How do pipes not burst when you turn off a faucet. Shouldnt all that stopped up water build up pressure and explode?
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Generally speaking, there are three kinds of physical energy stored in a fluid: Pressure, Flow and Elevation. I’ll go through them in context of our faucet.
The elevation energy of our faucet and source does not change regardless of the faucet being open or closed. This is typically the energy used to push water through your faucet out of a water tower or reservoir.
The pressure energy on our faucet is mostly negligible as well, as no pressure can be built in a system exposed to atmosphere. While some systems include some form of pressure element, the output of the faucet is not pressurized per say. This is still important for later, so don’t count it out yet.
The flow energy is the energy you are familiar with, and is directly converted from the elevation energy aforementioned. It’s a combination of the velocity the water is being pushed and the mass of water being moved.
Now earlier, I said there’s no pressure in the pipes. That’s only technically true: The elevation of the reservoir is converted into pressure in the pipes when the faucets are closed: Around 40 PSI in a properly regulated system.
When you open the faucet, that pressure energy is pushed into flow energy, as you would expect since water comes out with mass and speed. Now we’re done, and we close the faucet.
That flow energy doesn’t disappear, and is pushed back into the pressure energy in the piping system. This is in addition to the elevation energy from the water tower that normally goes into the system. This is the effect referred to as “Water Hammer”, as everything in your water system gets slammed by that pressure spike. In an unprepared system conveying significant mass and velocity of water, this can spike to 100+ PSI, doing exactly what you fear: Damaging fittings, loosening connectors, even eventually bursting a pipe.
It is however, entirely manageable: The simplest trick is to simply out-engineer it: If you build your pipes to handily ignore 100 PSI (Or include a regulator to put the general pressure well inside safe margins), you don’t even need to be concerned. In many cases however, fragile components are unavoidable, so instead, you add something to “catch” the shock. While water cannot be compressed, Air chambers or a membrane capped spring can be, and are typically installed near faucets to catch the pressure spike.
You can even exploit this effect in a device called a hydraulic ram: You let flow energy build until it slams shut a valve, and use the resulting pressure spike to push water up through a pipe. [https://en.wikipedia.org/wiki/Hydraulic_ram](https://en.wikipedia.org/wiki/Hydraulic_ram)
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