Efficient Removal of Leaked Air from Condenser at Coal-fired Power Station

Seoul, Korea In 2017, the state-owned Korean energy company, KOMIPO, commissioned the Shin Boryeong TPP coal-fired power station, which is the largest and most modern ultra-supercritical (USC) bituminous coal-fired thermal power plant in Korea. The power station is equipped with two units, each with a capacity of 1,000 megawatts. It is an extension to the Boryeong Thermal Power Site Division, the largest coal-fired power station complex in Korea. KOMIPO uses Busch vacuum systems equipped with the latest generation of Dolphin liquid ring vacuum pumps to evacuate leaked air from the condensers.

Shin Boryeong TPP is not only the largest and most modern coal-fired power station to be commissioned in Korea, it is also a facility which has relied solely on domestic partners and their technology for its construction. The existing power stations at the Boryeong Thermal Power Site Division have been used to create a national industrial complex. There are 10 units and all the coal-fired power stations generate 7,800 megawatts of energy in total. KOMIPO (Korea Midland Power Co. Ltd.) operates six different power plant complexes throughout Korea, and generates approximately 13 percent of the power consumed in the country. Two additional coal-fired power stations and two gas-fired combined cycle power plants (CCPP) are currently being constructed to take into account the increasing demand for power in the country.

The new Shin Boryeong coal-fired power station uses the latest ultra-supercritical energy technology to achieve considerable improvements in plant efficiency and drastically reduce the consumption of coal. This is achieved by increasing the steam pressure to 265 kg/cm2 (bar), as a result of the high temperatures of 610°C and 621°C used for the main and warming steam.  Besides highly efficient power generation, reducing environmental impact is also important for KOMIPO. It has been possible to considerably reduce CO2, SOx and NOx emissions in comparison with older power plant units. The first 1,000 megawatt unit was commissioned in June 2016, and the second unit has also been producing 1,000 megawatts of power since November 2016.

The use of Busch mechanical vacuum systems to evacuate both condensers has ensured an optimal efficiency factor for the steam turbines, and has therefore helped to increase efficiency and reduce energy costs. As well as for the constant evacuation process (holding) as for the initial evacuation during the start-up process (hogging) these mechanical vacuum systems equipped with Dolphin liquid ring vacuum pumps are used.

Fig 2: One of three vacuum systems with Dolphin liquid ring vacuum pumps for the constant evacuation of leaked air from the condensers (holding), each of them are equipped with two-stage Dolphin liquid ring vacuum pumps

Incidental leaked air is constantly evacuated (holding) from the condenser by means of three identical vacuum systems (Fig. 2), each with a two-stage Dolphin liquid ring vacuum pump. Each vacuum pump has a pumping speed of 3,700 cubic metres per hour and an ultimate achievable pressure of 33 millibar. Three vacuum systems are required because the power station operates in "winter operation" and "summer operation" modes. The power station is subject to both ambient temperature and seasonal fluctuations because the cooling water is taken from the sea.

During "winter operation", the ambient temperature and cooling water temperature are lower which means there is less need to evacuate leaked air because the condenser is operating under optimal conditions. Therefore, only one of the vacuum systems is operated during "winter operation". When temperatures are higher, a second vacuum system is additionally activated so that more incidental leaked air can safely be evacuated. This is usually the case during the summer months. The third vacuum system with Dolphin liquid ring vacuum pumps purely acts as a stand-by for holding as well as providing increased pumping speed during hogging phase when all three pumps operate in parallel. By implementing all three vacuum systems in the overall control system for the steam turbines, they can be hooked up or shut down in a fully automated manner. As air continually needs to be evacuated from the condenser, at least one of the three vacuum systems must be in operation at all times, and the control system must be programmed so that all three vacuum systems operate intermittently. In this way, all three vacuum systems will operate for the same number of hours. Maintaining a constant vacuum level in the condenser also helps to remove the vapour from the turbine, so that the turbine does not need to operate against back pressure when starting up, which is favourable for the efficiency factor.

Two additional vacuum systems with single-stage Dolphin liquid ring vacuum pumps (Fig. 3) are used for the condenser water box priming application. These systems are similar in terms of their construction, but the Dolphin sizes used here are smaller. They operate with a pumping speed of 1,230 cubic metres per hour in each case and an ultimate pressure of 130 millibar. They, too, have been integrated in the overall control system. During an initial degassing, both vacuum systems operate simultaneously. They create a vacuum and evacuate the condenser water boxes which lifts cooling water to flood the cooling side of the condense after which the pumps stop. In everyday practice, one of these two vacuum systems operates for a short time, twice or three times each day to re prime the condenser by removing additional dissolved air that is released from the cooling water as it rises in temperature through the condenser.

Multi-stage steam ejectors with intermediate condenser were used for these applications in the past. Nowadays, the mechanical Dolphin liquid ring vacuum pumps have become more and more established as reliable and effective vacuum generators. Power station operators and energy plant constructors are increasingly opting for liquid ring vacuum technology for obvious reasons. Multi-stage steam ejectors require considerably more space and are costly to install. They also require steam, which needs to undergo several intermediate condensing processes. 

Fig. 3: These two vacuum systems with single-stage Dolphin liquid ring vacuum pumps are used for the initial evacuation during the start-up process (hogging)

Sea water is used as the cooling fluid for the Dolphin liquid ring vacuum pumps. The vacuum systems heat exchangers are therefore made of corrosion-resistant materials and in the water box priming system the actual vacuum pumps and the entire systems are made of corrosion-resistant materials. The operating fluid is pumped through the circuit and cooled via a heat exchanger. A discharge separator allows pumped gases to be vented to exhaust whilst separating the operating liquid to be re used by recycling back to the vacuum pump. The operation of the vacuum systems is virtually maintenance-free. They are overhauled once a year, when the power station is taken offline for two to three weeks in order to complete general maintenance work.

As this project has only involved Korean companies, KOMIPO has decided to work with Busch Korea Ltd. KOMIPO has therefore found a partner that is part of the international Busch Group and which has earned an international reputation for vacuum technology in a wide range of industrial applications. Dolphin vacuum systems are specifically tailored to the requirements of the particular application in each case. The engineering, the design and the assembly of the vacuum systems are all carried out at Busch Korea. Even the advice and customer care are provided in person and on site. KOMIPO therefore has a local partner who also has a global presence with respect to generating vacuum. There has also been a focus on the European quality of the Dolphin vacuum pumps manufactured in Great Britain.

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