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Confidence in Fluid System Design

D O N M I L L E R


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Confidence in Fluid System Design D O N M I L L E R

MECHANICAL ANALYSIS

w w w . m e n t o r . c o m / m e c h a n i c a l

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Contents1. Due Diligence 3

2. 3D Loss Coefficient 4

3. Due Diligence not a Literature Review 6

4. Confidence in Internal Flow System 8

5. Origins of Internal Flow Systems 10

6. Importance of Static Pressure Measurements 11

7. Validation of BHRA Loss Coefficients 13

8. Cavitation 15

9. Compressible Flow 16

10. Transient Analysis and other Dynamic Events 17

11. The Origins of Flowmaster 20

12. Developments since the BHRA studies and the need for Standards 22

Postscript – Validation of fluid system 3D CFD studies 26

This eBook cove rs the his tor icalcontext which led to the creationof Internal Flow Systems.Encompassing both the motivesand the methods behind the book,the series will inevitably raisequestions about the reliability ofmany of the other available sourcesof available data. I also hope tomake it clear that this isn’t a

subject of academic interest alone;the subject of loss coeff icients andsystem design will prove to be acritical one as global pressuremounts for energy consumption tobe reduced. Don Miller


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1 Due Diligence The approach to engineering due diligenceis based on the ethical requirement of thecommon law in that all reasonable practicalprecautions are in place based on the balanceof the significance of the risks versus theeffort required to take all reasonable practicalprecautions.

This legalistic wording may seem irrelevantto most stakeholders involved with fluidsystems but “risk” can be related to the level ofcare/attention that one would reasonably beexpected to take to the generation and use ofloss coefficients. The level of care/attentiontrail extends from lectures who educate futureengineers, supervise researchers and writetext books on fluid mechanics to governmentdepartments who are now funding studies topersuade industry to optimise fluid machines/system performance in order to reduceenvironment pollution from needless energy

use.Virtually all validatable loss coefficient datawas generated by research teams who wereactive for more than 5 years from 1935onwards. Teams were able to build on priorresearch work and acquire the skills to measurecomponent loss coefficients under definedconditions. Much of the loss coefficient datain text books and design guides was gatheredprior to the 1960s. Without the understanding

of the variables affecting loss coefficients orwith the experimental facilities and techniquesneeded to generate validatable data. Oneshould bear in mind that many experimenterscould be considered to have exercised duediligence at the time of their experiments butthat knowledge gained since the experimentshas shown that parameters important togenerating replicable loss coefficients werenot understood or controlled at the time of theexperiments.

Through this series I aim to motivate

stakeholders involved with fluid systemsto adopt and/or to promote validated losscoefficients and calculation procedures insimulating fluid systems. For many engineersinvolved with the design of fluid systemssuch an aim should, rightly, be viewed withextreme scepticism. They may have tens ofyears of experience in designing complex andoften specialised fluid systems in successfuloperating plants. Why should they changefrom the loss coefficient data they have been

using? Well it’s because the “goal posts” havebeen moved. Plant owners have come tounderstand that they can increase profits bymeeting their moral responsibilities to reduceenvironmental pollution through minimumpower use, and that minimum power userequires fluid machines to operate close totheir design point. What this means in practiceis designing fluid machine installations forminimum lifetime costs.

To meet plant owners requirements forminimum life time costs engineers need topractice due diligence in regards to the dataand calculation procedures they use. Thisis different from past practices where thefact that a fluid machine was oversized wasoften considered a benefit as it guaranteeda fluid system would never be the cause ofthe output of a plant being restricted and anoversize machine would allow for future plantexpansion. Now if an owner wants an oversizedmachine it should be in the specification for

the plant.IFS was written before the terms validated dataand due diligence gained the prominencethey have today. The experimental work thatunderpins IFS was setup as a “once and forall” approach to establishing loss coefficientsfor common components in large fluidsystems. Some documented changes in losscoefficients were made between the firstand second edition of IFS. The need to recordchanges was brought home by comments

that loss coefficient data from the forerunnerpublication (Internal Flow) was used in safetycases and the presentation of the data hadbeen changed in IFS.

Since publication of the first edition of IFS usershave provided comments on the content of thebook along with details of typological errors.One could say these users were practicing duediligence to the benefit of other users.


