ABS OFFSHORE SUPPORT VESSEL GUIDELINE

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ABS的离岸供应船的指南中，第5章第6节关于溢油回收船的一处要求(5-6-4/17.1.2)
17.1.2 Portable Foam Applicator Units
Two portable foam applicator units are to be provided. A portable foam applicator unit is to c***ist
of an air-foam nozzle of an inductor type capable of being connected to the fire main by a fire hose
together with a portable tank containing at least 20 liters (5 U.S. gall***) of foam-making liquid.
The nozzle is to be capable of producing effective foam suitable for extinguishing an oil fire, at the
rate of at least 90 m3/hr (3180 ft3/hr). A total of at least eight portable tanks of foam-making liquid
are to be supplied on board. In instances where, due to the size of the vessel, only one fire hydrant
is required to be fitted, the vessel need only be provided with one portable foam applicator unit.
这段话该怎么理解。。前面要求船上装备2个泡沫灭火器，后面又说考虑到船体的尺寸，允许只装设一个。
这个尺寸又没有个明确的说法，怎么理解？

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无人解答下？