http://www.cdc.gov/niosh/face/in-house/full9014.html"After climbing down to the grating level the victim installed an inflatable rubber sewer plug into the inlet sewer pipe. The
pipe was flowing about one-third full with sewage. The plug was inflated (using an air hose attached to a cylinder of compressed air) until it closed off the pipe and stopped the flow of sewage."
How does a pipe "flow"? A liquid could flow through a pipe, but a pipe itself is fixed in space.