Under the Pressure Systems Safety Regulations 2000, no relevant pressure system may be operated without a current Written Scheme of Examination. The Scheme is the engineering document that defines what gets examined, how, and how often. This guide walks through the contents and explains the practicalities of getting one in place.
What is a relevant pressure system?
A relevant pressure system stores or processes a relevant fluid above 0.5 bar gauge. "Relevant fluids" include steam, compressed or liquefied gas (including air above the threshold), and any liquid which would change to a gas if released. Vacuum systems below atmospheric are out of scope.
What the Scheme must contain
Regulation 8 sets the minimum content. A complete Scheme names every protective device and every part of the system whose failure could cause danger, the type of examination for each, the maximum interval between examinations, and the nature of any preparation required before examination.
- Identification of the system — owner, location, schematic, design parameters (pressure, temperature, fluid).
- Listed parts — vessels, pipework, valves, gauges, safety relief devices. Each item identified by tag number.
- Examination type for each part — internal, external, or both; with or without pressure testing; visual or NDT.
- Interval for each examination, justified against the system's duty and condition.
- Pre-examination preparation — isolation, draining, cooling, opening of inspection covers.
- Protective device settings — set pressures of safety valves and trips.
- Identification of the competent person certifying the Scheme.
Who can certify it
The Scheme must be drawn up or certified by a competent person as defined in the regulations — someone with the practical and theoretical knowledge to examine the system, sufficient independence from day-to-day operation, and the resources (test equipment, NDT capability) to back the recommendation. In practice this is an external PSSR inspection engineer.
How long it takes
For a typical compressed-air installation — a receiver, distribution pipework, isolation valves and a safety valve — site survey and drafting take half a day. The first examination follows immediately, and the Scheme is in force from the date the certified document is issued.
Larger systems (steam plant, multiple receivers, process pipework) take longer to schematize but the principles do not change.
Keeping the Scheme current
The Scheme is a living document. It must be reviewed and re-issued whenever:
- The system is materially altered — new vessels, repiping, change of fluid
- Operating conditions change — higher pressures, hotter service
- An examination reveals deterioration that warrants a shorter interval
- Ownership transfers (the new owner is the duty holder for compliance)
Common pitfalls
- Assuming a manufacturer's commissioning document is the Scheme — it isn't
- Examining only the vessel and missing the protective devices, which are equally in scope
- Letting the interval drift past the certified date "because we'll do it next visit" — this lapses the Scheme
- Buying a second-hand vessel without checking that an in-date Scheme transfers with it
What happens at the examination
On the date set by the Scheme, our engineer attends with isolation paperwork agreed in advance. Each named part is examined in line with the Scheme — internal where required, external always. Defects are recorded with photographs; corrective actions and a re-examination date are issued in the report.
Where defects affect immediate safety, we issue a notification under Regulation 14 — the system must not be returned to service until the defect is remedied.