ORNLINRCILTR-91/25

Throttled Valve Cavitation and Erosion

Don Casada

Nuclear Plant Aging Program Oak Ridge National Laboratory Oak Ridge, Tennessee

December, 1991

NRC Technical Monitor: Bill Farmer /
ORNL/NRC/LTR-91/25

Throttled Valve Cavitation and Erosion

Don Casada

Nuclear Plant Aging Program

Oak Ridge National Laboratory

Oak Ridge, Tennessee

December 13, 1991

Research sponsored by the Office of Nuclear Regulatory Commission under Interagency Agreement DOE

1886402-81B with the US. Department of Energy under contract No. DE-AC05-840R21400 with the

Martin Marietta Energy Systens, Inc.

The submitted manuscipt has been authored by a contmrctor of the US. Government under contract No.

DE-AC05-840R21400. Accordingly, the U. S. Government retains a nonexclusive royalty-free license to

publish or reproduce the published form of the contribution, or aflow others to do so, for US. Government

pupose&
1. Background

In November of 1988, Brunswick plant maintenance personnel discovered significant

localized erosion of the valve body of a Unit I Residual Heat Removal (RHR) valve, 1-

El 1-F017B (the maintenance was being performed to repair the valve stem and back seat).1

The F017B valv'e is a 20-inch Rockwell angle globe valve that has been historically used to

throttle RHR flow. Excessive throttling of the valve had resulted in cavitation-induced

erosion damage to areas immediately downstream of the seat. Subsequent investigation

indicated that erosion of valve bodies was a generic concern for the other RHR valves used

in the same service (FO17A on Unit I and F017A and F017B on Unit 2).

Brunswick personnel expanded the investigation to include another set of RHR valves that

had also been used in throttling service. The F024A and F024B valves (from both Units),

which are used for Suppression Pool Cooling, were also found to have been damaged by

cavitation erosion. These valves aie 16-inch'Anchor Darling globe valves.

Testing of the F017 valves indicated that cavitation was most prevalent in lower flow

ranges, but existed throughout the range of 4,000 to 16,000 gpm. Testing of the F024

valves indicated cavitation present throughout the range of 4,500 to 15,000 gpm, and was

most prevalent at higher flow rates. It was also noted that the location of the cavitation

moved throughout the body as flow changed.

Subsequent investigation of seven other valves used in safety-related throttling service,

including Core Spray (CS), High Pressure Coolant Injection (HPCI), and Reactor Core

Isolation Cooling (RCIC) systems, revealed that one other valve, the HPCI system full

flow test isolation valve F008, had experienced notable erosion.

The Nuclear Regulatory Commission (NRC) issued Information Notice 89-01 in response

to this event. The Reactor Operations Analysis Branch also performed a review of

previous NRC reports and equipment failure data (Licensee Event Report and Nuclear Plant

Reliability Data System databases) and issued a technical report documenting the results.2

Because erosion is an aging-related concern, the NRC requested that Oak Ridge National

Laboratory perform an assessment of the significance of valve body erosion, with the

principal focus of the review to be the identification of valve types and applications

susceptible to erosion problems.

2. Valve Erosion Mechanisms

Four principal sources of erosion in valves have been identified:3

* abrasive particles,

* high liquid velocity impingement,

* erosion-corrosion, and

* cavitation.

I
Depending upon the nature of the service conditions, one or more of these erosive sources

may exist in nuclear plant valves.

Abrasive particles are primarily a concern in service and other raw-water systems.

Abrasion is not viewed to bea major source of concern relative to wall thinning, although

some plants have noted increased valve seat and disc wear problems due to high sediment

content of the water.

High liquid velocity impingement and erosive-corrosive wear occurs when high velocity

fluid impinges on valve or pipe surfaces. In the case of erosion-corrosion, the protective

corrosion layer that naturally forms is continually removed as it is formed. These

phenomena have been largely responsible for pipe and valve body failures that have

occurred in secondary plant (steam and feedwater) systems.

The last erosion mechanism, cavitation, is the principal factor of concern relative to the

wear of valve bodies and downstream piping in conjunction with throttled valve service.

Cavitation is a two-stage phenomenon involving:

(1) flashing of the liquid due to the pressure of the liquid dropping below the saturation

pressure, and

(2) subsequent collapse of the vapor back into liquid due to pressure recovery.

