Increasingly, power plants are being called upon to cycle frequently, rather than to operate in baseload, which was the assumed mode of operation for large power plants not so long ago. There is also the ever increasing pressure on plant costs, with shorter building times and greater constructibility.
Alstom has attempted to respond to this feedback from the marketplace by developing a heat recovery steam generator specifically designed to address the issues. Known by the acronym OCC™, which stands for Optimised for Cycling and Constructibility, the design aims to combine exceptional high-cycling performance with cost-effective construction. It essentially brings together a number of design features already used to a greater or lesser extent in existing Alstom installations and incorporates them in one package.

Cycling issues
Design features or practices that contribute to poor cycling tolerance in large HRSGs, which have been called, by J Michael Pearson, the “eight deadly sins” of HRSG design, can be summarised as follows:
• Multiple tube rows in one header.
• Bends in tubes resulting in high local thermal stress.
• Rigid piping links between pressure part sections.
• Common headers with division walls (partition plates).
• Thick walled headers with low creep strength.
• Large diameter headers with low fatigue resistance.
• Single nozzle connections on superheaters and reheaters.
• Inadequate drain capacity to prevent condensate flooding.
Conversely, to achieve good cycling tolerance, it is necessary to minimise the magnitude and impact of thermal stresses in critical components during transients. This is done by minimising component thickness wherever possible, maximising flexibility, maintaining uniform temperature distributions and being conservative in material selection.
One of the most crucial issues in achieving tolerance of cycling is design of the harps, which are the assemblies of tubes plus upper and lower headers that make up a heat recovery steam generator.
Most horizontal HRSGs use multiple row harp designs, consisting of one horizontal upper header and one horizontal lower header joined by two, three or more rows of vertical tubes. The temperature of exhaust gas passing across mutiple tube rows is successively reduced so that individual tube rows may operate at different temperatures, leading to differential thermal stress at the weld joints. These stresses can be reduced by having only one row of tubes between the headers. This allows the use of smaller diameter headers (Figure 1) and also minimises circumferential temperature gradients in the headers. Small diameter headers reduce thermal stresses by as much as 60 per cent when compared with headers having multiple tube rows. One of the first plants to employ the single row harp design was Taranaki in New Zealand.
One comparative simulation has examined the effects of a thermal quench during shutdown and provides an example of the benefits of the smaller header with a single row of tubes over a bigger header with two tubes. In the simulation both headers are of P91 steel. One is 4in in diameter with a single 1.5in tube welded to it, the other has a diameter of 10.75in and has two 2in tubes welded to it. In the simulated thermal quench, condensate forms in the tube, the tube cools to saturation temperature (545 ° F) while the bottom of the header remains at nominal steam temperature (800 ° F). The simulation shows that the predicted number of cycles to crack initiation are 18272 for the header with a single tube and 6156 for the header with two tubes, which experiences much higher stresses than the smaller header (Figure 2).
Use of finned tubes without bends and radial penetration of the header are also beneficial as they minimise stresses in the tube-to-header welds. Upflow and downflow in the same harp should also be avoided, so headers need to be designed without division walls to avoid unacceptable temperature differentials in tubes adjacent to the division wall, which may occur particularly during transients.
The OCC design of HRSG uses single-row harps with headers of 3in and 4in nominal pipe size and relatively thin walls. This configuration provides lower thermal stresses and faster start-up times, as required for high cycling operation.
Also important is achieving maximum flexibility in all piping used to connect sections operating at different temperatures, and at the high temperature piping terminals.
To meet lifecycle criteria the OCC design uses high creep strength materials (P91 and T91) in high temperature areas.
Multiple header connections are also employed, to promote uniform flow through tubes in a harp and thus uniform metal temperatures in the superheater and reheater sections.
Another important feature of OCC is generous drain provision (Figure 3) and careful drain placement to prevent condensate flooding of superheater and reheater tubes during a hot restart of the HRSG, since the gas turbine purge air flow will rapidly cool these sections. Condensate drain flows can be as much as 6 kg/s during a ten minute purge period.

Features to enhance constructibility
Among the main measures incorporated in the OCC HRSG aimed at improving constructibility are:
•Optimised modularisation to meet shipping and erection constraints. Typically this results in a reduced number of modules. In the extreme case, pressure part modules are shipped with structural columns, insulation and casing attached (see Figure 4). As a minimum, the pressure part modules and casing assemblies are built to maximum permissible shipping limits with pre-insulated and lined casing panels already attached to the the structural columns.
•Use of manifolded risers and piping connections between modules (Figure 5).
•Significant reduction in the number of field welds. About 70 per cent reduction in large welds, from about 1060 field welded pipe connections to about 300 in some cases (Figure 6).
•Steam drums shipped complete with internals.

Avoiding the exotic
As gas turbine outlet temperatures and supplemental firing rates have increased, HRSG materials and design have become increasingly exotic. The OCC design uses steam cooled supports (Figure 7) in the pressure part modules immediately downstream of the gas turbine outlet and the duct burners. This avoids the need for prohibitively expensive alloys to support the modules.

The basic features


OCC features for improved cycling


Examples of Alstom HRSGs in cycling service
OCC HRSG projects to date (all USA)

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