a case study of distillate pipework failures at hendrina power station

8/4/2019 A Case Study of Distillate Pipework Failures at Hendrina Power Station

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ht. .I. Pres. Ves. & Pipin g 66 (1996)v-98Elsevie r cience imitedFrlntcdn GreatBritain0308-0161(95)00086-O 030&0161/96/$15.00ELSEVIER

A CASE STUDY OF DISTILLATE PIPEWORKFAILURES AT HENDRINA POWER STATION

R J CROUSEskom Hendrina Power Station, EngineeringDepartment,Private Bag X1003, Pullenshope,1096,South Africaand

L PRETORIUSRand Afrikaans University, EngineeringFaculty, Laboratory or Systems,Private Bag 524, Auckland Park, 2007, South Africa

INTRODUCTIONSince the commissioning of Hendrina Power Station, numerousproblems havebeen encountered n thedistillate pipework from the lowest high pressure eater o the deaerator.From available general iterature,clearly it is common practice o design eedheater hells with a section o subcool he drains o approximately30C below satura tion empera tureof the water at heatershell pressure,hus avoiding the possibility offlashing. Unaccountably, he Hendrina design s fairly unique n the sense hat the heatersdo not have such asubcooling section and as the distillate flows up to the deaerator, rictional lossesand gravity causeareduction n static pressurewhich causes he distillate to flash with consequen t iolent pipe vibration anddifficulties in automatic control of the heater evel.

GENERAL INFORMATION ABOUT HENDRINA POWER STATIONHendrina Power Station has 10 x 200 MW pulverized fuel coal-fired units. The first unit was commissionedin 1970, and he last unit in 1976. The turbines are non-rehea tAEG machineswith a reverse-flowhighpressureand a single double-flow low pressurecylinder. The station is equippedwith 7 wet cooling towers.Unit 1 has a Babcock & Wilcox boiler of the once-through esign,but with a drum. Units 2 to 5 are Babcock& Wilcox El Pasoboilers, and Units 6 to 10 areof Steinmuller design,all being of the two-pass ype.Although the units were designed or base oad, the currentexcessgenerationcapacity n South Africa leadsto the need o operateat lower loads during off-peak periods.

OVERVIEW OF RELEVANT PLANT DESIGNThe feedheatingplant consistsof two low pressure eaters LPHl and LPH2), a deaerator ea terand twobanksof three high pressure eaterseach HPHl, HPH2, HPH3; banks A and B). For the purposesof thisstudy, only the HP heater rain needed o be considered. (See igure 1)The HP heatersare of the vertical tube-plate ype, with cascaded rains. The distillate from HPH3 cascadesdown through a flash box to HPH2, the HPH2 drains h rough a flash box to HPHl, and from the HPHl at 1m level up to the deaerator t fl0 m level. The heaterdramsare saturated iquid WITH NO SUB-COOLING.

71


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78 R. J. Crow L. PretoriusFor those not very familiar with boiler feed water heating, disti llate is (as the name implies) the watercondensed from the steam extracted by each heater. This condensate assists in deaerating and heating thefeed water from the last low pressure heater. It is standard practice to design the turbine with the deaeratorworking near atmospheric pressure, as this is most convenient for extracting the non-condensables (air,gasses, etc.). For various reasons, but mainly because it is preferable to have the boiler feed pumpdownstream of the deaerator, the deaerator is placed at a high level and therefore automatically providesadequate positive suction head to the booster feed pump. Thus, at Hendrina, the need for the vertical pipefrom HPH 1 to supply disti llate to the 28 m level.During very low load conditions, the pressure in the HPHl shell is not sufficient to sustain a flow of distillateto the deaerator. During such operating conditions, the so-called pitch-load valve opens, dumping the HPHldistillate to the LPI42 flash tank. This valve also opens. if the HPHl distillate control valve C (in figure 1)fails to cope with the distillate flow and thus prevents high HPHl distillate levels.

rlPH 1Fig. 1: HP heater 1 /distillate control layout

OVERVIEW OF DISTILLATE PROBLEMSIn essence, the problems encountered may be divided into two parts:

l Unstable control conditions. HPHl A & B distillate control valve cycling causing considerablemaintenance and related process performance problems.

l Severe vibration of the disti llate pipe from HPHl to the deaerator, with associated fatigue failures ofthe pipework and hangers.

Both problems are related to unstable flow conditions caused by the fact that the HPHl disti llate is saturatedliquid. Due to friction losses and the difference in height (28m) there is a drop in static pressure in the line tothe deaerator so a proportion of the disti llate flashes to steam.Severe cycling of control valve C leads to high rates of failure of both the valve and the actuator and causesthe pitch load valve to open frequently, dumping hot distillate into LPI-I2 that will impact on its reliabi lityand performance. Severe problems with the setting of the system have necessitated all units to be run withthe control valves fully open and on manual. This is detrimental to performance as HPH 1 runs without awater level and the consequential increased bled steam flow to the heater is expected to impact on the life ofthe heater.