In a valve (see Figure 1), the pressure is reduced as the fluid passes through the minimum

flow area (and thus the region of highest velocity), which is typically near the seat. If the

pressure reduction is sufficient, saturation conditions can be reached, even for relatively

low temperature service conditions (<100°1F), and at least a portion of the fluid is

vaporized. As the fluid exits the seat area, the flow area is rapidly restored, and as a result,

a portion of the pressure drop is recovered. If the downstream pressure is greater than the

saturation pressure, the vapor pockets are rapidly collapsed back to the liquid phase.

L.

pvc2

PI, -- --- -. ... ... ..

%_J 11

PV ------- ,\< -

PVC

Figure 1. Pressure Recovery Profiles for Globe and Butterfly Valves

2
The pressure profile shown in Figure 1 indicates that the fluid pressure, initially at an

upstream pressure of PI, drops in pressure to Pvc as it passes through the valve's vena

contracta. PVC is greater than the fluid vapor pressure (Pv) in the case of the globe valve

(on left), while Pvc for the butterfly valve (on right) is less than the vapor pressure,

resulting in fluid vaporization. As the fluid passes on through the butterfly valve until the

effective flow area is larger, its pressure is recovered to P2, which is greater than the vapor

pressure, resulting in collapse of the vapor back to liquid phase. Note that the recovered

pressure for the butterfly valve, that is P2 - PVC, is greater than for the globe valve.

Butterfly valves, along with conventional ball valves, are commonly referred to-as "high

pressure recovery valves" due to the fact that the pressure recovered is relatively high

(compared to, for example, globe valves).

It is primarily in the collapse of the vapor that damage is incurred. Localized pressures as

high as 100,000 psi have been reported in conjunction with cavitation.3 It is theorized that

when the bubbles collapse near a metal surface, such as the valve body wall, a

"hammering" of the metal occurs which locally fatigues the material. Other explanations

have been proposed; however, regardless of the mechanism, the fact remains that local

,cavitation removes metal.

3. Cavitation Results

As noted above, cavitation can result in erosion of valve parts, including the valve body,

and for some valve designs, ,such as butterfly valves, the erosion can occur in downstream

piping. The location of cavitation damage is dependent upon a variety of factors, including

valve geometry and flow conditions.

Incipient cavitation is characterized by a relatively low-level "hissing" sound. Fully

developed cavitation is much louder and sounds like marbles or gravel flowing through the

system. The cavitation may also result in considerable system vibration, depending upon

cavitation level, how well the piping is supported, and other factors. System vibration

effects may be manifested in other active components, such as other system valves or even

the system pump(s).4 The vibration levels encountered under severe cavitation have been

high enough to result in a variety of vibration-induced failures, including drain/vent line

cracking, valve leak-off tubing failure, and loosening of limit switches, packing followers

and other valve components.

Cavitation erosion damage is characterized by a very rough, sometimes pock-marked

appearance (as opposed to the smooth wear patterns often associated with high-velocity

wear). The damage may be localized or may cover a relatively broad area (a few square

inches).

At Brunswick, the erosion was found immediately downstream of the valve seats. For the

angle globe valves (F017 set), the erosion occurred in two regions on either side of the

center guide just above the seat (flow comes from under the plug, and makes a 90° turn

above the seat). In the case of the straight globe valves (1024 set), the erosion was again

just above the seat (flow comes from under the seat); however, it occurred in several

regions around the full circumference of the seat

3
Erosion damage associated with cavitation from throttled butterfly valves may be

manifested in the piping downstream of the valve, instead of the valve body itself, due to

the minimal axial length of butterfly valves. Erosion damage associated with pump

recirculation valves (particularly for high energy pumps, such as main feedwater pumps)

may occur farther downstream (the result of high velocity impingement in some cases).

Erosion can eventually result in through-wall failures. These failures can progress rapidly

if a system transient results in a sudden hydraulic or mechanical load (internal pressure

spike, for example). Alternatively, the failures may show up as pin-hole leaks that can be

corrected before the consequences become more significant.

4. Valve Characteristics and Cavitation Susceptibility

The classical pressure drop and flow relationship for single-phase, non-compressible liquid

flow through valves is expressed as:

q=C, AP (1)

where

q = Fluid flow rate

C, = Valve flow coefficient

AP = Valve pressure drop

gf = Fluid specific gravity

Figure 2 provides a typical relationship between valve flow and pressure drop for constant

vapor pressure and upstream pressure conditions.

