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Tips March - June 1998

Modeling Wind load

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.

Understanding a NEMA SM-23 Report from CAEPIPE

Please open this PDF file for this month's tip.

Determination of SA Values in CAEPIPE

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.

Sustained Loads and Limit stops in CAEPIPE

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.