1 

MEMORANDUM 

TO: Docket # EPA-HQ-OAR-2011-0512 

FROM: Alexis McKittrick and Mark DeFigueiredo, EPA/CCD

DATE: February 20, 2014 

SUBJECT: Greenhouse Gas Reporting Rule: Technical Support for Revisions and 

Confidentiality Determinations for Petroleum and Natural Gas Systems; Proposed 

Rule 

The EPA is proposing revisions and other clarifications to 40 CFR part 98, subpart W (hereafter 

referred to as “subpart W”). This memorandum provides background information on certain 

proposed revisions relating to the following emission sources or specific provisions1

: 

1. Well venting for liquids unloading 

2. Gas well venting during completions and workovers with hydraulic fracturing 

3. Blowdown vent stacks 

4. Dump valves not closing properly at onshore production storage tanks 

5. Associated gas venting and flaring from produced hydrocarbons 

6. Reciprocating and centrifugal compressor venting 

7. Best available monitoring methods (BAMM). 

1 WELL VENTING FOR LIQUIDS UNLOADING 

 The EPA is proposing to revise the calculation and reporting requirements for well 

venting from liquids unloading to allow for annualizing venting data for facilities that calculate 

emissions using a recording flow meter (Calculation Method 1). This proposed amendment 

would address reporter concerns and simplify reporting. 

 Some reporters have expressed difficulty in collecting well venting data using a recording 

flow meter for the exact period of January 1 to December 31, because they contend that it would 

require them to be physically present at each recording flow meter on December 31. These 

reporters described that the activity data gathered for liquids unloading associated with well 

venting events are often recorded locally at the well site. They asserted that the data are not 

 

1

 For background information related to other proposed revisions, please refer to documents contained in Docket No. 

EPA-HQ-OAR-2011-0512. 
 2 

transmitted to a centralized location and must be collected by a technician visiting each well site. 

As a result, they asserted that it is logistically difficult for some reporters with multiple wells in 

this configuration to collect data from all wells on exactly December 31st or January 1st. They 

suggested that it would be more practical for these reporters to collect the data over a period of 

several weeks, depending on the number of wells and the distance between wells. 

 Options to address this issue of allowing reporters to use time periods other than exactly 

January 1 to December 31 (e.g., a period that may cover 300 to 400 days, instead of exactly 365) 

include: (1) allow the use of an average estimated time that venting occurs in a year, using 

engineering estimates based on best available data; or (2) allow reporters to compute an 

annualized estimate based on data that are collected over at least 300 days of the reporting 

period. The first option increases information collection flexibility and requires minimal 

revisions to the current regulatory language. However, the option potentially compromises data 

quality due to use of inconsistent methods among reporters to perform the engineering estimates. 

The second option increases information collection flexibility as well, while still requiring the 

collection of venting data and avoiding both the need to use engineering estimates and to 

establish criteria for “best available data.” Additionally, the same procedure would be used 

across all reporters, resulting in a consistent emissions calculation method. Although the 

collected data may or may not represent the exact reporting year, if each monitoring period 

begins immediately following the last, then all venting events would be captured in one of two 

consecutive reporting years. For example, if a reporter chose to collect data for the period from 

January 1, 2014 to December 1, 2014 for the 2014 reporting year, then the data collection for the 

2015 reporting year would start on December 2, 2014 as opposed to January 1, 2015. 

The EPA is proposing to revise Calculation Method 1 (40 CFR 98.233(f)(1)) such that 

reporters may use an annualized value to determine the cumulative amount of time of venting 

(“Tp” in Equation W-7A and W-7B) if data are not available for the specific time period 

January 1 to December 31. The EPA is specifying that if an annualized value is used, the 

monitoring period must begin before February 1 and must not end before December 1 of the 

reporting year, and that a minimum of 300 consecutive days must be used by reporters to 

determine the annualized vent time. The EPA is also proposing that the date of the end of one 

monitoring period must be the start of the next monitoring period for the next reporting year, and 

that all days must be monitored and all venting accounted for. The EPA is proposing that if a 

reporter uses a monitoring period other than a full calendar year for any well, they must report 

the percentage of wells for which a monitoring period other than a full calendar year is used. 

