Motor Operaed Valve Analysis

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MOTOR-OPERATED VALVE ANALYSIS PROGRAM

TO ADDRESS

THE DESIGN BASIS REVIEW REQUIREMENTS

OF

THE US NRC's GENERIC LETTER 89-10

M. Recinella and F. Isaac

(Westinghouse Energy Systems International, Brusssels)

T. Matty

(Westinghouse Nuclear and Advanced Technology Division, Pittsburgh)

1.0 INTRODUCTION

The EPRI sponsored PORV block valve testing conducted at the Marshall S team

Station in 1980 following the Three Mile Island accident provided the first

indications o f a potential motor-operated valve (MOV) problem in the nuclear

industry. A three inch 1500 class Westinghouse Electromechanical D ivision (EMD)

MOV failed t o fully close against high pressure and high temperature steam flow .

Shortly thereafter, valves of the same size were tested against high pressure, low

temperature water flow in the safety injection system o f a european nuclear power

plant and some of these valves also failed to close completely.

In November 1980 Westinhgouse initiated a MOV flow testing program to investigate

the required operating loads and the crit ical parameters 1).

On June 9 ,1 985, a loss-of-feedwater event occured at the Davis-Besse nuclear

power plant. The MOVs in the Auxiliary Feedwater (AFW) system could no t be

reopened electrically from the c ontrol room after having them inadvertently

closed. It was later found that the MOVs failed to open because of inadequate

torque switch bypass.

As a

onsequen e

of these and other related events the NRC staff issued in

November 1985 Bulletin 85-03 (2) and in April 1988 a Supplement 1 to Bulletin

85-03 to ensure that switch settings on all safety-related M OVs and on the MOVs

that cou ld be inadvertently mispositioned in the High-Pressure C oolant Injection

(HPCI) and Emergency Feedwater Systems (EFS) were selected, set, and maintained

proper ly.

Tests performed as part of the RES (NRC Office of Nuclear Regulatory Research)

effort to resolve Generic Issue 87 (3) have reinforced concerns for the

operability of MOVs under design-basis condit ions.

Generic Issue 87 (GI-87) issued in December 1985 covers three Boiling Water

Reactor (BWR) process lines: the HPCI turbine steam supply line, the reactor core

isolation cooling (RCIC) turbine steam supply line, and the reactor Water Cleanup

(RWCU) process line. All three of the BWR process lines comm unicate w ith the

primary system , pass through containment, and normally have open isolation valves.

The concern w ith the isolation valves is whether they will close in the event o f a

pipe break outside of the containment.

Motor-Op erated Valve A nalysis Program to address the D esign Basis R eview

Requirements of the US NRC's Generic Letter 89-10

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The NRC-sponsored testing of MO Vs has been conducted in tw o phases by the Idaho

National Engineering Laboratory (INEL). Phase I was performed in 19 88 at the W yle

Laboratory fatifty. Hot wate r b bw do wn tests were performed on tw o flexible wedge

gate valves typical of those used for containm ent isolation in the su pply line to

the RW CU . The results are documented in NUREG/CR-54O6 (4). The phase II wa s

performed in 19 89 at the Kraftwerk Union facility, iix valves, were tested typical

of those used in RW CU and HPCI applications. One of the valves was also tested at

RCIC test conditions. The test results are documented in NURE G/C R-55 58 (5) .

The tests revealed that more thrust w as required to operate the valves under

design-basis conditions than would have been predicted from standard industry

calculations and typical friction factors.

In June 1 98 9 the NR C issued Generic Letter 89 -1 0 ( 6). By this letter the NRC

extends the scope o f the program outlined in Bulletin 85- 03 and Supplement 1 of

Bul let in 85-03 to include al l safety-related MOVs as wel l as al l

position-changeable MOVs. Later supplements to Generic Letter 89-10 changed the

position-changeable M OV s to only those valves that can be controlled from the

control room; and fbrBW Rs the mispositioning of M O Vs don't need to be considered.