Kc PI - PFL=K=(2

C P-p ~ ~ ....... > P <

where:

PI = Valve upstream pressure Pv = Fluid vapor pressure

P2 = Valve downstream pressure Kc = Cavitation index

Pvc = Pressure at valve vena contracts FL = Valve pressure recovery factor 2

AP = Valve pressure drop Km= Valve recovery coefficient = FL

Figure 2. Flow Rate vs. Pressure Drop

4
As is the case for other fluid system components, the flow through valves is generally

proportional to the square root of the pressure drop. This relationship is valid so long as

the fluid is not vaporized. However, as the pressure drops below the vapor pressure,

departure from the relationship occurs. As can be seen, the departure from the square root

proportionality occurs at point a of Figure 2. The dimensionless parameter Kc, called the

cavitation index, which is calculated at this point, has been used to indicate this point of

departure. It provides a somewhat limited indication of the point of cavitation initiation;

measurements that have been iiade using acoustical and vibration instrumentation have

indicated that cavitation begins at pressure drops lower than that associated with the valve

coefficient departure from the characteristically linear relationship of flow and Vi.

A characteristic of importance when considering the susceptibility of a valve to cavitation is

the valve pressure recovery factor, FL (alternatively may be expressed as Km which equals

the square of FL), which accounts for the influence of the valve geometry on its capacity at

choked flow conditions, and is calculated at the intersection of the extension of the constant

flow line corresponding to choked flow for the given upstream pressure and the extension

of the square root proportionality curve (see Figure 2). Note that FL can be calculated

based on readily measurable parameters, except for the vena contracta pressure, Pvc.

Because the vena contracta pressure is not readily measurable, a relationship between the

vapor pressure, the fluid critical pressure, and the vena contracts pressure under choked

flow conditions has been derived, and can be expressed as: 5

Pvc = FFPv (3)

where:

FF = Fluid critical pressure ratio factor

FF can be predicted by the following:5

FF= 0.96-0.28 -Pv (4)

where:

Pv = Fluid vapor pressure

Pc = Fluid critical pressure

Equations 2 and 3 can be combined and rearranged to the following expression:

,Pm= F 2 (Pi - FFPV) (5)

where:

* Note that for most practical applications involving wate. FF is equal to 0.96, since «t vapor presure is

negligible compared to the critical poiure.

5
APm= The maximum rP for which increased flow results for a given upstream pressure

(i.e., corresponds to the point at which flow becomes choked)

Equation 5 is referred to as the Choked Flow Equation. It has been used to determine

allowable pressure drops for valves to avoid cavitation damage. The BWR Owner's Group

has established a criterion of valve AP of < 0.8 * APm as the first cut in a set of evaluation

criteria for valves used in throttled service. If this criterion is not met further monitoring to

detect the presence and extent of cavitation is required. Based on conversations with utility

personnel, this has proved to be a reliable criterion for globe valves, although it was noted

that cavitation conditions can exist even if the criterion is met.

Typical valve pressure recovery factors are:5

Valve type FL

Single port globe 0.9

Contoured plug angle 0.9

Segmented ball 0.6

90-degree offset seat butterfly 0.6

It is important to note that high pressure recovery valves have low pressure recovery

factors. It can be seen from equation 5 and the above pressure recovery factors that globe

and angle valves can be operated with greater pressure drops than can ball or butterfly

valves. More specific information on allowable pressure drops for different valve types is

provided below in Section 5.

5. General Valve Type and Material Considerations

5.1 Valve Types

It was noted in Section 2 that some valves, such as butterfly and ball valves, are referred to

as "high pressure recovery valves" due to the fact that a significant portion of the pressure

drop that occurs from upstream of the valve to the valve's vena contracta is recovered in the

discharge.

As can be seen from Equation 5, the greater the pressure recovery factor FL, the greater is

the maximum AP. To illustrate the effect that the type of valve has on the maximum

allowable pressure drop for the BWR Owner's Group guideline of 0.8 * APm, consider the

results provided in Table 1, which are based on representative valves from a commercial

valve supplier's valve sizing program.6 Conditions that are typical of valve applications

for emergency service water (ESW) and RHR valves are used.

