ML20215M840
| ML20215M840 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 01/31/1983 |
| From: | NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20215M841 | List: |
| References | |
| TASK-AE, TASK-E302 AEOD-E302, NUDOCS 8611030356 | |
| Download: ML20215M840 (5) | |
Text
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AE00 ENGINEERING EVALUATION REPORT
- UNIT: Palisades EE REPORT NO. AE0D/E302 DOCKET NO 50-255 DATE: January 31, 1983 LICENSEE: Consumers Power Company EVALUATOR / CONTACT:
E. Y. Imbro WSSS/AE: Combustion Engineering /Bechtel
SUBJECT:
POTENTIAL LOSS OF SERVICE WATER FLOW RESULTING FROM A LOSS OF INSTRUMENT AIR EVENT DATE: August 19, 1982 i
S3 MARY i
A potential service water system (SWS) problem was discoverd at Palisades as a result of a review conducted as part of the Systematic Evaluation Program (SEP).
It was postulated that the potential problem could occur following a loss of coolant accident (LOCA) with a simultaneous loss of offsite power and the failure of a diesel generator.
The loss of offsite power at Palisades causes a loss of instrument air, resulting in the air-operated service water flow control valves on the discharge component cooling water heat exchangers going full open.
If, in addition, the single failure of diesel generator 1-2 is assmed, service water pumps P-7A and P-7C will also be lost since they are powered by this diesel generator. The remaining service water pump P-7B, powered by the 1-1 diesel generator, would then runout.
It was thought that the pep would trip on high motor amperage due to overload of the motor. The con-sequence of these assmed failures 'is the total loss of service water which provides the communication path for the post accident heat loads to the ultimate heat sink.
Although the throttling of the service water flow to the component cooling heat exchangers is commonly used to regulate component cooling water temperature, the Palisades control system is somewhat unique in that the air-operated " fail open" service water flow control valves must go to a 3
preset throttled position following a LOCA. This required that the non-safety-related instrument air supply to these valves be maintained.
The postulated problem was easily corrected at Palisades by installing hard stops on the operators of the service water flow control valves located at the discharge of the component cooling water heat exchangers. These valves now fail to a preset throttled condition as determined by the position of the hard stops, thereby preventing service water pump runout. In addition, technical reviews and testing indicated that the service water peps would not trip on runout.
Considering the facts that the Palisades system design, i.e., requiring the service water flow controls to fail in a throttled position, appears unique; and that the type of service water peps typically used do not produce motor over load at runout; there does not appear to be any cause for generic concern regarding the loss of service water resulting from a loss of instraent air.
This report supports ongoing AE00 and NRC activities and does not represent the position or requirements of the responsible NRC program i
office.
9611030356 830131
)
PDR ADOCK 05000255 S
DISCUSSION A potential service water system (SWS) problem was discovered at Palisades as a result of a review conducted as part of the Systematic Evaluation Program (SEP).
It was postulated that the potential problem could occur following a loss of coolant accident (LOCA) with a simultaneous loss of offsite power and the failure of a diesel generator.
The loss of offsite power at Palisades causes a loss of instruent air, resulting in the air-operated service water flow control valves on the discharge of the component cooling water heat exchangers going full open.
If, in addition, the single failure of diesel generator 1-2 is asstaned, service water pumps P-7A and P-7C will also be lost, since they are powered by this diesel generator. The remaining service water pump P-78, powered by the 1-1 diesel generator, would then runout and be expected to trip on high motor amperage due to overload of the motor. The consequence of these assumed failures would be the total loss of service water, which provides the communication path for the post accident heat loads to the ultimate i
heat sink following a LOCA. Loss of service water would therefore result i
in loss of diesel generator cooling and loss of component cooling, which in turn is the heat sink for the containment air coolers, the contain-ment spray heat exchangers, and other equipment necessary to mitigate the consequences of a LOCA. Similarly, if the failure of diesel generator 1-1 rather than diesel generator 1-2 is assumed, service water pump P-7B would be lost. Licensee calculations indicate that the remaining service water pumps P-7A and P-7C would runout and also be expected to trip on high motor amperage due to motor overload, resulting again in a total loss of service water flow.
The problem was easily corrected at Palisades by installing hard stops on the operators of the service water flow control valves located at the discharge of the component cooling water heat exchangers. These valves now fail to a preset throttled condition as detennined by the position of the stops, thereby preventing service water pump runout. Although it was initially thought that the service water planps would trip at runout due to motor overload, it was discovered through testing that the pumps would not trip at runout. The pump manufacturer, contacted by the licensee, confinned that the pump motor would not overload even when pumping against zero back-pressure. In retrospect, the addition of the stops to the service water l
control valves may have been unnecessary to prevent pump trip. However, the stops which are set to throttle the flow to that slightly higher than required following a LOCA, may be necessary from the standpoint of flow balancing, thereby assuring an adequate flow to all service water cooled components following the loss of instrument air.
SYSTEM DESCRIPTION In view of this unanticipated potential system interaction that, it was thought, could have caused a total loss of service water following a LOCA, an investigation was begun to determine if there was any generic concern.
