# Why does a PSA generator stop on low feed air pressure?

**Why does a PSA oxygen generator stop or alarm on low feed air pressure, and what should be checked?**

The ULTRACON PSA generator datasheet lists a low inlet pressure shutdown, and AIGA 113/20 asks for gas pressure and temperature to be monitored at each stage of air compression, with alarms and automatic shutdown when thresholds are reached. NHS England's HTM 02-01 (2026) explains that PSA separates oxygen by using pressure.

## Why the feed pressure matters

HTM 02-01 (2026), Part A, paragraph 6.110, says oxygen concentrators operate by using pressure to adsorb the other atmospheric gases on to materials with specific physico-chemical properties. The Global Fund and Build Health International briefing note on PSA, VPSA and VSA (4 March 2025) explains that nitrogen sticks to the surface of the zeolite under pressure; in a PSA plant an air compressor raises the incoming air to 4.5 to 7 bar, with an air storage tank before the oxygen generator, and because the incoming air pressure is high, the oxygen at the end of the system can go directly to wall-mounted outlets at 3.5 to 5 bar.

The ULTRACON datasheet says the generator produces oxygen from compressed air fed to the unit, and in the high pressure step the adsorbent retains the contaminants present in the compressed air.

## Shutdowns and alarms

The ULTRACON datasheet lists a low inlet pressure shutdown alongside a high pressure safety shutdown, safety valves and a time meter. AIGA 113/20, adopted from EIGA Doc 195, asks for the operating gas temperature and pressure to be monitored at each stage of compression, with alarms and automatic shutdown when thresholds are reached (section 6.12). The Global Fund's operator procedure template (9 December 2025) includes checking that all alarm systems work, naming pressure, power failure and purity alarms, and testing emergency stop buttons and interlocks, at a frequency set by national guidance. None of the documents cited here gives a trip setting for low feed air pressure.

## What to check

The documents do not give a single checklist for low feed air pressure. The tips for low pressure in the oxygen tank in a USAID RISE procedure written for PSA plants in hospitals in India (March 2022), and the Global Fund daily maintenance checklist, cover the air supply:

- Air path: any obstruction to the air flow near the compressor, pre-filters clean or changed, and all valves fully open (USAID, low pressure in oxygen tank).
- Intake: the inlet air filter in good condition, and air intake filter mats, intake cover and ventilation filters in place (Global Fund daily checklist).
- Compressor: condensate draining correctly, compressor temperature recorded, and any alarm codes identified and photographed (Global Fund daily checklist).
- Start-up order: start the air dryer before the air compressor where the dryer is followed by the air receiver, and open the receiver outlet valve gradually to equalise the pressure (USAID).
- Demand and leaks: production against consumption, leaks found with the leak test procedure, and the oxygen tank gauge or transmitter, which may need calibration. The USAID procedure warns that drawing more oxygen than the plant's capacity leads to a drop in purity and pressure.
- If the tips do not solve it, the manufacturer or supplier should be contacted (USAID).

On the supply side, HTM 02-01 asks for compressors for PSA systems to comply with medical air requirements, says the medical air plant should not be used for the PSA plant (6.117), and asks for a duplex compressor; where more than two compressors are installed, the system should be capable of delivering the design flow with one compressor out of service (6.116).

## On ULTRAOX plants

The ULTRAOX SC and ULTRAOX ST model tables give 7.5 bar at the compressed air inlet and 6 bar at the oxygen outlet on every model, and the ULTRAOX ST datasheet adds that compressor power and pressure can vary with the service conditions required for the facility. In both plants, the compressors switch and the generators start in cascade to keep a continuous supply of oxygen (datasheets, p. 2). See [how the air compressor for a PSA plant is sized](https://library.ultracontrolo.com/answers/how-is-the-air-compressor-for-a-psa-plant-sized/).

### What feed air pressure does an ULTRAOX plant use?

The ULTRAOX SC and ULTRAOX ST model tables list 7.5 bar at the compressed air inlet and 6 bar at the oxygen outlet for every model; the ST datasheet notes that compressor power and pressure can vary with the service conditions.

### Can the hospital's medical air plant feed the PSA plant?

HTM 02-01 (2026) says it should not: the medical air plant should not be used for the PSA plant, and compressors for PSA systems should comply with medical air requirements.

## Sources

- [ULTRACON datasheet, p. 2](https://www.ultracontrolo.com/products/ultracon)
- [ULTRAOX SC datasheet, p. 2 and p. 5 (model table)](https://www.ultracontrolo.com/products/ultraox)
- [ULTRAOX ST datasheet, p. 2 and p. 5 (model table)](https://www.ultracontrolo.com/products/ultraox-st)
- [NHS England, HTM 02-01 (2026) Part A, paragraphs 6.110, 6.116 and 6.117](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [Global Fund / Build Health International, Briefing Note: PSA, VPSA and VSA (4 March 2025)](https://resources.theglobalfund.org/media/dp3clsrr/cr_c19rm-psa-vpsa-vsa_technical-briefing-note_en.pdf)
- [AIGA 113/20 (adopted from EIGA Doc 195), On site generation of oxygen 93% for medicinal use, section 6.12](https://asiaiga.org/uploaded_docs/en_AIGA_113_20_Generation_of_Oxygen_93_for_Medicinal_Use.pdf)
- [USAID RISE, SOP: PSA Plant Operations and Preventive Maintenance (March 2022)](https://media.path.org/documents/SOP_on_PSA_Plant_USAID_RISE_04032022_F7fUBr6.pdf)
- [Global Fund / Build Health International, PSA plant Daily Maintenance Checklist (Project BOXER)](https://resources.theglobalfund.org/media/xbanfgrc/cr_c19rm-psa-plant-daily-maintenance_checklist_en.pdf)
- [Global Fund / Build Health International, Modèle de procédure opérationnelle standard pour les opérateurs de centrales de production d'oxygène (9 December 2025), alarm and safety device checks](https://resources.theglobalfund.org/media/atepwfny/cr_c19rm-sop-oxygen-plant_template_fr.docx)

Reviewed: 2026-10-23. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/why-does-a-psa-generator-stop-on-low-feed-air-pressure/