Real Pumps Have Curves

These are pumps. This is a pump. No, we’re not talking shoes (though those are great too).

Pumps move water (or other liquids). They provide flow like this. Each curve is a different pump or impeller size.

We’ve all been in the shower and said “there’s not enough pressure!”. So we’re all in agreement, that moving water has pressure. Like this.

The pump curve

To understand the relationship between the flow rate and the pressure, think of a garden hose. When you place your thumb over the nozzle, you increase the pressure by minimizing the amount of water that comes out. If your amount of water is held constant (along with other variables like the height of the water) you are confined to one curve. You also run into the equation Q=vA. So flow is equal to velocity times the flow area (which you’ve decreased by your thumb, increasing the velocity). Increase the height, or another variable, and you jump to another curve.

So how does it work?

Pumps are one way to add pressure to the system. To get real nerdy, here’s Bernoulli’s extended equation:

where, I is for temperature change, h for heat added or lost, z for elevation, P for pressure, v for velocity, p for pump, l for losses. So the hp represents pressure added by a pump. In this case, hp is what we call the pressure head. It’s a conservation of energy equation for fluids.

The system curve

Now let’s talk about why this curve is so cool. On top of the pump curves, we can overlay the system curve.The system refers to the distribution system–all those pipes carrying water (or wastewater) to and from your house. Because the distribution system is bound by the physics of pipes, it is also a function of pressure and flow. As designers, we want to make sure that the furthest houses in the system have enough of both and that we comply with regulations. In Texas, it is required that any point in the system be above 35 psi (pounds per square inch) but it can drop to 20 psi during fire flows (during fire fighting). It’s my understanding that this is a typical range for most systems. To keep the pressure within this range, we alter the system by adding elevated storage tanks (think of potential energy converted to kinetic energy) , different pumps, and on and on and on. Bernoulli is our best friend!

The ideal pump curve

Pumps all have real curves. You can turn a pump on and measure the pressure and flow to generate its own curve. However, a lot of the time, the pump curves we see on manufacturer’s websites are ideal curves. They don’t take into account efficiency losses or power losses. These are hard to measure in each situation and don’t provide much benefit by doing so. Instead, we like to add efficiency curves onto the pump curve so that we can tell at what range (flow and pressure) the pump is most efficient.

For the cause

One of my goals when creating this website was to collect funds for a charity supporting girls in STEM. I am so excited to tell you guys about this girl’s tank top that we created. It says ‘Real Pumps Have Curves’ and it’s amazingly soft!! Who doesn’t love a triple pun?! Puns on puns on puns.

Curves and curves and curves

Want a tank?

I have a limited supply of tanks available from sizes XS to L. Each shirt will send $5 to Girls, Inc. Head on over to The Mark-et for more swag details!

And just for some nerdy extras, if you hook pumps up in parallel you can increase the flow rate. If you hook pumps up in series you can increase the pressure. So sometimes it’s better to design for a system with multiple pumps than to specify a larger, more expensive pump that may have too much head or pressure. There are lots of variables in pump design which is what makes it so exciting! And confusing at times. What did we do without computer models?!

Hey engineers, have I gotten anything wrong? Please let me know! I’m looking to you guys for peer review (and I’ll give you credit!)

Reading: The Fifth Season – Just started and I am very, very intrigued. It’s a female-centric fantasy novel that’s won several awards.

Working: Coordinating the start of project work for investigating and designing rehabilitation on a(n almost) failed dam.

Cooking: The best vegan carrot cake recipe. My friend who helped me design the tanks found this amazing recipe. It’s the perfect season here in Texas to start making carrot cakes! Carrot cake here. Substitute soy milk for tofutti in the cream cheese icing. It will melt over the side of the cake. And trust me, the vegan butter is key. Real butter just doesn’t do it.

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