I made a video a while back about water hammer, a hydraulic phenomenon that can lead to major
problems in pipelines. Then I did a video on steam hammer, a somewhat related phenomenon
associated with steam piping systems that can be extremely dangerous. And then, I did
a follow-up to the water hammer video talking about transient vacuum phenomena that can
collapse pipes if they’re not designed and operated correctly. But even after those three
videos, it turns out I haven’t told the full story. Because even though water hammer
is generally a problem for engineers, there is a way to take advantage of this normally
inauspicious effect for a beneficial use. Hey I’m Grady and this is Practical Engineering.
On today’s episode we’re talking about hydraulic ram pumps.
This video is sponsored by NordVPN. More on that later.
A hydraulic ram is a clever device invented over 200 years ago that can pump water uphill
with no other external source of power except for the water flowing into it. No, it’s
not a free energy device, but if you search around, you’ll find lots great implementations
of this style of pump on YouTube, mainly from people doing homesteading and off-grid lifestyle
vlogs. And, it’s easy to see why ram pumps have such popularity among these groups. Because
if you’ve got a piece of land with an abundant source of water, a ram pump let’s you get
that water to a tank or location at a higher elevation with a really elegant design that
requires no electricity or fuel and only two moving parts. So of course, I built my own
so you can see how it works, but first we need to build a just a little bit of foundational
knowledge on the behavior of fluids. And this is something anyone can understand.
There are three types of energy that a fluid can have, and in civil engineering, we usually
convert them to their equivalents as the height of a static column. This distance is called
the head. Understanding the energy in a fluid is how we solve a lot of engineering problems,
because in most scenarios, the amount of energy stays the same, and the only thing that changes
is what form it takes. The first type is head from gravitational potential. It doesn’t
have an equivalent static column because it is a static column. The head is just the distance
from an arbitrary datum. This one is easy to demonstrate with a tank and tube. I can
move this tube around wherever I want, but the level in the tube and tank are always
going to be the same. They’re both exposed to atmospheric pressure at their surface and
they’re not moving so there’s no velocity. It’s just pure gravitational potential.
The second type of energy is pressure head. In this case, the head is the pressure divided
by gravity and the density of the fluid. So, if I close off the top of my tank and add
some air pressure, the level in the tube goes up. The new height is the pressure head, the
equivalent static column related to the pressure in the tank. For a given pressure, a denser
fluid like mercury will have a lower head compared to a lighter fluid like water because
they have different unit weights. A good example of measuring pressure head is a barometer.
We live at the bottom of an ocean of air, and we like to keep track of the air pressure
down here. One of the easiest ways to do that is to measure how high the pressure can push
a static column of a fluid, in most cases mercury.
The final type of energy is velocity head, which relates to a fluid’s kinetic energy.
The conversion for velocity head is velocity squared divided by 2 times gravitational acceleration.
I can demonstrate the equivalent column of water using a tool called a pitot tube.
That’s a lot of background, but it’s important in understanding the function of a ram pump.
Because without an external source of power, even though you can go from one type of energy
to another, you can’t get more energy out than you had at the start. For example, I
can convert a static column of water to one with some velocity, but I’m never going
to get the fluid to a higher elevation than where it started… with one exception. An
exception that the hydraulic ram pump takes advantage of beautifully.
A ram pump is essentially just two one-way check valves, one called the waste valve and
the other called the delivery valve. To get it started, you just momentarily open the
waste valve to allow water to flow. After that it’s working on it’s own to pump
the water uphill above the elevation of the source. Pretty amazing, I think. Let’s walk
through the path of the water to understand how it works. First, as the waste valve opens,
water flows into the pump and immediately out of the valve. But, as it picks up speed,
the flowing water eventually forces the waste valve to slam shut. Now the water is stopped
in the pump. It had kinetic energy… but now it doesn’t. That means the kinetic energy
was converted to something else, in this case pressure. This is the definition of water
hammer. Slamming a valve shut converts all that kinetic energy nearly instantly creating
a huge spike in pressure that can lead to stress and damage in pipe systems and connected
equipment.
In the case of the ram pump though, that spike in pressure has a different effect. It opens
the second check valve and forces water entering the pump into the delivery line. As you can
see from my digital pressure gauge, this process is cyclical, pumping some of the water and
wasting the rest each time the valve slams shut. You can see what’s happening here
in real time: the pump is robbing some of the kinetic energy from the flow and imparting
it to a smaller volume of water. It’s a redistribution of energy, converting low head
and high flow into high head and low flow. And this type of pump can really create a
lot of head. I ran my discharge line up to well above the roof of my shed, and my pump
is still able to get the water up there. Sometimes an air chamber is included in the pump to
smoothing out those sharp spikes in pressure and provide a more even flow rate out of the
delivery pipe, reducing wear and tear on the pump components.
If you like to think in terms of modern electrical devices, imagine we installed a hydropower
turbine on a pipe to spin a generator and then used that electricity to power a pump
to move the water coming out of the turbine. Obviously you wouldn’t be able to pump all
the water, and anyway that would be a pretty complicated setup for something the ramp pump
can do with a few very simple off-the-shelf plumbing parts. In fact there is a type of
pump that works from a water-powered turbine. Maybe I’ll build one of those next. For
now though, I think the ram pump is an ingenious way to take advantage of the properties of
fluids. We all need water for a variety of reasons, so being able to move it where we
need it without any fancy equipment or external sources of power is a pretty nice tool to
have in your toolbox.
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