ML18067A620

Forwards Description of Change in Commitments Made in 180-day Response to GL 95-07, Pressure Locking & Thermal Binding of Safety-Related Power-Operated Gate Valves.

ML18067A620
Person / Time
Site:	Palisades
Issue date:	07/01/1997
From:	Bordine T CMS ENERGY CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-95-07, GL-95-7, NUDOCS 9707080053
Download: ML18067A620 (10)

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A CMS Energy Company Thoma* t:. Bordlml Manager Licensing July 1, 1997 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 DOCKET 50-255 - LICENSE DPR PALISADES PLANT CHANGE IN COMMITMENTS MADE IN 180-DAY RESPONSE TO GENERIC LETTER 95-07 On February 13, 1996, the Palisades Plant submitted the 180-day response to Generic Letter 95-07, "Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves". Within that letter, three commitments were made to alleviate potential pressure locking or thermal binding situations involving the power operated relief valve (PORV) Block Valves (M0-1042A and M0-1043A) and the high pressure safety injection (HPSI) Pump Subcooling Isolation Valves (CV-3070 and CV-3071 ).

These commitments were:

1. The susceptibility of the PORV Block Valves to possible pressure locking will be eliminated by modification of the valve disc or by a system operating configuration change.

2. The operating procedures will be revised to reduce the temperature differential seen by the PORV Block Valves.

3. The HPSI Pump Subcooling Isolation Valves will be modified by drilling a hole in the valve discs to equalize the bonnet and high side line pressure. l Ii Ps-e61-e- {

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• Tel: 616 764 2913

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• 2 Commitment 1 has been completed. This was accomplished during the 1996 refueling outage by drilling a hole in each PORV Block Valve upstream disc to equalize the bonnet and the upstream line pressures.

Further evaluation has resulted in a need to revise the methods to be used to accomplish the actions of Commitments 2 and 3. These evaluations are contained in Attachments 1 and 2, respectively.

SUMMARY

OF COMMITMENTS This letter contains no new commitments and revisions to two existing commitments.

The revised commitments are:

2. The control switch trip of the PORV Block Valve motor operators will be changed from torque switch closed to limit switch closed in order to reduce the susceptibility for thermal binding.

3. The bonnets of the HPSI Pump Subcooling Isolation Valves will be connected, through existing bonnet leak off lines, to the upstream piping to equalize the bonnet and high side line pressure in order to eliminate the susceptibility for

.pressure locking.

• The completion date of the revised commitments remains as the end of the 1998 refueling outage.

Thomas C. Berdine Manager, Licensing CC Administrator, Region Ill, USNRC Project Manager, NRR, USNRC NRC Resident Inspector - Palisades Attachments 1 and 2

CONSUMERS ENERGY COMPANY To the best of my knowledge, the contents of this letter revising commitments made in the 180-day response to Generic Letter 95-07, are truthful and complete.

~~_L ThOfTlaSC, B~ ~

Manager, Licensing Sworn and subscribed to before me this ;d day of ~ 1997.

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Alora M. Davis, Notary Public Berrien County, Michigan (Acting in Van Bur~n County, Michigan)

My commission expires August 26, 1999

ATTACHMENT 1 CONSUMERS ENERGY COMPANY PALISADES PLANT DOCKET 50-255 CHANGE IN COMMITMENTS MADE IN 180-DAY RESPONSE TO GENERIC LETTER 95-07 COMMITMENT 2: THERMAL BINDING POTENTIAL IN PORV BLOCK VALVES 3 Pages

• COMMITMENT 2: THERMAL BINDING POTENTIAL IN PORV BLOCK VALVES The purpose of this evaluation is to provide justification to revise the current commitment for the reduction of PORV Block Valve thermal binding susceptibility by replacing the proposed operating procedure changes with a modification to the motor operators.

Thermal binding occurs in double disk gate valves which are closed hot and allowed to cool. Contractions occurring during cooldown can cause a valve to become thermally bound and may prevent opening at the cooler temperature. Temperature differences greater than 100°F are considered a threshold for thermal binding susceptibility.

