ML19308C230

Pressurizer Liquid Level Measurement by Differential Pressure Method, Tech Guide VII.C.1

ML19308C230
Person / Time
Site:	Crane
Issue date:	03/08/1971
From:	Harold Denton, Vaughn Thomas NRC OFFICE OF INSPECTION & ENFORCEMENT (IE), Office of Nuclear Reactor Regulation
To:
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ML19308C225	List: ML19308C224 ML19308C230
References
TASK-TF, TASK-TMR NUDOCS 8001210563
Download: ML19308C230 (18)

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S. ATCMIC ENERGY CCMi!SSION l

DIVISION OF COM.PLIACE

.I TECHNICAL GUIDE NO. VII.C.1 t

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II SUK'.ARY...................................................

4 III DETAILS...................................................

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General D es cription...................................

3, Instrument Channel Testing and Calibration E.

4 Requirements........................................

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6 Tc perature and Steam F. cad Effects....................

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Calib ration Pr o cedur es...............................

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S e t Po in t S e lect ion...................................

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Concluding Observations and Coc=ents..................

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A Figure I

- Pressurizer Level and Pressure Transmitter 14 Lines and P:cs sure Taps.........................

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Figure II - Actual Vs Indicated Pressuri cr H 0 Level.........

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Figure III - Tc=perature and Stea= Eead Effects on Indicated 16 P res suri:cr **'ater Level.........................

17 Fi:ure IIIA Pres surizer *Jater Level Correction Curve Data.....

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I CF.0D"0T10N I.icuid level n.easure=ents in a vessel which operate under elevnted '

pressure and tenperature conditions are difficult.

The perfor=ance re-c.uirc:en:s of accuracy and response ti=a at operating conditions of high

c :perature and pressure i= pose restrictions on the type of level de:cetor thc: can he used. A generally accepted =ethod is the use of dif ferential pressure =easure=ent.

Pressuricc water level =casurerents on P W s such as Connecticut Yankee end San Onofre c= ploy the dif ferentici pressure ec: hod. T if f erential eressure trans=itters serve to =easure the vn:cr level in the pressuri:cr.

'--aca used in this canner both pressure chanbers of the trans=it c: connec:

to :h pressuri:c: in such a way that the transmitter detects the dif ference cf height be veen tne water in the pressuri cr and a reference water colu=n.

A.s the v :er level ecves froni the full position, the differential pressure across the :rans=itter body is increased. This differential pressure is thcn eenver:cd to a preportienal electrical signal which is 'trans=1::cd to en indica:or or recorder in :he control roc =.

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Tr.c cbservatica and ce==ents in this guide are li=ited to the water level neccure=ent syste=s that e= ploy the cold reference colunn installation.

Inic ins tallation is the generally accerted =cthod of detecting water level in p cssuri:crs.

In addition, the observations cnd cc==ents are only j ;

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directed to reacter plants which provide the "two of three" control logic for dacir protection systc=s.

When this type of -installation is inspected, the most significant re riew points are piping arrangenent and calibration procedures.

Specific review items are listed below to supplement physical and ciectrical separation require:ents in the application:

1.

Obtain temperature infor=ation of the cold ref erence column.

Since all calculations and calibrstfon information is based on the assu=ption t ha t the reference colu=n temperature is 120*T, it is significant that one knows it. is being maintained.

Further, ' this tc=perature infornation provides the operator with additional infor=ation for deter =ining if less of water level in the reference cole =n occurred.

2.

Separate reference colu=ns should be provided for each pressurizer water level instru=entatica channel that is part of the reactor pro-te: tion syste=.

Adopting this type of installation helps to insure that the required physical separation of redundant channels is main-

ined.

This type installation is necessary for preventing the less cf protective function as a result of a single fcilure of a reference 01'*00.

3.

The method for deter =ining that the reference cele =n is filled with vnter during power operation should be examined.

This che:k should i.

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also be included in the instru=ent channel eclibration procedures.

1 Subscouent periodic check.s of the column water level should con -

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j tinue between channel calibration intervals.

A vorhing method for i

cb taining reference colu=n water IcVel infor=ation is discussed within the guide.

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4.

Several reactor plants have adopted additional precautionary steps during reae:or operation, for determining the adequacy of water level H

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in the ref erence column.

Typical of these steps include the following:

a)

'lisual er.arination of the piping installation for evidence of leaksge.

This check is done on a veckly basis.

b)

Channel drif t limits from two redundca: channels requires an cppropriate corrective action be taken.

~his action can be in the form of conplete instru=ent channel calibration.

L 3.

