EC- Essential Chilled Water Flashcards
What is the purpose of the Essential Chilled Water system?
Supplies chilled water to the essential air cooling units and air handling units in the control and aux building during essential equipment operation
Can the two trains of Essential Chilled Water systems be cross connected?
No
The chilled water supplied to the AHUs and ACUs will cool what spaces?
Control Room Essential ventilation
DC equipment Room Essential Cooling
Electrical Penetration Room cooling
HPSI Pump Room cooling
CS Pump Room cooling
LPSI Pump Room cooling
ESF switchgear and equipment room cooling
AFP Room cooling
Essential Cooling
Water Pump room cooling
What is the normal source of makeup water to the Expansion tank? Backup? Emergency?
Normal: DW
Backup: CT
Emergency: FP
What are the Essential Chilled Water pumps rated for?
400 gpm at 100 ft head
What powers the Essential Chilled Water pumps? Where are they located?
PH powered 20 hp electric motor
74’ control building next to essential chillers
Where are the Essential Chilled Water pumps operated from?
Control switch in the Control room, and locally at the breaker
What is the capacity of the Essential Chillers?
235 ton capacity
What major components makeup the chiller package?
Cooler Compressor and motor Condenser Flash Economizer Pump down unit
Where are the Essential chillers normally operated from? Where else can it be operated from?
Control Room
A three position spring return switch located at the 4160 V class switchgear for each chiller
When would I start the chilled water pump independently of the chiller from the local push button on the supply MCC breaker?
Routinely done to perform chemical sampling of the chilled water
What is the Chiller start sequence?
(T + 0) Chiller start signal
(T + 0) Program timer starts
(T + 0) Chilled water pump starts
(T + 23) Chiller LO pump starts ~23 seconds after the chilled water pump
(T + 51) Chiller compressor starts ~28 seconds after the chiller LO pump starts
When the chiller starts, what energizes to block a restart attempt?
A 20 min restart relay
For the essential chillers, If the start signal is from the ESF, how would that affect the restart relay?
A 2 min emergency time delay will bypass the 20 min restart relay after two minutes if the sequence permissive relay de-energizes
What is the Chiller Stop sequence?
(T + 0) Chiller stop signal
(T + 0) Program timer starts
(T + 0) The chiller compressor and chilled water pump both stop
(T + 0) The inlet guide vanes go closed
(T + 41) Approximately 40 seconds after the chiller stops, the LO pump stops
What is used to set and control the desired chilled water supply temp?
Thermostat
What is used to set and limit compressor motor current flow by positioning the compressor inlet guide vanes?
Electrical Demand Unit
What are the basic flow paths of the two fluids flowing through the Essential Chiller Cooler?
Low pressure, liquid refrigerant on the shell side absorbs heat from chilled water flowing through the tubes as the refrigerant flashes to vapor
What feature of the Essential Chiller Cooler maximizes efficiency at all load conditions through equal use of all tubing surface areas for heat transfer?
A distribution system delivers the liquid refrigerant uniformly to all parts of the cooler tube bundle
Concerning the Essential Chiller Cooler, what is installed above the tube bundle to provide balanced suction pressure throughout the cooler? What other function does it serve?
Flow equalizer plates
These plates also prevent liquid carry over into the compressor
What is the purpose of the Essential Chiller Compressor?
A two stage, centrifugal compressor that takes suction on the cooler shell, increases the refrigerant gas pressure, and discharges to the condenser
How is refrigerant flow controlled in the Essential Chiller Compressor?
By the inlet guide vanes installed in the suction. These vanes are controlled by the temperature of the chilled water leaving the chiller
What feature of the Essential chiller compressor increases efficiency?
An inlet connection is provided between the first and second stages to allow single stage compression of refrigerant gas returning from the flash economizer. This means the flash gas only requires one stage of compression
What is “Surging” and how does it occur? Why is it a concern?
A momentary gas flow reversal from the condenser through the compressor
It occurs when the system heat load is not great enough to evaporate a sufficient volume of refrigerant in the cooler.
The compressor alternately tries to deliver the gas to the condenser, but is unable to equal condenser pressure and the condenser returns the gas
Continued surging may cause compressor bearing damage due to overheating
What is a way to avoid “Surging” during low heat load conditions?
Use of the “hot gas bypass” that directs a portion of the discharge gas to the bottom of the cooler, flashing some of the liquid refrigerant to gas and increasing gas flow from the cooler to the compressor suction
What powers the Essential Chiller Compressor motor?
PB
How is the Essential chiller compressor cooled and lubricated?
Refrigerant cooled and oil lubricated
What maintains the motor cooling flow?
D/P between the condenser and the cooler
How long after the chilled water pump starts will the oil pump energize?
23 seconds
When a chiller is shutdown, why do we have an electric heating coil energized in the oil sump?
To ensure proper oil viscosity and to prevent excessive Freon absorption
While the compressor is running, excessive foaming in the oil is observed. What is causing this?
A result of Freon boiling out of the oil
In the Essential chiller condenser, the integrated thermal economizer consists of approximately how much of the condenser tubes?
12% of the condenser tubes
What component prevents “stacking” from occurring in the condenser?
Refrigerant Head Pressure Control Valve
What is the purpose of the Flash Economizer?
Provides an intermediate pressure reservoir for liquid refrigerant before it returns to the cooler
What is the purpose of the Essential Chiller Pump Down Unit?
