Vessels
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AutoPIPE Modeling Approaches
Vessels
Vessels are common components in piping systems. Most piping originates or terminates at a vessel. The vessel usually acts as a
support for attached piping, thus its flexibility can have a significant effect on piping because of its ability to move under loading. Exact
modeling of a vessel can be quite complicated due to the vessel shape, support type, or presence of internal features. Fortunately, a
complete model is not necessary for the purpose of a stress analysis of the attached piping system.
If it is determined that the overall flexibility of the vessel structure can be neglected, then only the local flexibility of the vessel shell
(at the nozzle/vessel interface) need be modeled. If the nozzle flexibility components have been pre-calculated, they can be modeled
directly by defining a flexible anchor. It is strongly recommended, however, that the local shell flexibility be modeled using
the Nozzle command. The following model describes the procedure for modeling a vessel which considers the overall flexibility of the
structure, and the local shell flexibility at the nozzle/vessel interface. AutoPIPE Nozzle can be used to analyze the local stress at the
vessel/nozzle pipe connection.
Example
Modeling
In AutoPIPE, a vessel structure is typically modeled using a pipe of equivalent diameter and wall thickness. Irregular sections of the
vessel such as skirts and cones can be modeled as reducers, or as pipes based on the average of the end diameters. Temperatures at
various sections of the vessel are specified as operating loads. The pipe/vessel connection is modeled as a nozzle. The base of the
nozzle is connected to the centerline of the vessel by a rigid pipe so that the movement of the vessel is transmitted directly to the
piping system.
For this example, assume that an 8" pipe is connected to a 12' diameter by 30' tall vessel at a height of 4' above the rigidly anchored
vessel base. The vessel has an internal stiffening diaphragm at the 10' and 20' elevations, and the shell wall is 2" thick. Assume that
the control valve is open so that both the vessel and the piping are subjected to 400 psi and 600°F. For simplicity, the pipe flanges will
be omitted.
1. Build the connecting piping system using an 8" standard schedule pipe called 8"STD. The global coordinates of the system
points are listed below (length units are feet, and offsets are measured from the preceding point):
2. Select Insert > Piping Components > Valve to open the Valve dialog. Define an GLOBE-F valve with a Pressure rating
of 300 to point A04 along the global X axis.
3. Select Insert > Piping Components > Run new point A05 and enter vessel nozzle length of 6" in DX direction.
4. Select Insert > Piping Components > Nozzle to open the Nozzle dialog. Define a 2" (i.e. same as wall thickness) long
nozzle, in the DX direction, to point A06. Specify a radius of 6' (units of the radius field are already in feet) and a wall
thickness of 2" (units of the thickness field are already in inches). Specify a Flexibility method of Bijlaard, then enter the
distances to the closest stiffened elements as shown (L1 = 4' and L2 = 24'). The direction of the vessel axis is Global Y.
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Vessels
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Note: The current version of AutoPIPE does not include the unbalanced pressure effect which tends to blow the pipe off
of the vessel. In order to model this effect, a concentrated force should be specified at point A04 (in the -X direction).
AutoPIPE Nozzle does include the pressure thrust force on the nozzle in the local stress calculations.
5. Now we will define the segment of pipe which models the vessel structure. Select Insert > Piping Components >
Segment to begin a new segment. Accept the new segment name (B). Define point B00 at the coordinates shown (offsets
are from the origin, A00). Next, specify a pipe called VESL.
Comp.
6.
Point
DX
DY
DZ
Type
B00
20.54
0
0
rigid anchor
The Pipe dialog is automatically displayed so that VESL can be defined. Specify a nonstandard (NS) nominal size, then enter
the Actual O.D. as 144" and a 2" thickness. Note that the material properties remain the same as the 8" pipe.
7. Select Insert > Supports > Anchor/NodeLink to open the Anchor/NodeLink dialog. Define a rigid anchor at B00. This
arrangement represents a continuous weld around the base of the vessel.
8. Define the critical points of the vessel. The global coordinates of the system points are listed below (length units are feet,
and offsets are measured from the preceding point):
Comp.
Point
DX
DY
DZ
RUN
B01
0
4
0
RUN
B02
0
26
0
Type
Note: All that remains to complete the model is to connect the nozzle with the vessel using a rigid element. This is done
so that the movement of the vessel (at point B01) is transferred directly to the nozzle at the vessel surface (point A05).
This can be done by creating a new segment to connect B01 to A05, or by continuing segment A as shown below.
9. Make A06 the current point, then select Insert > Piping Components > Run to open the Run Point dialog. Specify B01
as the point to run to. Since this point already exists, the Tee dialog will be displayed next. Specify 8"STD in the selection
list as the pipe identifier. Since this pipe exists, all the parameters are recalled (this pipe data set has been included with
AutoPIPE and is always available).
10. Change the default type of tee (Welding) to Other and set SIF = 1.0 when the Tee dialog is displayed.
11. Click on the pipe between A06 and B01 to select this pipe (highlighted Red) then select Insert > Properties > Rigid
Options Over Range with Include Weight = No, Include Thermal Expansion = Yes.
Note: To transfer loads to AutoPIPE Nozzle, currently the vessel must be modeled as an anchor at A06 with applied
displacements in the GR, T1 etc load cases. The anchor option "Report anchor results for Local Shell Stress Analysis" =
checked.
The model appears as shown below.
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Vessels
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Display of the Vessel Model
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