Issues with steam extraction from turbine condensers
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In condensing steam turbines, a steam jet ejector is used to reduce the pressure in the condenser to a negative value, of about 12 KPa (absolute pressure). After the high-pressure steam in the turbine wheel chamber is converted to low-pressure steam, it enters the condenser. May I ask if the extraction pump also draws out some of the low-pressure steam? If not, how should the condenser be designed in terms of structure?
Firstly, the vacuum is not created by the pump; it results from the change in specific volume as gas turns into water, with the pump merely serving to maintain the vacuum. If the vacuum level is too high, it will draw out the steam that has not completed its work in the final stage, increasing the steam consumption of the turbine; therefore, the vacuum level cannot be too high either
I see; in fact, this pressure is the saturated vapor pressure of water corresponding to the temperature of the condensed water (47 degrees), which is approximately 11 KPa.
It mainly relies on the phase change of steam to generate negative pressure, with the volume undergoing a change of nearly 1,500 times due to this phase change. Venting serves only as an auxiliary function to remove non-condensable gases.
I’ve learned it – how can I ensure that no steam is released, and how can I tell? Seeking help from experts
Some of the uncondensed steam can be extracted; this steam is condensed again in the extractor and returned to the valve chamber of the recondenser, while the non-condensable gases are released into the atmosphere.
The steam ejector serves two purposes: 1. To rapidly create the necessary vacuum level in the condenser, the low-pressure cylinder of the turbine, and the low-temperature reheater system during the startup of the power plant, thereby enabling the turbine to meet its startup requirements quickly. 2. Non-condensable gases are removed during operation to maintain the vacuum level in the condenser, thereby preventing a deterioration in the condenser’s performance and subsequent decline in the overall performance of the unit. While pumping out non-condensable gases, the ejector does indeed draw in some of the low-pressure steam inside the condenser. In the design of the condenser, a portion of the tube bank is located in the subcooling zone; when non-condensable gases and low-pressure steam are drawn out through this subcooling zone using an ejector, some of the steam condenses to prevent a large amount of low-pressure steam from being removed. The U.S. HEI regulations specify that the degree of supercooling in the supercooling zone should not exceed 4.17 degrees (4.17 degrees below the design saturated pressure of the condenser). Once the capacity of the ejector is determined, the ratio of non-condensable gases to steam that are extracted can be roughly determined as well. The mixture at the ejector outlet (including non-condensable gases, low-pressure steam, and auxiliary steam) is condensed in a small condenser designed specifically for ejectors; the non-condensable gases are released into the atmosphere, while the condensed steam is returned to the condenser for reuse. Furthermore, this vacuum extraction system comes in various configurations: water jet evacuation system, vacuum pump system, vacuum pump + air ejector system, vacuum pump + steam ejector system, and so on.