The PORV Block Valves are 4 inch, double disk Edwards gate valves with Limitorque motor operators. They are normally closed during power operation and are closed/opened during heatup/cooldown to disable/enable low temperature overpressure protection (LTOP). The PORV Block Valves are found to be susceptible to thermal binding because they are double disk gate valves and may become thermally bound during certain plant conditions.

For the Palisades PORV Block Valves, operating procedures permit closure at .

temperatures as high as 643°F with subsequent opening as low as 495°F, a maximum

• temperature difference of 148°F.

Valve closure occurs during plant heatup when LTOP is disabled. Operating Procedure SOP-1, "Primary Coolant System", specifies disarming LTOP and closure of the PORV Block Valves when PCS temperature is greater than 460°F. Using the Saturation Curve of the Pressure and Temperature Limit Curves in SOP-1, Attachment 2, the corresponding Pressurizer temperature can be as high as 643°F (temperature at the highest PCS pressure, 2100 psia, for saturated conditions). This high temperature, while procedurally permissible, represents an extreme departure from the normal way the plant is operated. The PORV Blocks are typically closed soon after heatup past the 460°F permissive temperature. The high temperature is also conservative since it assumes valve temperature follows Pressurizer temperatures at saturated steam conditions. It does not account for heat loss in the piping to the block valves or any lag time required for valve heat up.

PORV Block Valve opening occurs during plant cooldown to enable LTOP and can occur at a temperature as low as 495°F. SOP:-1, Attachment 2 requires LTOP to be enabled whenever PCS temperature is less than 430°F. Using the Pressure and Temperature Limit Curves in SOP-1, Attachment 2, this can occur at a minimum PCS pressure of 650 psia which corresponds to a Pressurizer temperature of 495°F (temperature at saturated steam conditions). As stated in the February 13, 1996, response to Generic Letter (GL) 95-07, this temperature is also conservative since it also assumes valve temperature follows the Pressurizer temperature at saturated

• 2 steam conditions and that no heat is retained in the valve during cooldown.

Furthermore, plant operators are especially sensitive to the potential for pressurized thermal shock (PTS) and directed in the EOPs to open the PORV Block Valves early in the accident.

In the February 13, 1996, response, it was stated that the 148°F temperature difference was not large enough to cause the valve to become thermally bound. This was based on engineering judgement, motor-operated valve (MOV) test experience, and past operating history. However, a commitment was made to revise plant operating procedures to reduce this temperature differential.

In correspondence dated June 21, 1996, the NRC requested additional information regarding the proposed procedural changes to reduce the temperature difference on the PORV Block Valves. Question 2 of the NRC request was directed to our planned corrective actions for elimination of susceptibility of the PORV Block Valves to thermal binding. Additional details describing the conditions for opening and closing as well as the description of the planned procedure changes were provided in the Palisades response dated July 22, 1996. It was also acknowledged that revising operating procedures would reduce the operators' ability to operate the plant within the SOP-1 Pressure and Temperature Limit Curve and that Palisades would review alternatives before making any procedure changes.

Palisades has performed a review of the proposed procedure changes, alternatives available for the elimination/reduction of thermal binding susceptibility, and the consequences of continued operation in the current configuration. In addition, a review of NRC responses to GL 95-07 from other Combustion Engineering plants was performed. Based on these reviews, it is recommended that operating prqcedures not be revised and that the control switch trip for PORV Block Valve motor operators be changed from torque switch closed to limit switch closed. This recommendation is made for the following reasons:

1. The proposed procedure changes would reduce the operators' ability to maneuver within the Primary Coolant System pressure and temperature limit curve. The procedure revision would create an inherent operator work-around and reduce the margin and operator flexibility gained when the plant installed the variable LTOP modification.

2. The temperature difference postulated to produce thermal binding is not large and is based on conservative assumptions. Additionally, the calculated temperatures are conservative since they are derived from system pressures at saturated steam conditions and assume valve temperature follows system temperature. This assumes that no heat is retained in the PORV or the Pressurizer piping.

