Thermal Binding Walter G Bischoff

Brunswick station Duke Energy

Background- Brunswick Feedwater system

• 2 Unit station

• 2 feedwater trains per unit

• 2 Condenser shells (A and B) each with North and South regions

• 2 FWH streams cascading 5 to 4, 4 to single common dearator

• 1 dearator with normal drain to pumps, and dump to condenser

• Steam Driven GE turbines driven pumps

• Total Feedwater flow is approximately 12.25 MLB/hr

• Each RFPT uses a Woodward 5009 controller and steam control valves to adjust speed to meet RPV demand

• Exhaust steam flows to the main condenser

• Moisture drains collect liquid and direct effluent to the condenser via gravitational force

Reactor Feed pump turbine

Conditions progressing to the event

• Unit 1 completed the B120R1 refueling outage

• Upgraded the RFP impeller design• More stable • Less efficient • Requires additional speed• Calculations support single pump operation as high as 70% RTP• Slightly less maximum RTP for single pump operation with new impeller

• Factory acceptance testing was satisfactory

• Pump rebuild satisfactory

Former Reactor Feed pump impeller

Issue: Elevated RFPT exhaust casing drains

• Unit 1 achieved 60% reactor power

• RFP Inservice testing was progressing as expected

• Control room operators received Hi-Hi RFPT casing drain alarm

• Normal casing drain levels are <2 inches

• Casing drain levels reached as high as 7 inches before tripping RFP/T

Troubleshooting the issue

• Unit 1 remained at 60% reactor power

• B RFP was used to feed the RPV

• Calibration check indicated satisfactory level instrument performance

• Isolation drain valve was found open

• Steam inlet stop/isolation valves were closed

• Level remained between 7-8 inches

• Condenser level was 12 inches

• Heater drain dearator was dumping back to the condenser.

Piping geometry and hotwell configuration

• The RFPT exhaust casing elevation is 24’ 1” plant elevation (el).

• From the 1A RFPT exhaust casing (24’ 1” el), there is a 1’ 3(1/2)” vertical drop to 22’ 9(1/2)” el.

• Drain piping flows horizontally through a total of 60’ 2(3/4)” with (3) 90 degree horizontal piping bends.

• Piping elevation slopes downward on a 45 degree angle and drops 4” in elevation.

• The remaining 5(1/2)” elevation drop occurs over a 10’ 2(7/8)” length of pipe.

• Drain pipe center point penetrates the condenser at 22’-0” el.

• The drain pipe is 4” in diameter

Hotwell indications

• The ‘Normal hotwell water level’ of 0” equals 21’ 1(5/8)” el

• Hotwell level of +8(3/8)” equals 21’ 10” el and will begin to submerge the 1A RFPT casing drain line.

• Hotwell level of +12(3/8) equals 22’ 2” el and will completely submerge the 1A RFPT casing drain line.

• Level reached 10.5” at the time of the issue

• Heater drain dearator dump valve is located immediately below the 1A RFPT drain line

Troubleshooting the issue

• Operations started a second dearator drain pump and closed V-57 dump to the condenser

• RFPT casing level immediately dropped to 0 inches

• Level in the condenser remained unchanged at approximately 10.5 inches

• Original theory was the drain liquid was blocked due to elevated condenser hotwell level

Communicating Vessel theory and the hydrostatic paradox

• RFPT drain case is 2’ 1” above the condenser penetration

• Hotwell level would have had to rise another 2 feet to block the drain line

• Communicating vessels is a name given to a set of containers containing a homogeneous fluid

• When the liquid settles, it balances out to the same level in all of the containers regardless of the shape and volume of the containers.

• If additional liquid is added to one vessel, the liquid will again find a new equal level in all the connected vessels.

Condenser arrangement

Saturation point

• RFPT moisture collected in the bottom of the tank is a liquid, however at or near saturation point

• Although the drain was partially submerged, liquid should have continued to drain

• Heater drain dearator was at 60 psi and 235 degrees F

• The dearator dump line discharges just below the RFPT drain penetration.

• For condenser vacuum 2.25 inHg boiling temperature is approximately 106 degrees F

2 phase flow

Hotter discharge

CoolerInlet

• Dearator dump liquid heats the hotwell condensate including liquid at the RFPT drain.

• RFPT drain liquid at or slightly below boiling point is heated by dearator dump liquid

• Long 60 feet run of pipe contributes to head losses and slows flow

• Steam flows up the pipe forming a ‘wall’ blocking liquid flow

• Liquid that approaches the condenser boils to steam and tries to flow up the pipe until it condenses back to liquid and the cycle repeats itself

Troubleshooting the issue

• Operations started a second dearator drain pump and closed V-57 dump to the condenser

• This stopped the flow of hot water to the condenser at the RFPT drain line

• Without a source of heat, the RFPT drain liquid no longer boiled to steam

• The drain case was able to drain freely again.

• Continued with power ascension and RFPT mod acceptance testing.

Failure mode relevant to equipment other than RFPT

• Steam binding may occur in other equipment such as heaters and other components that drain to the condenser

• Reduced drain flows lesser than anticipated with no explanation

• More likely to occur in smaller bore piping

• Could be seen in FWH drains and FWH dump lines