vent accumulated gas from the actuator when the valve spring pushes gas out of the actuator.
- Intermittent bleed controllers vent gas to the atmosphere only when the valve position is moved from open to closed and back.
The actual bleed rate and the resulting magnitude of emissions largely depends on the design of the pneumatic controller. Depending on the dynamics of the process, intermittent bleed controllers, functioning properly, may vent more or less gas than continuous bleed controllers. Intermittent bleed controllers that operate quickly provide more precision and generally vent more gas than slower, less dynamic operating systems. In a fast dynamic process, such as large liquid flow through a smaller gas/liquid separator, the intermittent bleed controller may cycle frequently and thus vent emissions frequently, whereas a continuous bleed controller may hold a liquid level reasonably steady with infrequent and little movement of the valve. On the other hand, slow dynamics may allow an intermittent bleed controller to be closed for long periods, with minimal emissions, while the continuous bleed controller is continuously venting. The bleed rate will also vary with the supply gas pressure and age or condition of the equipment. The condition of a pneumatic controller is a stronger indicator of emissions potential than age; well-maintained pneumatic controllers in clean, non-corrosive gas service may operate efficiently for many years.
Allen, D. T., Pacsi, A. P., Sullivan, D. W., Zavala-Araiza, D., Harrison, M., Keen, K., Fraser, M. P., Hill, A. D., Sawyer, R. F., & Seinfeld, J. H. (2015). Methane emissions from process equipment at natural gas production sites in the United States: Pneumatic controllers. Environ. Sci. Technol., 49, 1, 633-640. https://doi.org/10.1021/es5040156
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