PIPE18 Element Description

PIPE18, also known as an elbow element, is a circularly uniaxial element with tension, compression, torsion, and bending capabilities. The element has six degrees of freedom at each node: translations in the nodal x, y, and z directions and rotations about the nodal x, y, and z axes.

Options are available to include various flexibility and stress intensification factors in the formulation. The element can account for insulation, contained fluid, and a corrosion allowance. See PIPE18 - Elastic Curved Pipe for more details about this element.

Figure 18.1: PIPE18 Geometry

PIPE18 Input Data

The geometry, node locations, and the coordinate system for this element are shown in Figure 18.1: PIPE18 Geometry. The element input data include three nodes, the pipe outer diameter, wall thickness, radius of curvature, optional stress intensification and flexibility factors, internal fluid density, exterior insulation density and thickness, corrosion thickness allowance, and the isotropic material properties. The internal fluid and external insulation constants are used only to determine the added mass effects for these components.

Although the curved pipe element has only two endpoints (nodes I and J), the third node (K) is required to define the plane in which the element lies. This node must lie in the plane of the curved pipe and on the center-of-curvature side of line I-J. A node point belonging to another element (such as the other node of a connecting straight pipe element) may be used. Input and output locations around the pipe circumference identified as being at 0° are located along the element y-axis, and similarly 90° is along the element z-axis.

Only the lumped mass matrix is available.

The flexibility and stress intensification factors included in the element are calculated as follows:

where:

h = tR/r2

t = thickness

R = radius of curvature

r = average radius

E = modulus of elasticity

Xk = 6 (r/t)4/3 (R/r)1/3 if KEYOPT(3) = 1 and R/r 1.7, otherwise Xk = 0

P = Pi - Po if Pi - Po > 0, otherwise P = 0, Pi = internal pressure, Po = external pressure

Do not use KEYOPT(3) = 1 if the included angle of the complete elbow is less than 360/(π(R/r)) degrees.

Element loads are described in Element Loading. Pressures may be input as surface loads on the element faces as shown by the circled numbers on Figure 18.1: PIPE18 Geometry. Internal pressure (PINT) and external pressure (POUT) are input as positive values. The internal and external pressure loads are designed for closed-loop static pressure environments and therefore include pressure loads on fictitious "end caps" so that the pressure loads induce an axial stress and/or reaction in the pipe system. If a dynamic situation needs to be represented, such as a pipe venting to a lower pressure area or the internal flow is past a constriction in the pipe, these end cap loads may need to be modified by applying a nodal force normal to the cross-section of the pipe with the magnitude representing the change in pressure. Alternatively, the precomputed end cap loads can be removed using KEYOPT(8) = 1 and the appropriate end cap loads added by the user. Note that when using KEYOPT(8) = 1, the pressure load will be acting on only the wall of the elbow element so that the total pressure load will not be self-equilibrating. The transverse pressures (PX, PY, and PZ) may represent wind or drag loads (per unit length of the pipe) and are defined in the global Cartesian directions. Positive transverse pressures act in the positive coordinate directions. Tapered pressures are not recognized. Only constant pressures are supported for this element.

Temperatures may be input as element body loads at the nodes. Temperatures may have wall gradients or diametral gradients (KEYOPT(1)). The average wall temperature at θ = 0° is computed as 2 * TAVG - T(180) and the average wall temperature at θ = -90° is computed as 2 * TAVG - T(90). The element temperatures are assumed to be linear along the length. The first temperature at node I (TOUT(I) or TAVG(I)) defaults to TUNIF. If all temperatures after the first are unspecified, they default to the first. If all temperatures at node I are input, and all temperatures at node J are unspecified, the node J temperatures default to the corresponding node I temperatures. For any other pattern of input temperatures, unspecified temperatures default to TUNIF.

For piping analyses, the PIPE module of PREP7 may be used to generate the input for this element.

A summary of the element input is given below. A general description of element input is given in Element Input.

PIPE18 Output Data

The solution output associated with the element is in two forms:

Several items are illustrated in Figure 18.2: PIPE18 Stress Output.

The stresses are computed with the outer diameter of the pipe reduced by twice the corrosion thickness allowance. The direct stress includes the internal pressure (closed end) effect. Also printed for each end are the maximum and minimum principal stresses and the stress intensity. These quantities are computed at the outer surface and may not occur at the same location around the pipe circumference. Some of these stresses are shown in Figure 18.2: PIPE18 Stress Output. The direct stress does not include the axial component of the transverse thermal stress. The principal stresses and the stress intensity include the shear force stress component. Angles listed in the output are measured (θ) as shown in Figure 18.2: PIPE18 Stress Output. A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

Figure 18.2: PIPE18 Stress Output

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file jobname.out. The R column indicates the availability of the items in the results file.

In either the O or R columns, “Y” indicates that the item is always available, a letter or number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

The following tables list output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table of this manual for more information. The following notation is used in Table 18.3: PIPE18 Item and Sequence Numbers (Node I) through Table 18.5: PIPE18 Item and Sequence Numbers:

PIPE18 Assumptions and Restrictions

PIPE18 Product Restrictions

When used in the product(s) listed below, the stated product-specific restrictions apply to this element in addition to the general assumptions and restrictions given in the previous section.

Ansys Professional  —