I need to replace a failed electric valve in my drip irrigation manifold. The pipes leading to each valve are 3/4" PVC, but they in turn are enclosed in what looks like black ABS pipe, at least 1/2" wider than the PVC. These pipes are open at the top. What is the purpose of the black pipe? Protecting the PVC somehow? I need to cut some of the black pipe away in order to get to the PVC so I can replace the valves. Any ideas?