[Q&A Question 115] 2017.04.25

Ensure even heating of the rotor and cylinder block, preventing steam condensation

1) Ensure thorough preheating of all components of the unit for uniform expansion: prevent components such as cylinders, diaphragms, nozzles, main shafts, impellers, and shaft seals from deforming or loosening. (2) It enables a comprehensive inspection of the operation of the steam turbine generator set and other aspects. The specified warm-up speed of 300–500 revolutions per minute is due to the fact that if the speed is too low, the bearing oil film cannot be properly established, which can lead to bearing wear; if the speed is too high, it will result in an overly rapid warm-up process, generating significant thermal stresses within the metal components and thus reducing the turbine’s lifespan.

When warming the pipes, it is essential to strictly control the rate of temperature increase in the pipe walls to ensure uniform expansion of the pipes. First, low-pressure warming should be carried out, that is, steam at low pressure but high flow rate is used to heat the pipes. In this way, at the beginning of pipe warming up, the pipe is subjected only to temperature changes and not to pressure changes. When the wall temperature rises to the saturation temperature corresponding to the steam pressure of low-pressure pipe warming, pipe warming under increased pressure can be carried out, that is, the steam pressure and temperature are gradually increased until the rated parameters are reached.

This post was last edited by Brian.Duke.Lee on 2017-4-25 07:24. When starting a turbine, it is necessary to allow sufficient time for low-speed warming up. The purpose of this low-speed warming up during cold starts is to ensure that all components of the turbine expand evenly as they are heated, thereby preventing deformation and loosening in parts such as the steam drum partitions, nozzles, shafts, impellers, steam seals, and shaft seals. For turbines that have not been fully cooled, especially those without a barring device, low-speed warming up is also essential during startup, in order to prevent the shaft from bending and to avoid friction between the moving and stationary parts of the steam flow path. It is specified to warm up at 300-500 RPM; this is mainly because if the speed is too low, the bearing oil film cannot be properly established, which can lead to bearing wear, while if the speed is too high, the warming-up process will be too rapid.

It ensures that all parts of the unit are properly preheated and expand evenly, thereby preventing components such as cylinders, diaphragms, nozzles, main shafts, impellers, and shaft seals from deforming or becoming loose. Another aspect is inspection, namely the inspection of the mechanical system and the oil system. Mechanical systems include whether there is friction between moving and stationary parts, and whether the control system can stabilize the rotation speed. The oil system mainly checks the return oil conditions of each bearing shell, etc. A lubricating oil film is established at low speeds.

Answer: It is mainly to evenly heat the various components of the turbine, in order to prevent excessive temperature differences from causing deformation or loosening of the turbine components. If the warm-up rotation speed is too low, the lubricating oil film cannot be properly established, which can lead to bearing wear. If the warm-up rotation speed is too high, it will result in an excessively fast warm-up rate, causing excessive thermal stress within the metal components and thus shortening the lifespan of the turbine.

Low-speed warming up is beneficial for the machine; it takes into account the thermal expansion and contraction of the impeller, and helps to assess the performance of the turbine. Warming up too quickly can easily lead to bearing wear and increased vibration! The specified speed for low-speed warm-up is intended to prevent damage to the machine!

Reduce damage to the machine caused by sudden cooling and heating

Cold start ensures uniform thermal expansion of all components of the machine, preventing the internal parts from becoming loose or deformed due to heat

Low-speed warm-up is the preheating process of the turbine at low speeds, with two purposes: (1) When starting the turbine, it is necessary to allow sufficient time for low-speed warm-up. During a cold start, this process ensures that all components of the turbine are properly preheated and can expand, thereby maintaining the proper curvature of the rotor and ensuring that all components meet the design requirements. It also prevents deformation and relaxation of components such as cylinders, diaphragms, nozzles, shafts, impellers, steam seals, and shaft seals, as well as friction between moving and stationary parts. The turbine casing is very heavy, with uneven thickness in different sections. The inlet steam temperature reaches 430°C; if warming up the machine at low speeds is not done, the various components will not be heated evenly, leading to uneven expansion, which can cause the shaft to bend and changes in the clearances between different parts. This puts additional stress on the bearings, and it may even result in damage to the labyrinth seals, leading to serious accidents. Generally, a specific speed is specified for warming up the machine – if the speed is too low, the oil film cannot be properly established, which can damage the bearings. ⑵To completely remove the condensed water inside the casing, before starting up, the turbine is at room temperature; when high-temperature steam enters the turbine, it will condense, and in severe cases this can lead to water hammer. High-speed operation can damage the blades and increase vibration. During the warm-up process, preheating helps to remove condensate water, thereby preventing accidents. ⑶For turbines that have not been fully cooled, especially those without a barring device, it is also necessary to warm them up at low speed during startup, in order to prevent shaft bending and deformation as well as avoid friction between the moving and stationary parts of the steam-bearing surfaces. It is specified to warm up at 300–500 revolutions per minute; this is mainly because if the speed is too low, the bearing oil film cannot be properly established, which can lead to bearing wear, while if the speed is too high, the warming-up process will be too rapid. Therefore, the low-speed warm-up speed is 300–500 revolutions per minute. (The dry gas seal performs poorly because at speeds below 500 revolutions, the dynamic and static seals cannot form an effective air film, resulting in contact and friction between the sealing surfaces and thus damage to the dry gas seal. Therefore, the warm-up time at 500 revolutions should be kept within 30 minutes; warm-up can be carried out for a longer period at 1000 revolutions.)