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Case study of distillate pipework failuresEARLY HISTORY OF FAILURES

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The Power Station records show that during the early years of operation erosion problems were experiencedin the distillate pipes. In addition, the lines had plunger-type control valves which eventually became veryexpensive to maintain. A decision was made to replace the old valves with the newer ball type control valves.After approximately ten years of operation the pipes were found to be seriously eroded and consequently thepipe diameter was increased from 6 inch to 8 inch and manufactured from stainless steel. The intent of thismodification was to facilitate lower flow velocities. This was a VERY expensive modification.After implementation, far from providing a cure, alarming pipe vibrations were observed, and some of thepipes fractured. An investigation of the matter by an external authority concluded that the vibrations were notcaused by flow instabilities, but by inadequate hangers. Consequently the pipe hanger system wasredesigned. However, in retrospect the calculations for the pipe hangers show that they were designed forconstant loading, considering only the weight of the pipe filled with liquid, and thus sti ll prone to failure. Aproposal made at the time was to move the valves to the turbine hall basement and change the pipe diameterto 14, which was fortunately not adopted due to extremely high capital costs.During the past two years, due to consequent work-hardening of the material the disti llate lines of severalunits have experienced fatigue failures which although undesirable in themselves, give rise to concern forpersonnel safety.

ROOT CAUSE ANALYSISRather than attempt to deal with the effects of the vibration, this investigation was conducted with the intentof dealing with the cause. Even at the early stages of the investigation and survey of literature it was clearthat the vibrations were caused by flow instabilit ies, the nature of which is to be pursued in later sections.The analysis of these instabili ties required modelling of the system, and this proved to be a formidable task.An understanding of two-phase liquid-vapour flow is essential to the analysis of the system and its problems.The extract of two-phase flow technology presented here is only superficial, and does not deal with any of themany modelling complexities, its main purpose is to simply establish an understanding of the mechanismswith the reader who may welcome a brief introduction to the subject.

TWO-PHASE FLOWDefinition of multkhase flowThe Penguin Dictionary of Science defines the word phase as follows:

Separate part of a heterogeneous body or system, e.g. a mixture of ice and water is a two-phase system,while a solution of salt and water is one-phase.

We are concerned in this context with the flow of two-phase mixtures of vapour and liquid, or gas and liquid.Vapour- liquid mixtures, where the vapour and liquid phases are of the same fluid are referred to as two-phasesingle-component mixtures. Gas-liquid mixtures, where the gas and liquid phases are not of the same fluid,are referred to as two-phase two-component mixtures. In the absence of phase change the flow of a two-phase one-component mixture obeys the same physical laws as a two-phase two-component mixture.


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80 R. J. Crow L PretoriusMost engineers dealing with two-phase flow for the first time are surprised to find that pressure drops withsuch flows are often 10 times as great as they would have expected from their experience with single-phaseflow.For flashing flows of a chemically stable medium, the problem may be described intrins ically as:

A three-dimensional transient motion involving time-dependent compressible flow of two interactingmetastable phases at different velocities in the same conduit.With a liquid at saturation temperature, condensation and evaporation are continuously occurring, thus thephases are met&able. Even with homogeneous flows, the two phases may be flowing at differentvelocities.

Overview of Industrial occurrenceMany industrial processes involve two-phase flow systems. Some examples include:

l tubular boilersl boiling water reactorsl boiler blow down systems. oil, gas and saturated steam transportation lines

. refrigeratorsl process pipe linesl process heat exchangers

The natural circulation water-tube boiler was one of the first instal lations to create an industrial demand forimproved knowledge of two-phase systems. Due to the extreme precautions necessary with reactor designand control, the nuclear industry made a major contribution to this field of study. Currently the major usersof such knowledge are the nuclear industry, petrochemical industry and heat exchanger designers.While the knowledge in the field has increased dramatically, the approach followed by process designerswhen dealing with a plant that may potentially produce two-phase flow is to design away from it. This isperfectly understandable and avoiding it altogether is also the recommendation of this author. Unfortunately,at Hendrina we have fait-accompli and must find a solution that works yet satisfies the ever vigilantaccountants.

Accuracv of Drediction of two-Dhase flow calculationsChisholm2 writes:

Despite the large number of studies related to two-phase flow, there are many situations where predictionmay carry with it an uncertainty of 50%. In considering this figure it should be remembered that, for anair-water mixture at ambient conditions, for example, for the same mass flow rate the ratio of the frictionpressure gradient with gas to that with liquid is about 800: 1. In many branches of engineering science,experience enables engineers to guess a magnitude to within 50%; this is not the case with two-phaseflow. That the uncertainty in prediction remains high is due to the large number of variables encounteredin two-phase flow. An extremely vast and systematic test programme would be required to provide datafor a soundly based empirical correlation, and even our knowledge of single-phase flow is essentiallyempirical.

Most engineering problems may be simpli fied by identifying the primary variable that has the largest effect.With two-phase flow such a primary variable does not really exist and many variables have equal effect. Thehigh level of complexity of these studies is thus justif ied but most definitely is not an attempt to make it adifficult subject to understand.


