IMO BLG decided in March to agree with a Danish proposal for clarification of certain sections
of IMO MSC/Circ. 677. Accordingly, devices to prevent the passage of flame, i.e., high velocity
valves and flame arresters, including gas-freeing covers and vacuum valve inlets, shall be certified
using a test gas with the same flammability as the most dangerous one on the ship's cargo list.
Flammability is defined as Maximum Explosion Safe Gap (MESG) and shown in millimeter or as a gas
group. Of relevance for tankers, group IIA equals 0.90 mm and group IIB equals 0.65 mm.
The IMO decision in March 2009 deviates from then existing class practice, which was explained in
detail to the IMO.
Ship owners will therefore face sub-standard equipment are therefore granted a grace period
until first docking after 1/1 2013 to have the necessary equipment installed.
Manufacturers, however, should immediately take heed from the IMO decision in March and cannot
wait until 2013 before starting to deliver what is called for by the IMO decision in March.
As from July 21, 2009, all devices to prevent the passage of flame should - according to EN 12874 -
be marked with the approved explosion group. The typical high velocity p/v valve with integral gas-freeing
cover should therefore carry 3 such markings; one for the pressure side, one for the vacuum inlet protection,
and one for the integral gas-freeing cover.