Sulzer has applied its pump design expertise at a nuclear power facility to improve performance and reduce power requirements while continuing to comply with regulatory standards. 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. A request for quotation was issued for tender and those manufacturers that qualified as a potential supplier, including Sulzer, prepared their bids.
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.
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
Following more detailed discussions, Sulzer was awarded the contract to manufacture and deliver six new pumps for the power plant. The decision to reduce the specific speed required some changes to the impeller and bowl but otherwise the original specifications were met. Sulzer also included some material upgrades and improved the design of the stuffing boxes to eliminate leakage, which had been a standing long-term corrosion issue identified by the maintenance team.
Delivering power savings
The new pumps came from the SJT range of cooling water pumps, which 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 1’775 rpm and required a 300 horsepower (225 kW) drive motor to deliver 5’320 gallons per minute (20’100 litres 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.
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.