Advanced Benson HRSG makes a successful debut

Approaching and exceeding the 60 per cent efficiency mark for combined cycle plants requires further development of the water/steam cycle and its key components, the steam turbine and the heat recovery steam generator.

A horizontal flow, vertical tube Benson HRSG has been installed in Siemens’ most advanced V94.3A gas turbine combined cycle field development plant, at Cottam in England, which is now operating at close to 58 per cent thermal efficiency. This Benson type horizontal triple-pressure heat recovery steam generator manufactured by Babcock Borsig Power is a new development.

Among the key features and advantages of this HRSG design are the following:

once-through evaporator with horizontal gas path;

no thick-walled drum, with better dynamic characteristics;

improved cost effectiveness, with 15-25 per cent less weight of pressure parts;

attractive operating characteristics, with good flexibility and short start-up times;

standardised heat exchange modules; and

excellent suitability for future advanced combined cycle steam parameters.

The parallel tubes of the evaporator for the HP and IP stages arranged sequentially in the exhaust flow path are characterised by extremely different heat uptakes. In this concept, mass flows automatically adjust to the heat input, ie all parallel tubes of the HP evaporator show saturation temperature at first pass outlet and low temperature differences between the rows of the second pass. The thermoelastic construction of the Benson boiler significantly increases flexibility of the combined cycle power plant over that of a drum boiler, especially during start-up.

The Cottam generating plant as a whole was described in Modern Power Systems, September 1999, pp 40-43. It employs the single shaft combined cycle concept, in which the gas turbine, generator and steam turbine are all connected on a single line of shafting. The power plant has an expansion potential of up to 500 MWe. The current gas turbine is the advanced V94.3A machine.

HRSG thermodynamic design

The efficiency improvement due to additional pressure stages in a heat recovery steam generator decreases with increasing gas turbine exhaust gas temperature. The reason for this is that the effectiveness of the IP and LP steam stages becomes less and less in comparison with the high pressure stage.

At very high exhaust gas temperature and limited main steam temperature, the IP and LP steam flow rates even become negligibly small, with the result that multiple-pressure processes approach the effectiveness of a single-pressure process. However, as long as it is possible for main steam temperature to follow the gas turbine exhaust gas temperature with a difference of less than 60 K, the triple-pressure reheat steam cycle is the preferred solution.

Considering the current implementation range for high-temperature resistant 9 to 12 per cent chromium steels, experience with plants in operation and with the steam side oxidation behaviour of heated finned tubes, a steam temperature of 600°C was determined as an upper limit.

An increase in main steam pressure leads to better utilisation of the high-temperature heat due to the associated increase in evaporation temperature and the reduced heat of evaporation. For sufficiently large-volume flow rates, no decrease in expansion efficiency of the steam turbine is anticipated (Figure 1).

From the standpoint of cost-effectiveness, the optimum main steam pressure lies below the thermodynamic optimum. The dependence of efficiency on steam pressure is significantly less pronounced at lower exhaust-gas temperatures than at higher exhaust-gas temperatures. A main steam pressure of 160 bar was selected to yield the largest possible potential for further increases in efficiency.

Evaporator system selection

At the main steam pressures of 80 to 130 bar common to date, natural circulation or once-through systems are primarily evaporator dependent. With the increase in main steam pressure to 160 bar and above, the transition to a once-through evaporator becomes advantageous. Other considerations also favour the technology:

Once-through evaporators have the potential of adapting to future technical developments in the direction of higher steam parameters.

Elimination of the thick-walled drum results in a significant improvement in the dynamic characteristics of the entire plant. This advantage is especially important for heat recovery steam generators which have no bypass on the exhaust-gas side. The parallel start-up of the gas turbine and heat recovery steam generator thus required necessitates a thermoelastic design of the pressure parts with low wall thicknesses.

Once-through evaporators can be made cost effectively. The weights of the pressure parts are 15 to 25 per cent lower than for a natural circulation evaporator with its drum.

The once-through system design can also be used for the intermediate-pressure section. This remains a cost decision in individual cases.

Once-through evaporator design can be implemented in both vertical and horizontal heat recovery steam generators. Various configurations have been developed for both designs.

New configuration

For vertical heat recovery steam generators through which the exhaust gas flows from bottom to top, the countercurrent configuration yields the smallest heat exchange surfaces and the lowest pressure drop. However, stability problems arise in once-through evaporators with this configuration (Figure 2) which can only be eliminated with the installation of flow restrictors. In contrast, the parallel flow configuration exhibits flow stability but results in the largest heat exchange surfaces and the highest pressure drop of all configurations.

A combination of the two configurations yields intermediate values for the heat exchange surface and for pressure drop, while also providing the requisite flow stability and retaining a variable evaporation endpoint.

Various configurations are also possible for once-through evaporators for HRSGs with horizontal flow on the exhaust-gas side.

A new configuration of heat exchange surfaces has been implemented in the Cottam combined cycle power plant. Flow through the parallel, vertical evaporator tubes in this horizontal heat recovery steam generator is only upward, as in a natural circulation boiler with horizontal gas pass. In contrast to standard once-through evaporators, a special fluid dynamics design results in a higher mass flow rate in tubes with increased heat absorption. This leads to equilibration of enthalpy at the evaporator outlet.

Evaporator

In the evaporator tubes of a heat recovery steam generator with moderate heat fluxes, steam mass flux can be radically reduced without impairing the reliable cooling of the tubes. Introducing a once-through evaporator in the so-called low mass flux design results in a basic change in its flow characteristic. With increased heating of an individual tube, the flow rate does not decrease but rather increases. In the low mass flux design, the following conditions are established in a tube with increased heat absorption in comparison with a tube with average heating:

Hydrostatic pressure drop decreases due to increased steam generation. Therefore the pressure difference to which the overall heat exchange surface is subjected tends to increase mass flow.

