Size copper pipe for oxygen, medical air, nitrous oxide, nitrogen and medical vacuum from outlets, flow per outlet and diversity — with pressure drop and velocity for Type K and Type L tube to NFPA 99 and HTM 02-01 practice.
Sizing aid for preliminary design. Design flows, diversity, pressure drop limits and minimum sizes must follow NFPA 99 or HTM 02-01 as adopted, the medical gas consultant's design and the authority having jurisdiction. Systems must be installed, tested and certified by qualified personnel.
| Size | ID | Velocity | Pressure drop | Status |
|---|
Every section of a medical gas pipeline is sized for its design flow — the flow that can realistically occur at once, not the sum of every outlet at full flow. That flow is turned into an actual velocity at line pressure, and the pressure drop along the section is checked against the share of the total allowance given to it.
| Gas | NFPA 99 (US) | HTM 02-01 (UK) |
|---|---|---|
| Oxygen, medical air, N₂O, CO₂ | 50–55 psig (345–380 kPa) | 4 bar (400 kPa) |
| Surgical air / nitrogen (tool drive) | 160–185 psig (1,100–1,275 kPa) | 7 bar (700 kPa) |
| Medical vacuum | at least 12 inHg (41 kPa) at the inlet | -40 kPa at terminal unit (plant about -60 kPa or lower) |
NFPA 99 requires medical gas and vacuum piping in Type K or Type L copper tube cleaned for oxygen service (ASTM B819), with mains and risers at least ¾″ (DN 20) and drops to individual outlets at least ½″ (DN 15). The tool applies these minimums even when a smaller size would pass hydraulically.
A ward with 24 oxygen outlets will not have 24 patients on high flow at the same time. Diversity factors — set per department in HTM 02-01 and in the designer's own basis under NFPA 99 — reduce the connected flow to a realistic design flow. Critical areas such as ICU and theatres use high diversity (close to 100%), general wards much lower. Enter the factor your design basis specifies.
Oxygen main serving 24 ward outlets at 10 L/min each, 40% diversity, 80 m equivalent length, 380 kPa line pressure.
Seamless Type K or Type L copper tube to ASTM B819, cleaned and capped for oxygen service and joined by brazing under a nitrogen purge. Type M and plastic pipe are not permitted for positive-pressure medical gases under NFPA 99.
NFPA 99 requires mains and risers of at least ¾ in (DN 20) and drops to individual station outlets of at least ½ in (DN 15).
Vacuum flows are given in free air, but inside the pipe the gas is at less than half of atmospheric pressure, so the same free-air flow occupies more than twice the volume and moves much faster. Vacuum also has very little pressure difference available, so the drop allowance is small.
It depends on the department and the design basis. HTM 02-01 gives department-by-department design flow formulas; under NFPA 99 the designer sets the basis. Critical care and theatres are close to 100%, general wards much lower.
The codes do not set one number. Designers commonly keep velocities around 15 m/s or below to limit noise and pressure loss; the pressure drop allowance usually governs before velocity does.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
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