Do Pumps Create Pressure or Flow?

There’s a popular and persistent saying that  pumps only create flow in a fluid, and resistance   to that flow is what creates the pressure in  a pipe. That may be helpful in conceptualizing   what’s happening in a pump system, but it’s not  the whole story. In fact, it’s almost identical   to another popular but misleading belief, this one  about electrical safety, that says, “It’s not the   voltage that kills you. It’s the current.”  Well if you know anything about Ohm’s Law,   you know that voltage and current go hand and  hand, and the same is true about the pressure   and flow rate in pipe systems. This is not rocket  science, but it’s not common knowledge either,   even though almost everyone has used or  interacted with a pump before. I’m Grady   and this is practical engineering. In today’s  episode, we’re talking about how pumps work! This video is sponsored by  Curiosity Stream and Nebula.   Get 26% off at the link in the  description. More on that later! Let me just say right at the start that I love  pumps. They’re one of my favorite topics, so   this is the first of two videos I’m doing about  them. Let me know if you want to see more because   there are a ton of topics we can cover. Funny  enough, most engineers working with pumps aren’t   all that concerned with the physics of what’s  happening inside one. They mostly care about   performance. That’s because the most important  job of an engineer designing a pump system is   choosing the right one. That might sound silly  at first. For a small aquarium pump or sump pump,   you usually don’t have to be very thoughtful  about selection - the difference between them   on such a small scale is not that significant.  But, like most things in our industry, those small   variances turn into large ones at scale. As pumps  get bigger, and their roles become more important,   selecting the right one for the job becomes a  critical task. For example, choosing the wrong   pump to supply a city with fresh water or get  rid of floodwaters can be life-or-death. Today   we’ll walk through some of the considerations  engineers use to select the right pump using   demonstrations here in my garage and even give you  some tips if you ever have to choose one yourself. Most pumps used in civil engineering, and indeed  most pumps you’ll encounter in your everyday life,   are centrifugal pumps. That means they  use an impeller connected to a motor   to accelerate the liquid into the discharge  line. If you go searching for pumps online   for smaller applications, you’ll likely see them  listed according to flow rate. That makes sense   because it’s usually what you care about. How  many gallons or liters per minute can I move?   But, for centrifugal pumps, it’s not quite  that simple. Let me show you what I mean.   I have a small fountain pump here rated for 2  liters per minute. If I turn it on and pump this   water into a beaker, it does just about that. It  takes just about 30 seconds to fill one liter.   But watch what happens if I raise or lower the  vertical distance of the beaker above the pump.   In fact, through the magic of video compositing,  I can show you all three at the same time. It’s very easy to see the effect that the  discharge pressure has on the pump’s flow rate.   The higher the beaker, the greater the pressure.  And the greater the pressure, the lower the flow.   To illustrate this further, here’s a graph of my  little experiment with flow rate on the x-axis   and pressure on the y-axis. In this case, I’m  measuring pressure as the height of a fluid   column, also known as head. You can see that  my experiment created a curve on this graph.   In fact, all centrifugal  pumps have a curve like this,   called the characteristic curve. And, at the risk  of this just becoming a video about cool graphs   (even though some might argue that it has inherent  value on its own just by being a cool graph),   in this case, it’s also a means to an end.  Let me show you why it’s so important. No matter what you connect a pump to - whether a  single hose or a complex citywide network of water   mains - it is going to have its own curve  describing how much flow will occur under   different pressure conditions. You can see when  I change the supply pressure by adjusting this   valve, the flow rate through the pipe changes  accordingly. The graph of this relationship is   called the system curve, and it’s different for  every network of pipes from the simple to the   complex. A system with lots of constriction will  have a more vertical curve where, no matter what   the pressure is, the flow rate doesn’t change  much. A system with less constriction will have   a flatter curve where more pressure equals a lot  more flow. A system at a much higher elevation or   higher pressure will have a curve high up on the  graph. System curves can even change over time.   A city’s fresh water distribution system will  have a flatter curve during the day when more   people are using their taps and a steeper curve  at night when the demand for water is lower. Stay with me, because here’s why this matters:  If you plot a pump’s characteristic curve   on top of the system curve to which it is  connected, you can see they intersect. This   point of intersection tells you the pressure  and flow rate at which the pump will operate.   It’s conceptually both simple and confusing.  The pump doesn’t decide what pressure and flow   rate it will deliver. What it’s connected  to does. When I change my system curve by   opening or closing this valve, both the  pressure and flow rate created by the pump   respond accordingly. So, to select the  right pump for an application, you have   to know how your system will respond to being  supplied with a range of different pressures. Flow and pressure are important, but they’re  not the only considerations that go into pump   selection. A pump curve sometimes also  shows other important information like   efficiency. Even if a pump can operate in  the extreme ranges of its performance curve,   it usually can’t do it efficiently. Listen to  the sound of this pump as I close the valve   and you can tell that it’s not performing  its best over the full range of flow rates.   That might not matter in some applications, but  if it’s a big pump that requires a lot of energy   or one that will run 24/7/365, this is  something to be thoughtful about. Again,   think about scale. On your fish tank pump,  a little inefficiency is not a huge deal.   If you are delivering water to millions of  customers 24 hours per day, small inefficiencies   add up quickly. And it’s especially challenging  if your system curve changes over time. It might seem cheaper to use a single pump  that can handle a wide range of flow rates,   but it’s often more cost-effective to use multiple  pumps so that you can always operate in the most   efficient part of each one’s characteristic  curve. A pump curve also shows you the pressure   at which it can’t create any flow. Watch  what happens when I raise the tube from my   aquarium pump to above its maximum pressure.  The liquid reaches the maximum head and stops.   I have to say, despite what you will read  in nearly every internet forum about pumps,   this one is not creating flow,  but it is creating pressure. I’m being a little facetious here  talking only about centrifugal pumps   when there is another major category  that behaves a little bit differently.   Positive displacement pumps trap a fixed volume  of fluid and force it into the discharge line.   Unlike centrifugal pumps, where the impeller  can spin without actually moving any fluid,   positive displacement pumps directly couple  the motor to a fixed volume no matter what the   pressure is in the discharge line. As long as the  motor has enough power to force that fluid out,   it will happen at a constant rate. Essentially,  their characteristic curve is just a flat line.   I think, in most cases, people who say that pumps  only create flow and not pressure are specifically   referring to positive displacement pumps. But, if  the pressure wouldn’t be there without the pump,   I have to contend that the pump created it.  That said, I think I understand the sentiment   of this idea that pumps only create  flow and why it’s so often repeated. It is a little bit confusing that a pump  itself is not directly responsible for the   flow rate and pressures under which it operates.  Those properties depend on the characteristics   of the system to which the pump is connected.  In the case of a positive displacement pump,   only the pressure is determined by the  system curve. The flow rate is a fixed value.   Both are still created by the pump,  but only one is “decided” by it.   For a centrifugal pump, both the flow and  the pressure depend on the system curve.   Given this discussion, I’d like to propose this  new mantra for the internet pump enthusiasts   as a more correct answer to the question  of whether pumps create pressure or flow:   Pumps impart flow and pressure to a fluid in  accordance with their characteristic curve   and the corresponding system  curve. Not a great catchphrase,   but it is accurate. Maybe one of you can  come up with something a bit more catchy. Thanks for sticking to the end of the video. If  you’re here, I’m guessing that means you prefer   to spend your valuable free time learning new  things about the world. You probably also don’t   enjoy watching ads like this, which is great,  because Nebula doesn’t have any. Nebula is a   streaming service built by and for independent  creators like MinutePhysics, Real Engineering,   Wendover Productions, and a bunch of others  (including me). It’s a way for us to try new   ideas that might not work on advertiser-supported  platforms like YouTube. My videos go live there   the day before they publish here, with no ads  or sponsorships. And, we’re super excited to   be partnered with CuriosityStream, a service with  thousands of documentaries and non-fiction titles   on pretty much every subject you can imagine.  CuriosityStream loves independent creators and   wants to help us grow our platform, so they’re  offering free access to Nebula when you sign up at   CuriosityStream.com/practicalengineering. I know  there are a lot of streaming services right now,   and it can be tough to add another monthly  subscription. That’s why this bundle is such   an awesome deal. For a short time, CuriosityStream  is taking 26% off an annual plan - that makes it   only $15 for a year for access to thousands  of awesome documentaries on CuriosityStream   AND everything on Nebula as well. You can watch  awesome long-form content on CuriosityStream.   One of my favorites is The Secret Life of Lakes,  which covers fascinating details of both humanmade   and and natural lakes across the world. Or, watch  Tom Scott pit other YouTubers against each other   in his Nebula-exclusive gameshow, Money. It’s  a great way to support my channel and a whole   host of your other favorite educational creators.  Plus it’s just a good deal. Do us both a favor and   click that link in the description. Thank you  for watching, and let me know what you think!

