Total Hydrocarbon Leaker Emission Factors Calculated using 

Clearstone I and II Data 

This memo discusses the development of hydrocarbon leaker emission factors (LEFs) for 

the natural gas processing sector for use in Subpart W of the MRR. Clearstone 

Engineering Ltd. provided raw emissions data collected in 1998 and 2004 from leaking 

components in the processing sector. This data was collected for two studies referred in 

this document as Clearstone I1 and Clearstone II2 respectively. 

Clearstone Engineering Ltd. measured hydrocarbon emissions from components found in 

nine processing plants and categorized them by site, process unit, component type, and 

stream type. In 1998, 4 processing plants were surveyed for the Clearstone I report which 

was published in 2002; the resulting data will be referred to henceforth as Clearstone I 

data. In 2004, an additional five processing plants were surveyed and the 4 processing 

plants surveyed in 1998 were resurveyed for the Clearstone II report, which was 

published in 2005. The data from the 9 processing facilities surveyed for the Clearstone II 

report is henceforth referred to as Clearstone II data. 

Methodology 

The LEFs provided by Clearstone were filtered and consolidated for comparison with the 

LEFs in the current draft supplemental rule Subpart W. First, the Clearstone II data was 

re-categorized by converting Clearstone’s component categories to those found in the 

current draft proposed Subpart W, the key for which is found below in Table A-1. The 

Clearstone component categories were relabeled to fit: valves, open ended lines, 

connectors, pressure-relief valves, meters, compressor seals, and other. 

Furthermore, the Clearstone II data was separated into components which occur before 

and after the de-methanizer unit. This was done using the stream types provided by 

Clearstone. Each stream type was labeled according to the key found in Table A-2. For 

example, components labeled PG (Process Gas) and SR/PG (Sour Process Gas) were 

considered to occur before the de-methanizer while components labeled FG (Fuel Gas) 

and Residue Gas were considered to occur after the de-methanizer. Stream types such as 

AIR, C, and HC Liq were excluded from the data set because they are not methane 

related. Also, the bottoms streams from the de-methanizer, and all other fractionation 

unit streams (such as C2, C2+, and C3) were excluded because they contain negligible 

methane content. Since there is a significant methane composition change over the demethanizer, separating the data into before and after the de-methanizer groups together 

those leaker factors with similar methane compositions. 

The Clearstone II data was further categorized into compressor and non-compressor 

related components. The vibrations and thermal stresses created by compressors put 

additional stress on surrounding components making them more susceptible to leaking. 

 

1

 Clearstone Engineering Ltd. Identification and Evaluation of Opportunities to Reduce Methane Losses at 

Four Gas Plants. Calgary, Alberta: 2002. 2

 Clearstone Engineering LTD. Cost-Effective Directed Inspection and Maintenance Control Opportunities 

at Five Gas Processing Plants and Upstream Gathering Compressor Stations and Well Sites. (Draft): 2006 
This was done by filtering those components with process unit descriptions referring to 

compressors. For example, if the process unit description contained “compressor” the 

data point would be categorized as compressor, otherwise it would be categorized as noncompressor. 

Compressor components were further separated into reciprocating and centrifugal 

compressor related components. Reciprocating compressor pistons cause heavier 

vibrations than centrifugal compressor turbines. This was done by filtering those data 

points categorized as compressors and searching for “turbine” in the process unit 

description. If the process description contained “turbine”, the component was labeled 

centrifugal, otherwise it was labeled reciprocating. The above methodology is displayed 

as a flowchart in Figure 1. 

 

The same methodology described above was repeated for the Clearstone I data. A 

statistical analysis was then performed to determine whether the resulting LEFs from 

both data sets, Clearstone I and II could be combined. 
Emission Factor Calculation 

Component Grouping 

Source Categorization 

Clearstone II Raw Data 

Methane 

Streams 

Non-Methane 

Streams 

Before 

De-Methanizer 

After 

De-Methanizer 

Compressor NonCompressor 

Reciprocating Centrifugal Reciprocating Centrifugal 

Compressor NonCompressor 

Valves 

Connector 

Open-Ended 

Lines

Pressure Relief 

Valve

Meter 

Component 

Counts 

Hydrocarbon 

LEF 

Methane LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Methane LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Methane LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Methane LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Methane LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Methane LEF 

Figure 1 - Summary of Clearstone data analysis. 
Statistical Analysis 

Clearstone II data was segregated into specific component groups and total hydrocarbon 

leaker emission factors were calculated. Prior to stating definite LEFs, a statistical 

analysis that verifies our component grouping assumptions needs to be conducted. In 

addition, combining LEFs calculated using Clearstone I and Clearstone II data needs to 

be justified statistically. 