Lac =cn ation should be reviewed which indicates the instru=entation i

channel has been tes:cd as a co=ple c =easuring sys:c=.

Tnis er.acina-4 tien rheuld verify that :he channel transnit:ct was used as part of the i

I neaturing sys:c= when the system's hysteresis, repeatability, and set noint dead band (reset) are being observed and documented.

i e.

Tne level (; of calibrated span) ct which se: peints are being scle :cd to actua:e should be ex =ined.

Tne piping installation and test da c i

csso:icted with the se -point under studv should be included in the c:= na tion.

}'. ore de:cils relating to this ite: is' discussed within l

his guide.

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General Descrittien Prescurizer level is centinuously indicc:ed in the con:rci roca bv three separate va c: level instrument channels.

A typical channel of the three water level instru=ent channels censists of a level transmit:cr (d/p cell), power supply, a liquid level indicator (0-100% calibration), and several trip bistables whose functions ara:

(1) to previde a "All F.od Scra=" when a prese: high pressuricer va c

_ v:1 : ndi:icn is reached, (2) a safe:y inj ecticn syc:ce ini:1stien if icv precruricer vater level condition er.ists coincident with lov pressuri:cr pressure, (3) "All Pressuricer Heaters Of f" and (4) "Lc:down Sys:c; Ic _ _ : c;" when icv pressurizer va:cr level is reachef.

A tv0 pen re: Order loca:2d in the control roca pra/Mes water leve2 infor=ction in per=anent c::: f o rm.

3.

Ins:rument Channel Testin: and Calibra:icn Eccuirements Tn: prcs suriner level inc llations are very cinilar f rc= reactor pl:n: :: reactor pl:nt, but the =cthod and frequency of channel calibration

fif f er significantly.

For example, doeuran:ction revealed that one re :::r plan eclibrated the protection syster three tiece during the period.berveen refu211:gs while another plant perfor=ed this task caly durir: refueling.

Since nc technical specifica: ions of both plants require ch:nnel calibrc:lon " : each planned refueling outage" neither operatica ve c in nen:enfo r=ance.

HovcVer, docu=cntation fro = the less frec.uen:1v.

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-S-calibrcted prote:: ion sys:c: revealed that a period cf greater than 2-1/2 v ca r: ha: clapsed between channel calibration.

This is contrarv to good pl c.n: cpcra: ion procedures.

A scr.crally accepted practice is to perfor= instru=ent channel calibra:ica of the reac:cr pro:ection system during the reactor's extended thu:de n periode.

Eevever, all set points are tested usually at cir. Week intervcis in accordance with the Technical Specifications recuirements.

/ n irt :n:nen: channel tes t, consists of applying a sfrulated test sient.1 to

he inpu; cf the trip bis table located in the control ro0=, and documenting

.n : ::in: :: whi:h :he : rig function actuates. An ins tr=cn: chennel tect

ces nc include the sensing device (d/p transmitter) of the instru=ent c rc c l. A review of several instr =ent channel calibrr. tion procedures

ve:le d tha: the instr =cnts: ion is not being calibrated as a co=plete n r. n.r i n : systen, i.e.,

the transnitter's output signal being applied to the rensining conponents of the instru=ent loop (bis tables, indicators, r::::: crc, centrollers, etc.).

O=itting :his importan: step in calibration trace:ures does not enhance reliability of the cenpletc =casuring syste=.

4.

Figure I functionally illus tra:cs the typical unter level =casure-nen ins::llatien provided at several reactor plan:s such as Connecticu:

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Te=ocrature and Scen= Head Ef fcets J

q Figure I shows the dis:ance between the upper and lcr.rer pressure i,

- ps as 32.5' or 390" which is also the enlibrated span of the level 1

necsuring syste.

The reference colu=n (cold leg) is at a constant 1:hient tempera:ure of 120*F vhile the variable water eclu=n is at an opera:ing tecnerature of 640*F.

Generally, (see Figure I) three of the four icvel channels are tenperature compensated.

If the level trans-

.itter: cre not Octeerature conpensated, the measuring systen would be in error (:rans=i ter cu:put no: proportional to licuid level) at opera-i ting conditions.

For en =ple, the distance between pressure tap: is 390" l

and the specific gravi:y of 640*F vater (variable va cr colt =n) is.617, the weight.cf water on the lov side of the transmitter is 390" x.62 or 4

242" of cold va:er.

Tne high side of the trans=1::er vould have the weight cf v :c: of the reference colunn i.e., 390" 1.0 (specific gravi:y of water f

a: 120*F) or 390" of cold va:cr.

