Air Void Tests for Nuclear Power Plant Safety-Related Feed Pumps


Written by:
Dr. T. Ravisundar, Ravi Somepalli & Bill Nagle, HydroAire Inc.

 

Test lab engineers perform 40 void tests in 10 days with results that could impact NRC regulations.

A major nuclear power company approached an independent pump performance test lab in Chicago to discuss a series of tests for its Pacific 4-inch BFIDS in safety-related service. These auxiliary feed water (AFW) pumps were used in two pressurized water reactor plants to supply backup cooling water to the steam generators in the event the main feed water source was interrupted.

The plants were designed to use an air void between two motor-operated valves to keep the two different suction sources to the pump separate. The Nuclear Regulatory Commission (NRC) guidelines dictated that no more than a 2 percent air void could be passed through the pump to reliably assure its safety-related function.

The nuclear power plant engineers believed that the pump could ingest a greater margin of air without damage or impairment to pump performance. The NRC gave the nuclear power company an opportunity to demonstrate the capability of this pump by allowing them to conduct and monitor a series of transient air-void tests at the test lab.

Working Together to Define the Scope

The nuclear power plant engineers worked closely with the engineers at the test lab and a third party engineering consultant to develop the scoping document, which defined the tests needed to demonstrate the pump's capability under a range of scenarios. To design these tests, the team first reviewed the system configuration at the plants.

For added safety, each unit at each plant had one motor-driven and one diesel-engine-driven AFW pump. Each AFW pump was installed and aligned through valves and piping to take suction from either the non-safety related condensate storage tank (CST) or the nuclear safety related essential service water system (SX). The SX system is the nuclear safety related system that is connected to the plants ultimate heat sink (UHS), which is raw river water.

There is considerable difference in the purity of the water between the CST water and the SX water. Therefore, both plants intentionally built in the air void as a provision for separating these two systems to reduce the chance of SX water contaminating the clean condensate side of the system.

After thorough review, the team issued specifications for ten different sets of test cases which encompassed several operating conditions and more than 35 test scenarios.

The tests would cover injection of different void volumes into the operating pump with several variables, some of which included different flow rates, suction pressures and pump statuses (i.e. operating pump, idle pump with a pump start while suction is being transferred, etc.).

(Left) identical AFW pump (Right) booster pump

Configuring the Test Lab

Once the scope had been clearly defined and agreed upon, the test lab engineers set out to configure the lab in a way that would duplicate, almost identically, the plant's AFW suction piping set-up.

Within 10 days, the test lab was configured with a booster pump installed with a variable frequency drive to simulate the SX system as closely as possible so that the safety-related AFW pump could be operated within the same environment as it would function in the plant. The SX water source came from the test lab's 38,000 gallon suppression tank that was fed through the booster pump. The CST, which was simulated by the test lab's suppression tank, was not sent through the booster pump.

Both nuclear power plants knew that mussel shells would pass through the pumps due to the SX river water source, and the plants had been facing issues with pump downstream valves getting clogged with shell fragments.

At the test lab, two different types of shells from each plant's river water sources were purposefully run through the pump during testing. This was done to examine how the pump and valves might perform in a real-life plant situation.

The pump, a 10-stage, chewed up the mussels without any problems. The test lab engineers used a 3/32-inch strainer to capture the sizable shell fragments. Figure 6 shows the small portion of shells captured by the strainer. The remaining shell bits were ground by the pump to smaller than 3/32 inch.

Monitoring by the NRC

The independent Test Facility had set up a live video feed to show the pumps on the test stand as well as the real-time data streaming from their data collection software. The NRC and other members of the nuclear power company's team monitored the series of tests from a more comfortable room where visitor safety could be ensured.

The Results

Following the comprehensive test, the following results were documented:

The tests showed that the pump's hydraulic performance did not degrade for voids up to 2.70 cubic ft. with a low flow rate.

Momentary degradation in hydraulic performance occurs for voids at high flow rates.

Momentary degradation in hydraulic performance occurs for small voids at higher flow rates only if the voids are ingested while suction pressure is still increasing. If the suction pressure increases to a value close to the SX pressure prior to ingestion, no effects on the hydraulic performance were noted.

In each test, the pump performance recovered completely after the void was cleared through the pump.

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