Nuclear power facilities are equipped with large diesel generators that supply emergency power for important safety circuits in the event of a situation where a power outage requires critical pumps to operate and safety shutdown the reactor.
Nuclear power facilities are equipped with large diesel generators that supply emergency power for important safety circuits in the event of a situation where a power outage requires critical pumps to operate and safety shutdown the reactor. Of these circuits, the ultimate heat sink (service water) pumps have one of the highest priorities, but they can impose a significant load on the diesel generators.
Operational safety is of paramount importance in nuclear generating facilities, which are governed by strict standards and protocols that dictate design specifications and operational procedures. These plants are required to ensure that the diesel generators are capable of running the necessary equipment during a power outage.
For a nuclear power station in the Midwest of the USA operating two pressurized water reactors (PWR), the raw water pumps were identified for replacement.
On the face of it, this would appear to be a straightforward like-for-like replacement. The pump specifications were provided and, due to the importance of this equipment, very unlikely to be changed. However, a number of issues with the performance of these pumps existed, including high vibration levels, high power requirements and reduced flow rates when operating in unison. In this case, there were six pumps in total with three cooling water pumps per reactor.
Sticking to the rules
The safety protocols for the plant state that, in the event of a loss of power, safety-critical systems will be powered by emergency diesel generators. These are designed to produce sufficient power for all the safety circuits, which are brought back online in sequence to avoid overloading the generators. The raw water pumps are among the first pieces of equipment to be restarted, but the protocols require the pumps that start and operate, to remain in service and not be stopped by the operators.
On the face of it, this would appear to be sensible – applying the maximum pumping capacity to the reactor. However, the maximum flow is dictated by the heat exchangers and injection systems, not the pumps, so operating all the pumps at the same time results in each pump operating at a significantly reduced flowrate.
In this case, where high specific speed (Ns) pumps were installed, the power requirement increased at the highly reduced flowrates, overloading the generators. In fact, the combined operation of three pumps in parallel caused the safety relief valves to lift as the pumps approached shutoff head conditions.
Optimizing the design
For a pump with a high specific speed, the shape of the power curve is highest at low flows and it drops down as flow increases. The curve for a pump with a lower specific speed is almost the reverse, with a lower power requirement at low flows and increasing as the flow rate rises.
Art continues: “Once the history of the original pumps was understood, it was possible to establish why the plant was experiencing the issues described. This led to the design of a pump with a lower specific speed, which would not only deliver the required flowrates but also save a huge amount of power. Under normal design conditions, the power requirement remained unchanged, but when running on the diesel generators, it is critical.”
Delivering power savings
The cooling water pumps chosen are large-flow vertical pumps that are engineered to order to ensure the most efficient design is achieved. In this case, the pump operated at 1775 rpm and required a 300 horsepower (225 kW) drive motor to deliver 5320 gallons per minute.
The change in the pump design saved approximately 100 horsepower (75 kW) per pump, thereby freeing up an additional 600 horsepower (450 kW) from the emergency diesel generators, for no additional cost. To put that into context, a project to re-rate the generators and provide a similar amount of extra capacity, if feasible, would cost in the tens of millions of dollars.
The pumps have been installed and the improvement in performance has been significant. The levels of vibration have dropped considerably, especially at minimum flow and the safety relief valves are no longer being challenged. In addition, the material upgrades that were included in the new designs have brought further benefits, increasing the mean time between maintenance (MTBM).
However, by far and away the greatest benefit has been the reduction in power requirement at low flow conditions. Such a large gain in diesel margin has saved the power plant from a potential generator re-rate project, the cost of which would take years to recover.
Improving understanding
For installations that are experiencing concerns about exceeding the available power from the emergency diesel generators, it is possible to review existing pump curves and adjust the specific speed of a pump without compromising the specifications laid down in the original documentation. Reducing the load on the generators has the potential to save millions of dollars that would otherwise need to be spent on re-rating this equipment.
