Principle of operation

The electrical power emitted by the back-pressure steam turbine generator set is determined by the thermal load, and therefore cannot meet the needs of both thermal and electrical loads. Back-pressure steam turbines are generally not installed separately, but run side by side with other condensing steam turbines. The condensing steam turbines undertake the electrical load changes to meet the external electrical load requirements. The electric power of the front-mounted steam turbine is determined by the amount of steam required by the medium and low pressure turbines. The pressure regulator is used to control the steam intake to maintain its exhaust pressure unchanged; the low-pressure unit adjusts its steam intake according to the electrical load needs, thereby changing the steam output of the front turbine. Therefore, it is impossible to directly control the steam intake of the front-mounted steam turbine based on the electric load.

Since the power generation capacity of the heating back-pressure unit depends on the size of the heat load, it should be used in situations where the heat load is relatively stable. Otherwise, the adjustable extraction steam turbine should be used.

The back pressure steam turbine has a high exhaust pressure and a small steam enthalpy drop. Compared with a condensing steam turbine with a low exhaust pressure, it emits the same power and requires a large amount of steam. Therefore, the unit of the back pressure turbine is large. The amount of steam required for power is greater than the condensing steam turbine. However, the heat contained in the exhaust steam of the back-pressure turbine is mostly used by the heat users, and there is no loss of cold source. Therefore, from the perspective of the heat utilization coefficient of the fuel, the thermal efficiency of the back-pressure turbine is more high. Because the back-pressure steam turbine can pass a large steam flow, the first few stages can use larger-sized blades, so the internal efficiency is higher than the high-pressure part of the condensing steam turbine.

In structure, the high-pressure part of the back-pressure steam turbine is similar to the condensing steam turbine. Back pressure steam turbines mostly adopt the nozzle adjustment steam distribution method to ensure that the efficiency does not change much when the operating conditions change. Because the back pressure machine is often used in situations where the heat load is relatively stable, the single-row impulse stage is generally used as the adjustment stage.

Common faults and solutions

During the operation of the back-pressure steam turbine, the cylinder is deformed due to casting defects, deformation due to stress, the expansion gap of the partition plate and the steam seal sleeve or the lug pressure plate, the impurities in the cylinder sealant are excessive, the bolt tightening is insufficient or the tightening sequence is not For reasons such as correctness, deformation and leakage often occur on the joint surface, affecting the safe operation of the unit.

Back pressure steam turbine leakage treatment method

To solve the problem of cylinder deformation and leakage, first use long flat feet and feeler gauges to check the deformation of the cylinder joint surface, and then take different solutions according to the degree of leakage:

1. For the joint surface with large cylinder deformation or serious steam leakage, the method of grinding and scraping the joint surface is adopted

If the deformation of the joint surface of the upper cylinder is within the range of 0.05mm, the joint surface of the upper cylinder is the reference surface, and the red cylinder or embossed blue paper is coated on the joint surface of the lower cylinder, and the lower cylinder is scraped according to the traces. If the deformation of the joint surface of the upper cylinder is large, apply Hongdan to the upper cylinder, grind out traces with a large flat ruler, and grind the upper cylinder.

2. Use proper cylinder seal material

Since there is no unified national standard and industry standard for back pressure steam turbine cylinder sealant, the product quality is uneven. When choosing a steam turbine cylinder sealant, a regular manufacturer with guaranteed product quality should be selected to ensure that the cylinder is overhauled. Rigor. At present, the high temperature sealant is applied more in high temperature environment in Germany, which is suitable for the working condition where the temperature and pressure of the smooth and flat sealing surface (butt joint) require high.

3. Local repair welding or spray plating on the joint surface of the cylinder

Since the joint surface of the cylinder is eroded or corroded by steam, grooves are selected by using appropriate welding rods, the traces are ground with a flat plate or a ruler, and the welding bead and the joint surface are in the same plane. When the cylinder joint surface has a large area of ​​steam leakage and the gap is about 0.50mm, in order to reduce the workload of grinding and scraping, the coating process can be used.

4. Method for adding pad to joint surface

If the local gap leakage of the joint surface is not very large, 80-100 mesh copper mesh can be used to reduce the hardness after heat treatment, and then cut into an appropriate shape, laid on the steam leakage of the joint surface, and then equipped with cylinder sealant.

5. Method for controlling bolt stress

If the deformation of the joint surface of the cylinder is small and uniform, new bolts can be replaced at the gap, or the pretension of the bolts can be increased appropriately. Tighten the bolts from the middle to both sides at the same time, that is, from the place where the vertical arc is the largest or where the force is deformed the most.

6. Repair method of polymer composite materials

Because repair welding and brush plating both have major drawbacks, it is easy to cause parts to bend or deform. Brush plating is limited by thickness and is easy to fall off. At present, in the western countries, polymer composite materials are often used to repair the leakage of the back pressure steam turbine. It has super strong adhesion and excellent compressive strength, and is used for rapid on-site maintenance. The most mature application is foreign technology, and domestic companies are gradually replacing traditional methods.