The EPA is also proposing an additional change to Calculation Method 1 at 40 CFR 

98.233(f)(1) to separate the calculation and reporting of emissions from wells that have plunger 

lifts and wells that do not have plunger lifts. The calculation and reporting of emissions from 

these sources separately would allow the EPA and the public to better analyze the reported 

emissions and activity data. A clarification is also proposed for Calculation Method 2 at 40 CFR 

98.233(f)(2) to clarify that this method is used for wells without plunger lifts. 
 3 

The EPA also proposes to amend the definition of the term “SPp” in Equation W-8 (40 

CFR 98.233(f)(2)) to clarify that if casing pressure is not available for each well, reporters may 

determine the casing pressure using a ratio of the casing pressure to tubing pressure from a well 

in the same sub-basin where the casing pressure is known. This amendment would improve the 

consistency of the calculation method used to determine casing pressure across reporters. 

2 GAS WELL VENTING DURING COMPLETIONS AND WORKOVERS WITH 

HYDRAULIC FRACTURING 

The EPA is proposing to amend 40 CFR 98.238 to add definitions for “reduced emissions 

completion” and “reduced emissions workover.” Currently, reduced emissions completions and 

reduced emission workovers are mentioned in the relevant calculation method as equipment that 

separates natural gas from the backflow and sends this natural gas to a flow-line. However, the 

rule currently has no defined terms for reduced emissions completions and reduced emissions 

workovers. Since the time that subpart W was promulgated, the EPA promulgated new source 

performance standards for the oil and natural gas sector under 40 CFR part 60, subpart OOOO, 

that requires the use of a reduced emissions completion (following fracturing or re-fracturing) in 

specified circumstances. 

The EPA is proposing to add a definition for “reduced emissions completion” to subpart 

W that would be consistent with the description of that term in the new source performance 

standard (see 76 FR 52757-8, August 23, 2011). Specifically, the EPA is proposing to amend 40 

CFR 98.238 to define a “reduced emissions completion” as a well completion following 

fracturing where gas flowback that is otherwise vented is captured, cleaned, and routed to the 

flow line or collection system, re-injected into the well or another well, used as an on-site fuel 

source, or used for other useful purpose that a purchased fuel or raw material would serve, with 

no direct release to the atmosphere. We are proposing to amend 40 CFR 98.238 to define a 

“reduced emissions workover” as a well workover with hydraulic fracturing (i.e., re-fracturing) 

where gas flowback that is otherwise vented is captured, cleaned, and routed to the flow line or 

collection system, re-injected into the well or another well, used as an on-site fuel source, or used 

for other useful purpose that a purchased fuel or raw material would serve, with no direct release 

to the atmosphere. 

The EPA is also proposing to amend the definition of “well completions” in 40 CFR 98.6 

to delete the term “re-fracture” because this term applies to an already producing well and a “refracturing” is considered a well workover, not a well completion, for the purposes of Part 98. 

This amendment is intended to avoid potential confusion concerning whether a re-fracture is a 

completion or workover in the context of subpart W. This change would also better align the 

existing definition of “well completions” with the new proposed definition of a “reduced 

emissions completion” by clarifying that a reduced emission completion applies to only new 

fractures and that re-fractures are potentially covered under the new definition of “reduced 

emission workover.” The definition of “well workover” in 40 CFR 98.6 already refers to refractures, so no clarifying change is needed for that definition. 
 4 

The EPA is also proposing to revise reporting requirements for completions and 

workovers to differentiate between completions and workovers with different well type 

combinations in each sub-basin category. A well type combination is being proposed as a unique 

combination of the following factors: vertical or horizontal (i.e., directional drilling), with flaring 

or without flaring, and reduced emission completion/workover or not reduced emission 

completion/workover. Specifically, for well completions and workovers with hydraulic 

fracturing, the EPA is proposing to require separate counts and separate reporting of emissions 

for the different well type combinations. These revisions would improve data quality for 

emissions from wells with hydraulic fracturing. The EPA is also proposing revisions to 