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2 3D Loss CoefficientsA one-dimensional (1D) approach is taken tothe simulation of fluid system performance.A system is treated as nodes connectedtogether by pipes with the nodes representingcomponents. A component is eitherdynamic, such as a pump, or non-dynamic,such as a bend. Non-dynamic componentsare represented by a loss coefficient whichdepends on Reynolds number, and geometric

parameters.Most components are treated as having zerolength. All the pressure loss associated witha component is assumed to occur at a node,even if part of the loss occurs many diametersafter a component. If simulation of componentlength is important, such as in the study oflocal fluid transients in a heat exchanger withlong tubes, a component can be representedas a series of nodes joined by short pipes withthe component losses distributed across the

nodes.A non-dimensional bend loss coefficient (K) isdefined as:

Where (ΔP) is the total pressure loss

ΔH is the head loss

U is the bulk velocity

ρ is the fluid density

The Reynolds number is given by:

Re = UD/ν

Where ν is the kinematic viscosity

D is the pipe diameter or the hydraulicdiameter given by:

D = 4 x cross sectional area/perimeter

This definition attributes to a component

all the pressure loss ΔP caused by thecomponent. Although this loss coefficientis used in 1D fluid system simulations it is ineffect a 3D coefficient as it accounts for theeffects of complex 3D flows within a pipingsystem.

In Internal Flow Systems (IFS), losscoefficients are put into three classes. Class1 loss coefficients are for components withinstallation conditions that meet the followingrequirements:

• Flow passage geometries that are accuratelydescribed, usually by two non-dimensionalgeometric ratios and by one or two shapedescriptors.

• Hydraulically smooth component internalsurfaces

• A sufficiently long hydraulically smooth inletpipe to provide a developed frictiongradient prior to a component

• A sufficiently long hydraulically smoothoutlet pipe or passage to re-establish adeveloped friction gradient downstream ofa component

• Known Reynolds numbers

• Pressure loss over and above developedpipe friction loss attributed to a component.

To apply Class 1 coefficients to industrial fluidsystems correction factors are applied to theloss coefficients to account for:

• Geometric parameters including surfaceroughness that differ from those of Class 1.

• Installation parameters that affect inlet andoutlet conditions, such as inlet and outletpipe lengths, pipe surface roughness andinteraction effects with other componentscaused by flow conditions generated by onecomponent affecting another component.

• Reynolds number corrections if losscoefficients are provided at a fixed Reynoldsnumber.

• Applying corrections to Class I coefficientsputs them into Class 2.

Class 3 coefficients in IFS were consideredto be the best available loss coefficients butover which there are uncertainties of theirreliability.

P HK

U U 2

1

2

12 2t

D D= =


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Why use loss coefficients in theform in IFS in Fluid System Design

The method of calculating fluid systemperformance in IFS is the most scientificallybased method devised. It provides a simpleto understand and coherent approach forthe application of loss coefficients in thesimulation of fluid system performance. Losscoefficients are provided for basic componentflow geometries installed in defined pipingarrangements. Correction factors are

applied to these loss coefficients to accountfor departures in component geometricflow paths and from the defined pipingarrangements. This approach requires thatloss coefficient researchers think deeply aboutarriving at correction factors rather than justpublishing more loss coefficients that areunlikely to be accessed.

The most troublesome parameter indetermining fluid system pressure lossesis flow surface roughness. There are no

satisfactory methods of measuring pipewall roughness, as will be discussed in laterarticles. The best way to get around theproblem is to carry out definitive experimentswith hydraulically smooth surfaces, whicheliminates roughness effects, and thendetermine or estimate correction factorsto apply to account for expected surfaceroughness.

The scientific method of measuring losscoefficients goes back to research studies inthe 1920s at the Hydraulic Institute of theMunich Technical University in Germany onpressure losses caused by bends. Members ofthe Hydraulic Division of the American Societyof Mechanical Engineers (ASME) recognisedthe importance of the Munich work andin 1935 a translation of the Munich workwas published by ASME. The need to basereplicable loss coefficient on experimentswith pipes and components with hydraulically

smooth surfaces was a conclusion fromexperimental work at the US National Bureauof Standards on bend loss coefficients in thelate 1930s.

The history of the Munich work and thatat the Bureau of Standards, although oftenreferenced, has not been understood atthe fundamental level by researchers. As aresult numerous studies to determine losscoefficients over the past 70 years have notgenerated replicable loss coefficients.

When I started experimental work on losscoefficients I joined colleagues whoseexperimental practices were similar to thoseof the Munich researchers, and makingmeasurements with hydraulically smoothpipe surfaces was the norm. Many otherresearchers did not start out with theadvantage I had.

Adverse gradient

Adverse gradient

Secondary flows

Secondary flows

Bend outlet velocity contours (local/mean velocity ratios)

22.5º 45º 90º

Ideal flow through a bend

Inside

I n s i d e

O u t s i d

e


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3 Due Diligence not a LiteratureReview

Reviewers of papers submitted to refereed journals can often judge the submittersunderstanding of the paper’s subject by aquick look at the references and the commentsabout the references in the literature review.All too often it is clear that the writers havenot critically reviewed the papers and set uptheir experiments to both validate and extend

existing knowledge – it’s more a question ofgoing through the motions of including aliterature review.

