Выражение встречается в следующем контексте: Dispensers fitted with relief valve settings above shut-off head of the transfer pump will not lift in normal operation and therefore should not give rise to a primary grade source of release. However, small leakages of the relief valve or fittings within the dispenser result in a Zone 2 area of radius 2 m, around the outside of the dispenser. This distance is equivalent to a leak hole size of 1 mm diameter. The slumping effect of the vapour discharge results in a 2 m extension, at a nominal height of 1 m. The enclosed volume within the dispenser should then be uprated to Zone 1.

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