Understanding Ultrasonic Gas Detection

By Edward Naranjo
Product Manager, General Monitors

General Monitors is pleased to welcome the addition of Gassonic A/S (Ballerup, Denmark), a worldwide leader in the manufacture of fixed ultrasonic gas leak detectors, to the General Monitors family of companies. Gassonic was the first company to develop an ultrasonic gas leak detector and has over ten years experience in both offshore and onshore installations. Through the acquisition of Gassonic, we are now able to offer our customers an even broader array of integrated gas detection technologies to meet specific process safety monitoring requirements. The application of ultrasonic sensing complements our existing catalytic bead, infrared point, and open path gas monitoring solutions that help protect people, equipment, and plants worldwide.

Acoustic monitoring techniques use ultrasonic sensors to detect leaks based on changes in the background noise pattern. These sensors respond to the sound generated by escaping gas at ultrasonic frequencies. The ultrasonic sound level is directly proportional to the mass flow rate (leak rate) at a given distance. The leak rate, in turn, is mainly dependent on the size of the leak and the gas pressure.

What makes airborne ultrasound effective at detecting escaping gas? Most gas leaks — as well as operating mechanical equipment and electrical emissions — produce a broad range of sound that span from the audible to the ultrasonic range (approx. 20 Hz-10 MHz). The ultrasonic range itself extends from 25 kHz to 10 MHz.

Unlike other detection technologies, ultrasonic gas leak detection does not measure gas concentration (ex. %LEL or ppm) or a concentration over a sampling distance (ex. LEL-m or ppm-m). Rather, it defines gas leaks in terms of the sound pressure level (SPL). In general, the greater the leak rate, the larger the sound pressure level emitted by the escaping gas.

Leak rates can be divided into three categories according to dispersion models employed in the oil and gas industry:

Minor leak < 0.1 kg/s
Significant leak 0.1 – 2.0 kg/s
Major leak > 2.0 kg/s

These categories have been defined based on the speed of a gas cloud to accumulate into an explosive gas concentration. According to the above categorization, the performance standard of ultrasonic gas leak detectors for typical applications is based on gas leaks of 0.1 kg/s. For reference, a methane leak of this flow rate can be generated with gas pressurized to 650 psi (45 bar) and expelled through a hole measuring 4 mm in diameter. At 0.1 kg/s in average background noise conditions, the ultrasonic gas leak detector detects pressurized gas leaks in a radius of 8 – 12 m.

For special applications, the performance standard of the ultrasonic gas leak detectors may be changed in order to detect even smaller leaks. This is possible without increasing the risk of false alarms. For example, an ultrasonic gas leak detector can respond to a leak rate of 0.03 kg/s if the detection coverage is decreased to 4 – 8 m.

Ultrasonic noise, of course, can be generated by sources other than streams of jetting gas. Compressors, turbines, large fans, electric motors, for example, produce high levels of sound, which include frequencies in the ultrasonic range that can be detected by ultrasonic gas leak devices. In order to enhance immunity to these potential false signals, ultrasonic gas leak detectors are equipped with high pass electronic filters that screen frequencies from many man-made and natural sources. An alarm trigger level for the ultrasonic gas leak detector is set at least 6 dB above the ultrasonic background noise. Such margin reduces the likelihood that fluctuations in background noise cause the detector to alarm. Lastly, detectors are supplied with a built-in delay function that can be adjusted by users to screen controlled releases of pressurized air at a facility. Most air releases last a few seconds, whereas the ultrasound emitted from a gas leak lasts much longer.

A practical example of the difference between audible and ultrasonic sound levels emitted from machinery can be found in a survey performed on a platform in the North Sea. A test installation was set up next to a turbo expander unit and sound measurements were taken. The audible (20Hz – 25 kHz) sound level was 100 dB, but the ultrasonic level (> 25 kHz) was less than 78 dB. This resulted in setting the trigger level of the ultrasonic gas leak detector at 84 dB with a delay time of 15 seconds. These settings proved to be sufficient to mask out the ultrasonic sound levels and prevent spurious alarms.

The advantages of the system include instant detection of pressurized gas leaks and imperviousness to changes in wind direction or gas dilution. Ultrasonic detection applies to all types of gas, whether combustible, toxic, or inert, and it is thus, quite versatile on many applications.

Another advantage of ultrasonic gas leak detectors is that their performance can be verified with live gas leaks during commissioning. Using an inert gas, operators can carry out simulations of gas releases of a known leak rate and test the response of the detectors in potential locations.

Despite these advantages, the technology is unable to detect low pressure leaks (< 10 bar or 145 psi) that do not produce acoustic emissions at levels substantially higher than the background noise. Attempts to detect small leaks can be accomplished but it requires that the detector is placed closer to potential leak sources. Otherwise it may result in false alarms.

It should also be noted that the correct positioning of the ultrasonic gas leak detectors in the gas facility is important to ensure optimal performance. As a result, qualified personnel should be consulted during the implementation of these detectors. Gassonic or General Monitors have trained and experienced engineers, who can assist facility managers in the survey, installation, and implementation phases of these detectors.

SOURCE: General Monitors