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Case study of distillate pipmork failuresConsider some of the primary variables in two-phase flow:

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1. mass flow rate 6. vapour viscosity2. mass dryness fraction 7. surface tension3. liquid density 8. surface roughness4. vapour density 9. pipe inclination5. liquid viscosi ty 10. pipe diameter

For the two-phase correlations to be entirely empirical it would be necessary to obtain data with each variableat five different value8 (say) for all combinations of the other variables. The number of test points required inthat case would be 5 =9765625, yet only the case of incompressible flow is covered by this astronomicalfigure.Other parameters present in industrial plant that will affect the nature of the flow, and that may not bemeasured or accounted for include:

1. Changes in chemical composition of the fluids, e.g. through dosing.2. Presence of entrained debris, which wil l act as nucleation seeds.3. Changes in the surface roughness of the pipe, e.g. due to surface fouling by crud deposits.

PERSPECTIVE ON TWO-PHASE FLOWIt is clear from the study of the available literature that numerous years of field work would be required ofanyone who intends to produce substantial results. However, the primary intent of this project is simply tosuccessfully model a system with which to ultimately solve a specific problem. While the bulk of theavailable literature presents the results of research, very little material is available on the practical aspects ofdealing with two-phase flow.A substantial portion of the work done by others in this field is related to the integration of the momentum orenergy differential equations. To carry out such integrations, numerous simpl ifying assumptions are required,which may account for a sizable proportion of the relatively large uncertainty of prediction. While this isnecessary for hand calculations, it may not be so for computer based finite models where the differentialequations can be used as is. Using smaller and smaller elements, as their size approaches infinitesimaldimensions, the error becomes negligible.During the term of this investigation, several interested parties questioned the level of detail found necessaryfor it. ln particular, with mass qualities as low as 5% it may be difficult to visualise large volumes of steamin the flow. Furthermore, the effects of flashing on pressure drop required clarification. Considering thesystem to be modelled, with a saturated water-steam mixture at 7 bar, the properties of the substances are asfollows (using the ASME Steam Tables3):

P = 7bar T,, = 164.96C 9 L = 0.001108m~/R 8 G = 0.2727 m3/kgThe ratios of densities of water and steam are therefore:

i3,/SL =246.1 (1)Consequently, the gravitational pressure drop for vertical flow of the liquid phase is also 246 times as large asfor the vapour phase.


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82 R. J. Crow L.. PretoriusTo relate the spatial location of the phases consider the volumetric quality is defined as the volume occupiedby the vapour phase divided by the total volume occupied, (by analogy to mass dryness fraction or massquality):

p= XBGxS.+(l-x)9,

(2)

Figure 3 presents a plot of volumetric quality versus mass quality. From this plot it can be observed that at amass quality of 1% the volumetric quality is 70%, thus the vapour phase would occupy a significant portionof the conduit.The dramatic change in specific volume of the mixture would also have a signif icant impact on flowvelocities and consequently frictional pressure drop. Using a pipe surface roughness of 0.02 mm, a pipelength of 30m and a mass flow rate of 18 kg/s for this system, the following values apply:

D = 8 = 203.2mm E = 9.843. 10e5 p, = 1.640.10-4 Ns/m pG = 1.454. 10e5 Ns/m2

P(X)-

Fig. 3: Volumetric quaI@ versus mass quality for water-steam at 7 bar.

Evaluating the pressure drop due to friction for a pipe length of 30 m, for the total mass flowing as liquid:ApL = pJ, $5 0.353kPa (4 )

and for the total mass flowing as vapour:ApG = p&, $$ = 757kPa

The ratio of the frictional pressure gradients is therefore& G _ 75.7kPa = 2144-4% 0.353kPa .

(5)

(6)The results of this calculation are summarised in Table 1.


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Case study of distillate pipework failuresTable I: Summary of ana&sis of water versus steam pressure drops

83

Case

Total mass flows as liquidTotal mass flows as vapourR&i 0

Gravitational pressure Frictional pressuredrop drop

265.5kPa 0.353kPa1.08kPa 75.7kPa246.1 214.4Considering the magnitude of these ratios, the necessity for appropriate modelling is clear.

FLOW PATTERNSThe flow pattern, or flow geometry assumed by a two-phase flow system is one of the most problematicaspects of the subject because it is three-dimensional. In gas-liquid flow the two phases can distribute in theconduit in a wide variety of ways rarely under the control of the experimenter or designer. When changesoccur in flow rates, fluid properties, conduit shape or inclination, this distribution will vary. In addition, theindividual velocities and shapes of the interfaces are unknown. Because the spatial locations of the phases areunknown it is impossible to specify which fluid properties are applicable at each point in space and time.However, since the earliest visual observations of two-phase flow it has been recognised that there are naturalgroupings or patterns. Baketi (1954) published the first flow pattern map for horizontal pipes. Short lyafterwards, Baker5 (1958) also suggested that better correlations for pressure drop could be obtained bydeveloping equations to be applied separately to each regime. He was able to show that even with empir icalcorrelations, the accuracy of the pressure drop predict ions varied substantially with the flow pattern they wereapplied to.Of course the pressure drop in a two-phase flow system is not the only factor of interest. Where, as atHendrina the stability of a system is of concern, the pressure and mass flux oscil lations have to be studied andthese as well as others are of course highly dependent on the assumed flow pattern.If the models created for individual flow patterns are to be useful it is necessary to accurately predict the flowpattern that would actually exist. Numerous articles have been published on this subject, mostly usingempir ical mapping without any basis in the mechanisms that are responsible for the transitions. Each result istherefore only useful for a narrow range of pipe sizes and fluid properties with flow rates approaching theconditions of the experiments: Poss ibly due to financial limitations they tended to be carried out using smallpipe diameters, typical ly 2.5 cm to 5 cm. A very brief extract from some of the better-known two-phase flowpattern maps will be presented in this chapter, principal ly to illustrate the constraints. The most elegant flowpattern transition modelling to date is that by Taitel and Dukler7, 8,g, oa I.

