Mechanical Controllers

Process control automation throughout the natural gas industry is commonly done with natural gas-powered pneumatic controllers that vent methane to the atmosphere through their normal function. Replacing natural gas-powered pneumatic controllers with mechanical controllers can reduce methane emissions. 

A common process controller in the natural gas industry is a liquid level controller, which opens and closes a liquid dump valve to maintain a liquid (i.e., water and/or oil) level in a vessel. The close proximity of the liquid level measurement to the dump valve makes this process control amenable to use of mechanical linkages to translate the position of a liquid-level float in an oil and gas separator to the position of a liquid dump valve. In a mechanical liquid level controller, the float riding on the liquid phase contained within a gas/liquid separator or other vessel is coupled with a rod connected to the liquid dump valve; movement in the float up or down leads to direct movement in the rod, which translates to the opening or closing of the dump valve. For example, as the liquid height within a vessel increases, the float will rise and, via connection to the rod, the dump valve will open to release liquid such as oil, condensate, or water. Conversely, as the liquid is released through the valve, the float will drop, and then the dump valve will close to slow the release of liquid. Figure 1 shows a mechanical liquid level controller in use on a separator. Mechanical controllers are reliable and do not require the use of natural gas, thus eliminating methane emissions from the control process.

Figure 1. Mechanical Liquid Level Controller

Mechanical controllers can be used in place of natural gas-powered pneumatic controllers where the process measurement is near the flow control valve, where flowing pressure is low to moderate, and where fine throttling control of the process variable (i.e., pressure, temperature, flow rate, liquid level) is not necessary. The delicate nature of temperature (bi-metallic strips) and pressure (bourdon tube) process measurement and the proximity of the process measurement to the valve that is controlling the process (e.g., fuel gas to a burner, suction and discharge pressure around a compressor) make these applications less practical than liquid level control. Mechanical controllers have been successfully installed at low pressure, low volume oil and gas producing wells. 

Methane emission reductions can be determined by taking the difference in emissions from the source before and after the specific mitigation action was applied. For replacing natural gas-powered pneumatic controllers with instrument air controllers, this means calculating emissions from the natural gas controllers and subtracting zero (because mechanical controllers do not emit methane). While using actual measurements may provide a more accurate representation of emissions/reductions from individual equipment at a given time, emissions from natural gas-powered pneumatic controllers can be calculated using emission factors as follows:  

ER = C_i × EF_i

Where: 

ER = Emissions reduction estimate (kg CH₄/yr) 

C_i = Number of controllers of type i (i.e., count of controllers that use natural gas) 

EF_i = Emission factor for controllers of type i (kg CH₄/yr/controller) 

Assumptions/Constants: 

• Use the most current “controllers” emission factor. Emission factors are generally developed to be representative of long-term averages for all applicable emission sources. EPA updates the emission factors from the Natural Gas Systems section of the Inventory of U.S. Greenhouse Gas Emissions and Sinks (“Greenhouse Gas Inventory”, or “GHGI”) every year, so specific emission factors may change. To find the current emission factor, navigate to the GHGI website for Natural Gas and Petroleum Systems and click on the page for the most recent inventory. On that page, you will find links for Annex 3.5 (Methodology for Estimating CH₄, CO₂, and N₂O Emissions for Petroleum Systems) and Annex 3.6 (Methodology for Estimating CH₄, CO₂, and N₂O Emissions for Natural Gas Systems). Methane emission factors can be found in Table 3.5-3 (Petroleum Systems) and Table 3.6-2 (Natural Gas Systems).   
• Reductions should be estimated individually by controller type (i.e., High Bleed, Low Bleed, or Intermittent).

The calculation methodology in this emissions reduction section is based upon current information and regulations (as of August 1, 2023). EPA will periodically review and update the methodology as needed.

In addition to reducing emissions of methane, replacing natural gas-driven pneumatic controllers with mechanical controllers may:  

• Reduce product losses: Replacing natural gas-powered pneumatic controllers with mechanical controllers reduces the quantity of natural gas lost as emissions.  
• Reduce maintenance: Mechanical controllers are mechanically simpler than natural gas-powered pneumatic controllers and generally require less maintenance effort and operator time.

Kimray. How does liquid level control work with a lever-operated dump valve? https://kimray.com/training/how-does-liquid-level-control-work-lever-operated-dump-valve