1. Walter G Bischoff Brunswick station Duke Energy
Thermal Binding
Walter G Bischoff
Brunswick station Duke Energy

2. 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 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

3. Reactor Feed pump turbine

4. 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

5. Former Reactor Feed pump impeller

6. 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

7. 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.

8. 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

9. 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

10. 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.

11. Condenser arrangement

12. 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

13. 2 phase flow Cooler Inlet Hotter discharge
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
Cooler
Inlet
Hotter discharge

14. 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.

15. 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