Route Blowdown Gas to Low Pressure System

Prior to maintenance, testing, or equipment/pipeline installations, operators typically release, or “blow down,” pressurized natural gas from a station, equipment, or pipeline as a safety measure. Operators can reduce the amount of high-pressure natural gas that would otherwise be emitted to the atmosphere by first rerouting the gas to an existing low-pressure gas system. This practice can bring the volume of gas from the equipment to be evacuated to the pressure of the low-pressure system, resulting in reduced emissions.

Blowdowns are considered controlled venting; therefore, they are usually planned events that can be mitigated by eliminating or reducing their frequency or the volume of natural gas vented. Rerouting higher pressure natural gas from equipment and/or pipeline segments to an existing low-pressure gas system reduces the volume of gas that needs to be vented to the atmosphere. Examples of low-pressure systems that the blowdown gas can be routed to include nearby distribution mains or a fuel gas system.  

Several options, driven by operational considerations, exist for performing this practice.  

• Existing Connections: Gas can be rerouted to the low-pressure system by taking advantage of existing piping connections between high- and low-pressure systems. To reroute gas to a lower pressure system, the operator first isolates the equipment to be depressurized. Then, the downstream load can be used to draw down the pressure. Alternatively, an existing bypass can be used to divert the gas to another pipeline that operates at a lower pressure than the equipment being blown down.  
• Temporary Bypass: Operators can temporarily reset or bypass pressure regulators or install temporary connections between high- and low-pressure systems to reduce system pressure prior to maintenance. A portable regulator station or a valve may be required to connect two systems with different pressures. Welded, flanged, or screwed pipes or flexible hoses may be used to divert the gas.  
• Additional Evacuation Using Portable Compressors: When gas is rerouted to lower pressure systems, if the gas is only brought down to the same pressure as the receiving low-pressure system, not all gas may be removed from the station, equipment, or pipeline segment being evacuated. To further reduce the pressure of the gas (and the amount of gas that may need to be emitted to the atmosphere), operators can use portable compressors to divert most of the remaining gas into the receiving line. There may still be a small amount of gas remaining in the equipment which would be vented or flared.

This practice can be employed wherever there are low-pressure gas systems that remain in service when nearby higher-pressure systems are shut down. Compressors are routinely blown down for maintenance or operational purposes and offer a unique example of this mitigation option. The large volume of high-pressure gas between the compressor unit isolation valves may be blown down at a controlled rate to a low-pressure fuel gas line, such as for reciprocating compressor engines. The use of portable compressors requires advanced project scheduling and planning, and this equipment may need to be rented.

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 routing blowdown gas to low pressure systems, this means calculating emissions from normal blowdowns and subtracting zero (because routing blowdown gas to low pressure systems emits minimal amounts of methane). While using actual measurements may provide a more accurate representation of emissions/reductions from individual equipment at a given time, emissions from starts can be reasonably calculated using an emission factor as follows. It should be noted that various types of blowdowns can occur. The equation shown below is a generalized equation that should be adjusted for the relevant units of activity (e.g., facilities for LNG Station Blowdowns, terminals for LNG Import or Export Terminals Blowdowns, pipeline miles for vessels for Distribution Pipeline Blowdowns) and the associated emission factor. 

ER = A × EF 

Where: 

ER = Emissions reduction estimate (kg CH₄/year) 

A = Number of units of activity (e.g., number of facilities, terminals, pipeline miles) 

EF = Activity-specific emission factor (kg CH₄/yr/unit of activity) 

Assumptions: 

• Use the most current “blowdown” 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).

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, routing blowdown gas to low pressure systems may: 

• Increase revenue: Provides opportunities for selling gas that would otherwise be emitted to the atmosphere.  
• Increase safety: Improves worker safety conditions by reducing the amount of high-pressure gas releases.

Interstate National Gas Association of America. (2018, August). Improving methane emissions from natural gas transmission and storage.https://www.ingaa.org/File.aspx?id=34990&v=56603504

Lowell, D., Jones, B., Seamonds, D., & Russell, P. (2016, June). Analysis of Pipeline and Hazardous Materials Safety Administration proposed new safety rules – Pipeline blowdown emissions and mitigation options. https://blogs.edf.org/energyexchange/files/2016/07/PHMSA-Blowdown-Analysis-FINAL.pdf (1 MB)

Tangeman, J. (2017). Mitigating emissions from pipeline blowdowns. Kinder Morgan. https://www.epa.gov/sites/default/files/2017-11/documents/12.tangeman_2017aiw.pdf (881 KB)