Detection limits of optical gas imagers as a function of temperature differential and distance

Abstract

Optical gas imaging (OGI) is an effective tool for detecting gas leaks from process equipment. Despite the fact that OGI has been used for leak detection for over a decade, its detection limit is an elusive performance metric and has not been systematically characterized and quantified like other detection instruments. A substantial body of research has been performed that has shed some light on the OGI detection limits and the factors that dictate the detection limits. The OGI detection limit expressed as ppm-m and ppm now can be quantified as a function of ΔT (differential temperature between the gas plume and the background), the OGI camera type, and the specific gas in question. Furthermore, the OGI detection limit expressed as grams per hour can be calculated based on the ΔT and the distance from the OGI camera to the leak location under common leak survey conditions. For the same OGI camera, the detection limit can vary by several orders of magnitude due to ΔT and distance. The present work has demonstrated how different OGI detection limits can be. More importantly, this work has, for the first time, formulated equations that can be used to determine OGI detection limits with a given set of leak detection conditions. Being able to quantify OGI detection limit and understand the variables that dictate the detection limit is a significant advancement. It will help OGI to become accepted as a mature field instrument. The variables characterized in this work should have an impact on the development of OGI leak survey protocols, such as Appendix K to Code of Federal Regulations 40 CFR Part 60 in the United States. Established detection limits will also help emission inventory for fugitive emissions when OGI is used as the sole leak detection method.Implications: Optical gas imaging (OGI) has been used for leak detection and control of fugitive volatile organic compound (VOC) emissions and methane emissions due to equipment leaks. However, detection limits of OGI have not been characterized and quantified like other detection instruments. The lack of well-understood detection limits has hindered broader applications of OGI. The work presented in this paper represents important steps that will enable OGI users and policymakers to establish (1) OGI detection limits under various conditions, (2) OGI leak survey criteria for a desired minimum detectable leak size, and (3) maximum potential emissions from the nondetect sources in emission inventory studies.

Detection Limits of Optical Gas Imaging for Natural Gas Leak Detection in Realistic Controlled Conditions

“…This study focuses on the dominant type of OGI system used in O&G LDAR applications: A video camera using a cooled detector filtered to the 3.2 to 3.4 μm spectra band where normally invisible HC emissions absorb light. − In practice, both the HC emission plume and the background scene (e.g., equipment, sky, or ground) absorb and emit infrared radiation in the spectral window of the camera. If enough image contrast exists between the HC plume and the background scene, the emission can be detected by the OGI surveyor . While other OGI cameras are available, all surveyors in this study used the FLIR GF320 (GF320) camera (FLIR Inc., North Billerica, MA).…”

Understanding oil and gas pneumatic controllers in the Denver–Julesburg basin using optical gas imaging

“…For these reasons, the detection threshold of the OGI is not a specific number but forms a continuum (a performance band). Current research suggests that closedistance OGI inspection by an experienced operator with the equipment and procedures used here should detect continuous ONG hydrocarbon emissions with relatively high confidence at emission rates >30 g/hr (about half of the 60 g/hr EPA OGI base sensitivity requirement) under typical conditions on a working well pad (Ravikumar et al, 2018;Zeng and Morris 2019). We define this as the detection threshold band upper limit for this study and equates to 1.3 scf/hr for typical ONG gas compositions related to PC operation (assumed molecular weight of 20 g/mol).…”

“…It is known that many factors including leak size, observation distance, difference between emitted gas and background temperature (ΔT), gas dispersion effects (e.g., wind speed) and operator skill/diligence, affect each OGI camera observation (Concawe 2017;Ravikumar et al, 2018;U.S. EPA 2015;Zeng and Morris 2019;Zeng et al 2017). Together these method factors determine the ability of a camera operator to successfully detect a specific leak/ emission of a given ER in actual operating conditions.…”

“…As discussed subsequently, the OGI detection threshold lower bound can be well below 1.0 scf/hr under favorable conditions. Under certain conditions, such as high wind or low (ΔT), the emission detection capability of OGI can seriously degrade and approach zero (Zeng and Morris 2019), invalidating the stated detection threshold upper bound assumption. For inspection procedures used and conditions encountered in this study, these OGI method limitations were not believed to play a major factor.…”

Implications of Sampling Methods on Geospatial Mapping of Methane Sources

“…This is due to the temperature difference of the surface and sky and the sky providing constant radiance different from methane. The maximum allowable distance using a OGI is a function of the differences in temperature [45].…”