ABS的近海供应船规范求解

lmw0320

ABS的近海供应船规范中（大船规范应该也是一样的)，关于无火星型风机是这样描述的： 4-8-3/11.1.1 Air Gap
The air gap between the impeller and the casing is to be not less than 10% of the shaft diameter in way of the impeller bearing but, in any case, not to be less than 2 mm (0.08 in.). It need not be more than 13 mm (0.5 in.).
而4-8-3/11.3.3 iv) 又写道 Any combination of ferrous (including austenitic stainless steel) impellers and housings with not less than 13 mm (0.5 in.) tip design clearance.  
4-8-3/11.3.4 iii) 也写道 Any combination of ferrous impellers and housings with less than 13 mm (0.5 in.) design tip clearance.
感觉有点前后矛盾了。。
求高手解释下。。谢谢