4.2.3 Feedwater de-aeration
Feedwater chemistry requirements are stringent with regard to the permissible level of dissolved oxygen. The main concern is that of minimising waterside corrosion of boiler tubes. Waterside corrosion and fireside corrosion lead to thinning of tube walls and the subsequent risk of rupture by the internal fluid pressure.
A significant feature of British designed feedheating plant has been the combined 'feedheating/de-aerating/ water storage plant'. This is illustrated in Fig 1.64 which shows the position of the de-aerating/feedheating plant within typical turbine plant and Fig 1.66 is a simple illustration of a typical high level de-aerating/ feedheating plant.
Feedwater from LP feedheaters is admitted to the twin towers on the tank. Steam, typically bled either from the main turbine or boiler feed pump turbine, or taken from the feed pump turbine exhaust, is supplied for heating and de-aerating the ingoing feed-water. Feedwater, de-aerated to a boiler quality standard of typically 5 ^g (of oxygen) per kg (of water), i.e., five parts per billion.
Design practice has been to install high level plant in an auxiliary plant annexe between the boiler house and turbine hall, at a level set by boiler house steelwork. Provision of generous margins over the steady state NPSH requirements of suction stage feed pumps has proved possible.
Typically, tanks have had a total volume equal to about ten minutes feedwater flow rate, with an eight minute store to provide a buffer for transients and time to unload the plant should the condenser extraction pumps fail. The mass of water stored is about half a tonne per megawatt of plant capacity (i.e., about 330 t for current 660 MW plant).
In summary, this type of combined plant, which is described in detail in Chapter 3, fulfils important primary functions:
- It serves as a feedwater heater.
- Feedwater is de-aerated down to the necessary low gas content.
- It acts as a 'buffer' to the fluctuations of condensate feedwater flow that occur in service.
- The tank height provides a hydrostatic head to satisfy the suction pressure requirements of the boiler feed pumps.
Reliable and effective service over several decades has been obtained from plant of the designs described, but changing economics and competitiveness have now led to a change in design.
The design illustrated in Fig 1.67 involves the direct injection of steam into the body of stored water through a series of vertical perforated tubes immersed in the water, and the admission of con-densate through self-regulating sprays. The condensate is heated almost to saturation temperature; steam rises through the water and gases are liberated and conducted away through small vents adjacent to the sprays.
The water passes at low velocity through the tank at a rate corresponding to the plant load and is almost at saturation temperature. The remaining dissolved gases are liberated: they rise to the surface and steam space and are then conducted away through small vents, the water being brought up to saturation temperature by the injected steam. This design and its function are covered fully in Chapter 3.
For thermodynamic economy, the bled-steam used in the de-aerator should ideally be without superheat — the use of superheat is a wasteful way of heating water when it could be more effectively used by being converted to mechanical work in a turbine.
British practice for the steam supply to the boiler feed pump turbine has been, as explained earlier, to use steam partly expanded through and exhausted from the HP cylinder of the main turbine (sometimes known as ''cold reheat steam'). This steam, after further expansion through the boiler feed pump turbine, contains only a modest amount of superheat and this exhaust steam is used to supply the combined feedheating/ de-aerating plant.
Because the steam requirement of the boiler feed pump turbine cannot be exactly matched to the steam requirement of the feedheating/de-aerating plant throughout the load range, arrangements are necessary to pass any excess steam, or to supply any steam deficiency from another part of the cycle.
An effective means of accomplishing these requirements is to design the boiler feed pump to exhaust at a pressure approximately equal to the pressure of the main turbine IP cylinder exhaust. This exhaust steam is supplied to the LP cylinders and the boiler feed pump turbine exhaust, the heater/de-aerator and the main turbine IP/LP crossover pipes are connected together.
The steam pipework is arranged so that, during plant operation, the feed pump turbine exhaust steam is preferentially used by the heater/de-aerator, with any excess being 'spilt' into the crossover pipe. If a steam deficiency exists, that deficiency is supplied from the IP/LP crossover, at the same pressure but with greater superheat.
This configuration, where the heater/de-aerator 'floats' on the boiler feed pump turbine exhaust and the IP/LP crossover interconnected piping, has proved very successful. If the feed pump turbine is unavailable and standby electric pumps are used, the heater/ de-aerator can be supplied entirely from the IP/LP crossover and operation can continue unimpaired. Such 'flexibility' of operation is advantageous and removes the 'interdependence' of one plant item upon another.
For plants involving a steam turbine drive for the feed pump, the economic benefits and the flexibility of the described system are clear. If boiler feed pump drives are not used, the system described above would involve steam supply from the main turbine only.