Best Practices for Enclosed Cabin Pressurisation

 

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 “The first priority and concern of all in the coal and other mining industry must be the health and safety of its most precious resource - the miner” US Federal Coal Mine Health and Safety Act 1969 - amended 1977


Breathesafe aims to protect operators’ health when working inside a cabin or enclosure with the next key points being best practice, recommendations and topics to consider when selecting a protective system. As renewed health focus of mining operations is welcomed with imperatives to protect the health of operators whom are working and exposed to respirable mine dust in above and below ground situations.


Respirable mine dust definition: airborne particulate which is <10 microns in diameter (PM10). These invisible particles can remain suspended in air for days or weeks with higher exposure levels when machinery is near areas of blasting, shovelling, screening, long wall mining and handling minerals.


These man-made particles can penetrate and deposit in air exchange part of the lung; the contributing cause is that our bodies lack of defence mechanisms to expel these particles that will in effect remain in our lungs continuously damaging tissue and potentially entering our bloodstream (Particulate that is >10 microns is normally filtered out in nasal passages, mucus membranes and conductive airways).


In the US, the Occupational Safety and Health Administration (OSHA) has directed a new rule to lower Silica exposure level for US construction workers. The new rule establishes an exposure limit of 50 micrograms per cubic meter of air (50 ug/m³) as an 8-hour time-weighted average. Silica is one the most common materials on the Earth’s surface but it is found as fugitive dust at mineral processing situations when mining materials have been crushed, milled, blasted and transported. This leads to the production or airborne respirable mine dust with particularly Silica dust that is around twenty times more toxic that coal dust.


The American National Institute of Occupational Safety and Health (NIOSH) has conducted several research projects with new and old equipment. The conclusions of these studies clearly show that high efficiency filters must be installed with fresh air pressurisation in adequately sealed cabins with high levels of confidence on the results; the evidence shows significant reduction of respirable mine dust contamination inside these cabins.


The following NIOSH table compares three markers of the effectiveness of cabin protection factors:

Filter efficiency chart

 

NIOSH protection factor is a calculation of the following principles:

Protection Factor (PF) = Co/Ci                     Efficiency (E) = (Co-Ci )/Ci
Co = outside respirable dust & Ci = inside respirable dust [Concentration]

These following are the main elements for protective systems in fixed and mobile plant equipment that is operating in hot, dust laden and harsh mine site environments:

Recommendation 1.                                                                                                                                                                                                                                                       Cabin Sealing is required to limit external pollution from entering the cabin from wind penetration; door and window seals must be regularly inspected as part of a dust management plan and must be replaced along with fixing gaps and leaks inside the cabin. The higher the level of cabin sealing; will in fact make the system more efficient.


An earlier study by Cecala et al. [2002a,b] demonstrates the importance of Cabin Sealing and Pressurisation with a case study regarding respirable dust inside a drilling cab application:

  • Vehicle Cabin with new Air Conditioning System installed did not improve any dust controls inside Cabin.
  • Vehicle Cabin with new filtration with Pressurisation returned on average a protection factor of 52.

A cabin with a leakage model of 10% will require a higher volume of air to pressurise at 50 pascals than a cabin with a leakage model of 4% though ensuring fresh air volume exceeds a minimum of 30 m3/h to limit CO2 build up inside the enclosure.                                                                                                                                                                                                    

The situation for a pressurised cabin with poor cabin sealing or broken seals is often a case where the HVAC system is struggling to cool down the cabin due to dust ingress and a higher volume of hot air due to leaks. Even though positive pressure may be achievable; it’s the hot air loading that is much higher than the ability of the HVAC system to control its climate. Ultimately delivering hot air to the face of operator whom will often open a door or window for additional ventilation rendering a protective system obsolete.               

The other hazardous to health exposure that exists is when a cabin has become extremely contaminated with mine dust. Is the fact that the HVAC system will continue to recirculate respirable mine dust at the breathing zone of the operator inside the cabin.                                                                                                              

Recomendation 2.                                                                                                                                                                                                                                                            Fresh air Pressurisation is the method to push against external particulate pollution from entering the cabin or enclosure which means that the operator must remain inside with doors and windows closed when the protection system is turned on. The studies conducted by NIOSH demonstrate that protection factors start at 10 pascals of positive internal air pressure. The European model for a one person sealed cabin is >20 pascal when there is a pressure gauge and >50 pascals for no pressure gauge operating a machine with hazardous airborne substances in agriculture.