Classification of flow patternsWith a sufficient supply of adjectives it would be possible to define innumerable flow patterns. As anillustration, consider the followin 1P lis t of pattern descriptions extracted from only a handful of the moresuccessful papers 8, 12, 13, 14, 15, 16, 18, 19,20.

Horizontal tubes:Stratified, stratified smooth, stratified wavy, laminar-stratified, stratified-roll wave, stratified-inertia, wave,plug, slug, elongated bubble, proto slug, wavy annular, annular flow-through, pulsating froth, semiannular,annular mist, spray, homogeneous, bubble, dispersed bubble.


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84Vertical tubes:

R. J. Crow, L. PretoriusBubble, dispersed bubble, homogeneous, slug, plug, slug-annular, chum, froth, frothy slug, quiet slug, piston,pulsating annular, annular, annular-mist, wispy annular.Developing more detailed descriptions of flow patterns would not serve a very useful purpose as the maincentre of interest is in the approximate spatial location of the phases in order that pressure drops and stabilitycan be adequately modelled.

Flow patterns for horizontal pipesFigure 4 shows the generally accepted flow patterns for horizontal tubes from which it wil l be observed thatthe most unstable patterns are those classified as intermittent.

bSTRATIFIED SMOOTH

4

STRATIFIED

STRATIFIED WAVY

ELONGATED BUBBLE-1 INTERMITTENT

SLUG

ANNULAR / ANNULAR-MIST ANNULAR

WAVY ANNULAR

DISPERSED BUBBLEFig 4: Flowpatterns in horizontaljlow (FromTaitel and DuklerO 1986)

tJ0 B01

0OO

fQ0

00 0

&0

+

: . .. .._. . .: .. :..). .. . .. .. .. .

;1

.. ...... : .. .; . .:... ..; .: ; ::. ._..,..tFig. 5: Flow patterns in verticalflow (From Taiteland DuklerO 1986)

Flow patterns for vertical pipe8Figure 5 shows the generally accepted flow patterns encountered in vertical pipes, the most unstable beingslug flow and chum flow. Chum flow is the most chaotic and character&d by severe changes in value anddirection of the velocities.

FLOW PATTERN DETECTIONFlow pattern detection is complicated by the fact that certain flow patterns contain arbitrary elements and thetransistions between some of them is a gradual process, so there is some diff iculty in defining the boundary.By the very nature of the problem, there is litt le point in attempting to make fine distinctions.The simplest method for detecting flow pattern is by visual observaipg t@ygh transparent test sections.This method may be refined through the use of high-speed cameras . Where the fluids are nottransparent, X-ray photography has been used24 r . For practical reasons these methods are not very usefulfor the problem considered in this paper.


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Case study of distillate pipework failures 85The observation hat the time-aveT 2t 27d ressure radient curve changedwith flow patternwith a systematicchange n liquid or vapour flow rate , probably prompted Hubbardand Dukler28 1966) o suggestamethod basedon the spectralanalysis of wall pressure luctuations. Although condensation r vapourisationis considered o occur at a specific saturationpressure or a fixed temperature, n practice this doesnothappen. For vapourisation o develop, a local condition of liquid superheatmust first be established n orderto producenucleation, and vice versa. It is thus observed hat the wall pressuresluctuate n sympathy withthe physical distribution of the phases n the conduit.The work was conductedwith a horizontal tube using analysis of the time traceof the static pressure t thewall to obtain the power spectraldensity of the wall pressure luctuations. The time variation of thefluctuations are obtainedas follows:

P(t)= P(t)- P (7)where P is the time-averageof the static pressures. f the autocorrelation unction is defined as

q(7)= him; j P(t)P (t +z)lt0

(8)

then the power spectraldensity is the Fourier transform of the autocorrelationS,, = i R; (+i2=Bdz (9

The measgments showed hat, despitea wide variety of apparent hasedistributions only three basic spectraexisted,as shown in Fig. 6 and hesecan be used as ingerprints for the flow patterns. Type A, where hespectra s a maximum at f = 0 with decayas f increaseswas found to be characteristicof separatedlowpatterns,such as annularor stratified flow with low entrainment ates. Intermittent flow patternsdisplayedcharacteristicssimilar to curve B and representedhe mean frequencyof slug passage.Dispersedordistributed flow patternssuch as bubbly flow revealedspectrasimilar to band-limited white noise as shownby curve C. In p ractice what is observed s a superpositionof spectra.

Fig 6. Power spectral density of wall pressure fluctuations.(From Taitel and DuklerO (1986) ).

INSTABILITIES IN TWO-PHASE FLOW SYSTEMSTwo-phase lows are prone o a variety of instabilities leading o oscillations or excursions n flow rate,voidfraction and quality. Reviews of suchunstablephenomena regiven by Yadigaroglu2g 1981) and Cho30(1989).


