High-reliability boiler last-stage superheater and transformation process thereof

Abstract

The invention discloses a high-reliability boiler last-stage superheater and a transformation process thereof, and belongs to the field of power station boiler equipment. Each heating surface tube panel is composed of a low-temperature section and a high-temperature section which are formed by sequentially connecting tube sections made of at least two different materials in the steam flowing direction; the invention further comprises a process for transforming an existing boiler by adopting the last-stage superheater. By means of the transformation process, the defects that due to the fact that materials of a traditional supercritical / ultra-supercritical boiler last-stage superheater are oxidized and aged at high temperature, oxide skin falls off to block a pipeline, and then potential safety hazards and economic losses of over-temperature pipe explosion are caused are overcome.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of power plant boiler equipment, and in particular, relates to a high-reliability boiler final superheater and a reforming process thereof. BACKGROUND

[0002] In order to pursue high efficiency, the main steam temperature and pressure of supercritical and ultra-supercritical coal-fired boilers are very high (for example, 571℃ / 25.4MPa), and the final superheater in the boiler is a key component that bears the highest temperature and pressure.

[0003] In the prior art, the final superheater is usually composed of multiple high-temperature alloy steel materials, such as SA213-T91 steel for the low-temperature section, SA213-TP347H steel for the high-temperature section, and even SA213-T23 steel for some areas.

[0004] However, under the harsh working conditions of long-term high temperature, high pressure, flue gas scouring, and frequent start-stop, these materials (especially T23 steel and early TP347H steel) have exposed the following serious problems:

[0005] Material performance degradation: the material microstructure ages (such as the 3.5th and 4th levels of microstructure aging mentioned in the report), and the high-temperature creep strength decreases.

[0006] Internal wall oxide skin generation and shedding: a relatively thick oxide skin (such as 0.2-0.35mm mentioned in the report) is generated on the internal wall of the tube, and under the action of temperature change stress during unit start-stop, the oxide skin will fall off in pieces.

[0007] Pipe blockage and over-temperature pipe explosion: the oxide skin debris shed with the steam flow, which is easy to accumulate at the "U" bend at the lower part of the heating surface tube panel, causing pipe blockage. After blockage, the steam flow in the pipe is greatly reduced, the cooling effect is lost, leading to a sharp rise in the temperature of the pipe wall (i.e., "over-temperature"), and eventually the pipe explodes due to insufficient strength.

[0008] Safety hazards and economic losses: pipe explosion not only leads to unplanned unit shutdown (non-stop), causing huge power generation losses and maintenance costs, but also poses a serious threat to equipment and personal safety. In order to avoid risks, power plants have to adopt the mode of reducing parameters and load operation, sacrificing the economy of the unit.

[0009] Therefore, there is an urgent need for a reforming technical solution that can fundamentally solve the problem of oxide skin blockage and pipe explosion of the final superheater. SUMMARY

[0010] In view of the above defects, the application provides a high-reliability boiler last-stage superheater, which comprises an inlet header, an outlet header and a plurality of heating surface tube panels connected between the inlet header and the outlet header, each of the heating surface tube panels is composed of at least two tube segments made of different materials and connected in sequence along a steam flow direction to form a low-temperature section and a high-temperature section;

[0011] The low-temperature section is close to the inlet header and is made of heat-resistant steel; the high-temperature section is close to the outlet header and is made of austenitic stainless steel.

[0012] A transition section is arranged between the low-temperature section and the outlet section.

[0013] Further, the austenitic stainless steel used in the high-temperature section is fine-grained austenitic stainless steel SA213-TP347HFG.

[0014] Further, the austenitic stainless steel used in the high-temperature section is Super304H steel with a shot-blasting strengthening treatment on an inner wall.

[0015] Further, the austenitic stainless steel used in the high-temperature section is HR3C steel.

[0016] Further, the heat-resistant steel used in the low-temperature section is SA213-T91 steel.

[0017] Further, a U-shaped elbow is arranged at a lower part of each of the heating surface tube panels, and a bending radius of the U-shaped elbow is greater than a conventional design bending radius.

[0018] A high-reliability boiler last-stage superheater reforming process comprises the following steps:

[0019] S1, the original last-stage superheater whole heating surface tube panel is removed and replaced;

[0020] S2, a new heating surface tube panel is processed in a manufacturing plant by using a whole-panel prefabrication process, and a dissimilar steel welding joint between a low-temperature section and a high-temperature section of the tube panel is prefabricated;

[0021] S3, the prefabricated heating surface tube panel is hoisted to a boiler site, and is connected to short tubes of the inlet and outlet headers by welding between the same steel materials.

[0022] Further, before the step S3, the prefabricated heating surface tube panel is internally cleaned.

[0023] Further, the application further comprises an inlet header composed of a SA335-P91 main tube and a SA213-T91 short tube.

