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Taming the Time Bomb Inside Pressure Vessels · Mechanical Engineering Made Simple

• 0:00
  Welcome back to mechanical engineering made simple.
• 0:02
  It's good to be here. Today we're looking at something
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  that, honestly, most people drive past every single day
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  without giving it a second thought.
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  Oh, yeah. You see them at refineries,
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  chemical plants, maybe even behind a large brewery.
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  Big steel tanks, silent stationary.
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  They look, well, they look pretty boring.
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  On the surface, sure, they just look like big metal buckets
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  sitting in a field. But if you look closer, those
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  tanks are actually pressurized vessels and that I mean that
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  changes everything. That is the hook for today,
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  because while they look simple, the source material makes it
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  pretty clear a pressure vessel is essentially A ticking time
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  bomb that we've managed to tame. Tamed with a whole lot of math
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  and physics. It's a dramatic way to put it,
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  but you're not wrong. The potential energy stored
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  inside one of those is it's immense.
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  You're compressing gas or liquid to hundreds, sometimes thousands
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  of PSI. You are taking a massive amount
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  of energy and try to hold it inside a thin metal skin.
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  If that design fails, it doesn't just leak, it can result in a
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  catastrophic energy release. We're talking shock waves that
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  can level buildings. Which brings us to the central
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  conflict of this deep dive. It seems like engineering these
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  things isn't just about making it strong, it's this constant
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  cage match between two forces, safety and economy.
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  It always is, so you could make a pressure vessel infinitely
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  safe. It's actually easy from an
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  engineering standpoint. You just make the walls 5 feet
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  thick. And it would never burst.
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  Never. You could shoot it with a tank
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  and it wouldn't burst. But but I'm guessing you'd go
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  out of business trying to build it.
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  Immediately it would be so heavy you would sink into the ground
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  and the steel bill alone would bankrupt the project.
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  So engineering is the art of finding that sweet spot.
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  Thin enough to be affordable, but thick enough not to kill
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  anyone. That's the job.
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  And to find that spot you need a rule book or, as the note
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  suggests, a really expensive calculator.
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  Yeah, usually both. Let's unpack that.
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  The Bible for this seems to be the ASME Code Section 8.
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  But there's this fork in the road.
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  Right away you have Division One and division two.
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  To an outsider, that just sounds like bureaucracy.
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  What's the actual difference here?
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  It's not just bureaucracy, it's a business decision.
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  Think of Division One is designed by rules.
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  It's the old school cookbook approach.
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  OK, so it gives you the recipe it.
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  Gives you explicit formulas. You plug in your pressure, your
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  diameter, your material, and it spits out a wall thickness.
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  OK, so it's fast plug and play. It is efficient, but because
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  it's A1 size fits all approach it has to be conservative.
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  It uses a higher safety factor to account for stresses we
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  aren't explicitly calculating. So you're adding extra metal
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  just in case. Just in case there are forces at
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  play that the simple formula missed, you save time on math,
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  which means fewer billable hours for the engineer, but you spend
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  more money on the actual steel plate.
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  Right, a penalty on the design for taking the easy route.
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  Exactly. Then you have division two.
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  This is design by analysis. The expensive calculator.
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  The supercomputer more likely Division two.
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  Let's use thinner walls, less metal, lighter vessels, cheaper
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  fabrication, but in exchange the code demands you prove it's safe
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  through rigorous analysis. So you're talking finite element
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  analysis, fatigue studies, all of that.
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  Oh yeah, you're getting into the weeds of the physics.
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  You're trading metal for math. So if you're building one small
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  propane tank, you just use Division One and buy the extra
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  steel because hiring a team of analysts is too expensive.
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  Exactly. But if you're building a massive
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  reactor that weighs 400 tons, the cost of that extra steel is
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  astronomical. So you pay the engineers to run
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  the division two analysis to shave down the thickness.