6
Table 1. Comparison of Maximum Pressure Drops for Valve Types

System Valve Type % Open APm APaut/APm

ESW Globe 69 73 0.68

Butterfly 19 57 0.88

Full-Bore Ball 29 65 0.77

Q-trim Ball 43 74 0.67

RHR Case 1 Globe 66 227 0.66

Butterfly 18 178 0.84

Full-Bore Ball 25 201 0.75

Q-trim Ball 39 232 0.65

RHR Case 2 Globe 71 225 0.67

Butterfly 21 175 0.86

Full-Bore Ball 31 199 0.75

Q-trinm Ball 46 227 0.66

*0ap = Actual aP

The assumed conditions for Table 1 are:

Application q, gpm d, in. Pi, psia P2, psia T, SF

ESW 5000 16 80 30 80

RHR Case 1 3000 10 250 100 150

RHR Case 2 10000 16 250 100 150

'It can be seen from Table 1 that the maximum allowable pressure drops across simple ball

and butterfly valves are less than those for globe or Q-trim ball valves (which have a unique

valve trim which provides several stages of orificed pressure drop, thus helping avoid

cavitation problems). Note that, in the case of the conventional butterfly valve in all three

applications in Table 1, the actual pressure drop exceeds that suggested by the BWR

Owner's Group as the limit for purposes of requiring further investigation.

In order to provide a perspective on the significance of upstream pressure, Figure 3

presents a plot of 0.asJAPm for a spectrum of pressure conditions for four valve types.

The comparative abilities of the valve types to handle the pressure conditions can be readily

seen.

7
1.0

0.9 Q-Trinm Ball n

BmlR ownees Grop F~~~ast Cut CHImeion e

CE z

s0.72 o

/ ~~~~~~~~~~~~~AssuondConditions:

0.6 ' I Isia = 

60 70 80 90 100 110

Pi (psia)

Figure 3. Fraction of AP. for Various 16-inch Valves as a Function

of Upstream Pressure

5.2 Valve Materials

In general, harder materials are more resistant to cavitation-induced erosion damage.

However, hardness is not a perfect indicator of susceptibility to erosion, and a variety of

factors have been reported to affect erosion resistance, including alloy composition and heat

treatment. Table 2 provides relative cavitation resistances of several materials used as

valve plugs under cavitating conditions (the higher the number, the more resistant the

material is to erosion damage). The resistances of Table 2 are referenced to 316 stainless

steel.

Table 2. Resistance of Materials to Valve Plug Cavitation Erosion 3

Mate ki

Stellite No. 6 over Type 316 stainless steel

316 stainless steel

Cast iron (A126 Class C)

Carbon steel (WCB)

Carbon steel (AISI C1213)

Brass

Aluminum

Relative Resistance

20

1.00

0.75

0.38

0.17

0.08

0.006

8
The erosion rates of several materials (relative to type 308 stainless steel) subjected to a

cavitating jet are provided in Table 3. Note that for Table 3, the lower the value, the higher

the cavitation erosion resistance.

Table 3. Resistance of Materials to Cavitating Jet Erosions

Maial Relative Erosion Rate

316 stainless steel 0.8

304 stainless steel 0.7

Stellite 21 weld overlay 0.1

308 stainless steel weld 1.0

Carbon steel 1.6

Aluminum Bronze weld 3.7

The cavitating jet erosion rate of some intermetallic compounds of nickel aluminide has

been reported to be 4 to 10% of that of 308 stainless steel weld material; however, the

abrasive wear resistance (using dry sand) of 304 stainless steel has been reported to be

superior to that of nickel aluminide.9 Abrasive wear resistance would primarily be a

concern for nuclear plant applications only when considering the adoption of erosionresistant materials in service water systems which have a heavy silt level. Some iron-based

cobalt-free alloys, developed specifically to provide utilities with weld overlay alternatives

to cobalt containing alloys for use in nuclear applications (and thereby reduce radiation

levels resulting from 60Co), have been shown to have relative cavitating jet erosion rates of

14 to 19% of 308 stainless steel.10

6. Historical Failure Experience

Failure data from the Institute of Nuclear Power Operation's Nuclear Plant Reliability Data

System (NPRDS) database was reviewed to provide at least a qualitative indication of

historical erosion and cavitation problems. Two separate sets of failure data were

compiled. The first set was for through-wall failures of valves and pipes in all NPRDS

f systems, and the second set was for failures in the RHR systermi. Discussion of the results

follows.