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86Momentum flux oscillations

R. J. Crow, L. Pretorius

With intermittent flow patterns, very large reaction forces may be induced even in a straight pipe due tomomentum flux oscillations. The magnitudes of these forces are dependent on the shape of the bubble whichalso determines the rate of change of momentum.

Excursive of Ledinegg instabilityVarious dynamic instabil ities may occur independent of the flow pattern. A static effect known as theLedinegg instability follows from the pressure drop versus flow rate characteristics of the system. For avertical channel with constant heat flux a multi-valued curve that characteristically passes through amaximum and then a minimum is obtained. For a fixed pressure drop, excursions can occur between thethree possible values of flow rate (see Fig. 7).

St ami

AP

Flow rateFig. 7: Flow rate versus pressure drop characteristicsof a system dibiting the Ledinegg instability

Chueeiw instabilityVapour formation does not necessari ly occur when the fluid reaches its saturation temperature. Eventually,and sometimes with the presence of considerable liquid superheats, vapour nucleation can occur at the pipewall and a situation can develop in which the vapour bubble grows rapidly into the superheated liquid causinga violent ejection of liquid downstream of the point of nucleation. Once the ejection has occurred, the tuberefills with liquid and the process is repeated cyclically . Liquid metal systems are particularly prone to thisform of instabili ty. Reviews of the subject are given by Coll ier3 (1968) and Fauske32 (1968).

Dvnamic instabilitiesPossible dynamic instabilities include:

l Low frequency oscil lations due to compressible volumes either in or connected to the two-phasechannel

l High frequency acoustic oscillations associated by the propagation of pressure waves travelling at sonicvelocity

l Density wave instabil ity caused by feedback from individual pressure drop components operating outof phase with the flow perturbations.

Davies and Potter33 (1966) gave an explanation of density wave instability. In steady state, the variouscomponents of pressure drop are additive. With a given fluctuation in inlet velocity at a given frequency,


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Case study of distillate pipework failures 87consequential scillations may occur n the pressuredrop components hat are out of phasewith thesefluctuations. A situation can arise where he total pressure rop doesnot vary, in which case here s nothingto oppose he rapid growth of fluctuation in inlet velocity at this frequency. For example, t may occur thatthe single-phase ariations in pressure rop in the inlet region (which are n phasewith the velocityfluctuations) are counter-balanced y two-phasepressureoscillations, 180degrees ut of phasewith thevelocity fluctuations at the outlet. Linear theory would predict that the velocity fluctuations would reachaninfinite amplitude due o this resonance, ut non-lineareffects would dampen his to a finite (though oftenvery large) value.

CONCEPTUAL SOLUTIONSThe ideasconsideredarebasedon the following:

l Subcooling the distillate to ensuresingle-phaselow up to the control valve C in its curren tpositionl Increasing he distillate pressure o ensuresingle-phase low up to the valvel Re-routing he distillate elsewherel Stabilising the flow /control of flow patternassumedby the phases,which may be achieved hrougheither or a combination of

n Placing the valve C in the turbine basement,orn Changing he pipe diameter

Subcooling may be achievedby taking condensaterom the extractionpump dischargeand njecting it intothe distillate line, in effect lowering its temperature o such an extent as o ensureno flashing before hecontrol valve. The installation of an expansionnozzle at the deaeratornlet would ensureproper flashing ofthe distillate as t entersand also increases he static pressure n the distillate line thus reducing he requiredattemperation low. This would increase he heat ate of the turbine cycle, as the cooling flow bypassesheLP heatersand owers the deaeratoremperature. Due to its simplicity sucha modification is expected o bethe cheapestn terms of capital cost, but the efficiency losseswould also have o be considered.Alternatively HP 1 heatersmay be replacedwith a designhaving an integratedsub cooling section hat wouldunfortunately decreasehe cycle efficiency due to lowering of the deaeratoremperatureand pressure.Thecapital costs nvolved would undoubtedlybe prohibitive.Subcoolingmay also be achievedby the installation of a heatexchanger etween he distillate line and hecondensateine to the deaerator,with a bypassvalve to control distillate temperature. The requiredheatexchanger ould be very large yet it must be small enough o be accommodatedwithin the existing plant, andthis would thereforebe a limiting factor. In this case herewould be no effect on cycle efficiency due to thefact that the mass and energy lows to the deaerator emain unchanged .Heaterdrain pumps may be used o increase he static pressure n the distillate line, thus preventing flashing.Due to the very low suction headavailable, he pumps would be very expensiveandhave been oundelsewhere o be extremely maintenance ntensive. Apart from this, operatingexperience t Amot PowerStation shows that when a drain pump trips, the forcesgenerated y the suddenonsetof flashing can fmcturethe pipe. There s also an mpact on cycle efficiency through he pumping power required.In order o ensure hat the distillate control valve C dealsonly with single phase low, it could be moved tothe basement f the turbine hall. This should also prevent he possibility of density wave instabilities.Increasing he pressuredrop across he valve will encourage lashing due o the reduction n static pressure nthe distillate line. Another possible advantagewould be a higher dryness raction that would increase hevapourvelocities and ead to a more stableannular low but the vapourvelocity may still not be high enoughto sustain his. In thesecircumstancesa smaller pipe diametercould provide the increased apourvelocityrequired o achieveannular flow.