Equation W-10A that would add clarity and increase the accuracy of emissions calculations for 

gas well completions and workovers with hydraulic fracturing. In the final subpart W rule, the 

measurement or calculation for determining the ratio of flowback during well completions and 

workovers to 30-day production rate in Equation W-10A (40 CFR 98.233(g)) begins 

immediately upon initiating flowback of a well. Some reporters have asserted that the flowback 

characteristics of a well following hydraulic fracturing do not enable measurement or calculation 

to begin immediately upon initiating flowback due to a lack of sufficient gas being present, and 

the calculation needs to be revised to account for this fact. Therefore, the EPA is proposing to 

modify the calculation to require the measurement of flow rate only when sufficient gas is 

present to enable flow rate measurement. In addition, some reporters have asserted that the 

accuracy of emissions calculations could be affected by the combined use of sales gas volume 

and approximations on flow rates for non-measured wells. To resolve this apparent issue, the 

time variable “Tp” in Equation W-10A and W-10B is being modified. Time that the gas is routed 

to production would no longer be included, so it would no longer be necessary to subtract the 

volume of gas being sent to sales. 

The EPA is also proposing changes to 40 CFR 98.233(g) and (h) to support the separate 

calculation of emissions from completions and workovers that are vented, flared, or use 

equipment that separates natural gas from the backflow and sends this natural gas to a flow-line 

(i.e., reduced emissions completions or reduced emissions workovers). Reporters currently 

calculate emissions from all completion and workover activities, but the equations do not 

facilitate the classification of the activity needed for separate reporting. The EPA is proposing to 

revise Equation W-13 in 40 CFR 98.233(h) to separate the calculation of emissions from 

workovers from the calculation of completions into two equations. The EPA is also proposing to 

clarify that reporters must calculate the annual volumetric natural gas emissions from each gas 

well venting during workovers without hydraulic fracturing using Equation W-13A and from 

each gas well venting from completions without hydraulic fracturing using new Equation W13B. The proposed methodology also requires the addition of parameter “Es,p” for Equation W13B to specify the annual volumetric natural gas emissions in standard cubic feet from well 

completions. We are also proposing to revise 40 CFR 98.233(g)(1) to clarify the number of 

measurements or calculations that must be taken to estimate the average ratio of flowback rate 

(FRM). 
 5 

3 BLOWDOWN VENT STACKS 

During implementation of the final subpart W rule, some reporters requested that the 

EPA allow the use of a factor to adjust for compressibility when calculating emissions from 

blowdown vents. The calculation method for blowdown vents included in the existing subpart W 

rule assumes natural gas is an ideal gas with a compressibility factor of 1, and does not include 

an adjustment for compressibility in the calculation. 

The compressibility of methane at standard conditions is close to 1. However, the 

compressibility of methane at low temperatures and high pressures is significantly different, as 

illustrated in Table 3.1. The natural gas processing and transmission compression segments 

covered by subpart W can operate systems at pressures higher than 25 atmospheres.2 

Table 3.1. Compressibility of Natural Gas at Sample Temperatures and Pressures3

Temperature (deg K) 1 atm 5 atm 10 atm 40 atm 

150 0.9854 0.9225 0.8275 0.1411 

200 0.9936 0.9676 0.9339 0.6784 

250 0.9965 0.9838 0.9680 0.8682 

350 0.9991 0.9954 0.9911 0.9662 

Although the EPA had previously considered including the compressibility term (76 FR 

56010, September 9, 2011), the EPA ultimately did not propose including the factor. The EPA 

concluded at that time that including a compressibility adjustment could create a degree of 

uncertainty among reporters when trying to compare their reported blowdown values on a 

volume basis. The EPA noted at that time that although the compressibility of pure light 

hydrocarbon substances is well known, the compressibility of hydrocarbon mixtures is less well 

known and the composition of natural gas throughout the segments covered by subpart W can be 

variable. At that time, we determined that ideal gas law calculations were adequate for reporting 

purposes under Part 98. 