6.1 Review of Through-Wall Failures

A review of the NPRDS database records of through-wall failures of pipes and valves in

which erosion' was indicated as the cause of the failure was completed. The results of the

* The search was conducted by acquiring those failure ccori for which the narative descnipon included the

term erosion (or oxfe words based on the root word erode). Failure records which involved body-to-bonnet

or packing leaks were manually eliminated during the review process. It should be noted that some known

erosion related failures of valves did not appear in this type of search, due to the use of a more generic term,

such as iwar. to describe thea

9
search and review are provided in Table 4, which indicates the distribution of reported

degradation and failure, by system. Half of the valve failures and almost three-fourths of

the pipe failures occurred in the Service Water system. Many, but not all, of the failures

were clearly related to valve throttling. Note that this table is not a tabulation of all

cavitation or pipe/valve body erosion related problems from the NPRDS data, but only a

listing of those which resulted in external leakage due to body/pipe failure. It should be

recognized that the number of failures reported should not be considered to be an accurate

quantitative representation of actual historical experience. In fact, discussions with a utility

valve expert indicated that his utility alone had probably experienced as ntany through-wall

failures as those shown below. However, the distribution of failures provides a relatively

accurate indicator of failure distribution among systems, valve types, etc.

Table 4. Through-wall Pipe or Valve Body Failures

System Valves Pe

Component Cooling Water (CCW) 0 1

Condensate/Feedwater 4 30

Chemical Volume & Control 1 2

Service Water (SW) 7 90

Main Steam 2 2

Total 14 125

Nine of the 14 through-wall valve failures occurred in globe valves. It is important to

recognize that erosion induced by the throttling of valves (and particularly butterfly valves)

would often be manifested in the piping downstream of the valve, and not in the valve body

itself. Many of the pipe failures recorded in the data were clearly associated with throttled

valves, although the valve type was not identified. For some of the service water system

failures, the erosive/corrosive properties of the water (as opposed to, or in addition to valve

operating conditions) were also cited as factors, particularly for plants with high silt content

water.4, Clearly, the systems of primary concern are condensate/feedwiter and service

water. Almost 80% of the valve failures and over 95% of the pipe failures which resulted

in through-wall leakage occurred in these two systems.

'There were no reported through-wall failures of the front line safety systems, such as high

or low pressure coolant injection/residual heat removal, which were directly attributed to

erosion. Probably the'most significant factor in the absence of through-wall failures in

these systems is the fact that they are typically operated only a small fraction of the time.

6.2 A More Detailed Review of Cavitation-Related Valve Failures

In recognition of the fact that all valve cavitation problems would not necessarily result in

through-wall erosion, a qualitative review of a portion of all valve failures in the NPRDS

database was performed, and discussions with utility personnel were held to identify the

10
valve applications most susceptible to cavitation and erosion problems and the ways in

which the problems were manifested. The discussion in Section 7 provides information on

the valve applications observed to be most often affected. Examples of symptoms of

throtled valve cavitation are:

* Erosion of the valve body, seat, obturator, guides, cage

* Vibration induced cracks in valve packing leak-off lines or other small-bore lines (such

as vent/drain lines or instrumentation tubing)

* Loosening of valve packing follower bolts, handwheel key set screws, limit switch

attachment screws, and other threaded fasteners

* Excessive fluttering of downstream check valves

It was determined that a review of all the NPRDS failure data for valves to assess the extent

to which these symptoms were present would not be feasible because of the extensive

number and variety of valve failures in the NPRDS database. As an alternative, it was

decided that a more detailed review of all reported valve failures in a single system would

be conduct&L. The RHR system was selected for three reasons:

* it is a critical safety-related system,

* the erosion identified at Brunswick was in the RHR system, and

* it is operated more often than the other standby safety-related systems (though not as

much as some of the normally operating safety-related systems such as service water).

6.2.1 RHR System Functions

The RHR system at most BWR and PWR plants is designed to be used for a variety of

functions. The system is most frequently used in support of decay heat removal during

plant shutdowns. At least one train is normally in service at all times when the plant is in

the hot or cold shutdown condition or refueling.

The RHR system is also used at most plants to provide the engineered safety feature

function of low pressure injection of water to the reactor in the event of a loss-of-coolant

accident (LOCA). It may also be used for post-LOCA recirculation of containment sump or

suppression pool inventory to the reactor. -For some plants, it may also lbeused for other

functions, such as containment cooling, suppression pool cooling, etc. Finally, the RHR

system is periodically tested at flow rates ranging from recirculation only (in some cases,

this is less than 10% of pump best efficiency point flow) to flows rates exceeding the

design basis flow rate.

6.2.2 Distribution of Throttling Valve Types in the RHR System

A search of the NPRDS database to determine the extent of use of two commonly used

throttling valve types, globea and butterfly, in the RHR system was conducted. The

numbers of these valve types ranging in nominal size from 4 to 40 inches are provided in

Figure 4.

