I intend to make a glossary of basic terms for nuclear pipeline design. We'll see how much enthusiasm I have.
Four operating modes
Piping elements can operate in four operating modes marked with the letter A to D. Mode A (normal) is the normal operating mode, in which the system is to operate at full capacity. The typical design stress is SH.HMode B (upset) is an abnormal situation that the pipeline must withstand without having to repair it. The loading in this mode, in addition to the typical mode A load, is a basic earthquake. The typical design stress is 1.2 SHHIn mode C (emergency) we have an emergency situation after the occurrence, the pipeline requires inspection because of visible large deformations. In this scenario, however, the pipeline remains intact. As for the load combination, in this case, exceptionally strong earthquakes can be expected. The typical design stress is 1.8 SHHIn the last mode of operation (faulted), the pipeline is in a state of complete failure, although the required level is to remain more or less intact. The aim is to ensure that the destruction of the pipeline and its disintegration does not lead to a cascading increase in the size of the failure. Possible load combinations include a missile strike, an act of terrorism or an aircraft crash. The typical design stress is 2.4 SHH.
Component classes
The standard divides all pipeline elements into the following classes: Class 1 constructed in accordance with the principles of subsection NB, Class 2 in accordance with NC, Class 3 in accordance with ND, Class MC (metallic safety tanks) in accordance with NE, Class CS (main supporting structures) in accordance with NG. Section NF contains the principles of support construction and Section NF describes the principles for internal structures.
The question is what are the elements of classes 1 to 3 and how are they classified? This is the task of the safety department. Class 1 will include all systems (apparatus and pipelines) whose failure could lead to the most serious consequences. As a rule, these are the primary steam pipelines from the reactor to the turbine and the main cooling system. Class 2 includes the remaining systems of the main technology. Class 3 usually concerns auxiliary systems such as sprinkler systems, compressed air, flooding and water purification. All other pipeline systems not covered by the above classes are classless and can be designed on the basis of codes other than nuclear, for example from hour B31. An example would be building installations such as chilled water for air conditioning.
Typical loads
Designing a pressure element begins with a review of the possible loads occurring at the site of the system. Typical ones that must be considered first include:
- Internal and external pressure;
- Occasional impact loads
- Rapidly changing pressures;
- Weight under operating or testing conditions
- Wind loads, snow loads, vibrations and earthquake loads
- Reactions at supports
- Temperature influence
Design pressure
This parameter is defined as not less than the maximum pressure difference between the inside and outside of the pipeline or between any enclosed areas of the pipeline that occurs under the most severe loading conditions in service limit A. The design pressure must include allowances for pressure surges, control errors, and system configuration effects such as static pressure. It is worth noting that this definition of design pressure includes surges that would otherwise be in the occasional range.
The design pressure must be used in calculations made according to paragraphs NB-3221, NB-3227.1, NB-3227.2, NB-3227.4, NB-3228.1, NB-3228.2 and NB-3231. The specified working pressure at the relevant time must be used in calculations made according to NB-3222, NB-3228.3 and NB-3232.
Design temperature
The design or design temperature shall not be less than the expected maximum average metal temperature through the thickness of the part under consideration for which Level A service limits are specified. If the component is locally heated, for example by induction or internal heat generation, the effect of such heat input shall be taken into account in establishing the design temperature. The design temperature shall take into account control errors and system configuration effects.
The design temperature shall be used in calculations in conjunction with the design pressure and mechanical loads. The actual metal temperature at the point under consideration shall be used in all calculations where the specified working pressure is required.