[0024] Compared with the prior art, the application has the following beneficial effects:

[0025] 1. Fundamental improvement of safety: By selecting the inner wall shot peening Super304H steel with strong antioxidant and eliminating T23 steel, the generation of oxide skin is greatly reduced from the source, combined with the structure optimization of increasing the bend radius, the core problems of oxide skin peeling, blocking and over-temperature tube burst are completely solved, and the major safety hazards are eliminated.

[0026] 2. Significant improvement of operation reliability: The main cause of unit "non-stop" is eliminated, ensuring that the boiler can realize long-period, safe and stable operation, improving the overall reliability and availability of the equipment.

[0027] 3. Recovery and improvement of economic benefits: The unit can be restored to full load or high load operating conditions, avoiding the huge economic losses caused by parameter reduction operation, at the same time, reducing the high maintenance cost and power generation loss caused by tube repair.

[0028] 4. Advanced process and controllable quality: The strategy of whole screen replacement and factory completion of dissimilar steel welding is adopted, which simplifies the on-site construction difficulty and greatly improves the welding quality and the reliability of the overall reconstruction project. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 is a schematic view of the end overheat surface in embodiment 1 of the present application.

[0030] Figure 2 is a schematic view of the end overheat surface in embodiment 2 of the present application.

[0031] Figure 3 is a schematic view of the end overheat surface in embodiment 3 of the present application.

[0032] Figure 4 is a schematic view of the end overheat lower inner circle pipe in the present application.

[0033] Figure 5 is a schematic view of the bend radius of the U-shaped bend in the present application.

[0034] Figure 6 is a schematic view of the bend radius of the U-shaped bend in the present application. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present application will be described in detail below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0036] The following examples are all based on a supercritical boiler (main steam parameters 571℃ / 25.4MPa) of a power generation company No. 1.

[0037] Example 1

[0038] The present embodiment is as shown in the following: Figure 1 The specific modification process steps are as follows:

[0039] S1, first remove the original last-stage superheater integral heating surface tube panel, inlet header and connecting pipes in the boiler furnace, it should be noted that the last-stage superheater inlet header and outlet header should be fixed to avoid displacement of the header due to weight reduction when a large number of old pipes are removed;

[0040] S2, design and manufacture a new last-stage superheater heating surface tube panel (using the "whole panel prefabrication" process mode, all new heating surface tube panels complete pipe bending, panel assembly and welding in the factory):

[0041] S2-1, the tube panel uses a regional material design, and the material is selected according to the calculated tube wall temperature:

[0042] In the low-temperature section where the calculated tube wall temperature is below 595℃, the tube material used is SA213-T91 steel;

[0043] In the high-temperature section where the calculated tube wall temperature is above 595℃, the present embodiment selects fine-grained austenitic stainless steel SA213-TP347HFG as its tube material, which has more excellent high-temperature creep strength and oxidation resistance than conventional TP347H steel;

[0044] S2-2, the dissimilar steel welded joint between the low-temperature section (SA213-T91) and the high-temperature section (SA213-TP347HFG) is pre-completed under strict process control in the factory, and is subjected to non-destructive testing to ensure the welding quality;

[0045] In addition, the low-temperature section and the high-temperature section are connected by a transition section, which is a dissimilar steel welded joint of T91 steel and TP347HFG steel. The position of the transition section is accurately calculated by heat and force to ensure that it is in a relatively safe temperature range to ensure the long-term reliability of the joint;

[0046] S2-3, the burst pipe is mainly due to the generation of oxide scale on the heating surface during operation. The generated oxide scale accumulates in the elbow part of the heating surface after peeling off during the start and stop of the unit, and cannot be removed during operation, resulting in a decrease in the flow cross section of the pipe. After long-term operation, over-temperature occurs, so the structure of the U-shaped elbow at the lower part of the tube panel is optimized (such as Figure 4The bend radius is significantly increased, making the bend transition much smoother to prevent potential scale accumulation (detailed shape optimization parameters are as shown in Figure 5 and Figure 6 two shapes shown);

[0047] S3, after the pre-fabricated tube panel is internally cleaned, it is hoisted to the designated position in the boiler furnace:

[0048] The on-site installation work only includes welding the T91 pipe end of the low-temperature section of the tube panel with the T91 short pipe of the inlet header, and welding the TP347HFG pipe end of the high-temperature section with the pipe seat of the outlet header, and all on-site welding is between the same steel materials, which simplifies the on-site process and improves the installation reliability.

[0049] Through the modification of the embodiment, the material SA213-TP347HFG with better performance is used to replace the original material, and the structure and process are optimized, which effectively solves the problems of scale blockage and over-temperature tube explosion.

[0050] Embodiment 2

[0051] As shown in Figure 2 The difference from Embodiment 1 is that the high-temperature section adopts Super304H steel with inner wall shot peening treatment, which uses high-performance Super304H steel and forms a dense protective layer on the inner wall of the high-temperature section made of Super304H steel, thereby fundamentally inhibiting the generation of scale.

