لَمْ يَجِدِ الْعَطَبَ فِي الْأُنْبُوبِ، بَلْ وَجَدَهُ فِي الصِّمَامِ.
He did not find the defect in the pipe; rather, he found it in the valve.
Bal corrects a presumed location. The pronoun -hu in wajadahu refers to the same defect; the expression does not require two defects. The contrast rejects the first location and affirms the second.
A construction coordinating 'in the pipe and in the valve' would assert defects in both locations. Here the communicative purpose is correction of attribution, so the adversative relation matters.