Methane Leaker Emission Factors Calculated using Canadian Energy 

Partnership for Environmental Innovation Data 

Emission surveys for the Canadian Energy Partnership for Environmental Innovation 

(CEPEI) were used in the development of methane leaker emission factors (LEF) for the 

transmission and distribution sectors under Subpart W of the current proposed rule. This 

document discusses the methodology used to develop the methane LEFs from the CEPEI 

data and the statistical analysis conducted. 

The CEPEI data consists of emission surveys conducted by Clearstone Engineering 

Limited in 1998 and 2007 and leak measurement surveys submitted by individual private 

sector companies between those years. The surveys examined numerous natural gas 

transmission, storage and distribution facilities in Canada. The results of these surveys 

were used to write two separate reports for CEPEI: Handbook for Estimating Methane 

Emissions from Canadian Natural Gas Systems1 (referred to as CEPEI 1998) and 

Measurement of Natural Gas Emissions from the Canadian Natural Gas Transmission 

and Distribution Industry2 (referred to as CEPEI 2007). CEPEI 1998 uses data from the 

survey conducted in 1998 to present available methods for assessing methane emissions 

from non-combustion sources of natural gas transmission, storage, and distribution 

systems. It was designed as a detailed reference guide for use by both management and 

practitioners. CEPEI 2007 uses data from both the 1998 and 2007 surveys, as well as 

data from surveys conducted for private sector companies, to develop a protocol for 

detecting and measuring emission. Its objective was to improve the estimation of fugitive 

methane emissions from the Canadian natural gas transmission, storage and distribution 

industries. 

The data used in this analysis includes the emission surveys from 1998 and 2007 and 

those conducted for individual companies. Specific facilities were segregated and 

methane leaker emission factors were calculated for component categories covered under 

Subpart W. 

Methodology 

The collective data is a representative sample of block valve stations, booster stations, 

border meter stations, compressor stations, control valve stations, transmission farm taps, 

receipt meter stations, sales meter stations, storage stations, and storage wells for the 

transmission/storage sector; and commercial meter sets, gate stations, distribution farm 

taps, district regulating stations, industrial meter sets, residential meter sets, and 

miscellaneous pipeline equipment in the distribution sector. As specified by Subpart W, 

the EPA isolated the data for compressor stations, storage stations, and storage wells to 

develop leaker factors for transmission and storage, and isolated gate stations and district 

regulating stations for distribution. 

 

1

 Clearstone Engineering Ltd. Handbook for Estimating Methane Emissions from Canadian Natural Gas 

Systems, (for CEPEI). 1998 2

 Clearstone Engineering Ltd. Measurement of Natural Gas Emissions from the Canadian Natural Gas 

Transmission and Distribution Industry, (for CEPEI). 2007. 
Components, in the raw data, were associated with a site identifier, tag identity, source, 

component count and leakage rate. The site identifiers designate the facility types and the 

tag identities distinguish the leaking components from the non-leaking components. The 

components were categorized as: blowdown vent, connector, controller, centrifugal 

compressor seal, reciprocating compressor seal, control valve, open-ended line, orifice 

meter, other flow meter, pressure relief valve, regulator and valve. The leakage rates 

provided in the raw data indicates the total hydrocarbon leaker rates. In cases where there 

were more than one component associated with a leakage rate, it was assumed that the 

leakage rate was a total of those components. 

Using the site identifiers, the components from the compressor stations, gate stations and 

district regulating station were separated out from the raw data. Unlike the Clearstone I 

and Clearstone II studies, it was not possible to separate the compressor station 

components into centrifugal, reciprocating and non-compressor related. Consequently, it 

was assumed that any component found at compressor stations was compressor related. 

In addition, the compressor unit could not be identified as centrifugal or reciprocating 

given the labeling system used in the raw data. Hydrocarbon LEFs for distribution were 

derived from components found in gate stations and district regulating stations as 

specified by Subpart W. 

Components found at compressor stations were grouped together to formulate 

hydrocarbon LEFs for the transmission sector where as components found at gate stations 

and district regulating stations were grouped together to calculate hydrocarbon LEFs for 

the distribution sector. The data includes non-leaking and leaking components in their 

raw data requiring the isolation of leaking components using their tag identities. The 

leaking components were then grouped according to source category. Finally, 

hydrocarbon LEFs for the transmission and distribution sectors were calculated by 

dividing the sum of the leakage rates by the sum of the component counts in each source 

category. To convert the hydrocarbon leaker emission factors to methane LEFs, a 

composition factor of 0.934, as stipulated by the GRI report3

 for transmission gas quality, 

was used 

The data analysis process is summarized visually in Figure 1. 

 

3

 GRI (1992) GRI Report, Methane Emissions from the Natural Gas Industry, Volumes 1- 15. 

Energy Information Agency (U.S. DOE) 
Emission Factor 

Calculations 

Source Categorization Facility Type Leaking 

Components 

Raw Data 

Compressor 

Stations 

Leaking 

Components 

Non-leaking 

Components 

Component 

Counts 

Hydrocarbon 

Leaker Emission 

Factors 

Connector 

Block Valve 

Control Valve 

Compressor 

Blowdown Valve 

Pressure Relief 

Valve 

Orifice Meter 

Regulator 

Open-ended Line 

Methane Leaker 

Emission 

Factors 

TRANSMISSION DISTRIBUTION

Component 

Counts 

Hydrocarbon 

Leaker Emission 

Factors 

Connector 

Block Valve 

Control Valve 

Compressor 

Blowdown Valve 

Pressure Relief 

Valve 

Orifice Meter 

Regulator 

Open-ended Line 

Methane Leaker 

Emission 

Factors 

District 

Regulating 

Stations 

Gate Station 

Leaking 

Components 

Non-leaking 

Components 

Figure 1 – Summary of CEPEI data analysis
Statistical Analysis 

The CEPEI source categories were reduced to the categories found in Clearstone I and II 

to facilitate a comparison of hydrocarbon LEFs between the studies. CEPEI LEFs for 

transmission were compared to LEFs for compressor related components that occur after 

the de-methanizer in Clearstone I and II. The CEPEI LEFs for distribution were not 

compared to Clearstone I and II because these studies had no data points from 

distribution facilities. 

