This is my understanding in accordance with BS6891, and if anyone has anything to add then please feel free. If the pipework is exposed in the protected area, ie not inside it’s own purpose made duct, then there are a couple of options, but compression fittings cannot be used in any circumstance.
1. If there is ventilation at both high and low level direct to outside air in the protected area then BS6891 clause 8.19.1 states that the following can be used:
a) steel pipework with screwed joints;
b) steel pipework with welded joints;
c) continuous length of copper (no joints) or;
d) continuous length of pliable corrugated (stainless-steel) tubing manufactured to withstand fire test A of BS EN 1775:2007, Annex A.
2. If there is no ventilation at high and low level in the protected area then the pipework can still be exposed to, and run through the protected area in accordance with clause 8.19.4, whereby the pipe work must be continuous or welded and meet the requirements of 8.19.1b), c) or d), provided that the protected area is normally occupied. This means normally used common lobbies or corridors that have regular passers through.
There are some different requirements where the pipe runs through suspended ceilings and if it is run in its own purpose built duct.