Crankcase ventilation systems are designed to help maintain an engine’s reliability and lower long-term maintenance costs. They also reduce engine oil consumption, improving engine efficiency and performance. The system does so by regulating crankcase pressure within a specific range by capturing oil inside blow-by gases. Pressure regulation is typically managed through automatic vacuum regulation valves. The system includes a filter that captures oil in the blow-by gases and directs it back to a waste oil sump. The oil inside the crankcase pollutants contains a variety of volatile organic compounds and hazardous air pollutants that can be minimized using the ventilation system. 

This Delve explores the new requirement and emissions calculation methodologies for facilities affected by this update. 

Subpart W Updates: Crankcase Venting 

All source categories in Subpart W (except offshore petroleum and natural gas production and onshore natural gas transmission pipeline) with reciprocating internal combustion engines greater than 130 horsepower will be affected by the new crankcase venting source type.   

Methane is the primary GHG emitted from crankcase ventilation systems. The emitted methane may either be released into the atmosphere or routed to a flare to reduce emissions.  

EPA has provided operators the option to estimate representative methane emissions either by directly measuring volumetric flow or by using emission factors. If crankcase vents are routed to a flare, CH4, CO2, and N2O emissions must be calculated and reported for the flare stack pursuant to §98.236(n). Calculation methodologies are summarized below: 

Crankcase Venting Calculation Methodology 1: Direct Measurement Method 

1. Select measurement equipment

Choose an appropriate measurement tool for collecting data from the crankcase vent:

• An appropriate meter 
• A calibrated bag
• A high-volume sampler (the procedure for using the high-volume sampler follows the same requirements as those outlined for compressor vents in sections 98.233 (o) and (p)) 

2. Screen vents with optical gas imaging (OGI) instrument 

If available, use an OGI camera to screen the vent for emissions. If emissions are detected, a flow measurement will be required. If no emissions are visible, the flow rate will be considered zero. Alternatively, Method 21 or infrared laser beam-illuminated instruments can also be used to screen the vent for emissions. 

3. Measure volumetric flow and methane concentration 

Using the selected equipment, measure the volumetric flow of gas venting from the crankcase in cubic feet per hour (cfh). Then, determine the concentration of methane in the venting gas stream using the appropriate method for detecting methane levels. 

4. Record total operating time 

Record the total operating time of the internal combustion engine in hours.  

5. Calculate volumetric release of methane 

Multiply the three variables to calculate the volumetric release of methane in standard cubic feet and convert to metric tons: 

• Volumetric flow of gas (cfh) 
• Methane concentration in the gas
• Total operating time (hours) 

Crankcase Venting Calculation Methodology 2: Default Emission Factor Method 

Calculate the mass release of CH4 emissions from crankcase venting using the proper emission factor and the total operating time. Operators must still consider the specific industry segment in which the internal combustion engine is operating. Crankcase venting systems and internal combustion engines differ depending on the industry, and as a result, their emission factors also vary.  

To calculate the mass emissions of methane, multiply the appropriate emission factor by the total operating time (in hours) and then multiply by 0.001 to convert from kilograms to metric tons. This will yield the mass emissions of methane in metric tons. 

Emission Factors: 

• Use 0.083 kilograms methane per hour per reciprocating internal combustion engine for sources in the onshore petroleum and natural gas production and onshore petroleum and natural gas gathering and boosting industry segments.
• Use 0.11 kilograms methane per hour per reciprocating internal combustion engine for sources in all other applicable industry segments. 

New Crankcase Venting Reporting Requirements 

The 2025 calendar year (to be reported by March 31, 2026) will mark the first time emission reporting for crankcase venting is required.  

Questions?  

Trihydro's air quality and regulatory specialists can help you manage the complexities of Subpart W compliance. With our expertise in emissions regulations and industry best practices, we can assist you in developing effective strategies for monitoring, calculating, and reporting compliance data. 

Contact us to develop your Subpart W compliance strategy.