Problem:

What are some typical modeling approaches for Victaulic / Grinnell grooved fittings?

Solution:

Most grooved coupling come in two types, rigid and flexible. The rigid coupling rigidly connects components / pipe together, while the flexible coupling allows both axial and angular movements within limits

The Bentley CAE development team has developed methods of modeling grooved fittings using Bentley's AutoPIPE stress analysis program.

Model Rigid Coupling

The Rigid Coupling is easier to model using one of the following techniques:

Note:  Beam properties, some values cannot be extremely large: (ex. setting modulus as 1e8 in a field that has units of e6, stored value would become 1e14). Setting the modulus value for the rigid to a more reasonable value (say 1000 times steel modulus = 200 E6 N/mm2) resolves the problem

Method #1: Model Rigid coupling as a Flange component

Model coupling using regular flanges with socket or threaded connection (SIF between 2.1 and 2.3). You would need to enter the proper flange weight equal to that of the coupling.

Note: modeling the coupling as a flange would cause connected bends SIF and flexibility to decrease since flanges prevent bend ovalling. Couplings will not have same effect as flanges on bends, so you may model couplings near bends as added weights with user SIF (Insert > Xtra Data > Joint Type and User SIF).

Method #2: Model Rigid coupling as a Flexible Joint component

Model coupling using flexible joint component. The Flexible Joint component allows the user to specify rigid axial, shear, and bending stiffnesses. However, also allows the user to specify a smaller more realistic torsional stiffness. 

Note: recommend weight to be equal to that of a coupling, and Pressure Area should be set to 0.00 to avoid added pressure thrust component to the system.

Model Flexible Coupling

The bases of these modeling approaches was presented in a published article; "Flexible Coupling: modeling of bi-linear moment rotation relationships in AutoPIPE"  by Nasir Zulfiqar (see attachment, Flexible Coupling_Nasir Zulfiqar.pdf)

The flexible coupling model is more complicated and involves modeling axial and angular flexibility.  The use of a flexible joint element is recommended. The shear stiffness’s of the joint can be set to rigid and the torsion and bending stiffness’s estimated.  The axial and bending flexibility is modeled using 4 rigid spider beams on each end of the flexible joint and these are connected using 4 tie-link supports with gaps.  This model is similar to flexible joints model with tie rods.
 
The determination of the stiffness and gap settings for the 4 tie-link supports is somewhat involved and is explained a little better in the articles mentioned on this WIKI page:

a. Flexible Coupling_Nasir Zulfiqar.pdf

b. Modeling Victaulic Flexible coupling Method-2_Rev 01.pdf

Note:

1. Modeling every flexible coupling in a model (say 10 or more flexible couplings) could be a cause of nonlinear convergence issues.

2.  Check Beam properties, some values cannot be extremely large: (ex. setting modulus as 1e8 in a field that has units of e6, stored value would become 1e14). Setting the modulus value for the rigid to a more reasonable value (say 1000 times steel modulus = 200 E6 N/mm2) resolves the problem

Method #1: Model Flexible coupling using  Tie-links, rigid frame members and a Flexible Joint

The 1st method has the following attributes:

• Captures the rotational gapped movement

• Captures axial gapped movement

• Gravity load is transferred across the joint

• Pressure thrust automatically calculated on Anchors with axial expansion joint

• No weights will be considered for the rigid frames, as the weight of the coupling has been defined by the expansion joint

Bentley visited Victaulic in 1998 and gave a presentation on the use of AutoPIPE to model Victaulic couplings. The AutoPIPE model for a Victaulic coupling is based on using tie-links, rigid frame members and a flexible joint.

The orientation of the frame members and tie-links can be in any direction including relative to the local pipe cords and resulting model gives a bi-linear spring rate action where the gaps on the tie-links can be set to simulate a limit on angular rotation before the spring rate of the tie-link becomes active.

Discussions with the Victaulic people felt this modeling approach reasonable but the only problem was the lack of bending stiffness data available at that time for Victaulic joints. Bentley cannot provide the exact stiffness values to be used, recommend contacting Victaulic Engineering Manager. Since Victaulic does not publish stiffness data in their literature, the CAE development team approximated all the values used when developing the modeling approach.

Basic type of construction:

1. Insert a flexible joint with information gathered from Victaulic's catalog / personnel.
2. On each side of the flexible joint insert 4 rigid beam members (Up, Down, Left, & Right)
3. Connect the far ends of coinciding beam member together with a tie-link support.

Method #2: Model Flexible coupling referring to attached document procedure

The 2nd method has the following attributes

• Does not have the limitations of Method 1 of having to define a spring stiffness to obtain the exact moments and rotations at the center of the flexible joint

Please read the attached document (Modeling Victaulic Flexible coupling Method-2_Rev 01.pdf) for complete information on this method of modeling a grooved fitting.

Basic type of construction:

Similar methodology as Method #1, but with different values based on coupling geometry and movement.

Note: Setting the beam modulus value for the rigid to a more reasonable value (say 1000 times steel modulus = 200 E6 N/mm2) prevents analysis problems from an unstable model.

Example Model:

Please see the attached Victaulic2.DAT model.

See Also

Model Different Types of PIPING Components

Bentley AutoPIPE