The statistical analysis consists of a paired, two-tailed t-test. The test consists of a null 

hypothesis and an alternate hypothesis which are validated using the t-statistic, t0. 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 (study 1) and 

Clearstone II (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 the separate 

studies 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 

Component Grouping Assumptions

There were three major assumptions that were made when grouping the components: 

centrifugal compressors will have different LEFs than reciprocating compressors; 

compressor related components will have different LEFs than non-compressor related 

components; and components placed after the de-methanizer will have different LEFs 

Raw Data 

Combinable or 

Not Combinable 

Statistical Analysis 

Statistical Data 

Clearstone I Raw 

Data 

Clearstone II 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 
than components placed before the de-methanizer. In each of these cases, the t-test should 

fail, suggesting that the assumptions made and the component groupings are correct. 

1. Centrifugal/Reciprocating Compressors vs. Non-compressor

Compressor components were separated from non-compressor components 

because the vibrations and thermal stresses created by compressors put additional 

stress on surrounding components making them more susceptible to leak. Table 1 

and 2 show the results of comparing centrifugal compressor and reciprocating 

compressor to non-compressors. 

Table 1 – Comparison of centrifugal compressor LEFs to non-compressor LEFs.

Clearstone I Clearstone II Condensed 

Categories Before

De-methanizer

After

De-methanizer

Before

De-methanizer

After

De-methanizer

Valve FAIL FAIL Cannot Compare i FAIL 

Connector FAIL FAIL Cannot Compare i PASS 

Open-ended Line Cannot Compare i FAIL Cannot Compare i PASS 

Pressure Relief Valve Cannot Compare i Cannot Compare i Cannot Compare i Cannot Compare i

Meter Cannot Compare i Cannot Compare i Cannot Compare i Cannot Compare i

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

Table 2 – Comparison of reciprocating compressor LEFs to non-compressor LEFs.

Clearstone I Clearstone II Condensed 

Categories Before

De-methanizer

After

De-methanizer

Before

De-methanizer

After

De-methanizer

Valve FAIL FAIL PASS FAIL 

Connector FAIL FAIL PASS FAIL 

Open-ended Line FAIL PASS PASS PASS 

Pressure Relief Valve Cannot Compare i Cannot Compare i Cannot Compare i Cannot Compare i

Meter Cannot Compare i Cannot Compare i Cannot Compare i Cannot Compare i

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

2. Centrifugal Compressors vs. Reciprocating Compressors

Centrifugal compressor components were separated from reciprocating 

compressor components because the rotating pistons in reciprocating compressors 

cause heavier vibrations than the centrifugal compressor’s turbines. As a result, 

the centrifugal compressors are expected to have higher LEFs than reciprocating 

compressors. Table 3 shows the results of comparing centrifugal compressor and 

reciprocating compressor to non-compressors. 

Table 3 - Comparison of centrifugal compressor LEFs to reciprocating LEFs.

Clearstone I Clearstone II Condensed 

Categories Before

De-methanizer

After

De-methanizer

Before

De-methanizer

After

De-methanizer

Valve FAIL FAIL Cannot Compare i FAIL

Connector FAIL FAIL Cannot Compare i FAIL
Open-ended Line Cannot Compare i FAIL Cannot Compare i PASS 

Pressure Relief Valve Cannot Compare i Cannot Compare i Cannot Compare i Cannot Compare i

Meter Cannot Compare i Cannot Compare i Cannot Compare i Cannot Compare i

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

3. Before the De-Methanizer vs. After the De-Methanizer 

Streams after the de-methanizer would be expected to have the same emission 

factor in total hydrocarbon as those before the de-methanizer. To determine 

whether this is true, the t-test was performed comparing components before and 

after de-methanizer. Table 4 and 5 show the results of the t-test. 

