Four major piping in thermal power plants
Four major piping in thermal power plants
Generally speaking, the four main pipeing in power plants refer to main steam pipes, reheated steam pipes, reheated steam pipes and Main Feedwater pipes.
The definitions of the four main pipes are as follows: main steam pipes, reheat and heat section steam pipes, reheat and cold section steam pipes, high pressure bypass pipes, low pressure bypass pipes, high pressure water supply pipes, water supply recycling pipes and high side reducing water pipes, for short, four pipes.
1. Main steam piping (two high temperature and high pressure steam pipelines between the superheater outlet header and the high pressure main steam valve interface);
2. Hot reheat steam piping (two high temperature and high pressure steam pipes connecting the reheater outlet header to the medium pressure main valve port);
3. Cold reheat steam piping (two high temperature and high pressure steam pipelines between the high pressure cylinder exhaust port and the reheater inlet header interface);
4. High-pressure water supply piping (the high-pressure boiler supply water pipe between the electric feed water pump outlet and the economizer inlet header interface).
Selection of materials for four large steam water pipes
1 material selection for main steam pipe.
The main steam system supervisor of 300MW unit recommends A335--P22 pipe.
A335--P91 pipe is also the main material to be selected.
A335--P91 steel is an improved 9Cr--1Mo high strength martensitic heat resistant steel. Because of its high temperature, high oxidation resistance and high temperature resistance to steam corrosion, P91 steel has been widely used in power plant equipment in Britain, the United States and Germany in 80s.
The strength of P91 steel is higher than that of P22, and its strength decreases less with the increase of temperature. At 20 C, the tensile strength of P91 steel is 41.6% higher than that of P22 steel. At 538, the allowable stress of P91 steel is 83.3% higher than that of P22 steel. It is because P91 steel has much higher allowable stress than P22 steel at high temperature. The thickness of the main steam pipe is much thinner than that of P22 steel. This is the main reason for the more and more extensive application of P91 steel in large units. The primary steam pipe is compared with the use of P91 and P22 steel. When the main steam pipe is made of P22, the specifications are 368 x 82 and the specification of the branch pipe is 273 * 62.23. When using the P91 material, the specifications are 368.3 x 40, and the specification of the branch pipe is 273 * 30.
The comparison shows that the thickness of the main pipe wall is thinned by 42mm, the thinning rate is 51.2%, the thickness of the branch pipe wall is thinned by 32.23mm, and the thinning rate is 51.8%. The total weight of the pipeline is significantly reduced, and the total weight of the pipeline is P91 / P22=1 / 2.18.
In addition, the thrust and torque of the pipe to the equipment interface can be reduced due to the thinner wall thickness of the pipeline. At the same time, because the weight of the pipeline is reduced, the load of the hanger and hanger decreases correspondingly. It not only saves the cost of hangers and hangers, but also saves the cost of pipe installation and civil construction.
2. selection of pipe pipe for Reheated Steam Heat section.
The reheated hot pipe is a large diameter thin-walled tube. If P91 tube is used, the thinner wall will be thinner. As the tube wall is too thin, the actual value of the wall thickness is much larger than that of the calculated value from the safety point of view. In this way, compared with the P22, the P91 pipe is less economical.
When the reheat pipe is used for P22 material, the specifications of the pipe are 635 x 31 and the specification of the branch pipe is 508 x 24.8; when the P91 material is used, the specification is 727.96 x 21.03 and the specification of the branch pipe is 632.97 x 17.98., the wall thickness of the main pipe is reduced by 10mm, the wall thickness of the pipe is thinned by 6.8mm, the total weight of the pipe is not changed, the total pipe weight of the pipe is not changed. P91 / P22=1 / 1.39 is much larger than price ratio, so P22 pipe is recommended.
3. selection of pipe material for reheating steam cold section.
According to the experience of the 300MW unit, the A672B70CL32 steel pipe welded by reheat steam cooling section can replace A106B seamless steel pipe, which can also meet the technical requirements.
The cold section director uses A672B70CL32 slotted steel instead of A106B seamless steel pipe. Seamless welded steel pipe is much cheaper than A106B seamless steel tube, which is about 1 / 3 of seamless steel pipe. The thickness deviation of welded pipe is less than that of seamless steel pipe, and its quality is no less than that of seamless steel pipe.
4. High pressure water supply pipe pipe selection.
The high pressure water supply pipe usually uses St4518/III pipe. The project is optimized to 15NiCuMoNb5. The main pipe specification is changed from Φ406.4×50 to Φ355.6×25, and the pipe size is changed from Φ298.6×36 to Φ244.5×20.
The elastic modulus and linear expansion coefficient of 15NiCuMoNb5 and St4518/III are very similar, but the allowable stress of 15NiCuMoNb5 is much larger than that of St4518/III. Therefore, when 15NiCuMoNb5 is used for high-pressure water supply pipeline, the wall thickness of the pipeline can be reduced by 50%. The amount of material used for the support and hanger can be greatly saved, and the economy is more reasonable, which can effectively reduce the construction cost. Therefore, 15NiCuMoNb5 is used for the high-pressure water supply pipe.