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88 R. J. Crow, L. PretoriusA common configuration in modem power stations is to pump the heater drains into the feedwater line afterthe heater. Since these drains contribute to maintaining feed water temperature, the impact on the heat rateand performance of the deaerator needs to be considered. The concept is not suitable for the Hendrina systemas the disti llate assists in heating the deaerator and so its loss would impact on deaeration efficiency andproduce a considerable reduction in cycle efficiency.Although technically feasible, the subcooling concepts would add to the complexity of the existing plant.Furthermore, additional instrumentation would be required and maintenance of this new plant would mostcertainly be greater than for current requirements. The available pressure drop from HPHl to the deaeratorand space limitations would mainly determine the feasibility of this solution.Because of its simplic ity and from a practical point of view, the stabilised two-phase flow option, is thuspreferred and would not complicate the design of the system. As this solution is passive, no extrainstrumentation is required. It is however the most difficult to design due to the uncertainty associated withsuch calculations.

RESULTS OF CURRENT MODELLING AND MEASUREMENTSThe total two-phase pressure drop is the sum of the gravitational, frictional and accelerational pressure drops.For the model, the generalized frictional pressure drop correlation of Chisholm34 (B-coefficient) (1983) wasused. To determine gravitational pressure drop, knowledge of the void fraction is required so for the model,the void fraction correlation of Permol i et al35 (1970) was used. The mechanistic models of Taitel andDukler*O (1986) was used for predicting flow pattern transitions. Coding was done in Borland Turbo Pascalusing object-oriented programming techniques.Early modelling suggested that moving the control valve C closer to the heater outlet may result in achievingannular flow due to the increased mass dryness fraction. As a test, this modification was performed on oneunit (see Appendix A). In addition, pressure tapping points were instal led at various points for the purpose ofcarrying out a spectral power density analysis of the wall pressure fluctuations and to compare the results tothe model developed. Pressure transmitters were then connected to these tapping points and connected to adatalogger. A number of tests were then conducted with pressures being logged for each specific load andvalve position.In the following sections the results of the modelling and measurements wil l be discussed, whilst comparingthe results to the evidence found in the plant history.With the original system using 6 pipes, the modelling reveals high flow velocities so agreeing with therecords of erosion problems. It is also clear that the pipe vibration problem is certainly not a newphenomenon as the intermittent flow pattern is dominant, especial ly in the vertical sections. It may bespeculated that the smaller diameter system had a higher natural frequency due to stifIbess considerations and/or different geometry.The model shows that with the current configuration, all loads the intermittent flow pattern manifests itself invirtually the whole length of the lines. Consequently a high level of pipe vibration may be expected.With the valve in the turbine hall basement the intermittent flow pattern is sti ll observed, but transition to themore stable annular flow takes place before the bends are reached, with the exception of the lowest load of160 MW. Pipe vibrations are thus reduced. As described in the next section the modelling results presentedwere based on the measured pressures. Upon processing of these results, it was clear that the isolating valveafter the old valve position near the deaerator was not ful ly open during the tests, possibly as a consequence


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Case study of distillate pipework failures 89of the temporary mode of operation of the plant, to limit the flows. If this valve is to be opened fully, orremoved completely, a further 2 bar of pressure drop becomes available for utilisation across the controlvalve, .which wil l result in annular flow. The only portion of the pipe that is then still prone to intermittentflow is the last section downstream of the original control valve position to the deaerator inlet, this could beobviated by the replacement of this section with 8 pipe.During the tests with the valve in its old position, severe level control problems were encountered whichvanished when the valve was moved to the basement. This indicates the existence of a density waveinstabili ty. Pipe vibrations were substantial ly reduced with the new configuration. From the measurements,it is clear that the deaerator pressure is extremely sensitive to the distillate flows and subcooling the distillatewould have had a serious impact on cycle efficiency.The power spectral density of each signal was obtained using a fast Fourier transform according to themethod proposed by Hubbard and Dukler15 (1966). The results of this analysis are summarized in Table 2and the detail spectra are included in the appendix.

Table 2: Analysis of power spectral density of wall pressure fluctuationsConfiguration Position 1 Position 2 Position 3

Line A200 MW, valve at the deaerator No evidence of

intermittent flowSlight tendency tointermittent flow

Strong evidence ofintermittent flow

160 MW, valve at the deaerator

200 MW, valve in the basement160 MW, valve in the basement

Line B

No evidence of Slight tendency tointermittent flow intermittent flowNo evidence of No evidence ofintermittent flow intermittent flowNo evidence of No evidence ofintermittent flow intermittent flow

No evidence ofintermittent flowNo evidence ofintermittent flowNo evidence ofintermittent flow



200 MW, valve in the basement

No evidence ofintermittent flowNo evidence ofintermittent flowNo evidence ofintermittent flow

Strong evidence ofintermittent flowStrong evidence ofintermittent flowNo evidence ofintermittent flow

Very strong evidenceof intermittent flowNo evidence ofintermittent flowNo evidence ofintermittent flow

160 MW, valve in the basement No evidence ofintermittent flow

Very slight tendency Very slight tendencyto intermittent flow to intermittent flow