11
It is noteworthy that the General Electric (GE) plants have more butterfly and globe valves

in service in the RHR system than at the other three nuclear steam supply system (NSSS)

plants combined. In fact, there are more globe valves in GE plant RHR systems than globe

plus butterfly valves at the other three NSSS supplied plants combined. Because Babcock

& Wilcox (BW), Combustion Engineering (CE), and Westinghouse (W) plants all use

borated water in the RHR system, most of the RHR valves have either stainless steel

bodies or are stainless steel lined. ,Most of the RHR valves in GE plants are carbon steel.

The material of manufacture is important from an erosion standpoint, since stainless steel

is, as discussed in Section 5, more resistant to cavitation damage than carbon steel.

500-

400- ____

300 - _ _ _ _ _ _ _

200-

100- u

EJ Carbon Steel

Butterfly

a Carbon Steel a Stainless Steel

Globe Butterfly

l Stainless Steel

Globe

Figure 4. Number of Globe and Butterfly Valves Used in the RHR-System

6.2.3 Discussion of RHR System Valve Failures

Brunswick is a BWR plant, and the valves identified at Brunswick as having incurred

throttling-related erosion damage are routinely opmated in a throttled position at other BWR

plants as well. Some valves in the RHR system at PWR plants are also routinely throttled

(such as the heat exchanger outlet and/or bypass valves). Since, as noted in Section 6.1

above, there are no recorded thuxmugh-wall failures of valves in the RHR system, all failures

i of RHR valves were reviewed for other manifestations of cavitation damage, such as

erosion/wear related degradation of internal valve parts and problems related to vibration.

Failure reports from the NPRDS data for four-inch and larger valves used in the RHR

system were individually reviewed for failure symptoms similar to those discussed

previously. The results of this review follow.

12
A total of 83 failures of globe or butterfly valves which appeared to be at least partially the

result of cavitation were found. The results, by NSSS vendor and method of discovery,

are provided in Figure 5. The number of reported valve failures at GE plants is

substantially higher than for the'PWR plants. This is not unexpected, in light of the larger

population of valves (see Fig. 3). Also, note that the principle means of failure detection at

BWR plants was by leak testing. Since the extent of leak testing performed at BWR plants

is typically greater thin that at PWR plants, this particular result is also not unexpected.

30-

20 -

0--

BW CE GE W

* Operational Abnormality 0 Other Testing/Inspection

M Lea testing IN Maintenance 0 Unknown

Figure 5. Cavitation and/or Erosion Related Valve Failures in the

RHR System: Methods of Detection

The principle failure mechanism identified for these failures is indicated in Figure 6.

Clearly, erosion and wear are the dominant problems found among the valve failures. It

should be recognized that some vibration problems might result in failures of other

components, such as auxiliary piping, pipe supports, etc., which were not considered in

this particular part of the review.

50

g 40

o 3

0

H 3

, * Erosion/wear a Vibration

Figure 6. Cavitation and Erosion Related

0 Other

Failures by Mechanism

13
7. Valve Application Considerations

Most valves used in nuclear power plant systems are normally positioned in either the full

open or full closed position (i.e, not throttled). In most cases, these valves would not be

subjected to the erosive mechanisms previously discussed. Certain system conditions,

such as high fluid velocity and/or operation near saturation could result in cavitation even

for fully open valves. For valves that are normally fully closed, a slight seat leak,

particularly for steam system or operation near saturation, can result in steam cutting that

progressively worsens. This is not uncommon for steam isolation or pressure relief

valves.

There are relatively few valves that are normally operated in the throttled position. -Those

valves that are throttled are primarily those used for flow, pressure or other control

purposes. Examples of commonly throttled valves are:

* Outlet and/or bypass valves for heat exchangers such as:

* CCW heat exchanger (SW side)

* RHR heat exchanger (on both the RHR and CCW or SW sides)

* Emergency diesel jacket water cooler

* Containment fan coolers (located outside of containment, and throttled to maintain

line pressure greater than containment pressure following a LOCA

* Main generator hydrogen gas cooler (non-safety)

* Main turbine lube oil cooler (non-safety)

* Chemical and volume control system control valves, such as letdown pressure control

valves

* Pump test line valves for systems such as:

* High Pressure Coolant Injection

* Low Pressure Coolant Injection

* Core Spray

* Steam generator blowdown flow, pressure, and flash tank level control valves

* Secondary system level control valves (e.g., for the condenser hotwell and heater drain

tank)

* Feedwater control valves

There are variations in throttled valve applications from one plant to another that are

controlled by such factors as system pump capacity, pipe sizing, and the use of other flow

control devices (such as orifices). Some noteworthy areas of variation are:

* The valves used in a given application may be either manually or automatically

operated. For example, heat exchanger outlet and bypass valves may be automatically

temperature controlled at one plant, while the equivalent valves at another plant require

manual positioning.