The EPA notes that the circumstances surrounding this issue are now different, because 

the EPA is proposing the option of using site-specific data for gas compositions, if available. 

Therefore, the original concern regarding the variation in the composition of natural gas at 

subpart W facilities and its effect on compressibility would be addressed through data collection 

on actual gas composition. The EPA has determined that at high pressures and low temperatures, 

the accuracy of the emission estimate would be improved if a compressibility factor were 

included in the calculation. 

 

2

 Natural gas transmission compressor station components and main line pressures are in the range of 500 to 1000 

psi (34 to 68 atmospheres). See page 6-32, American Petroleum Institute (August 2009), Compendium of 

Greenhouse Gas Emission Methodologies for the Oil and Natural Gas Industry. 3

 Values derived from Table 3-172 of Perry, R. H. (1984). Perry's Chemical Engineers' Handbook, Sixth Edition.

New York, New York: McGraw-Hill Book Company. 
 6 

Options to address the compressibility factor include: (1) include the compressibility term 

“z” into Equations W-14A and W-14B with the requirement that “z” can be assumed to be “1” if 

the temperature is above -10 degrees Fahrenheit and pressure is below 5 atmospheres (or if the 

compressibility factor at the actual temperature and pressure is 0.98 or greater) or (2) allow the 

compressibility to be used at the reporter’s discretion. The EPA is proposing the first option 

above to ensure clarity and consistency with any variations in the compressibility term from “1” 

are applied among reporters. 

The EPA is proposing to remove the current reporting requirements for blowdown vents 

in the rule related to “unique physical volumes,” but would retain the requirement to calculate 

and keep records for blowdown vent stack emissions from each unique volume. Examples of the 

proposed equipment types that would replace the requirement in 40 CFR 98.233(i) to track and 

report by unique volume are identified as follows: 

 Station piping 

 Pipeline venting (inside the compressor station fence line) 

 Compressors 

 Scrubbers/Strainers 

 Pig launchers and receivers 

 Emergency shutdowns 

 All other blowdowns greater than or equal to 50 cubic feet. 

These equipment categories cover all blowdown venting from equipment or event types 

associated with the three (3) source categories (i.e., onshore natural gas processing, onshore 

natural gas transmission compression, and liquefied natural gas (LNG) import and export 

equipment). With this proposed option, the reporting burden would be reduced and detailed 

information on equipment type would provide the EPA with sufficient detail to determine the 

source of emissions.

The EPA is not proposing a change to the “unique volume” calculation methods, because 

an accurate estimate of emissions can be obtained only by either estimating the actual volume of 

equipment vented, or by directly measuring the emissions. The calculation methods in 40 CFR 

98.233(i)(2) (i.e., Equations W-14A and W-14B) would not need to be revised, because all 

reporters would still need to determine annual venting emissions for each unique volume. 

It is possible that some facilities are currently using (or would prefer to use) a flow meter 

on some of their blowdown vents. If a flow meter were used, Equation W-14A or W-14B would 

not be necessary for calculating the annual volumetric amount of natural gas vented. This option 

would allow reporters to use an existing flow meter, or to install a flow meter in locations where 

emissions are a routine occurrence rather than calculating individual volumes. The accuracy of 

flow meter measurements is equal to or greater than the current unique volume method. 

Therefore, the EPA is also proposing an optional calculation method for blowdown emissions 
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where a flow meter is in place to measure the emissions directly. If a blowdown vent is equipped 

with a flow meter, there would not be an advantage to calculating the emissions using the unique 

volume, temperature, and pressure conditions of the equipment instead of the directly measured 

flow rate. The EPA is proposing this alternative calculation method in 40 CFR 98.233(i), along 

with associated reporting requirements in 40 CFR 98.236. 

4 DUMP VALVES NOT CLOSING PROPERLY AT ONSHORE PRODUCTION 

STORAGE TANKS 

Currently, subpart W reporters must calculate annual emissions from onshore production 

storage tanks, including emissions during periods when the liquid dump valve on gas-liquid 

separators does not close properly (40 CFR 98.233(j)(8)). 

