Flow and Pressure in Pipes Explained

All pipes carrying fluids experience losses  of pressure caused by friction and turbulence   of the flow. It affects seemingly simple  things like the plumbing in your house   all the way up to the design of massive, way  more complex, long-distance pipelines. I’ve   talked about many of the challenges engineers  face in designing piped systems, including   water hammer, air entrainment, and thrust forces.  But, I’ve never talked about the factors affecting   how much fluid actually flows through a pipe  and the pressures at which that occurs. So,   today we’re going to have a little fun, test  out some different configurations of piping,   and see how well the engineering equations  can predict the pressure and flow.   Even if you’re not going to use the equations,  hopefully, you’ll gain some intuition from seeing   how they work in a real situation. I’m Grady and  this is Practical Engineering. In today’s episode,   we’re talking about closed conduit  hydraulics and pressure drop in pipes. This video is sponsored by HelloFresh,  America’s number 1 meal kit. More on that later. I love engineering analogies, and in this case,  there are a lot of similarities between electrical   circuits and fluids in pipes. Just like all  conventional conductors have some resistance   to the flow of current, all pipes impart some  resistance to the flow of the fluid inside,   usually in the form of friction and turbulence.  In fact, this is a lovely analogy because   the resistance of a conductor is both a function  of the cross-sectional area and length of the   conductor—the bigger and shorter the wire, the  lower the resistance. The same is true for pipes,   but the reasons are a little different. The fluid  velocity in a pipe is a function of the flow rate   and the pipe’s area. Given a flowrate, a  larger pipe will have a lower velocity,   and a small pipe will have a higher velocity.  This concept is critical to understanding the   hydraulics of pipeline design because friction and  turbulence are mostly a result of flow velocity. I built this demonstration that should help  us see this in practice. This is a manifold   to test out different configurations of pipes and  see their effect on the flow and pressure of the   fluid inside. It’s connected to my regular  tap on the left. The water passes through   a flow meter and valve, past some pressure  gauges, through the sample pipe in question,   and finally through a showerhead. I  picked a showerhead since, for many of us,   it’s the most tangible and immediate connection  we have to pressure problems in plumbing. It’s   probably one of the most important factors in the  difference between a good shower, and a bad one.   Don’t worry, all this water will be given  to my plants which need it right now anyway. I used these clear pipes because they  look cool, but there won’t be much to see   inside. All the information we need will show  up on the gauges (as long as I bleed all the   air from the lines each time). The first one  measures the flow rate in gallons per minute,   the second one measures the pressure  in the pipe in pounds per square inch,   and the third gauge measures the difference  in pressure before and after the sample   (also called the head loss) in inches of water. In  other words, this gauge measures how much pressure   is lost through friction and turbulence in the  sample - this is the one to keep your eye on. In   simple terms, it’s saying how far do you have to  open the valve to achieve a certain rate of flow.   I know the metric folks are giggling at these  units. For this video, I’m going to break my   rule about providing both systems of measurement  because these values are just examples anyway.   They are just nice round numbers that are easy  to compare with no real application outside the   demo. Substitute your own preferred units if you  want, because it won’t affect the conclusions. There are a few methods engineers use to estimate  the energy losses in pipes carrying water,   but one of the simplest is the Hazen-Williams  equation. It can be rearranged in a few ways,   but this way is nice because it has the variables  we can measure. It says that the head loss (in   other words the drop in pressure from one end of a  pipe to the other) is a function of the flow rate,   and the diameter, length, and roughness of  the pipe. Now - that’s a lot of variables,   so let’s try an example to show how this works.  First, we’ll investigate the effect the length   of the pipe has on head loss. I’m starting  with a short piece of pipe in the manifold,   and I’m testing everything at three flow rates:  0.3, 0.6, and 0.9 gallons per minute (or gpm). At 0.3 gpm, we see pressure drop across the pipe  is practically negligible, just under half an   inch. At 0.6 gpm, the head loss is about an inch.  