6.1 Central Supply Locations, Cylinder Manifolds, Line Controls & Relief Valves
Key Takeaways
Indoor rooms for oxygen, nitrous oxide, or other oxidizers must be separated from the rest of the building by 1-hour fire-rated walls and floors, with 3/4-hour rated doors.
Supply locations must be lockable, have no fuel-fired equipment, use heating elements no hotter than 266°F (130°C), and secure every cylinder, full or empty, against falling.
Final line pressure controls must keep pressure within Table 5.1.11, each flow 100% of peak demand, and be redundant so any component can be serviced without interrupting supply.
Central supply relief valves sit between each final line regulator and the source valve, are set 50% above normal operating pressure, and are vented outdoors except for small compressed air systems.
A gas cylinder manifold has two equal headers, each holding one average day's supply but never fewer than two cylinder connections, with automatic changeover that signals the master alarms.
Types of Central Supply Systems
NFPA 99 permits central supply systems to consist of:
- Cylinder manifolds for gas cylinders
- Manifolds for cryogenic liquid containers
- Cryogenic fluid central supply systems (bulk)
- Medical air compressor systems
- Medical-surgical vacuum producers
- WAGD producers
- Instrument air compressor systems
- Proportioning systems for medical air USP
- Oxygen central supply systems using concentrators
Central supply systems must come from a supplier or manufacturer familiar with their construction and use, and must be installed per the manufacturer's instructions. Installers of medical gas piping that supports pump, compressor, and source gas manifold systems must be ASSE 6010 certified. Permanently installed bulk source systems are excluded from that requirement.
Locations for Cylinders and Gas Manifolds
Locations for central supply systems (other than cryogenic fluid systems and motor-driven equipment), and locations where positive-pressure gases are stored, must:
| Requirement | Detail |
|---|---|
| Access | Allow cylinders and equipment to be moved in and out on hand trucks |
| Security | Have lockable doors or gates, or otherwise be securable |
| Outdoors | Be well drained, with a noncombustible enclosure such as a wall or fence; keep cylinders off prolonged contact with soil |
| Indoor finishes | Use noncombustible or limited-combustible interior finishes |
| Oxidizer rooms | Separate rooms with oxygen, nitrous oxide, or other oxidizers from the building with 1-hour fire-rated walls and floors and 3/4-hour rated doors and other opening protectives |
| Electrical | Meet NFPA 70 for ordinary locations; protect electrical devices from damage; supply power from the essential electrical system |
| Fuel-fired equipment | None in the room |
| Heat | Where heat is needed, in-room heating elements may reach at most 266°F (130°C) |
| Cylinder restraint | Racks, chains, or other fastenings for all cylinders, connected or not, full or empty |
| Shelving | Racks, shelves, and supports of noncombustible or limited-combustible materials |
| Delivery | Allow access by delivery vehicles and for cylinder management |
Ventilation: indoor central supply locations, medical gas storage rooms, and transfilling rooms are ventilated per NFPA 99's HVAC chapter. Rooms with motor-driven sources (medical air, vacuum, WAGD, instrument air) must be ventilated to prevent heat buildup. Outdoor enclosures with solid walls need protected ventilation openings at the base of each wall. Indoor supply systems must vent all relief valves outdoors as described below.
Storage quantity limits per room or enclosure
| Gas | Outdoor enclosure | Indoor, nonsprinklered | Indoor, sprinklered |
|---|---|---|---|
| Oxygen and nitrous oxide (sum of all oxidizing gases in the room) | No limit | 10,000 ft3 (283 m3) | 20,000 ft3 (566 m3) |
| Carbon dioxide, helium, medical air, nitrogen | No limit | Per Table 5.1.3.3.2.5 | Per Table 5.1.3.3.2.5 |
These limits may be exceeded only where an approved risk assessment by the governing body documents it.