Generic Letter 89 -1 0 requires mainly that each licensee develop a program that

provide a review o f the M O V design basis, methods for correct sw itch settings,

field testing a t design-basis conditions, procedures to ensure sw itch setting is

maintained, . . .

The present paper describes the elements and the methodologies used by

Westinghouse to address the M O V Design Basis Review requirements of the NRC

Gener ic Let ter 89-10 .

2 . 0 PRO G RAM DESCRI PT IO N

2.1 Da ta col lection

Gathering data is time consuming and the related effort is often unde restimate d.

How ever this is the first and probably the m ost important step in the M O V design

basis review program and therefore major attention should be given to this task.

The information wh ich is needed in a first place are the pressure and temperature

conditions w hich the valve must stroke against for all modes of o peration. This

data is the result of a fluid system evaluation.

Since the am ount of M O V 's in a Nuclear Power Plant is considerable it is wise t o

review the valve inventory and to group M O V s in order to reduce duplication of

analysis.

Further in order to bs able to perform thrust and torque calculations and valve

we ak link analysis, drawings of the valve internals an d valve design reports are

necessary. How ever in most cases this information, except a M O V general assembly ,

drawing, is not in the possession of the utilities and will have to be requested >;.

from the valve manufacturer. '

Motor-Operated Valve Analysis Program to address the Design Basis Review




especially for older plants, the general assembly drawing may be th e only piece o f

existing information. In such cases an approach of measuring on available va lve

spare parts or a disassembly of the valve itself may be required.

To assess the operator capabilities a series of operator characteristics are

needed. These are l isted in paragraph 2.4 of this paper.

2. 2 Thrust and torque calculation

Th e seco nd e lement in the analysis involves the calculation of the required thru st

and/or torque to op en and close the valve a t the defined pressure conditions.

In the thru st and torque calculation equations there are, w her e applicable

depending on the type of valve and operator, three variables which are o f

importance because they are difficult to define and w hich c an o nly be determined

by test ing .

The first is the V alve Factor defined by the s eat friction coefficient. The second

is the S tem Factor which contains the stem /stem nut friction coefficient as a

variable; and the last one is the Packing Load.

Westinghouse has performed extensive flow testing (1) to determine the critical

parameters that effect the required operating toads for th e Westinghouse gate

valves.

The results of the testing performed in the early 1 98 0 's recognized tha t

the calculations could be under predictive of the actua l required thrusts bec ause

the valve factors ma y be greater than the standard valve factors. In addition t he

test program included, amoung oth ers, investigation of th e s tem/stem nut friction

coeff icient and packing drag loads.

2.3 Va lve W ea k Link Analysis

This analysis consists in determining the va lve w eak est part or parts w hich are

subjected to the operating thrust and/or torque.

In general the valve pans, where applicable depending o n the type of va lve, wh ich

are affected by the thrust and/or torque are :

- valve stem

- stem nut

- valv e disc

- stem /disc l ink

- valve main seat

- back seat

- valve yoke

- yok e to bonn et bolts

- yok e to actuator bol ts

• valve bonnet

- valve body

- bonne t/body bolts



162



In order to perform th e analysis of th e above p a rs some detailed dimensional

information is required as w e* as the parts material specification. As men tioned

in the v alve data collection task, this information is not a lway s available and

cannot be readi ly obtained.

However to reduce the impacts of the limited information and so to limit the

amount o f calculations, the above valve part list can be reduced based on valve

engineering experience.

The valve w eak ink evaluation consists in calculating backwards in the compo nents

stress formulas w here by th e material allowables are considered as applied stress.

In this w ay for eac h valve part under analysis a maximum allowable thrust and/or

torque is calculated. Th e lesser of these maximum values w i determine the valve

weakest part (Valve W ea k Link Analysis) and so determine the valve capability to

suit for the speci f ied pressure condit ions.

2 .4 Mo tor Operator Evaluation

Th e objective of this evaluation is to summ arize relevant operator characteristics

in order to determine operator capabilities to suit for the specified pressure

condit ions.

