Reducing emission of toxic and explosive gases is an ongoing challenge for industries that process, store or transport high volumes of potentially hazardous substances. Concerns for plant safety, environmental compliance and prevention of production loss drive the need for timely, reliable monitoring. However, installing and maintaining wired detectors has limited coverage and limited return on leak monitoring investments. However, wireless approaches prove to be cost-effective in extending coverage.
Only about 20 percent of assets produce most gas leakage, but finding that 20 percent requires checking 100 percent of the assets. Since few companies can afford to deploy fixed sensors on all potentially vulnerable assets, most supplement by sending operators or third-party contractors to visit potential leak sites to check for leaks. An oil producer that operates five wellheads, for example, might send someone to each wellhead but find leakage at only one site. The time traveling to the other four wellheads is wasted, except for the assurance that no leaks are detectable, at least at the time of the visit.
A wireless gas detection system can improve monitoring capability at 10 percent of the cost of a wired system. With drop-in-network mobility, wireless detectors can be positioned anywhere close to leak-prone assets without wiring. Leaks can be detected cost-effectively before manual inspection, streamlining the number of assets and locations an oil producer has to inspect, for example. At the same time, leaks can be detected quickly because of the detector’s proximity to the asset. Prompt detection enhances safety by preventing the occurrence of gas accumulations and explosions that might otherwise endanger lives. Recent tests conducted at Central Valley Gas Storage LLC (CVGS), a wholly owned subsidiary of Southern Company gas that operates underground natural gas storage facilities in California, indicate that wireless gas detectors can detect and alert operators to the presence of combustible gases — in this case, methane.
Putting wireless to the test
Once a natural gas field, CVGS’s facility was converted into a storage reservoir for gas reinjection. The conversion process involved drilling many new wells to enable rapid withdrawal of gas from the reservoir storage. With methane being a primary constituent of natural gas, these wells are a potential source of hazardous leakage.
“We needed a way to detect small methane leaks around the wellhead so we could fix small leaks before they got bigger,” said Patrice Mbeukeu, instrumentation and control specialist for CVGS. “I had been skeptical of gas detectors because of their limitations in coverage, but when I heard of the benefits wireless solutions would bring, I developed an idea of deploying multiple units around a wellhead to create a robust gas detection envelope for early detection.”
Natural gas is usually between 95 and 97 percent pure methane, but establishing the presence of a leak requires detecting a small percentage of the lower explosive limit (LEL). The system CVGS tested was a gas detector with WirelessHART. The test involved simulating a small leak by opening a quarter-inch valve for 10 minutes, releasing four standard cubic feet of methane into the air, and repeating this process every day for 40 days (see Figure 2). The amount of gas released into the air during the test is negligible but nonetheless, tests were conducted in communication with the California Division of Oil, Gas and Geothermal Resources.
The team used optical gas imaging cameras to optimize sensor placement by tracking the path of the passing methane plume on the camera. Four detectors were mounted on the well chasses at heights of about 8 feet northeast, northwest, southeast and southwest of a valve. Deploying multiple units ensured that even if the wind blew in different directions, at least one sensor would detect the fugitive emissions when the valve was opened. Testing temperatures were cool, with wind speeds between five and 10 mph.
Figure 3 shows the communications architecture. The sensors exchange information with the plant control system via a WirelessHART gateway. Operators manage the system via the plant supervisory control and data acquisition software residing on a workstation accessed via a human machine interface.
CVGS already had a WirelessHART communications network in place, so deploying the gas detectors was simply a matter of mounting them and configuring them for the wireless network.
Figure 4 shows the results from the first day of testing. The Y axis represents the parts per million (ppm) of methane. The red vertical line marks the moment of the first controlled gas release, which had a density of 5,000 ppm. The X axis is the time elapsed in minutes from the discharge. Prior to the discharge, the sensors detected no methane, but within the first minute after opening the valve, the sensor in the southwest corner picked up a plume of methane measuring 3,000 ppm (approximately 6 percent LEL). In minute six, the southeast unit picked up a 3,500-ppm plume of methane, followed by a report of a 500-ppm concentration in minute seven. That only two of the sensors detected methane was likely caused by a specific direction the wind was blowing.
Similar results were achieved during subsequent days of testing. Although considerable variability was caused by changing wind speeds and direction, the sensors picked up gas concentrations every day. For example, in day four, sensors detected concentrations in seven out of the 10 minutes the valve was open. Further testing conducted at various intervals over the next three weeks was consistent with these findings. No gas release went undetected, confirming the conclusion that wireless point gas detectors can pick up leaks as small as 500 ppm or 1 percent LEL within minutes and communicate the data reliably across a wireless gateway.
Once leaks are detected, the onus is on the facility owner to deploy teams to locate the exact leak location. Without the wireless sensors, repairing any leak outside the detectors’ limited range would likely require two visits — one to identify the leak and one to repair it.
Safety with in-house detection
Wireless gas detectors provide a low-maintenance, in-house solution, which reduces the cost of using third parties for leak detection. Safety can be implemented on a broader, more affordable scale. At least three factors contribute to the low maintenance requirements.
One is the extended battery life now possible with lithium technology, which can be five years or more. A second contributor is the increasing lifespan of the sensing units, which can be up to three years. The third is the ease of deployment and calibration, which reduces commissioning time from days to minutes. These add up to better protection for employees and neighboring communities, greater control over production loss because of detected leakage and improved regulatory compliance — all in ways that would not likely be cost-feasible with wired gas detectors alone.
Andrew Liptak is a research engineer at United Electric Controls. He is responsible for new product and business opportunity research.
Julian Yeo is a product manager for United Electric Controls. He is responsible for the gas detection as well as electromechanical switch product lines.
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