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Update: CAEPIPE (starting v5.1x) has the ability to accept a wind speed profile that is a function of elevation. Please see CAEPIPE User's Manual (v5.1J) for details.
Did you know you can use the guidelines in ASCE (7-88 or
newer) "Minimum Design loads for Buildings and other Structures,"
sections 6.4 and 6.5, to calculate the input required for modeling Wind
load in CAEPIPE? Here is how.
Note that your specific situation may warrant a more careful evaluation
according to the latest revision to this document, dated 7/1995. Contact
ASCE for a copy.
In CAEPIPE, you need three input parameters: Wind Speed,
Shape factor and Direction of Wind (at the piping).
Wind Speed: Look up Fig. 1 to determine the basic wind speed in your region.
Example: San Jose, CA, has a 75mph speed shown for Category C 33ft. (10m)
above grade. Category C is "open terrain with scattered obstructions
having heights generally less than 30ft. This category includes flat open
country and grasslands."
Shape factor: Before we get to it, let us review the following
equation CAEPIPE uses internally to calculate the Wind Force. It is
F (lbs.) = 0.00256 • I^2 • V^2
• Shape factor • Area
Where,
I = Importance factor, I^2 internally set to 1.25, (based on I=1.12 for
non-emergency piping in chemical plants, refineries, industrial facilities
and power plants), and,
V = Basic Wind speed (mph) from step 1 above.
Shape factor = Kz • Gz • Cf,
and,
Area = Pipe's projected area = (Pipe OD+Insulation) • Length of pipe
So, the equation becomes (after plugging in I^2),
F (lbs.) = 0.0032 • V^2 •
Shape factor • Area, or
Wind pressure (lbs./ft^2) = 0.0032
• V^2 • Shape factor, (see page 79
of CAEPIPE User's Manual (Rev 20)).
Then resolve the computed force with respect to the applied wind direction
at the piping (Xcomp, Ycomp and Zcomp).
Table below lists approximate values for the shape factor for different
heights of piping.
| Height of piping | Kz.Gz.Cf (Shape factor) |
| At or below 50' above ground | 1.23 |
| Above 50' to 100' | 1.44 |
| Above 100' to 200' | 1.68 |
Kz, Gz, Cf are Velocity, Gust and Force coefficients respectively.
Tip for the Month (Apr 1998)
Please open this
PDF file for this month's tip.
SA (Allowable displacement stress range) should be calculated
only once during the analysis of each piping component. Typically (and
required by the current rules of ASME B31.3 code) SA is calculated for
the maximum displacement [limited] (or secondary) stress range. The maximum
displacement [limited] stress range by custom and (normal conditions)
is the startup-shutdown thermal expansion stress range (in accordance
with ASME B31.1 nomenclature) and is defined as SE.
The effects of other displacement [limited] stress ranges are evaluated
by calculating the number of effective full stress range cycles and determining
a single value of f (the stress range reduction factor) to be used in
the calculation of SA for each component. Note that the value of SA =
f (1.25 [Sc+Sh]–SL) will be different for each component based on
the value of SL from component to component.
Also, as an engineering approximation, the effective full stress-range
calculation has also been used to incorporate the cyclic effects of primary
[non self-limiting] stress ranges. This is a reasonable approximation
when there are a significant number of primary stress range cycles and
the primary stress magnitude is likewise limited to Code allowables.
In CAEPIPE, if you put in
T1=100F, T2=700F and T3=212F,
SA would be calculated based on the maximum range for the component.
Here, it would T2–T ref.
Let us assume f=0.9 (based on Number of cycles between 7000-14000),
For pipe material, A 335 Grade P5, we have
Sc=14400 psi, and Sh=13700,
We get SA = 0.9 (1.25 [14400+13700]-SL) = 0.9 (35125 –SL) .
So, as SL changes for each component, so will SA based on the above equation.
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.
The analysis methods of most piping analysis computer programs
follow the convention of hand calculations used in the 1950s and as expressed
by requirements published in the various books of the ASME B31 Code for
Pressure Piping (B31.1, B31.3, etc.). In the 1950s, nonlinearities such
as limit stops, were simply too complicated to evaluate except in very
simple cases. The Code books express methods understood by practitioners
of that time and not always understood today.
