Yes, when the maximum allowable pressure PS they can be exposed to is above 0,5 bar, flame arresters and flash back arresters are covered by the PED and, in general, should be considered as pressure accessories.
Such flame arresters are generally also covered by ATEX directive. In that case, they are excluded from PED if they do not exceed Category I (Article 1 paragraph 2 (f)).
Specific solutions to essential safety requirements shall take account of the potential explosion, as a result of the hazard analysis; the essential safety requirements from ATEX directive need also to be taken into account.
Note 1:
In accordance with Article 2 (8), PS would be the maximum pressure for which the flame arrester housing is designed. PS is not necessarily the explosion pressure; in any case the explosion pressure shall be taken into account and may be considered as a load case following the hazard analysis (see Annex I section 2.2.1).
Note 2:
In general, the flame arresters will be classified using Annex II Table 6.
Note 3:
For the definition of flame arresters, see EN ISO 16852:2010