The difference between the reference I

colun. and the variable cole =n is therefore 390" eines 242" or 148" of cold va:cr.

Inis dif ference of weight between ecle=ns is what is factored in the calibratica of :he level channel and referenced as being ":c=pera-ture censensated."

Another calibration f cetor that cust be incorporated into the 1

liquid icvel =ecsure=ent c:libration procedures is the weight of the st"-

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I head bc:veen the upper cnd lover pressure taps of the installation.

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i-t 9.c dir,tence being 390" and the weight of ste:= at 6.'.0* r is the vapor I

dcncity..,, or 390" x.039 vhich is ec,ual to 34.7" of cold va:cr.

Inis

• 6 cit:erence of weight =ust be included in the enannel calibration; i.e.,

3 %-30.7

353" E 0.

lhe equivalent air pressure (353" x.0361 psi) 2 sig:n1 of 12.7 psi vould be applied to the high side of the differenrial tra:scitter.

Both co=pensation factors described above are included in the

.librction procedures of the pressurizer level channels.

Inis is a tva

in:

cro vnter level and full vater 1cyci) calibrc: ion and only correct

crur::ing cenditien:.

If :he water (variable icg) vae ~attined at c

i:ferent operating tenperature, the actual va:c: level vould always bc ciff :c:.: than :he indicated.

'r, hen operating under these conditions a f.-r_y :f tc=percture and s:can conpensation curves should be provided for

he :pera:c: so thc: he can de:cr=ine the correct water level.

Figure II illustrates this difference between actual (compensated) it.;1:ated (caco pensated) water level =casurc=cnts c ploying the ccid

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t f o r en:c colt =n = thod.

• he uncompensated channel vill always indicate 1c r c.

n_ n the three redundant (conpensated) protection system channels wh:n :ne pressurized va:cr icvel is high or near full, cnd indicate ni7 c

.+.en the vc cr icvel is lov or near =cro.

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D.

Calibrction Procedures Genernlly, the pressurizer water level reasurc=cn: syste: provides a carect rea c-o u t.

Increasing water level causes an increasing electrical

1 1:1 cu:put (10 to 50 =illienpers) to a 0-1001 calibrated secle indicator 5

in the ::ntrol room.

The celibration procedures to compensate for the te:perature and s:cc: head errors that exist are as follows:

1.

Apply a pressure of appror.iestely 14S" H 0 or an eauivalent air pressure of 5.34 psi to the dif ferential pressure transmitter high

'da

--> adjus: (the ::ansmitter) for a 50 millic=per (=a) ou:put.

This s:cp co=pensatas for difference in water density of the vari-able and ref erence eclu=ns at 640'F and 120*F, respectively.

2.

/.pply c pressure equivalent to 353" d,0 (12.7 psi) to the trcns-I nit:cr high side and adjust the transmitter for a 10 ca (0 on readout device) output.

This step cc=pensates for the error introduced by the vciph: of s:can head of 390" :: 640*F.

As previcusly ten:1cned, one arca of concern with this type tas teilc:icn (cold reference leg) is the errer bc:vcen actual and indi:::cd when no: operating at calibrated conditicas.

Figure III illustrates this d;fferen:e.

~hree temperature curves (640*F, 500*F, and 120*F), with calcula:ien to demonstrate the =2thod for correc:ing the difference, are rhevn.

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E.

Set Point Selection Scle::1:n cf set points is very i=per: ant.

Docunen:ation a several rest::: plants shov thc the trip points, in some instanets, are required

c epcrc:e a: < 2% of calibrc:cd span.

This is poor practice.

If a trip at:1cn is sele :ed at :he entrc=c ends of the calibrated span, the trip cys :c= : nno: : lerate =uch c ror (deviation) in the nensuring syste=.

This dev-. :1on can be introduced (=any tines unknown to the operator or ins :r u=ent technician) by :cro shift, nonlinearity, hysteres ts, air trapped in : e s e r s ing lines, or tenperature changes.

A tc=perature shif t in the

cierence colu n could well have been the cause of the instability end experienced at one PL*R.2' /

e v i2 :ica

?.e:alibration of the 7easuring systen, in mos t cases. can casily tvcid havin: se: points a: the extreme icv end of the calibrated span.

7:r in ::n=e. the pressuri:cr pressurc =:asuring sys:c= (channels 1, 2 and

3) has 2 overall range of apprcrinately 0-3000 psi.

The calibrated span

::e ncesurin; sys ten is 1700-2500 psi (800 psi spca),1.c.. equivalent

o :: cu:pu: of 10 to 50 na de.

