ML13330B179
| ML13330B179 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 03/25/1987 |
| From: | Medford M SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 8704010458 | |
| Download: ML13330B179 (5) | |
Text
A0 Southern California Edison Company P. 0. BOX BOO 2244 WALNUT GROVE AVENUE ROSEMEAD. CALIFORNIA 91770 M.O.MEDFORD TELEPHONE MANAGER OF NUCLEAR ENGINEERING (818) 302-1749 AND LICENSING March 25, 1987 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlemen:
Subject:
Docket No. 50-206 Standby Diesel Generators San Onofre Nuclear Generating Station Unit 1
References:
A) Letter from R. F. Dudley (NRC) to K. P. Baskin (SCE), dated May 27, 1986, Transamerica-DeLaval (TDI) Diesel Engine Reliability and Operability B) Letter from G. E. Lear (NRC) to K. P. Baskin (SCE), dated January 28, 1987, Safety Evaluation Report on the Operability/Reliability of Emergency Diesel Generators Manufactured by Transamerica Delaval, Inc. (TDI)
This letter is regarding Southern California Edison's (SCE's) commitments to investigate alternatives and associated corrective actions to reduce transient torsional stresses on the San Onofre Unit 1 standby diesel generator crankshafts and to propose Technical Specification (TS) changes making "slow" starts of the diesel generators at least 24 seconds in duration. These commitments are consistent with your Reference A letter.
With respect to the proposed TS changes, we notified your staff by telephone that the changes involve hardware modifications and suggested that submittal of these changes should be deferred until after specific corrective actions and associated modifications to reduce crankshaft stresses have been determined and agreed upon between SCE and the NRC.
The enclosure sets forth the results of SCE's investigation of means to reduce crankshaft transient torsional stresses that can cause cracking.
For this purpose, different options were evaluated. Our conclusion is that modified automatic slow starts of the diesel engines during surveillance testing are cost effective and adequate to achieve the long term safety objectives. However, the refueling interval SISLOP surveillance test will still retain the automatic fast start.
8704010458 870325 PDR ADOCK 05000206 S
Document Control Desk March 25, 1987 In accordance with Reference B, we will incorporate this modification into the Integrated Living Schedule (ILS).
The proposed TS changes referred to in the first paragraph of this letter will be submitted for your approval after preliminary design and engineering relating to the modification have been completed.
If you have any questions, please let me know.
Very truly yours, Enclosure cc:
- 3. B. Martin, Regional Administrator, NRC Region V F. R. Huey, NRC Senior Resident Inspector, San Onofre Units 1, 2 and 3 R. F. Dudley, NRR Project Manager, San Onofre Unit 1
Enclosure INVESTIGATION OF ALTERNATIVES TO REDUCE CRANKSHAFT TRANSIENT TORSIONAL STRESSES STANDBY DIESEL GENERATORS SAN ONOFRE NUCLEAR GENERATING STATION UNIT 1
Background
PNL-5600 (Reference 1) proposed various alternatives to mitigate crankshaft transient torsional stresses for the San Onofre Unit 1 standby diesel generators. These alternatives include installation of a viscous damper or a pendulum damper, changes in rotating masses, and firing order changes. SCE contracted with Imo Delaval to evaluate the alternatives proposed by PNL and include other potential improvements. Other alternatives evaluated by Imo Delaval include cylinder disabling, replacing the existing crankshaft (low carbon steel) with a high strength crankshaft (low alloy steel), and modifying the engine for automatic slow surveillance starts.
Classification and Discussion of Alternatives The alternatives fall into three basic categories:
- 1. Those offering no improvement.
- 2. Those completely eliminating cracking potential.
- 3. Those which will satisfactorily reduce cracking potential.
Category 1 alternatives include modification of the mass elastic system and engine retiming, neither of which will offer any improvement. Installation of pendulum dampers also falls into this category because of the risks associated with technical complexity, questionable reliability and lack of industry experience.
Installation of a viscous damper falls into category 2. The calculated stresses in the shaft extension keyway and oil holes are well below the material endurance limit of 35,000 psi.