• 3

3. Plant operation to achieve the postulated temperature difference does not represent normal operation. During heatup, LTOP is disabled and the PORV Block Valves closed soon after reaching 460°F. At this point, plant operators normally maintain PCS pressure well within the Pressure and Temperature Limit Curves. Conversely, during cooldown LTOP is enabled early when PCS pressure is still high. The probability of reaching postulated plant conditions for the temperatures required to produce thermal binding is unlikely.

The proposed modification is to change the closed control switch trip from torque closed to limit closed. This would assure that excessive closing forces do not exist on the valve disc when the valve is closed and would reduce the amount of unwedging force required for opening. This corrective action will reduce the susceptibility for thermal binding for the PORV Block Valves. Limit seating is commonly used throughout the industry and is used on the low pressure safety injection (LPSI) Loop Injection MOVs at Palisades.

ATTACHMENT 2 CONSUMERS ENERGY COMPANY PALISADES PLANT DOCKET 50-255 CHANGE IN COMMITMENTS MADE IN 180-DAY RESPONSE TO GENERIC LETTER 95-07 COMMITMENT 3: PRESSURE LOCKING POTENTIAL IN HPSI PUMP SUBCOOLING ISOLATION VALVES 2 Pages

.*. c*oMMltMENT 3:

PRESSURE LOCKING POTENTIAL IN HPSI PUMP SUBCOOLING ISOLATION VALVES The purpose of this evaluation is to provide justification to revise the current commitment to modify the valve discs of the HPSI Pump Subcooling Isolation Valves to eliminate the possibility for these valves to become pressure locked. The intent is to achieve the same result by another method.

The original commitment was to modify the valve disc by drilling a hole to equalize the pressure between the bonnet and the high side line pressure. This modification was to be done during the 1998 refueling outage when the core was scheduled to be off-loaded. This was needed because the modification to the valve discs requires disassembly of the valves. Disassembly of these valves requires draining the header connected to the Shutdown Cooling Heat Exchanger and would render HPSI, Containment Spray, and Shutdown Cooling inoperable. After the commitment was made, plar:is changed and the core is no longer scheduled to be off-loaded during the 1998 refueling outage.

Since full core off-load is no longer scheduled for the 1998 refueling outage, other*

options for resolving the pressure_ locking concern were explored. Implementation of the valve disc modification by the establishment of a freeze seal to provide boundary isolation was considered. While a freeze seal was possible, the plant staff evaluated the consequences of a freeze seal failure. Such a failure would create a PCS leak outside of containment which could potentially flood a room containing the safety injection and shutdown cooling pumps. The consequences of such a failure create an unacceptable risk, therefore the freeze seal option was not pursued.

An additional option considered was an analytical evaluation of the actuator to overcome the pressure locking forces. This was ruled out since there has been no validation testing to date which could justify an analytical approach for these valves.

The plant conditions which led to pressure locking of these valves was also re-examined. Results of this evaluation confirmed the conditions leading to pressure locking. In addition, no changes to plant procedures were identified that could mitigate all of the conditions leading to pressure locking of these valves. Two procedure changes were recommended and have been implemented which assures the valves do not have pressure trapped in the bonnet after either a LPSI pump or containment spray pump are secured.

• The safest, most feasible optio_n presented has been an alternate modification to eliminate the pressure locking potential. This alternate modification consists of externally connecting the bonnet, through the existing bonnet leak-off line, to the upstream piping. The connection to the upstream piping would be accomplished by drilling a hole in the upstream piping through a full ported valve. When the pipe wall penetration occurs, water would flow out of the hole and would be isolated by closing the valve. This (commonly called a hot tap) could be done without having to isolate the

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• 2 upstream piping. Therefore the modification could be performed with the plant on line and Shutdown Cooling secured. This modification would have the same effect as drilling a hole in the upstream disc and would remove the possibility of these valves becoming pressure locked. Special precautions will be taken to prevent detrimental foreign material from entering the line.