A method of relieving a condition of over-pressure in a vessel (14, 15) having a burst pressure and a vessel gas outlet, the method comprising: communicating in parallel a plurality of rupture discs (33a-d) with the vessel gas outlet; operatively pairing a first pair (33a, b) of rupture discs amongst the plurality of rupture discs by: providing a first rupture disc with a capacity to rupture at a predetermined lowest level of over-pressure; providing a second rupture disc with a capacity to rupture at a predetermined intermediate level of over-pressure; locating the first and second rupture discs opposite of each other symmetrically with respect to an axis of the vessel gas outlet; and operatively pairing a second pair (33c, d) of rupture discs amongst the plurality of rupture discs by: providing a third and a fourth rupture disc with a capacity to rupture at a common predetermined higher level of over-pressure, the higher level of over-pressure being higher than the intermediate pressure and lower than the burst pressure of the vessel; and locating the third and fourth discs opposite of each other symmetrically with respect to the axis; whereby the plurality of rupture discs are symmetrically disposed about the axis such that thrust induced moments during a relief of an over-pressurized gas from the vessel through one or more of the rupture discs are abated. communicating in parallel a plurality of rupture discs (33a-d) with the vessel gas outlet; operatively pairing a first pair (33a, b) of rupture discs amongst the plurality of rupture discs by: providing a first rupture disc with a capacity to rupture at a predetermined lowest level of over-pressure; providing a second rupture disc with a capacity to rupture at a predetermined intermediate level of over-pressure; locating the first and second rupture discs opposite of each other symmetrically with respect to an axis of the vessel gas outlet; and providing a first rupture disc with a capacity to rupture at a predetermined lowest level of over-pressure; providing a second rupture disc with a capacity to rupture at a predetermined intermediate level of over-pressure; locating the first and second rupture discs opposite of each other symmetrically with respect to an axis of the vessel gas outlet; and operatively pairing a second pair (33c, d) of rupture discs amongst the plurality of rupture discs by: providing a third and a fourth rupture disc with a capacity to rupture at a common predetermined higher level of over-pressure, the higher level of over-pressure being higher than the intermediate pressure and lower than the burst pressure of the vessel; and locating the third and fourth discs opposite of each other symmetrically with respect to the axis; whereby the plurality of rupture discs are symmetrically disposed about the axis such that thrust induced moments during a relief of an over-pressurized gas from the vessel through one or more of the rupture discs are abated. locating the third and fourth discs opposite of each other symmetrically with respect to the axis; whereby the plurality of rupture discs are symmetrically disposed about the axis such that thrust induced moments during a relief of an over-pressurized gas from the vessel through one or more of the rupture discs are abated.