Maintaining positive pressure inside a refuge chamber is extremely important. Positive pressure helps to prevent the ingress of airborne contaminants that may compromise the breathable atmosphere for occupants.
Pressure systems are one of the vital life support systems installed in refuge chambers to provide a safe, breathable atmosphere for occupants.
Positive pressure is normally achieved through the use of compressed air. In mining, Compressed air is fed into the chamber until the internal pressure reaches 200 Pascals (Pa). The positive pressure is controlled by the use of check valves, which will open should the positive pressure inside the chamber exceed 250Pa. Too much positive pressure will prevent the chamber door from opening, or risk damaging the chamber.
In cases where a refuge chamber or safe haven is not connected to a compressed air source, positive pressure is achieved by use of a compressed air cylinder venting directly into the chamber. The amount of air required to reach the necessary level of positive pressure is determined by chamber internal volume and the relationship between the internal and external atmospheric temperatures. It is not sufficient to set an arbitrary pressure as a target for internal chamber pressure, as a fixed value inside the chamber cannot be assumed to be positive relative to the external air pressure.
Pressure System Components
The positive pressure system requires a 24 volt DC power source and comprises the following components:
- Compressed Air Cylinder(s) and Regulator – These provide the compressed air required to obtain and maintain positive pressure inside the refuge chamber. The air in the cylinder is typically pressurised to 15000-20000kPa. The regulator controls the pressure of the gas exiting the cylinder.
- Adjustable Differential Pressure Switch – The differential pressure switch is a device that measures the difference in pressure between two sources. When the difference falls above or below a certain range, the switch will either open or close electrical contacts.
- Solenoid Valve – The solenoid valve controls the release of the compressed air into the refuge chamber. It operates via an electromagnet. When power is applied to the coil, the valve will open.
Upon initial entry into the refuge chamber, the pressure inside and outside the chamber will be equal. Switching on the scrubber blower provides power to the positive pressure system. The solenoid valve will then open, allowing compressed air to flow into the chamber until such time as the interior pressure reaches 200Pa above the outside pressure. The switch will then open, cutting power to the solenoid and therefore shutting off the compressed air supply. If the inside pressure drops below approximately 170Pa above outside pressure, the switch will re-close, sending power back to the solenoid and allowing compressed air into the refuge chamber, again shutting off at 200Pa positive pressure. This process will repeat until the supply of compressed air is exhausted.
Postive Pressure Systems
Pressure systems are designed to help maintain a safe, breathable atmosphere within the refuge chamber. Systems include the Compressed Air Management System (CAMS) to regulate the release of compressed air into the refuge chamber; the Pressurised Access Safety System (PASS) to ensure safe entry into the refuge chamber; and, the Positive Pressure Maintenance System (PPMS) to maintain positive internal pressure within the refuge chamber.
Compressed Air Management System
CAMS contains a differential air pressure sensor and a shut-off valve allowing air flow into the chamber to be regulated, automatically emitting periodic ‘bursts’ of compressed air into the refuge chamber when the pressure inside drops below 200Pa. This process optimises mine air usage and guarantees against over-pressurisation of the refuge chamber. CAMS maintains a positive pressure ‘seal’, ensuring contaminants cannot enter the refuge chamber from the outside.
CAMS Electrical Controller
The electronic controller of CAMS performs a number of functions, depending on the mode of operation; chamber occupied and chamber non-occupied.
Unoccupied Mode: Its main function is to keep the chamber positively pressurised. Using a differential pressure sensor to switch off the flow of compressed air when the chamber reaches 200Pa above ambient pressure reduces compressed air usage by as much as 95%, while still maintaining protection against the ingress of smoke and other contaminants into the refuge chamber.
Occupied Mode: The chamber requires a constant stream of air as its primary source of breathable air. While in occupied mode, the incoming compressed air is constantly monitored for quality by an oxygen sensor. If the level of oxygen drops below 18%, indicating contamination with smoke or other gases, the system will go into alarm and shut off the compressed air supply until such time as the oxygen levels go back above 18%.
Pressurised Access Safety System (PASS)
MineARC’s PASS remote activation unit is located next to the door on the front exterior of the refuge, allowing personnel to pre-prepare the chamber for safe entry. Should the chamber’s fresh compressed air supply become disconnected or compromised, the system’s external LED light will display red, indicating that the chamber is not positively pressurised and therefore unsafe for entry. Once activated, the PASS will disperse controlled quantities of compressed air into the chamber until the internal pressure reaches 200Pa. By ensuring that the pressure inside the refuge is slightly greater than outside, toxic contaminants are prevented from infiltrating the chamber during entry of personnel.
Positive Pressure Maintenance System (PPMS)
MineARC’s PPMS is securely mounted to the interior wall of the refuge chamber. Powered by a 24VDC power supply, the electric solenoid valve opens and closes to release measured amounts of breathable air from compressed air cylinders in order to maintain a positive internal pressure.
The quantity of compressed breathable air cylinders is configurable to suit various internal volumes and durations of operation.
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