Edwin Gobrecht

Advanced Steam Turbine Bypass Valve Design for Flexible Power Plants

The authors company has had extensive experience providing steam turbines including auxiliary systems as a turn key contractor for more than 40 years. Bypass systems are an integrated part of modern Combined Cycle Power Plants (CCPP) [1]. Bypass systems contribute a major part for operational flexibility. They allow the shortest start-up times by minimising mismatches between boiler/HRSG and turbine. Bypass systems also lead to predictable and repeatable start-up times, as well as reducing solid particle erosion of component, to a great extent. The functional requirements for bypass valves are: • Control mode for an accurate control of the IP and LP bypass steam flow during the unit start-up and shut-down, as well as during normal operating transients. • Fast closing mode for bypass-trip (supported by spring force) when required for condenser protection. • Combined mode for fast reaction on pressure increase to a define set point and further action in control mode. In the past, a combined stop and control valve design, each with a separate stem, was common. The challenging objective for the bypass valve design was to integrate the control function and the trip function with a single stem design. The authors company has developed this advanced steam turbine bypass valve that incorporates hydraulic actuator with a single stem design. The valve bodies have noise reduction fittings and are equipped with large extensions on the outlet side to reduce vibration throughout the bypass system. The bypass valve body has an integrated steam strainer which protects both valve parts and the condenser from external debris. The bypass design is prepared for Power Plants with elevated temperatures which allow for the highest plant efficiencies [2]. Surface coating protect moving components against oxidation and reduce friction by means of a surface coating. Steam at high temperature passes through the bypass to the condenser. An incorporated water attemporating flow control system reduces the steam temperatures before entering the condenser. Condensate water is injected through an orifice in the bypass system. The orifice is located down stream in the pipe between the bypass valve and condenser. Electro-hydraulic supply units deliver the control fluid to the bypass valves. An optimized bypass system has to provide: • Long service life with low maintenance costs; • High stroke speed; • Pressure control by unit set point; • High actuation forces; • Accurate positioning; • Very short trip time into closed position. By means of bypass station, one can get highest flexibility of power plants use of the new valve one will get highest control performance and shortest reaction time.Copyright

Flexible and Economical Operation of Power Plants: 25 Years of Expertise

Within the last years the idea of running a conventional power plant has changed. Fluctuating power generation by solar power and wind parks creates a need for highly flexible backup power plants. This need quickly arose within the last 5 years and the market is still searching for a solution.Single steam turbine manufacturers can provide features to react more flexibly, quickly and to prolong component life time. Thus, considerable operating experience has already been in existence for many years.New highly efficient steam turbines are already equipped with solutions to serve an ambitious market. But also, for existing units, different modernization packages can be provided along with hardware and software modifications which allow power plants to supply power at a moment’s notice.This paper presents the overall approach and the possible field of application for the well-established features of Siemens steam turbines.Starting a power plant within a short time to fill the gap of fluctuating power generation is an important capability in order to participate in today’s and tomorrow’s energy market. A fully automated start-up procedure to avoid any delays contributes in fulfilling this requirement. Optimized component geometries guarantee the shortest start-up times. Furthermore, a parallel start-up of gas and steam turbines (Hot Start on the Fly) has already been proven for many years.Regarding flexibility, the improvement of start-up time is only one major aspect. Another important task is to provide the opportunity to influence scheduled maintenance outages. Therefore, steam turbines can be equipped with software which allows the customer to plan the power plant’s outages in accordance with single components requirements, e.g. GT outages. The lifecycle counter enables customers to evaluate the optimum between start-up time and life time consumption based on dynamic equivalent operating hours.In addition, fast cooling procedures help to keep outage times to a minimum.Copyright

High Steam Turbine Operating Flexibility Coupled With Service Interval Optimization