CONCLUSIONSThe analysis of the wall pressure fluctuations confirms the results obtained with the model of the system.When comparing the spectra obtained with the spectral analysis of the pipe vibrations as obtained by theindependent authority, the similarit ies are clear and virtually identical. It is thus concluded that the pipevibrations are indeed caused by flow instabi lities, and that the modification of moving the valve to the turbinehall basement effectively and economically solves the problem by removing the cause. It is fortunate that the


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90 R. J. Crous, L. Pretoriusexisting pipe diameter is suitable for the alternative valve position, as this saves considerable capitalexpenditure.As a point of interest, the modification proposed earlier to move the valve to the basement and replace thepipework with 14 lines was analyzed. It is clear that this diameter was chosen by considering the pressuredrop of the disti llate flowing in the steam phase. The model shows intermittent flow for all load cases, thusthis modification would have been unsuccessful.The tests indicate that the dearator pressure is extremely sensitive to the state of the disti llate entering thevessel, thus subcooling would not be a sensible option, and the stabil ized two-phase flow option is superiorboth in terms of capital expenditure, technical feasibility and cycle efficiency.

ACKNOWLEDGEMENTS

I must give credit to a colleague Dr Fossil Mudguard for his insights into engineering philosophy who,although he is not an expert in two-phase flow has helped me stay on the right track. To my family, whichhas made peace with the terminal condition of curiosity that I suffer from, my sincerest apologies for notsaying no to this challenge. Many thanks to Dr Hewitt and Dr Moore for coming to darkest Africa to look ata pipe, and supplying me with literature and confidence with the modelling done. Thank you to H. Pomeroyfor reading all this and more, preventing me from inflicting mortal injury on the English language.

1.2.3.4.5.6.7.8.

9.

REFERENCES

Uvarov, E B and Chapman, D R, A dictionary of Science, Harmondsworth, Middlesex: Penguin BooksChisholm, D, Two-phase flow in pipelines and heat exchangers, George Godwin, London and New Yorkin association with the Institution of Chemical Engineers: Longman Inc., New York (1983).ASME, ASME Steam Tables Sixth Edition, ASME Press, New York, (1993).Baker, 0, Simultaneous Flow of Oil and Gas. Oil Gas J. (1954) Vol53, p 185.Baker, 0, Multiphase Flow in Pipel ines 1958, Pipeline News, (1958) June, 23Taitel, Y, Flow pattern transition in rough pipes. Int. J Multiphase Flow, (1977) Vo13, p 597Taitel, Y, Bamea, D, and Dukler, A E, Modelling flow pattern transitions for steady upward gas liquidflow in vertical tubes. AIChE J. Vo126, (1977) p 345Taitel, Y and Dukler, A E, A model for predicting flow regime transitions in Horizontal and near-horizontal gas liquid flow. AIChE J. Vo13, (1976) p 585Taitel, Y, and Dukler, A E, A model for slug frequency during gas liquid flow in horizontal and nearhorizontal pipes. Int. J Multiphase Flow vol. 3, (1977) p 585

10. Taitel, Y and Dukler, A E, Flow Pattern Transitions in Gas-Liquid Systems: Measurement andModelling, Multiphase Science and Technology, Vo12, (1986) p l-94, Hemisphere PublishingCorporation.

11. Taitel, Y, Lee, N and Dukler, A E, Transient gas liquid flow in horizontal pipes modeling flow patterntransitions. AIChE J. Vol 24, (1978) p 920

12. Griffith, P, and G B Wallis, Two-phase slug flow. J Heat Transfer., Vol. 83, (1961) p 30713. Duns, Jr, H and N C J Ros, Vert ical flow of gas and liquid mixtures from boreholes. Proc. 6th World

Petroleum Congress, Frankfurt, (1963).14. Stemling, V C, Two phase flow theory and engineering Decisions. Award lecture presented at AIChE

annual meeting, (1965).15. Hubbard, M G , and A E Dukler, The character isation of flow regimes for horizontal two phase flow.Proceedings of the heat transfer andjluid mechanics institute (Saad, M.A. and J.A. Moller, eds.), (1966)Stanford: Stanford University Press.


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Case tudyof d istillatepipeworkailures 9116. Wallis, G B, OneDimensional Two-PhaseFlow. New York: McGraw-Hill, (1969)17. Hewitt G F , and D N Robers,Studiesof two-phase low patternsby simultaneousX-Rays and FlashPhotography. Atomic Energy ResearchEstablishmentReport M-21 59; Harwell England, (1969).18. Govier, G W , and K Aziz, The flow of complex mixtures in pipes. New York: Van Nostrand Reinhold,(1972).19. Oshinowo, T and M E Charles,Vertical two-phase low. II. Holdup and pressure rop. Can. J Chem.Eng. (1974) Vol56, p 438.20. Spedding,P L, and V T Nguyen, Regime Maps for Air water wo phase low. Chem.Eng. Sci. Vo135,(1980) p 77921. Raisson,C, Flow regime studiesup to critical heat lux conditions at 80 kg/m2. CEA Grenoble,ReportNo 7722, 1965).22. Hsu, Y Y, and R W Graham,A visual study of two phase low in a verical tube with heataddition. NASATechnical Note D-1564, (1963).23. Bergles, A E, and M Suo, Investigation of Boiling Water Flow Regimes at High Pressure.DynatechReport No. NYO-3304-8,mS 1909, 1966).24. Derbysh ire, R T P, G F Hewitt, and B Nicholls, X-Radiographyof Two-PhaseGas-Liquid Flow.

