Document ID: EPA-HQ-OAR-2021-0299-0040
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2023-02-10T05:00Z

The following information is provided as follow-up to FHR and Molex conference call with EPA on 3/22/22. 

Detection Floor
 Current AMEL Draft verbiage - Section IV. A (a)(2): 
            The sensor must be capable of maintaining a detection floor of less than 10 ppbe on a rolling 10-minute average, when adjusted for the system response to the most recent successful bump test conducted in accordance with IV.A(e)(2). The detection floor is determined at three times the standard deviation of the previous 10 minutes of data excluding excursions related to emissions peaks. 
            

In comments submitted by Molex and FHR during Nov 2021,  we requested to remove the bump test factor from the equation shown above. The need to remove the bump test factor was described in detail as part of those comments. It is important to note that the sensor network which was installed in these process units was shown to be equivalent or better than the emission control provided in the applicable rule. However, that system would not meet the detection floor requirements that were proposed by EPA in the AMEL draft.  FHR would not be able to implement the AMEL as written.

In addition to returning to an equation without bump test adjustments, after thoroughly reviewing the available data, Molex/FHR also requests that 2 minutes of data (instead of 10 minutes as originally proposed) be used for the detection floor calculation. The detection floor calculation utilizes baseline modeling, i.e.  correction for baseline drift associated with environmental effects such as temperature and humidity changes. Because the baseline drift is generally more pronounced over a longer time period, the calculation is considered to be more accurate and representative by using data from a shorter duration. Therefore, Molex/FHR requests the following changes to the specification:

            The sensor must be capable of maintaining a detection floor of less than 10 ppbe on a rolling 102-minute average, when adjusted for the system response to the most recent successful bump test conducted in accordance with IV.A(e)(2). The detection floor is determined at three times the standard deviation of the previous 102 minutes of data excluding excursions related to emissions peaks. 
            
            

 Current AMEL Draft verbiage - Section IV. A (e)(3): 
            The health of each sensor must be confirmed for power and data transmission at least once every 15 minutes. Data transmission, which includes data recorded by the sensor every second as noted in IV.A(a)(3), must occur at least once every 15 minutes. The rolling 10-minute average detection floor data collected in accordance with IV.A(a)(2) must be updated with each new minute of data every 15 minutes.
            
Because a sensor can be continuously exposed to gas for several hours during the automated  PSL generation process and until the leak is corrected during the DRF work practice , Molex/FHR  proposes the detection floor be validated once every 72 hours which is in sync with the minimum time required for the generation of the PSL and detection notification. Thus, at least one of the calculated detection floor values within the past 72 hours should meet the 10ppbe specification, starting at 00:00 UTC of the 1[st] day and ending at 23:59 UTC of the 3[rd] day. For a sensor that is continuously experiencing positive detections during the 72 hour period, FHR will utilize the sensor data to demonstrate that the detection floor calculation was not feasible. Due to the time required to obtain valid data, the sensor detection floor is not included in the continuous sensor health monitoring, which is to occur once every 15 minutes.  

Also, Molex's comments from Nov 2021 included "If a sensor reset or repair is necessary, FHR must test the responsivity and wireless communication of the sensor through a bump test ...". To better clarify this comment, Molex asks to remove "reset or" from the verbiage since a sensor reset is often caused by brief power outage or automatic WiFi reconnection. These types of resets do not negatively impact the gas sensor's performance and dispatching a technician to perform a bump test in the field is unnecessary for these types of resets.  
 
Molex/FHR propose the following clarification. 

