The West Closure Complex is a billion-dollar piece of infrastructure that protects parts of New Orleans from flooding during tropical storms. Constructed partly as a result of Hurricane Katrina, it features one of the largest pumping stations in the world, capable of lifting the equivalent of a fully-loaded Boeing 747 every second. When storm surge threatens to raise the levels of the sea above developed areas on the west bank of the Mississippi River, this facility’s job is to hold it back. The gates close and the pumps move rainwater and drainage from the City’s canals back into the Mississippi River and out to the gulf. This pump station may be the largest of its kind, but its job is hardly unique. We collectively move incredible volumes of fresh water, drainage, and wastewater into, out of, and around our cities every day. And, we mostly do it using pumps. I love pumps. But, even though they are critical for the safety, health, and well-being of huge populations of people, there are a lot of things that can go wrong if not properly designed and operated. I’m Grady, and this is Practical Engineering. Today, we’re exploring some of the problems that can happen with pumps.

The first of the common pitfalls that pumps can face is priming. Although liquids and gases are both fluids, not all pumps can move them equally. Most types of pumps that move liquids cannot move air. It’s less dense and more compressible, so it’s often just unaffected by impellers designed for liquids. That has a big implication, though. It means if you’re starting a pump dry - that is when the intake line and the housing are not already full of water, like I’m doing here - nothing happens. The pump can run and run, but because it can’t draw air out of the intake line, no water ever flows. This is why many pumps need to be primed before starting up. Priming just means filling the pump with liquid to displace the air out of housing and sometimes the intake pipe. When you raise the discharge line to let water flow backwards into the pump, it happens quickly. As soon as the air is displaced from the housing, the pump is primed and water starts to flow. There are a lot of creative ways to accomplish this for large pumps. Some even have small priming pumps to do this very job. “But what primes the priming pumps?” Well, there are some kinds of pumps that are self-priming. One is submersible pumps that are always below the water where air can’t find its way in. Another is positive displacement pumps that can create a vacuum and draw air through. They may not be as efficient or convenient to use as the main pump, but they work just fine for the smaller application of priming.

However a pump is primed, it’s critical that it stays that way. If air finds its way into the suction line of a pump, it can lose its prime and stop working altogether. When you lift a pump out of water, the prime is lost. And if you put the pump back down into the water, it doesn’t start back up. This can be a big problem if it goes unnoticed, not just because the pump isn’t working, but also because running a pump dry often leads to damage. Many pumps depend on the fluid in the housing for cooling, so without it, they overheat. In addition, the seals around the shaft that keep water from intruding on the motor depend on the fluid to function properly. If the seals dry out, they get damaged and require replacement which can be a big job.

The next problem with pumps is also related to the suction side. Pumps work by creating a difference in pressure between the inlet and outlet. In very simple terms, one side sucks and one side blows. A problem comes when the pressure gets too low on the suction side. You might know that the phase of many substances depends not just on their temperature, but also on the ambient pressure. That’s why the higher you are in elevation, the lower the temperature needed to boil water. If you continue that trend into lower and lower pressures, eventually some liquids (including water) will boil at normal temperatures without any added heat. It’s a pretty cool effect as a science demonstration, but it’s not something you want happening spontaneously inside your pump. Just like they don’t work with air, most pumps don’t work very well with steam either. But, the major problem comes when those bubbles of stream collapse back into a liquid. Liquids aren’t very compressible so these collapsing bubbles send powerful shockwaves that can damage pump components. This phenomenon is called cavitation, and I have a blog covering it in a lot more detail that you can check out after this one to learn more. It usually doesn’t lead to immediate failure, but cavitation will definitely shorten the life of a pump significantly if not addressed.

The solution to this problem at pumps is known as Net Positive Suction Head, and with a name like that, you know it’s important. Manufacturers of large pumps will tell you the required Net Positive Suction Head (or NPSH), which is the minimum pressure needed at a pump inlet to avoid cavitation. The engineer’s job is to make sure that a pump system is designed to provide at least this minimum pressure. That NPSH depends on the vertical distance between the sump and inlet, the frictional losses in the intake pipe, the temperature of the fluid, and the ambient air pressure. Here’s an example: With a valve wide open, the suction pressure at the inlet is about 20 kPa or 5 inches of mercury. Now, when you move the pump to the height of a ladder, but leave the bucket on the ground, the suction pressure just about doubles. A constriction in the line also decreases the available NPSH. If you close the valve on the intake side of a pump, you immediately see the pressure in the line becoming more negative (in other words, a stronger vacuum). This pump isn’t strong enough to cavitate, but it will make a bad sound when there isn’t enough Positive Suction Head at the inlet. I think it easily demonstrates how a poor intake design can dramatically affect the pressure in the intake line and quickly lead to failure of a pump.

The last problem that can occur at pumps is also the most interesting: vortices. You’ve probably seen a vortex form when you drain a sink or bathtub. These vortices occur when the water accelerates in a circular pattern around an outlet. If the vortex is strong enough, the water is flung to the outside, allowing air to dip below the surface. This is a problem for pumps if that air is allowed to enter the suction line. We talked a little about what happens when a pump runs dry in the discussion about priming, but air is a problem even if it’s mixed with water. That’s because it takes up space. A bubble of air in the impeller reduces the pump’s efficiency since the full surface of the blades can’t act on the water. This causes the pump to run at reduced performance and may cause it to lose prime, creating further damage.

The easiest solution to vortexing is submergence - just getting the intake pipe as far as possible below the surface of the water. The deeper it is, the larger and longer a vortex would have to be before air could find its way into the line. This is achieved by making the sump - that is the structure that guides the water toward the intake - deeper. That solution seems simple enough, except that these sumps are often major structural elements of a pump station that are very costly to construct. You can’t just indiscriminately oversize them. But how deep is deep enough?

It turns out that’s a pretty complicated question because a vortex is hard to predict. Even sophisticated computational fluid dynamics models have trouble accurately characterizing when and if a vortex will form. That’s an issue because you don’t want to design and construct a multi-million-dollar pumping facility just to find out it doesn’t work. And there aren’t really off-the-shelf designs. Just about every pumping station is a custom-designed facility meant for a specific application, whether it’s delivering raw water from a reservoir or river to a treatment plant, sending fresh water out to customers, lifting sewage to be treated at a wastewater plant, pumping rainwater out of a low area, or any number of other reasons to move large volumes of water. So if you’re a designer, you have some options.

First, you can just be conservative. We know through lots of testing that vortices occur mostly due to non-uniform flow in the sump. Any obstructions, sharp turns, and even vertical walls can lead to flow patterns that evolve into vortices. Organizations like the Hydraulic Institute have come up with detailed design standards that can guide engineers through the process of designing a pump station to make sure many of these pitfalls are avoided. Things like reducing the velocity of the flow and maintaining clearance between the walls and the suction line can reduce the probability of a vortex forming. There are also lots of geometric elements that can be added to a sump or intake pipe to suppress the formation of vortices.

The second option for an engineer is to build a scale model. Civil engineering is a little bit unique from other fields because there aren’t as many opportunities for testing and prototyping. Infrastructure is so large and costly, you usually only have one shot to get the design right. But, some things can be tested at scale, including hydraulic phenomena. In fact, there are many laboratories across the world that can assemble and test scale models of pump stations, pipelines, spillways, and other water-handling infrastructure to make sure they work correctly before spending those millions (or billions) of dollars on construction. They give engineers a chance to try out different configurations, gain confidence in the performance of a hydraulic structure, and avoid the pitfalls like loss of prime, cavitation, and vortices at pump stations.