Atomic Energy ResearchEstablishmentReportM-1 321; Harwell England, (1969).25. Govier, G W, B A Radford, and J SCDunn, The upwardvertical flow of air-watermixtures: I. Effect ofair and water rateson flow patternholdup andpressure rop. Can .I Chem.Eng. Vo135, (1957) pp. 5%70.26. Isbin, H S, R H Moen, R 0 Wickley, D R Mosher, andH C Larson,Two phasesteam water pressuredrop. Chem. Eng. Symp. Ser., (1959)Vol. 55, no 23, p 7527. Chaudry, A B, A C Emerton, and R Jackson,Flow regimes n the concurrent pward flow of water andair. Paper presentedat the symposiumon two-phase$ow, Exeter, England, (1965).28. Hubbard, M G, and A E Dukler, The characterisation f flow regimes for horizontal two phase low.Proceedingsof the heat transfer andjluid mechanics nstitute (Saad,M.A. and J.A. Moller, eds.)Stanford: Stanford University Press, 1966).29. Yadigaroglu, G, Two-phase low instabilities and propagationphenomena. Thermo-hydraulicsof Two-Phase Systemsor Industrial Design and Nuclear Engineering, HemispherePublishing Corporation,NewYork, (1981).30. Cho, S M, Single component, wo-phase boiling and condensation)low instability. Heat transfe requipmentdesign, HemispherePublishing Corporation,New York, (1989).3 1. Collier, J G, Boiling of liquid alkali metals. Chem.Proc. Eng. Heat TransferSurvey, (1968)pp. 167-173and 180.32. Fauske, Super-heating f liquid metals n relation to fast reactorsafety. Reactorand Fuel ProcessingTechnology, 11, No. 2, (1968)pp.84-89.33. Davies, A L and Potter, R Hydraulic instability. An anlysis of causes f unstableflow in parallelchannels.UKAEA Report, AEEW-R446, (1966).34. Chisholm, D, Two-phase low in pipelines andheat exchangers,GeorgeGodwin, London andNew Yorkin associationwith the Institution of Chemical Engineers:Longman Inc., New York, (1983).35. Premoli, A, Francesco,D and Prina, A, An empirical correlation or evaluating wo-phasemixturedensity under adiabaticconditions, European Two-phaseFlow GroupMeeting, PaperB9, Milan, (1970).


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92 R. J. Crous, L. PretoriusAppendix A: General layout of distillate lines, configurations and positions of pressure tapping points

40 level

HPH 1B

-- Dea 1

219.1 x 6 mm


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Case study of distillate pipework failuresAPPENDIX B: RESULTS OF MODELLING FOR THE LOAD CASES OF THE TESTS

Test 1:

Test 2

Lord : 200 rwN l : 166.3 mme : 219.1 mm#a : 355.6 nnFlow : 18.58ks/secControl valve aosit ion I TODP : 84.7kPa

POS Cl 8ibrr>1 7.367 7.3672 6.821 6.9673 5.967 6.0834 5.611 5.9375 5.528 5.8466 1.780 1.7807 1.738 1.738

Flow pattern legend:S - StratifiedA - AnnularI - IntermittentI B - Bubble

LOad : 160 PWCl : 168.3 nnc2 : 219.i nne : 355.6 nmFlaw : 13.84kdsecControl valve aosit ion : TooP : 84.7kPa

POS cI< bar > B1 6.429 6.4292 6.449 6.0533 5.400 5.2184 5.008 5.0815 4.926 4.9976 1.614 1.8147 1.575 1.575

Flow pattern legend:S - StratifiedA - AnnularI - IntermittentI B - Bubble

93


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94Test 6:

R. J. Crow, L.. Pretorius

Test 5:

Load : 200 Wl 1 : 166.3 nnl : 219.1 H"# :355.srrmFlow : 16.7lks/secControl value oosit ion : BottonPCatn> : B3.6kPa

Pos CI B< bar >1 7.283 7.2632 2.519 2.33s3 2.263 2.0834 2.003 2.0035 1.694 1.8946 1.694 1.8947 1.652 1.852

I - Intermittent

Load : 162 WI+I : 166.3 nnl 2 : 219.1 "l lrra : 355.6 mmFlow : 14.2iks/secControl value wsit ion : BottonP : 83.6kPa

POS Cl 6ibar>1 8.481 8.48i2 2.180 2.0383 1.952 1.8154 1.752 1.7515 i -676 1.6756 1.676 1.6757 1.636 1.636

Flow pattern legend:S - StratifiedA - AnnularI - IntermittentB - Bubble


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Case study of distillate pipework failuresAppendix C: Spectral analysis of wall pressure fluctuations

Test 1: 200 MW, valve at the top

Pos Line A

95

Line B

1


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96

Pos

R. J. Crow L.. Pretor iusTest 2: 160 MW, valve at the top

Line A Line B


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Pos

Case study of distilZate pipework failuresTest 6: 200 MW, valve in the basement

97

Line Bine A


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R. J. Crow L.. PretoriusTest 5: 160 MW, valve in the basement

Line A Line B

a case study of distillate pipework failures at hendrina power station

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