And, at 0.9 gpm, the head loss is just over 3   inches. Now I’m changing out the sample for a much  longer pipe of the same diameter. In this case,   it’s 20 times longer than the previous example.  Length has an exponent of 1 in the Hazen-Williams   equation, so we know if we double the length,  we should get double the head loss. And if we   multiply the length times 20, we should see the  pressure drop increase by a factor of 20 as well.   And sure enough, at a flow rate of 0.3 gpm, we  see a pressure drop across the pipe of 7.5 inches,   just about 20 times what it was with the short  pipe. That’s the max we can do here - opening   the valve any further just overwhelms the  differential pressure gauge. There is so much   friction and turbulence in this long pipe that I  would need a different gauge just to measure it. Length is just one factor that influences the  hydraulics of a pipe. This demo can also show   how the pipe diameter affects the pressure  loss. If I switch in this pipe with the same   length as the original sample but which has  a smaller diameter, we can see the additional   pressure drop that occurs. The smaller pipe  has ⅔ the diameter of the original sample,   and diameter has an exponent of 4.9 in our  equation. That’s because, as I mentioned before,   changing the diameter changes the fluid velocity,  and friction is all about velocity. We expect the   pressure drop to be 1 over (⅔)^4.9 or about 7  times higher than the original pipe. At 0.3 gpm,   the pressure drop is 3 inches. That’s  about 6 times the original. At 0.6 gpm,   the pressure drop is 7.5 inches, about  7 times the original. And at 0.9 gpm,   we’re off the scale. All of that is to say, we’re  getting close to the correct answers, but there’s   something else going on here. To explore this  even further, let’s take it to the extreme. We’ll swap out a pipe with a diameter 5 times  larger than the original sample. In this case,   we’d expect the head loss to be 1 over 5^4.3,  basically a tiny fraction of that measured with   the original sample. Let’s see if this is the  case. At 0.3 gpm, the pressure drop is basically   negligible just like last time. At 0.6 and 0.9  gpm, the pressure drop is essentially the same as   the original. Obviously, there’s more to the head  loss than just the properties of the pipe itself,   and maybe you caught this already. There is  something conspicuous about the Hazen-Williams   equation. It estimates the friction in a pipe,  but it doesn’t include the friction and turbulence   that occurs at sudden changes in direction or  expansion and contraction of the flow. These   are called minor losses, because for long pipes  they usually are minor. But in some situations   like the plumbing in buildings or my little  demonstration here, they can add up quickly. Every time a fluid makes a sudden turn (like  around an elbow) or expands or contracts (like   through these quick-release fittings), it  experiences extra turbulence, which creates   an additional loss of pressure. Think of it like  you are walking through a hallway with a turn. You   anticipate the turn, so you adjust your path  accordingly. Water doesn’t, so it has to crash   into the side - and then change directions.  And, there is actually a formula for these minor   losses. It says that they are a function of the  fluid’s velocity squared and this k factor that   has been measured in laboratory testing for any  number of bends, expansions, and contractions.   As just another example of this, here’s a sample  pipe with four 90-degree bends. If you were just   calculating pressure loss from pipe flow, you  would expect it to be insignificant. Short,   smooth pipe of an appropriate diameter. The  reality is that, at each of the flow rates tested   in the original straight pipe sample, this one has  about double the head loss, maxing out at nearly   6 inches of pressure drop at 0.9 gpm. Engineers  have to include “minor” losses to the calculated   frictional losses within the pipe to estimate the  total head loss. In my demo here, except for the   case of the 20’ pipe, most of the pressure drop  between the two measurement points is caused by   minor losses through the different fittings in the  manifold. It’s why, in this example, the pressure   drop is essentially the same as the original. Even  though the pipe is much larger in diameter, the   expansion and contraction required to transition  to this large pipe make up for the difference. One clarification to this demo I want to make:  I’ve been adjusting this valve each time to keep   the flow rate consistent between each example  so that we make fair comparisons. But that’s not   how we take showers or use our taps. Maybe  you do it differently, but I just turn the   valve as far as it will go. The resulting flow  rate is a function of the pressure in the tap   and the configuration of piping along  the way. More pressure or less friction   and turbulence in the pipes and fittings  will give you more flow (and vice versa). So let’s tie all this new knowledge  together with an example pipeline.   