What happens to off-spec oxygen when PSA purity is low?

How the gas is kept out of the pipeline

• Analysers and the main shut-off valve: AIGA 113/20 asks for two oxygen analysers that continuously monitor and record the concentrator output and can close the main shut-off valve automatically if the concentration falls outside the pharmacopoeia specification (its reference table, taken from Ph. Eur. 2455, gives 90.0% to 96.0%). Carbon monoxide, carbon dioxide and moisture analysers can close the same valve when impurities exceed the specification. The valve allows the oxygen 93 source to be isolated from the hospital pipeline when the gas is out of specification, or for maintenance or fire, and a non-return valve prevents back flow from the pipeline into the concentrator unit.
• Proved at validation: during validation, AIGA asks for the system to be challenged to verify that the supply from the concentrator units stops when critical process parameters are outside their limits, and that the back-up source, secondary or reserve, then supplies the demand automatically.
• England: NHS England's HTM 02-01 (2026) asks for the monitoring system to isolate the PSA from the pipeline when the concentration falls below 94%, so that the emergency or reserve supply operates, with an independent monitoring system that also isolates the plant below 94%.
• Portugal: the ACSS specification ET 03/2006 (revised June 2024) places the oxygen analyser and the shut-off valve upstream of the Oxygen 93 reservoir, and asks for alarms connected to the building management system and a shut-off valve that closes automatically if the analyser detects gas outside the European Pharmacopoeia specification.

Purging and venting

The WHO interim technical specifications for PSA plants (8 June 2020), a COVID-19 document that stated it would expire two years after publication, listed a function to purge low-concentration oxygen as optional, alongside an alarm below 90% and an alarm when the automatic back-up engages. AIGA 113/20 gives the oxygen 93 buffer a means of purging, and during operational qualification records the value at which each alarm or control activates or resets, naming automatic shut-off and vent valves among them. AIGA adds that when a generator shuts down there is usually a built-in, fail-safe sequence of shutdown events; if there is no automatic system, or it fails, a manual shutdown procedure lists the necessary actions, such as product disposal and equipment venting. It asks for all vents to be directed to a safe location outside the buildings. The USAID RISE operating procedure (2022), written for hospitals in India, lists purging the whole plant, as the manual describes, among its tips for low purity.

After an out-of-specification event

AIGA asks the responsible person to notify the clinical staff when any parameter out of specification is identified, so that they can investigate any impact on patients, and asks for production records, including analysis results, to be kept for at least five years. See what quality control a hospital producing oxygen needs.

ULTRAOX SC plants measure purity with one or two zirconium oxide analysers, and on ULTRAOX plants all alarms are on an interface for remote signalling, according to the ULTRAOX SC and ULTRAOX 2022 datasheets. See what backup to keep with a PSA plant.