Some reporters have asserted that the current requirements for calculating emissions from 

separator dump valves that are not closing properly requires a high effort to monitor and 

document that separator dump valves are closing properly. They have also asserted that the 

current equations require them to keep unnecessarily large amounts of records to document 

periods when the valves are closing properly, as well as not closing properly. 

According to these reporters, onshore production companies typically have equipment 

maintenance and monitoring programs in place to maintain the integrity of their equipment and 

to minimize losses of natural gas from various types of equipment leaks, including dump valves 

that are not closing properly on gas liquid separators on onshore production storage tanks. 

Large companies may be responsible for several thousand dump valves and some may 

use remote monitoring to determine whether or not the dump valves are operating properly, 

while other companies may rely on technicians visiting a well site to test the equipment and 

ensure it is working properly. Companies that use remote monitoring can determine if a dump 

valve is not closing properly, as indicated by a drop in pressure, a low level in a gas/liquid 

separator, a decrease in gas production rates, or a sudden increase in the level of a storage tank. 

Reporters have asserted that field technicians usually undertake corrective actions within 

24 hours of when companies detect abnormal operations at remotely monitored wells. 

Other companies may rely on field technicians alone to ensure that equipment such as 

dump valves are operating correctly. Field technicians will take corrective actions when they 

detect faulty equipment. Reporters have asserted that the frequency with which a technician 

visits a well will depend on the company’s standard operating procedures, but it will range from 

once every few days to no more than once every two weeks. 

In the case of either remote monitoring or field monitoring of equipment, some reporters 

further asserted that they should not need to record information documenting the occurrences of 

dump valves that are closing properly and should, instead, record only those times when the 

dump valves are not closing properly. They asserted that because dump valves operate properly 

the vast majority of the time and because many companies are responsible for large numbers of 
 8 

dump valves that are monitored continuously or visited frequently, documenting that they are 

operating properly entails making and keeping large amounts of records that can become 

burdensome. 

Equation W-16 in the current rule also does not calculate separately emissions for those 

periods when the dump valves are not closing properly. Instead, the current equation calculates 

total annual emissions from each storage tank, and combines emissions during normal dump 

valve operation with emissions when dump valves are not closing properly. Under the current 

equation, the data do not distinguish emissions that occur when dump valves are not closing 

properly from the total emissions from the separator on the storage tank. Therefore, the EPA is 

proposing to simplify Equation W-16 to calculate emissions for only periods when the dump 

valve is not closing properly. The EPA is clarifying that the revised equation calculates only the 

additional emissions that result when the dump valve is not closing properly. 

Because the revised Equation W-16 would calculate only the additional emissions from 

dump valves not closing properly, it was necessary to adjust the correction factors (CFn) in order 

to simplify the equation. The EPA is proposing to decrease the correction factors for crude oil 

production and gas condensate production by “1” to reflect emissions originating only from the 

dump valve not closing properly. 

5 ASSOCIATED GAS VENTING AND FLARING FROM PRODUCED 

HYDROCARBONS 

Subpart W reporters must calculate annual emissions from associated natural gas venting 

and flaring (40 CFR 98.233(m)). 

Some reporters have requested that a term be added to Equation W-18 to account for 

situations where a portion of the natural gas is going to the sales line and a portion of the natural 

gas is flared. These reporters described situations in which new well production could exceed the 

capacity of the sales line until the downstream operator makes adjustments, or where temporary 

equipment problems limit the sales line capacity. In these situations, some of the natural gas 

production from a well may be flared while some of the production goes to sales. In this case, 

these reporters assert that the associated gas emissions calculation method, found in Equation W18, would overestimate the associated gas emissions, because the equation estimates emissions 

based on the gas to oil ratio (GOR) and volume of oil produced during the flaring period. The 

GOR is based on total gas from the well, which means all the gas would be reported as flared, 

despite some of the gas going to the sales line. 

