×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

How to draw 2:1 Elliptical Head
3

How to draw 2:1 Elliptical Head

How to draw 2:1 Elliptical Head

(OP)
Hello folks. I want to see if anyone has a good reference for drawing a 2:1 elliptical head. I always draw them in CAD and get the volume to calculate the weight, but it's kind of a pain. I've got two xeroxs out of old layout books and they offer slightly differently rounded versions of the same approximation. Sometimes that's close enough but other times I'm left having trouble b/c my polylines won't join to make a revolution solid, or I can't make a region out of the shape, etc. That's AutoCAD specific problems as I don't have Solidworks and I still suck at Inventor.

So, good folks of the internet, does anyone have any older style ways to draw these suckers i.e. laying the thing out with arcs and bisecting etc? Or do I stick with my approximations that I've got. I'm not interested in gaining a potential tiny tiny tiny bit better approximation, I just want to find a way that makes the drawing easy.

THANKS!

RE: How to draw 2:1 Elliptical Head

Have you tried constructing per UG-32(d)?

You can just draw 2:1 ellipse in ACAD, but yeah, can't join the plines.

Regards

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

RE: How to draw 2:1 Elliptical Head

(OP)
SnTMan, I think that's where the approximations I have come from, and it's usually not too bad, just wondered if there was a 'traditional' drafting method that people used. Thanks for the reply.

FWIW, you can set the system variable PELLIPSE = 1 and get a polyline approximation instead of a mathematical ellipse in AutoCAD. Or, if you set PELLIPSE = 0 and draw a mathematical ellipse, you can offset it some amount and it degrades to a spline which you can then polyedit into a polyline. Not sure if you knew that, but I hope it helps someone.

RE: How to draw 2:1 Elliptical Head

If I remember right, ASME says a 2:1 head can be constructed as a torospherical head with crown radius of 0.9D and knuckle radius of 0.17D. That's the way I draw them. Calculate inside crown radius, knuckle radius, flange radius. Draw a circle with crown radius, draw a line through the center, offset it to each side for the flange radius. Offset both those lines and the crown radius by the knuckle radius and that intersection is the center of the knuckle arc. Draw it in, and start trimming.

I figure since a head manufacturer can make a head either "perfectly" elliptical or this shape, this is just as likely to be "right" as a true elliptical shape and simpler to draw.

The problem is that if you offset an ellipse, you don't get a bigger ellipse, and that messes things up.

RE: How to draw 2:1 Elliptical Head

"An acceptable approximation of a 2:1 ellipsoidal head is one with a knuckle radius of 0.17D and a spherical radius
of 0.90D." UG-32
This is only for drawings, not for design, manufacturing and inspections.

Regards
r6155

RE: How to draw 2:1 Elliptical Head

(OP)
Thanks for the input folks. Looks like I've pretty much been doing the same thing others do. Y'all have a good day!

RE: How to draw 2:1 Elliptical Head

Quote (r6155)

This is only for drawings, not for design, manufacturing and inspections.
That's an interesting claim. Please explain.

RE: How to draw 2:1 Elliptical Head

If you get tired of redrawing it every time, you can just copy/paste/scale from other drawings. Just keep a standard block for a 2:1 head and scale as required. Both the radii are related to the ID, so scaling will be accurate (except for straight flange, but that's easy to fix).

RE: How to draw 2:1 Elliptical Head

r6155, yes, please cite your reference.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

RE: How to draw 2:1 Elliptical Head

ASME VIII Div. 1 My last ed. 2011
My example: DP= 1000 psi, ID=60 in L= 0,9ID= 54 in, r=0,17ID= 10,2 in, S= 20000 psi SA 516-70; E=1

Ellip 2:1 tmín= 1,43 in PWHT: not required, only preheat

Torispherical tmín= 1,8 in PWHT: Yes

Hence, ellip 2:1 head is not equivalent to a torispherical with L= 0,9ID and r=0,17ID
They are equivalent only for drawings.

Regards
r6155

RE: How to draw 2:1 Elliptical Head

r6155, no need to drag torispherical heads into the discussion. If an elliptical head is approximated per UG-32(d) in it's actual fabrication it meets Code and is acceptable.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

RE: How to draw 2:1 Elliptical Head

SnTMan and r6155, interesting that this issue should be discussed at this specific time. At the ASME Boiler Code Week, next week, I have an item to discuss exactly this approximation provided in UG-32(c).

[As an aside, in the editions and addenda prior to 2015, this paragraph was sub-paragraph (d). However, in the 2011 Edition paragraph UG-32(b) was deleted, and the UG-32 subparagraphs were subsequently re-numbered for the 2015 Edition. If you look at the 2015 Edition, you will see that it is UG-32(c) now.]

As r6155 rightly points out, the calculated head thickness for the "approximate" torospherical head is higher than the "actual" 2:1 semi-elliptical head. So, the point that I will be bringing up is whether or not that approximation is appropriate. Before I do my presentation there, does anyone here have any thoughts or comments?

RE: How to draw 2:1 Elliptical Head

I insist. UG-32 in not used as tolerance. ¿Which is the meaning of "approximate" here?. The correct term is "out-of-true", hence UG-81 "tolerance of formed head" must be applied.