Frictional pressure drop increases due to the increased steam fraction. Therefore pressure difference to which the overall heat exchange surface is subjected tends to decrease mass flow.

As all parallel tubes are subject to the same overall pressure drop, however the influence of frictional losses on flow rate is only slight, mass flow through the tube with increased heat absorption increases. The outlet enthalpy or outlet temperature of this tube thus increases only slightly despite the increased heat absorption. This behaviour, comparable with that of a drum boiler, is called a natural circulation or positive flow characteristic.

Design and construction

Figure 3 shows a simplified view of the HP and IP heat exchange surfaces. The HP evaporator consists of two partial sections, connected in series on the exhaust-gas side. Flow in each stage is from bottom to top in cross-flow to the exhaust gas. On the water side all the tubes of one partial stage are designed in parallel resulting in low mass fluxes. The flow leaving the outlet of the first stage is wet steam.

The water/steam mixture is led downwards through headers and down tubes, and from there it is distributed into the second evaporator stage. The steam at the outlet of the second stage is already superheated. Tubes lead steam from the outlet of the second stage to the separators and then to the superheater. Configuration of the IP section is analogous. However the IP evaporator has three stages of heat exchange surfaces, enabling placement of yet another heat exchange surface between the first and second stage in connection with heat exchange surface optimisation.

The upflow tube rows of the horizontal heat recovery steam generator on the exhaust-gas side are subjected to very different heat absorption (Figure 4). The low mass flux design enables good adaptation of flow rate in the individual tube to its heat absorption. Only relatively small differences in outlet steam fraction or temperature occur between the individual rows of tubes.

This new evaporator concept enables the use of standardised modules for all heat exchange surfaces with the same design principle which has been used with proven operating records over many years in natural circulation boilers. Only the tube and header dimensions need to be adjusted for the corresponding steam data.

Before implementation of the concept, extensive theoretical and experimental investigations have been performed to address questions of static and dynamic flow stabilities as well as uniform distribution of water/steam mixtures in a parallel tube system.

Static flow stability in a parallel tube system means that every tube has the same flow rate for a given pressure drop under all operating conditions. The flow characteristic must therefore be positively inclined over the whole range of evaporation. If the system is unstable, conditions are possible under which there is only lower flow and thus insufficient cooling in some of the tubes in the parallel tube system, while other tubes are subjected to a higher flow than that in the design. Figure 5 shows a 3D image of the HP stage of the Cottam HRSG. Figure 6 shows the stable flow conditions in the first HP evaporator stage even with the assumption that there is a significant flow stratification on the gas side.

Dynamic flow stability means that the flow rate exhibits good damping behaviour in response to external disturbances such as excess heat absorption in individual tubes, ruling out the possibility of uncontrollable oscillations. It was also possible to predict outstanding operating behaviour in this regard by using computer simulations.

Wet steam distributors are well known from fossil-fuelled Benson boilers. These boilers generally operate with high mass fluxes and high steam fractions (x > 0.8) at the distributor inlet. However, as other conditions prevail for the evaporator in the Cottam plant, such as phase separation in the headers and connecting tubes, phase slip in tubes with upward flow, low mass fluxes and water/steam mixtures with intermediate steam fractions, the design of these distribution elements had been optimised in the Benson water/air test facility in Erlangen.

The experimental investigations included star type distributors as well as pipe type distributors with and without internals. For the star type distributor the desired uniform distribution of mass flow and mixing is achieved in the twelve outgoing tubes.

Possible differences in inlet enthalpy are reduced in the evaporator tubes on the way to the outlet, as a tube with a higher inlet steam fraction has a higher flow rate than a tube with a low steam fraction.

A special feature of the start-up system is the connecting line between the water collecting bottle and the evaporator inlet. The natural circulation characteristic of the heat exchange surface enables the water recirculation from the water collecting bottle back to the evaporator inlet without a recirculation pump.

Initial operating results

The gas turbine in the Cottam power plant was synchronised with the grid for the first time in March 1999. The entire power plant was on line in the summer of 1999. Initial operating performance of the heat recovery steam generator can be summarised as follows:

The natural circulation characteristics of the HP and IP evaporators have been clearly confirmed. Under all operating conditions no tube temperature exceeded allowable values.

The design values for steam turbine output and for exhaust-gas temperature of the heat recovery steam generator were achieved. This means that the planned heat transfer to the fluid was achieved despite differing flow rates through the individual evaporator tubes.

The simulation results from the start-up process agree very well with the initial operating measurements. This is shown in

Figure 7 for the temperatures at HP-superheater and at evaporator outlets. Start-up time for a cold start (120°C) up to attainment of steady-state operating parameters (at around 70 per cent load) is about 45 minutes, which is thus significantly shorter than that required for a natural circulation HRSG (Figure 8). The difference exists mainly in the difference between allowable temperature transients in the drum and in the separator in the once-through boiler. This enables a significant increase in overall plant flexibility during start-up.

The water level control in the water bottle has operated without trouble right from beginning of and during the start-up process, in particular due to successive water swell from the individual tube rows.

Prospects

The HRSG in the Cottam power plant has provided an innovative concept which offers great potential for future developments in combined cycle power plants. Its special advantages are suitability for advanced steam parameters, high flexibility, a simple start-up system and cost-effective construction.

Standardised heat exchange modules are used, in which solely the tube and header dimensions need to be adjusted for the corresponding steam conditions. Initial operating results confirm the expected fluid dynamics which are in agreement with the simulated dynamic behaviour during start-up.

The Cottam HRSG is thus a prototype for standardised steam generators with great potential and outstanding operating characteristics for future combined cycle plants