ML20099K829

Final Part 21/deficiency Rept Re Pressurizer PORV Failing to Close During Preoperational Test.Initially Reported 841011. Resolution Will Be Provided by 841119

ML20099K829
Person / Time
Site:	Wolf Creek
Issue date:	11/13/1984
From:	Koester G KANSAS GAS & ELECTRIC CO.
To:	Denise R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
References
REF-PT21-85-214-000 KMLNRC-84-196, PT21-85-214, PT21-85-214-000, NUDOCS 8411290584
Download: ML20099K829 (6)

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KANSAS GAS AND ELECTRIC COMPANY THE: ELECTFaC COMPANY GLENN L KOESTER vice oneseormt.muctram November 13, 1984 Mr. R.P. Denise, Wolf Creek Task Force b Reactor Projects Branch 2 )j U.S. Nuclear Regulatory Commission Region IV { g g 611 Ryan Plaza Drive, Suite 1000 Arlington, Texas 76011 i y KMLNRC 84-196 Re: Docket No. STN 50-482 Subj: Final 10CFR50.55(e) Report - Pressurizer Power Operated Relief Valves (53564-K155)

Dear Mr. Denise:

The attachment to this letter provides the final report submitted pursuant to 10CFR50.55(e) concerning the pressurizer power operated relief valves (PORV) at Wolf Creek Generating Station (WCGS). This matter was initially reported Ly Mr. it.K. Chernoff of Kansas Gas and Electric Company (KG&E) to Mr. John Boardman of the Nuclear Regulatory Commission, Region IV, on October 11, 1984.

Kansas Gas and Electric's plans for resolution of the pressurizer PORV preoperational test discrepancies will be provided to the NRC under separate cover by November 19, 1984.

If you have any questions concerning this matter, please contact me or Mr. Otto Maynard of my staff.

Yours very truly, I

V Glenn L. Koester Vice President - Nuclear GLK:jlh xc:RCDeYoung l FO'Connor (2)

IIBundy At tach .

8411290504 841113 -

qDRADOCK 05000482 PDR l h,yt 201 N. Market -Wichtra, Kansas - Mad Address: PO. Box 208 I W1chita, Kansas 67201 - Telephone: Area Code (316) 2616451 1

I WOLF CREEK PRESSURIZER POWER OPERATED RELIEF VALVES (PORV)

Safety Evaluation Sunnary The Wolf Creek PORV's failed to close during a specific preoperational test. This safety evaluation summary report is intended to define the conditions under which the valve malfunctioned; describe the reason for the failure to close; explain the corrective action that was taken; and provide assurance of valve operability.

Introduction The Wolf Creek pressurizer power operated relief valves (PORVs),

manufactured by Garrett, are 3" x 6" and are solenoid operated. They are intended to control pressurizer pressure to a value below the fixed high-pressure reactor trip setpoint for a 110% load rejection assuning failure of the pressurizer spray system. They also provide a safety grade means for reactor coolant system depressurization to achiev e cold shutdo wn .

Additionally, they serve as part of the cold overpressure mitigation system

( COMS) .

The PORVs are not required to open in order to prevent overpressurization of the reactor coolant system for the loss of load event discussed in the Overpressure Protection Report. The pressurizer safety valves perform this function assuming pressurizer spray and PORVs fail to operate.

The PORVs are electrically actuated valves which respond to a signal from the pressure sensing system or to manual control. They are provided with Class 1E direct position indication in the main control room. For each valve there are indication li6 hts and alarms that are activated by stem-actuated limit switches.

Figure 1 shows the functional schematic of the PORV. The mode of operation of the valve is as follows:

The valve is a line-pressure actuated , solenoid-controlled , relief valve of the caged-plug type. The schematic diagram of Figure 1 shows the unit with the solenoid de-energized and the valve closed. Inlet pressure (either vapor or water) flows into the valve inlet connection and is ported through the solenoid seat to the actuator head chamber of the valve. Inlet pressure is also ported underneath the piston and through the cage holes to surround the plug. The forces tending to hold the valve closed incluie the pressure in the actuator head chamber acting on the entire piston area and the actuator spring load. Inlet pressure also acts on the annular area beneath the piston (and outside the seat d iameter) in a direction to open the valve. Since the annular area is less than the total piston area, the closing force predoninates and the plug is held down against the seat with a force equal to the value of inlet pressure multiplied by the seat area.

