By: S. Zaheer Akhtar, P.E., Bechtel Power Corporation
Combined-cycle power plants operate by integrating a combustion turbine generator (CTG) and heat recovery steam generator (HRSG) with a steam turbine generator (STG). However, during some modes of plant operation — e.g., start-up, STG trip, simple-cycle operation — it is desirable to isolate the STG from the CTG/HRSG. The isolation of the CTG/HRSG from the STG is facilitated by means of the steam bypass system.
Without a bypass system, the steam generated in the HRSG has to be discharged to the atmosphere until the STG is available to accept steam. Steam is discharged by means of vent valves and/or atmospheric dump valves (sky valves) installed on the HRSG steam headers. The dumping of steam to atmosphere is not desirable as it results in loss of valuable condensate and also raises environmental concerns due to noise pollution.
On some combined-cycle plants, the isolation of the CTG/HRSG from the STG is provided by the application of an HRSG bypass damper and bypass stack. In this case steam generation in the HRSG can be controlled/eliminated by venting the CTG exhaust gas through the bypass stack. This scheme facilitates simple-cycle operation and avoids condensate loss, but it is capital intensive and presents the problem of noise pollution.
Modes of Operation
The steam bypass system is generally used during the following modes of operation: start-up and shutdown, steam turbine trip, steam turbine no-load or low-load operation, and simple-cycle operation.
On start-up, the isolation of the CTG/HRSG from the STG allows the CTG to be placed on load without delay and well before the heat-up and roll-off of the STG. In addition, a faster start-up of the STG is possible since the bypass system provides the capability of close temperature matching between the steam inlet temperature and the steam turbine metal temperature. This is achieved by continuous steam dumping to the condenser until the optimum temperature, pressure and flow requirements are achieved for starting and loading the steam turbine.
For a combined-cycle plant with multiple CTGs/HRSGs and a single steam turbine, the bypass system allows for a sequential startup of the CTGs. If one or more CTGs are already on-line and an additional CTG/HRSG needs to be brought on-line, the start-up bypass system facilitates start-up by allowing the gradual warming of the lagging HRSG and allowing for steam temperature matching between the leading and lagging HRSGs.
The steam bypass system is also used when the CTG/HRSG is up on load while the steam turbine is off-line or under no-load/low load conditions. Because STG start-up takes much longer than CTG start-up (due to larger mass of metal that needs to be gradually heated prior to start-up), the STG could be at no-load or low-load conditions while the CTG is at a significantly higher load. This results in excess steam generation from the HRSG that is diverted to the bypass system.
During shutdown, the steam bypass system enables the steam turbine to be taken off-line independent of the operation of the combustion turbine. The combustion turbine/HRSG remain loaded while the steam generated in the HRSG is gradually diverted to the condenser through the bypass system. In case of a steam trip, the bypass system is placed in full service immediately. However, in a controlled shutdown, the STG load is gradually reduced and excess steam generation is diverted to the bypass system. The steam turbine can then be tripped at a reduced load and subsequently isolated. If the steam turbine is isolated while the CTG/HRSG load is comparatively high, the steam turbine metal temperatures remain high. This enables the steam turbine to be ready for a hot start, which is preferable because it minimizes start-up time and start-up stresses in the turbine metal.
The steam bypass system can also be used to operate a combined-cycle plant in simple-cycle mode before the combined-cycle portion of the plant is completed. However, this assumes that the CTG and HRSG are in place and a heat sink (condenser) is available for condensing the steam passing through the bypass system.
Sky valves and electromatic relief valves (ERVs) dump steam to the atmosphere and can be used to supplement the capacity of the steam bypass system. The response time of the ERVs is faster than the conventional, pneumatically operated sky valves and are typically used to prevent the boiler safety relief valves from lifting. However, the drawback with both the sky valves and the ERVs is that their use results in loss of valuable condensate and generates noise concerns.
Steam Bypass Configurations
Two types of steam bypass systems are generally used in combined-cycle power plants:
- Parallel bypass (also known as "direct" bypass or "dry" reheater bypass)
- Cascade bypass (also known as "European" bypass or "wet" reheater bypass) with or without the "Start-up Bypass"
The choice of the bypass system is based on economics and the characteristics of the steam turbine. Most modern plants use the cascade bypass system.
The bypass system consists of steam bypass piping, a steam conditioning station, and a dump tube to the condenser. The steam conditioning station consists of a pressure reducing valve and an attemperator supplied with spray water from the condensate pump or the feedwater pump discharge. The dump tube (sparger) is installed downstream of the steam conditioning station at the condenser inlet.
