## Total Hydrocarbon Leaker Emissi on Factors Calculated using Clearstone I and II Data

This memo discusses the development of hydr ocarbon leaker emission factors (LEFs) for the natural gas processing sector for us e 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 I 1 

### and Clearstone II 2 

### 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 resulti ng 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 Subp art W. First, the Cl earstone II data was re-categorized by converting Clearstone’s co mponent 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 rela beled to fit: valves, open ended lines, connectors, pressure-relie f valves, meters, compressor seals, and other.

Furthermore, the Clearstone II data was se parated into components which occur before and after the de-methanizer unit. This wa s 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 Ga s) 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-methani zer. Stream types such as AIR, C, and HC Liq were excluded from th e 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 si milar methane compositions.

The Clearstone II data was further categor ized into compressor and non-compressor related components. The vibrations and th ermal stresses created by compressors put additional stress on surrounding components maki ng them more suscep tible 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 categori zed as compressor, otherwise it would be categorized as noncompressor.

Compressor components were further separa ted into reciprocat ing and centrifugal compressor related components. Reciproc ating compressor pistons cause heavier vibrations than centrifugal compressor turb ines. This was done by filtering those data points categorized as compressors and sear ching for “turbine” in the process unit description. If the process description cont ained “turbine”, the component was labeled centrifugal, otherwise it was labeled recipr ocating. 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 a nd 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 sp ecific component groups and total hydrocarbon leaker emission factors were calculated. Prio r to stating definite LEFs, a statistical analysis that verifies our component gr ouping assumptions needs to be conducted. In addition, combining LEFs calculated using Clears tone I and Clearstone II data needs to be justified statistically.

The statistical analysis cons ists of a paired, two-tailed t -test. The test consists of a null hypothesis and an alternate hypothesi s which are validated using the t -statistic, t 0 

### . The null hypothesis states that both the population means ar e equal and the alternate hypothesis refutes this by stating the populati on means are not equal. In this case, the population means are the leaker emission f actors from Clearstone I (study 1) and Clearstone II (study 2), (subscripts in form ulas). Stated mathematically, the hypotheses

are:

Null Hypothesis, H o 

### : μ 1 

### = μ 2 

### , i.e., the two means (LEFs) are combinable Alternative Hypothesis, H 1 

### : μ 1 

### ≠ μ 2



### 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 2 1



### n S

n S M M t

o



### + −

= ,



### where
M 1 

### and M 2 

### are the means from studies 1 and 2,
S 1 

### and S 2 

### are the standard deviations from studies 1 and 2,

n 1 

### and n 2 

### are the component counts from studies 1 and 2.

The population sizes (i.e. compone nt counts) and the standard deviations of the separate studies were not equal. Therefor e, a corrected degree of freedom, Dof , that accommodates such differences is calculated using



# ( ) ( )

1 1 2 2

2 2
2

1 2

1 2
1 2

2 2
2

1 2
1



### − +

− ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛

+

=

n n S

n n S n S

n S

Dof

,



### where
S 1 

### and S 2 

### are the standard deviations from studies 1 and 2,

n 1 

### and n 2 

### 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 t 0 

### . 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 st udies can be combined. However, if t 0 

### was above the standard t -value, then the null hypothesis is re jected 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 di fferent 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

M 1 , Emission

Factors S 1 , Standard Deviation

Test statistic, t 0 Standard t -value at 95% confidence

If t 0 > t , then do FAIL Degree of Freedom, Dof

If t 0 < t , then PASS n 1 , Component Count M 2 , Emission

Factors S 2 , Standard Deviation n 2 , Component Count

### than components placed before the de-met hanizer. 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 stress es 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 FAILOpen-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 be fore 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 LEF s 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 LEF s 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 separate d into centrifugal compressor, reciprocating compressor and non-compressor related. In a ddition, the centrifugal and reciprocating compressor components were separated into be fore 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 Clearst one I data and Clearstone II data provides a statistical base for combining the emission fact ors derived from either study. Clearstone I was grouped in exactly the same way as th e Clearstone II data to facilitate the comparison between the two studies. The result s of the comparison are shown in Table 6 and 7.Table 6 – Comparison of Clearstone I a nd 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 Clears tone 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 - Reciprocatin g 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