All From One Source Hydraulic Closed Loop Control Systems for Gas and Steam Turbines

Transcription

1 All From One Source Hydraulic Closed Loop Control Systems for Gas and Steam Turbines

2 1 Introduction All From One Source Hydraulic Closed Loop Control Systems for Gas and Steam Turbines ensuring maximum operational efficiency. Trip of a turbine group due to a hydraulic component failure is to be avoided for a minimum of five to six years. The application of hydraulic systems in gas and steam turbines is described below in more detail using a Siemens turbine group as an example. The trend in the development of power station technology is today more and more towards the so-called gas and steam combination systems (combined cycle). Hydraulic control systems have a long tradition in the construction of turbines and are surely one of the founders of oil hydraulic control technology in general. (e.g. the centrifugal force control by J. Watt) Approximately 20 years ago Rexroth began to replace the low pressure hydraulics control concepts based on bearing lubrication systems with high pressure systems. Turbines with pressure ranges of 1450 to 2300 psi (100 to 160 bar) are considered high pressure systems. This application of hydraulics has been described in publications Hydraulics in gas turbines and High pressure hydraulics in modern turbine systems for gas and steam processes (Rexroth RE /07.90). The more important aspects in hydraulic turbine control are safety, availability and service life. Safety unsafe operating situations should be 100% under control. Not being able to control faults can lead to injury to personnel, machine damage, or even catastrophic accidents. Availability and service life is necessary so that the uninterrupted continuous operation of the generators are made possible. The most important aspect for power station applications is fault-free power supply. Thus With the combined cycle process hot exhaust gas from the gas turbine is passed through a waste heat boiler where steam is generated, which is then passed on to a steam turbine that is again used to generate electricity (efficiency of up to 58 %). In combined cycle power stations one or more gas turbines are used in conjunction with steam turbines. An application range of steam turbine power of 300 MW can be expected. For this, the gas turbines are each installed with their own generator; a single shaft system (gas turbine + steam turbine + generator) can, however, also be selected. The turbine sets are laid out to suit the frequency that the power supply requires, for example 50 Hz or 60 Hz.

3 Gas Turbines 2 Premix Control Valve Premix Shut-off Valve 1 and 2 Gas Turbines To ensure the controllability of the gas turbines with regard to the control technology and safety requirements, various systems in the fuel system circuit are necessary. Generally it is possible to fit a gas turbine with two types of fuel systems, these are natural gas and/or fuel/oil. (Other fuels are also possible, e.g. coal gas or naphtha). The type of gas turbine used is dependent on the customer. A relevant fuel and gas circuit is shown in Figure 1. As can be seen from this schematic, it is possible to drive the gas turbine in various modes of operation (pre-mix operation or the diffusion mode of operation). The modes of operation require the use of control and safety systems as shown in the schematic. These systems comprise fuel and gas valves that are driven by hydraulic actuators manufactured by Rexroth. For relevant nominal sizes and pressure ratings, see Figure 2. These components do cover a gas turbine power range from 60 MW to 240 MW. Shut-off Valve Fuel Pumps Diffusion Control Valve Diffusion Shut-off Valve 1 and 2 Return Line Shut-off Valve Premix Control Valve with Shut-off Function Diffusion Control Valve with Shut-off Function Pilot gas Control Valve with Shut-off Function Figure 1 Premix Burner Diff. Burner Return Line from Diff. Burner Premix Burner Diff. Burner Pilot gas Burner Pilot Control Shut-off Control Shut-off control valve valve valve valve valve Medium Fuel Fuel Fuel Fuel Fuel Fuel Naphtha Gas Gas Gas Gas Gas Gas Gas Size [mm] Size [inch] Size [bar] Size [Ansi] Cv [gpm] Kvs [m 3 /h] T [ C] T [ F] Leakage class V V V V V V V VI VI VI VI VI VI VI [Ansi Bl ] Figure 2