f P:go Two When the solenoid is energized , the magnetic force acts on the solenoid armature to move the ball from the vent seat (as shown) to the opposite seat, thus sealing off inlet pressure from the actuator head chamber. At the same time, the actuator heed pressure is vented to discharge through the vent seat of the solenoid. With the actuator head chamber now at discharge pr essur e , inlet pr essure acting on the annular area is sufficient to overcome the actuator spring load. The plug moves away from the seat in the direction to open the valve.

As the valve opens, pressure inside the case builds up underneath that portion of the plus exposed to discharge pressure. Because of the pressure drop through the cage flow holes, this pressure is less than inlet pressure but higher than the discharge pressure. The targe seating force that exits when the valve is closed is thus turned into an opening force, causing the plug to move to the full-lift position.

When the solenoid is de-energized, the ball moves back to the seat as shown, sealing off the path to discharge and repressurizing the actuator head chamber with inlet pressure. With the plug in the full-lift position, the opening force consists of inlet pressure acting on the annular area and cage pressure acting on the base of the plug. The closing forces (consisting of inlet pressure in the actuator head chamber and the actuator spring load) overcome the opening forces and cause the plug to move toward the seat.

Discharge pressure drops to a minimum as the valve reseats, and the valve is once more held in the closed position by a force that is equal to inlet pressure multiplied by the seat area.

Discussion of Valve Malfunction It was in the closing mode, described above, in which the valves m al func tion ed . Speci ficall y, the valves were being operated in the manual mode, discharging steam, and being held open for a period of approximately 32 seconds. Prior to opening the valve, the inlet piping (consisting of approximately fourteen feet of vertical downward run loop seal) was filled with cold water as were the valves thenselves. The valves are located in a c&npartment which is below the top of the pressurizer. This location away fran the bap of the pressurizer results in valves being substantially colder than if they were at the top of the pressurizer. Valve ambient temperature at Wolf Creek is approximately 90 degrees fahrenheit.

l The preoperational test itself required approximately 32 seconds or continuous operation to achieve pressure relief of 200 psi. The pur pose o f the test is to verify valve stroke time and leakage after the valve has been opened for more than two seconds. This test simulates certain conditions l

which may be encountered during plant operation such as loss of load. The valve equipnent specification contains requirements such as t valve cycle time; discharge fluid rates; nunber of design cycles; etc. These design

, requirements are adequate to assure that the valve will perform its intended I function.

Page Three In addition to assuring operability through equipment specification requirements, considerable testing has been performed on these valves. This testing includes preoperational tests at other foreign and domestic plants and the following successful tests at Wolf Creek. At Wol f Cr eek , tests performed in the automatic mode, during which the valve remained open for a period of approximately two seconds, were successful. Additionally all Wolf Creek testing per formed without a cold loop seal was completed successfully. Further, a number of isothermal tests have been performed on the Garrett Power Operated Relief Valves. These include the EPRI Safety and Relief Valve Test Program, and Garrett operability tests. In these tests, the valves closed as required.

When the valves failed to close when signaled after the discharge period of approximately 32 seconds, the motor-o perated block valves, which are installed upstream of the PORVs and whose function is to preclude the loss of reactor coolant if a leak should develop in a PORV, were closed. Closing of the PROVs was observed to occur simultaneously with block valve closure. This occurred because the head actuator chamber ( which was isolated) was at approximately 500 psis, the normal discharge pressure.

Closure of the block valve reduced inlet pressure. Since the active area above the piston is three times greater than that below the piston, the 500 psig was sufficient to overcome the falling inlet pressure.

Summary of Investigation The postulated causes for the valve malfunction considered were: solenoid failure, plug to cage binding, and failure to get required fluid pressure to actuator head chamber. Proper solenoid operation was verified. The fact that the valve operated as designed in the autonatic mode and inspection of the valve internals showed no evidence of binding (i.e., gouging etc.),

eliminated the binding supposition. There fore , there was strong indication that the third postulated cause, that of failure to get required fluid pressure to the actuator head chamber, was the source of the mal func tion .

