Vacuum pumping system is an important part of direct air cooling system. Its function is to establish and maintain low back pressure of steam turbine and vacuum of condenser. When the unit is started, the accumulated air in some steam, water pipe systems and equipment shall be removed to speed up the start-up. During normal operation, the steam, drain water, air leaked into the vacuum system and other non condensable gases shall be removed in time to maintain the vacuum of the air condenser and reduce the corrosion to the equipment. The shaft seal and low-pressure heater of the low-pressure part of the steam turbine also rely on the normal operation of the vacuum extraction system to establish the corresponding vacuum.
The vacuumizing system is generally composed of vacuumizing tooling, vacuumizing device, vacuum measurement, temperature measurement and control components. Half of the evacuation device includes the extraction pipeline, valve, vacuum pump or vacuum unit, etc. Vacuum measurement mainly refers to vacuum gauge. The selection of main pump and front pump is mainly based on the required pumping speed and time. The materials used for the pipelines involved in vacuum pumping shall meet the following conditions:
(1) Good air tightness;
(2) The suction capacity of the internal surface shall be as small as possible. If there is suction, it shall be easy to degas;
(3) Stable chemical properties;
(4) Good thermal stability;
(5) Avoid dead ends that can hold air.
Vacuum pumping system importance：
Whether the most favorable vacuum can be maintained is a standard to measure the operation of vacuum pumping equipment, which has a great impact on the safety and economy of steam turbine. During the operation of the condensing equipment, if the vacuum system is not tight, air will leak in and the vacuum will be reduced. At the same time, the partial pressure of air is increased. Because the solubility of air is directly proportional to its partial pressure, more air is dissolved in water, and the oxygen content of condensate is increased.
The quality of vacuum technology directly affects the obtaining and maintaining of vacuum degree. The current vacuumizing technology, which combines heating and displacement, can achieve high vacuum quickly, but it also has some limitations in application.
(1) Selection of heating mode. For small vertical low-temperature insulated vessels, the recommended internal heating and vacuumizing process can be used, but for horizontal tanks or large low-temperature vessels, the external heating method is mainly used. If a reasonable internal pipe layout is adopted, the internal heating and vacuumizing can also be used.
(2) Control of interlayer temperature. The higher the degassing temperature is, the easier it is to activate the intercalated adsorbent. The higher the temperature is, the worse the performance of the brazed part of the vessel and the multilayer insulation material is. So it is very important to choose the ideal heating maximum temperature. Secondly, at present, it is difficult to monitor the temperature in the interlayer and determine the actual temperature of the interlayer. Therefore, how to control and ensure the temperature of the interlayer is constant and the flow field distribution of the heating gas is in a reasonable state is also a direction of future research.
(3) Large scale design of vacuum pumping process. In order to realize the large-scale vacuum pumping process in the batch production line of low-temperature insulation vessel, it is necessary to design the process flow of vacuum system reasonably, including the planning of replacement gas heating system, gas flow control system, interlayer heating system, etc.
With the improvement of vacuum theory and deep understanding of vacuum process by EVP vacuum pump manufacturer, the existing problems and limitations in the process will be solved and improved gradually with the accumulation of theory and experience, further solving the bottleneck problem of vacuum in the manufacturing process of low-temperature pressure vessel and reducing the energy consumption in the process of vacuum.