My brother is a professional plumber and I'm a physics major, so he often comes to me for explanations of some of the physics underlying his job. I'm familiar with the basics of fluid mechanics, e.g. hydrostatic principles, continuity equation, Bernoulli's law, Poiseuille's law. But often I struggle to help him because I am ignorant of how real-world water distribution works. So I have a few questions, mostly regarding water pressure regulators/reducing valves (don't know if those two are the same). If you're kind enough to answer, please don't get TOO technical. Please stick to the simplest possible regulator that can be used to answer my questions. In fact, here is a diagram of the simple regulator I'm using to think about it: Here is my basic understanding of how these regulators work. As downstream demand changes, this changes the pressure in the pipes. But the pressure in the pipes ideally would stay the same. The regulator's operation allows demand to be met while counteracting accompanying pressure changes to keep pressure within acceptable limits. As for how this is accomplished, when someone opens a tap, pressure (downstream from the regulator) drops. So the regulator needs to increase the pressure. This happens because, since the pressure dropped, the spring force downward is now able to push the valve down, which actually enlarges the opening, allowing more water flow, which increases the pressure. This happens until the pressure increases enough so that the spring force is now counterbalanced by the upward pressure, and the valve stays in place, i.e. constant pressure. And similarly for an increase in pressure, but everything is reversed. Finally, all of this can be calibrated to a desired constant pressure by adjusting the initial force of the spring. Here are my questions. 1. When the tap is opened - e.g. someone is using the faucet - the water pressure in the piping drops, correct? Is this due to Bernoulli's principle, i.e. as the water's velocity increases, its pressure decreases? In other words, after opening a tap, we would expect a drop in the pressure in the pipes, but the reason we don't observe a drop is because the regulator is in place to counteract that drop? 2. My basic understanding is that open tap > lower downstream pressure > enlarged opening > more flow > increased downstream pressure is this correct? 3. In the literature, when 'flow' is mentioned - as in, increased flow leads to increased pressure - how exactly is this flow defined? Is it flow rate, in units of volume per second? Or is it something else? 4. My biggest question is the physics of why increased 'flow' results in increased pressure. I know that, according to Poiseuille's law, flow rate and pressure difference across a section of flow are directly proportional to each other. Is that what's happening here? I'm more used to thinking of an increase in pressure difference as giving rise to an increase in flow rate, but it could work the other way around, couldn't it? 5. To what extent does Bernoulli's principle apply to this situation? I did read in one of the texts I found that Bernoulli's principle does not apply to flow around a valve, which is what I'm considering here. At the same time, I feel like it must enter into the relationship between flow and pressure in some way. Thanks for any help anyone can give in understanding these issues!