In an earlier section, ‘‘Definition of the Term Design Bases,’’ loading conditions were identified as one of the five principal elements in the definition of the term design bases. This section will identify some of the more common loading conditions and discuss the way in which they are considered in design.
Loading conditions may be classified as either sustained or occasional. Sustained loads act on the piping system during all or at least the great majority of its operating time. These loads are time-invariant. Examples of sustained loads include the dead-weight of the pipe plus its contents or the pressure load, including the effects of static head. Occasional loads are transient and act during relatively small percentages of the system’s total operation time. Examples of occasional loads include surges due to pump start-up and shutdown or pressure depressions and/or peaks due to sudden valve actuations.
Design Pressure
The design pressure is the maximum sustained pressure that a piping system must contain without exceeding its code-defined allowable stress limits. In single-compartment systems the design pressure is the maximum differential pressure between the interior and exterior portions of the system. In multi compartment systems the design pressure is the maximum differential pressure between any two adjacent compartments. The design pressure is the pressure that results in the heaviest piping wall thickness and/or the highest component pressure rating. The design pressure is not to be exceeded during any normal steady-state operating mode of the piping system.
In formulating the design pressure, the designer must consider all potential pressure sources. Among the more common sources to be considered are
The hydrostatic head due to differences in elevation between the high and low
points in the system
Back-pressure effects
Friction losses
The shutoff head of in-line pumps
Frequently occurring pressure surges
Variations in control system performance
Variations in System Pressure. As previously indicated, the system design pressure is the steady-state or sustained maximum pressure. Sustained conditions are those that remain constant over the majority of the total operating time. It is reasonable to expect that short-duration transient system pressure excursions in excess of the steady-state design pressure will occur during normal system operation. These transients, or occasional pressure excursions, may be tolerated without increasing the basic system design pressure, provided that the pressure increase does not exceed predefined limits and provided that the amount of time that the transients act does not exceed a specified percentage of the total system operating time. A number of, but not all, piping design codes provide rules to account for over pressure transients. Among the codes that provide design criteria or guidance are
The ASME Boiler and Pressure Vessel Code, Section III, Rules for Construction of Nuclear Power Plant Components
Various sections of the ASME Code for Pressure Piping, including
The methods used to qualify over pressure conditions for service vary from code to code. The ASME Code, Section III, uses a rather complex approach in which the range of acceptable over pressure transients is related to the nature of the loading combinations being investigated. The loading combinations are known as service conditions, and depending upon their severity and frequency of occurrence, pressure transients of up to 2 times the design pressure may be tolerated. The interested reader is referred to sub-articles NB, NC, ND-3600 of Section III for the details. In contrast to the complex methods adopted by Section III, ASME B31.4 and ASME B31.11 allow pressure transients of up to 10 percent over the system design pressure without restricting the amount of time that the transients may act.
ASME B31.1 and ASME B31.3 provide rules that are about midway in relative complexity from the extremes indicated above. As an example, the acceptance criteria for occasional loads specified in Paragraph 102.2.4 of the ASME B31.1 Code for Power Piping are reproduced below:
Ratings: Allowance for Variation from Normal Operation. The maximum internal pressure and temperature allowed shall include considerations for occasional loads and transients of pressure and temperature.
It is recognized that variations in pressure and temperature inevitably occur, and therefore the piping system except as limited by component standards referred to in Para. 102.2.1 or by manufacturers of components referred to in Para. 102.2.2, shall be considered safe for occasional short operating periods at higher than design pressure or temperature. For such variations, either pressure or temperature, or both, may exceed the design values if the computed circumferential pressure stress does not exceed the maximum allowable stress from Appendix A for the coincident temperature by:
A. 15% if the event duration occurs less than 10% of any 24 hour operating period; or B. 20% if the event duration occurs less than 1% of any 24 hour operating period.
Referring to Paragraph 104.1.2 of the ASME B31.1 code, one finds Eq. (4) for the maximum allowable pressure in a straight pipe
It can be seen from Eq. (B2.1) that the maximum allowable pressure P varies directly with the allowable stress S. Therefore, the net effect of Paragraph 102.2.4 is to allow short-term pressure excursions of from 15 to 20 percent in excess of the design pressure, as long as the respective time criteria are met.