By reviewing the valve design in conjunction with detailed manufacturing drawings it was determined that differentici thernal expansion between the valve cage and the valve body bore in which the cage is housed, would cause the cage-to-body annulus to be reduced in size even to a point of total closur e . This annulus serves as a path for inlet fluid to travel to the solenoid port and eventually to the actuator head chamber as defined i prev iousl y. To verify this supposition, a subsequent manual test, similar to the tests in which malfunction occurred , was per formed with the v alve body heated to 228 degrees fahrenheit. The valve functioned as required providing strong support to the prenise that differential thermal expansion was the cause of the malfunction. It should be noted that by heating the valve body to 223 degrees fahrenheit the valve body bore was increased by six mila which results in an additional annular clearance under the flow conditions.

In review of the valve manufacturing drawings, it was determined that the maximum and minimum radial annular clearance at ambient temperature when the parts (valve body and cage) are machined to within specified tolerances are nine and six mils (0.009 - 0.006) respectively with diametral clearance being eighteen to twelve mils (0.018 - 0.012).

Page Four Based on the in formation from the testing described above and the small manufacturing tolerances, an analysis was performed to determine the effects of differential temperature on the valve body and cage. Figure 2 is a plot of the results and shows that for 100 degrees fahrenheit of temperature differential the annular gap is reduced by approximately three and one half mils ( .0035) . This is based on the expansion of the cage with no expansion of the valve bod y . By heating the valve in the successful test, approximately six mils (0.006) annular clearance was added due to the thermal expansion of the valve body at its initial condition of 228 degrees fahrenheit.

This phenonenon was then analyzed to determine the effects of gap closure on fluid flow with a homogeneous flow model. The results substantiate the heated test results and the differential expansion premise. Specifically, for all relief conditions, the minimum annular diametral gap between the body and the cage necessary for the PORV to function properly is 1.12 mils.

With the valve starting cold (90 degrees fahrenheit) and suddenly exposed to high pressure steam (650 degrees fahrenheit) it will take 5.75 seconds for the annular orifice gap to be reduced from 15 mils to 1.12 mils. In 7.47 seconds, the annular orifice is completely closed off. If the annular orifice gap started out at 13 mils, it would take approximately 9 seconds for the gap to be reduced to 1.12 mils. By 11 seconds, the 18 mil gap would be canpletely clo sed . In this analysis the cage expands as a function of time and temperature and the thermal expansion of the valve body during these time intervals is negligible.

Corrective Action Taken The valves under discussion were disassembled and dimensions of the body bore I.D. and cage 0.D. were taken. This showed that the diametral annular clearances at ambient temperature were nominally 15 mils and 18 mils for the two v alv es . A field change notice (FCN) was prepared to machine the cages to an 0.D. of 4.55 to 4.57 inches, thereby provid ing a final diametral annular clearance of 114 mils and 111 mils respectively. This action was taken with full cognizance and technical support / assurance by the valve designer / manufacturer (Garrett) and Westinglause.

In designing the valve to meet the specification requirements, the designer kept the annular clearance sna11 so that it would serve as a filter to prevent any debris that may be entrained in the fluid from fouling the three-way ball valve of the solenoid. Ibwev er , the v alv e manufacturer (Garrett) has deternined that the clearance provided by this design need not be this sna11. Garrett has also confirmed that the machining to resize the cage is a product improvement.

Summary and Conclusion The Walf Creek pressurizer PROVs failed to close after a discharge of water followed by steam which was conducted manually for an extended period of time. Ihe cause of this malfunction was determined to be differential

Page Five thennal expansion (valve bod y to cage) resulting in a restriction of an essential fluid flow path to the valve actuator head assembly.

Identification of the cause of the malfunction is supported by testing in other operating modes, successfully repeating the failed test with red uced differential tenperatures, and a detailed engineering analysis.

A valve modification, specified by Westinghouse and concurred with by Garrett (the valve designer / manufacturer) has been made which corrects the malfunction without having any deleterious effects on valve function.

Based on the infonnation contained herein and supporting documentation, it is concluded that the Garrett pressurizer power-operated relief valves will function under all design conditions.

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