In this arrangement (Figure 1), the steam generated at start-up in the HP drum and the IP drum of the HRSG is sent directly to the condenser after being attemperated with spray water from the condensate pump discharge. As such there is no flow through the reheater, which operates "dry" when the bypass system is in service. The steam generated in the LP drum can be similarly disposed of to the condenser but this is generally not required for CTGs that do not use rotor air cooling for LP steam generation. Instead the LP steam production is suppressed by bypassing the condensate pre-heater and introducing the cold condensate directly in to the LP evaporator or de-aerator (if the HRSG is designed with an integral de-aerator). Any excess steam that may be produced is vented to the atmosphere through the vent/drain valves on the HRSG LP section and LP steam piping.
With the reheater operating in the "dry" mode at start-up, the reheater tube metal tends to overheat. As a result the metallurgy of the reheater tubes has to be enhanced, which adds cost to the HRSG.
Another drawback of the parallel bypass system is that it requires the use of long lengths of steam piping from the HRSG to the condenser. With the pressure letdown valve and attemperating station located close to the condenser, most of the bypass piping for HP steam is expensive alloy piping that adds to the capital cost of the plant.
In a combined-cycle plant with a multiple train configuration (2x2x1 or 3x3x1), the parallel bypass pipe lengths increase with the number of HRSGs installed. To minimize capital cost, the parallel bypass piping from multiple HRSGs is often combined into a single line with a single steam conditioning station. With such a design, the overall capacity of the bypass system is curtailed, introducing operational constraints during STG trip/start-up. For example, it may not be possible to (i) keep all the CTGs on baseload following a STG trip, or (ii) start-up the lagging CTG/HRSG without decreasing load on the leading CTG/HRSG.
In this arrangement (Figure 2), the HP steam generated at start-up is bypassed around the HP section of the turbine to the cold reheat (CRH) line. The bypass line is equipped with a pressure reducing valve and an attemperator using spray water from the feed water pump discharge. The bypassed steam then mixes with the steam from the IP drum and sent through the reheater. The hot reheat line (HRH) is provided with another pressure reducing/attemperating station that directs the steam to the condenser. The attemperating station on the HRH line uses spray water from the condensate pump discharge. With this system, a continuous flow is maintained through the reheater ("wet" reheater mode) that provides a cooling effect for the reheater tube metal. As such, upgraded metallurgy is not required for the reheater tubes.
The cascade bypass system uses comparatively short lengths of steam piping. This is because the HP to CRH bypass piping is located near the HRSG while the HRH to condenser piping is located near the condenser. These pipe lengths are not dependent on the distance between the HRSG and the condenser, which tends to increase proportionally with multiple HRSGs.
Since the cascade bypass utilizes spray water from the feed water pump as well as the condensate pump, some additional energy debits are incurred. Note that the parallel bypass system uses only the condensate pump discharge for source of spray water and hence the energy debit is less.
The main disadvantage of the cascade bypass is that the STG has to start up with a pressurized reheater, which raises concerns about overheating of the HP turbine back-end, especially during low flow conditions experienced during start-up.
Cascade Bypass fitted with a Start-up Bypass System
Several methods have been devised to alleviate turbine back-end overheating problems during start-up. One is to keep the reheater pressure low by equipping the cascade bypass with a start-up bypass system (Figure 3). The start-up bypass connects the HP turbine exhaust to the condenser from upstream of the CRH line non-return valve. The non-return valve in the CRH line isolates the HP turbine from the pressure in the reheat section. This way even though the reheater is at a high pressure, the HP turbine exhaust pressure is low (at around the condenser pressure). The drawback of the start-up bypass is added capital cost.
HP Turbine Back-End Overheating Concerns
The HP turbine back-end and the cold reheat piping are generally made from carbon steel, which experiences degradation due to graphitization at temperatures above 800 F. If this temperature limit is expected to be exceeded, the construction material needs to be upgraded to low alloy steel, thereby increasing the capital cost. To keep the temperature below 800 F, the turbine steam inlet parameters parameters can be adjusted, as discussed below under "Overheating Control Parameters."
When high-temperature conditions exist at the HP exhaust, the turbine is unable to extract sufficient energy from the steam and convert it to work. As a result, the HP turbine exhaust temperature remains high, resulting in overheating at the back-end. High exhaust pressures at the HP turbine back-end also result in higher windage heating during periods of low steam flow (as experienced during start-up).
