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What Is a Sustained Load in Piping Engineering?
What Is a Sustained Load in Piping Engineering?
A sustained load is a force that acts on a piping system continuously and at full magnitude throughout the operating life of the plant. Internal pressure and deadweight are the two sustained loads that ASME B31.3 explicitly addresses in its code stress equations. Both are present from the moment the plant starts up until it shuts down. Neither varies with time or with the thermal state of the system. Sustained loads therefore represent the baseline structural demand that the piping system must always satisfy, regardless of any additional occasional or expansion loads that may also be present.
Why Sustained Loads Are Classified Separately
ASME B31.3 separates piping loads into three categories: sustained loads, occasional loads, and displacement loads. This separation matters because each category uses a different allowable stress and a different code equation for the stress check. Sustained loads generate primary stress, which is a load-controlled stress that does not self-limit. If primary stress exceeds the material yield strength, plastic collapse occurs without relief. Consequently, ASME B31.3 requires that sustained load stress stays within the basic material allowable stress Sh at the operating temperature, with no relaxation for the infrequent nature of a load event.
The Sustained Load Code Equation
ASME B31.3 Paragraph 302.3.5(c) requires that the sum of the longitudinal stress from pressure and weight in each piping component shall not exceed the basic allowable stress Sh. The equation combines the pressure-driven axial stress, calculated from the internal pressure and the pipe cross-sectional area, with the bending stress from the pipe weight acting across the span between supports. The stress intensification factor at each fitting amplifies the bending stress component at elbows, tees, and reducers where local geometry increases stress concentration. The combined sustained longitudinal stress must remain below Sh at every location in the system.
Applications in Piping Engineering
Deadweight Analysis
Deadweight is the gravitational load from the combined weight of the pipe, its contents, insulation, valves, and fittings. It acts vertically downward at every point along the pipe run. The pipe sags between supports under this load, generating bending stress that is highest at the midspan between adjacent supports and at the support attachment points. The stress engineer calculates the deadweight bending stress at every critical location in the model and verifies that it stays within the ASME B31.3 sustained load allowable. The pipe span between supports is the primary variable the engineer controls to keep deadweight stress within limits.
Hoop Stress and Pressure Design
Internal pressure generates stress in two directions in the pipe wall. The circumferential stress, called hoop stress, is the primary pressure stress that governs pipe wall thickness selection in the pressure design calculation. A secondary longitudinal stress from pressure also acts axially along the pipe. ASME B31.3 uses this longitudinal pressure stress as one of the two components in the sustained load equation. The engineer selects the pipe schedule and wall thickness in the pressure design step before the sustained load stress check begins. Consequently, an adequately thick pipe wall satisfies the pressure component of the sustained load equation automatically, leaving the weight stress component as the design variable.
Support Spacing and Weight Stress
Support spacing is the distance between adjacent pipe supports along a run. It directly determines the bending moment under deadweight loading and therefore the weight component of sustained stress. ASME B31.3 does not explicitly specify maximum support spacing, but the sustained load code check implicitly constrains it. Most engineering companies use standard pipe support spacing tables that give allowable spans as a function of pipe size, schedule, fluid density, and insulation weight. These tables are pre-calculated to keep the weight stress within the sustained load allowable for standard conditions. Non-standard conditions, such as very heavy valves, dense fluid, or large bore pipe, require specific calculations rather than table values.
Primary Stress Classification and Consequence
ASME B31.3 classifies sustained load stress as primary stress because it is generated by an external force that the pipe must resist continuously. Primary stress is fundamentally different from secondary stress from thermal expansion. Thermal stress is self-limiting: as the material yields locally under thermal expansion, the stress redistributes and the driving force is relieved. Primary stress does not self-limit. If the deadweight or pressure load exceeds the material strength, the pipe continues to deform until it fractures. This is why the sustained load allowable is set at the basic material allowable Sh, the most conservative of the three stress categories, rather than the higher allowable permitted for displacement loads.
Creep and High-Temperature Sustained Load
At temperatures above the creep range for a given material, sustained load stress causes time-dependent plastic deformation called creep. The pipe gradually deflects further between supports over the design life, and the stress redistributes in ways that simple elastic analysis does not capture. ASME B31.3 addresses creep by reducing the allowable stress Sh at elevated temperatures based on the long-term creep rupture and creep rate properties of the material. Consequently, high-temperature piping systems typically require closer support spacing and more carefully controlled sustained load stress levels than the same pipe would require at ambient temperature.
