Nuclear know-how gets to heart of a critical pumps issue

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.

When a nuclear power station in the Midwest of the USA operating two pressurized water reactors (PWR), invited tenders for replacing its raw water pumps, nuclear technical support manager for Sulzer Art Washburn was concerned that the specification did not appear to address potential operational concerns.

Protocols dictated that, in the event of a loss of power, safety-critical systems would be powered by emergency diesel generators. These were 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 them remain in service and not be stopped by the operators.

Applying the maximum pumping capacity to the reactor appeared sensible yet the maximum flow is dictated by the heat exchangers and injection systems, not the pumps. Thus, operating all the pumps at the same time results in each pump operating at a significantly reduced flowrate.

Where high specific speed 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 had caused the safety relief valves to lift as the pumps approached shutoff head conditions.

The combined operation of three pumps in parallel had caused the safety relief valves to lift as the pumps approached shutoff head conditions

Washburn 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.”

Contracted to manufacture six new pumps, Sulzer made some changes to the impeller and bowl to reduce the specific speed required. IT also included material upgrades and improved the design of the stuffing boxes to eliminate leakage – a long-term corrosion issue identified by the maintenance team.

The replacements were from the SJT range of cooling, large-flow vertical pumps water pumps. Here, 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).

This saved approximately 100 horsepower (75 kW) per pump, freeing an additional 600 horsepower (450 kW) from the emergency diesel generators, for no additional cost.

Vibration levels have dropped considerably, especially at minimum flow and the safety relief valves are no longer being challenged. Material upgrades included in the new designs have brought further benefits, increasing mean time between maintenance.

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 millions of dollars cost of which would take years to recover.

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.