Innovative electric-powered units offer high-pressure, high flow rates along with advanced capabilities beyond traditional pneumatic and hydraulic units
In the underground mining industry, gas boosters are ubiquitous products working hard and with little fanfare behind the scenes to pressurise gases to several thousand bar (3,000 to 10,000 psi) for recharging rebreather cylinders to protect miners, fire suppression, environmental cooling or gas scavenging.
Unfortunately, traditional pneumatic and hydraulic gas boosters have some inherent limitations. Pneumatic (air-powered) units work well to boost pressures at intermittent, low-flow rates, but are extremely noisy during operation. At higher flow rates the sound is further increased, since multiple units must operate in parallel. This also increases the amount of electricity required since compressing air is inefficient. Hydraulic-powered units, on the other hand, are more suited to continuous operation and are slightly quieter than pneumatic options, but come with the potential risk of hydraulic oil leaks and spills.
Now, a new category of advanced electric gas boosters is promising to provide quieter, cleaner, high-pressure, high-flow rates up to 6,500 psi – along with improved monitoring and controls for a variety of mining applications.
Gas booster applications in mining
Whether used as standalone devices or in larger OEM equipment, gas boosters are integral components in many mining applications. In underground mining, gases such as methane (CH4), nitrogen dioxide (NO2), and hydrogen sulphide (H2S) can be present; these gases can be highly concentrated, toxic and undetectable. Inhaling such gases can cause burns, poisoning or even death. For this reason, miners and mine rescue teams depend on safety equipment such as breathing apparatus that can utilise both oxygen and breathing air. These rebreathers absorb CO2 from exhaled breath to permit the recycling of the substantially unused oxygen content of each breath. Oxygen is added to replenish the amount metabolised by the user.
Gas boosters provide a safe way to pressurise flammable gases when charging or refilling oxygen and breathing air cylinders. However, rebreather gas manufacturers supply their gases in high pressure gas bottles that are usually larger than portable breathing apparatus. When full, gas is cascaded from the high pressure in the supply bottles to a lower pressure in the breathing gas cylinders. At some point, the pressure in the supply bottle become too low to cascade into the breathing cylinder. Gas boosters allow this lower pressure supply gas to be almost completely drawn from the supply bottle. Referred to as “scavenging,” this ensures the maximum value for the operator by using all the supplied and expensive gas.
There are further applications in mining as well. According to the SME Mining Engineering Handbook by the Society of Mining, Metallurgy, and Explosion Electronic Edition, “[gas] booster applications include minerals exploration drilling using reverse circulation techniques and coal-bed methane, where drilling very deep holes, up to several miles deep, can be demanded.”
Other applications for boosters at mines include nitrogen injection for mine fire suppression, eliminating the risk for explosion in underground mines. Liquid nitrogen injection, an engineering control developed to reduce heat stress and spontaneous combustion hazards in mines, is used for environmental cooling and combustion prevention in an underground mining site.
Quieter, cleaner, more efficient gas boosters
Despite the many applications for gas boosters in underground mining, there can be significant drawbacks to pneumatic and hydraulic powered units, starting with the sound levels produced during operation. In some cases, many multiples of the units are used in parallel, particularly when higher flow rates are required. The combined noise generated can be excessive.
“Pneumatic-driven gas boosters are extremely loud during operation, and even louder if multiple units work in parallel, which can make complying with OSHA regulations related to sound levels in the plant more difficult,” says George Volk at Haskel, a division of Ingersoll Rand that manufactures gas/liquid transfer and pressurisation technology.
According to Volk, the operation of typical air-operated and hydraulic-driven gas boosters can exceed the 85dBA threshold, which can cause hearing loss. In fact, OSHA requires employers to implement a hearing conservation programme when noise exposure is at or above 85 decibels averaged over eight working hours, or an eight-hour time-weighted average.
Hearing conservation programmes strive to prevent initial occupational hearing loss, preserve and protect remaining hearing, and equip workers with the knowledge and hearing protection devices necessary to safeguard themselves. Under OSHA’s Noise Standard, the employer must reduce noise exposure through engineering controls, administrative controls, or hearing protection devices (HPDs) to attenuate the occupational noise received by the employee’s ears to within levels specified.
In contrast, advanced industrial electric gas boosters such as the Q-Drive from Haskel, are much quieter (<77dBA) during operation, while still offering up to 6,500 psi for high-pressure applications.
“This eliminates the need for regulatory scrutiny, along with hearing conservation programmes,” says Volk. “The use of the electric units can also streamline production, since workers can spend more time in the vicinity without worrying about exceeding the regulations or potential hearing loss.”
In addition to the noise produced (even though it is less than air-powered units), there can be some concern that hydraulic gas boosters might leak or spill hydraulic oil. This can be a deterrent for applications that mandate a certain level of cleanliness, including cleanrooms.
“Whenever you have hydraulics, there is the potential for leaks or spills,” explains Volk. “That is essentially the reason the automotive industry moved away from hydraulics in their production line – because of the potential contamination issues.”
Energy efficiency of the advanced electric gas booster
Electric energy consumption is also a concern. Despite being electric-powered, the more advanced units are more energy efficient than both pneumatic and hydraulically driven boosters.
“Compared to pneumatic gas boosters, advanced electric units use one-third of the energy and offer flow rates 10 to 20 times higher,” says Volk. “Compared to hydraulic boosters, the electric units also provide energy savings due to lower cooling requirements.”
Although there are several electric-driven gas boosters on the market, even within the category there can be significant design differences. Some of the early market entrants are designs that employ a gearbox to convert the rotary motion of the motor to reciprocating, which increases complexity and the amount of maintenance required.
More advanced units are built using a simplified linear actuator drive which enhances reliability and reduced the mean time between failure (MTBF).
Both pneumatic and hydraulic gas boosters can also be difficult to control with much specificity, which makes their operation less efficient. Today’s more advanced unit include sophisticated remote and self-diagnostic capabilities. Units such as the Q-Drive come with human machine interface (HMI) and touchpad control to allow operators to monitor and control pressure and temperature closely and easily change setpoints.
Given the inherent drawbacks of pneumatic and hydraulic gas boosters, Volk believes quieter, cleaner, easier operation of electric-powered units will have considerable appeal for mining applications. “With the considerable R&D investment in these more advanced electric gas boosters, many of the shortcomings of pneumatic and hydraulic units have been resolved and this opens up new possibilities for mining applications at high flow rates and pressures,” he concludes.