Detailed information – Conventional Power Plants

Detailed information – Conventional Power Plants

Detailed information – Conventional Power Plants

In general a conventional power plant primarily comprises of a steam boiler which produces steam at high pressure and temperature (pressures of 250 bar and temperatures of between 550-560°C are not uncommon). The most common fuels for the boilers are coal, natural gas, fuel gas and oil . In addition, a certain percentage of different organic fuels, such as pyrolysis oil, verge grass and wood shavings, are presently also used to fire boilers.

The high pressure steam is channeled to a steam turbine. The turbine generally has HP, MP and LP sections. Steam from the HP section is returned to the boiler and reheated to the original steam temperature. This step is added to the process in order to improve the output of the plant. This reheated steam is passed through the MP and LP sections of the turbine to the condenser. Efficiency is increased because the average temperature in the cycle (Rankine cycle) is higher due to the reheating feature. The condensed steam is raised to the required boiler pressure by means of the condensate pumps and boiler feed pumps. In order to optimize the output of the installation, the condensate is pre-heated to a level of around 250°C before it is heated further in the boiler itself. This heating takes place in low-pressure (LPPH) and high-pressure pre-heaters (HPPH). The feedwater deaerator also forms part of the LP pre-heating step. The steam needed for the pre-heating process is taken from the steam turbine at various extraction points (drains). As a rule, the installation comprises of 3 HP pre-heaters and 4 or 5 LP pre-heaters, of which one functions as a mixture pre-heater and deaerator. In addition, there is another small pre-heater which condenses stuffing box steam that escapes from the turbine glands (gland steam condenses).

In practical terms, the following operating situations can be distinguished for the deaerator:

Normal operation

Normal operation falls generally between 30%-100% of MCR (Maximum Continuous Rating). The superheated steam from the steam turbine is unregulated which means that the pressure depends on the load of the steam turbine. The drain line to the deaerator does not contain a control valve and therefore the deaerator pressure will follow the pressure in the drain line (sliding pressure).

Operation with 1 or more pre-heaters out of service

If one or more HP pre-heaters are not in use, the flow of HP condensate to the deaerator is reduced. This only results in a slight increase in the turbine drain steam how (less than 5%).

The situation with LP pre-heaters is very different. The reduced heating of the LP condensate causes the demand for steam in the deaerator to increase considerably. The increase is primarily dependent on the course of the heating route through the LP pre-heaters and which of the LP pre-heaters are out of service. The steam demand when the last LP pre-heater is switched off can therefore increase by a factor of 1.5 to 3. If, for example, the last pre-heater is still service, then the effect is much less because the downstream pre-heater(s) provide(s) for a considerable portion of the shortfall.

Steam turbine out of service

Generally a steam turbine by-pass or steam turbine trip has the most consequences for the deaerator. The diagram below shows the situation when the steam turbine is by-passed.

Since the re-heater in the boiler has to be sufficiently cooled, the HP steam is passed through the pre-heater via the HP bypass. To do this, the steam is cooled and its pressure reduced. The MP steam from the re-heater is then reduced in pressure, cooled and sent to the condenser. The steam from the deaerator is now drawn from the cold re-heater line and added to the deaerator in a controlled manner. As the LP pre-heaters are no longer supplied with drain steam, the condensate temperature falls at the inlet of the deaerator to a value that is ultimately the same as that in the condenser. This causes a sharp increase in the flow of steam to the deaerator. Since there is now no production of electricity, the boiler load is reduced. The final load also depends on whether any other users still need to be supplied with steam (e.g. district heating), but as a rule this will not amount to more than 30-50% MCR. The process pressure in the deaerator is reduced to a value of between 2 and 3 bar.

A number of steps can be distinguished for the deaerator when the steam turbine trips;

Step 1
Situation before the steam turbine trip.

Step 2
This is the situation at the moment the turbine trips. A fast-closing valve (valve A) will cut off the steam supply to the steam turbine in less than 3 secs. Prior to the HP bypass (valve B) and control valve C open sufficiently, there is zero or insufficient steam available for the deaerator for a short period of time. As an indication, this period of time is approx. 0.5 to 1 min.

Step 3
The steam required for the deaerator is drawn entirely from the cold re-heater. Control valve C regulates the pressure in the deaerator. The condensate from the LP pre-heaters already has a significantly lower temperature, which increases the amount of steam required by the deaerator. In general this phase is also short, (2 to 3 minutes), and commences with the regulation of the boiler. The valve in the            connection between the operational rake and the start-up rake (valve E) remains open.

Step 4
Regulation of the boiler. During this phase, the condensate temperature reduces further and the pressure in the deaerator drops (controlled pressure gradient). This phase lasts 15 to 20 minutes.

Step 5
The final condition. The pressure is regulated to a fixed value of approx. 2 bar.

Starting the deaerator

An auxiliary boiler is available for starting the power plant. Before fuel is supplied to the main boiler and the first steam is produced, the auxiliary boiler supplies steam in order to bring the critical sections of piping, including the high-pressure and re-heat steam lines, to a specific temperature and pressure. The steam from the auxiliary boiler is also used to heat the deaerator. The entire process takes place in a number of phases. Generally, the start-up procedure is set by the customer’s system engineers in consultation with Stork Thermeq.