Benefits of Sustained Load Analysis
Preventing Weight-Induced Sagging and Pocket Formation
A sustained load analysis that correctly controls the weight bending stress between supports also prevents excessive midspan deflection. In piping systems carrying vapour, condensate, or multiphase fluids, a sagging span creates a low-point liquid pocket. Liquid pockets cause slug flow, flow-induced vibration, inadequate drainage during shutdown, and corrosion from standing liquid. Consequently, correct support spacing driven by the sustained load check delivers both code compliance and practical operability benefits.
Protecting Equipment Nozzles from Weight Load
Pipe deadweight transfers into equipment nozzles wherever the pipe connects to pumps, compressors, vessels, and heat exchangers. Rotating equipment manufacturers publish allowable nozzle loads that represent the maximum force and moment the nozzle can accept without compromising shaft alignment or casing distortion. The sustained load analysis in the pipe stress model calculates the actual weight load on each nozzle and compares it to the manufacturer’s allowable. Where the calculated load exceeds the allowable, the engineer adds a support close to the nozzle to carry the deadweight and reduce the nozzle load to within acceptance.
Establishing the Baseline for Combined Load Checks
The sustained load check establishes the baseline stress level that the pipe carries at all times. This baseline is then used in the occasional load check. ASME B31.3 requires that the sum of sustained load stress plus occasional load stress from seismic or wind stays within 1.33 times the sustained load allowable. A pipe already operating close to its sustained load allowable has very little margin remaining for occasional loads. Consequently, the sustained load analysis result directly governs how demanding the seismic and wind load checks will be.
Limitations to Consider
Simplified Code Equation Conservatism
The ASME B31.3 sustained load equation uses simplified beam bending formulas that assume the pipe acts as a continuous beam on simple supports. This assumption is inherently conservative for most real piping configurations, which include anchors, guides, and partially restrained supports that reduce the effective bending moment at midspan. However, stress analysis software such as CAESAR II models the actual support conditions and therefore calculates a more accurate and typically less conservative sustained stress than the simplified formula. Engineers working with more complex support arrangements benefit from using software rather than manual code equations.
Creep Deflection at High Temperature
Standard support spacing tables and pipe stress analysis programs calculate elastic deflection under deadweight loading. At high temperatures in the creep range, the pipe continues to deflect over time beyond the initial elastic deflection because the material slowly flows under sustained stress. Creep deflection over the plant design life can be many times larger than the initial elastic deflection. Standard span tables do not account for creep deflection. Engineers must therefore apply more conservative support spacing or perform explicit creep deflection calculations for piping operating in the creep regime to prevent excessive long-term sagging between supports.
Interaction with Thermal Loads
Although sustained loads and thermal expansion loads use separate code equations and separate allowable stresses, they interact physically in the real piping system. Support loads, nozzle loads, and pipe stress levels under the combined operating condition all reflect both deadweight and thermal effects simultaneously. Designing for sustained load alone, without checking the combined operating condition, can miss situations where thermal expansion unloads a support that was carrying weight in the cold condition, transferring that weight to adjacent supports or equipment nozzles during operation. Consequently, the sustained load analysis must always be evaluated in conjunction with the full operating load case.
Sustained Load FAQ
What is a sustained load in piping engineering? A sustained load is a force that acts on a piping system continuously throughout its operating life. ASME B31.3 identifies internal pressure and deadweight as the two sustained loads. Both generate longitudinal stress in the pipe wall that must be checked against the basic material allowable stress Sh at operating temperature. Sustained loads generate primary stress, which does not self-limit and can cause plastic collapse if it exceeds the material yield strength. Consequently, the sustained load allowable is the most conservative of the three ASME B31.3 load categories, with no relaxation for the infrequent nature of a loading event.
How does ASME B31.3 check sustained load stress? ASME B31.3 Paragraph 302.3.5(c) requires that the combined longitudinal stress from pressure and deadweight at every location in the piping system stays below the basic material allowable stress Sh at operating temperature. The engineer calculates the pressure axial stress from the internal pressure and pipe cross-section dimensions and the weight bending stress from the pipe deadweight acting across the span between supports. The stress intensification factor amplifies the weight bending stress at fittings such as elbows and tees. The sum of these two components must stay below Sh at every point in the system.
What is the difference between sustained load and occasional load in ASME B31.3? Sustained loads act continuously throughout plant operation and include only internal pressure and deadweight. Occasional loads act infrequently for short durations and include seismic forces and wind loads. ASME B31.3 checks sustained load stress against the basic allowable Sh. It checks the combined sustained plus occasional stress against 1.33 times Sh, reflecting the low probability that a design-level seismic or wind event will coincide with maximum operating conditions. Both sustained and occasional loads generate primary stress, but the higher allowable for the combined check reflects the reduced likelihood of the most severe load combination occurring simultaneously.
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