Boiler operation at very low pressure carrying high load leads to problems in the boiler performance as well as overall plant heat rate.
Boilers are used for steam production in process plants and in power plants for electricity generation. These boilers are designed for specific operating parameters and one of the critical parameters is that of steam. The steam parameters are mainly superheated steam pressure, superheated steam temperature and the superheated steam flow. While carrying any load the boiler steam parameters have to be maintained as designed to achieve the optimal performance.
Deviations in steam parameter has implications on:
- Changes in the steam pressure and temperature change the enthalpy of the steam available for Rankine cycle.
- Temperature of superheated steam plays a major role in the available enthalpy however pressure also changes enthalpy and if the reduction in pressure is significant there will be considerable increase in enthalpy.
- The problem is mainly the specific volume of steam; reduction in pressure increases the specific volume. This leads to higher steam pipe and tube velocities thus increasing the pressure drop by a square function.
- Due to increase in superheated steam pressure drop at lower operating pressure and higher steam flow the changes in drum pressure from design will be to a reduced extent. However it will depend on the amount of reduction in operating pressure and the amount of increase in steam flow.
- The saturation temperature of water drops with pressure and so the sensible heat addition required reduces
- The latent heat of evaporation increases with reduction in pressure – only a gradual change
If turbo separators are used in drum this is also will have a very high velocity due to specific volume change which has a very impact on carryover.
- The performance of screen dryers in the drum also deteriorates and increases carryover
- As the operating pressure is lowered to a large extend, it is possible the heat input and the heat absorption pattern changes to that an extent that it will be difficult to make the super heater steam temperatures. No operating group would lower the operating pressure to this extend under any condition. However some operating engineers try to load the unit with steam flow higher than the designed limit during the cold startup conditions for meeting the electrical load demand.
Boilers are designed for both constant pressure operation and sliding pressure operation. In constant pressure operation the boiler superheater outlet pressure is designed to be maintained constant from as low as 40% of the rated maximum continuous load. In sliding pressure boilers the superheater steam outlet pressure is slowly taken up at a particular ramping rate acceptable to both boiler and turbine designers. Operations of the boiler at lower than the designed pressure in both the types of boilers has all the implications at different levels depending upon the deviation.
Boilers operated at a reduced pressure in power station lowers the overall heat rate of the power plant. It is found that in a 176 kg/cm2 steam cycle with 540 degree C superheated steam temperature for a 500 MW power plant lowers the overall plant heat rate around 1.2 kcal/hr. However a reduction of 10 degree C in superheated steam temperature lowers the overall plant heat rate around 9 kcal/hr.