Here you can see the blowup section of the pump and its immediate pipeline. The pipelines are opaque and if one has to check, whether the liquid is flowing through the pipeline, then the sight Glass is a useful device. It has two  transparent opposing faces, through which one can see if the liquid flow is there or not. The sight Glass is flange mounted and becomes an integral part of the piping assembly. Here as the pump rotates the blue coloured liquid flows through the pipeline and can be seen through the sight glass.