Vacuum systems for steam power plants

Vacuum systems during the test run at the Busch power plant expertise centre in Spain
( | Press Release | 2019-10-10 13:43:43 )
Busch Vacuum Pumps and Systems has now set up three expertise centres especially for the construction of vacuum systems for power plants worldwide. Vacuum systems are important components in power plants for extracting the large quantities of leakage air from the capacitors downstream of the steam turbines. They thus indirectly influence the economic efficiency and environmental compatibility of steam power plants. In these vacuum systems, Busch uses the latest generation of Dolphin liquid ring vacuum pumps, which has considerable advantages over vacuum systems using steam ejectors.

A capacitor is connected downstream of each steam turbine operated in a power plant. This condenses the steam that was previously generated in a combustion chamber in order to drive the steam turbine. The water is fed back from the capacitor to the combustion chamber, where the cycle starts again from the beginning. The task of the vacuum system is to extract the leakage air from the capacitor so that it can be operated as effectively as possible. This holding process takes place constantly during turbine operation. If a turbine needs to be restarted after a standstill, it must first be evacuated from atmospheric pressure to a certain vacuum level. Vacuum systems are also used for this evacuation process, called hogging. Depending on the size and design of the turbine and the capacitor, two or three vacuum systems are required for each, which achieve the required ultimate pressure and whose size must be adapted to the capacity and operating conditions of the turbine and capacitor. Depending on the design and operating principle of the power plant, the same vacuum systems can be used for the holding and hogging processes, as they are technically identical. If different vacuum levels are required for both processes, different vacuum systems can also be used.

Two identical vacuum systems are usually installed for hogging, one of which fulfils a purely stand-by function. A third system may be necessary if the power plant is located in a climatic zone with strongly fluctuating ambient or cooling water temperatures. Operation is then performed in winter or summer mode. At lower temperatures in winter, less leakage air has to be extracted and the capacitor can run under optimum conditions due to the colder cooling water. A vacuum system is usually designed for these ideal conditions. If the ambient temperature and water temperature exceed a certain value, a second vacuum system must be connected because more leakage air is produced. Then, together with the stand-by system, three vacuum systems are required in total.

Busch Vacuum Pumps and Systems has now established three expertise centres in order to meet the increasing worldwide demand for effective vacuum systems for holding and hogging processes. They have started operations at the Busch companies in Korea, the USA and Spain. In addition to specially equipped assembly and test rooms, engineering is also accommodated at the three interconnected locations. The project engineers in turn work together with all Busch companies worldwide, so that Busch can provide local support to operators of power plants, manufacturers of capacitors or engineering offices entrusted with the construction of power plants. It is always the responsibility of the vacuum specialists at Busch to design a vacuum system in such a way that the required performance in terms of ultimate pressure and pumping speed is guaranteed as effectively as possible and a vacuum system is configured accordingly.

As modules for these vacuum systems, Busch uses the latest generation of Dolphin liquid ring vacuum pumps. They are available in single- and two-stage versions as standard. An ultimate pressure of 130 millibar absolute is achieved in the single-stage version, while the ultimate pressure in the two-stage design is 33 millibar absolute. The ultimate pressure can be reduced even further by using additional gas ejectors. This allows the vacuum systems to be precisely adapted to the individual conditions and operated with the greatest possible economic efficiency. Dolphin liquid ring vacuum pumps are available with pumping speeds of up to 27,000 cubic metres per hour.
In the past, multi-stage steam ejectors with intermediate capacitors were often used for these applications. The reasons are obvious why more and more power plant operators and energy plant constructors are opting for liquid ring vacuum technology. Multi-stage steam ejectors require considerably more space and are difficult to install. They also require water vapour, which has to be intercondensed several times.
Liquid ring vacuum pumps can be connected to the turbine's overall control unit, so that the entire process runs fully automatically. Steam ejectors, on the other hand, can only be manually adapted to changing requirements. Complete automation is not possible.

Water is used as the operating fluid for the Dolphin liquid ring vacuum pumps. When seawater is used, both the vacuum pumps themselves and the complete vacuum systems are made of corrosion-resistant materials. The water is circulated and cooled by a heat exchanger. A liquid separator prevents condensates – from water vapour being sucked in – from remaining in the vacuum pump. The vacuum systems are nearly maintenance-free. An overhaul is performed once a year – when a power plant is disconnected from the mains for general maintenance.

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Jasmin Markanic

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