Isn’t this kind of the same thing as saying the pump creates the pressure?
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Say you put a pump in the middle of a huge swimming pool, it will create a flow of water around the pump. The water won’t really be at a higher pressure than the rest of the pool though.
Now suppose you put the pump in a pipe. The pressure is there. So it can’t be the pump alone that creates the pressure. The pipe must be doing something.
On the other hand, if you *just* had the pipe with no pump, the pressure wouldn’t be there. So really it’s a bit of both.
Pumps physically move fluid. They create flow. There’s nothing in the pump mechanism itself that creates pressure, they just move fluid around…if you leave the outlet open on a pump you’ll have full flow and zero (ambient) outlet pressure. It’s only by restricting the flow that you get pressure.
Kinda, but no. The kinda is because it matters where you measure it. Sometimes they stress these definitions so you don’t get confused later. This sounds like they are clarifying the difference between the words “flow” and “pressure”, as much as “pump.”
Imagine a pump hooked up to nothing, so that the water just falls out onto the ground. That water is under almost zero pressure aside from, like, the atmosphere.
But, attach a 2″ dia. pipe to it, say 20 feet long. The pump is making water flow into the pipe, and if it’s pumping enough water to basically fill part of the pipe’s diameter, the water is under SOME pressure, because water closer to the pump has to move the other water along before it can flow. Resistance, see?
Now, say we angle the pump upward diagonally. That makes the water farther from the pump harder to move, so pressure in the pipe rises. Cap it off to a tiny hole, and pressure rises again.
Yes, the pump needs to generate pressure inside itself to make the fluid flow, BUT, no matter how hard a pump pumps, how much volume it moves, or how much flow it creates, once the fluid leaves the pump, there is only pressure if there is resistance.
For rotodynamic (spinny) pumps, the impeller technically doesn’t pressurize (squeeze) the fluid. Instead it adds velocity / kinetic energy to the fluid, making it flow. The volute (pump casing / housing) geometry reduces in area as the fluid moves around to the exit of the pump, which converts some of that flow energy to pressure energy.
Think of putting your thumb over the end of a hose. In either case you have flow, but when you restrict the opening you get a much higher pressure stream coming out of the hose.
You can contrast that with a piston pump, which directly squeezes the fluid.
Of course, to a certain extent this is an academic distinction. Ultimately the pump works by adding kinetic energy to the fluid, but the pump also converts that energy into pressure, so in a sense the pump is creating pressure. There’s also the fact that flow, pressure, and resistance are all interrelated and affect each other. If you are running a pump at a fixed speed and you increase the resistance in the system (piping) then the flow will go down and the pressure will go up. So they’re interrelated.
If you are at all familiar with electrical circuits, flow and pressure are analogous to current and voltage. You can’t have flow without pressure and you can’t have current without voltage, and it’s the system resistance that links the two.
A pump moves water. Ambient pressure causes water to enter the pump. The pump captures or spins a volume of water, and physically moves it to the outlet. If the outlet just dumps into a trough, then the outlet water has no pressure. However, if the outlet leads to a pipe, friction with the pipe causes water to resist moving, blocking the passage of water following. Because water is incompressible, this pressure reaches all the way back to the pump mechanism.
Either the pump is strong enough to keep moving water in-spite of this pressure, or it is not. Either way, the pressure’s source is friction of water moving within the pipe and not from the pump itself.
Think of a garden hose. Water exiting the hose has real no pressure until you press your finger over the end of the hose and create a restriction.
It’s not the same thing. If you have to fix, troubleshoot, adjust, maintain, design, the pump.
What makes the pump a pump is that it pumps, moves, displaces, the fluid. As long as this movement is performed the pump is doing the job.
It’s not true for pressure, there are several reasons for a pump to fail to provide pressure or to be unsatisfactory in pressure, but none of this make the pump stopping being a pump.
Example axial compressor: does air come out of it? Yes? It’s working. If the pressure is low there may be inefficiency, the system to which it is connected is malfunctioning, etc. But the compressor is truly dead only if it stops, or it stalls. Both cases, dead flow dead pump.
Centrifugal compressor or pump: same, no flow no pumping. As long as there is flow, the output will have a greater pressure than the inlet. It’s dead only if it stall or stop. And again, stall is a fancy name to say the flow has stopped.
Piston or gear pump: again, what does kill the pump? Stop its movement, or lock the outlet (which will stop the pump or break it). Both cases it would not pump anymore. All the other problems will affect the pressure but not kill the pump.
Seems trivial, but really, if you get this you get what pumps should do all time and how to keep ‘em working properly. You preserve the flow, in any way needed, you preserve the pump’s scope.
And if you know that a pump makes flow not pressure, your can make it really efficient in design, as you know exactly what the pump does and what it needs to be a good pump.
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