4 3 Gas Turbines Gas Turbines Due to the special design feature of the fuel and gas valves (valves are internally pressure balanced, therefore, actuation forces are practically independent of valve size) it is possible to use the same hydraulic actuator for most sizes of turbines. Figure 4 Figure 3 This means, for example, that the nominal size 5 in. (125 mm) gas control valve (turbine power MW) (Figure 3) can be actuated by the same cylinder as that used for the nominal size 10 in. (250 mm) control valve (turbine power MW). A further feature is that in the gas circuit all of the control valves are equipped with a priority shut-off function. This combined control and shut-off function in one valve housing has the same sealing capacity as the standalone shut-off valve (Figure 3). The principle of dual safety has been achieved by using a fast-close valve and a combination of fast-close valve and control valve. The need for a second isolator valve (e.g. when a non-leak-free control valve is used) is eliminated. All of the hydraulic actuators (Figure 4) are servo actuator fitted with a direct mounted control block. The cylinder chambers are directly connected with each other via the control block (no pipework). All of the valves are hydraulically held open against the spring package which is located in the rear of the actuator. The closing is purely mechanical via the spring package. The cylinder is designed as a synchronizing actuator with a double rod. Since the oil volume on both sides of the actuator piston is the same (equal area cylinder), hydraulic oil can be directly passed to the back side of the actuator via the control block during fast closing (switching time ms) operation. By using this concept (re-gen), it is not necessary during fast switching movements to return the hydraulic oil to the hydraulic power unit. The result of this is that field piping needs only be sized for the low-flow control operation. This reduces the size of the hydraulic supply power unit, horsepower, and interconnecting piping. All of the hydraulic valves used, with the exception of the servo valve, are of a leak-free design. The positive shutoff safety of the system is guaranteed (all solenoid valves are seat valves). Therefore, the leakage flow of the entire system (i.e. the fast closing cylinders are totally leak-free) is drastically reduced so that a hydraulic power unit with redundant electric motors (10 HP [7.5 kw]) is adequate to supply all actuators.

5 Gas Turbines 4 Natural Gas Control Valve Figures 3 and 5 show an example of a gas control valve fitted with a hydraulic actuator. Figure 6 shows a fully assembled valve with cylinder in a Berlin test site. Technical Data: Size DN 125 Size DN 200 Size DN 250 Pressure PN psi (30 40 bar) Temperature 392 F (200 C) Flow-rate DN 125 = 125 m 3 /h DN 200 = 200 m 3 /h DN 250 = 250 m 3 /h Leakage in closed position 0.001% Figure 5 Figure 6

6 5 Gas Turbines Gas Turbines Figure 7 is an example of a Rexroth hydraulic power unit for control of fuel and gas valves. The design features include redundant motor pump groups and valving for uninterrupted continuous operation. This design also includes verticallymounted, submerged pump design for reduced noise (less than 80 dba) and improved pump life. Figure 8 shows production units of Rexroth hydraulic power units for fuel and gas valve control. Figure 7 Figure 8

7 Steam Turbines 6 Steam Turbines When compared with a gas turbine, steam turbine safety and control systems have to be controlled over the entire thermic power range of the turbine. With a gas turbine, the fuel quantity flowing into the turbine is controlled (e.g. similar to an injector pump on a diesel engine); a steam turbine has to have the entire steam flow controlled by steam valves. The nominal sizes of the steam valves, the steam pressures and temperatures that the components are subjected to, are much higher than those of a gas turbine. This results in much higher actuation forces and thereby the diameters and spring forces of the actuator are increased. A typical design of a steam turbine group complete with the necessary steam valves is shown in Figure 9. Via two main steam control valve combinations, items 1.1 and 1.2, the steam is passed from steam supply to the high pressure component of the turbine. After the first expansion, the steam is passed to the medium pressure component via the intercept valve items 2.1 and 2.2. From the medium pressure component, the steam passes over the low pressure stages to the condenser and returns as feed-water back into the process. For the control of fault cases the high pressure bypass station item 4, the medium pressure bypass station item 5 and the low pressure bypass station item 6 are provided. By using the bypass stations, it is possible to operate the steam supply independently from the turbine. The built-in valves for turbine control are designed and manufactured by the turbine manufacturer. The temperature can range up to 1022 F (550 C) and the pressure range from psi (8 300 bar) depending where the valve is located within the system. Each valve combination is comprised of a fast closing valve and a control valve. The safety and control functions are combined into one valve housing. Two hydraulic actuators are attached to this housing (see Figure 10). 1 Main steam control valve combination 2 Intercept valve combination 3 Supply steam butterfly valve combination 4 HP Bypass-station 5 MP Bypass-station 6 LP Bypass-station Figure 9