Rather than just knowing the total  pressure drop from one end to another,   engineers like to draw the pressure continuously  along a pipe. This is called the hydraulic grade   line, and, conveniently, it represents the  height the water would reach if you were to tap   a vertical tube into the main pipe. With a  hydraulic grade line, it’s really easy to see   how pressure is lost through pipe friction.  Changing the flow rate or diameter of the pipe   changes the slope of the hydraulic grade line.  It’s also easy to see how fittings create minor   losses in the pipe. This type of diagram  is advantageous in many ways. For example,   you can overlay the pressure rating of the pipe  and see if you’re going above it. You can also   see where you might need booster pump stations  on long pipelines. Finally, you can visualize how   changes to a design like pipe size, flow rate,  or length affect the hydraulics along the way. Friction in pipes? Not necessarily the  most fascinating hydraulic phenomenon. But,   most of engineering is making compromises, usually  between cost and performance. That’s why it’s so   useful to understand how changing a design can  tip the scales. Formulas like the Hazen-Williams   and the minor loss equations are just as useful  to engineers designing pipelines that carry   huge volumes of fluid all the way down to  homeowners fixing the plumbing in their houses.   It’s intuitive that reducing the length of a pipe  or increasing its diameter or reducing the number   of bends and fittings ensures that more of the  fluid’s pressure makes it to the end of the line.   But engineers can’t rely just on intuition.  These equations help us understand how much   of an improvement can be expected without having  to go out to the garage and test it out like I   did. Pipe systems are important to us,  so it’s critical that we can design them   to carry the right amount of flow without too  much drop in pressure from one end to the other. It’s time for everyone’s favorite segment of   me trying to cook while my wife  tries to capture that on video. “And… Action!” “Who cut this tiny hole in the cheese?” Goofing around in the kitchen is one  of our favorite things to do together.   That’s why we’re thankful for  HelloFresh, the sponsor of this video,   for converting cooking from a chore into  our favorite thing to do on date night. “So delizioso!” Sometimes, the hardest part about  dinner is just deciding what to have,   so it’s nice to have HelloFresh curating  delicious and healthy recipes so we don’t have to. “How’s it feel?” The pre-portioned ingredients mean there’s less  prep and less food waste, and the packaging is   mostly recyclable or already recycled content.  HelloFresh also helps us get dinner ready quickly   on the days we don’t feel like planning, prep, and  shopping. We get to skip straight to the fun part. “Ewww!” Go try it yourself at HelloFresh.com and use   code PRACTICAL12 to get 12 free  meals, including free shipping. Supporting our sponsors helps support this  channel. That’s HelloFresh.com and use code   PRACTICAL12. Thanks, HelloFresh, and thank  YOU for watching. Let me know what you think.

Channel: Practical Engineering

Views: 184,500

Rating: 4.9632306 out of 5

Keywords: pipes, hydraulics, friction, pressure, turbulence, flow, pressure drop, fluid velocity, hazen willaims, hydraulic grade line, Practical Engineering, Civil Engineering, Engineer, Grady

Id: ZQKpu-obzlU

Length: 12min 42sec (762 seconds)

Published: Tue Apr 06 2021

Reddit Comments

[u/turbodeeznuts]

I wondered whether he was going to mention Darcy-Weisbach.

[u/Bigmitch2]

Grady's videos are so consistently high quality, yet accessible

[u/evanparker]

former process engineer here, this video is GREAT

[u/Bunce1260]

Man, I needed this last term. He does make good videos though.

[u/Ravaha]

Saw this just as I got done modelling a Water Network and doing Fire Hydrant Residual pressure analysis.

[u/Adam38932]

Shouldn't he be taking the second pressure reading at the section of pipe with the smaller/larger diameter rather than at the section with with same diameter as the original pipe? I feel like the math he described only accounted for one size step change, but the reading was actually showing two. Either way, still a really good video!