            IV.A.(e)(3): The health of each sensor must be confirmed for power and data transmission at least once every 15 minutes. Data transmission, which includes data recorded by the sensor every second as noted in IV.A(a)(3), must occur at least once every 15 minutes. Appropriate corrective actions must be taken for any sensors that fail to collect data in accordance with IV.A(a)(2) and (3) and transmit data in accordance with this paragraph to ensure any errors or malfunctions are corrected in a timely manner. Such periods are considered downtime until corrected. If a sensor reset or repair is necessary, FHR must test the responsivity and wireless communication of the sensor through a bump test according to the procedure specified in IV.A(e)(2). FHR must maintain records of sensor health in accordance with IV.C(f). 
            The sensor detection floor shall be reviewed at 00:00 UTC each day to confirm each sensor detection floor remains below established threshold of 10 ppbe on a rolling 2-minute average during at least one 2-minute period in the past 72 hour period. If a sensor does not pass the detection floor review, then a sensor fault notification shall be issued, and the sensor issue shall be corrected through recalibration, repair, replacement, or another appropriate measure, unless FHR can demonstrate the sensor was continuously experiencing positive detections during this time.
 Current AMEL DRAFT - Recordkeeping Section IV. C (g): (g) Raw sensor readings. Additionally, for each sensor, the percent of time positive detections were registered during the 72-hour lookback must be recorded each day and the minimum, average, and maximum detection floor.
            Molex/FHR requests a change to the above specification to remove the requirement for recording the maximum and average detection floor values due to the difficulty in calculating these values in the presence of gas detections. The detection floor is more accurately represented by the minimum detection floor value, i.e. the sensor output without gas.    
                  (g) Raw sensor readings. Additionally, for each sensor, the percent of time positive detections were registered during the 72-hour lookback must be recorded each day and the minimum, average, and maximum together with the minimum detection floor.
                  
Bump Test
 Current AMEL Draft verbiage - Section IV. A (e)(2)(ii): 
            (ii) The bump test is successful if the response of the sensor exceeds 50 percent of the nominal value of the standard and the adjusted detection floor does not exceed 10 ppbe. The bump test may be repeated up to two additional times if the first bump test is unsuccessful.

Molex/FHR requests to delete the phrase, "and the adjusted detection floor does not exceed 10ppbe." A bump test is a brief gas exposure test conducted quarterly to document responsiveness. The detection floor check is required to be done in a relatively clean environment when active leaks are not occurring in close proximity to the sensor. The detection floor will be checked at least every 72 hours and does not need to be included as part of a quarterly bump test.

The sensors were designed, installed and tested in a manner to ensure that responsiveness of each individual sensor could be confirmed easily through a brief in-the-field bump test. During the pilot studies, 50% of the nominal value of the gas standard was used as the pass/fail threshold. Sensors that were unable to perform at or above the threshold were replaced.  That system and the associated maintenance practices were shown to be equivalent or better than the emission control provided in the applicable rule. Using a more stringent but untested threshold would likely increase cost for LDSN adopters, increase calculated down time, but have limited/no additional emission reductions. 

Molex/FHR requests the following changes to the specification.
            (ii) The bump test is successful if the response of the sensor exceeds 50 percent of the nominal value of the standard and the adjusted detection floor does not exceed 10 ppbe. The bump test may be repeated up to two additional times if the first bump test is unsuccessful.

 Current AMEL Draft verbiage - Section IV. A (e)(2)(iv): 
            (iv) FHR must maintain records of the bump test in accordance with IV.C(f) and records of the detection floor must be maintained in accordance with IV.C(g).
      Based on the information included above, Molex/FHR requests the following change.
            (iv) FHR must maintain records of the bump test in accordance with IV.C(f) and records of the detection floor must be maintained in accordance with IV.C(g).

Non-LDAR Emission Sources
Although the LDSN has clearly shown the benefits of its capability to detect emissions from non-LDAR components, those benefits were not included in any equivalency representations. However, earlier detections will logically lead to earlier repairs. Non-LDAR leaks and emission events that are discovered due to PSL notifications will be handled just as they would if the detection had originated from AVO, process alarms, or process parameter changes. EPA has been clear that the scope of the alternative method addresses traditional calendar based M-21 monitoring and does not replace AVO, CVS, NDE, drain, or other inspection requirements. FHR seeks no change to current requirements for emission event evaluation, notification, record-keeping, or reporting. To augment information previously provided to EPA, FHR reviewed the data since March in the Mid-crude and Meta-xylene units. Since that time, there were 9 non-LDAR emission events recorded. Of those, 4 were corrected within 1 day, 1 in 2 days, 1 in 4 days, and 1 in 11 days. One was later identified as a permitted emission source and there is one event that remains ongoing and will require a process unit shutdown to repair. Note that each of these (with the exception of the permitted source) were recorded as emission events under the state rules (30 TAC 101) and will be included in Title V deviation reporting. FHR is already highly incentivized to make repairs as soon as practicable, minimize emissions, and return to normal conditions when emission events occur. Adding a 30-day repair requirement for emission events alerted via LDSN with no allowance in the AMEL for managing within the current regulatory framework, or, at minimum,  providing LDAR-type work practices, including delay of repair for unit shutdown, for these identified emissions events remains out of scope for this AMEL request that is specific to an alternative for M-21 monitoring of LDAR emission sources.