	Applies To
	Product(s):	AutoPIPE
	Version(s):	ALL
	Area:	Modeling
	Original Author:	Bentley Technical Support Group
	Date Logged & Current Version	Jan 2015 09.06.01.10

Problem:

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.

Solution:

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).

Note:

1. When modeling a Sockolet or Threadolet, insert a node point at the precise location of the socket or threaded connection. At this connection node point, use AutoPIPE command to Insert > Xtra Data > Joint type and set User SIF > Joint End Type (select the appropriate end type connection). This procedure will ensure that the socket or threaded connection is accurately represented.

Method #2 – 2 Point model

Large bore header, small bore branch (does not include pipe wall flexibility)

Model the branch connection as usual. Set Tee type = Other, SIF = 1.0 in and out.
On the branch, add a run node point where the fitting (i.e. Sockolet, Weldolet, Threadolet, etc..) attaches to the header OD, referred as “point 2”; in the sketch below = B02. Run length should be 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 out-plane 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 to specify both in-plane and out-plane SIF’s).

NOTE:
1. It is not advisable to make the header rigid; only the branch running from A01 to B02 should be made rigid. At the Tee node point A01, recommend setting Tee Type = Other and to enter a SIF = 1.0, as the SIF is transferred to the connection point at B02. Upon reviewing the results, it was noted that no stress was calculated on the tee header. AutoPIPE will disregard stress on rigid pipes, confirm the setting for Tools > Model options > Results > “Show rigid tee stress”.

2. When modeling a Sockolet or Threadolet, insert a node point at the precise location of the socket or threaded connection. At this connection node point, use AutoPIPE command to Insert > Xtra Data > Joint type and set User SIF > Joint End Type (select the appropriate end type connection). This procedure will ensure that the socket or threaded connection is accurately represented.

Method #3 - 3 point model

Large bore header, large bore branch (including the pipe wall flexibility). This method can also be applied to small bore pipe branches, if desired.

Model the branch connection as usual. Set Tee type = Other, SIF = 1.0 in and out.
On the branch, insert a run in the direction of the branch piping, length header pipe ID radius, “point 2”, B02.
Next, insert a Flexible joint component with length equal to the wall thickness of the header, “point 3”; B03.

Note:

1. The data for the flex joint will be determined in the steps below.

2. Point 3, B03, is at the header pipe OD radius or surface of the pipe.

Highlight the pipe element between the header Tee point, A01 and “point 2”; B02, and make this element rigid (In AutoPIPE, use “Insert / Rigid Options Over Range”, Include weight = checked OFF, and thermal expansion = checked ON).
On “point 3”, B03, insert a User SIF. See the following steps for the SIF details.

Note

1. AutoPIPE cannot determine the in-plane or out-plane SIF values for “point 3” since it 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 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 Static analysis in AutoPIPE.

STEP 5: Generate Results Output report.

Prior to generating a report in AutoPIPE, open Results Model Options dialog, set Force (Global/Local) = L.

Create a “Result / Output report “ that includes Forces & Moments and Code Compliance (remove / add other subsections as needed).

Optional: Before generating a report, highlight parts of the piping system and use Report Option “Limit reports to highlighted points” for a focused information.

In our case, node B03 is our “point 3”, 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

02. How to model branch fitting (i.e. tee, weldolet, sockolet,etc..) on strait pipe using one of 3 m