How to model branch fitting (i.e. tee, weldolet, sockolet,etc..) on strait pipe using one of 3 methods:
1. Single Point Method
2. 2-Point Method
3. 3-Point Method.
Background:
Piping stress analysis programs such as AutoPIPE and CAESAR model branch connections as a single point on the centerlines of the intersecting pipes. This becomes less and less accurate as piping diameters increase. Larger pipes begin to act like thin shell cylinders. For straight runs and elbows, the stress program model is adequate, but at branch connections, the behavior of the pipe does not match the single point model of the stress analysis programs. In order to more accurately model a small bore branch connection on large bore pipe, a 2-point or 3-point model must be used.
This modeling approach assumes the use of a weld on fitting (integrally reinforced; aka sockolet, weldolet, threadolet, etc..) is required to attach the branch piping to the header pipe. However this procedure can be adapted for other types of branch connections (i.e. welded-tee fitting, reinforced fabricated, Unreinforced fabricated, extruded, welded-in contour, etc..).
Vocabulary:
In-plane = Longitudinal = the branch pipe bending in the direction of the axis of the header pipe (the moment vector is perpendicular to both the branch axis and header axis).
Out-plane = Circumferential = the branch pipe bending about the circumference of the pipe (the moment vector is parallel to the header pipe axis).
Axial (to the branch) = radial (to the header wall) = the branch pipe pushing or pulling in and out of the header pipe wall. The user can determine the local in-plane and out-plane directions for the flexibility input. The Flex Joint input window uses local coordinates for X, Y, and Z.
• The local x axis is always axial to the branch pipe axis (flex joint axis).
• For pipes that are not vertical, the local y axis always points in the direction of the Global Y axis, and the local z axis is perpendicular to the local x and local z axis.
• For vertical pipes, the local y axis points in the direction of the Global Z axis, and the local z axis is perpendicular to the local x and local y axis. Using the definition for In-plane (longitudinal) and Out-plane (circumferential), the user can orient the proper flexibility numbers to the correct local axis.
Method #1 –1 Point model
Large bore header, small bore branch (does not include pipe wall flexibility)
• Model the branch connection (aka Tee) as defined by AutoPIPE’s online help, and set Tee type = Fitting (AutoPIPE will calculate the correct SIF).
Method #2 –2 Point model
• Model the branch connection as usual. Set Tee type = Other, Sif = 1.0 in and out.
• On the branch, add a run point where the fitting (i.e. weld-o-let or sock-o-let) attaches to the header OD, referred as “point 2”; in the sketch below = B02. Run length should equal to the radius of the header (additional length required for reinforced branches.).
• Highlight the pipe element between the header branch point, A01, & “point 2”, B02; make this element rigid (In AutoPIPE, use “Insert / Rigid Options Over Range”, Include weight = checked OFF, and thermal expansion = checked ON).
• On “point 2”, B02, insert a User SIF. To keep things simple, and a little conservative, calculate the outplane SIF for the branch type being modeled, and apply it to both the in-plane and out plane SIF’s (see method #3 - 3-point model in order to specify both in-plane and out-plane SIF’s).
NOTE:
1. It is Not recommend in making the header run rigid, only the branch run from A01 to B02 should be made rigid. For the intersection point A01, suggest changing the Type of Tee = Other and enter an SIF of 1.0; since SIF is moved to connection point at B02. In reviewing the results reports no stress was calculated on tee header, AutoPIPE will ignore stress on rigid pipes or in new version of the program, turn ON/OFF Tools> Model options> Results> Show rigid tee stress.
2. When modeling a Sock-o-let or Thread-o-let, recommend modeling length of fitting from B02 to B03. Insert> Xtra Data> Joint type and User SIF> Joint End Type = (select correct end type connection).
Method #3 - 3 point model
Large bore header, large bore branch (includes the pipe wall flexibility). This method can also be applied to small bore pipe branches, if desired.
• On the branch, add a point where the small pipe attaches to the header OD, “point 2”. (This point is at the header pipe radius for reinforced or unreinforced branches.)
• Starting at “point 2”; B02, insert a flex joint in the direction toward the header centerline with a length equal to the wall thickness of the header, “point 3”; B03. The data for the flex joint will be determined in the steps below.
• Highlight the pipe element between the branch point, A01 and “point 3”; B03, and make this element rigid (In AutoPIPE, use “Insert / Rigid Options Over Range”, Include weight = checked OFF, and thermal expansion = checked ON).
• On “point 2”, B02, insert a User SIF. See the following steps for the SIF details. Note that the Stress Analysis Program cannot determine the in-plane or out-plane directions for “point 2” since “point 2” is not an elbow or tee point. See step #2 from Method #4 below for procedure to apply the SIF and flexibility values.
Method #4 -PROCEDURES FOR MODELING BRANCH CONNECTIONS USING FE ANALYSIS OUTPUT
STEP 1:
Run FE analysis outside of AutoPIPE and record output. An example is shown below.
Table 1: Output from FE Analysis (converted to correct units for AutoPIPE)
STEP 2:
Input In-plane & Out-plane SIF’s. As a first guess, you may input SIF’s in the same order as outputted by FE analysis.
Fig. 1: AutoPIPE SIF input window
STEP 3:
Using the procedure described above, input the corresponding flexibilities from table 1 into AutoPIPE
Fig. 2: Flexible Joint Input window from AutoPIPE
STEP 4:
Perform the analysis. In “Tools / Model Options / Result…”, change the second entry Force (Global/Local): to L for local. Create a “Result / Output report “and select Forces_Moments and Code Compliance. In our case, node B01 is our “point 2”, and we have previously figured out that the local z axis is the in-plane direction; therefore 1170 is the in-plane moment. Similarly, the local y axis is the out-plane direction, so 1376 is the out-plane moment.
Table 2: Local Force & Moment table from AutoPIPE Output
STEP 5:
Look at code stress and determine if the in-plane moment 1170 is being multiplied by the correct SIF of 6.8, which is our in-plane SIF. (Be aware that the in-plane and out-plane moments we determined in the previous step might not be listed under the In-Pl. and Out-Pl. columns in the Code Compliance report, since AutoPIPE is ‘guessing” about which one is which! The important thing is that the numbers match up.) Are the SIF directions correct? If they are not, go back to step 2 and reverse SIF directions. In this case SIF of 6.8 matches the in-plane moment of 1170, and the SIF of 20.3 matches the out-plane moment of 1376, so our assumption in step 2 was correct;
Table 3: Code Stress from AutoPIPE Output
Written By: Alan S. Lucas & Mike Dattilio
Note: See models here.
Tee, Cross, or Branch Piping Components - Modeling Approaches
Bentley AutoPIPE