What happens to low-purity oxygen during start-up?

WHO's 2020 interim specifications (superseded by WHO's technical specifications of 31 December 2024) listed a purge function for low oxygen concentration as optional, and AIGA 113/20 asks for analysers that can close the main shut-off valve automatically when concentration falls outside the pharmacopoeia specification.

Does a standby plant start fast enough to take over?

AIGA 113/20 makes the standby unit's buffer cover its whole start-up at nominal flow and required pressure, and does not accept using the reserve source during the changeover.

How quickly does a PSA plant reach full purity after starting?

Q: How long does a PSA oxygen plant take to reach specified purity after a cold start?

Operating procedures bring a PSA plant on line when purity and pressure are confirmed rather than after a set time: in a USAID RISE procedure for Indian hospitals, the operator waits until the display shows more than 4.2 bar and 93% ± 3% purity, then 5 more minutes, before opening the oxygen receiver to the pipeline.

How long does a PSA oxygen plant take to reach specified purity after a cold start?

The ULTRAOX datasheets do not give a start-up time, and the guidance documents below set conditions and checks rather than a duration. The USAID procedure described below itself refers to the manufacturer's manual for the air receiver pressure at which the PSA time cycle starts, and for troubleshooting.

A typical start-up sequence

The USAID RISE standard operating procedure for PSA plants, written for plants where the air dryer comes before the air receiver, sets out these steps:

Check the voltage in the power panel, and keep the oxygen receiver inlet and outlet valves and the air receiver outlet valve closed.
Start the air dryer before the air compressor and wait until its dew temperature reaches the set value, usually less than 7 °C.
Start the compressor. At around 70% of the required pressure (3.5 to 4.9 bar in the air receiver), open the air tank drain for 5 seconds and check the filter drains for water, every 5 minutes for 5 cycles; if water appears, do not let air into the generator.
Open the air receiver outlet gradually, check that the valves follow the PLC programme, and watch for abnormal sounds, exhaust blowdown or powder discharge at the silencers.
Watch purity on the operator screen while raising pressure gradually. Once oxygen pressure exceeds 4.2 bar and purity is 93% ± 3%, wait 5 minutes, then fully open the oxygen receiver outlet if the plant feeds the hospital pipeline.

Start-up time and the backup

AIGA 113/20, adopted from EIGA Doc 195, says that when a concentrator unit is the secondary source, the duration of its start-up has to be taken into account in sizing its oxygen buffer, which must supply the nominal flow at the required pressure throughout the start-up, the worst case being considered. It also requires a written start-up procedure: before any restart, all process parameters, including online analysis, are verified within the specified limits; after starting, operating conditions are checked before the plant runs unattended and re-checked after several hours of running.

At commissioning

Global Fund commissioning guidelines (28 August 2024) ask for an uninterrupted run of at least 24 hours, with 3 days recommended, at the plant's maximum flow rate and with purity above 90% at all times. WHO's Foundations of medical oxygen systems (2023) lists a running test to reach nominal performance, with a minimum of 3 days of continuous running, among the tests and reports a vendor provides.

ULTRAOX plants are parameterised and tested in the factory and ready to start on site, and they start and stop oxygen production automatically in response to demand; the ULTRAOX SC switches between compressors and starts its generators in cascade to keep the supply continuous, according to the datasheets. See also why a PSA plant needs an oxygen receiver.