The new power station is situated in the immediate vicinity of the open-cast lignite mining sites in the Cologne/Aachen area. Its power output of 1,000-MW and net efficiency of 43 percent make it the world's most modern and efficient power station. This excellent performance comes from all the components of the power station unit being perfectly matched to the process as a whole.
The CHTA is a so-called “hundred percent pump,” meaning it is not backed up by a stand-by pump. Plant outages are every power station operator’s nightmare because of the immense costs involved. This pump, therefore, has to be hard-wearing, insensitive to extreme duty conditions, and provide reliable service for long periods without maintenance.
That’s not all. Energy supply companies expect very high efficiency because every kW not consumed by the pump – which operates around the clock – can be sold to their clients.
Turbine Drives Pump
The pump is driven by its own steam turbine with continuously variable speed control. The turbine has a power rating 44-MW and is equipped with two shaft ends (see Figure 1). One shaft end is directly coupled to the CHTA pump.
Figure 1. Simplified schematic of the steam turbine and the two pumps.
The other, on the opposite side of the turbine, drives a YNK 500/800 booster pump which supplies the main pump with the necessary inlet pressure (see Figure 2). The booster is not coupled to the turbine directly but to a gear which reduces the speed at a ratio of 3.5 to 1. Running up and low flow operations are normally taken care of by two electric motor-driven start-up pumps, each with an output of approximately 35 percent of that of the CHTA.
Figure 2. A YNK 500/800 booster pump supplies the main pump with the necessary inlet pressure.
During these phases of operation, the main boiler produces too little steam to operate the turbine. In exceptional circumstances, the steam needed to start up the pump and run up the unit while the pump is still cold can also be supplied from an auxiliary boiler.
Special Vane Profile Required
As the pressure in the system can change enormously in response to the operating conditions of the power station unit, the first stage impeller is the most sensitive part in every boiler feed pump. The plant operator’s specification called for a minimum of 50,000 hours of operation without cavitation-induced wear. Cavitation is the term generally used to describe the formation and subsequent implosion of cavities in a fluid.
To meet this requirement, a special vane profile was designed for the suction stage impeller, using numerical flow modeling that allowed the pump to be operated without cavitation across the entire relevant operating range and with a sufficient safety margin from the system’s NPSHA, the energy provided by the system immediately upstream of the pump.
To meet the taxing demands on the pump set, a number of special components were designed. For example, the floating ring seal was incorporated into the pump design. It is very reliable, even at peak running speeds of 50-m/s (164-ft/s) and more, which are common on high specific speed pumps. Other exceptional features of this seal are its thermal characteristics and the way these positively affect the pump’s operation. We will come back to this aspect later.
Figure 3. Assembly of the CHTA 140/5 for Niederaussem.
At the 100 percent duty point, when the power station operates at 100 percent rated load, the main feed pump has the following performance data:
&mring; = 698 kg/s
(Q = 2,898 m³/h (12,760-gpm))
Head H = 3,361 m (11,024-ft)
Δp = 285.7 bar
(H = 3,361-m (11,024-ft))
n = 4,620 rpm
P = 26.47 MW (35,500 hp)
η = 86.9 percent
At the so-called TÜV point, the point at which the safety valves are activated, the pump set reaches a maximum speed of n = 5,217-rpm. It delivers hot water at a rate of 893-kg/s and has a power input of just under 42-MW (around 2-MW of which are required for the booster).