To start the steam turbine with the cascading bypass configuration, the reheater pressure must be maintained at a low value as recommended by the STG vendor. Typically, the maximum reheater pressure should be about 20 percent of rated pressure during a cold start and at 35 percent of rated pressure during a hot start. Somewhat higher reheat pressures may be possible by lowering the main steam temperatures, but this should be based on STG vendor recommendations.
If the maximum reheater pressure cannot be limited to the recommended value during start-up, and HP turbine back-end temperatures are expected to exceed the 800 F limit, then other control parameters (discussed in next section) should be evaluated and adjusted as required. If the overheating problem still persists, then a start-up bypass can be considered along with the cascade bypass system.
Some steam turbines that operate with the cascade bypass configuration utilize a start-up procedure by rolling off on the IP turbine. In this process, the steam turbine is rolled and synchronized with the intercept valves. The HP turbine remains isolated and is cooled by back-flow of cooling steam through the CRH line. The cooling steam is then vented through a bleed line/valve to the condenser. Reverse flow through the HP turbine continues until the turbine is placed on minimum load. The turbine is then switched over to forward flow through the HP section. Even with this arrangement, low reheat pressure is important to avoid overheating due to abbreviated steam expansion and windage heating of the HP turbine back-end during the transition to forward flow.
Overheating Control Parameters
As discussed earlier, the main control parameter for controlling HP turbine exhaust temperature at start-up is the HP back-end pressure. Other parameters that affect the HP turbine exhaust temperature include HP turbine inlet steam enthalpy, steam flow through the HP turbine, and rate of ramp-up allowed on the HP turbine. For a given reheat system pressure and a fixed inlet steam flow to the HP turbine, the higher the inlet steam enthalpy, the higher the HP turbine exhaust temperature. Also, the higher the steam flow through the HP turbine, the lower the HP turbine exhaust temperature.
At a fixed inlet steam temperature, the enthalpy increases as the steam pressure is reduced. High enthalpy of inlet steam to HP turbine is experienced during various modes of operation, such as 1) single train operation of a 2x2x1 plant or 3x3x1 plant and 2) unfired HRSG operation with STG designed for fired HRSG operation. In these scenarios, the HP steam inlet is at a relatively lower pressure with normal operating temperature resulting in high enthalpy steam. Based on this, the start-up conditions require that the steam pressure should be above the minimum "floor" pressure.
As mentioned above, the inlet steam enthalpy is determined by the inlet steam temperature and pressure. The inlet steam temperature is usually based on the turbine rotor metal temperature, which determines the type of start procedure (hot start, warm start or a cold start). A cold start requires lower steam inlet temperatures to the steam turbine as compared to a hot start. As a result the problem of HP back-end overheating during a cold start is not as severe as in the case of a warm start or a hot start. Generally a warm start is more critical than a hot start from the viewpoint of HP turbine overheating. A hot start allows fast ramp-up rates, enabling the steam flow through the turbine to increase rapidly, negating the overheating effect at the HP turbine back-end.
In cases where the steam turbines roll-off on the HP turbine, the HP control valve regulates the flow through the HP turbine to control the exhaust temperatures. During a hot start, when inlet steam temperature is high, a high flow through the HP turbine is maintained. On a cold start, when inlet steam temperature is low, a lower flow through the HP can be sustained based on the HP exhaust temperature.
Steam Bypass System Design Considerations
At a minimum, the steam bypass system should be designed for the start-up cases (cold start, warm start, hot start) and the load rejection (STG trip) case.
In the basic design, the bypass system should be designed to handle the steam generated in the HRSG during start-up and prior to STG roll-up/synchronization. At this point the CTG will be operating at a stable load point with HRSG steam conditions suitable for admission to the steam turbine. Generally it is expected that the STG will be rolled-up/synchronized with CTG load not in excess of 40 percent for any type of start (hot, warm or cold). At the same time the reheater pressure will be at around 20-35 percent of rated pressure. The bypass system should be checked to ensure that it can handle the steam flow associated with the low reheater pressure and the start-up load on the CTG.
In the load rejection (STG trip) case with no reduction of CTG load, the reheat system will operate at rated pressure and the bypass system will be required to pass the normal operating steam flow. However, since the steam flow will be attemperated, the total mass flow rate through the bypass system will be somewhat higher due to the spray water flow. Under these conditions, the condenser pressure could be higher than normal since the STG is not in service. However, on STG re-start, the condenser pressure will have to be reduced by cutting back on the CTG load.