The description “literature review” does notconcentrate the mind sufficiently on theobjectives for accessing and assessing prior art.If a research study aims to produce loss datathat is to be used in industry to design systems,that may be life critical, then the term duediligence study rather than literature reviewis appropriate in determining how relevant

literature is accessed and interpreted.

An important reason why I stress the needfor due diligence is researchers all too oftenquote design guides as their main references.Quoting design guides is acceptable providedthe sources of the data on which a designguides is based are accessed and criticallyassessed. Unfortunately, accessing the originalsource of data is seldom carried out.

Numerous literature reviewsConcern over the lack of agreement betweenloss coefficients generated by differentresearchers has resulted in numerousliterature reviews aimed specifically at arrivingat definitive loss coefficients. The mostextensive of these reviews were funded bygovernment related organisations and carriedout by acknowledge fluid dynamics experts.Common conclusions of reviews:

• Results from different researchers are not

reconcilable• Further research is necessary in whichimportant component geometric and flowparameters should be varied systematically.

My observations of reviews of loss coefficientdata include:

• Reviewers are seldom critical of individualexperimental studies

• Reviewers do not specify how experimentalstudies should be carried out to generate loss

coefficients that others could replicate

• Reviewers do not comment on the fact thatresearchers do not experimentally replicateother’s loss coefficients before proceedingwith their own measurements; that is to saythey had no reference points to anchor theirresults in the real world.

Further Reading:Dean, W. R., 1927, “Note on the motion of fluidin a curved pipe,“ Philosophical Magazine,4(20), pp. 208-223.

Beij, K., 1938, “Pressure Losses for Fluid Flow in90 Degree Pipe Bends,“ Journal of Research ofthe National Bureau of Standards, 21, pp. 1-18.

Ito, H., 1960, “Pressure Losses in Smooth PipeBends,“ J Fluid Eng, 82(1), p131-140.

Coffield, R. D., Hammond, R. B., Koczko, J. P.,

McKeown, P. T., Zirpoli, P. J., 1998, “Irrecoverab-le Pressure Loss Coefficients for a Short Radiusof Curvature Piping Elbow at High ReynoldsNumbers,“ Bettis Atomic Power Laboratory: USDepartment of Energy, Pennsylvania.

dos Santos, A. P. P., Andrade, C. R., Zaparoli, E.L., 2014, \CFD Prediction of the Round ElbowFitting Loss Coefficient,“ International Journalof Mechanical, Industrial Science and Enginee-ring, 8(4), p94-98.


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Who is a fluid dynamics expert?

A web survey of CVs of professors of fluiddynamics throws up hundreds of fluiddynamics speciality areas. Steady statesingle phase flow of Newtonian fluids inindustrial piping systems – the dominatetype of industrial flows - is not one of thesespecialities. The lack of specialists in pipingsystem flows has meant that reviewers of losscoefficient data may have strong credentials,as regards one or more areas of fluid dynamics,but not in fluid system performance. Withouta deep knowledge of the subject there is aproblem for supervisors of research studieson fluid systems. Not only do academics havea problem in supervising research programsrelated to fluid systems there is also a problemwith inappropriate loss coefficients andunscientific methods of calculating systempressure losses being included in modern fluidmechanics text books.

Due diligence reviews of the

literature

When someone is spending many tensof thousands of dollars of someone else’smoney - usually taxpayer’s – on measuringloss coefficients one should at least expectthat researchers carry out a literaturereview diligently. This requires setting clearobjectives, one of which is to establish that theexperiments are anchored in the real world. To

do this the experimenter needs to accurately

replicate one, and preferably more, validatedsets of data.

One of the few measurements in fluid systemresearch that is readily reproducible is thehydraulically smooth pipe friction coefficientversus Reynolds number relationship. If thisrelationship is established an experimenter hasa check on the accuracy of flow measurementand on the pressure measuring systems.Next an experimenter should, if possible,reproduce validated data for a component of

similar geometry to those to be used in theirexperimental programme.

If you are carrying out a literature review youshould not accept uncritically loss coefficientsin published work. Informed scepticism isrequired given the frequency of even quitebasic errors. The fact that a paper has beenthrough a journal’s review process andhas been referenced numerous times is noguarantee that the loss coefficients it containsare for known component geometries, tested

under defined conditions. You are likely to carryout a literature review at the start of the projectwhen your understanding of the subject maybe inadequate to perform a due diligencestudy. You should return to the literaturestudy at a later date when you have a betterunderstanding of the subject. Some of themost valuable contributions have come fromobservations of what one should have doneexperimentally not what was actually done.

BHRA literature reviews

Prior to starting the experimental studies atBHRA four literature reviews were carried outrelated to:

• Bends – turning flow

• Diffusers – diffusing flow

• Junctions – combining and dividing flow

• Friction losses in large diameter straight pipes.

