According to the WSH Act, the following air receivers must be registered:

• Any vessel (other than a pipe or coil or an accessory, a fitting or part of a compressor) containing compressed air and connected with an air compressing plant.
• Any fixed vessel for containing compressed air and used for the purpose of starting an internal combustion engine.
• Any blast air bottle.
• Any vessel in which any substance in the form of solid or liquid is stored and from which it is forced by compressed air.

You do not have to register air receivers where the safe working pressure does not exceed 0.5 bar or the product of its safe working pressure and volume does not exceed 100 bar litres.

This information is sourced from MOM.