* Valve applications that require throttling at one plant may not require throttling at

another. For example, the emergency diesel jacket water cooling outlet valves

(normally a service water cooled heat exchanger) are normally throttled at some plants

and full open at others, depending upon overall system balance.

14
* The type of valve used for a particular application varies. For example, butterfly valves

are used as control valves for component cooling water heat exchangers at some plants,

while ball valves are used at others.

* The extent of temperature swings of open systems (such as service water) has a

substantial impact on the amount of throttling that is required. Plants located in milder

climates see less variation in service water temperature, thus requiring less flow

adjustment during the course of the year to maintain proper temperature control of

systems cooled by the service water.

* Design provisions are made at some plants to minimize control problems, such as the

use of parallel lines of varying sizes to provide flow control without excessive throttling

of any valve throughout a broad range of flow rates, while others depend totally upon

individual valve throttling.

* The amount of entrained air can have an effect on the severity of cavitation problems.

All else being equal, a system containing water which has been aerated (for example, a

service water system drawing suction from a cooling tower basin) will experience

fewer cavitation problems than a relatively de-aerated system (for example, service

water system drawing its suction from a lake).

* Upstream pressure, particularly for open systems, can play a significant role in service

conditions, as can be deduced from Figure 3.

The extent of throttling required for a given valve can also vary substantially, depending

upon operational demands. For instance, the outlet valves on the CCW (or SW, depending

upon plant specific design) side of RHR heat exchangers which require little throttling

shortly after shutdown when decay heat levels are high may be substantially throttled

weeks (or months, for a protracted outage) later when the decay heat level has diminished.

Figure 7 shows the extent to which the CCW side outlet valve on an RHR heat exchanger

was found throttled during a visit to a plant that was in a protracted outage. As can be

seen, the butterfly valve is throttled to about 10% open (there was significant flow through

the valve - over 2000 gpm - when the picture was taken).

The system/valve applications that are most lily to experience significant erosion

problems are those that are routinely operated under throttled,-cavitating conditions. This,

in part, helps to explain why there have been significantly more through-wall failures in the

service water system than in other systems. Even though valves in some other systems

may see harsher conditions when the system is operated, such as the RHR valves at

Brunswick and other plants, the fact that the system is normally not in operation

substantially reduces the effective rate of degradation. Some valves operated under

particularly severe conditions, such as feedwater recirculation valves which normally see

flow only briefly during plant startup and shutdown, may experience failure in a relatively

short period of time (i.e., weeks or months) if operated under continuous flow (during

protracted testing, such as power ascension testing, or inadvertently, due to seat leakage).

The same valves would remain intact for several years when used infrequently (which

would normally be the case).

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Figure 7. A Heavily Throttled Butterfly Valve

8. Corrective Actions

Substantial cavitation-induced vibration and some erosion of valves used in certain

applications has been experienced. As a result, some plants have taken or are talking

actions to eliminate or minimize the problem by the use of anti-cavitation or cavitation

control trim. These types of trim are designed toward minimizing cavitation and/or

directing the cavitation such that it does not occur in close proximity to the valve material

surfaces.

Cavitation can be minimized by dropping pressure in stages by directing flow through a

series of restricted paths. A comparison of the pressure profiles of a multi-stage valve and

a single-stage, high pressure recovery valve are shown in Figure 8.

16
\P_ 1 P2

PIS - - - - - - - efsg - - - - -

Single-stage, trim

Py-ti-stagc trim

Multi-stage trim

Figure 8. Pressure Profiles for Single and Multi-stage Trim Valves

IFor the multi-stage trim valve, the pressure breakdown occurs in steps, with minimal

pressure recovery. As a result, the minimum pressure occurring in the valve is greater than

for the single-stage design, resulting in less susceptibility to cavitation.

An alternative, or supplemental design feature that can reduce the consequences (if not the

severity) of the cavitation is to change the valve trim design such that the cavitation does not

occur in the immediate vicinity of the valve body or seat. Such a design may use a trim

with multiple opposing orifices such that the flow from one orifice collides with that from

the one diametrically opposite it, thereby avoiding impingement on valve metal surfaces.