BHRA’s library services identified 700 relevant

references. With the benefit of the web andthe passage of time many times this number ofreferences could easily be found.

The most important aspect of the reviewswas their use by the researchers at BHRAto identifying experimental programmeswhere sufficient detail was given to be ableto replicate the experimental results. Aninteresting observation is that replicablecomponent loss coefficients were generatedby research teams who had been active for

more than five years. What this means is thatgenerating replicable loss coefficients is askill based activity by a team; several peoplehave to gain extensive experience, be theytest facility builders, instrument technicians,researchers or supervisors.


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4 Confidence in Internal FlowSystems

If we transport back to 1950s, we would findourselves at the brink of a substantial shift inthe world’s energy needs. The current powerstation units simply weren’t large enough, andto meet the population’s need for power, newdesigns were delivering ever increasing unitsizes. This continued until the mid-60s whenthe sizes stabilized but at nearly ten times the

1950s designs.

Cooling water pipes and culverts to and fromthe condensers of 660 MW generating setsare over 2 m in hydraulic diameter and theconnections are built into a power station’sfoundations. Space is limited and componentssuch as bends are closely spaced causingflows to be highly 3D. Construction is largelyin reinforced concrete with shuttering againstwhich concrete is cast allowing for flowpassage designs that would be prohibitively

expensive to make in steel - provided of coursethat one knows what the shape should be.

Funding to establish 3D losscoefficients

At the time of rapid growth in generating setsize the British Hydromechanics ResearchAssociation (BHRA) had built up a reputationfor modelling large fluid systems using air asthe working fluid. Air models construction

allowed for rapid modification of flow passage

shapes and for studying flow behaviour. Usingair models of cooling water systems BHRAdemonstrated that pump power requirementscould be reduced by a third by improveddesign practices. For a 660 MW generatingunit the saving in pump power was over 1 MW.Significant improvements were also possiblein other power station fluid systems involvingfans/blowers as well pumps.

At the time of the sustained growth in powerstation size the generation and distribution

of electricity in many countries was stilleffectively state monopolies. For Englandand Wales the Central Electrical GeneratingBoard (CEGB) in the UK, was responsible forthe design, construction and operation ofpower stations. With the size of the newpower station building programme the CEGBengineering department was in a position tofund large scale research studies that todaywould unlikely to be considered. One of thesestudies was awarded to BHRA to extend theirexperimental modelling work on cooling watersystems to provide a guide for the design oflarge fluid systems.

The study commissioned by the CEGBwas continued by funding from industrialcompanies and the UK Department of Industryculminating in a study for the CompressorCommittee of the American Gas Associationrelated to compressor yard piping. Thelater study was particularly interesting asgas compression in the US and Canada was

consuming an estimated 1 billion US dollars

per year by the end of the 1970s. 10% ofenergy added at some compressor stationswas being dissipated within the compressoryard piping.

The CEGB had its own extensive researchlaboratories that it could have used and theCompressor Committee of the American GasAssociation had the choice of laboratories. Intoday’s terminology you could say they bothexercised due diligence in funding BHRAgroup.

Transition from the analogue to thedigital age

In the 1960s the changeover from analogueto digital methods began to take off inengineering; out with the slide rule andnomographs and in with main framecomputers and software. Computers had adramatic effect on aeronautical research which,in the pre CFD age, relied almost exclusively

on wind tunnel tests involving extensive staticpressure measurements. By linking electronicpressure scanning systems and computers thecollection of wind tunnel pressure data wasrevolutionised.

BHRA was able to use the pressure scanningtechnology developed for aeronauticalresearch to both improve and speed up theprocess of measuring static pressures whentesting fluid system components. Computeranalysis of pressure measurements provided


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rapid feedback on fluid system component

pressure losses. Any unusual results could thenbe investigated using measurements of flowvelocities and turbulence levels and by flowvisualisation studies.

With computer analysis of static pressuresthe pressure gradients along a component’sinlet and outlet pipes or ducts could be

established, allowing a direct measurement

of a component loss coefficient. This replacedthe method used by most experimenters ofmeasuring the pressure difference between alocation upstream and a location downstreamof a component and subtracting a calculatedpipe friction pressure loss equivalent to thestraight pipe length between the upstream

and downstream measuring points. Many

of the loss coefficients in the literature weregathered using water flows in pipes in variousstages of corrosion and in which friction losseswere likely to have been changing with timeso that friction loss values used in determiningloss coefficients were unreliable.

