A Written Scheme of Examination is the engineering document, required under the Pressure Systems Safety Regulations 2000 (PSSR), that sets out which parts of a pressure system must be examined, how, and how often. It is drawn up or certified by a competent person, and a relevant pressure system cannot legally be operated without a current one. This guide explains what a PSSR Written Scheme contains, who can certify it, and how to keep it valid.
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 document that defines what gets examined, how, and how often, and the sections below walk through its contents and 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.
Frequently asked questions
What is a Written Scheme of Examination?
It is the document required under PSSR 2000 that identifies the parts of a pressure system to be examined, the type and interval of each examination, and the preparation needed. It must be drawn up or certified by a competent person.
Is a Written Scheme of Examination a legal requirement?
Yes. Under PSSR, a relevant pressure system must not be operated without a current, certified Written Scheme covering it. Operating without one is an offence.
Who can draw up a PSSR Written Scheme?
A competent person with the practical and theoretical knowledge to examine the system, and enough independence to certify it objectively. In practice this is an external inspection engineer.
How often must the examinations happen?
The Scheme sets the interval for each part, based on engineering judgement rather than a single fixed figure. Intervals commonly range from around 14 months for some systems to longer for others, always as specified in the Scheme. Our inspection frequency guide puts this alongside the other regimes.
What if I have inherited a system with no Written Scheme?
It must be drafted and certified by a competent person, and the first examination completed, before the system is returned to normal operation. See our PSSR inspection service for how we handle this.