Now the calculation of nozzle load has become the content that must be considered in the design of pressure vessel. After checking the nozzle and equipment body, is it necessary to convert the pipe load F and m into equivalent internal pressure to check whether the flange pressure level is sufficient?
Let’s start with the conclusion. No need.
The pipe load is generally caused by temperature. The stress specialty models and calculates the load at each node according to the design conditions. The software is CAESARII, and the calculated load is generally located on the shell of the equipment (the software can also be set on the flange sealing surface, but it is rare, because the value of the nozzle extension height of the stress specialty may be inaccurate), The shell is generally rigid (flexible nozzle may also be used, as shown in the figure below). If the pipeline load is too large, the stress will adjust the pipeline and control the flexibility of the pipeline.
The working condition of pipeline is W + T + P, W is the dead weight, t is the thermal displacement caused by temperature, and P is the axial stress of internal pressure. Generally, the load generated by P is very small. Therefore, it can be considered that the main part of the nozzle load is self weight and temperature.
The stress specialty calculates each pipeline and calculates the load values under multiple working conditions. As shown in the figure below, the load values under 14 working conditions are compared with the allowable table after each calculation. If it is larger than the allowable value, the stress specialty will adjust it. If it is smaller than the allowable value, it will be regarded as qualified.
In the load above, WIN is wind, U is earthquake, 6-9 is superimposed earthquake, and 10-13 is superimposed wind. It belongs to accidental load. For accidental load, the allowable stress of pipeline will be enlarged by 1/3 (20% for vessel), and the allowable load of flange will be enlarged by 2 times when calculated according to NC3658.
Because the stress specialty calculates the root, when we calculate the load, the SW6 nozzle extension length h is 0. Pvelite is the root of nozzle by default.
In CAESAR II, there is the flange checks option, which can calculate the leakage of all the flanges on the pipeline.
In the calculation of stress specialty, the flange load on all pipelines will be automatically evaluated and the calculation sheet will be generated. The flange on the pipeline contains the flange connected by the equipment. If the calculation fails, the stress will be adjusted.
Sometimes, even with the help of advanced software, it is a heavy workload to calculate all the pipe flange, so the stress specialty will check the flange on the pipeline according to the severity (important pipeline, high temperature and high pressure, compressor inlet and outlet, medium flammable and explosive, etc.).
There are two kinds of flange calculation, one is equivalent pressure PEQ, the other is NC: 3658.3 method. The method of NC3658 is easy to pass, which is generally used.
When the stress specialty checks the flange, the corresponding calculation is carried out according to the actual load of various working conditions and whether there is wind and earthquake. It is equivalent to 14 kinds of working conditions calculated separately, and evaluated according to 1 or 2 times (wind and earthquake conditions) separately. The calculation is more detailed and accurate.
If the flange does not pass the check, then the stress will try to improve the flexibility of the pipeline and make the flange qualified. Because there are many flanges in a pipeline, the stress specialty will not let the material specialty modify the flange pressure rating to pass the calculation, which will increase the workload and cost. So at present, it is rare to improve the flange pressure level due to pipeline load.
If the load in the table is calculated according to the equivalent pressure, it has great limitations.
- First, the pipeline stress of this flange has been considered in the modeling, and the pipeline stress has been calculated, which is equivalent to repeated work. From the convenience of specialty division and implementation, this work should be done by stress specialty.
- The second is to use all the most conservative loads according to the equivalent pressure method. As a result, 80% of the flange can not be used, which is not in line with the actual project.
- The third is the calculation of flange equivalent pressure by equipment specialty, the result is not credible, the tool is not professional, and the flange pressure level is improved with the result of non professional, so it is difficult to persuade other specialties to implement, especially the material specialty.
- Fourth, even if the scope of the pressure vessel is the flange sealing surface, there is still a flange on the opposite side which belongs to the piping, even if the failure is not the cause of the equipment.
- Fifthly, from the perspective of engineering practice, there is no precedent for equipment specialty to do so according to the equivalent stress method.
The conclusion is that the equivalent stress method is not recommended to check the flange pressure rating.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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