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  It makes sense. It's like busing a suit off the
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  rack versus paying a tailor. That is a great analogy. 1 uses
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  more fabric to fit everyone, the other is trimmed perfectly to
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  fit just you. OK, so let's get into that math
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  stress analysis. The text makes an interesting
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  point that I struggled with a bit.
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  It says the goal isn't to find every stress.
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  No, that would be a waste of time.
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  A vessel has infinite points of stress.
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  The goal is to find the governing stresses, the specific
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  ones that actually dictate how thick the plate needs to be.
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  And the notes break this down into two main flavors, membrane
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  stress and bending stress. Help me visualize membrane
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  stress. Picture a balloon.
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  When you blow it up, the rubber skin stretches evenly, right?
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  That tension, that pulling a part of the fibers along the
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  surface is membrane stress. It's just stretching.
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  But steel isn't rubber. It's rigid.
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  It fights back. And that's where bending stress
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  comes in. This happens at structural
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  discontinuities. Yeah, Think about the corners
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  where a head meets the shell, or where a nozzle sticks out.
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  The shape changes and the metal has to bend.
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  OK, so membrane is stretching. Bending is, well, bending.
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  But here's where it gets nuanced.
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  The notes talk about a thin wall assumption.
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  Yes, and there's a specific check for this.
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  It's the radius to thickness ratio.
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  We look at R sub M over TR. Sub M / t OK.
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  If the radius of the tank is more than 10 times the thickness
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  of the wall, we call it a thin walled vessel.
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  Why 10? What's magical about that
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  number? Because it simplifies the
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  physics. If it's thin walled, we assume
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  the stress is biaxial. It's only pulling in two
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  directions, lengthwise and around the girth.
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  We get to ignore the third direction.
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  The radial stress pushing into the wall itself.
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  Right in a thin wall, that squeezing stress is so small
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  compared to the stretching that we just we ignore it.
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  It's a family landing on an elephant.
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  But if that ratio drops below 10, yeah, if the wall gets
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  really thick. Now you have a thick walled
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  vessel, and now that squeezing stress becomes significant, it's
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  no longer a family, it's another elephant.
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  The stress state becomes triaxial and the math gets much,
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  much harder. So Division One just kind of
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  assumes you're building thin walls.
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  It does. If you're building a cannon
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  barrel, Division One formula simply won't work for you.
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  OK, so we know how the stress is distributed.
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  Now how do these things actually die?
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  Because my movie brain thinks huge fireball but the source
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  material says failure is way more nuanced.
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  It is. We use failure theories for this
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  and again it splits Division One versus Division 2.
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  Division One uses maximum stress theory.
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  What's the logic there? It's the oldest, simplest
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  theory. It basically says failure
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  happens when the pull in any One Direction hits the materials
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  yield point. Imagine pulling a piece of chalk
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  from both ends. Eventually it just snaps.
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  Exactly. It's intuitive, but it assumes
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  the material is brittle like chalk.
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  Modern steel isn't, it's ductile.
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  Which is why Division 2 uses maximum shear stress theory.
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  Precisely. The ductile materials don't just
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  snap, they slide the yield. This theory assumes yielding
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  starts when the shear stress reaches a critical point.
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  It's like sliding a deck of cards.
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  The failure happens at a 45° angle.
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  So the molecules are sliding past each other, correct?
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  And calculating for that sliding failure is much more accurate
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  for the ductile steels we use today.
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  So, knowing how we predict it, what are the actual real world
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  failure modes? Well beyond bursting, you have
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  elastic deformation. The vessel gets to bendy or
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  buckles, then there's brittle fracture.
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  The text mentioned that with hydrotest that sounded scary.
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  It is an engineer's nightmare. Steel gets brittle when it's
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  cold. If the water you use for testing
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  is too cold, say 40 or 50°F, and you have a tiny flaw, the vessel
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  doesn't stretch, it shatters like dropping a glass vase.
