Information Notice 1994-66, Overspeed of Turbine-Driven Pumps Caused by Governor Valve Stem Binding

Overspeed of Turbine-Driven Pumps Caused by Governor Valve Stem Binding
	ML031210648
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	09/19/1994
From:	Grimes B; Office of Nuclear Reactor Regulation
To:	;
References
	IN-94-066, NUDOCS 9409160230
	Download: ML031210648 (7)
	v • d • e

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UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 September 19, 1994 NRC INFORMATION NOTICE 94-66: OVERSPEED OF TURBINE-DRIVEN PUMPS CAUSED BY

GOVERNOR VALVE STEM BINDING

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purnose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to recent problems regarding binding of governor

valves for turbine-driven pumps that have resulted in ovcrspeed trips. It is

expected that recipients will review the information for applicability to

their facilities and consider actions, as appropriate, to avoid similar

problems. However, suggestions contained in this information notice are not

NRC requirements; therefore, no specific action or written response is

required.

Description of Circumstances

River Bend

On September 8, 1994, the River Bend Station experienced an automatic reactor trip on a reactor water level high (Level 8) trip signal. In the course of

the recovery, the operators initiated the reactor core isolation cooling

(RCIC) system, but the RC'C pump turbine tripped on overspeed. The oper'ators

then initiated the high pressure core spray (HPCS) system to provide

condensate makeup to the reactor vessel. The reactor vessel level was

maintained above the emergency core cooling system (ECCS) actuation setpoints, and no ECCS initiations occurred.

The RCIC turbine tripped on overspeed due to the governor valve failing in the

open position. Attempts to manually stroke the valve were unsuccessful. The

valve bonnet was disassembled. The valve stem was found to be stuck and had

to be forcibly removed. Close inspection indicated that the apparent failure

mechanism was galvanic corrosion between the valve stem and spacers in the

packing assembly. The licensee reviewed the maintenance history of the valve

and found that the stem and spacers had been recently replaced during the

refueling outage that ended in July 1994. The valve, which is on a quarterly

test schedule, had not been tested since the outage. The licensee has

con'-acted the valve manufacturer, Dresser-Rand Steam Turbines, for information

on the material specifications for the valve stem and spacers. In addition, the licensee has submitted the spacers for chemical analysis and is presently

awaiting the results.

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IN 94-66 September 19, 1994 North Anna

On July 29, 1994, the North Anna Power Station Unit 2 turbine-driven auxiliary

feedwater pump (AFW) pump tripped on overspeed during an initial startup

following maintenance. During valve cycling by hand, the licensee noted that

the valve appeared to stick and the stem would only move approximately

one-half inch. After several more attempts, the valve was freed and traveled

an additional one-half inch. The valve was disassembled, and the licensee

noted pitting and corrosion on the valve stem. The licensee replaced the stem

and packing assembly. The post-maintenance test was successfully completed.

Surry

On January 11, 1992, the Surry Power Station Unit 1 turbine-driven AFW pump

tripped on overspeed during shutdown of the pump; it was noted that the valve

stem was sticking. On December 2, 1993, the Surry Power Station Unit 1 turbine-driven AFW pump tripped on overspeed after being started for a monthly

surveillance test. The pump was started a second time and again tripped on

overspecJ. The governor valve linkage was discQuinected from the turbine

governor and the governor valve stem was checked for freedom of movement. The

valve stem was found stuck in the open position. In this condition, the

turbine governor was unable to reposition the governor valve and the turbine

tripped at the overspeed trip setpoint. The governor valve bonnet, stem and

packing were replaced and the turbine-driven AFW pump operated satisfactorily.

On June 15, 1994, the Unit 1 turbine-driven AFW pump again tripped on

overspeed after being started for a monthly surveillance test. The pump was

started a second time and operated satisfactorily. Corrective maintenance was

not performed between the first and second starts. The governor valve linkage

was disconnected from the turbine governor and the governor valve stem was

checked for freedom of movement. The valve stem was difficult to move and it

was concluded that governor valve stem binding caused the turbine-driven AFW

pump to trip on overspeed during the initial start. The governor valve

bonnet, stem and packing, which were previously replaced in December 1993, were again replaced and the turbine-driven AFW pump operated satisfactorily.

To evaluate whether a similar condition existed in Unit 2, the Unit 2 turbine- driven AFW pump governor valve linkage was disconnected from the turbine

governor and the governor valve stem was checked for freedom of movement on

June 25, 1994. It also had been checked on January 17, 1994. In both

instances governor valve stem binding was noted. The Unit 2 turbine-driven

AFW pump had satisfactorily passed its monthly surveillance in this condition.

The governor valve stem and packing were replaced each time stem binding was

identified.

Based on metallurgical analysis, the licensee determined that the corrosion

products are formed due to galvanic corrosion, crevice corrosion, and pitting

corrosion. Dresser-Rand stated that the packing binder material has

approximately 1.29% sulfur and traces of other chemicals. The licensee and

the vendor are currently investigating to determine what effect the sulfur and

other chemicals have in accelerating the corrosion process.

IN 94-66 September 19, 1994 Arkansas Nuclear One

On May 9, 1991, an event occurred at Arkansas Nuclear One where a turbine- driven emergency feedwater pump tripped on overspeed. The cause of the

overspeed trip was determined to be corrosion of the governor valve stem. The

buildup of corrosion products on the stem caused binding which prevented the

valve from closing as the turbine accelerated on startup. In this instance, the valve stem had been in service for one month prior to the failure.

Discussion

Turbine driven pumps used in both RCIC and the auxiliary feedwater systems are

classified as engineered safety features which are designed for removing decay

heat and for providing a redundant means of supplying water to either the

reactor vessel or the steam generators.