BHRA council meeting circa 1979


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5 Origins of Internal FlowSystems

My first task when I joined the project at BHRAto measure loss coefficients for a design guidefor large flow systems was to commissiontwo test rigs. I was aware that commissioninga new wind tunnel to have acceptable flowconditions in the test section could takeresearchers months, particularly wheninstabilities in diffusing sections gave rise to

flow fluctuations. I naively thought it wouldbe much easier to commission a test rig formeasuring component loss coefficients. Thereality proved to be very different as it turnedinto a rapid learning exercise into many aspectsof fluid systems.

Most studies of loss coefficients have beenmade with water as the working fluid. Theneed for a fixed water source and somemeans of catching the water discharge froma test rig places serious restrictions on test rig

geometries. At BHRA the preference was to useair as the working medium for fluid system testrigs. Using air frees up how a test rig can beconfigured and the speed of reconfiguration;major factors when hundreds of single andcombinations of components were to beinvestigated.

The decision had been made to base the testrigs on 0.3 m (12 inch) hydraulic diameterpipes and ducts and to use a Reynolds numberof 106 as the reference Reynolds number.

Air velocities through the pipes and ducts

to achieve Reynolds numbers of 106 weresufficiently low (Mach numbers < 0.25) thatair compressibility corrections were small. Alltests were to be conducted with hydraulicallysmooth wall surfaces.

Size and Stability

First it is useful to get an idea of the size ofthe test facility. Straight pipe lengths of 90diameters were needed, plus inlet and outletarrangements, resulting in a test rig lengthof some 35 m; the wing span of a Boeing737. Tests involving components in 3Darrangements required a height of over 6 m. These dimensions are aircraft hangar sizes,which was not a problem as part of BHRAlabs were located in an aircraft hangar on theCollege of Aeronautics (later to become theCranfield Institute of Technology and thenCranfield University).

The test rigs were powered by 45kW fixedspeed fans. The original assumption was thatflow rate would be controlled by throttlingeither on the inlet or the outlet side of thefan. Here begins a lesson on why one shouldnot operate fluid machines away from theirbest efficiency point. Running a fluid machineaway from its best efficiency point resultsin high frequency pressure waves (noise)propagating through a system accompaniedby longer term fluctuations in mass flow ratewith accompanying pressure fluctuations.Fluctuations in flow rate are common in

fluid systems but since they are usually not

important, as regards the functioning of asystem, they go unnoticed. When one is tryingto establish reliable and reproducible losscoefficients fluctuations in flow and pressureare unacceptable; some of the componentstested suffered violent flow instabilities andcaused significant flow fluctuations, a featurethat needed to be noted to provide advice oncomponent flow geometries to avoid.

The solution for the test rig instabilities wasto bleed air into or out of a test rig local to a

fan to maintain conditions across a fan so thatit operated close to its best efficiency point.Substantial flow structures incorporatingcontrollable air bleeds were built to provideappropriate conditions into and out of a fan.Within the structures vanes, screens andhoneycombs were located in areas of diffusingflow – always an unstable process - to reducepressure fluctuation to a negligible level.

Today with cheaper variable speed drives forfans one would be strongly advised not to use

a fixed speed fan for an experimental programof the sort carried out at BHRA. Much is nowpublished about the benefits to industry ofusing variable speed drives for fluid machineryto reduce lifetime costs of pump and fanownership.


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Test Cross-Sectional Shapes

An aim of the loss coefficient studies was toprovide loss coefficients for large flow systemscovering tunnels, pipes, ducts and culverts.Large fluid systems, such as power stationcooling water systems, have pipes/culvertsthat are either circular or rectangular with anaspect ratio of 2:1 or less. The decision was touse 0.3 m diameter pipes, 0.3 m x 0.3 m squareducts and 0.23 x 0.46 rectangular ducts, allof which had a hydraulic diameter of 0.3 m(12 inches). Square and rectangular culverts

constructed in reinforced concrete have smallcorner fillets so the square and rectangular testducts were constructed with 25 mm filets inthe corners; in practice the corner fillets hadno important effect on loss coefficients. The2:1 aspect ratio rectangular ducts allowedaspect ratio 0.5 and 2 components to be testedincluding interactions between aspect ratio 0.5and 2 components. For components involvingdifferent inlet and outlet pipe diameters 0.2and 0.36 m pipe diameters were used.

BHRA had a team of pattern and model makerswith experience of fabricating componentssuch as bends, diffusers and junctions, to tighttolerances.

Measuring System

Flow rates were measured using nozzlesat inlet to the test rigs and when requiredorifice plates for dividing and combiningflow tests. Numerous measurements of flowdistributions were made using pitot tubes andhot wire anemometers, and some of thesemeasurements were integrated to determine

flow rate as a check on the calibration of the

flow metering devices.

