There are two elements to this 1)
The whole story is told from the perspective of Red, who wasn't there at the time and told it with dramatic effect.
2)
The pressure may have built up and the explosion was a sudden release, afterwards the pressure was less.
Either way, it is pretty unpleasant.
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I've done some more digging on this according to moviequotesandmore When the top of the pipe is broken open, it shoots a fountain, indicating that it is pressurized (if only by gravity).
But then the pipe drains nearly empty.
It should only have drained to the lowest point in the hole broken at the top.
and there's an even more detailed discussion on Slipups That discusses such things as fluid dynamics , the angle of the pipe and the effectiveness of the shot i.e. getting the audience to feel revolted for what Andy had to go through.
I think that this was a deliberate mistake that the director decided would enhance the shot.
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I worked in underground utlities and sewage.
There is no reason.
The pipe diameter was not large enough for the fountain, so to speak.
The only way that the pipe would have produced this would be by a pump, and the pipe was breeched when the pump was working to release accumulated waste in some sort of tank.
I also think it was just for effect.
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Dual use sewage system To keep from having open grates in the yard (security issue), and pipes along the outside of the building (climbing holds), rainwater from the roof might have been linked to go down the sewage line.
If there was a heavy rain that created a backup pooled on the roof before he broke the pipe, and now the rate of rain had diminished, that backup would only started to diminish but still be putting pressure in the pipe.
In this case, the water would fountain until the backup water on the roof was fully released.
Sewage systems are designed to not have waste just sitting in a pipe.
Waste sitting in a pipe causes sedimentation leading to a clog.
If he is breaking out of prison in the middle of the night, what caused there to be a flow of anything in the system at all?
Rainwater would explain why there was a flow in the sewage system.
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I agree it was "just for effect".
If the pipe had had a DIP in it (and Andy had been operating at the "dip"), then pressure could have built up to create the fountain.
The dip would have to have been at least as deep as the fountain was high.
Problem with this is that the pipe would have remained filled and been unnavigable.
Yes a pump in operation at the exact moment of opening could have produced the fountain (and then astonishingly conveniently ceased five seconds later) -- but it seems clear the system was "gravity fed" and not pumped (therefore the pipe carefully laid without dips).
There is no conceivable way for there to have been fluid pressure sufficient for the fountain, AND then for the pipe to be mostly empty.
The "shock value" was for audience sympathy ("EEEWWWWWWW!"), most not realizing it was after all just water, sawdust, and chocolate syrup...
I want to know how he knew there was going to be a STORM that night -- escape would have been so much harder without the covering thunder (and natural bathing).
And how did he get to town, where did he change,
when did he buy the car, why there weren't roadblocks, etcetera...
Let's pretend the town was near enough for walking
(he had nearly all night), the rain ceased, and midway he found a closed gas station with an unlocked bathroom for changing & grooming.
He visited one bank in town under the first alias ("Stevens") receiving enough cash to buy a car; which he did, under his second alias.
He then drove to other TOWNS (the movie states "nearly a dozen banks in the Portland AREA"), therefore eluding road blocks.
As he drove away from the first bank, he stopped long enough to buy the volcanic-glass-rock and the tin, and plant the already-written letter at the Buxton field
(he had a lot of time to prepare as he worked in Norton's office).
The glass rock could have been left 20 years ago as a memento, but he couldn't have been sure it would still be there; he likely knew where he could buy one to mark the tin.
THEN he drove to other towns and emptied the other accounts as fast as he nonchalantly could, then he headed South to Fort Hancock in his new convertible with with the sun sparkling off the ocean in his eyes, cool breeze stroking his hair, and an incredible song of freedom singing in his heart. ;-)
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I have a question regarding the end of  The Shawshank Redemption  when Red is narrating Andy's escape - can some one explain why the pipe erupts when Andy breaks it when he is escaping from Shawshank?  I would understand if the pipe was under constant pressure, but in the next scene we see that the pipe is only a partially filled.  Was this a goof or is the a logical reason for it?
Why did the pipe explode in The Shawshank Redemption?
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Pipes along the outside of the building to remove rainwater from the roof might have been linked to a sewage line. So, if there was heavy rainfall that had created a backup in the pipes after breaking the pipe the water wouldn't stop coming out of it until  all the  water from the roof was released. Sedimentation occurs when waste sits in a pipe, resulting in a blockage.
Rainwater from the roof might have been linked to go down the sewage line. Heavy rain could have created a backup pooled on the roof before he broke the pipe. Sewage systems are designed to not have waste just sitting in a pipe. The water would fountain until the backup water on the rooftop was fully released.Sedimentation occurs when waste sits in a pipe, resulting in a blockage.