8 7 Steam Turbines Steam Turbines For use with various nominal sizes and pressure ratings, Rexroth offers a complete series of hydraulic actuators and spring packages for steam valves. The diameter range is 4-1/4 9-1/2 in. ( mm) and stroke ranges from 2 10 in. ( mm). Figure 10 The actuator incorporates a modular design, the diameter range is covered by three housing sizes and three sizes of spring packages (see Figure 11). The actuator sizes and spring packages of the medium and large range can be interchanged, thus making it possible, with this series, to cover a range of steam valves on turbines with ratings of MW. Mini Medium Large Independent of the actuator, it is possible to integrate all of the required control functions into a control block which can then be fitted onto the actuator. Both actuator ports are directly connected to the control block. As can be seen (Figure 12), valve closing is ensured (isolation of the steam feed and thereby the tripping of the turbine) via a spring package, therefore, a mechanical closing is guaranteed. The steam valves are hydraulically held open against the spring package by a servo valve held in position in closed loop control. Modular interchangeable Figure 11

9 Steam Turbines 8 Figure 12 Each valve combination is comprised of a fast closing and a control component. Due to the reasons previously described, steam isolation can be achieved with control actuators also fitted with a priority fast close function. Similar to the gas turbine actuators, the high oil flows generated with switching times of ms (with large cylinders GPM [ l/min]) are passed directly to the opposite side of the cylinder. Figure 14 The connections to the hydraulic power unit need only be sized to handle the flow for normal control operation. During fast closing, no oil flows back to the hydraulic power unit. Figure 13 and 14 depict a typical Rexroth hydraulic power unit for control of main steam valves. Features include redundant motor pump groups, water/oil heat exchanger and valves, and fluid conditioning filters for phosphate ester hydraulic fluid. Figure 13

10 9 Steam Turbine Bypass Steam Turbine Bypass In addition to the hydraulic control of the primary components of combustion and steam turbines, Rexroth supplies similar control systems for auxiliary functions of turbines such as gas diverter control and turbine bypass stations. Figure 15 shows a typical Rexroth power unit for control of steam turbine bypass valves. Major features include redundant motor pump groups vertically mounted with submersed pumps for reduced noise and longer pump life, accumulator capacity for emergency (loss of electrical power) operation, programmable logic controller (PLC) (see Figure 16) for closed loop control of servo or proportional valves, and monitoring of power unit Figure 15 Figure 16 functions which are communicated back to main distributed control system (DCS). Figure 17 shows a spray water control valve actuator with combined control and shutoff function features. Figure 17

11 Steam Turbine Bypass 10 Figure 18 demonstrates Rexroth s flexibility in custom designs of packaged equipment with this example of an emergency accumulator rack segmented to match the radii of the customer s wall where the rack will be mounted in order to optimize the limited floor space within the plant. As can be seen to this point, Rexroth is able to offer an economic modular system to meet all of the control and safety function requirements for gas and steam turbines covering the power range of MW. Extensive references are available upon request. Figure 18 Training Bosch Rexroth Corporation has a worldwide service and support network, whether it be in-house repairs or field service and start-up. Training classes are offered at Rexroth Hydraulic Training Centers. In-plant training catered to your specific application is also available.

12 Bosch Rexroth Corporation Industrial Hydraulics 2315 City Line Road Bethlehem, PA Telephone (610) Facsimile (610) Bosch Rexroth Corporation Corporate Headquarters 5150 Prairie Stone Parkway Hoffman Estates, IL Telephone (847) Facsimile (847) Bosch Rexroth Corporation Electric Drives and Controls 5150 Prairie Stone Parkway Hoffman Estates, IL Telephone (847) Facsimile (847) Bosch Rexroth Corporation Linear Motion and Assembly Technologies 816 E. Third Street Buchanan, MI Telephone (616) Facsimile (616) Bosch Rexroth Corporation Pneumatics 1953 Mercer Road Lexington, KY Telephone (859) Facsimile (859) Bosch Rexroth Corporation Mobile Hydraulics 1700 Old Mansfield Road Wooster, OH Telephone (330) Facsimile (330) RA /11.03 Replaces: RA /11.98 Printed in the United States