Note: The letters in parentheses
in the terms S(L), S(A), S(av), S(y), S(c), S(h) are subscripts.
Recently the Boiler and Pressure Vessel Code (BPVC) has passed Code Cases
to change the allowable stress bases for some of the "design by rule"
codes (Section I and Section VIII, Division 1) from the lesser of 1/4
of the tensile strengths or 2/3 of the yield strengths at temperature
to 1/3.5 of the tensile strengths or 2/3 of the yield strengths at temperature.
Both B31.1 and Section III, Subsections NC and ND are considering conforming
to the BPVC changes. There has been considerable discussion about the
effect of these changes on piping design, with considerable misunderstanding
as well. The following comments are offered to help in understanding the
real effects of the proposed change.
Note, the change only reduces the factor of safety against the material
tensile strengths, not yield strengths. The proposed change for piping,
then, will reduce the pipe wall thickness a bit. This reduction will primarily
affect piping designed for lower pressures and temperatures, e.g., below
600ºF. Piping failures due to internal pressure (burst) correlate
best with material tensile strengths.
Regarding cyclic effects, since the 1950s, based on shakedown concepts,
B31.1 has limited piping stress ranges due to thermal expansion (and other
fatigue producing loads) to something less than 2 S(y). According to Markl
[1], S(A) + S(L), which he equates to 1.25 (S(c) + S(h)) in the rules
proposed for B31.1 – 1955 edition, "utilize at most 78 percent
of the available stress range S(av)", S(av) being equal to 2 S(y)
which was "considered the maximum stress range ... to which a system
could be subjected without producing flow [yielding] at either limit,"
i.e., a piping system subjected to 100 percent shakedown. [Note: the scope
of the B31.1 – 1955 edition included all pressure piping, i.e., the
present B31.1, B31.3, etc. However, the pipeline codes have never embraced
shakedown concepts because the pipeline grade materials were, until recently,
not adequately tough.].
In the 1980s, the allowable stress basis for B31.1 (and BPVC SC I, SC
III Class 2/3, and SC VIII Div.1, all) changed from the lesser of 1/4
tensile or 5/8 of yield to the lesser of 1/4 tensile or 2/3 of yield.
This increased maximum S(A) + S(L) values in some cases, but did not exceed
"78 percent of the available stress range" for those materials
which were controlled by tensile stresses or 83 percent of the available
stress range for those materials which were controlled by yield stresses,
(1.25)(2)(5/8) = 1.56/2 = 0.78 vs. (1.25)(2)(2/3) = 1.67/2 = 0.83.
For the allowable stress basis change proposed now, for seamless Grade
B carbon steel operating at ambient temperatures, the current change will
increase the maximum possible S(A) + S(L) value from 37,500 psi to 42,800
psi, but this is still a good deal less than (0.78)(2)S(y) = (0.78)(2)(35,000)
= 54,600 psi. For seamless Type 304 stainless steel at operating ambient
temperatures, the current change will increase the maximum possible S(A)
+ S(L) value from 47,000 psi to 50,000 psi. This, on the other hand, is
equal to but not greater than (0.83)(2)S(y) = (0.83)(2)(30,000) = 50,000
psi. [Note: Obviously the S(A) values available for thermal expansion
(and other fatigue producing loads) will be less than the maximum possible
S(A) + S(L) values because of the existence of the S(L) term, either on
the right side of the thermal expansion
(fatigue) equation (13) in B31.1 or on the left side of the equation (11)
in NC/ND-3653.2. This S(L) requirement in the fatigue equation could be
discussed in future comments, if there is enough response interest.].
Thus, while affecting the wall thickness required for pressure design,
the proposed change cannot ever increase allowable stress ranges above
the traditional 2 S(y) limit, which is the current basis for shakedown
concepts in ASME B31.1 and SC III NC/ND-3600.
Reference:
[1] A.R.C. Markl, "Piping-Flexibility Analysis", American Society
of Mechanical Engineers Paper 53-A-51
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.
Tip for the Month (Feb 2000)The purpose of piping design, layout and the supporting
elements should focus on preventing the following:
1. Piping stresses under allowables,
2. Joint leakage,
3. Excessive thrusts and moments on connected equipment (such as pumps
and compressors),
4. Resonance with imposed or fluid-flow induced vibrations,
5. Excessive interference with thermal movement which is otherwise adequately
flexible,
6. Unintentional disengagement of piping from its supports (like a pipe
shoe disengaging from a beam),
7. Excessive piping sag in piping requiring drainage slope,
8. Excessive distortion or sag of piping subject to creep under conditions
of repeated thermal cycling,
9. Excessive heat flow, exposure of supports to temperature extremes outside
of their design limits.
Paraphrased from B31.3-1999
In case you did not know, CAEPIPE is REALLY an efficient
program, provided you use the different features to your advantage.
In this multi-part tip, we will cover the various features in "bitesizes."
In this tip, we will look only at one aspect of Input (Layout window),
keyboard shortcuts for inputting the Element types.
Several studies over the years have shown that the keyboard is still the
more efficient and ergonomic method for inputting data compared to the
mouse with the exception of voice technology (whenever that goes mainstream!).
There are always exceptions!
For the mouse-heavy user, this discussion should be interesting
Consider the following scenario:
-Assume your fingers just typed some data through the keyboard,
-now, move your right (or left) hand to the mouse from the keyboard,
-position the mouse on the field where you need to enter more data,
-click in the field to position cursor,
-move your hand back to the keyboard,
-enter new data.
Compare the above scenario with this one:
-Assume your fingers just typed some data through the keyboard,
-now, use a keyboard shortcut to position cursor in the new field,
-enter new data.
Which one is more efficient? In most cases, it is the keyboard.
CAEPIPE has been designed with this in mind. To provide the keyboard shortcuts
so that the user can zoom through input. CAEPIPE is also ready and waiting
if you choose to use the mouse.
As an example, let us input a node number followed by a Bend element using
the mouse and the keyboard.
Using the mouse: After typing the node
number in the Node field (on the keyboard),
-move your hand from the keyboard to the mouse,
-position the mouse in the Type field (of that row) where you need to
enter a Bend,
-right click in the field to open the Element types dialog,
-move the mouse pointer to Bend,
-click once on Bend (CAEPIPE puts a bend and moves the cursor to the next
field, DX)
Using the keyboard: After typing the
node number in the Node field (on the keyboard),
-Press TAB to move to the next field (that is the Type field),
-Type "b" and press TAB (CAEPIPE puts a bend and moves the cursor
to the next field, DX)
See how efficient (and fast) keyboard shortcuts in CAEPIPE can be?
So, here is the list of keyboard shortcuts for you to try out.
| B(TAB) | Bend |
| BEN | Bend (no TAB required, and typing BEN moves the cursor over to the DX field.) |
| BEA | Beam |
| BEL | Bellows |
| BA | Ball joint |
| C | Cut pipe (Cold spring) |
| E | Elastic element |
| F | From (starting point for the line, the beginning row must have a From) |
| H | Hinge joint |
| J | Jacketed pipe |
| JB | Jacketed Bend |
| L | Location (to insert Data at that node number, for which the node must have been defined earlier) |
| M | Miter Bend |
| P | Pipe (you don't have to type this because a Pipe is the default element type in CAEPIPE and an empty field/cell indicates a Pipe element). |
| R(TAB) | Reducer |
| RE | Reducer (no TAB required) |
| RI | Rigid element |
| S | Slip joint |
| V | Valve |
TOP |
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