Table 4 – Comparison of Clearstone I LEFs calculated for components before the 

de-methanizer and components after the de-methanizer.

Clearstone I Condensed 

Categories Centrifugal Reciprocating Non-Compressor

Valve FAIL FAIL FAIL 

Connector FAIL FAIL FAIL 

Open-ended Line Cannot Compare i FAIL FAIL 

Pressure Relief Valve Cannot Compare i Cannot Compare i Cannot Compare i

Meter Cannot Compare i Cannot Compare i FAIL 

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

Table 5 – Comparison of Clearstone II LEFs calculated for components before the 

de-methanizer and components after the de-methanizer. 

Clearstone II Condensed 

Categories Centrifugal Reciprocating Non-compressor

Valve Cannot Compare i PASS PASS 

Connector Cannot Compare i PASS FAIL 

Open-ended Line Cannot Compare i PASS PASS 

Pressure Relief Valve Cannot Compare i Cannot Compare i Cannot Compare i

Meter Cannot Compare i Cannot Compare i Cannot Compare i

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

In summary, our final LEFs were separated into centrifugal compressor, reciprocating 

compressor and non-compressor related. In addition, the centrifugal and reciprocating 

compressor components were separated into before and after the de-methanizer based on 

the surprising statistical differences in emission factors. 

Combining Clearstone I and Clearstone II

Performing a t-test study between Clearstone I data and Clearstone II data provides a 

statistical base for combining the emission factors derived from either study. Clearstone I 

was grouped in exactly the same way as the Clearstone II data to facilitate the 

comparison between the two studies. The results of the comparison are shown in Table 6 

and 7. 
Table 6 – Comparison of Clearstone I and Clearstone II LEFs for centrifugal and 

reciprocating compressor components.

Centrifugal Reciprocating Condensed 

Categories Before

De-methanizer

After

De-methanizer

Before

De-methanizer

After

De-methanizer

Valve Cannot Compare i PASS PASS PASS 

Connector Cannot Compare i PASS PASS PASS 

Open-ended Line Cannot Compare i PASS FAIL FAIL 

Pressure Relief Valve Cannot Compare i Cannot Compare i Cannot Compare i Cannot Compare i

Meter Cannot Compare i Cannot Compare i Cannot Compare i Cannot Compare i

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

Table 7 – Comparing Clearstone I and Clearstone II LEFs for noncompressor components. 

Non-Compressor Condensed 

Categories Before

De-methanizer

After

De-methanizer

Valve PASS PASS 

Connector PASS PASS 

Open-ended Line PASS PASS 

Pressure Relief Valve PASS Cannot Compare i

Meter PASS PASS 

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

In categories that passed the t-test, the LEFs were combined using a weighted average. 

On the other hand, LEF’s calculated using Clearstone I data was used in categories that 

failed the t-test. 

Leaker Emission Factor 

Based on the results of the statistical study, the resulting emission factors are shown in 

Table 8. 
Table 8 – Table W-2 of Subpart W – Default total hydrocarbon emission factors for 

processing. 

Processing

Before De-methanizer

Emission Factor 

(scf/hour/component)

After De-methanizer 

Emission Factor 

(scf/hour/component)

Leaker Emission Factors - Reciprocating Compressor Components, Gas Service

Clearstone (2002); Clearstone (2005)

Valve 15.88 18.09

Connector 4.31 9.10

Open-ended Line 17.90 10.29

Pressure Relief Valve 2.01 30.46

Meter 0.02 48.29

Leaker Emission Factors - Centrifugal Compressor Components, Gas Service

Clearstone (2002) ; Clearstone (2005) ; EPA Lessons Learned (2006)

Valve 0.67 2.51

Connector 2.33 3.14

Open-ended Line 17.90 16.17

Dry Seal 105 105

Leaker Emission Factors - Other Components, Gas Service

Clearstone (2002) ; Clearstone (2005)

Valve 6.42

Connector 5.71

Open-ended Line 11.27

Pressure Relief Valve 2.01