In {HEAD ~ preferred practice}, {TAIL ~ the pairs} of injectors 524, 526 and 528, 530 are operated in such a manner that, one of the injectors of each pair is permitted to defrost while its mate is in active service 
In { HEAD ~ preferred practice } , { TAIL ~ the pairs } of injectors 524 , 526 and 528 , 530 are operated in such a manner that , one of the injectors of each pair is permitted to defrost while its mate is in active service
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