Why Nuclear Power Pipeline System Is Inseparable from Metal Expansion Joints- -Triple Constraints of Thermal Displacement, Vibration and Safety Margin
The pipeline system of nuclear power plants, to put it bluntly, is something that runs desperately under high temperature and high pressure. The main steam temperature can soar above 300℃, and the pressure of the coolant circuit is often more than ten MPa. Calculated by thermal expansion and contraction of the metal pipe, the displacement is often tens of millimeters. Hard carry? No way. Cracked welds, leaky flanges, broken supports, all of which are outside the red line of nuclear safety.
What about that? Rely on metal expansion joints to "flexibly resolve". It can not only absorb axial expansion and contraction, but also handle lateral and angular displacement, and at the same time, it can isolate vibration and noise. More importantly, the safety margin of nuclear power plants is extremely high, and the pipeline cannot collapse under the design benchmark accident, so the metal expansion joint becomes the last flexible line of defense. If you choose or install it incorrectly, the consequence is not as simple as shutdown for maintenance.
Selection of Nuclear Grade Metal Expansion Joints: Matching Logic of Materials, Structures and Pressure Grades
"Does your power plant industry use corrugated expansion to save energy directly in nuclear islands?" I replied directly: Brother, there are several levels of standards between nuclear grade and non-nuclear grade.
Let's talk about the materials first. Nuclear-grade bellows must be made of austenitic stainless steel, such as 316L or even higher nickel-based alloy. The key is to control carbon and cobalt to avoid neutron activation. 304 for ordinary industrial use? No, stress corrosion cracking is too common in boric acid-containing water.
Structurally, the high temperature and high pressure of the main steam pipeline often requireHigh temperature axial expansion jointOrExternal pressure single axial expansion jointBecause the external pressure design can avoid bellows instability. And the cooling circuit pipeline is complicated, so it has to be matchedStraight pipe pressure balanced expansion jointAndCompound hinge transverse expansion joint— — One tube compensates in the axial direction and does not drop the blind plate force, while the other tube eats lateral displacement and does not twist the tube.
Not to mention the pressure grade, it has to be calculated according to ASME BPVC or RCC-M nuclear grade specifications, and the number of bellows layers, wall thickness, wave height and wave pitch have to be checked step by step. The shell takeover has to be filmed for non-destructive testing, which is far from the conventional power station set.
Typical Application in Main Steam Pipeline and Cooling Circuit- -Matching Idea of Straight Pipe Pressure Balance Expansion Joint and Double Hinge Transverse Expansion Joint
The main steam pipeline of the nuclear island has a large diameter and high pressure. The biggest headache is the huge axial thrust generated by the thermal expansion of the pipeline and the load of the fixed support. Axial type expansion joint for light No, the blind plate force generated by the internal pressure of the bellows will directly damage the equipment interface. That's when you have to ask outStraight pipe pressure balanced expansion joint-It comes with its own balance ring, the pressure in the inner cavity cancels each other, and the force on the pipe is almost zero. CooperateCompound hinge transverse expansion jointAbsorbing the lateral displacement in the pipe system, when the two brothers set up, the stress of the pipe system instantly dropped.
What about the cooling circuit? Space is limited, there are many turns, and it is often necessary to absorb three-dimensional displacement at the same time. Compound hinge transverse expansion joints can be used in pairs to form "Z" or "Π" shaped tube segments, with equivalent transverse displacement by combining angular displacements of the hinges. PlusDouble hinge expansion joint for air-cooled island vacuum pipelineThis type of special model is specially designed to deal with the risk of instability under vacuum conditions.
There is another detail when selecting the model-the deflector. Bellows without guide tubes are directly exposed to high-speed steam, and perforation occurs after several years of erosion and corrosion. We talked about it in the Q&A at the station,Specific Function of Expansion Joint Guide TubeIt is to divert the medium, reduce the vortex current and protect the bellows. In nuclear power projects, the thickness, material and fixing method of the deflector must be according to the requirements of nuclear grade, and the height of the welding foot has rigid regulations.
Special Quality Requirements for Metal Expansion Joints in Nuclear Power Plants: From Design Specification to Factory Inspection, to Tie Rod Adjustment and Guide Tube Function for Site Installation
Nuclear grade metal expansion joint supply process, documents alone can pack a box. In the design stage, stress analysis report, fatigue life assessment and seismic load check should be submitted. Factory inspection is not only water pressure and air tightness, but also cycle life test-simulating the number of displacements in the whole life cycle, and the bellows can't have the slightest crack.
The pull rod is not adjusted in place, or the screw that should be removed is not removed. When the expansion joint leaves the factory, it will be equipped with a tie rod and a limit screw for transportation protection. After installation, it must be adjusted step by step according to the instructions. How exactly to adjust? We wrote in our Q&AHow to adjust the tie rod nut of expansion joint— — First loosen the nut, let the bellows be in a free state, then tighten or loosen according to the actual displacement direction, and finally lock the anti-loosening nut. Don't weld to death directly to save trouble, and the expansion joint will be wasted.
The direction of the arrow must point to the flow direction of the medium, otherwise the guide tube will become a throttle, and it is easy to catch foreign matters. We have a special article in our Q&AThe arrow direction of the expansion joint refers toThat's what it's talking about.
Common failure modes and preventive measures-stiffness calculation, fatigue life assessment, and engineering experience with periodic joint replacement
In the final analysis, the failure of metal expansion joints in nuclear power plants is in several categories: fatigue cracking of bellows, stress corrosion, instability bulging and deflector tube falling off. Fatigue cracking is most common – and guess what? In order to save money, many projects count the number of bellows layers less, or choose the wave height smaller, and as a result, the number of cycles is wasted if it is less than the design value.
Calculate the stiffness. In our station aboutStiffness and Calculation Formula of BellowsThere is detailed deduction. Simply put, stiffness determines the ability of the expansion joint to absorb displacement. If the stiffness is too large, the thrust of the pipeline exceeds the standard, and if the stiffness is too small, it is easy to become unstable. Article 2: Do sufficient fatigue life assessment. The fatigue curve in EJMA standard gives the average value, and the safety factor of nuclear power projects should be above 3, and several more rounds of verification tests have to be done.
Establish and improve replacement nodes. The expansion joint on the main steam line has to be replaced when it needs to be replaced after 10 to 15 years of operation. Don't wait for a leak before stopping the reactor. Pay attention to check the model size when replacing-we have it in our product libraryModel and size of expansion jointJust follow the table and place the order directly. In addition, check the locking state of the tie rod regularly, don't let the nut loose without knowing.
Nuclear power projects are not laboratories, and every link of metal expansion joint selection and installation is gambling on safety. You can't afford to gamble, so don't save that little design fee. Choosing the right structure, using the right materials, calculating the stiffness, and managing the installation can be considered to be the ground.