Feed pump drive turbine
A feed pump drive turbine (short form: SPAT ) is a ( steam ) turbine that is used to drive a feed water pump for a steam boiler . In this case, the associated feed pump is also called a turbo pump .
A turbine instead of an electric motor is used to drive the feed pumps for two main reasons:
- Safety: Since the steam required to drive the turbine comes from the same boiler, which in turn draws the feed water supplied by the pump, it can be assumed that drive energy is always available as long as the steam generator still needs water as a coolant. In addition, the steam supply can be supported by a buffer storage tank or auxiliary boiler . Safe shutdown without overheating of the steam generator is thus possible at any time, which is absolutely essential for nuclear steam generators in particular. With an electric motor as the drive, an emergency power system would have to be provided in the event of a "blackfall" (power failure) , which tends to be more complex and error-prone. However, this assumption is a fallacy. Since very complex auxiliary systems are required to operate a SPAT (e.g. oil systems, condensate pumps, cooling water pumps and their associated motor fittings), the availability and, above all, safety-oriented operation of a SPAT is only possible with significant reductions in relation to an electric pump. In the nuclear sector in particular, this knowledge has led to the fact that turbopumps are used as main feed pumps, but have almost not been used in systems in emergency cooling systems since around 1980.
- Cost-effectiveness : Since the conversion into electrical energy is bypassed as an intermediate stage and since the electricity fed in is remunerated differently than own consumption, a SPAT can be economically advantageous compared to an electric motor.
However, since a SPAT is far more complicated in structure than an electric motor, the advantages mentioned above are offset by higher investment costs, higher maintenance costs and lower availability. The use of turbines is only worthwhile from a certain size, since very small turbines are significantly less efficient than large ones. (The SPAT has only approx. 2-3% of the output of the main turbine.) For these reasons, SPATs are only used for selected boiler and power plant types.
When starting up a steam boiler with a SPAT, it must be remembered that the boiler does not yet supply steam to drive the SPAT when it is cold. The steam must therefore either be drawn from another boiler (possibly an auxiliary boiler) or the system must be started with an electrically driven pump that is fed from the network. Once the boiler has built up the necessary operating pressure, the transfer can be made to the SPAT.