4.3.5. Example of the results of an overall comparison of the through-life costs of four feed pump system options
Figure 1.75 shows the capital and running costs of the four feed pump options that were the subject of detailed investigation for a typical future large coal-fired turbine-generator unit. This histogram summarises the effects of the factors already discussed in this section, the main points of which are repeated below.
From this study, the three x 50% feed pump option became the only viable one for many reasons, but mainly because it is the best compromise between feed pump system availability (no interruption to full-load output with one feed pump unavailable) and capital cost. Operations data from CEGB sources suggests the same level of pumpset availability regardless of the type of drive, so for these four options (all three x 50%) the loss of availability' costs shown on the histogram are all the same.
Repair and maintenance costs are significantly higher for the two options with two steam turbines and one variable-speed motor (VSM) — options (3) and (4) — due to not having three identical pumps (increased maintenance times and spares holdings), and the fact that operating data shows that repair and maintenance costs are directly related to capital costs, which are higher for the steam options.
Capital costs reflect the higher cost of steam turbines and their associated bled-steam pipework, valves, etc., compared to electric motors, with the condensing steam turbine (option 3) being the most expensive, due to the larger steam volume and the design problems involved with the last-stage blade. The VSM and converter equipment (option 1) is more expensive than the simple induction motor with fluid coupling (option 2), but there is more uncertainty in the latter cost due to the development costs of equipment to cater for the high starting current with existing 11 kV station electrical systems.
Running costs are presented in Fig 1.75 relative to the condensing steam turbine-driven pumps (3) which was found to be the most efficient option. For turbine-driven pumps, the differential heat rates, calculated by computer program using data from CEGB reference designs, have been used to obtain the equivalent lifetime running costs between the turbine and electrically-driven options. The back pressure turbine option is based on the most efficient designs but without bled-steam tappings for HP heaters, to avoid possible feed system stability problems and loss of heater availability.
For the motor-driven pumps, the lifetime running costs take account of the power input required at the pump couplings (calculated for all pumps, whatever the type of driver, using pump efficiencies consistent with what might be expected for international standard machines), augmented by efficiency losses in the drive package, associated cabling, and transformer. The traditional pump head and flow margins (3%/5%) and the requirement to be able to produce full output with a minimum electrical system frequency of 49.5 Hz ensure that the pump duty point is at a significantly lower load than its rated output. Since the efficiency of the induction motor with fluid coupling falls off with decreasing load much quicker than the VSM efficiency (due to coupling slip losses — see Fig 1.74), the through-life running costs of this option (2) are greater than for the VSM option (1).
An example of how an electric motor-driven feed pump system can give a better overall steam cycle efficiency, i.e., a better heat rate, than a back pressure turbine-driven system is shown in Figs 1.72 and 1.73, which are largely self explanatory. This efficiency gain can lead to the improvement in through-life running costs that can be seen on the histogram if options (1) and (2) are compared with option (4).
The above conclusions relate to a specific set of economic circumstances. Different applications, studied against a scenario of changing fuel prices, capital costs and operating costs may result in different conclusions. Nevertheless, these considerations exemplify the need to analyse not only the capital cost, but also the other components of lifetime cost in arriving at a solution.