6.3 Medical Air Compressor Systems
Key Takeaways
Medical air compressors must serve peak calculated demand with the largest compressor out of service, and there must never be fewer than two compressors (Category 1).
Dryers, filters, and regulators must be at least duplexed and sized for peak demand with the largest of each out of service; filters must be 98% efficient at 1 micron or greater.
Medical air intakes must be at least 25 ft from exhausts, vents, and vacuum or WAGD discharges, at least 20 ft above ground, and at least 10 ft from any door, window, or other opening.
Dew point monitoring activates local and all master alarms above +35°F (+2°C); carbon monoxide monitoring activates a local alarm above 10 ppm.
Medical air dryers must provide air with a dew point below the frost point, 32°F (0°C), at 50-55 psi at any level of demand.
What the System Must Produce
Medical air must meet the USP medical air requirements, contain no detectable liquid hydrocarbons, contain less than 25 ppm gaseous hydrocarbons, and contain no more than 1 mg/m3 of permanent particulates 1 micron or larger. A medical air source may feed only the medical air distribution system, for breathing and calibrating respiratory devices.
Compressor Designs That Keep Contaminants Out
Medical air compressors must be designed to keep contaminants or liquid out of the pipeline by one of three methods:
- No oil anywhere in the compressor, for example liquid ring compressors and compressors with permanently sealed bearings
- Reciprocating compressors with the oil-containing section separated from the compression chamber by at least two seals. The space between the seals is open to atmosphere so the shaft can be seen directly, through openings at least 1.5 shaft diameters wide, and operators can confirm the seals work without taking the compressor apart.
- Rotating element compressors with an oil-free compression chamber, at least one seal with atmospheric vents on each side, a gravity drain on the vent nearest the oil, and vents that operators can inspect visually
Liquid ring compressors must use treated seal and service water to control waterborne pathogens and keep chlorine from hyperchlorination out of the air. They must also have a reserve medical air standby header or a backup compressor. A standby header, where installed, holds enough cylinders for 1 hour of normal operation.
Required Components and Arrangement
| Component | Requirement |
|---|---|
| Compressors | Enough to serve peak calculated demand with the largest compressor out of service; never fewer than two |
| Backflow | Automatic means to keep on-cycle compressors from backflowing through off-cycle ones |
| Isolation | A manual shutoff valve to isolate each compressor without losing system pressure |
| Intake protection | Dry-type intake filter-muffler on each compressor |
| Relief | Pressure relief valve(s) set 50% above line pressure |
| Mounting | Antivibration mountings as the manufacturer requires; flexible connectors to intake and outlet piping |
| Aftercoolers (where required) | Individual condensate traps; the receiver may not be used as an aftercooler or trap; duplex with isolation valves, or one per compressor |
| Receiver | Corrosion-resistant; ASME Section VIII; relief valve, automatic drain, manual drain, sight glass, pressure indicator; sized to prevent short-cycling; valved so it can be bypassed during service |
| Dryers | Dew point below the frost point, 32°F (0°C), at 50-55 psi at any demand; sized for 100% of peak; a sample port downstream of each dryer |
| Filters | Upstream of the final line regulators; sized for 100% of peak; at least 98% efficient at 1 micron or greater; a continuous filter-life indicator |
| Dryers, filters, regulators | At least duplexed, each sized for peak demand with the largest out of service, with valves so each can be serviced without shutdown. Only one dryer-filter-regulator sequence is open in normal operation. |
| Sample port | NPS 1/4 (DN8) valved sample port downstream of the final line regulators, dew point monitor, and CO monitor, and upstream of the source valve |
| Source valve | Per the valve rules |
| Different pressures | Piping separates after the filters, with separate regulators, dew point monitors, relief valves, and source valves for each pressure |
All components must be arranged so service continues and medical air keeps flowing through any single fault.
Monitoring and Local Alarms
| Monitor or alarm | Trigger |
|---|---|
| Dew point (downstream of regulators, upstream of piping) | Activates the local alarm and all master alarms above +35°F (+2°C) at system delivery pressure |
| Carbon monoxide | Activates the local alarm above 10 ppm |
| Monitor power loss | Dew point and CO monitors must signal their alarms if they lose power |
| Reserve capacity | Local alarm when the unused capacity is less than one compressor's; requires manual reset |
| High discharge temperature | Non-liquid-ring compressors shut down and alarm locally (set point per manufacturer) |
| High water | Liquid ring and water-cooled designs: receiver and separator high-water shutdown and alarm |
| Seal-type compressors (methods 2 and 3) | High-temperature shutdown, coalescing filters with change indicators, a charcoal absorber, and quarterly gaseous hydrocarbon monitoring |
Controls and Power
- Automatic activation of compressors as demand requires, and equalized wear. If wear is equalized manually, staff must schedule manual alternation.
- If an electrical device could stop medical air when it fails, an automatically activated alternative is provided. Isolating one compressor must never interrupt the others.
- Automatic restart after a power interruption without manual intervention
- A dedicated disconnect switch, motor starter, and overload protection for each compressor motor
- Essential electrical system power, and wiring per NFPA 70
The Intake
| Rule | Requirement |
|---|---|
| Air source | Clean air; air already filtered for operating room ventilation may be used if it is available 24/7 and has no fans with motors or belts in the airstream |
| Exhausts and vents | At least 25 ft (7.6 m) from ventilating exhausts, fuel storage vents, combustion vents, plumbing vents, vacuum and WAGD discharges, and areas that collect vehicle exhaust or other noxious fumes |
| Height | At least 20 ft (6 m) above ground level |
| Openings | At least 10 ft (3 m) from any door, window, or other building opening |
| End treatment | Turned down and screened with noncorroding screen |
| Combined intakes | Allowed if sized to minimize back pressure and each compressor can be isolated with a valve, blind flange, or cap |
| Piping | Materials and joining methods permitted for vacuum piping may be used |
| Labeling | Labeled so it is clearly identified as a medical air intake |
Proportioning Systems
Medical air can also be reconstituted from oxygen USP and nitrogen NF by a proportioning system. The system must produce 19.5-23.5% oxygen over its whole flow range, operate automatically, analyze the mixture continuously with a dedicated analyzer that has recording capability, and alarm and switch to the reserve if the mixture goes out of specification.
Category 2 note: medical air compressors, dryers, aftercoolers, filters, and regulators may be simplex if facility staff have an emergency plan for losing medical air.
What is the minimum height above ground for a medical air compressor intake under NFPA 99?
10 ft
15 ft
25 ft
20 ft
A medical air plant's carbon monoxide monitor reads 12 ppm. What does NFPA 99 require the monitor to do?
Activate the local alarm, because CO exceeds 10 ppm
Nothing until CO exceeds 25 ppm
Shut down every compressor automatically and vent the receiver
Signal only at the next scheduled quarterly test
How must the dryers, filters, and regulators in a Category 1 medical air system be arranged?
A single set sized for 150% of peak demand
One set per compressor, with every set open to flow at the same time for capacity
At least duplexed, each sized for peak with the largest out, one sequence normally open
Duplexed only for the dryers, while the filters and regulators may remain simplex
Sections you finish are checked off in the contents.