What is the reserve supply for a medical air system?

ISO 7396-1, the international standard for medical gas pipeline systems, requires a supply system with at least three sources of supply; ISO 7396-3 leaves that rule to Part 1. National guidance uses different names for them. HTM 02-01 in England calls them primary, secondary and tertiary, and the Portuguese ACSS ET 03/2006 calls them primary, secondary and reserve. In a compressor plant the compressors are the primary source, and the question is what takes over when they stop.

What the NHS England guidance specifies

HTM 02-01 (2026) says medical compressed air can be obtained from compressor systems and cylinders, and backs the compressors with automatic cylinder manifolds. Its examples include:

• Triplex compressor plant: two compressors as primary, the third compressor as secondary, and automatic manifolds plus locally based ready-to-use cylinders as tertiary.
• Combined medical and surgical air plant: two automatic manifolds as secondary, one for each service, which come online if the plant fails and hold enough cylinders for 4 hours of supply at average use; tertiary supply from automatic manifolds and locally based ready-to-use cylinders.
• Where the compressors could draw in smoke during a fire: an automatic shutdown linked to local smoke detectors, with the automatic emergency manifold sited well away from the fire-risk area and coming online automatically.

What the Portuguese specification asks for

ACSS ET 03/2006 (revised in June 2024) asks for three automatically operating compressor units, each rated at 100% of the nominal flow, and an emergency set of medical air cylinders able to supply the hospital for 24 hours. It also asks for permanently connected reserve systems for oxygen, medical air and vacuum in operating theatres and intensive care units. For medical air, that reserve is a service cylinder and an emergency cylinder with automatic changeover, and it must start automatically if the central supply fails.

Points to check in a design

• The reserve contains medical air: ACSS specifies cylinders of medical compressed air for the emergency set.
• Changeover is automatic and alarmed: HTM 02-01 raises a cylinder reserve pressure fault when the duty bank falls below 50% of normal cylinder pressure.
• A non-return valve upstream of the emergency cylinder connection stops the reserve from backfeeding the compressor plant (HTM 02-01).
• Reserve duration depends on the guidance applied: 4 hours at average use for the HTM manifold examples, 24 hours for the ACSS emergency set.

In Ultra Controlo plants, ULTRAAR SC and ULTRAAR ESW are configured with one, two, three or more oil-less compressors, and their receivers are prepared to receive an additional emergency compressor. For critical areas, the ULTRAGEC emergency cabinet holds B50 cylinders of oxygen and medical air and connects to the secondary network upstream of the area, so that supply continues if the network feeding it fails.