Three options to address these reporters’ concerns are: (1) allow the use of an “adjusted” 

gas to oil ratio, where reporters would use engineering estimates to revise the gas to oil ratio to 

account for average amount of gas sent to sales; (2) require reporters to meter the amount of 

associated gas vented or flared; or (3) add a definition of “Associated gas venting or flaring” to 

clarify what is covered by this term and add terms to Equation W-18 to exclude gas sent to sales 

and emissions that are calculated and reported elsewhere under subpart W. 
 9 

Allowing reporters to use engineering estimates in the first option to revise the gas to oil 

ratio to account for average amount of gas sent to sales could provide flexibility, but would result 

in inconsistent data collection among reporters and could potentially compromise data quality. 

The second option to require reporters to meter the amount of associated gas vented or flared 

would result in greater data quality but could potentially increase burden on reporters. 

The EPA is proposing the third option of adding a definition of “Associated gas venting 

or flaring” and adding terms to Equation W-18 to subtract the volume of associated gas sent to 

sales from the annual volumetric natural gas emissions from associated gas venting. The EPA is 

also proposing to include in the equation a term for emissions that are calculated and reported in 

other sections of subpart W. These proposed changes would minimize the risk of over-estimating 

emissions, clarify the emissions to be reported, and are not expected to significantly increase the 

reporting burden. 

6 RECIPROCATING AND CENTRIFUGAL COMPRESSOR VENTING 

To address the issue of measuring compressors every 3 years in the not-operatingdepressurized-mode, options include: (1) allow measurement in not-operating-depressurizedmode to be taken at the next scheduled depressurized compressor shutdown (for reciprocating 

compressors, this measurement can be taken during the next scheduled shutdown when the 

compressor rod packing is replaced); and (2) allow measurement in not-operating-depressurizedmode to be taken every 5 years (instead of once every 3 years). The option to allow measurement 

in not-operating-depressurized-mode to be taken at the next scheduled depressurized compressor 

shutdown would require minimal revisions to regulatory text, allow reporters flexibility, provide 

consistent data collection among reporters, and facilities would not potentially have to shut down 

a compressor every 3 years for the sole purpose of measuring that compressor in not-operatingdepressurized-mode. The option to allow measurement in not-operating-depressurized-mode to 

be taken every 5 years would result in minimal revisions to regulatory text, and would eliminate 

facilities having to shut down a compressor prior to every 5 years for the sole purpose of 

measuring in not-operating-depressurized-mode. However, this option may not provide enough 

flexibility for reporters who do not shut down their compressor(s) within a 5-year period. 

Therefore, the EPA is proposing to revise the requirement to measure each compressor in the 

not-operating-depressurized-mode at least once every 3 years. In place of the current 

requirements, the EPA is proposing that each compressor must be measured in not-operatingdepressurized-mode at least once in any 3 consecutive calendar years, provided the measurement 

can be taken during a scheduled shutdown. If there is no scheduled shutdown within 3 

consecutive calendar years, then a measurement would have to be made at the next scheduled 

depressurized compressor shutdown. For reciprocating compressors, this measurement could be 

taken during the next scheduled depressurized shutdown when the compressor rod packing is 

replaced. 

Some reporters have contended that it is not practical to require a measurement from each 

individual compressor for groups of compressors that are routed to a common vent manifold (or 
 10 

flare header), because this would require the entire group of compressors that are connected to 

the common manifold (or flare header) to be shutdown, blown down, and purged in order to 

safely install meters (or ports for temporary meters) and enable individual measurements. The 

reporters stated that it is extremely rare that entire groups of compressors are shutdown at the 

same time. In the November 2010 response to public comments on the subpart W final rule 

(Docket ID No. EPA-HQ-OAR-2009-0923), the EPA noted that commenters requested that the 

EPA allow direct measurements of common manifolded vent lines on compressors. At least one 

commenter stated that if continuous measurement of manifolded vent lines and aggregate annual 

emissions reporting were allowed as an option for measuring compressors, they would be able to 

safely collect and report to the EPA continuously measured data. The EPA did not include this 

option in the 2010 final subpart W rule because it was not clear whether measurements at a 

common vent outlet could be used to correctly characterize annual emissions from individual 

compressors. 