Regards
r6155

RE: How to draw 2:1 Elliptical Head

(OP)
Hey Marty007, I never really thought about it but I guess the dimensions are all linearly related to teh ID as you say, so I will definitely keep head lying about to copy/scale... thanks!

RE: How to draw 2:1 Elliptical Head

TGS4-In regards to the torispherical approximation, it is worth noting that this is how Division 2 handles the design of ellipsoidal heads. The required thickness for r6155's head would be about 1.37 inches using these rules with the Div. 1 allowable stress at 70°F. While they may be considered the same from a code calculation perspective, I'm sure the collapse pressures predicted by a FEA would be different.

RE: How to draw 2:1 Elliptical Head

Code approximation of crown 0.9D & knuckle 0.17D doesn't result exactly in 1/4D depth of the head which is basic dimension as the name of the head is 2:1. The reason for this is that the 0.17 has some other digits. So, I'm using SolidWorks to draw the head. I set the crown dimension and the depth, then knuck radius will be decided automatically by the geometry. It is normally about 0.171875D.

I can help you by giving you a sketch (*dwg format) generated by Solidworks in a dimeter, let's say, 1000 mm. Then you can scale it later on for any diameter will use.

Regards,
M.Salaheldin
Static Equipment Mechanical Design Engineer

RE: How to draw 2:1 Elliptical Head

I have assumed the intent of the approximation statement is that heads can be produced to approximately either shape and be considered equivalent.
Whether the thickness calculates the same applying the torospherical equation to the approximated elliptical head is a moot point. A similar discrepancy occurs when applying flange rating equations to 150# flanges, I believe.

TGS4, I think a major consideration would be to check how head producers actually produce heads. If they all produce "true" 2:1 form or all produce 90/17 or whether some do it one way and some the other.

RE: How to draw 2:1 Elliptical Head

Ellipse has infinite radius. When check tolerances with UG-81 it is impossible to measure infinite radius, hence an approximation 0,9D / 0,17D is used. Not exist spherical portion of ellipsoidal head.
If purchase order state ellip 2:1 head the manufacturer shall produce ellip 2:1 , not 0,9D / 0,17D(torispherical)

Regards
r6155

RE: How to draw 2:1 Elliptical Head

I went ahead and submitted a request for Interpretation on that point. #15-2512.

RE: How to draw 2:1 Elliptical Head

Thanks JStephen - you can also follow my item as 15-18341104 2177. If you don't object, I am going to request that your item be covered under my item, as well.

RE: How to draw 2:1 Elliptical Head

(OP)
abuanaselmasry , I would appreaciate a dwg if you don't mind. I've done them myself, but never had the option to check it against someone else's. Thanks!

RE: How to draw 2:1 Elliptical Head

Of course. This is ID 1000 mm internal head profile that I use to make any diameter of head by scaling the geometry, and offset the thickness and SF.
https://www.dropbox.com/s/0to5vjb6vmn95pa/Ellip%20...
Please check as well. Your feedback is most welcome.

Regards,
M.Salaheldin
Static Equipment Mechanical Design Engineer

RE: How to draw 2:1 Elliptical Head

(OP)
I see glaring differences between the two... we are an ENTIRE 1/256" off! Kidding, of course, looks essentially the same as what I have. Thank you.

RE: How to draw 2:1 Elliptical Head

I just got an email back on my request for interpretation above, thus- note the second question asked in particular:

Dear Sir,

Thank you for request for interpretation. After review, it appears that a previously issued interpretation may address this:

Interpretation: VIII-1-01-57
Subject: Section VIII, Division 1 (1998 Edition, 2000 Addenda); UG-32(d) and UCS-79
Date Issued: 06/11/2001
File: BC01-288

Question (1): A 2:1 ellipsoidal head is being fabricated from flat plate having a thickness t with a spherical radius of 0.90D and knuckle radius of 0.17D, where D = inside diameter of the head, based on UG-32(d) of Section VIII, Division 1. When applying UCS-79, where there is not an intermediate heat treatment performed during the forming operation, is the original center line radius equal to infinity and the final line radius equal to 0.17D + t/2?
Reply (1): Yes.

Question (2): Does the head described in Question (1), which is manufactured as a flange and dished head, and a head formed having the exact dimensions of a 2:1 ellipsoidal shape both meet the requirements of UG-32(d)?

Reply (2): Yes.

RE: How to draw 2:1 Elliptical Head

JStephen-

Thanks for closing the loop on this one!

RE: How to draw 2:1 Elliptical Head

I just wanted to add to this discussion that of the manufactures that we buy heads from, they all produce 2:1 heads as 90/17 heads.

Also, to the OP I would suggest contacting your head supplier to see how they handle 2:1 head manufacture. Then make your CAD model match what they do. We have found considerable differences between the two shapes in the "knuckle" area, which can impact the size and location of clips and other minor details.

RE: How to draw 2:1 Elliptical Head

(OP)

Thank you pperlich. That was kind of what I was thinking. We rarely use heads in items that we manufacture as our scope ususally is demo of old and installation of new (prefabricated) equipment. That usually involves welding on lifting lugs which brings up the issue you highlighted.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources