4.2 Supports, Buried Piping, Flexible Connectors & System Interconnections
Key Takeaways
Piping is supported from the building structure with hangers per MSS SP-58; supports in contact with copper in damp locations must be plastic-coated or otherwise insulated.
Maximum support spacing ranges from 5 ft for DN8 (3/8 in. OD) to 10 ft for DN40 (1-5/8 in. OD) and larger; risers are supported at every floor but not more than 15 ft apart.
Buried piping outside buildings needs 36 in. of cover (18 in. where there is no potential for surface-load damage), a named tape or marker just above the pipe, and a warning means at about half the burial depth.
Metallic flexible joints must be bronze, copper, or stainless steel on wetted surfaces, factory-cleaned for oxygen, rated for 300 psig and 1000°F, and supplied with brazing extensions.
Two medical gas or vacuum systems may not be interconnected for installation or testing; leak testing charges and tests each system separately.
Supports and Hangers
| Rule | NFPA 99 requirement |
|---|---|
| Support from | The building structure, never from other piping, ductwork, or ceiling wires that belong to other trades |
| Hanger standard | MSS SP-58 |
| Copper tube | Supports must be sized for copper tube |
| Damp locations | Hangers or supports in contact with copper must be plastic-coated or otherwise electrically insulated from the tube by a material that will not absorb moisture. This prevents galvanic corrosion |
| Seismic | Where required, piping must be seismically restrained per the applicable building code |
Maximum support spacing
| Pipe size | Outside diameter | Maximum hanger spacing |
|---|---|---|
| DN8 (NPS 1/4) | 3/8 in. | 5 ft |
| DN10 (NPS 3/8) | 1/2 in. | 6 ft |
| DN15 (NPS 1/2) | 5/8 in. | 6 ft |
| DN20 (NPS 3/4) | 7/8 in. | 7 ft |
| DN25 (NPS 1) | 1-1/8 in. | 8 ft |
| DN32 (NPS 1-1/4) | 1-3/8 in. | 9 ft |
| DN40 (NPS 1-1/2) and larger | 1-5/8 in. and larger | 10 ft |
| Vertical risers, all sizes | Every floor, but not to exceed 15 ft |
Worked example: a 1 in. nominal (1-1/8 in. OD) medical air branch runs 64 ft along a corridor ceiling. At 8 ft maximum spacing, 64 / 8 = 8 spaces, so the run needs at least 9 supports counting both ends. More supports are added at valves, changes in direction, and heavy components. Valves and flexible joints add weight, and NFPA 99 requires metallic flexible joints to be supported with hangers as required for their additional weight.
Buried Piping Outside Buildings
| Requirement | Detail |
|---|---|
| Frost | Install below the local frost penetration depth |
| Backfilling | The installation procedure must protect the pipe from damage while it is backfilled |
| Conduit, cover, or enclosure (if used) | Provide access at joints for visual inspection and leak testing; the enclosure must be self-draining and must not hold groundwater against the pipe |
| Surface loads | Bury deep enough to protect the pipe or its enclosure from excessive stress |
| Minimum cover | 36 in. (914 mm) above the top of the pipe or enclosure; may be reduced to 18 in. (457 mm) where there is no potential for damage from surface loads or surface conditions |
| Trench bottom | Excavate so the pipe or enclosure has firm, substantially continuous bearing |
| Backfill | Clean, free of material that can damage the pipe, and compacted |
| Identification | A continuous tape or marker immediately above the pipe or enclosure that names the pipeline |
| Warning | A continuous warning means about halfway between the pipe and the surface |
| Wall sleeve | Where the piping passes through a wall sleeve, seal the outdoor end so groundwater cannot enter the building |
Example: an oxygen line runs from an outdoor bulk farm under a parking lot to the hospital. The surface carries vehicle loads, so the minimum cover is 36 in. Above the pipe goes a continuous marker reading "OXYGEN," and a second warning tape goes about 18 in. below grade. If the line runs only under a landscaped area with no possibility of surface loading, the cover may be reduced to 18 in.
Hoses, Flexible Connectors and Metallic Flexible Joints
- Hoses and flexible connectors, metallic or nonmetallic, must be no longer than necessary, must not penetrate or be concealed in walls, floors, ceilings, or partitions, and must have a minimum burst pressure of 1000 psi gauge.
- Metallic flexible joints are permitted in the pipeline where needed for expansion joints, seismic protection, thermal expansion, or vibration control. They must be:
- Bronze, copper, or stainless steel on all wetted surfaces
- Factory-cleaned for oxygen service and received with certification of cleanliness
- Suitable for 300 psig or higher and able to withstand 1000°F (538°C)
- Supplied with brazing extensions for brazing into the pipeline
- Supported with hangers sized for their added weight
Never Interconnect Different Systems
Two or more medical gas or vacuum piping systems may not be interconnected for installation, testing, or any other reason. The one exception: systems with the same contents, such as two oxygen systems, may be interconnected through an in-line valve installed between them.
Leak testing must be done by separately charging and testing each individual piping system. A temporary jumper that lets nitrogen from the oxygen test rig feed the medical air piping "just for the pressure test" is an interconnection. It also defeats the cross-connection test that follows, because the test is meant to prove the systems are isolated from each other.
Changing a System's Use
- A positive-pressure distribution system converted to another gas or another pressure must meet the material, joining, and installation provisions as if it were new.
- A vacuum system may never be converted for use as a gas system.
Breaching Existing Piping
When positive-pressure patient gas or medical support gas piping is breached or penetrated, for example to add a branch to a live main, the method must not leave copper particles or other debris inside the piping or affect its oxygen-clean interior. Any debris created must stay contained in the work area. Once a system is breached, the affected zone must be inspected and tested again (Section 7.4).
What is the maximum hanger spacing NFPA 99 allows for horizontal DN25 (1 in. nominal, 1-1/8 in. OD) copper medical gas tube?
6 ft
7 ft
8 ft
10 ft
An oxygen line is buried outside the building under a driveway that carries delivery trucks. What is the minimum backfilled cover above the pipe or its enclosure?
12 in.
18 in.
24 in.
36 in.
To save time, a crew connects the oxygen and medical air piping with a temporary jumper so both can be pressure tested from one nitrogen cylinder. How does NFPA 99 treat this?
It is permitted for initial pressure tests but not standing pressure tests
It is permitted if the jumper has a check valve
It is permitted if the systems are separated again before the cross-connection test
It is prohibited; each system must be charged and tested separately
Sections you finish are checked off in the contents.