As indicated above, not all piping codes provide rules for accepting transient pressure excursions in excess of the design pressure. Sections of the ASME Code for Pressure Piping which have no such rules include
When designing to a code which has no rules for acceptance of over pressure transients, the designer must increase the design pressure to envelop the transient condition. If, however, no specific design code is being used as a basis for design of a project, the designer may make a reasonable engineering judgment concerning the handling of transient over pressure events. In the absence of any other governing criteria, the following may be considered:
For transient pressure conditions that exceed the design pressure by 10 percent or less and act for no more than 10 percent of the total operating time, the transient may be neglected and the design pressure need not be increased. For transients whose magnitude or duration is greater than 10 percent of the design pressure or operating time, the design pressure should be increased to envelop the transient.
Determination of the Piping Wall Thickness. The determination of the piping wall thickness is one of the most important calculations of the piping system design process. In arriving at the final specification of the piping wall thickness, the designer must consider a number of important factors:
Pressure integrity
Allowances for mechanical strength, corrosion, erosion, wear, threading, grooving,
or other joining processes
Manufacturing variations (tolerance) in the wall thickness of commercial pipe
Wall thickness reduction due to butt-welding of end preparation (counter boring)
While a number of different pipe wall thickness design formulas have been proposed over the years, the ASME piping codes have adopted one or the other of the following formulas for pressure-integrity design:
For the precise definition of the method by which either equation is used by the codes, the particular code of interest should be consulted. Most construction codes require the provision of additional wall thickness, over and above that intended to ensure pressure integrity. This additional material allowance is provided in accordance with Eq. (B2.4):
The additional material allowance c is made up of a number of individual allowances that are provided to address different loads or conditions the piping system will see during fabrication, installation, and operation. Each allowance is figured separately, and their sum is added to the pressure-integrity wall thickness to arrive at the final design minimum wall thickness. The major constituents of c include
Wall thickness added to account for progressive deterioration or thinning of the pipe wall in service due to the effects of corrosion, erosion, and wear.
Wall thickness added to account for material removed to facilitate joining of the various segments of the piping system. Typical joining methods include threading, grooving, and swagging. If a machining tolerance is required as a part of the joint manufacture, this tolerance must be accounted for in the most conservative manner.
Wall thickness added to provide mechanical strength. This additional strength might be required to resist external operating loads or loads associated with shipping and handling
The effects of pressure result in pipe wall stresses in both the longitudinal and circumferential (hoop stress) directions. Typically, the circumferential stress is twice the longitudinal stress. Piping wall thickness selections made using hoop stress-type formulas, such as (B2.2) and (B2.3), result in excess-material availability in the longitudinal direction. In most cases, this excess material is adequate to resist bending stresses associated with the dead weight of the pipe, its contents, and in-line components such as valves, flanges, and piping specialties. In some cases, such as extremely long spans between pipe hangers and piping which is required to support unusually large concentrated loads, it may be necessary to increase the wall thickness to control bending stresses.
Once the design minimum wall thickness tm is determined, the only remaining step is to specify the actual or purchase wall thickness. Pipe is manufactured to one of two wall thickness dimensioning procedures: minimum wall thickness and nominal wall thickness. Pipe purchased to a minimum wall thickness specification will be manufactured using special processes to control the wall thickness. These processes may include custom made dies, extra rolling passes, or final boring of the inside diameter. Most minimum wall pipe is custom-manufactured. The use of minimum wall pipe is normally limited to high-pressure, high-temperature applications where the savings in material weight is sufficient to offset the additional manufacturing cost. Pipe purchased to a nominal wall thickness specification is manufactured in accordance with the dimensional criteria specified in ASME Standards B36.10M and B36.19M. These standards provide predetermined nominal wall thicknesses, or schedules, for various standard outside diameters of commercially manufactured pipe.
The tolerance on the wall thickness of pipe varies with the particular manufacturing process employed and with the relevant manufacturing specification. Rolled seamless and seam-welded (without filler metal) pipe has a normal wall thickness tolerance of +0, -12¹⁄₂ percent. Forged and bored pipe has a wall thickness tolerance of +¹⁄₈ in (3.2 mm), -0. Piping manufactured from rolled and welded plate has a wall thickness tolerance of -0.01 in (0.25 mm). There is no plus tolerance for this type of pipe. The ASTM (or ASME) specification for the particular piping material should be consulted to determine the wall thickness tolerance. Refer to App. E5. When piping is to be joined by butt-welding, the pipe ends are frequently counter-bored to facilitate fit-up. Counter bore dimensions for standard pipe wall thicknesses are given in ASME B16.25. It is important that the net minimum wall thickness resulting from the counter boring process be compared with the code minimum wall thickness tm to be sure that an under-thickness condition does not occur at the joints.Example B2.1 demonstrates how the previously discussed concepts associated with the design pressure may be applied to a typical problem.