The use of erosion resistant materials is another design feature that can minimize the erosive

effects of cavitation. Many valves originally installed in services that were expected to be

rather harsh utilized relatively erosion resistant materials; however some did not. For the

latter, use of improved materials may be used to extend valve life. In addition, piping

downstream of the valves may be a candidate for replacement with improved materials.

The installation of smaller bypass lines around valves avoids operation that would

otherwise involve extreme throttling under some modes of operation. This system design

feature is commonly used in main feedwater and steam systems to provide better control

during startup evolutions. Some plants have implemented this type of design (often with

an orifice installed in the bypass line to reduce the burden borne by the valve) to alleviate

cavitation problems.

Another type of system design change which has been used has been the use of two valves

in series to distribute the energy dissipation.

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-4L

One plant contacted had added globe valves downstream of existing butterfly valves in the

containment fan cooler return lines. Previously, the butterfly valves had been throttled,

with significant cavitation and erosion resulting. The globe valves are now being throttled

(to maintain pressure in the lines greater than hypothetical containment pressure following a

LOCA) instead of the butterfly valves, with significantly reduced or non-existent cavitation.

In conjunction with the types of design corrective actions noted above, or in some cases as

a stand-alone measure, changes in administrative controls (e.g., operating procedures) are

made to ensure that valves are not throttled inappropriately in order to minimize valve

damage.

9. Summary

Based on a review of historical operating data, discussions with utility personnel and onsite observations, it appears that some valves are operated under conditions that are beyond

the intended design use of the valve (for instance valves that are throttled beyond what the

vendor recommends for continuous service). The result is that the valves and the

associated system reliability is less than desired. The number of reported through-wall

failures relative to the number of valves used is small. In most cases, the through-wall

failures have been manifested as pin-hole leaks which, from a rate of leakage standpoint,

are no more significant than gross packing or bonnet seal ring failures. However, where

there has been sufficient erosion to result in a pin-hole leak, the general integrity of the

valve or piping may be questionable.

Changes in valve and pipe materials can minimize the erosion rate of valve trim and bodies

and downstream piping. However, it is important to recognize that erosion is not the only

negative result of valve cavitation, and in fact may not be the principal concern. Cavitationinduced vibration can have a negative impact on not only the valve, but its operator,

adjacent components and piping supports. Changes to and-cavitation or cavitation control

trim, modification of system design, and/or implementation of administrative controls can

substantially mitigate these consequences of severe duty applications.

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References

1. Letter from J. L. Harness to the U. S. Nuclear Regulatory Commission, Brunswick

Steam Electric Plant Unit 1 Supplement to licensee Event Report 1-88-033, April 28,

1989.

2. Technical Review Report AEODIE908, September29,1989.

3. Instrument Society of America Handbook of Control Valves, 2nd Edition, 1976, J.

W. Hutchison, editor. Published by the Instrument Society of America.

4. J. Ozol and M. Horbaczewski, "Pump Full-Flow Test Valve Cavitation Problems and

Their Solutions," EPRI Third Valve Symposium and Equipment Exhibition, pp. 395-

429, August 20-23, 1990.

5. Instrument Society of America, "Flow Equations for Sizing Control Valves:' Standard

S75.01, 1985.

6. Neles-Jamesbury Nelsize, version 2.05U, February, 1991.

7. A. Akhtar, A. S. Rao and D. Kung, "Cavitation Erosion of Stainless Steel, Nickel and

Cobalt Alloy Weld Overlay Materials," Coatings and Bimetallics for Aggressive

Environments: Proceedings, Conference on Coatings and Bimetallics for Energy

Systems and Chemical Process Environments, American Society for Metals, pp 125-

142.

8. P. A. March, "Evaluating the Relative Resistance of Materials to Cavitation Erosion: A

Comparison of Cavitating Jet Results and Vibratory Results," Proceedings of the 1987

ASME Cavitation and Multiphase Flow Forum, FED-Volume 50, American Society

of Mechanical Engineers, 1987a.

9. M. Johnson, D. E. Mikkola, P. A. March, and R. N. Wright, 'The Resistance of

Nickel and Iron Aluminides to Cavitation Erosion and Abrasive Wear," Wear, Volume

140, pp. 279-289, November 1990.

10. P. A. March, "A Preliminary Assessment of the Cavitation Erosion Resistance for

EPRI/AMAX's NOREM Alloys," Report No. WR28-4-900-233, TVA Engineering

Laboratory, January 1989.

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