We use pipes to carry all kinds of fluids. Pretty much anyone can tell you how they work.
You put a liquid or a gas in one side and it comes out the other. But, designing pipe
systems is not always as simple as it seems. Pipes don’t float in the air on their own;
they have to be held in some way. We often bury pipes to protect them and keep them out
of the way, but the ground isn’t always that good at holding pipes together. Hey I’m
Grady and this is Practical Engineering. On today’s episode, we’re talking about thrust
forces in pipe systems.
Designing systems of piping might seem intuitive. I think most people have a general understanding
about how pipes work because most of us have them in our home delivering fresh water to
the taps and carrying our waste away. But, the bigger a pipe gets and the more pressure
it contains, the more complicated it becomes. Engineers design systems of pipes that can
be enormous - sometimes big enough to drive a car through - and that can hold many times
the pressure of your typical household plumbing. Those larger diameters and higher pressures
create greater forces, and those forces need to be accounted for in design. There are two
types of forces in pipelines that engineers need to consider: hydrostatic and hydrodynamic.
Hydrostatic forces are the ones that don’t require any fluid to be moving. They result
just from the pressure within a pipe. A fluid’s pressure is its force applied over an area.
Pressure works in every direction at the same time. So, within a section of pressurized
pipe, you have forces acting on the walls of the pipe. This force is resisted by the
hoop of pipe material. But, you also have forces acting along the axis of the pipe.
This force is equal to the pressure times the area of the pipe, and it’s resisted
by the fluid in the adjacent section of pipe. I can demonstrate this with clear tubing.
Even though the tube slides into this straight coupler fairly easily, I can pressurize it
without too much issue. If you ignore the small leaks from the imprecision of my demo,
you’d hardly know anything was happening at all if you weren’t paying attention to
the pressure gauge. That’s because, in this example, all the hydrostatic forces are balanced.
But, there’s not always an adjacent section of pipe to resist this longitudinal force.
Eventually, you get to the end of the pipe where you need a cap, or you get to a place
where you need to make a bend, a tee, or a wye. These are places where you end up with
an imbalance in hydrostatic forces within the pipe. Let’s try pressurizing this demo
for a couple of cases where the hydrostatic forces aren’t balanced to see what happens.
With a tee, you have two thrust forces that do balance each other out, and one that doesn’t.
Can you guess what happens when I pressurize the tubing? The force from the top tube has
nothing to resist it, so it easily separates the fitting from the tube. With an elbow,
there are unbalanced forces in both directions. It doesn’t take much pressure for the fitting
to pop right off. Now, this is a pretty cool demo if I do say so myself, but maybe it’s
a little simplistic and perhaps even a bit self evident. Plus, it only shows the hydrostatic
forces that occur within pipes. Actually, there’s a pretty cool demonstration of both
hydrostatic and hydrodynamic forces: a water rocket. I’m okay explaining this concept
to kindergartners, but I’ve asked for some help from the team behind the water rocket
altitude world record and awesome YouTube channel, Air Command Rockets, to show how
these two types of force work in an entirely different setting than pipelines.
Thanks Grady. Let’s have a look at how water rockets produce thrust. Now, it doesn’t
matter if you’re a conventional rocket or water rocket, your life is governed by the
thrust equation, which is derived from Newton’s second law. And here it is in its simplified
form. Over here you’ve got the thrust, or the force, that the rocket produces to propel
it upwards. And that’s made out of two terms. This one is the momentum thrust, and that’s
just the mass flow rate, in other words the rate at which the water or air flows through
the nozzle, times the velocity at which it exits. And, over here is the pressure thrust,
and that relates to the exit pressure versus the ambient pressure. So, while the rocket
is sitting on the pad pressurized, this term is zero because there’s no flow out of the
nozzle. So, we end up with the pressure inside versus the outside times the nozzle’s cross
sectional area. That’s the actual force of the rocket trying to get off the pad. So,
when you release the rocket, the momentum thrust comes back into play. The compressed
air is pushing the water out through the nozzle. And, the water comes out at probably about
one tenth of the speed of sound for regular types of rockets, which is quite low. But,
the mass flow rate is high because the water is so heavy. Now, when the water runs out
and the compressed air starts coming out, the mass flow rate really drops because air
is so much lighter than water. But, the exit velocity gets very high because the air comes
out at the speed of sound. So as it turns out during the air phase only, you get about
two thirds the amount of thrust as you get with the water phase. And this is in fact
why water rockets use water for improved performance.
