How to Improve Your Efficiency Using CAEPIPE?
Part 4b - The "Generate" Command - More examples

Tip of the Month July 2000

Part 4a introduced the Generate command (under the Edit menu in Layout) with two examples.

This part will finish this topic by providing two more examples.

For those of you who have not read Part 4a, this command lets you duplicate or copy existing data in the Layout window (spreadsheet). Examples could be copying existing expansion loops, piperacks, creating multiple segments of uniform sloped piping, or buried piping, or stacks of piping, among other creative uses.

Let the pictures tell the story!

Third example: Expansion loop

Objective is to create one or more set(s) of an existing section of piping (in this case, an expansion loop). As before, create one, and let Generate do the rest! See figures below. The first one is "before" and the next one is "after" the Generate command. In the graphics figure below, the numbers in cyan color (3 4, 5...) are row numbers that correspond with the rows in the Layout window.

We want to duplicate the section of piping from node 20 (bend) to node 60 (end of section). Let us make three copies.

The "Increase node numbers by" field needs to be understood. Essentially, we want to have the bend at node 20 repeat after node 60. So, the new bend should be node 70. So, the difference in the node numbers is 50 (70-20) which is the input for "Increase node numbers by" field.

A few of the node numbers will be duplicated such as node 60. The new loop starts "From" node 60. This is the same as continuing from the previous node 60.

Fourth example: Piping Stack

Starting from the figure above (node 10 to node 210), say we wanted to duplicate this whole length of piping but above it, as a stack; As if we need to "bodily lift" the bottom one. That is precisely what the Generate command does. The "before" figure is the figure above. The result of the Generate command is shown below. The piping from node 310 through node 510 was generated in one operation as shown by the Generate dialog below.

That is it for the Generate command. You may want to play with it to find more creative uses.

Modeling Tied Bellows

Tip of the Month August 2000

Tie rods are used in expansion joints to absorb the thermal growth in piping and eliminate pressure thrust. Tie rods are used on a number of expansion joint types.

How to model in CAEPIPE:

A bellows expansion joint may be modeled by typing "bel" in the Type column and entering the requisite information (see manufacturer's catalog for the technical data).

Tie rods symmetrically placed around the bellows may be simulated by using one limit stop (press "L" in the Data column) that connects the two bellows nodes ("From" and "To" nodes). This limit stop must act as a "tension only" element, i.e., have stiffness when in tension and no stiffness when in compression. The limit stop is in "tension" when the two nodes (limit stop node and the connected node) are moving away from each other which corresponds to the Upper Limit of the limit stop. Hence, the Upper Limit should be zero and the Lower Limit should be None.

Note that the stiffness for the limit stop is the collective stiffness of the four symmetrically placed tie rods. The data for a tie rod in this example are: E=30x10^6 psi, A=(pi)xd^2/4, (d=0.5"), and L=10". Stiffness = 4 x (AE/L).

Example: Please examine the data for the limit stop in the figure below.

Commentary on Stress Intensification Factors (SIF)

Tip of the Month September 2000

A B31 (B31.1, B31.3, B31.4, B31.5, B31.8, B31.9, B31.11) stress intensification factor (SIF) is an empirically derived parameter that allows the designer to estimate the fatigue performance of a piping component or joint. The concept of the parameter was developed in the late 1940s and early 1950s and was incorporated into B31.1 - 1955, which had separate chapters for different piping applications which eventually were published as B31.1, B31.3, B31.4, etc. The original background discussion of SIFs was published in ASME Paper 53-A-51, "Piping-Flexibility Analysis" by A.R.C. Markl. (This paper is highly recommended reading for any serious student of piping design.) In the 1955 B31.1 document, SIFs were tabularized in Chapter 6, but subsequent separate publication of the B31 books (B31.1, B31.3, B31.4, etc.) have in many cases resulted in different SIFs for the same piping components and joints. However, there is absolutely no justification for having different SIFs for different applications. The differences are from a lack of communication between the ASME committees responsible for the development of the B31 books.

The SIF (i) parameter was and still is determined through testing by applying a reversing displacement to, i.e., developing an alternating stress in, an assembly of piping elements containing the piping component or joint under consideration. The reversing displacement is applied to the assembly until a fatigue failure occurs – fatigue failure being defined as crack initiation and propagation through the wall such that a leak occurs in the pressure boundary in or near the piping component or joint under consideration. The process is repeated by applying various reversing displacements to similar assemblies, developing an alternating stress/number of cycles to failure curve for the piping component or joint. The resulting curve is compared to a reference curve to determine the SIF. The reference curve is the failure curve for an as-welded circumferential butt weld, where the SIF for the butt weld is assigned a value of unity (i = 1.0).

But, the actual stress in an as-welded circumferential butt weld in nominal pipe intuitively and logically must be greater than the actual stress in the same size nominal pipe without a weld. However, the B31 "intensified" bending stress at a butt weld is equal to the SIF (i = 1.0) times the nominal stress (M/Z) at the butt weld. Thus, at a butt weld the B31 "intensified" stress is equal to the calculated nominal stress. This has caused and continues to cause considerable confusion in pressure component design, especially amongst serious analytic types who often claim that B31 "intensified" stresses are wrong, i.e., too low, when compared to actual or theoretical stresses. B31 acknowledges that B31 "intensified" stresses (iM/Z) are less than actual or theoretical stresses. However, in this case, what is not understood is that the B31 allowable fatigue (flexibility) stresses are correspondingly low when compared to the allowable fatigue stresses for components when actual or theoretical stresses are evaluated – the factor of safety in a B31 fatigue analysis is comparable to the factor of safety in an actual or theoretical analysis.

When actual or theoretical fatigue stresses are evaluated in nuclear Class 1 piping services, stress indices (not SIFs) are used. Stress indices are theoretically developed parameters that are (like SIFs) a measure of the fatigue performance of a nuclear Class 1 piping component or joint. In this case, however, fatigue failure is defined merely as crack initiation in the piping component or joint (as opposed to through-wall crack propagation). As noted above, the allowable fatigue stresses for nuclear Class 1 services are correspondingly higher than B31 allowable fatigue (flexibility) stresses.

What the B31 designer must remember is that he/she is calculating an "intensified" (or "effective") stress, not an actual or theoretical stress.

But why, in a time approaching an analytic utopia, is the B31 designer still using these "ancient" SIFs? The answer is simple. B31 SIFs are simple to determine and apply (simplified methods are necessary to gain the widest possible application and there's a lot of pipe to design). B31 SIFs are consistent with the technology of the piping industry (we are typically not building watches). Determining actual or theoretical stresses is usually more expensive and complicated (compare a nuclear Class 1 piping analysis with a B31.1 analysis or, if you're not conversant with things nuclear, compare a Section VIII, Division 2 analysis to Section VIII, Division 1 analysis, it's the same difference). And perhaps the best reason for using B31 SIFs is that the piping industry has almost 50 years of validating experience with them (most of the SIFs proposed by Markl are still accurate within the context of good engineering practice and good engineering is more than a worthwhile commodity).

Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering (www.ppea.net) is a member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others). He consults with us in the capacity of Nuclear QA Manager.