This is the West Closure Complex, 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.
In today’s episode, we’re exploring some of the problems that can happen with pumps.
This video is sponsored by HelloFresh, America’s number 1 meal kit. More on that later.
I’ve got some colored water and clear pipe out here in my garage to demonstrate a few common
pitfalls that pumps can face, and the first one 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. Watch what happens when I 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 I lift the pump
out of the water, the prime is lost. And now if I 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 video 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 this valve wide open, the suction pressure at
the inlet is about 20 kPa or 5 inches of mercury. Now watch what happens when I move the pump to the
top of the 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 I close this valve on
the intake side of my 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 does 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 like I have here.
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.
It’s time for everyone’s favorite segment of
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