Channel: Practical Engineering

Views: 757,899

Keywords: pump, curves, sump pump, centrifugal pump, flow rate, pressure, flow, head, characteristic curve, system curve, postive displacement pump, friction, Practical Engineering, Civil Engineering, Engineer, Grady

Id: m3i_5xP9PYU

Length: 10min 38sec (638 seconds)

Published: Tue May 04 2021

Reddit Comments

[u/B0SSMANT0M]

Did not watch video, but technically both are true. However in practice, positive displacement pumps are classified as pressure devices and velocity pumps are classified as flow devices. You can also run multiple velocity pumps in series to build correct working pressure.

[u/mikebrown33]

Pumps are positive displacement devices

[u/Hyperbowleeeeeeeeeee]

If we're going to get into correcting ol' uncle bumblefuck about everything he says, we're going to be here a long time. Doesn't necessarily detract from the experience, such as it is.

[u/tysonfromcanada]

yes

[u/Gears_and_Beers]

Its all semantics. The number one take away from this video is that the pump operates where the pump curve and the system curve intersect.

This is something that engineers, owners and operators alike forget, and the marketing wanks never even learned it.

[u/Rental_Car]

Yes.

[u/ShuRugal]

Dude used a shitload of words to say not much.

Pumps input energy to a fluid. If a fluid which has energy added to it is not confined, it will flow. If it is confined, it will increase in pressure.

We simplify that to "pumps create flow" because the natural state of a fluid is to flow. You can have flow with zero pressure, and you can have pressure with zero flow, but you can't have pressure with zero restriction. Restriction is required for pressure to exist.

[u/MrAlanBondGday]

That's like asking do engines make power or torque.

[u/RollingZepp]

The more unintuitive thing for me, when I took fluid dynamics, is that pressure decreases as the fluid velocity increases.