When Andy breaks into the sewrage pipe, sewrage spurts out all over the place. Then when he crawls out of the end we see that the sewer is open-ended, so why would there be so much pressure in the pipe? It's also only about a quarter of the way full!
Sorry, but you are right - you aren't doing very well at that course! The pipe is an open system, and fluid dynamics (or hydraulics) are irrelevant. In any closed system even the tiniest fracture will equalize inner and outer pressures (given enough time) and the opening in the pipe from which Andy emerges is as wide as his shoulders! There is simply no way the fluid pressure in the pipe could be any higher than atmospheric pressure. One possible solution: a solid blockage of congealed sewage is being forced down towards the end of the pipe, and behind it is a large fluid mass. This is passing under Andy just as he hits the pipe. Big coincidence (and we saw no evidence of such a blockage) but that's Hollywood!
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Hydraulics aside, the director may just want to emphasize that Andy is about to crawl through human sewage in order to escape. If he were to break the pipe and nothing came out, viewers might think it is an empty pipe or a water pipe. Also, the gushing sewage gave Tim Robbins the chance to show the disgust on his face that wouldn't have been as apparent to the camera once he entered the pipe.
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The pipe could certainly be under pressure in one spot and open to the atmosphere elsewhere IF there was a change in elevation. Sewage can be heavier than water. A few feet of standing column could cause a real gushing.
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I'd like to know how 'Andy' knew...after crawling 'five football pitches' that he could quite simply kick off the grate at the end of the sewer? Could you imagine if he couldn't and had to crawl back???
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First, when Andy first comes into contact with his escape pipe, we can see that this section of pipe is level. If we assume for the sake of argument that the pipe is off level for some length of its total run, one could argue that the pipe could burst forth with some pressure. However, we need to remember that water seeks its own level. If the pipe was clogged at some point downstream from Andy's entry point, AND there was a considerable portion of pipe at a higher elevation from where Andy gained entry, then yes, the pipe would burst forth. However, if such was the case, the eruption would stop as soon as the water reached a static level. Further, the eruption would stop at a point when the water was equal in height to the top of the pipe; meaning that Andy would have to be submerged until he reached the clog downstream.
The camera clearly shows Andy breaking through the pipe and then immediately looking inside to see the pipe partially full. Not possible. Fluid dynamics can not support this turn of events.
Lastly, consider how a sewage pipe, so full of pressure, would allow anybody to successfully flush their toilets. Flushing aside, the toilets would backflow into the cells.
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The pipeline Andy sits on is perfectly level, and the outflow is only slightly lower than the hole he cuts. It is shown clearly in a long shot. None of this matters - the pipeline must be level or very close to it as it is a quarter full of liquid sewage. It is moving slowly, or not at all. If it was "tilted" up or down relative to ground level one end would be full of liquid sewage and the other totally dry, and the sewage would be flowing down the pipe very rapidly. We see a gentle outflow gurgling away as Andy slides out - therefore the pipe is level, or the next best thing to it.
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I'm taking a fluid dynamics class right now, unfortunately I'm not doing so well in it, but I think I did pick up on a few things...the pressure at one point in a pipe isn't necessarily the same somewhere else in the pipe and lots of variables can be responsible for that, especially in a long pipe.
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