Design Temperature
The design temperature is the temperature at which the allowable stresses for all pressure-retaining parts of the piping system are assigned. The design temperature must be equal to or greater than the maximum sustained temperature that the pressure-retaining components will experience during all normal and expected ab-normal modes of operation.The design temperature of the system’s pressure-retaining metal parts is normally assumed equal to the maximum free-stream fluid temperature. The effects of any internal or external heat sources such as heat tracing must be considered, as must any temperature excursions occurring as a result of control system error.The design temperature should be set at or above the peak of these temperature excursions.While the pressure-integrity design is based upon the design temperature, most other thermally related aspects of the design are based upon the normal operating temperature. The normal operating temperature is the temperature achieved by the system fluid while the system is operating in full-load, steady-state, non-transient conditions. It is lower than the design temperature. The normal operating temperature is used as the basis for all thermal design analyses that relate to the structural integrity of the piping system, including the thermal flexibility analysis, the spring hanger sizing and setting calculations, and the thermally induced anchor movement calculations. If a system has more than one ‘‘normal’’ operating mode (i.e., the system runs at different temperatures or has branches that run at different temperatures for different operating modes), then multiple thermal analysis calculations a tall normal operating temperatures may be necessary to fully qualify the design.
Dead Weight
The dead weight (self-weight) of a piping system consists of the sum of the distributed loads from the weight of the pipe itself, its thermal insulation, and/or other uniformly applied covering materials, plus the sum of the weights of any permanently installed concentrated loads such as valves, strainers, or other in-line appurtenances.
External loads on the piping system such as wind loads, snow and ice loads, and the weight of the fluid contents are considered as live loads. They are distinct from dead weight in that live loads may be variable both in magnitude and/or in the percentage of the total system operating time during which they act. An additional distinction is that the effects of live loads may be removed from the piping system while those of dead weight may not (without dismantling the system, of course).
Both the ASME Boiler and Pressure Vessel Code, Section I, Power Boilers, and the ASME B31.1 Code, Power Piping, require that the effects of dead weight and other sustained loads be considered in verifying the pressure integrity of components. Sub-part PG-22 of ASME Section I, Power Boilers, provides the following generalized rule:
Stresses due to hydrostatic head shall be taken into account in determining the minimum thickness required unless noted otherwise. Additional stresses imposed by effects other than working pressure or static head, which increase the average stress by more than 10% of the allowable working stress, shall also be taken into account. These effects include the weight of the component and its contents, and the method of support.
ASME B31.1, Power Piping, specifies more definitive rules to account for the effects of dead weight. Paragraph 104.8 provides closed-form equations to evaluate the effect of the simultaneous application of the internal pressure, dead weight, and other sustained loads on the design of a piping system. Equation 11 is used to calculate the piping system stress and to compare the calculated stress with code acceptance criteria.
Wind Load
The majority of all piping system installations are indoors where the effects of wind loading can be neglected. However, there are sufficient numbers of outdoor piping installations where wind loading can be a significant design factor. Wind load, like dead weight, is a uniformly distributed load that acts along the entire length, or that portion of the piping system that is exposed to the wind. The difference is that while dead weight loads are oriented in the downward vertical direction, wind loads are horizontally oriented and may act in any arbitrary direction. Since wind loads are oriented in the horizontal direction, the regular dead weight support system of hangers and anchors may have little or no ability to resist these loads. Consequently, when wind loading is a factor, a separate structural evaluation and wind load support system design is required.
Determination of the magnitude of the wind loadings is based upon empirical procedures developed for the design of buildings and other outdoor structures. Analysis of piping system stresses and support system loads is accomplished by using techniques that are similar to those applied for dead weight design.
Seismic (Earthquake) Loads
Under certain circumstances it is necessary or desirable to design a piping system to withstand the effects of an earthquake. Although the applications are not extensive, piping system seismic design technology is well developed and readily accessible. Many currently available piping stress analysis computer programs are capable of performing a detailed seismic structural and stress analysis, in addition to the traditional dead weight and thermal flexibility analyses. Most of these programs run on desktop microcomputers. Because of the higher construction costs and design complexities introduced by the application of seismic design criteria, this type of work is normally done only in response to specific regulatory, code, or contractual requirements.