Why not add up the flows of all outlets?

Because not all outlets are used at once. HTM 02-01 uses diversified flow formulas for each department, and no diversity in critical care, where it assumes all bed spaces are occupied.

What pressure loss is allowed?

HTM 02-01 (2026) asks for pipe sizes that keep pressure loss below 5% of nominal pipeline pressure, from the source to and including the outlet.

How is medical gas pipe sized?

Q: What inputs are used to size medical gas pipes: flow, diversity and pressure drop?

Under HTM 02-01, the NHS England guidance, medical gas pipe is sized from diversified flow, not the sum of all outlets: designers set a design flow for each outlet, apply a diversity formula for each department, and choose diameters that keep pressure loss from source to outlet below 5% of nominal pipeline pressure.

What inputs are used to size medical gas pipes: flow, diversity and pressure drop?

Precise prediction of pipeline flow is not possible, so design guidance gives rules that have proved adequate in practice, and WHO notes that every design is facility-specific. The method below follows HTM 02-01 (2026) Part A, the NHS England guidance; other countries use their own references.

The inputs

Space definitions: each clinical space is classified at the start of design, because the flow needed at each outlet depends on how the space is used.
Design flow per outlet: HTM 02-01 Table 2 gives indicative flows to be achieved at each terminal unit.
Diversity: HTM 02-01 Table 3 gives a diversified flow formula for each department. Operating departments, for example, are based on 50 L/min for the first operating room and 10 L/min for each of the others. In the formulas, n is the number of beds or treatment spaces in use, not the number of outlets.
Where no diversity applies: HTM 02-01 assumes that all bed spaces in critical care, coronary care and high-dependency units will be occupied and will need oxygen, so Q is 60 L/min per bed space where high-flow ventilation is required, or 10 L/min per bed space without it.
Permissible pressure loss: from the source of supply to, and including, the outlet, below 5% of the nominal pipeline pressure.
Layout: the length of each pipe run, taken from isometric drawings drawn full size, which are used to calculate flows and pressure drops in each section.

The method step by step

Classify every room and assign the design flow per outlet.
Calculate the diversified flow for each ward and department.
Select pipe diameters so that the pressure loss stays within the limit, with pressure-drop calculations from several locations, including the furthest points.
Allow for high-flow devices: CPAP flows should be included in pipeline sizing, flow loops should be at least 22 mm, and loops in critical care with high-flow ventilation go up one pipe size.
Confirm at commissioning that each outlet reaches the specified pressure at the specified flow.

Other references

In Portugal, general note II in the annex of the ACSS specification ET 03/2006 (revised 2024) asks for one of FD S 90-155, HTM 2022, AS 2896-1998 or SIS HB 370 to be applied for sizing, using the confirmed number of points of use and real utilisation coefficients; HTM 2022 has since been superseded by HTM 02-01 (2026), and section 16 of the same specification lists AS 2896-2011. The ACSS also recommends that all intensive care and emergency observation beds can take high-flow oxygen up to 60 L/min, and 20% of ward beds. In France, FD S90-155 (November 2024 edition, current according to AFNOR) gives additional elements for the design and acceptance of medical gas and vacuum pipelines.