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  A massive steel vase? That's terrifying.
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  It has happened. Then you have excessive plastic
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  deformation where just stretches and doesn't bounce back.
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  And finally fatigue. This is the silent killer.
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  Cyclic service, so filling and emptying over and over.
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  Pressurizing and depressurizing. It's like bending a paper clip
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  back and forth. You do it 100 times and it
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  snaps. And the notes highlight that bad
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  design details can cut your strength in half here.
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  Oh, easily. We call them stress razors,
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  sharp corners, bad welds. In fatigue service, those little
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  details are where the cracks start.
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  The vessel doesn't fail from the general stress, it fails at that
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  local concentration. OK, we've covered internal
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  pressure. But there's another enemy, one
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  that seems even sneakier. External pressure.
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  The Invisible Crusher. This is usually vacuum right?
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  The tank is empty and the atmosphere is trying to crush
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  it. Correct, or it's jacketed with
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  steam pressure on the outside and the physics are completely
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  different. Internal pressure makes a vessel
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  stable. It wants to be a circle.
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  External pressure wants to flatten it.
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  Exactly. And here's the surprising fact.
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  A vessel under external pressure can fail substantially below the
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  yield strength of the material. How is that possible?
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  Because it's not a failure of material strength, it is a
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  failure of geometry. It's called buckling.
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  The soda can analogy. The best one?
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  Take an empty soda can. You can stand on it if it's
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  perfectly upright, but tap the side just a little.
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  And crunch, it collapses instantly.
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  That is buckling. It's an unstable state, and for
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  external pressure the length of the cylinder matters immensely.
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  A long cylinder is much easier to buckle than a short 1.
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  So if you have 100 foot tall tower under vacuum, how do you
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  stop it from crumpling you? Cheat.
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  You add stiffening rings around the outside.
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  The hoops you see on those towers.
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  That's what they're for. They're structural cheats.
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  They act as boundaries. So the physics doesn't see 100
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  foot tower, it sees 1010 foot towers stacked on top of each
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  other. You chop the effective length
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  down. That's.
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  Clever. OK, another invisible threat,
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  the cold. We touched on brittle fracture,
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  but let's talk about the design parameter.
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  MDMT, minimum design metal temperature.
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  This is a huge battleground in design.
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  A battleground. Why?
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  It's about money. To prove a metal is tough enough
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  at low temperatures, you have to do Sharpie impact testing.
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  You freeze a sample and smash it with a pendulum.
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  That sounds expensive. Very expensive and time
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  consuming, so the goal for many designers is to play the
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  exemption game. I saw the charts and the notes.
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  How do you win this game? You have a few levers.
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  You can upgrade the material or, and this is counterintuitive,
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  you can increase the thickness. Wait, making it thicker helps
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  avoid testing? I thought thicker metal was more
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  prone to brittle fracture? It is, but if you make it
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  thicker than required, the stress in the metal is lower for
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  the same pressure. Lower stress means you're
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  further from the danger zone, so the code lets you operate at a
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  lower temperature without testing.
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  You're buying your way out of the test with extra.
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  Steel. Exactly.
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  And there was one other loophole that really caught my eye.
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  Stress relief. Post Weld heat treatment, yeah.
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  Even if the code doesn't require you to bake the vessel to
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  relieve welding stresses, if you do it anyway, the code rewards
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  you. You can reduce your minimum
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  allowable temperature by 30°F just for doing that.
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  So it's often cheaper to cook the tank in a giant oven than to
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  run all the tests. Often yes.
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  It's a game of trade-offs. OK, let's step back and look at
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  the vessel as a structure. These things are sitting
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  outside, exposed to the elements.
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  Right. A tall tower isn't just a tank,
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  it's a cantilever beam stuck in the ground.
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  It has to resist wind and earthquakes.
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  I was surprised wind isn't just about pushing.
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  No, that's the static part. The dangerous part is dynamic.
