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Get a tailored quotation for pressure vessel heads, stainless spheres, or firepit bowls. Our engineers review your drawings, materials, standards, and quantities to provide accurate pricing and delivery options.
An “ellipsoidal head” is a 3D head shape based on an ellipsoid (most commonly a 2:1 ellipsoid of revolution), while “elliptical head” is often a loose industry shortcut that may mean the same thing—or may mean “semi-elliptical” without specifying the ratio.
That one sentence explains 90% of the headaches I see on RFQs.
Because people mix 2D and 3D terms—and because “elliptical head” became shop slang.
But in day-to-day manufacturing talk, buyers will say elliptical head when they really mean 2:1 ellipsoidal head (also called a semi-elliptical head in some contexts). And yes, that “some contexts” part is the trap.
In many pressure-vessel shops, “elliptical head” = “2:1 ellipsoidal head”… unless someone says otherwise.
If you’re working to common pressure vessel expectations (ASME-style terminology in a lot of global trade), the default “elliptical” people intend is usually 2:1.
But I don’t trust defaults. I’ve seen “elliptical” used for:
So if your spec only says “elliptical head,” you’re leaving room for interpretation. That’s not a fun game when tooling, forming depth, and inspection plans are already tight.
An ellipsoidal head is typically a segment of an ellipsoid of revolution, where the meridian curve follows an ellipse.
Translation: take an ellipse and spin it around the vessel axis. That rotated surface is your head.
“2:1” means the ellipse’s major axis is twice the minor axis (in the classic convention used for these heads).
In practical shop talk, the key takeaway is the proportion you can sanity-check fast:
So if your vessel ID is 2000 mm, you’re roughly looking at ~500 mm inside depth (before you get picky about straight flange, thinning, and tolerances).
That quick check catches a lot of drawing mistakes.
Because “elliptical” without a ratio can lead to the wrong forming depth, different stress behavior, different material thinning patterns, and code/design verification headaches.
And because procurement reality is brutal: once a shop starts forming, your “clarification” becomes a change order.
Here are the real consequences I see most often:
Simple? Not really. But avoidable? Yeah.
In proper engineering language, there isn’t a “vs” if “elliptical” is being used as shorthand; the “vs” only appears when the ratio or definition is unclear.
So the more useful comparison is: 2:1 ellipsoidal versus “some other implied ellipse/ratio.”
As the shape gets “deeper” or “shallower,” you change stress distribution, forming difficulty, and head height.
In the real world, most buyers choose 2:1 ellipsoidal because it’s a widely recognized middle-ground:
Ask for the ratio and the drawing callout—or infer it from the given depth and diameter.
If you already have numbers, do this quick sniff test:
If yes, you’re probably looking at a 2:1 ellipsoidal head.
If not, you need clarity. Don’t guess.
Use “2:1 ellipsoidal head” (or explicitly state the ellipsoidal ratio) and include dimensional definitions like ID, thickness, straight flange, and code.
If you want fewer emails and fewer “please confirm” messages, write it like you mean it.
State the geometry, ratio, code, and critical dimensions in one place. For example:
That’s not “extra.” That’s how you stop surprises.
This table is basically a translation layer between shop-speak and spec-speak.
| Term you’ll hear | What it often means in practice | What to write if you want zero ambiguity |
|---|---|---|
| Elliptical head | Usually 2:1 ellipsoidal head | 2:1 ellipsoidal head (state ratio) |
| Ellipsoidal head | An ellipsoid-based head (often 2:1) | 2:1 ellipsoidal or (ratio):1 ellipsoidal |
| Semi-elliptical head | Sometimes 2:1 ellipsoidal, sometimes loosely used | Avoid slang; write 2:1 ellipsoidal |
| Ellipse / elliptical | 2D curve description | Only use for sketches; not the final head spec |
| Ellipsoid / ellipsoidal | 3D surface description | Best term for the actual formed head |
No—an ellipsoidal head follows an ellipsoid-based curve, while a torispherical head is built from a spherical crown plus a toroidal knuckle.
I’m bringing this up because I’ve seen RFQs where “elliptical” was used, but the drawing proportions looked torispherical-ish (shallower crown, defined knuckle). That’s a separate head family.
If your drawing shows a clear crown radius and knuckle radius, you’re drifting into torispherical territory. If it’s a smooth continuous ellipse-like profile, you’re likely in ellipsoidal territory.
If both terms refer to the same 2:1 geometry, manufacturing is the same; the risk is that the spec ambiguity causes the wrong tooling, wrong depth, or wrong QC checks.
Typical forming routes (depending on thickness/diameter/material):
Where it gets real:
Again: same process if same geometry. Different process if the “elliptical” word hid a different ratio or expectation.
Confirm geometry definition, minimum thickness after forming, and inspection criteria before production starts.
Here’s my practical checklist:
If you want to be extra safe, request a dimension report (depth, diameter, flange) and photos of measurement setup. Not fancy. Just disciplined.
If you’re using “elliptical head” on your drawings right now, I’m not here to shame you. I get it. It’s common. But tell me this: when you say “elliptical,” do you always mean 2:1? Or have you seen it interpreted differently on a real job? Drop a comment—those real-world miscommunication stories are where buyers learn fastest.