The investigation commenced with the evaluation of the service water flow control system for the component cooling water heat exchangers. The service water flow is controlled in order to maintain a specified component cooling
water outlet temperature. The component cooling water temperature, if unregulated, would experience wide swings depending on the system heat load and the seasonal temperature variation of the ultimate heat sink.
i Figure 1 shows the valving and control system for the service water flow to the component cooling water heat exchangers. As can be seen from the figure, the service water flow to the heat exchangers is controlled by throttle valves in the heat exchanger discharge lines. Each heat exchanger has two throttle valves in parallel, CV-0823 and CV-0826 provide coarse adjustment and valves C-0821 and CV-0822 provide the fine control of the component cooling water outlet temperature. During normal plant operation, valves CV-0823 and CV-0826 are manually adjusted using hand indicating controllers (HIC) HIC-0823 and HIC-0826, respectively. The HICs are pressure regulating devices that vary the air pressure applied to the pnematic valve operators to adjust valve position. Yalves CV-0823 and CV-0826 are air-to-close/ fail open valves; i.e., increased air pressure to the operator tends to close the valve, and loss of air pressure would cause the valves to open to the full extent of their travel. Fine temperature control is automatically provided by bypass valves CV-0821 and CV-0822. These valves control service water flow by responding directly to the component cooling water temperature measured in the common discharge line of the component cooling water heat exchangers. This te:::perature, measured by temperature indicating controller j
(TIC) TIC-0821, tends to modulate open the bypass valves as the temperature in the component cooling water system increases. Valves CV-0821 and CV-0822 are air-to-open/ fail closed valves.
As previously mentioned, during normal operation the flow valves CV-0823 and CV-0826 are controlled by HIC-0823 and HIC-0826, respectively. Therefore, solenoid valves (SV) SV-0823A and SV-0826A are open and SV-0823B and SV-0826B are closed. Following a LOCA, these solenoid valves are deenergized on a low level signal from the refueling water storage tank (RWST).
In the deener-gized state the "A" solenoid valves close and the "B" solenoid valves open.
Instrument air to CV-0823 and CV-0826 is then controlled by HIC-0881 and 4
HIC-0882, respectively. These controllers are preset such that the service water flow is controlled to its post-LOCA requirement. Apparently, it was not realized during the initial design that while the solenoid valves fail to their " safe" position in the deenergized state, a loss of instrument air would cause CV-0823 and CV-0826 to fail open, resulting in service water l
pump runout and a system flow imbalance. The hard stops now installed on these control valves limit the valve travel so that on loss of instrument air the valves fail to their throttled post-LOCA position.
Limiting valve travel by installation of hard stops may cause some operational problems, since the component cooling system heat load during normal operation j
could at times be greater than the post-LOCA heat load due to the simultaneous l
operation of the waste evaporators and other non-safety-related equipment.
It is expected that the plant can compensate for the now limited capacity of the component cooling water system by running multiple service water pumps during
)
peak loads, or limiting the system heat load by judicious selection of which components can be run concurrently. A possible benefit to be derived from I
the installation of hard stops is the simplification of the control system for valves CV-0823 and CV-0826. The mechanical limitation of valve travel, l
for example, obviates the need for HIC-0881, HIC-0882 and solenoid valves l
I
SV-08238, and SV-0826B, if the "A" solenoid valves are replaced by 3-way solenoid valves set-up to vent the pneumatic operators on CV-0823 and j
CV-0826 on low RWST level. It is my understanding that the licensee is considering some change along these lines.
1 FINDINGS Although the throttling of the service water flow to the component cooling heat exchangers is a corunonly used way to regulate component cooling water temperature, the Palisades control system is somewhat unique in that the air-operated " fail open" service water flow control valves must go to a preset throttled position following a LOCA. Prior to the installation of the hard stops, this required that the non-safety-related instrianent air supply to these valves be maintained. A brief survey of a few other plants indicated that similar air-operated service water flow control valves performing the same function either had no preset throttled position following a LOCA and failed open on loss of instrument air, or received open signals concurrent with the safety injection actuation signal.
The primary safety concern at Palisades was that the service water pumps would trip due to motor overload at the runout condition if the flow control valves were allowed to fail to their fully open position on loss of instrument air.
It has subsequently been deteratined by the licensee that the motor would not overload even with the pump operating against zero back pressure. A review of characteristic pump perfomance curves indicates that this phenomenon is typical for the type of service water pumps used at Palisades.
Vertical wet-pit pumps are used at Palisades and most, if not all, other plants to pump water from the intake bay throughout the plant where it is used to cool a variety of mechanical components. This type of pump generally has what is referred to as a " mixed flow" impeller. The power versus capacity characteristic for this type of impeller is such that the pumping power reaches a maximum at or near rated capacity. As the capacity incteases to runout, the power required to drive the impeller typically remains relatively constant. Therefore, pump trip due to motor overload at the runout would not be expected for this type of pump.
CONCLUSIONS There does not appear to be any cause for generic concern regarding the loss of service water resulting from a loss of instrument air since the Palisades system design appears to be unique in requiring that service water flow controls fall in a throttled position, and the type of service water pumps typically used do not produce motor overload at runout.
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