Primary, Secondary and Occasional Loads
From a piping stress analysis point of view the following are the main loads to be considered for the design:
- Primary load occurs from Sustained loads like dead weight, live weight, internal pressure etc. and are called non-self-limiting loads. Pressure thrust from an expansion joint is used in this article.
- Secondary loads occur from thermal expansion loads like temperature change, anchors and restraints etc. and are called self-limiting loads. Thermal expansion in a horizontal pipe loop is used in this article.
- Occasional loads occur from static wind and seismic loads and are considered to act occasionally. Seismic load on a vertical pipe loop is used in this article.
Primary Stress is generated by internal and external force and moments. Primary stress is not self limiting – even if a part moves, the load causing it does not reduce. In this example, an expansion joint without restraining hardware creates a primary stress on a pipe.
This primary stress is caused by pressure of the fluid multiplied by the area of the pipe. It occurs all the time the system is pressurized. No matter how much the pipe displaces, the untied bellows keeps pushing on it.
Because primary stresses are not relieved by the piping moving or yielding, primary stress limits are set lower than other allowable stresses. For example, if primary stresses managed to get above the yield point, the piping would balloon out and explode. The piping codes keep the primary stresses below the yield point by a factor of safety.
Thermal expansion and contraction happens when a pipe heats up and cools down. The piping system must have enough flexibility to handle the expansion.
The stress is caused by the pipe pushing against some fixed restraint. Thermal stresses are “secondary stresses” because they are self-limiting. That is, yielding or deformation of the part reduces the stress.
Higher stresses are allowed than in the primary case because of this self limiting behavior. Stresses above the yield point can be acceptable, however, over time, an accumulation of highly stressed thermal cycles can exceed the fatigue capacity of the material leading to failure.
Wind and seismic loads are occasional. In this example sideways seismic loads create the stress.
The supports for a loop like this would have to be carefully considered to maintain the flexibility while also providing the required support for seismic loads.
Loads like those induced by seismic events are not expected to occur frequently, so the generated stresses are allowed to be higher than primary loads. Seismic stresses are typically allowed to be 20% higher than primary stresses in ASME piping codes. The equipment should be able to survive these rarely occurring stresses without damage.
We offer pipe stress analysis services.
- Caesar II thermal, flexibility and dynamic pipe stress analysis
- Seismic analysis for British Columbia CRN registration
- Water hammer, flow induced vibration & integrity review
- Fitness For Service using API 579
- Layout design, hanger, guide, anchor and expansion joint location and specification
- P.E. / P. Eng. stamping
Finite Element Analysis (FEA) – We use FEA to design and validate fittings and vessels that cannot be designed by rule-based codes like VIII-1 or B31.3.
ASME Code Design – We work to many ASME standards to design and validate pressure vessels, boiler, fittings and piping systems.
Canadian Registration Number (CRN) – We are Canada’s largest independent registrar of fittings, vessels and piping under the CRN program registering for more than a thousand customers.
Pressure Vessel Engineering has twenty years of successful experience in the pressure vessel field working for more than a thousand customers.
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