Pressures were measured using a scanningsystem with a single pressure transducer.Calibration was against Betz manometers -high accuracy manometers commonly usedas pressure calibration instruments in industry.Reference Betz manometer readings weremade during each set of pressure scans to beused as a check of the pressure transducerreadings. Output was to punch tape whichat the end of a day was taken to the nearest

commercial mainframe computer installationalong with the analysis program on punchedcards. Later, when Cranfield University put up anew building to house an ICL 1900 computer,analysis runs were made overnight on thiscomputer (how computers have changed astoday a portable computer have more powerthan a 1970s main frame computer).

6 Importance of Static PressureMeasurements

Holes drilled through the skin of modelaircraft and connected to monometers orpressure gauges played a crucial role in thedevelopment of the aerospace industry. TodayCFD is fundamental to aircraft design but thehumble static pressure tapping still plays amajor role in aerodynamic and hydrodynamicadvances. Wind tunnel model makers arehighly skilled in making static pressure tapings,

which they can view under magnification toensure the tapings were free from defects.

What cannot be seen?

Measuring loss coefficients for fluid systemcomponents similarly relies on static pressuretapings but now the taping termination isinside a pipe or duct. Anyone who has drilledthrough metal knows that when the drillbreaks through the finish is usually not a prettysight. You can ream a hole but this is difficultfor small holes, and since the inner surfaceof a pipe in the vicinity of a taping can bedifficult or impractical to access considerableuncertainty can exists over whether a good

taping is achieved. During checking ofpipe wall tapings I have experienced errorsexceeding 10% of the mean dynamic head andthat is from tapings that at first sight lookedOK.

In the case of tests with water in steel pipes thesharp edges of static pressure tappings tendto cause concentrated corrosion. Ridges formaround a tapping entrance so there are alwaysuncertainties over static pressure readings incorroding pipe.

Piezometric Rings

Concern over tapping errors have resultedin the widely adopted practice of using atapping ring with four tappings at a particularlocation; referred to as a piezometer ring.If all four tappings record the same valuethen connecting the tappings together canprovide an accurate and a more responsiveconnection to a pressure sensing device,albeit replacing one joint where leakage canoccur with typically thirteen joints in makingthe piezometer ring. If tappings are not well


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made flow circulation is set up in a piezometric

ring and a pressure somewhere between thehighest and lowest value is sensed. ASHRAEStandard 120-2008 “Method of Testing toDetermine Flow Resistance of HVAC Duct andFittings” requires tappings of a piezometric ringto agree within 2% of the dynamic head, whichfor measuring 3D loss coefficients is muchtoo high. For hydraulically smooth pipes oneshould be aiming for 0.2%.

In the BHRA tests at each measuring locationthere where 4 tappings evenly spaced around

a pipe. Each of the tappings was checkedagainst the other 3 and remedial action takenif difference were observed. One of the fourtapping was chosen and the other threetappings blanked off. Loss coefficients weredetermined based on a set of 8 tappingsspaced 2 diameters apart upstream of acomponent and 8 tappings spaced 2 diametersapart starting 40 downstream of a component.Any problem with a tapping or leakage froma connection between a tapping and pressuretransducer or manometer was detectable frominspection of the pressure gradients along apipe.

The most critical static pressure tappingreadings are those for the flow measurementdevices. In the case of the BHRA studies ASMEelliptical nozzles taking flow from a large spacewere used at the pipe inlets. The nozzles had apiezometer ring with all the tapings checked.A leakage check on the tapping connectionswas carried out on each day of testing.

Choose test pipes/ducts carefully

The first tests of the BHRA studies were madewith 2:1 rectangular ducts and components.BHRA had adopted a practice of using highgrade hardboard with support frameworkfor making ducts for air models of fluidsystems. The hardboard surface finish wassuch that hydraulically smooth surfaceswere guaranteed. During trials to check thetest duct static pressure tapings small butpersistent inconsistencies in static pressurereadings were detected between tapings on

the long and short sides of the duct. The 0.23x 0.46 m duct cross-section was larger thannormally used for fluid system models and thepressure differences from atmospheric greater.A small amount of wall deflection, which variedwith pressure differential, was the cause of thestatic pressure inconsistencies. Reinforcingthe duct wall would still have left doubts sothe duct was scrapped and 25 m of new ductconstructed in high grade 12mm thick birchplywood. A hydraulically smooth surface was

achieved by 5 coats of varnish, with the coatsrubbed down in between coats.

For the tests involving 0.3 m diameter pipesextruded PVC was originally chosen. Whenchecking the static pressure tapings smallinconsistencies were found. Although a firstsight the internal surface of the pipe appearedto be very smooth and free of defects onclose inspection small ripples on the boresurface were detected and slight variationsin bore diameter. To remove all doubt aboutthe validity of static pressure readings the

decision was made to abandon the use of 0.3

m diameter PVC pipe and make fibreglasspipe. Layup of fibreglass was onto a precisionformer so that a hydraulically smooth boresurface was achieved with tubes for staticpressure measurements included in the pipewall. The decision to use fibreglass turned outto be a very fortuitous because when it cameto component interaction tests, with 3D pipelayouts, substantial lengths of the light weightand ridged pipe could be supported using amobile structure. This allowed changes in 3Dpipe layout to be made rapidly.