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  You get vortex shedding. As wind flows around a cylinder,
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  it creates these alternating swirls on the backside.
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  It pushes left, then right, then left like a.
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  Flag flapping. Exactly, and this can make the
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  power vibrate. If that vibration matches the
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  towers natural frequency, you're in real trouble.
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  And there's a ratio for that too.
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  If the height to diameter ratio over D is greater than 15, you
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  have to investigate that dynamic stability.
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  The wind could literally shake the vessel part.
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  Wow. Now what about where it meets
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  the ground? The skirt?
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  What if the vessel is at 800°? And the skirt is anchored to
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  concrete at 60°. You have a massive thermal fight
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  at that Weld. The vessel wants to expand, the
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  skirt wants to stay small. The Weld will RIP apart.
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  How do you? Fix that.
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  The text mentioned a hot box. It's a clever, simple solution.
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  You build an insulated chamber at the top of the skirt.
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  It traps heat from the vessel itself to warm up the top of the
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  skirt. So the top of the skirt gets hot
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  and expands with the vessel. Right.
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  It creates A gentle temperature gradient instead of a thermal
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  shock. It saves the Weld.
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  Smart. What about horizontal vessels?
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  The propane tanks? They sit on saddles.
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  Right, and you can't just rest a heavy tank on a concrete block,
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  it would puncture the shell. So how big should the saddle be?
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  The optimum is generally 168°. It wraps almost halfway around
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  the bottom and distributes the weight perfectly.
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  And I saw where plates mentioned yes.
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  You don't Weld the rigid saddle directly to your pressurized
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  shell. You Weld a where plate to the
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  shell first. It's like a second skin that
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  distributes the load. OK, final segment, the weight of
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  steel. You think with all this math
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  we'd know exactly what these things weigh, but the text says
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  the calculated weight is always less than the real weight.
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  Why? Several reasons, but the biggest
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  is plate overage. Overage.
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  Steel mills are businesses. If you order 1/2 inch thick
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  plate and they roll it to .499 inches, you'll reject it.
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  It's out of spec so they always roll it slightly thick to be
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  safe. So you're paying for steel you
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  didn't technically asked for? And lifting it.
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  Then you have thinning allowance for the heads, the curved ends.
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  When you shape it, it gets thinner.
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  Right, so to get a one inch thick head, you might have to
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  start with a 1 1/2 inch thick plate.
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  That's a lot of extra metal. It is, but here's the kicker.
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  The heaviest the vessel will ever be is usually during
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  testing. The water trap.
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  Right. We test them by filling them
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  completely with water. Water is heavy, often much
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  heavier than the gas or oil it will eventually hold.
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  So if you design the foundation just for the operating weight.
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  You are in for a bad day. The moment you fill it with
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  water for the test, the whole thing might sink into the mud or
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  collapse the supports. That sounds like an incredibly
• 14:02
  expensive mistake. It's a classic rookie error.
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  You have to design for the wind, the cold, the vacuum, the
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  pressure, and even the water you use to prove it works.
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  It really is a balancing. Act.
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  It's a compromise between internal pressure trying to
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  burst it, external pressure trying to crush it, heat trying
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  to expand it, and the accountant trying to make it cheaper.
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  And the engineer is sweating in the middle.
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  Always. Next time you drive past a
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  refinery or chemical plant, realize those tanks aren't just
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  static metal buckets. They are carefully engineered
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  balloons, constantly fighting the urge to either burst or
• 14:36
  crumple. And luckily for us, the math
• 14:38
  usually wins. Usually, if you haven't already,
• 14:42
  get over to our YouTube channel Mechanical Engineering Made
• 14:44
  Simple, where we post hands on how to videos, visual
• 14:48
  breakdowns, and practical engineering walkthroughs.
• 14:50
  You can actually use a family that solves together, stays
• 14:53
  together. See you next.
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  Time.