To address the issue of having to measure individual compressors that are manifolded to 

a single vent, options include requiring reporters to take either one, two, or three measurements 

per year on the common vent (or flare) stack that is associated with the manifolded group of 

compressor sources (before emissions are comingled with other non-compressor emission 

sources). The option to require one or two annual measurements from a single point in the 

manifold would likely reduce overall burden on reporters. However, the process variation in a 

manifold fed by several sources over the year could be high enough that overall data quality 

could be reduced if one or two measurements are not representative of the entire year’s 

operations. The option to require three annual measurements from a single point in a manifold 

would likely reduce overall burden on reporters, while ensuring a more representative data set 

and better data quality than collecting one or two data points. Any of these options to require 

annual measurements would resolve the reporter’s concerns about the technical and safety issues 

with installing and utilizing sampling ports in manifolded lines. The accuracy of these options 

would vary with the number of annual measurements that would be required. 

For compressors that are manifolded to a common vent (or flare header), the EPA is 

proposing to require measurements to be taken at least three times per year at a single point in 

the manifold, downstream of all compressor inputs. These measurements would need to be taken 

where emissions cannot be comingled with other non-compressor emission sources. This option 

would maintain accurate characterization of annual emissions from compressors at the facility. 

Annual volumetric emissions would be determined for each manifolded group of compressors 

combined for all operating conditions (mode-source combinations). Reporters would still be 

required to report activity data for any individually measured sources (i.e., non-manifolded 

sources) at the compressor level. However, the EPA proposes that operators that choose to 

conduct continuous measurements at a single point in the manifold downstream of all 

compressor inputs would report emissions at the manifold level. Activity data reported would 

include information about the individual compressors included in the manifolded vent. This 

proposed measurement option would allow the EPA to correctly characterize and analyze total 
 11 

compressor emissions at individual facilities in the petroleum and natural gas source category 

while reducing burden to the industry. 

Currently, for most emission sources (including compressors), subpart W requires that 

flare emissions be calculated separately for each source type, backed out of the total flare 

emissions, and reported with each source type separately. For other emission sources that do not 

have a requirement to report as a specific source type emission, the remaining flare emissions are 

reported in the flare source type. To address the issue of having to measure each compressor 

when it is routed to an operational flare, facilities could calculate and report emissions with other 

flare emissions. This would address reporters’ safety concerns, provide reporters flexibility, and 

would reduce burden by not requiring reporters to sample compressors individually to calculate 

the portion combusted in the flare. 

 The EPA is proposing to allow operators to calculate and report emissions from 

compressors that are routed to a flare with other flare emissions (in lieu of backing these 

emissions out of the total flare emissions and reporting them as compressor emissions).

7 BEST AVAILABLE MONITORING METHODS (BAMM) 

In order to provide facilities with time to adjust to the requirements of the rule, subpart W 

has provisions allowing the optional use of BAMM for unique or unusual circumstances. Where 

a facility uses BAMM, it is required to follow emission calculations specified by the EPA, but is 

allowed to use alternative methods for determining inputs to calculate emissions. Inputs are the 

values used by facilities to calculate equation outputs. Examples of BAMM include: Monitoring 

methods used by the facility that do not meet the specifications of subpart W, supplier data, 

engineering calculations, and other company records. Facilities are required to receive approval 

from the EPA prior to using BAMM and these facilities are required to specify in their GHG 

annual reports when BAMM is used for an emission source. 

The EPA has previously noted that the Agency intended to “approve the use of BAMM 

beyond 2011 only in cases that are unique or unusual” (76 FR 59538). Furthermore, the EPA 

limited the approvals of BAMM to one reporting year in keeping with the intent to allow use of 

BAMM as a transitional provision until facilities come into compliance with the final rule. While 

the EPA occasionally uses BAMM for targeted, short-term monitoring flexibilities (i.e., 

provision for reporters who become subject to Part 98 from the recent GWP changes to subpart 

A to have automatic BAMM for the first three months of reporting), no industry-specific subpart 

within Part 98 continues to use the BAMM flexibility except subpart W. 