Now, let’s have a look at a couple of examples of the water
rocket. This one is a low pressure one. This one would be a typical one that you’d launch
and it produces about 100 N peak thrust.
And, this one over here is a higher pressure one
(if you really crank up the pressure) and this one generates about 2,500 N peak thrust,
so that’s a lot more. And, here’s what happens when you crank up the pressure too much.
Okay back to you Grady before we blow something else up.
Just like in rockets, engineers call these forces in pipelines “thrusts.” But unlike
those aerospace guys and gals, civil engineers don’t want the things they design to go
flying through the air. We want our pipelines to stay put, which means in this case thrust
is a bad thing and must be resisted. I know what you’re probably thinking after seeing
all these demonstrations. “Just glue the joints.” And I promise we’re getting there,
but the reality is that a lot of the pressure piping we use underground, particularly in
municipal settings - such as water mains for drinking water and force mains for sewers
- use push-on fittings. These joints use gaskets and tight tolerances to achieve a watertight
seal, but they don’t provide longitudinal restraint. The pipes can still slide fairly
freely in and out of the joint. We use these types of push-on fittings because they are
inexpensive, reliable, and most-importantly, they are easy to install speeding up the construction
time which benefits everyone, from the contractor to the owner to even the citizens waiting
on a road to open back up after a main break. In plumbing we use glue or threaded connections
for pipes, but those options are a lot less feasible for certain types of large diameter
pipes. But, because push-on fittings don’t offer any longitudinal restraint, we have
to provide that restraint somewhere else. In most cases, that comes from burying the
pipe. Encasing the line in compacted soil holds it in place to prevent the pieces from
slipping apart.
But, it’s not that simple. These pipelines can be under enormous pressure, sometimes
two or three times the pressure at the tap in your house, and in some industrial settings
many times higher. Also - and this is straight from geotechnical engineering 101 - soil isn’t
that strong. Anyone who’s ever tried to walk through the mud knows this. So, we rarely
trust soil on its own to hold our pipelines together underground. Relying on soil for
restraint is essentially asking the soil to be as strong as the pipe material. If it doesn’t
hold the pipe still against hydrostatic and hydrodynamic forces, you can get separation
of joints and leakage from the pipes. Fixing this can be a huge endeavor, leading to loss
of service and creating significant expenses. For water mains, it takes a maintenance crew
closing traffic, excavating the line, repairing the damaged section, backfilling, and restoring
the pavement. And, although public works crews are awesome at this job, most people would
agree that it would be better to avoid the need in the first place if possible.
So, what do we do? The classic solution to this problem is thrust blocks: masses of concrete
that distribute thrust forces over a larger area against the soil. If you could make the
subsurface invisible so you could see all the water mains below your city, it’s a
fairly sure bet that at each and every bend, tee, wye, or reduction there is an adjacent
block of concrete transferring thrust forces to the soil through the larger bearing area
of the block so that the strength of the soil isn’t exceeded. In fact, one very important
job of a pipeline engineer is sizing the thrust blocks based on the type of fitting, test
pressure of the pipe, and soil conditions at the site. But, thrust blocks aren’t a
panacea for thrust forces in pipes. They’re big and bulky, they get in the way of other
subsurface utilities, they make it difficult to excavate and repair lines when needed,
and because they’re made of concrete, they often take several days to cure before you
can pressurize and test the line before backfilling. So, the other way we deal with thrusts in
pipelines is to take a cue from the plumbers and provide longitudinal restraint at the
joints themselves.
A wide variety of pipe fittings that can provide longitudinal
restraint are becoming more popular. They’re still usually more expensive than using concrete
reaction blocks, but they have a lot of other benefits as described. Of course, in certain
situations, it makes more sense to fully restrain a subsurface pipe. Most petroleum pipelines
are fully welded at every joint, and you can fuse polyethylene pipe at the joints as well.
It’s the engineer’s job to decide what type of restraint is needed based on all the
considerations involved. Next time you see a crew working on a pipeline, try to sneak
a peek into the trench and see which type of restraint system they’re using, or ask
one of the workers if they’re installing thrust blocks or restrained fittings (or both)
to make sure the pipe stays put.
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