Socket-welded fitting dimension problem

[razzendahcuben]

The fittings catalog I have gives the following hub diameter for a 1/4" elbow for use with standard/sch. 40 pipe:

B = 0.90625

B is shown here:

http://www.bonneyforge.com/images/specs/fu-24a.gif

I should be able to get the outer diameter of the hub by multiplying the wall thickness by 2 and adding it to OD.

OD + 2*wall = 0.54 + 2*0.88 = 0.716

Why is this not equal to B?

Obviously my understanding of fittings is lacking so if someone could please explain what's going on I'd be appreciative. I'd also like to know if the wall thickness in a socket welded and screwed fitting remains equal throughout. Thanks.

 

[racookpe1978]

No. The wall thickness of the pipe has nothing to do with any dimension in the socket weld fittings.

Every socket welded fitting is sized (its ID only!) to smoothly slip around the designated pipe OD. The fitting ID will be slightly larger than the OD of the pipe to allow a little manufactoring tolerance in pipes and fittings. This also gives the pipefitter foreman some "wiggle" needed every now and then to actually get the things to fit together.

The "hole" in the socket weld fitting is for proper flow from inlet pipe ID through the fitting to outlet pipe ID - but since different schedule pipes with ID's can be used on different pressure-rated forged fittings with different ID's, this hole is not always an exact match.


The OD of the socket "cup" doesn't matter to anybody in real life, except to finicky CAD programmers who had to model it. (A previous life of mine.) The OD of the cup is always large enough to acccept the fillet weld that will join the pipe to the socket weld fitting.

The depth of the socket weld cup is sized for penetration + 1/16 expansion margin (of the pipe) as it is fillet welded. If there were no expansion margin, the pipe would expand when heated, push on the bottom of the socket weld fitting, and overstress the fitting.

 

[racookpe1978]

Threaded fittings:

The outside dim's of threaded fittings are about the same sizes as socket weld fittings of the same pressure rating - one suspects this is for convenience of casting and forging or catalog printers, but it doesn't really matter.

The ID of threaded fittings IS smaller than the OD of the pipe because standard male NPT pipe threads are cut into the outside of cut-off lengths of 21 foot standard pipes. So the (female) fitting threads ID is smaller than the pipe OD by the tapered thread depth.

 

[razzendahcuben]

I appreciate the help. I am modeling fittings with SolidWorks, actually, which is why I need to know these things.

Fair enough about the "hub" OD. I'll use the catalog value and then assign an slightly smaller, arbitrary value to the diameter of the rest of the elbow.

But I still need an ID for the elbow channel itself. If I am not supposed to use the schedule then I'm at a loss as to where this dimension is supposed to come from. Suggestions?

Thanks for the info on threaded fittings; that helps, too. I'm not going to model the threads, however, so I should be able to use my socket-weld models as is, no?

 

[racookpe1978]

From the standard diagrams, you know the centerline bend radius from end to end of the fitting.

You know the Pipe OD for each size fitting. As very, very good approximation, create a torus from inlet to outlet with the torus radii equal to pipe OD. It will work very well.

The "visible ID" of the forged fitting in a generic plan view is a result of the pipe torus curving around the centerline of the elbow from end to end. Unless you are making the actual forgings, castings, and molds for the 90 deg elbows - nobody will ever worry about these small difference from the finished part.

 

[racookpe1978]

Sorry - That should be torus dia = pipe OD.

Torus radii = dist A in your diagram, post nbr 1

 

[razzendahcuben]

Thanks for the response.

However, and please excuse my imprecision, but I was talking about the flow ID, not the "hub" ID. The latter is equal to the pipe OD, but the former is smaller. So, once again, if I'm not supposed to use the schedule then I'm not sure where this value comes from.