To determine whether the emission factors can be combined, a paired, two-tailed t-test 

was performed. The test consists of a null hypothesis and an alternate hypothesis which 

are validated using the t-statistic. The null hypothesis states that both the population 

means are equal and the alternate hypothesis refutes this by stating the population means 

are not equal. In this case, the population means are the leaker emission factors from 

Clearstone I or II (study 1) and CEPEI (study 2), (subscripts in formulas). Stated 

mathematically, the hypotheses are: 

Null Hypothesis, Ho: μ1 = μ2, i.e., the two means (LEFs) are combinable 

Alternative Hypothesis, H1: μ1 ≠ μ2

The test statistic represents the difference in the two means from the two studies under 

consideration and is calculated using 

2

2

2

1

2

1

1 2

n

S

n

S

M M

to

+

− = ,

where 

M1 and M2 are the means from studies 1 and 2, 

S1 and S2 are the standard deviations from studies 1 and 2, 

n1 and n2 are the component counts from studies 1 and 2. 

The population sizes (i.e. component counts) and the standard deviations of study 1 and 2 

were not equal. Therefore, a corrected degree of freedom, Dof, that accommodates such 

differences is calculated using 

( ) ( )

1 2 1

2

2

2

2

1

2

1

2

1

2

2

2

2

1

2

1

− + −

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

=

n

S n

n

S n

n

S

n

S

Dof , 

where 

S1 and S2 are the standard deviations from studies 1 and 2, 

n1 and n2 are the component counts from studies 1 and 2. 
A standard t value found at a 95% confidence interval from a t-distribution curve is 

compared to t0. If the test statistic is below the standard t-value then the null hypothesis is 

accepted. Therefore, the population is considered to have similar characteristics and the 

emission factors from the compared studies can be combined. However, if t0 was above 

the standard t-value, then the null hypothesis is rejected and the emission factors cannot 

be combined. 

The steps followed are summarized visually in Figure 2. 

Figure 2 – Summary of comparative statistics methodology 

The results of the t-test are shown in Table 1 and 2. 

Raw Data 

Combinable or 

Not Combinable 

Statistical Analysis 

Statistical Data 

Clearstone I or II 

Raw Data 

CEPEI Raw Data 

M1, 

Emission 

Factors 

S1, 

Standard 

Deviation 

Test statistic, t0 Standard t-value 

at 95% 

confidence 

If t0 > t, then do 

FAIL 

Degree of 

Freedom, Dof 

If t0 < t, then 

PASS 

n1, 

Component 

Count 

M2, 

Emission 

Factors 

S2, 

Standard 

Deviation 

n2, 

Component 

Count 
Table 1 – Comparing Clearstone I LEFs to CEPEI 1998/2007 LEFs. 

Condensed 

Categories 

Clearstone I 

2002 

(scf/hr) 

CEPEI 

1998/2007 

(scf/hr) 

t-test result 

Valve 18.26 11.14 Fail 

Connector 13.25 2.87 Fail 

Open-ended Line 29.61 23.06 Pass 

Pressure Relief Valve 30.46 39.82 Cannot Compare i 

Meter 36.27 14.13 Pass 

Table 2 – Comparing Clearstone II LEFs to CEPEI 1998/2007 LEFs. 

Condensed 

Categories 

Clearstone II 

2006 

(scf/hr) 

CEPEI 

1998/2007 

(scf/hr) 

t-test result 

Valve 17.78 11.14 Fail 

Connector 8.31 2.87 Fail 

Open-ended Line 7.00 23.06 Fail 

Pressure Relief Valve - 39.82 Cannot Compare i 

Meter 72.34 14.13 Cannot Compare i 

i. There are not enough data points to make a comparison

In most cases, the test indicated that the CEPEI data cannot be combined or compared 

with Clearstone I and II data. 

Leaker Emission Factors 

The resulting hydrocarbon emission factors for transmission and distribution are shown 

in Table 3 and 4. 

Table 3 – Table W-3 of Subpart W - Default methane emission factors for transmission. 

Transmission Emission Factor 

(scf/hour/component) 

Leaker Emission Factors - All Components, Gas Service 

CEPEI (Picard 1998) & CEPEI (Picard 2007) 

Connector 2.7

Block Valve 10.4

Control Valve 3.4

Compressor Blowdown Valve 543.5

Pressure Relief Valve 37.2

Orifice Meter 14.3

Other Meter 0.1

Regulator 9.8

Open-ended Line 21.5
Table 4 – Table W-7 of Subpart W – Default methane emission factors for distribution.

Distribution Emission Factor 

(scf/hour/component) 

Leaker Emission Factors - Above Grade M&R Stations Components, Gas Service 

CEPEI (Picard 1998) & CEPEI (Picard 2007)

Connector 1.67 

Block Valve 0.557 

Control Valve 9.34 

Pressure Relief Valve 0.270 

Orifice Meter 0.212 

Regulator 0.772 

Open-ended Line 26.131