The turbine-driven pumps at River Bend, North Anna, Surry, and Arkansas

Nuclear One were manufactured by Terry Steam Turbine rnmpdY. Dresser-Rand

Steam Turbines is the current vendor for the equipment. The governor valve

for the turbines is designed such that the valve stem travels through a

packing assembly made up of carbon spacers and stainless steel washers with

very tight clearances. The governor valve stem material is 410 stainless

steel that is nitrided for hardness. The packing space is composed of carbon

discs and stainless steel washers. A gland packing leakoff is located in the

outer portion of the packing/stem assembly. In one case, the licensee has

determined that there was corrosion and a buildup of mineral deposits on the

governor valve stem in the area adjacent to the packing leakoff. The deposits

were extremely hard and adherent to the stem. The corrosion and deposits

interfered with the movement of the stem through the packing causing the stem

to bind. The clearance between a new stem and packing assembly is

approximately 0.005 cm [0.002 inches]; therefore, only a small buildup of

deposits on the stem will result in interference between the stem and packing.

One licensee reported that the ambient temperature of the governor valve is

significantly warmer, which may affect the corrosion rate. (The steam supply

valve is in close proximity to the governor valve.)

In a letter dated March 24, 1993, to Surry Nuclear Power Station from Dresser- Rand Steam Turbines, nuclear plant experience with corrosion-related valve

stem binding was discussed. The letter stated that eight plants had reported

governor valve stem binding problems. Dresser-Rand stated that Terry model ZS

and GS turbines using two and one half-inch veeport and three-inch venturi

governor valves with 410 stainless steel stems and carbon packing rings were

subject to this phenomena. The letter also stated that the environment of the

governor valve was critical and that a dry environment for stem and packing

assembly was desired when the equipment was not operating. When unable to

assure a dry environment the letter recommended using a chrome plated stem.

However, in conversation with the staff, the Dresser-Rand Steam Turbines

representative indicated that the new stems may also be subject to binding.

'-' IN 94-66 September 19, 1994 The root cause for the formation of deposits and corrosion on the governor

valve stems is under review by several licensees, Dresser-Rand Steam Turbines, and the EPRI Terry Turbine Users Group. Until a root cause is determined, some licensees have elected to perform more frequent operational checks or

inspections of the stem and packing. At Surry Power Station, a maintenance

procedure to stroke the governor valve stem is performed weekly at Unit 2 and

every few days at Unit 1. In addition, the speed of the turbine and the

initial movement of the governor valve stem are correlated during monthly

surveillance testing. Trending has identified that turbine speed at initial

governor valve stem movement increases as the deposits on the stem are formed.

Also, the governor valve packing drain line at Surry is periodically removed

and the area of the stem inside the governor valve bonnet is inspected for

corrosion and deposit buildup with mirror/flashlight and mini-camera.

Related Generic Communications

Turbine-driven pump overspeed trip events attributed to various causes are

dezzribed in the following information notices:

° NRC Information Notice 93-51, Repetitive Overspeed Tripping of Turbine- Driven Auxiliary Feedwater Pumps.

o NRC Information Notice 90-45, Overspeed of the Turbine-Driven Auxiliary

Feedwater Pumps and Overpressurizaton of the Associated Piping Systems.

° NRC information Notice 88-67, PWR Auxiliary Feedwater Pump Turbine

Overspeed Trip Failure.

° IE Information Notice 86-14, Supplement 1, Overspeed Trips of AFW, HPCI,

and RCIC Turbines.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

one of the technical contacts listed below or the appropriate Office of

Nuclear Reactor Regulation project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contacts: M. Branch, RII G. Hornseth, NRR

(804) 357-2101 (301) 504-2756 S. Tingen, RII F. Grubelich, NRR

(804) 357-2101 (301) 504-2784 Attachment:

List of Recently Issued NRC Information Notices

i4A g bledf lieJb#cLgr

Attachment

IN 94-66 September 19, 1994 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Subject Issuance Issued to

Notice No.

Potential Errors in 09/12/94 Al U.S. Nuclear Regulatory

94-65 Commission medical

Manual Brachytherapy

Dose Calculations licensees.

Generated Using a

Computerized Treatment

Planning System

Reactivity Insertion 08/31/94 All holders of OLs or CPs

94-64 Transient and Accident for nuclear power reactors

Limits for High Burnup and all fuel fabrication

Fuel licensees.

Boric Acid Corrosion of 08/30/94 All holders of OLs or CPs

94-63 for pressurized water

Charging Pump Casing

Caused by Cladding Cracks reactors.

Operational Experience 08/30/94 All holders of OLs or CPs

94-62 for pressurized water

on Steam Generator Tube

Leaks and Tube Ruptures reactors.

Corrosion of William 08/25/94 All holders of OLs or CPs

94-61 for nuclear power reactors.

Powell Gate Valve Disc

Holders

Potential Overpressuriza- 08/22/94 All holders of OLs ^ CPs

94-60 for pressurized-water

tion of Main Steam System

reactors.

Leaking Shutdown Cooling 08/19/94 All holders of OLs or CPs

94-30, for nuclear power reactors.

Supp. 1 Isolation Valves at

Cooper Nuclear Station

Accelerated Dealloying of 08/17/94 All holders of OLs or CPs

94-59 for nuclear power reactors.

Cast Aluminum-Bronze

Valves Caused by Micro- biologically Induced

Corrosion

Reactor Coolant Pump 08/16/94 All holders of OLs or CPs

94-58 for pressurized water

Lube Oil Fire

reactors.

OL = Operating License

CP = Construction Permit

IN 94-6F

Septemb,_j19, 1994 The root cause for the formation of deposits and corrosion on the governor