The European best practice is: >20 pascals + HEPA media + Sealed cabin = High Protection Factor [EN156954-3 Cabin equipped with pressure gauge]                    

Breathe-Safe Auto Pressure controls cabin pressurisation in real time in a dynamic process as pressurisation pre-set level is continually adjusted to cabin environment which is pre-set at >35 pascals.

Auto Pressurisation key points:

  • If a small loss of pressure is detected the system will automatically compensate.
  • If high level pressure loss is detected or pressure is below set point: low-pressure warning light will turn on to alert operator (option to add audible alarm is available plus further option for remote monitoring with remote alarm).
  • Event data will be recorded for future reference and/or record keeping.
  • System is only filtering volume of air required, on demand pressurisation.
  • Decreases heat load with HVAC components - Heat exchangers, compressor, expansion valves.      

Breathe-Safe’s extensive onsite testing has demonstrated high system efficiency and extended filter service lifecycles when pre-set pressure is dialled in at >35 pascals positive pressure.

Recommendation 3.
HEPA filtration means installing fresh air pressurisation with HEPA filters to control respirable mine dust in the cabin. HEPA media is tested to European standard (EN1822) with 0.3-micron diameter particles to demonstrate an absolute level of performance at this particle size range defined as being the “Most Penetrating Particle Size” (0.3 micron MPPS). When testing for example: if 10000 particles (0.3 micron) are introduced to HEPA filter media a maximum of 3 particles can get through.


The best practice system delivers the highest protection factor when designed as two stage HEPA: fresh air (inlet) and HEPA (inside cabin) return air filter which captures contaminants introduced in the cabin from shoes or clothes.

Recommendation 4.                                                                                                                                                                                                                                                          The pressure display is designed to inform operator that the protective system is working correctly and to alert operator when the system requires maintenance of if there is a fault that has been detected. Breathe-Safe pressure display has data logging functions that can assist maintenance personnel to check for when filter media is approaching end of service life and replace filters accordingly. This data can also be used as chain of evidence that pre-set pressurisation [safe] level has been maintained over time inside the cabin.


Best practice protective system design is:

  1. Cabin sealing: eliminate any contamination exposure from wind penetration.
  2. Pre-Cleaner: separate up to 90% of coarse airborne debris & extend filer service life from premature dust loading
  3. Pressuriser is to increase internal air pressure to push against airborne particulate.
  4. HEPA filter (Fresh air) High Efficiency Particulate Arrestance filter media to separate respirable dust from entering the cabin.
  5. HEPA return air filter: High Efficiency filter to clean air inside the cabin from shoes and clothes.
  6. Digital display with warnings to inform operator that protective system is working and when it requires maintenance.

Breathe-Safe system is readily available to be remotely monitored with further options to include automated data logging and alerts for mine site management.


Summary
The international best practice methods are for cabins / enclosures which must be pressurised above 20pascals to be effective at pushing against external pollution. Considering that positive pressurisation will only occur in a reasonably airtight cabin environment. The operator is required to keep doors and windows closed always with site operations carrying out an effective mine dust maintenance plan to replace worn out cabin seals and to detect any air pressure leaks that can potentially be a source of cabin mine dust contamination.
In situations related to airborne respirable particles, to protect the operator, the requirements are to fit a fresh air cabin pressuriser system with HEPA filters rated at H13 or above. In situations with corrosive gases then an activated carbon filter is added to HEPA filters.
Breathesafe has developed innovative technology to deliver safety for the health of operators working in harsh environments. High efficiency is required to reduce costs with proven outcomes providing the most reliable products for demanding 24/7 mine site operations. Breathe-Safe ensures a cabin that delivers high protection factors along with high levels of comfort due to matching pressurising protective system to HVAC performance.

*Refer Euro Standard EN15695 for one person safety cabin fitted with high efficiency cabin pressurisation intended for mitigation of dusts and aerosols in agricultural sector. It is expected this standard will extend to construction equipment in future.

References

Cecala AB, Organiscak JA, Heitbrink WA, Zimmer JA, Fisher T, Gresh RE, et al. [2002a]. Reducing enclosed cab drill operators’ respirable dust exposure at surface coal operations with a retrofitted filtration and pressurization system. SME preprint 02-105. Littleton, CO: Society for Mining, Metallurgy, and Exploration, Inc.

Cecala AB, Organiscak JA, Page SJ, Thimons ED [2002b]. Reducing silica exposure in aggregate operations. AggMan Jan:24-28.