How to Quantify Leaks with Optical Gas Imaging (OGI)

March 30, 2021

By: Terence Trefiak

The role of OGI in leak detection and repair (LDAR) is growing, but before you add this technology to your LDAR program, you have to address the elephant in the room – quantification.

Myth #1: You can’t quantify a leak with OGI.

This is a common LDAR industry belief – the Method 21 Correlation Equations are the best way to measure a leak. However, this method is not a quantitative emission rate. It’s an estimate based on a point source concentration measurement. What the OGI method can tell us that the Method 21 Correlation Equations cannot, is the magnitude of the leak. OGI does a much better job of identifying larger, more problematic leaks.

Methods to Quantify Leaks with OGI

Method 21 PPM Correlation Equations
This method involves taking ppm readings and using a correlation equation to get a flow rate. This is the most widely used method in the US.
Efficiency
Because the equipment has to be passed over each individual component, this technique can be time consuming.
Cost
$••
Accuracy
It’s difficult to take a single point concentration reading across a large leak interface. Most Method 21 devices also have a maximum (pegged) range of 100,000 ppm, so if the concentration is more than that, you cannot read it accurately.

 

Bagging
This is a simple method where you capture the leak in a bag with a known volume as you time how long it takes to fill the bag.
Efficiency
Bagging tests can take a lot of time to set up and more time to run – especially for small leaks.
Cost
$$
Accuracy
It can be difficult to capture all the leaking gas depending on the type and location of the leaking area.

 

Optical Gas Imaging (OGI) Qualitative Estimation
While this is not a widely accepted method, we have found that with a properly trained camera operator this method can provide an accurate estimated rate based on the video image of the leak or vent.
Efficiency
The benefit of OGI estimation is that it provides the facility with a level of severity regarding the leak – very small, small, medium, large, or very large. This data allows them to prioritize which leaks should be repaired first.
Cost
$••
Accuracy
These estimates can be quite accurate when compared to actual quantification and can be done at no additional cost.
Using OGI estimates is far more accurate than the ppm correlation – especially with medium to large leaks.

 

OGI Quantitative Algorithms
OGI Suppliers Opgal and Flir have their own software to quantify emissions based on the pixels in video images. Their algorithms can determine the density and rate of a gas plume and provide a leak rate. These methods have regulatory approval in Canada but are not widely used in the US.
Efficiency
It can take time to properly set up the camera and position it fairly close to the emission source. The source needs to be clearly visible with a good background, which is not always possible.
Cost
$$
Accuracy
In our experience, this method can provide an accurate rate, but results can vary based on the leak scene – distance, background, etc.

 

Hi-Flow Sampler*
This method relies on an internal vacuum pump that sucks in the leaking gas and background air. The device then measures the total flow rate of the stream. Then, a sensor measures the percent of hydrocarbon gas in the stream.
*The manufacturer of this device (Bacharach) has discontinued this product, but there are more in development.
Efficiency
The method is efficient, but the sensors are prone to drift/damage and need to be calibrated often with an experienced technician that can identify malfunctions.
Cost
$$
Accuracy
This device is the most accurate technology available to measure a hydrocarbon gas leak or vent.

 

Myth #2: Some leaks are smaller, some are bigger, but they are all roughly the same size.

If you’re only using the Method 21 Correlation Equations, then this is basically true. Because of the limitations of using the correlation equations, there is not much difference between a small and large leak. But in reality, there can be a massive difference. It is common knowledge in LDAR that a minority (10-20%) of larger leaks make up the majority of the leak volumes (80-95%), so it’s important these large leaks be found and fixed as soon as possible.

The table below shows the calculated leak rates based on the Method 21 correlation method, but then compares the actual leak rates measured with a Hi-Flow sampler. From the data it’s clear – the Method 21 correlation method is severely underestimating leak rates, especially for leaks with high ppm readings.

PPM
Reading
EPA Emission Factor (EF)
Calc (Table 2-10) lbs/hour
Emission Factor ft3/min
Actual ft3/min
% Error
Leak Weight
Severity
500
0.0003
0.0001
0.0001
18%
1
Minute
10,000
0.0029
0.0011
0.0010
9%
10
Very Small
50,000
0.0096
0.0036
0.01
-180%
100
Small
100,000
0.0160
0.0060
0.25
-4098%
2,500
Medium
500,000
0.0521
0.0194
1
-5045%
10,000
Large
1,000,000
0.0867
0.0323
2 to 200+
-5,653% to -575,187%+
20,000 to 200,000+
Very Large

How Montrose Can Help

Since 2007 Montrose Environmental Group has specialized in accurate leak and vent rate measurement. We use a combination of OGI detection and quantification with actual flow rate measurement using Hi-Flow Samplers. We have the most OGI cameras in North America, and we pair them with the best trained technicians in the LDAR industry.

Terence Trefiak
VP, LDAR Canada
Terence brings over 19 years of industry experience to the Montrose team where he currently serves as Vice President of LDAR, Canada. He is an expert in fugitive emission management and specializes in Optical Gas Imaging for GHG and LDAR compliance.

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