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Old 07-19-2010, 06:36 PM   #1
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Water pressure vs velocity

I'm wondering what the difference is, and if they're related somehow. I know that higher velocity water=low pressure, but that doesn't mean low pressure=high velocity.

Due to Bernoulli, the faster water travels, the lower the pressure is, but can't pressure and velocity both be used to the same end? For example, in head height, water can be pumped using pressure and velocity.. right?

I'm having trouble relating the two in my head... High pressure water at low velocity... high velocity water at low pressure... I don't think I get it
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Old 07-19-2010, 07:07 PM   #2
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Ughh too much for my brain. Next?
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Old 07-19-2010, 07:14 PM   #3
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You need some type of pressure to develop flow (velocity), but other than that the two are not related.

As with head height like you mentioned, the pressure is doing the work (kinda) and generating the flow. For a given pump, if you increase the head height you are pumping, the flow will decrease. Eventually, when you max out the allowable pressure (head) the pump can put out, you will have no flow.

If electrical stuff makes more sense to you, you can use the following analogy: pressure=voltage and flow=current.

{Edit: thinking more about your original question, your confusion might be coming from thinking about "velocity" versus "flow". Flow is a measurement of volume per unit of time... like gallons/hour. Velocity is a measurement of distance per unit of time... like feet/second. Pumps are rated for a maximum flow, and a maximum pressure. Now what the actual "velocity" of the liquid is... that all depends on your tubing or piping diameter. Bigger pipes = slower velocity, smaller pipes = faster velocity... but the "flow" rate is the same regardless. Hopefully that makes sense...]
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Old 07-19-2010, 07:28 PM   #4
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I spent alot of time thinking about this, and I think I may be confusing myself, but I'll walk through it anyways

You have a pipe that narrows. For the flow to be constant, velocity must increase. When velocity increases pressure decreases. (Bernoulli's says when a fluid has more velocity, it has less pressure, which is why planes fly). You can have a large pump that can pump alot of water, but at a low pressure. From what I've read (mainly for mazzei), you can't just get a pump with alot of flow, decrease the diameter of the pipe, and substitute with velocity. You need a high pressure pump because.. I guess to get through the small opening at the center of the mazzei injector.

What's getting me is, shouldn't an amount of water be able to get through the hole at low pressure, but high velocity, because its momentum should be able to carry it through. I know I'm wrong, but I want to know why.
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Old 07-19-2010, 07:45 PM   #5
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I see what you're driving at now...

Again... velocity and pressure are NOT related to each other. You're trying to make them dependent, and I think that's where you're getting messed up. You can't "substitute" velocity for pressure... they're two different things. Think of it like electricity - you need a certain amount of Volts (voltage) for something to work... just having a lot of Amps (current) doesn't do the trick.

Regarding getting fluid past an obstruction, like narrowing pipes or an injector, yeah... you get a certain pressure drop as the fluid passes through the obstruction. In your example of a narrowing pipe, if you picture that fluid with a given pressure coming through the pipe, going in to a smaller pipe (which increases the velocity, but not the flow), and then going back into the original size pipe again, the velocity will be the same as it was before but the pressure will be lower. That's why you need a certain amount of pressure to get through the obstruction - it's not the velocity that is "pushing" the water through... it's the pressure. And if you have too low of pressure you can't get the water through to start with.
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Old 07-22-2010, 05:01 PM   #6
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Firstly, it should be speed, not velocity, unless you are talking about a definite direction.

And secondly, I think you are talking about flow rate but not speed.

Thirdly, higher flow rate = higher pressure (think fire hydrant); lower flow rate = lower pressure (think... peeing?). That's the general rule. But other factors do apply, such as the direction of flow, height of water above ground, change in elevation during flow, change in diameter during flow, viscosity, divine intervention, surface friction between fluid and tubing, etc.
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Old 07-22-2010, 06:39 PM   #7
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There's a lot of undefined terms in the original post.

For water, the basic equation is v^2 + 2gz + p = constant where v is velocity along a streamline, g is gravitational acceleration, z is elevation, and p is pressure at a point. If the pipe is horizontal, z = 0, so the equation becomes v^2 + p = constant. Basically, this breaks down to pressure being related to the velocity squared.

Think of it this way: You're pushing a shopping cart. When it's empty, you can go really fast, but it won't make a big dent when you run it into a parked car. Now fill that shopping cart with bricks. Can't push it as fast, but it sure makes a mess out of that car.

It's a rough example, but you get the picture.
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Old 07-22-2010, 11:35 PM   #8
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Quote:
Originally Posted by agp View Post
Firstly, it should be speed, not velocity, unless you are talking about a definite direction.

And secondly, I think you are talking about flow rate but not speed.

Thirdly, higher flow rate = higher pressure (think fire hydrant); lower flow rate = lower pressure (think... peeing?). That's the general rule. But other factors do apply, such as the direction of flow, height of water above ground, change in elevation during flow, change in diameter during flow, viscosity, divine intervention, surface friction between fluid and tubing, etc.
You can have alot of flow and low pressure. Conversely you can have high pressure and relatively low flow. This is one thing that confused me, as to me, pressure naturally seemed like it was related to momentum of water, ie think ball on slope. More momentum (KE or velocity) = more potential energy (head height).
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Old 07-22-2010, 11:39 PM   #9
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Quote:
Originally Posted by BigJim View Post
There's a lot of undefined terms in the original post.

For water, the basic equation is v^2 + 2gz + p = constant where v is velocity along a streamline, g is gravitational acceleration, z is elevation, and p is pressure at a point. If the pipe is horizontal, z = 0, so the equation becomes v^2 + p = constant. Basically, this breaks down to pressure being related to the velocity squared.

Think of it this way: You're pushing a shopping cart. When it's empty, you can go really fast, but it won't make a big dent when you run it into a parked car. Now fill that shopping cart with bricks. Can't push it as fast, but it sure makes a mess out of that car.

It's a rough example, but you get the picture.
So, the thing that gets me, is that when you reduce pipe diameter, velocity goes up and pressure goes down. Shouldn't pressure end up being proportional to the negative of velocity squared?
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Old 07-23-2010, 11:43 AM   #10
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Quote:
Originally Posted by BigJim View Post
...
For water, the basic equation is v^2 + 2gz + p = constant where v is velocity along a streamline, g is gravitational acceleration, z is elevation, and p is pressure at a point. If the pipe is horizontal, z = 0, so the equation becomes v^2 + p = constant. Basically, this breaks down to pressure being related to the velocity squared.

Think of it this way: You're pushing a shopping cart. When it's empty, you can go really fast, but it won't make a big dent when you run it into a parked car. Now fill that shopping cart with bricks. Can't push it as fast, but it sure makes a mess out of that car.

It's a rough example, but you get the picture.
This is true with gravity fed systems, but doesn't hold when the prime mover of the fluid is something other than gravity. The amount of power a pump puts out will determine your pressure and flow. You just need to make sure the pump is sized to give you both the pressure and flow you need - whether it's high pressure/low flow, high pressure/high flow, low pressure/low flow, or low pressure/high flow. You can have whatever you want with the use of flow control valves, pressure reducing valves, etc. The pump will just dictate what the maximums are.
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