I've seen flowable fill used to seal gravity pipes (storm sewer or sanitary sewer) without any problem. A pressure pipe would require a different method.
you dont really even need to fill the pipe with flowable fill. Concrete brick and nonshrink grout bulkhead/plug would do it. How large is the line? i assum this is a gravity sewer line? The hardest part is getting in/stoping flow. If you are trying to kill a line comming out of a manhole the best way to do it is float an air plug into the line you want to kill blow it up then walk through the plug on the dry side or dig down on it and build a bulk head. depending on the side of the line it may be more practical to bypass the manhole and build your plug from the inside of the manhole.
Any pipe can be filled with flowable fill. All abandoned pipes should be filled. Saves a lot of time when you find a 'strange 'pipe in an excavation. Pull the hose from the upstream end to 10 feet from downstream end. Place a plug in downstream end with a 1 inch pipe on the top of the plug past the end of the plug. Start neat cement grout pumping. Once grout is coming out of the 'overflow' pipe, cap it. Proceed to retrack hose as you pump. watch volume being pumped and extract hose at a rate to always have 25 feet of hose buried in grout. You can use a CCTV camera to wtch if you are concerned about your calculations for volumes.