In the case of the BHRA studies all of thecomponents were accurately made by skilledcraftsman experienced in model building.Flanges of pipes and ducts were drilled usingtemplates so that bores were free of steps.Pipe bends and diffusers were of fibre glasslaid up on individually moulds to provide bothgeometric similarity and hydraulically smoothflow surfaces. Square and 2:1 rectangular ductcomponents where made of plywood with insome cases one flat side made of Perspex.


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7 Validation of BHRA LossCoefficients

BHRA had over 15 years of experience inmeasuring component loss coefficients whenwork started on the project to produce losscoefficient data for the design of large fluidsystems. In this time facilities, experienceand knowledge had accumulated related tomeasuring loss coefficients. It was only laterthat I came to understand how much this

contributed to setting up and carrying out theresearch project to provide data and guidanceon the design of large flow systems.

My perception on joining the project waswe were carrying out “once and for all”experiments on loss coefficients. That is to saythat:

• First and foremost the loss coefficientsmeasured and the way they were presentedhad to meet industry needs when simulatingand designing fluid systems,

• Appropriate precautions were taken toensure the accuracy of the test facilities, themeasurement and recording instrumentationand the data analysis methods, and

• The work of other research teams, who wereknown to have measured loss coefficientsaccurately and under defined conditions, wasreplicated and that sufficient information wasprovided for others to replicate the BHRA losscoefficients.

In today’s parlance we validated the loss

coefficient data and made it available in a formthat engineers could readily use and otherresearchers could use to validate their work.

Testing Models

Experimental fluid dynamics is all about testingmodels and using non-dimensional numbersthat allow the flow physics at different modelscales and with different fluid properties tobe compared. A well known use of modellingis measuring the drag coefficient of a modelaircraft in a wind tunnel. There are veryrestricted boundaries to a wind tunnel test,however, if the interest was to study the crashof a light aircraft that flew into the wake some5 kilometres after a large aircraft, then usinga normal wind tunnel would be of little use. Itwould be necessary to go from studying theflow physics local to the model to studying thecomplex flow swept downstream of the model.

In piping systems disturbances caused bycomponents are swept downstream and,

at high Reynolds Numbers, are only slowlydamped. The presence of some componentsis detectable two hundred pipe diametersdownstream. For true similarity a pipingsystem test rig should be sufficiently longenough to isolate all disturbances that couldaffect a loss coefficient, in practice this isimpractical and unnecessary.

From experimental observations it has beenfound that if :

• Flow conditions within a pipe are such that

the friction gradient prior to a component isessentially constant, and

• A constant friction gradient is established inthe pipe downstream of a component,

then:

A 3D loss coefficient can be defined as thedifference between the inlet and downstreampipe friction gradients projected to thecomponent location. For all intents andpurposes geometric similarity, as regards test

rig length, is satisfied by this loss coefficientdefinition. This definition is easily adapted forcomponents without inlet or outlet pipes andfor dividing and combining flows.

For the BHRA studies an inlet pipe lengthof 30 diameters after an inlet nozzle and 55diameters of pipe after a component weretypically used to satisfy the pipe/duct lengthrequirement. Friction gradients were measuredover 15 diameters before a component andover the last 15 diameters downstream.

There are components where theestablishment of an essentially constantfriction gradient upstream of a componentis not a sufficient criteria to achieve areplicable loss coefficient and there are othercomponents that cause friction gradients tobe above the steady state value for surprisingdistances downstream. Examples of theseeffects are discussed below as they illustratesome of the cross-checking carried out at BHRAin validating loss coefficients.


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Internal flow phenomena still to beunderstood

Over the 15 year period of investigations usingthe two BHRA test rigs, a number of researchengineers carried out studies using the rigs.If a researcher was to use combinations ofcomponents previously tested the first testswere to re-test the components singularly.On one occasion the loss coefficient for a7 degree half angle diffuser differed fromprior measurements by about 0.05 of theinlet velocity head, which was 5 times theexperimental tolerance set for measurements. The area ratio of the diffuser was 1.37 andwith a half angle of 7 degrees meant it wasoperating close to what is known as thetransitory stall region where areas of separationform and are then washed out of a diffuser. Test on other components gave results thatagreed with prior measurements.