The evaluation for sustained loads is a simplified method to assure that
piping collapse will not occur. By limiting stress magnitudes of relevant
loads (typically pressure plus weight) to stresses less than yield, it
is anticipated that collapse will be avoided. In the creep regime, the
sustained load evaluation is also a simplified method to assure that creep
rupture will not occur. By limiting stress magnitudes of relevant loads
(again, typically pressure plus weight) to stresses less than creep rupture
stresses (based on system life assumptions), it is anticipated that creep
rupture will be avoided.
It was understood in the 1950s that the evaluation of sustained loads
occurred in the supported condition. This assumed that no (or negligible)
lift-off occurred and that adjustments of the supports would be made to
support the pipe in the operating (hot) condition. Adjustment may be necessary
to limit stresses to avoid the failure modes noted above or to maintain
pipe slopes to facilitate venting or drainage. It was standard practice,
where significant lift-off would be expected when the piping is initially
operated, i.e., the piping is heated to operating temperatures from the
as-installed (cold) condition, to provide for manual (threaded hangers,
turnbuckles) or intrinsic (spring) adjustment of the piping in the operating
(hot) condition. Slight gaps at supports, necessary for construction,
would be tolerated because during shakedown these gaps could be expected
to close due to local yielding or creep. However, even if the gaps remained,
standard practice would be for the engineer to design supports such that
the pipe would engage its support when necessary, i.e., limit excessive
sag.
The practice of supporting the pipe in the operating (hot) condition is
clearly inferred by noting that ASME B31.1, Para. 101.6 requires that
“Piping shall be carried on adjustable hangers or properly leveled
rigid hangers or supports,...” and more specifically that Para. 121.4
requires that “hangers used for the support of piping, NPS 2½
and larger, shall be designed to permit adjustment after erection while
supporting the load.” Note, that this is consistent with standard
practice in the design of variable spring hangers, where the design load
is supported in the operating (hot) condition.
If the evaluation for sustained loads is an evaluation of the supported
condition, a computer analysis of the pipe in the cold condition to determine
support loads is entirely appropriate. Calculating (cold) weight stresses,
adding operating pressure stresses, and comparing the combined stresses
to the hot allowable stress, Sh, is equivalent to evaluating the piping
system in the hot supported condition. The engineer is then expected to
develop a pipe support system that reflects his analysis assumptions,
i.e., design a support arrangement with springs that will self-adjust
or other devices that plant personnel can adjust so that the hot piping
system will be properly supported.
Assuming that the pipe will be supported in the operating condition renders
any possible lift-off to the cold (or not operating, non-pressurized)
condition. Evaluation of this condition should normally be unnecessary.
Sustained weight stresses without pressure could possibly be evaluated
against a higher allowable stress, Sc, but this higher stress would seldom,
if ever, be exceeded. Even then, evaluating the cold piping does not appear
to have much significance, especially for safety.
Considering the above discussion, the following recommendations regarding
computer aided piping analysis with limit stops are offered:
1. Sustained load analyses, combining the “cold” pipe weight
stress (without any lift-off) and the operating (or conservatively the
design) pressure stress and comparing the resultant stress against the
hot allowable stress, is an acceptable method of complying with the ASME
Code for Pressure Piping, B31.
2. The pipe support system should be designed to support the pipe in the
operating condition, with no gaps or negligible gaps.
3. If the pressure plus weight plus thermal expansion, the so-called “operating”
load case, discloses lift-offs, they should be evaluated based on whether
the lift-offs are negligible or not. Negligible lift-offs would normally
be construed as being lift-offs less than ¼ inch. Lift-offs greater
than ¼ inch may suggest that a spring hanger or relocation of a
rigid support may be necessary. Lift-offs greater than ¼ inch may
be acceptable if the contents of the pipe are relatively benign. Lift-offs
less than ¼ inch may not be considered negligible in the vicinity
of sensitive equipment.
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.
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