It depends on the pressure, not on the size or the contents. The Pressure Systems Safety Regulations 2000 apply to systems containing a relevant fluid, and a relevant fluid includes a gas or a liquid under pressure greater than 0.5 bar above atmospheric pressure. A pressurised cryogenic vessel operating above that threshold falls within scope. An open necked, non pressurised storage dewar does not.
Where the regulations apply, the user must have a written scheme of examination drawn up or certified by a competent person before the system is operated, and the system must then be examined in accordance with that scheme. HSE guidance INDG178 sets out what a written scheme covers and what the duties are.
Two points are commonly misunderstood. First, the written scheme is not the same thing as a service visit. It is a document specifying which parts are examined, how, and at what intervals, and it is the basis on which examinations are carried out. Second, the duty sits with the user of the system, not with the supplier. Buying a compliant vessel does not discharge it.
If you are unsure whether a particular vessel is in scope, the operating pressure on the nameplate settles it. Where a vessel sits close to the threshold, or where the configuration has been altered since installation, take the question to a competent person rather than reasoning it out internally.
