Practice English Speaking&Listening with: Why Pipes Move Underground

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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 dont 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 isnt always that good at holding pipes together. Hey Im

Grady and this is Practical Engineering. On todays episode, were 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 dont require any fluid to be moving. They result

just from the pressure within a pipe. A fluids 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 its 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,

youd hardly know anything was happening at all if you werent paying attention to

the pressure gauge. Thats because, in this example, all the hydrostatic forces are balanced.

But, theres 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. Lets try pressurizing this demo

for a couple of cases where the hydrostatic forces arent balanced to see what happens.

With a tee, you have two thrust forces that do balance each other out, and one that doesnt.

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 doesnt 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 its

a little simplistic and perhaps even a bit self evident. Plus, it only shows the hydrostatic

forces that occur within pipes. Actually, theres a pretty cool demonstration of both

hydrostatic and hydrodynamic forces: a water rocket. Im okay explaining this concept

to kindergartners, but Ive 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. Lets have a look at how water rockets produce thrust. Now, it doesnt

matter if youre a conventional rocket or water rocket, your life is governed by the

thrust equation, which is derived from Newtons second law. And here it is in its simplified

form. Over here youve got the thrust, or the force, that the rocket produces to propel

it upwards. And thats made out of two terms. This one is the momentum thrust, and thats

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 theres no flow out of the

nozzle. So, we end up with the pressure inside versus the outside times the nozzles cross

sectional area. Thats 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, lets 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 youd 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 thats a lot more. And, heres 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 pipelinesthrusts.” But unlike

those aerospace guys and gals, civil engineers dont 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 youre probably thinking after seeing

all these demonstrations. “Just glue the joints.” And I promise were 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 dont 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 dont 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, its 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 isnt

that strong. Anyone whos 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 doesnt

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, its 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 isnt 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 arent a

panacea for thrust forces in pipes. Theyre 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 theyre 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. Theyre 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.

Its the engineers 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 theyre using, or ask

one of the workers if theyre installing thrust blocks or restrained fittings (or both)

to make sure the pipe stays put.

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The Description of Why Pipes Move Underground