Historically steam turbine operations were designed for a market that was typically either base load or intermediate duty load operation. The optimal steam turbine start-up profile was established using the maximum allowable component stress and therefore optimizing service time consumption. Over the last few years, the market requirements have changed significantly. The market requires plant start-up flexibility with the ability to accurately predict start-up time, and reliably meet the start-up time. Applying the historical steam turbine start-up philosophy either limits the operating flexibility of the plant or exceeds steam turbine allowable stresses increasing service time consumption. Innovative concepts are being presented on how steam turbines can achieve reduced start-up times while minimizing service time consumption thereby improving availability. These concepts allow the customer to be able to accurately predict start-up times and reliably meet the dispatch bid. Therefore, an economic calculation may be performed to determine the most effective start-up mode. This economic calculation will evaluate the impact to service life (inspection and test intervals) versus the benefits of power generation. The new concepts provide one solution for base load, intermediate duty load operation, and plants requiring fast start up capability. The new market needs for flexible operation including fast start-up times require plant operability enhancements [1]. Some of the operability enhancements that can be implemented include: • steam turbine stress controller and stress monitoring systems which allow a selection of the start-up mode determining the start-up time, thermal stress and service time consumption; • high level of plant automation; • plant systems designed to provide steam conditions necessary for selected start-up mode. The benefit of these solutions will be presented by means of examples from recently modified power plants. It is possible to achieve a significant improvement in the plant operation and start-up with low costs.© 2003 ASME

State of the Art Steam Turbine Automation for Optimum Transient Operation Performance

The growing share of renewable energies in the power industry coupled with increased deregulation has led to the need for additional operating flexibility of steam turbine units in both Combined Cycle and Steam Power Plants. Siemens steam turbine engineering and controls presently have several solutions to address various operating requirements: - Use of an automatic step program to perform startups allows operating comfort and repeatability. - 3 start-up modes give the operator the flexibility to start quickly to meet demand or slowly to conserve turbine life. - Several options for lifetime management are available. These options range from a basic counter of equivalent operating hours to a detailed fatigue calculation. - Restarting capabilities have been improved to allow a faster response following a trip or shutdown. - In addition to control of speed, load and pressure, special control functions provide alternative work split modes during transient conditions. - Optimum steam temperatures are calculated by the steam turbine control system to achieve optimum startup performance. - Siemens steam turbines are also capable of load rejection to house load, some even to operation at full speed, no load. Several plants are already equipped with these solutions and have provided data showing they are operating with shorter start-up times and improved load rejection capabilities. Finally Siemens of course continues to pursue future development.Copyright

Peterhead Power Station: Parallel Repowering Innovative Steam Turbine Enhancement

In the year 2000 one of Europe’s most flexible power stations was commissioned by the authors’ company. The existing fossil fired power station was modified by a “Parallel Repowering”. With that concept three gas turbines (GT) in combination with three heat recovery steam generators (HRSG) were tied-in additionally to the fired boiler. This concept is compelling especially for large steam power plants and offers more flexibility than “Full Repowering” in matching GTs with the existing steam turbine (ST). The key to maintaining reliability of the repowered unit is the ST modernisation. Plant operability enhancements provide the flexibility of the fired boiler and ST for load following and peaking purposes. The authors’ company was responsible for the complete conversion of the fossil fired power station into a modern combined cycle unit. This comprises the tie-in of new steam pipes, bypass stations and the upgrade of the steam turbine auxiliaries as well as the implementation of a new automation system parallel to the existing one. The “Parallel Repowering” offers a maximum of operation variations: •Conventional (Rakine cycle) mode. •Open cycle mode (only GT). •Combined cycle mode. •Hybrid mode. The non-OEM steam turbine needed to be modified for the combined cycle operation with GTs. The condenser load had to be kept as low as possible because of the existing condenser design. Auxiliary systems like the gland steam system and the drain system had to be modified for all different operating modes. Special design features, like the IP rotor cooling system and the flange heating system, had to be extended to operate under all circumstances. One essential difference to the existing operational mode is the necessity of a steam bypass operation. Existing cold reheat (CRH) piping is of carbon steel, so the ST needs to be started with an isolated HP cylinder. The following modifications for the HP turbine were necessary: •For the isolated HP cylinder operation non-return valves (NRV) were built into the CRH line at the HP turbine exhaust. •The HP cylinder will be automatically isolated by closure of the HP valves and the non-return valves in the CRH line, and the simultaneous opening of the HP vent line. •As no instrumentation was available for a reliable monitoring of the isolated operation, a controlled reverse flow from the CRH to the HP vent line was established. •The HP cylinder evacuation is controlled by a dedicated control logic.Copyright