The safe:y inj ection poin: of cetuation is sat :: 1715 psi, which is less than 2 of span.

If the sys:c= vere

.ihrc:cd fer a range of 1500-2500 (this recalibration requires approxi-

c v__y r a hours) then the actuntion pein

veuld be 215 (1715-1503) ever

- P. r:3, Den:en to F.oscley, Conne :icut Yanhec A: sis: Inspection Report 3

d::ed February 26, 1970.

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1000 (cclibrated span) or greater than 20% of calibrated scale.

Furth er,

the trip actionc are gencrclly initiated by histchles which are part of an ins trumen: channel that also includes indicators, recorders, controllers, etc.

C,enerally, all cc penents of the reasuring sys:c cre connected in a series current loop so that each co=ponent is calibrated to the 10 to 50 na (cr !.

20

) signal which in this case, is preper:ional to 1700 to 2500 pci.

During the res: of :he se: point at 1715 psi it appears that sone difficulty cight be encountered to accurately read the corresponding indicator and/or recorder. Another area of concern would be the repeat-ability of :' e bistable and the dif ferential span (rese: of trip action)

hc: is e charceteristic cf the ceasuring systen.

2.is information becc::s cos: important when operating trip points are a: the lower end of eclibrated spans.

F.

Concludinc Observations and Coc=ents rne following observations and co= men:s apply specifically to pig's th:: c ploy the "tvo of three" control logic for :he protection syste: and he " cold reference column". piping arrange =ent shown in Figure I.

1.

Tc=ocrcture of the reference colt =n is not being monitored.

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ceperaturc =casurc=cn of the colt =n (a fe skin thernacouples i

applied to the standpipe and conitored en enisting =ul:1 point recorder in the centrel roc =) would alert the operator if a valve (byoass) was inadvertently lef t open and' lcahing, or if the m

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_n-1 venting syste= vas accidently heativ; the reference cole==. The j

Co--a--dau: Yankee report discusses this poscibility, i.e., since i

a the venting sys:c= is in close preni=ity of the reference colu=n and the vent line can reach a point of 640*F during a venting 4

procedure, the reference les vould exucrience an increase in te=parature.

As previously discussed, this could introduce an crror of 148" E 0, not to =ention the fact that due to water croansion the reference colu=n could experience a decrease in r

f inventory when the venting was ~ ter=inated and the temper 5ture v::c i

re urned to nor:al.

k'ith " Tee" reservoirs rather than condensate pots. this loss could be quite significant.

Since San Onofre

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stated their pressuricer venting systc= is not routed near the reference coluan as it is in the Connecticut Yankee installa-

icn they do no: enoect any proble=s resulting fro = venting of the pressuricer.

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2.

A meth:d for de:erninicg if the reference cole =n is in fact ' full l

sh uld be included as part of the surveillance testing procedures so dat do:e=en:ation vould be available for the inspector's revicv.

In addi:icn, periodic checks of the reference cele =n should con-

inue in the interval between the instrument channel calibra:1ons.

I= cloying a s=all capacity pu=p provided with a re cte =anual control J

feature -is one method that. can be used for de:cr=ining reference c:le:n varer level adecuacy.

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change in va:cr icvel reading indica ce reference colu=n is full.

The pump should retain en until the vater level reading re=ains constant.

2.

At several reactor plants (P. R's) two of the three preccure trans-cittcrc which initiate hich and low pres sure scrar and cafety injection arc counted on the channel 1 and channel 2 reference column s tandpipe.

Since the piping installation was not reviewed, it could not be de:cr=ined whether a cerron valve existed such that when closed could prevent a protective action.

In a two of three cysics, which is used in this application, one can postulate a single failure cad subsequen: loss of protection function; havtver, in the tsu of four or a one of two times two protection cystc : on2 rould not le c the redundant protective syc cc with a single failure, e.

1ith trip unit cetuating near the resitive cero of neasuring sys:cas, one has to be cuare of such rys:ca charnetcristics as repen: ability of trip units and the differential span (reset of

rip unit) cf the sys ce.

Severcl FRR's calibration procedures

no; r; quire the transritter cnd crecciated corponents to bc tested as a cc=plete sys

ct. L'ith the generally accep

cd fact taa: rast cacsuring systen devic:f ons caused by hys:cresis, cero chift, drif ting, air entrain ent, etc., are found to originate

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! i frc: the sensing device further substantiates the need for a 2:ntinual review of the set point ac:uatien selection area.

Tae co _ ants rclating to set point actuation and calibration proce-dures apply dire::1y to other process parc=eters throughou: the reacter power plant.

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