Instrument Air Controllers

Pneumatic controllers are process control devices that can be powered by natural gas or compressed air. Natural gas-powered pneumatic controllers, either continuous or intermittent bleed, can be converted to instrument air-powered pneumatic controllers by substituting the pressurized natural gas with compressed air. Natural gas-powered pneumatic controllers are one of the largest sources of methane emissions from the oil and gas industry and using instrument air eliminates the pneumatic control system methane emissions.

Pneumatic controllers can be powered by compressed air instead of natural gas, eliminating controller methane emissions. Figure 1 shows a typical instrument air controller system where atmospheric air is compressed, stored in a volume tank, dried, and then introduced into the existing gas supply piping to each pneumatic controller. Retrofitting natural gas-powered pneumatic controllers to use instrument air requires new equipment (e.g., air compressors, volume tanks, air dryers). Air compressors are typically driven with electrical motors. However, the pneumatic controllers and other existing components of the natural gas-powered pneumatic controller system (i.e., pneumatic gas supply piping and valve actuators) can be reused in an instrument air controller system. The compressed air can also be used for onsite utility services (e.g., small pneumatic pumps, gas compressor engine starters, pneumatic tools). 

The major components of an instrument air conversion project include the air compressors, electrical power source, air dryers, and a volume tank. The following are descriptions of each of these components along with installation considerations.  

• Compressor: Compressors used for instrument air delivery are available in various types, from rotary compressors to reciprocating piston. The size of the compressor depends on the number of pneumatic controllers operated by the system and the typical bleed rates of these controllers. A conservative rule-of-thumb for sizing air systems is one cubic foot per minute (cfm) of instrument air per control loop, which consists of a process measurement device and control valve. The compressor is typically sized for double the required instrument pneumatic gas demand so that the compressor runs only half the time as the volume tank is depleted.  
• Power Source: A critical component of the instrument air controller system is the power source required to operate the air compressors. The reliability of an instrument air controller system, however, depends on the reliability of the compressor and the associated electric power supply. Most large natural gas plants are either tied to an existing electric power supply or have their own power generation system. For smaller facilities and remote locations, however, a reliable source of electric power can be difficult to ensure. In these instances, alternatives will need to be considered, such as solar power.  
• Air Dryer: Air dryers are an essential part of the instrument air system. Water vapor present in atmospheric air condenses when the air is compressed and cooled and can cause problems to these systems, including corrosion of the instrument parts and blockage of instrument air supply piping and controller orifices. Therefore, it is necessary to remove as much water from the air as possible. For smaller systems, membrane dryers are an option. These are molecular filters that allow oxygen and nitrogen molecules to pass through the membrane and hold back water molecules. For larger applications, desiccant (alumina) dryers are available. Multiple air dryers are required for periodic maintenance without interrupting the supply of dry pneumatic air to the process. 
• Volume Tank: The air is stored in a volume tank, which should hold enough air to allow the pneumatic control system to have an uninterrupted supply of pressurized air without having to run the air compressor continuously. The volume tank should also allow for a large withdrawal of compressed air for short periods (e.g., for an engine starter, pneumatic pumps, or pneumatic tools), without affecting the process control functions.

Figure 1. Schematic of Typical Instrument Air Controller System.

The conversion of natural gas-powered pneumatic controllers to an instrument air controller system is applicable to many natural gas facilities. Instrument air systems will require new capital investments for the compressors, volume tank, air dryers, and other related equipment, as well as a supply of electricity. As such, an initial step in a successful instrument air conversion project is screening existing facilities to identify the most suitable locations. In general, three main factors should be considered during this process: 

• Available Power Supply: Since most instrument air systems rely on electric power for operating the compressor, a continuous electrical energy source is essential. While major facilities (e.g., large gathering and boosting stations, gas processing plants, and offshore production platforms) are often tied to an existing power supply or their own power generation system, many smaller and remote facilities are not. In addition, facilities with dedicated generators need to assess whether the generators have enough available capacity to support an air compression system. Remote facilities should examine alternatives for power generation. 

• Number of Pneumatics: The more pneumatic controllers converted to instrument air, the greater the potential for reduced emissions and increased company savings. Conversion to instrument air is most cost-effective when a company is planning a facility-wide change. 

• Facility Layout: The layout of a natural gas facility can affect equipment and installation costs for an instrument air system. Instrument air is most suitable when used at facilities where a large number of pneumatic controllers are consolidated within a relatively small area. 

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 instrument air 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, converting natural gas-powered pneumatic controllers to instrument air controllers may: 

• Increase the life of controllers and improve operational efficiency: Natural gas used in pneumatic controller devices and instruments often contains corrosive gases, such as carbon dioxide and hydrogen sulfide, that can reduce the effective operating life of these devices. In addition, natural gas often produces by-products of iron oxidation, which can plug small orifices in the equipment resulting in operational inefficiencies or hazards. When instrument air is used, and properly filtered and dried, system degradation is reduced and operating life is extended. 

• Improve safety: Using compressed air as an alternative to natural gas eliminates the use of a flammable substance, increasing the safety of facilities. This can be particularly important at offshore installations, where risks associated with hazardous and flammable materials are greater.

Carbon Limits. (2016, August 1). Zero emission technologies for pneumatic controllers in the USA: Applicability and cost effectiveness, Version 2.0. https://www.carbonlimits.no/wp-content/uploads/2017/01/Report_FINAL.pdf

Carbon Limits. (2021, November). Zero emission technologies for pneumatic controllers in the USA: Updated applicability and cost effectiveness, Final. https://cdn.catf.us/wp-content/uploads/2022/01/31114844/Zero-Emissions-Technologoes-for-Pneumatic-Controllers-2022.pdf

EQT. (2022, January). Pneumatic device replacement: Low-cost opportunity for methane abatement. https://www.eqt.com/wp-content/uploads/2022/01/Pneumatic-Device-Replacement-FINAL.pdf