If plant operation requires the bypass system to cope with other plant specific operating scenarios, the bypass system and condenser should be designed accordingly. Specific scenarios include: 1) bringing a second train in operation with the first one already operating at a high load, 2) operating the CTG/HRSG at high loads while the steam turbine is down or at no-load/low load operation, or 3) placing the STG in service after a trip without reducing load on the CTG(s).
In summary, there are several parameters that can affect the HP turbine exhaust temperature. These parameters should all be evaluated and adjusted to avoid high exhaust temperatures at HP turbine outlet. The use of low alloy steel construction at the HP exhaust or installing a start-up bypass in addition to the cascade bypass incur higher capital cost and should be used only after determining that adjustments to the inlet steam conditions and reheater pressure will not solve the problem. Also, the bypass system design basis should be clearly defined and take into consideration all operating scenarios expected to be included in the plant design.
Steam Bypass in the field
The Intergen North America Standard Plants used at the Cottonwood (Texas), Redbud (Oklahoma) and Magnolia (Mississippi) sites feature 1x1x1 configurations utilizing GE-7FA combustion turbine generators followed by triple-pressure-level HRSGs and axial exhaust steam turbine generators. All units are equipped with a start-up bypass line (from the HP turbine exhaust to the condenser) to control HP turbine exhaust temperatures at start-up. The HRSGs are duct fired, and in the unfired condition, the HP steam inlet enthalpies are high, resulting in HP turbine exhaust temperatures exceeding 750 F during normal operation. This temperature tends to be even higher during start-up when flow rate is low, approaching the 775-800 F limits for carbon steel. The start-up bypass valve and line (14-inch NPS) maintain the start-up temperatures below the carbon steel limits by reducing the HP exhaust pressure at start-up to less than 50 psig. A de-superheating station in the start-up bypass line reduces the steam enthalpy to around 1200 Btu/lb (which includes some superheat) at the condenser inlet.
The closing logic of the start-up bypass valve is based on estimating the temperature after the valve is closed. If the estimated temperature is less than 800 F, the valve will close. Also, the lower the reheater pressure, the lower the minimum flow requirement through the HP turbine, which means that the start-up valve can close sooner.
The LaRosita Power Plant in Mexico is a 3x3x1 configuration utilizing Siemens Westinghouse W501F combustion turbine generators followed by triple-pressure-level HRSG and a steam turbine generator. The plant experienced problems with overheating of the exhaust during hot start only. Instead of using a start-up bypass line, the problem was mitigated by maintaining low reheater pressure and low steam enthalpy at the HP turbine inlet during hot start-up. To keep the steam enthalpy at low levels, the plant increased the start-up steam pressure to around 1450 psig, while maintaining the reheater pressure below 160 psig. In addition, LaRosita relied on flow trimming control, which adjusts the flow ratio between the HP control valve position and the intercept control valve position, to increase the HP turbine flow and limit the HP exhaust temperature.
The GE design for the 3x3x1 power island at the Hsin Tao Power Plant in Taiwan and several other steam turbine generator vendor designs have overcome the start-up problem of HP exhaust overheating by starting the STG with steam admission to the IP turbine instead of the HP turbine. Forward flow steam admission to the HP turbine is introduced only after the STG is synchronized and placed on load with steam to the IP turbine. The heat generated in the HP turbine due to windage during a hot start is removed by bleeding a small quantity of reverse-flow cooling steam. The cooling steam enters the HP turbine exhaust (through the bypass line to the CRH line check valve) when a set speed is achieved and back-flows through the HP turbine. The cooling steam is then discharged to the condenser through a 4-inch NPS ventilating line. The back flow of cooling steam is no longer required once the forward-flow steam is admitted to the HP turbine. This design with an IP turbine start thus precludes the problem of overheating HP turbine exhaust during start-up.
S. Zaheer Akhtar, P.E., is a Senior Engineering Specialist with Bechtel Power Corporation, Frederick, Md. He has more than 25 years' experience in power generation and process plants, including positions with SCECO's Ghazlan Power Plant in Saudi Arabia, Exxon Chemical Co., General Physics Corp. and BE&K Inc. Akhtar received a bachelor's degree in chemical engineering from the University of Engineering and Technology in Pakistan and a master's degree in chemical engineering from the University of Manchester Institute of Science and Technology in the U.K. Akhtar is a member of the ASME PTC-4.3 Code Committee on Air Heaters.