2.1 Tube Specifications for Medical Gas and Vacuum Piping
Key Takeaways
Positive-pressure tube must be hard-drawn seamless copper ASTM B819 medical gas tube, Type L.
Type K is required only where operating pressure exceeds 185 psi and the pipe is larger than DN80 (NPS 3, 3-1/8 in. OD); both conditions must apply.
ASTM B819 tube is marked OXY, MED, OXY/MED, OXY/ACR, or ACR/MED, in blue for Type L and green for Type K.
Vacuum piping may be ASTM B88 Type K, L, or M; ASTM B280 ACR; ASTM B819; or 304L/316L stainless steel tube or pipe.
Mains and branches must be at least DN15 (5/8 in. OD) for medical gases and DN20 (7/8 in. OD) for medical-surgical vacuum; drops must be at least DN15.
Positive-Pressure Piping: ASTM B819 Medical Gas Tube
For field-installed positive-pressure medical gas systems, NFPA 99 requires hard-drawn seamless copper tube made to ASTM B819, the standard specification for seamless copper tube for medical gas systems. The default is Type L. Type K is required only when both of these are true:
- the operating pressure is above 185 psi (1276 kPa) gauge, and
- the pipe size is larger than DN80 (NPS 3, which is 3-1/8 in. OD).
Tip
Exam writers like to test the word "and." A 4 in. oxygen main at 55 psi stays Type L. A 1 in. nitrogen line at 185 psi also stays Type L. Type K is mandatory only for a pipe that is both large and above 185 psi.
Hard-drawn matters. Medical gas piping comes in straight, rigid lengths. Soft (annealed) coils are not the specified product for field-installed medical gas distribution.
Manufacturer markings
ASTM B819 medical gas tube carries the manufacturer's markings "OXY," "MED," "OXY/MED," "OXY/ACR," or "ACR/MED" in blue for Type L or green for Type K. These markings tell you three things: the tube was made and cleaned as medical gas tube, it is suited to oxygen service, and its wall type. ASTM B819 tube arrives from the mill cleaned for oxygen service and capped. Section 2.2 covers cleanliness and delivery.
Nominal size versus outside diameter
Copper tube is ordered by nominal size, but its outside diameter is 1/8 in. larger than the nominal size. NFPA 99 lists all three ways of naming a size, so you must translate between them quickly:
| Metric DN | Nominal (NPS) | Actual OD | Type K wall | Type L wall |
|---|---|---|---|---|
| DN8 | 1/4 | 3/8 in. | 0.035 in. | 0.030 in. |
| DN15 | 1/2 | 5/8 in. | 0.049 in. | 0.040 in. |
| DN20 | 3/4 | 7/8 in. | 0.065 in. | 0.045 in. |
| DN25 | 1 | 1-1/8 in. | 0.065 in. | 0.050 in. |
| DN32 | 1-1/4 | 1-3/8 in. | 0.065 in. | 0.055 in. |
| DN40 | 1-1/2 | 1-5/8 in. | 0.072 in. | 0.060 in. |
| DN50 | 2 | 2-1/8 in. | 0.083 in. | 0.070 in. |
| DN65 | 2-1/2 | 2-5/8 in. | 0.095 in. | 0.080 in. |
| DN80 | 3 | 3-1/8 in. | 0.109 in. | 0.090 in. |
| DN100 | 4 | 4-1/8 in. | 0.134 in. | 0.110 in. |
The wall dimensions are the standard copper tube dimensions published for Type K and Type L. Type K always has the heavier wall at a given size. That extra wall is why NFPA 99 calls for it where high pressure and a large diameter combine. Hoop stress in a tube wall rises with both pressure and diameter.
Vacuum and WAGD Piping
Vacuum piping never carries an oxidizing gas under pressure, so NFPA 99 allows more materials. Piping for vacuum systems may be any of the following:
- Hard-drawn seamless copper:
- ASTM B88 copper water tube, Type K, L, or M
- ASTM B280 copper ACR tube
- ASTM B819 copper medical gas tube, Type K or L
- Stainless steel:
- ASTM A269 TP304L or 316L tube
- ASTM A312 TP304L or 316L
- ASTM A312 TP304L/316L Schedule 5S pipe with ASTM A403 WP304L/316L Schedule 5S fittings
When ASTM B819 medical gas tube is used for vacuum piping, the special vacuum marking the code otherwise calls for is not required.
Warning
The reverse is never allowed. Type M water tube or ACR tube that is perfectly legal on a vacuum main can never be used on an oxygen, medical air, nitrous oxide, or nitrogen line. When you see Type M or ASTM B88 on a positive-pressure line in an exam question, look for the code violation.
Minimum Pipe Sizes
NFPA 99 sets minimum sizes so outlets receive adequate flow even when the design calculation would allow something smaller:
| Piping element | Minimum size |
|---|---|
| Mains and branches, medical gas | DN15 (NPS 1/2, 5/8 in. OD) |
| Mains and branches, medical-surgical vacuum | DN20 (NPS 3/4, 7/8 in. OD) |
| Drops to individual station outlets and inlets | DN15 (NPS 1/2, 5/8 in. OD) |
| Runouts to alarm panels and connecting tubing for gauges and alarm devices | DN8 (NPS 1/4, 3/8 in. OD) permitted |
Beyond these minimums, the system must be designed and sized to deliver the required flow rates at the utilization pressures. The designer does the sizing, but the installer must install exactly the sizes shown. Swapping a 3/4 in. vacuum branch for 1/2 in. because the fittings were on the truck is a code violation.
CMT and Other Newer Products
NFPA 99-2024 recognizes corrugated medical tubing (CMT) systems. Changes in direction in CMT may be made by bending the tubing within the manufacturer's minimum bend radius, or by using permitted fittings. CMT must be installed by ASSE 6010-qualified installers following the CMT manufacturer's instructions. Use only products that the drawings specify and the adopted code edition permits.
Converting an Existing System
If a positive-pressure distribution system built for one gas or pressure is converted to another gas or pressure, all of the piping-material, joining, and installation provisions apply as if the system were new. A vacuum system may never be converted for use as a gas system. Vacuum materials such as Type M or ACR tube, extruded tees, and uncleaned components were never qualified for positive-pressure oxygen-service duty.
A 4 in. nominal oxygen main will operate at 55 psi. Which ASTM B819 tube type does NFPA 99 require?
Type K, because the pipe is larger than 3 in. nominal
Type K, because oxygen is an oxidizer
Type L, because the pressure does not exceed 185 psi
Type M, because it is a main rather than a branch
An installer finds a bundle of medical gas tube with blue continuous markings reading ACR/MED. What does the marking indicate?
ASTM B280 refrigeration tube that is limited to vacuum service
ASTM B819 medical gas tube, Type L
ASTM B819 medical gas tube, Type K
ASTM B88 water tube that must be cleaned on site
Which material is permitted for a medical-surgical vacuum branch but prohibited for a medical air branch?
ASTM B88 Type M hard-drawn copper tube
ASTM B819 Type L medical gas tube
ASTM B819 Type K medical gas tube
Hard-drawn copper tube of any type, if cleaned on site
Sections you finish are checked off in the contents.