The chart in Appendix 1 of the ACOP, which is L122, is extremely useful. Relevant fluids include gases, steam, and liquid nitrogen. Examples of things that require Written Schemes include Fancy coffee machines that deliver steam Liquid nitrogen tanks that are pressurised where the liquid volume x the pressure exceeds 250 bar litres Compressors, according to the same pressure volume product rule. Fixed pipework attached to gas cylinders which are themselves greater than 250 bar litres. Note that the gas cylinder, if it is rented from a gas supplier does not belong to you, so the responsibility for the written scheme remains with the supplier.