Storage rule: cylinders, full or empty, may not be stored in enclosures that contain motor-driven machinery. The one exception is cylinders for an instrument air reserve header, which may share a room with the instrument air compressor when it is the only machinery there. Locations for motor-driven central supply systems must be lockable, meet NFPA 70, stay within the manufacturer's temperature limits, and run on essential electrical power.
Final Line Pressure Controls
Every positive-pressure supply system must control final line pressure at the source with equipment that:
- Keeps pressure within the limits of Table 5.1.11
- Can flow 100% of the peak calculated demand through each control mechanism
- Is redundant, so each component can be isolated for service while normal operation continues
- Is protected against overpressure
- Is built from materials the manufacturer deems suitable
For cryogenic fluid central supply systems, these line pressure regulators must be of a balanced design. Where one gas is piped at two pressures, each pressure needs its own pressure-regulating equipment, relief valve, and source shutoff valve.
Relief Valves
All pressure relief valves must:
- Be brass, bronze, or stainless steel, designed for the specific gas
- Be set no higher than the maximum allowable working pressure of the weakest component in the portion of the system they protect
- Vent outside the building. Relief valves on compressed air systems holding less than 3000 ft3 at STP may diffuse locally by means that do not restrict flow.
- Have a vent discharge line no smaller than the relief valve outlet or NPS 3/4, whichever is larger. Where several relief valves share a vent, its cross-sectional area must be at least the combined area of all of them.
- Discharge where escaping gas will not hit people, equipment, or structures or enter enclosed spaces, and have the vent end turned down and screened
- Be designed to ASME B31.3
Plumbing codes that extract NFPA 99, such as the Uniform Plumbing Code's health care chapter, add that a relief valve may not be isolated from its intended use by a valve.
Central supply systems for positive-pressure gases must have one or more relief valves between each final line regulator and the source valve, set 50% above the normal system operating pressure.
Headers
Each header in a system using cylinders or containers includes:
- Cylinder or container connection points
- A connection lead for each point, with end fittings per CGA V-1
- A filter to keep debris out of the manifold controls
- A header shutoff valve
- A pressure indicator for header contents
- A check valve that prevents backflow into the header and allows the header to be serviced
- For gas cylinders: a check valve at each cylinder lead connection, and a pressure regulator that reduces cylinder pressure to an intermediate pressure
- For cryogenic liquid containers: a pressure relief valve
- Any header vent valves piped outdoors
Manifolds for Gas Cylinders
- Location: outdoors in an enclosure used only for that purpose (sited per the separation table), or indoors in a room used only for such manifolds. The room or enclosure must be built to the location rules above.
- The primary and secondary headers are in the same enclosure.
- Two equal headers, each with enough cylinder connections for one average day's supply but not fewer than two connections, connected so either header can supply the system
- Intermediate relief valve(s), piped outdoors, protecting the piping between the header regulator and the line regulators
- Automatic alternation: one header is primary and the other secondary, and either can take either role. The secondary is held off while the primary supplies, then takes over automatically when the primary is depleted.
- A local signal that shows operating status, and an indicator at all master alarm panels when, or just before, the secondary header begins to supply the system (changeover)
An indoor room houses an oxygen cylinder manifold. Which construction does NFPA 99 require between this room and the rest of the building?
1-hour walls and floors, with 3/4-hour doors and opening protectives
Walls and floors with a 2-hour rating and doors with a 1-1/2-hour rating
Noncombustible finishes only, with no rating required
Walls and floors with a 1-hour rating and any solid-core wood door
How must the two headers of a gas cylinder manifold be sized under NFPA 99?
The primary header for one week of supply and the secondary for one day
Equal headers, each holding one average day's supply, two connections minimum
Each with exactly six cylinder connections
The secondary header only needs half the primary header's cylinder capacity
Where must the relief valves for a positive-pressure central supply system be located, and at what setting?
Downstream of the source valve on the patient side, set at normal operating pressure
At each zone valve box on every floor, set 20% above the normal operating pressure
Between each final line regulator and the source valve, set 50% above normal
At the header inlet, set at cylinder pressure
Sections you finish are checked off in the contents.