However, a viscous damper is not risk-free and experimental testing must first be conducted to confirm suitable transient response. If the damper is not fully functional in the first ten revolutions of the engine, even higher stresses could result in permanent damage to the shaft. In addition, actual installation of the hardware is complex and expensive. Misassembly may occur, greatly increasing installation time and cost.
Cylinder disabling, automatic slow surveillance starting, and installation of a high strength crankshaft all comprise category 3 alternatives. (A high strength crankshaft could be considered in Category 2 since the finite life of the crankshaft extends well beyond the expected life of the plant.)
Cylinder disabling is not desirable, as it requires extensive hardware changes and may create excessive vibration due to the unbalanced forces and moments created, yet still yields higher nominal stresses than the modified start.
-2 The installation of a high strength crankshaft is technically feasible, but requires a major disassembly and high cost. The endurance limit in the high strength crankshaft case that was evaluated is greater than 50,000 psi but is not sufficient to bound all possible starting stresses.
Automatic slow surveillance starting adds equipment which is typically included on commercial generators and the cost is moderate. The current crankshafts were analyzed and should withstand on the order of 5,000 automatic slow starts and 100 high stress starts without initiating a crack. Delaval developed and benchmarked new computer simulations for calculating actual combined stress in each crank member using both frequency domain and direct integration techniques. The combined stress analysis along with determination of stress concentration and damping factors conservatively found slow start/coast down stress to range from 33,000 to 40,000 psi.
The highest stress fast start ranged from 61,000 to 73,000 psi.
Method and Cost of Implementation o
Torsional Damper:
The torsional damper needs to be mounted at the front end attached to an extension shaft and outboard self-aligning bearing. Special assembly techniques are required to obtain the proper interference fits between the crankshaft and extension shaft hub. Moderate risk in the assembly operation exists since the hub could seize part way on the shaft if not properly aligned. If this occurs, the hub would have to be cut off and the operation repeated with a new component. Extensive modification of existing auxiliaries and piping is also required. Delaval has not installed a damper hub on a crankshaft in this manner.
After completion of the reassembly, the system would be tested to assure suitable damper operation.
The fully installed cost including both units is estimated to be
$4,600,000.
o High Strength Crankshaft:
Installation of a new, high strength crankshaft requires the following work:
The old crankshaft, all heads, pistons and rods would be removed and the flywheel and generator disconnected. All fluid piping (including that attached to turbochargers) would be disconnected at the engine, and the intake manifolds, gear case and idler gears removed. The crankcase would then be unbolted, cribbed upward and slid laterally toward the near wall about four feet. The crankshaft would then be rolled out over the open portion of the base and removed through the louvered door. (The starting air compressors and dryers may have to be removed to clear the
-3 crankshaft.)
The new crankshaft could then be installed, and the engine reassembled. Moderate risk results from the major disassembly involved and increased shaft damage should a bearing fail.
The fully installed cost including both units is estimated to be
$3,300,000.
o Modified Automatic Slow Surveillance Starting:
Automatic slow surveillance starts require installation of a pneumatic cylinder which limits fuel rack travel during acceleration, and then retracts upon reaching synchronous speed. In addition, some minor control panel modifications are required. Delaval has installed this equipment on commercial power plant engines.
The fully installed cost including both units is estimated to be $260,000.
Summary and Resolution Three alternatives were found that can effectively reduce transient torsional stresses and permit the diesel generators to meet their functional requirements for standby service:
o Installation of viscous damper.
o Retrofit with a high strength crankshaft.
o Addition of automatic slow surveillance start.
The preferred alternative is the addition of an automatic slow surveillance start capability. The current crankshafts are estimated to withstand 5,000 automatic slow surveillance starts and 100 high stress SISLOP test starts without initiating a crack. The cost of adding automatic slow surveillance starts is 12 times less than installing a new crankshaft and 17 times less than adding a viscous damper.
Reference:
- 1. PNL-5600, Review of Resolution of Known Problems in Engine Components for Transamerica Delaval, Inc., Emergency Diesel Generators, dated December, 1985 1515L