For this evaluation important operator parameters are :

- Mo tor size, speed, performance curve, operator unit gear ratio and operator

efficiency in order to estimate the operator torque ou tput capabilities an d

to estimate stroke t imes and to compare to requirements.

- Operator rated torque and thrust for a specified number of cyc les.

It has been experienced that the outpu t thrust of som e operators has

exceeded the operators' nominal thrust ratings because they w ere set too

high or additional thrust is required to operate the valves. Due to the

overthrust operating conditions, a qualification tes t program wa s developed

and rnptememed by W estinghouse in order to increase the present nominal

ttvuft ratings of iheSMBOOO, SMBOO, SMB O, SMB-1, SMB-2, SBOOand SBO

Limitorque operators.

- Operator stall torque considering voltage variations. Th e stall torq ue is

whether obtained by calculation or from manufacturer shop tes ting, or from

in si tu bench test.

- The operator torque switch setting range and torque switch repeatability.



163



Inertia of the operator and the increase in thrust du e to torque sw itch

delay times are important parameters because they can be excessive large.

This information can be easily obtained during in situ testing of th e M O V s

by measuring thrust, motor current and torque sw itch close coil current.

Rate of Loading .

One of the conclusions of the NRC sponsored testing (4) (5 ) w as that s tem

factor changes with loading or loading rate resulting in less thrust being

delivered at higher differential pressures (slower load rate) for the sam e

torque switch setpoint.

While the changes observed w ere aH attributed to stem factor ch ang es, it

wa s recognized that some of the changes may be caused by differences in

torque output for a given spring pack displacement. The term Ra te of

Loading

has been used to categorize this variation in output thrust. Rate

of Loading ef fects o f about 2 0 % were measured.

The Westinghouse com pany m -M O VA TS has performed testing to study this

pnenomenum (7) and is currently performing further investigation on this

field.

2 .5 M O V recommended torque swi tch set ting

Whether the MOV will succesfully operate under the specified pressure and

temperature conditions depends on the relative importance of the required

operating thrust/torque, va lve maximu m allowable thrust/torque and the operator

thrust/torque capabil i ties.

It is the purpose of this task to point out th e upper and the lower bound of the

thrust and torque setting, to identify the margins to consider; and so to

determine the available thrust and torque setting range. A setting range which

wi l l assure proper and safe functioning of the MOV.

It is evident that the lower bound is determined by the maximum required thrust

and torque under the specified pressure conditions. The upper bound is the lesser

of the m aximum allowable valve thrust and torque, the operator rated thrust an d

torque; and operator stall torque at reduced voltage conditions.

The margins to consider on the upper and lower bound values are torque switch

repeatability, torque and thrust measurements errors, operator inertia, torque

switch delay t imes and Rate of Loading phenom ena.

Motor-Operated Valve Analysis Program to address the Oesign Basis Review

f, Requirements of the US NRC's Generic Letter 89 -1 0

I,

164



3.0 REFERENCES

(1 ) Westinghouse B v D report EM 56 72

Rev.

1 :

Motor Operated Gate Valveaosure

Problem F low Test ing Program, September 23 , 1982.

(2) NRC Buletin85<X3:Mo<a<3peratBdVate common n r ^

transients due to improper switch sett ings.

(3) NRC Generic Issue 8 7 : Failure of HPCI Stea m Line W ithou t Isolation.

(4 ) NURE G/CR-5406: BWR Reactor WaterCleanup System Flexible Wedge Gate

Isolation Valve Qualification and High Energy Flow Interruption Test,

O c to b er 1989 .

(5 ) NUR EG/CR -5558: Generic Issue 8 7 Flexible Wedge Gate Valve Test Program,

J a n u a r y 1 9 9 1 .

(6 ) NRC Generic Letter 8 9 -1 0 : Safety-Related Motor-Operated Valve Testing and

Survei l lance.

(7) mMOVATSEnoneeririg Report B ^ 5 .0 F ^ 3 : m MOVATS Equipment Accuracy

S u m m a r y , O c t o b e r 1 9 9 1 .



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Motor Operaed Valve Analysis