[0052] Embodiment 3

[0053] As shown in Figure 3 The difference from Embodiment 1 is that the high-temperature section adopts HR3C austenitic stainless steel, which uses the extremely high high-temperature strength and excellent oxidation resistance of HR3C steel to solve the problem.

[0054] It should be noted that in Embodiment 1, Embodiment 2 and Embodiment 3, the inlet header also needs to be modified. The original header main pipe adopts SA335-P91 steel, but the connecting pipe of the tube panel adopts SA213-T23 steel. The dissimilar steel welding joint between the T23 steel and the P91 steel has a very high cracking tendency under long-term high-temperature operation:

[0055] The original inlet header welded by the SA335-P91 main pipe and the SA213-T23 short pipe is replaced by a new inlet header, and the new inlet header is welded by the SA335-P91 main pipe and the SA213-T91 short pipe, which eliminates the T23 steel and the related dissimilar steel welding crack risk in the original structure.

[0056] The tools used in the modification process include, but are not limited to: hoisting equipment such as truck crane, winch, monorail crane, snap ring, chain, pallet car, plasma cutting machine, etc. The required instruments and meters include, but are not limited to: steel tape, vernier caliper, oxygen meter, acetylene meter, argon meter, etc. The required materials include, but are not limited to: channel steel, grinding wheel, fine steel wire, angle steel, round steel, steel plate, etc. The required safety tools include, but are not limited to: double-hook safety belt, safety helmet, vertical zipper, speed difference protector, fire extinguisher, insulating gloves, protective glasses, etc.

[0057] In summary, in Example 1, fine-grained austenitic stainless steel SA213-TP347HFG is used in the high-temperature section, focusing on selecting a material with stronger performance to "resist" oxidation and delay the occurrence of the problem; in Example 2, Super304H steel with inner wall shot peening treatment is used in the high-temperature section, focusing on using surface modification technology to "eliminate" the source of the oxide scale and prevent the problem from occurring; in Example 3, austenitic stainless steel HR3C is used in the high-temperature section, focusing on selecting another high-end material with excellent comprehensive performance to "resist" oxidation. It can be seen that the material used in Example 2 can meet the requirements of the operating parameters after the modification, and the modification effect is better than that of Example 1, the price is close to that of Example 1 but lower than that of Example 3, and the overall cost performance is good, so in actual use, the modification process in Example 2 is adopted.

[0058] It should be noted that the structure described in the present application can be implemented in many different forms, and is not limited to the embodiments described, and any equivalent transformation made by those skilled in the art using the contents of the present application specification and drawings, or direct or indirect application in other related technical fields, such as other article loading and unloading, are all included within the scope of protection of the present application.

Claims

1. A high-reliability boiler final stage superheater, comprising an inlet header, an outlet header, and a plurality of heating surface tube panels connected between the inlet header and the outlet header, characterized in that: Each heating surface tube panel is composed of at least two tube sections made of different materials, which are connected in sequence along the steam flow direction to form a low-temperature section and a high-temperature section; The low-temperature section is close to the inlet header and is made of heat-resistant steel; the high-temperature section is close to the outlet header and is made of austenitic stainless steel; A transition section is provided between the low temperature section and the outlet section.

2. The high-reliability boiler final stage superheater according to claim 1, characterized in that: The austenitic stainless steel used in the high temperature section is fine-grained austenitic stainless steel SA213-TP347HFG.

3. The high-reliability boiler final stage superheater according to claim 1, characterized in that: The austenitic stainless steel used in the high-temperature section is Super304H steel whose inner wall has been shot peened.

4. The high-reliability boiler final stage superheater according to claim 1, characterized in that: The austenitic stainless steel used in the high temperature section is HR3C steel.

5. The high-reliability boiler final stage superheater according to claim 1, characterized in that: The heat-resistant steel used in the low-temperature section is SA213-T91 steel.

6. The boiler pressure-bearing heating surface with gradient temperature resistance according to any one of claims 1 to 5, characterized in that: A U-shaped elbow is provided at the bottom of each heating surface tube panel.

7. A high reliability boiler final stage superheater modification process, characterized in that: The high-reliability boiler final stage superheater according to claim 6 further comprises the following steps: S1. Remove and replace the original last stage superheater integral heating surface tube panel; S2. Using the whole panel prefabrication process, the new heating surface tube panel is processed in the factory, and the dissimilar steel welding joints between the low-temperature section and the high-temperature section of the tube panel are pre-finished; S3. Hoist the prefabricated heating surface tube panel to the boiler site and connect it to the short pipes of the inlet and outlet headers by welding the same type of steel.

8. A high-reliability boiler final superheater modification process according to claim 7, characterized in that: Before step S3, the interior of the prefabricated heating surface tube panel is cleaned.

9. The high-reliability boiler final superheater modification process according to claim 7, characterized in that: It also includes an inlet header made of SA335-P91 main pipes and SA213-T91 short pipes welded together.

Images