Used to transfer refrigerant to and from the refrigerant storage tanks and the chiller package as necessary for maintenance
What are some of the functions of the EC Expansion Tank?
Provides convenient location for adding makeup water
Accommodates changes in system volume due to temperature changes
Provides a means of detecting leakage into or out of the system by monitoring level changes
What is the capacity of the EC Expansion Tank? Where is it located?
80 gallons
74’ control building
How much Nitrogen is charged into the EC Expansion Tank? What is the basis for that number? What is the minimum pressure in the system to prevent flashing at the highest point in the system?
> 25 psig
Added to keep oxygen out
10 psig with pumps not in service
We maintain it at this pressure since the tank is lower (elevation of surge tank relative to motive force elevation) than most surge tanks, so the additional pressure helps to maintain that surge volume throughout the rest of the system
Why are the EC Chemical Addition Tanks normally isolated from the EC system?
Because they are a different piping classification than the EC system
What signals would automatically start the EC water system?
SIAS, CSAS, CREFAS, CRVIAS, LOP, AFAS 1 and 2
The Essential Chilled water supply temperature is maintained at what value?
44F
What happens if the Essential Chilled water supply temperature drops to 38F?
Chiller will stop on auto load recycle trip to prevent circ water pump trip & freezing
After an auto load recycle trip of 20 minutes, when will the chiller automatically restart?
Chilled water exceeds 45F
Approximately what temperature is the chilled water return?
57F
What will happen to the Essential Chilled Water system if I lose EW or SP?
EC chillers will trip on Condenser high refrigerant pressure
How is the system affected by a loss of PB?
The compressor will have no power
How is the system affected by a loss of PH?
The chilled water circ. pump and lube oil pump will lose power
What system serves as the essential standby power source for the EC system?
DG system
How does EC interface with PK?
PK powers EC expansion tank make up water solenoid control valve
How does EC interface with NH?
powers the Essential chiller pump out unit
What is the back up make up water supply for EC?
CT
What is located inside the Essential Chiller local control panel?
thermostat- controls chilled water supply temperature
electrical demand unit- limits the compressor motor current by positioning the compressor guide vanes
Where are the essential chillers located?
control building 74’
In reference to the essential chillers, what is stacking and where does it occur?
occurs in the condenser
stacking is when the refrigerant accumulates in the condenser due to low differential pressure
AT what condenser pressure do the Refrigerant head pressure control valves close? Full open?
14 psig
23 psig
throttled between those two numbers
What position are the RHPCV bypasses in normally?
locked closed
How is the RHPCV manually operated?
pilot bypass stem valve or manual override screw
What are the two paths for liquid refrigerant to get from the condenser to the flash economizer on the essential chiller?
flash gas chamber
high side float valve chamber
What is the purpose of the high side float valve chamber and low side float valve chamber in the essential chiller flash economizer?
high side- serves as a level controller for the condenser
low side-controlled by flash economizer level, allows refrigerant to enter the cooler and maintains the D/P between the economizer and the cooler
In the flash economizer of the essential chiller, where does the flash gas get directed to and how does it affect the liquid refrigerant in the economizer?
its directed to the compressor 2nd stage inlet
latent heat of vaporization occurs, absorbing heat from surrounding liquid refrigerant
What signal closes the make up valves to the expansion tank in the EC system? Why?
lo lo level
to prevent feeding a major leak in the system
For how long after a seismic event can the CST feed make up water to the EC expansion tank?
24 hours
Where are the essential chiller system circulating water pumps powered from?
PH
If a train of EC is lost, what equipment is considered INOPERABLE?
that trains ESF equipment
How is EC affected by the loss of the EW system?
EC chillers will trip on High condenser refrigerant pressure
How is EC affected by the loss of GA?
potential oxygen intrusion into the system and possibly trip the chillers on a flow oscillation
How is EC affected by the loss of the SP system?
chillers will trip on condenser high refrigerant pressure
Per tech spec how many EC trains must be OPERABLE?
two
What conditions could lead to “stacking”?
Under low cooling load and low condenser cooling water temperature conditions
What is “stacking”?
The liquid refrigerant remains in the condenser since there is not enough condenser refrigerant gas pressure to drive the liquid through the flash economizer to the cooler
What are the adverse effects of “stacking”?
Coolers pressure and temperature drop due to a reduction in cooler refrigerant. If it drops too low, the chiller will trip on low temperature (34F)
What is installed to help prevent “stacking”?
Refrigerant head pressure control valves on the EW lines downstream of the condensers
At what pressures will the RHPCVs be open or closed? Why?
Closed at cooler pressures below 14 psig and full open at pressures higher than 23 psig
Reducing condenser cooling flow when the EW temperatures are at their lowest values, stacking is prevented.
Why are bypasses installed around the RHPCVs?
They ensure adequate condenser cooling flowrate during the most limiting postulated EW system condition of a 17% degraded EW pump while being currently aligned to fuel pool cooling loads
What flow requirement is associated with the EW system?
720 gpm through the condensers is necessary to provide sufficient cooling capacity to accommodate a design basis accident in the hottest part of the summer when the EW water temp is 132F
What is the preferred method of overriding the RHPCV?
Hydraulically by using the Pilot Bypass Stem located on top of the valve bonnet
What is the other method available of overriding the RHPCV?
Manual Override Screw located on the bottom of the valve
