What is the diversity factor in medical gas design?

HTM 02-01, the NHS England guidance on medical gas pipeline systems (2026 edition), states that pipeline flow cannot be predicted precisely, but gives guideline flows that have proved adequate in practice. The pipework is designed on the diversified flows and on the permissible pressure loss from the source to the terminal unit, with pipe sizes chosen to keep that loss below 5% of the nominal pipeline pressure.

How HTM 02-01 expresses it

Each terminal unit has a design flow, and each department has a formula for its diversified flow Q. In these formulas, n is the number of beds, treatment spaces or rooms in which the procedure is performed, not the number of terminal units, and nT is the number of theatres. Examples for oxygen:

Some areas are designed with no diversity. HTM 02-01 assumes that all critical care bed spaces are occupied and using oxygen, and calculates Q at 60 L/min per bed space, reduced to 10 L/min where high-flow ventilation is not required. For dental vacuum, it assumes that all chairs are in use in surgeries with up to ten chairs and 60% of the remaining chairs above ten, presents these diversity factors for guidance only and asks for each installation to be considered individually.

Supply lines are treated differently: the pipeline from the supply plant to the distribution system is sized for the full design flow of the system plus 25%. HTM 02-01 also notes that high oxygen use during the COVID-19 pandemic highlighted the risk of starving terminal units downstream of a significant load.

Other countries, other methods

Diversity rules are not international. General note II in the annex of the Portuguese ACSS specification ET 03/2006 (revised 2024) asks designers to size distribution with one of FD S 90-155, HTM 2022, AS 2896-1998 or SIS HB 370; HTM 2022 has since been superseded by the 2026 edition of HTM 02-01, section 16 of the same specification lists AS 2896-2011, and the French FD S90-155, whose edition in force according to AFNOR dates from November 2024, is an informative document giving additional elements for design and acceptance. ACSS also recommends that all intensive care and emergency observation beds can deliver high-flow oxygen up to 60 L/min, and that 20% of inpatient beds can do the same.