The EPA has proposed several revisions to the subpart W calculation methods and 

reporting requirements addressing technical feasibility. Table 7.1 summarizes examples of such 

proposed revisions to the rule, by emission source type. The proposed changes would modify the 

subpart W data collection and calculation requirements, resolving the need for the BAMM 

flexibility beyond the effective date of the final rule. 
 12 

Table 7.1. Examples of Proposed Revisions. 

Emission Source Type Issue Proposed Revisions 

Centrifugal and reciprocating 

compressors 

Compressor emissions are 

manifolded together and individual 

compressor vents are not accessible 

or practical to measure. 

Language was added to subpart W to 

allow emission measurement at the 

manifold vent outlet, rather than at each 

individual compressor. 

Reporting emissions for notoperating-depressurized-mode. Measurements in the not-operatingdepressurized-mode should be taken 

during scheduled depressurized 

compressor shutdowns. If there is no 

scheduled depressurized shutdown within 

three consecutive calendar years, then the 

measurement must be taken at the next 

scheduled shutdown. 

For reciprocating compressors, reporters 

can take these measurements during the 

next scheduled shutdown when the 

compressor rod packing is replaced. 

Compressor emissions routed to 

flares must be sampled individually 

and reported separately as a portion 

of the flare emissions. 

Allow reporters to calculate and report 

compressor emissions sent to an 

operating flare in combination with other 

flare emissions. Reporters would still 

report compressor-related activity data. 

Onshore production storage 

tanks 

Reporting of emissions from dump 

valves that are not closing properly. 

Facilities must still report emissions from 

dump valves that are not closing 

properly. Equation W-16 was revised to 

calculate emissions that result from only 

the dump valves that are not closing 

properly. 

Recordkeeping revised to keep records 

only for the dump valves that are not 

closing properly; removed the need to 

keep records for properly closing dump 

valves. 

Blowdown vent stacks Method of reporting data by unique 

physical volume is not consistent 

with industry practices. 

Blowdown emissions may be grouped by 

equipment type for reporting, but 

emissions must still be calculated for 

each unique physical volume. 

Alternatively, facilities may use a flow 

meter.

Well venting for liquids 

unloading 

Not feasible to gather vent hours or 

number of unloadings from all 

controllers on one day, i.e. January 1 

or December 31. 

Language was revised to allow facilities 

to use an annualized value and gather 

data for monitoring periods with start/end 

dates between December 1 and January 

31 that are at least 300 days. 

Parameters such as well depth and 

casing or tubing internal diameter 

are not available. 

No change in rule because facilities have 

been able to use existing BAMM periods 

to collect needed data. 

Uncertainty regarding definition of 

shut-in pressure in Equation W-8. 

Shut-in pressure definition was revised to 

clarify when it should be determined. 

Definition was also revised to describe 

how to estimate casing pressure for a 

well if it is not known, using data from 

other wells in the same sub-basin. 
 13 

Emission Source Type Issue Proposed Revisions 

Completions and workovers Uncertainty regarding the point in 

time when a completion or workover 

begins. 

Equations W-10A and W-10B were 

revised for clarity regarding start times to 

require the measurement of flow rate 

only when sufficient gas is present to 

enable flow rate measurement. 

Parameter “FRMs” in Equation W10A is not well-specific. “FRMs” was modified to be specific to 

each sub-basin and well type 

combination. 

Flowback measurement equipment 

may not be available to meet 

demand. 

No change in rule because facilities have 

been able to use existing BAMM periods 

to address equipment availability issues. 

The EPA is proposing to remove all provisions in 40 CFR 98.234(f) for BAMM. The 

removal of BAMM would improve data quality by requiring consistent reporting for each 

segment in subpart W. The EPA is proposing to remove the BAMM provisions, because it is 

expected that facilities would be able to comply with the monitoring and QA/QC methods 

required under subpart W if the proposed amendments are finalized. It has been the EPA’s intent 

throughout implementation of subpart W that BAMM be available as a limited, transitional 

program to serve as a bridge to full compliance with the rule for cases where reporters faced 

reasonable impediments to using the monitoring methods in the rule. The proposed amendments 

are in keeping with the EPA’s stated intent to transition to reporting without BAMM.