ML20236K970
| ML20236K970 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 11/03/1987 |
| From: | Clark R NRC |
| To: | Butler W NRC |
| Shared Package | |
| ML20236K951 | List: |
| References | |
| NUDOCS 8711100040 | |
| Download: ML20236K970 (3) | |
Text
_ - _ _ _ _ _.
November 3, 1987 1
NOTE T0:
Walter R. Butler FROM:
Richard J. Clark i
SUBJECT:
FOLLOW-UP INSPECTION OF LIMERICK EHC SYSTEM My note to you of October-14, 1987 summarized the severe vibration in the Limerick Unit 1 electrohydraulic control (EHC) piping and control systems as a result of modifying the turbine logic from " full arc emission" to " partial arc emission." Limerick found that the vibration increased significantly and 3
proportionately at power levels above 87%.
For over a month, PECo has been holding power to 83% to 85% while they have conducted an extensive test program to determine the root causes of the vibration and evaluate the safety impifcations.
On October 23, 1987 I inspected the EHC and turbine control systems.
PECo installed a variety of test instrumentation on the systems. The electrical and electronic signals from all sensors on the steam line, the turbine controls and the EHC controls, along with outputs from the test i
instrumentation, are fed into the sophisticated plant computer used during the l
startup test program, j
i The basic cause of the EHC vibration at Limerick is a second-order resonance j
set up in the steam line when using only one valve to control steam flow to d
the turbine. A very simplified arrangement of the EHC control system is shown in the attached sketch (figure 1).
The frequency of the resonance is 2.5 hz.
}
With a gain of 100:1 in the amplifier, a small change in input voltage i
caused by the resonance (e.g., from approximately 1 v to 3 to 4 v) is greatly i
exaggerated. The EHC system operates at 1600 psi.
When controlling with only one control valve, the valve has to open or close more than with full arc emission to effect a change in steam flow.
The perturbations in signals to the servo valve are creating a " water" hanner effect in the EHC oil system and thus creating the vibrations.
The situation is a lot more complex than I can i
explain in a few sentences.
You have to take into account the valve l
characteristics, the slopes used to approximate demand curves, etc.
The important point is that you can't calculate what second order resonances (frequencies) you may get if you modify a turbine to partial arc emission.
I understand that one of the Fukushima BWRs in Japan that had converted to partial arc emission ruptured the EHC system.
The resulting pressure transient caused damage to both fuel and plant s.ystems.
1 3
ard J.
lark
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