Piping Loading Conditions | Calgary, AB


Piping Loading Conditions

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

  1. B31.1, Power Piping

  2. B31.3, Process Piping

  3. B31.4, Liquid Transportation Systems for Hydrocarbons, Liquid Petroleum Gas, Anhydrous Ammonia and Alcohols

  4. B31.11, Slurry Transportation Piping Systems

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

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:

pressure integrity design
pressure 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):

minimum thickness for pipe

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