To try and establish the reason for thedifference in the measured diffuser losscoefficient, the two test rigs where assembled

with 30 diameters of inlet pipe and thediffuser tested on each rig. On one rig the losscoefficient was the same as recorded severalyears before and on the other 0.05 lower. The complete inlet pipes were exchangedbetween the two test rigs without alteringthe measured values. The inlet measuringnozzles, with their calming and conditioningsections (these were mounted on castors toallow rapid repositioning of an inlet when thetests involved air being drawn through the test

rigs), were exchanged between the test rigs.

The diffuser performance followed an inletnozzle, so what was causing the difference inperformance was linked to the inlet nozzleswith their calming sections. Velocity surveysat the nozzle outlets and 30 pipe diametersdownstream showed no anomalies.

At his stage no further work was done, partlybecause of experience with diffusers operatingclose to the transitory stall region. BHRA hadmodelled a number of large civil engineeringdiffusers – a jumbo jet could be parked in some

civil engineering diffusers. To minimise thecost of such diffusers they are usually vannedand operate with some areas of transitory stall.Seemingly small disturbances well upstreamof these diffusers have been found to havea marked effect on their performance. It isnow known that the turbulence structure inpipes is quite different from that assumed atthe time of the BHRA tests. Very large scaleorganised structures occur in turbulent pipeflow. To visualise such structures requires 3Dinstantaneous measurements of turbulencestructures which has only recently beenpractical. Whether or not the arrival of suchlarge scale turbulence structures coincidedwith or affected the growth and washout ofdiffuser stall regions is unknown.

Propagation of disturbances

Information about disturbed conditions at alocation in turbulent pipe flow can take 30 orso diameters to propagate half way across a

pipe and since the turbulent structure will not

be “developed” another 30 or so diameters forinformation that the structure is not matureto propagate back. This probably goes onthrough a number of cycles of decreasingamplitude.

The effect of flow development towards a “fullydeveloped” structure is usually not of interestin the design of fluid systems. However, inthe measurement of friction and pressureloss coefficients some interesting effectsare observed. One of these was the high

length of square duct required to achieve adeveloped friction gradient after a transitionfrom a smaller area pipe to a square duct.With a 0.2 diameter pipe connected by adiffusing expansion to 0.3m square duct thefriction factors measure over a duct section62 hydraulic diameters, starting 17 diametersdownstream of a transition, were significantlyhigher than when a 0.3m diameter pipe wasconnected by a transition to the duct.

The first check was whether the flow nozzle

calibrations were correct. With a transitiondown to 0.2m diameter after a 0.3 nozzlefollowed by a transition from 0.2m tothe 0.3m square duct similar friction lossmeasurements as with a 0.2m diameter nozzleso flow measurement was not a problem. Theobservations provided another validation ofthe experimental rigs.


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8 Cavitation The Reynolds number is the only non-dimensional parameter involved in the flow ofNewtonian fluids through geometrically similarcomponents until:

• Dynamic events cause the pressure withina liquid flow to decrease sufficiently below aliquid’s vapor pressure for cavitation to occur

• Flow velocities exceeds 20% of the speed of

sound in a fluid and compressibility begins tobecome important.

Cavitation is defined as the formation of abubble or void in a liquid, but in engineeringwe use the term to encompass all aspectsof the growth and collapse of bubbles andcavities. In particular it is the collapse of vaporfilled bubbles and cavities that is of concernin fluid systems as these events cause noise,damage, deterioration in performance andflow instabilities.

The serious problems caused by cavitation influid machinery, valves, hydraulic structures,etc. has resulted in a vast literature onthe subject but we are a long way fromunderstanding cavitation at the fundamentallevel. The number of variables involved,the speed at which events occur and thedifficulty of measuring these events, are allrelative unknowns. Cavitation is usually anunavoidable consequence of what a system isrequired to do or how it must operate under

some conditions, such as start up or shut down.

If cavitation could give rise to problems weneed to be able to predict this and then decideon measures to avoid or alleviate problems.

Cavitation prediction and design proceduresin IFS are based on the velocity in thepipe upstream of a component. This isconsistent with the pressure loss calculationprocedures in IFS as loss coefficients aremainly based on a component’s inlet velocity.It is also compatible with the results froma major source of reliable cavitation test

data generated by studies funded by theMetropolitan Water District of South California(MWD) - the largest bulk water supplier formunicipal in the world. MWD needed pressurebreakdown stations located in hilly populatedcountry that dissipated substantial amounts ofhead without nuisance noise. The MWD nowuses structures in which multi-ported slidevalves are located. These valves produce manysmall jets that discharge into a large volumeof water; the best way of dissipating excesshead without generating excessive noise.

The fact that cavitation takes place aroundsmall diameter jets in a large volume of watermeans there is no cavitation damage and thetotal energy converted to nuisance noise frommultiple small jets is very low compared tothat from a single or a few large jets. In pipingsystems the most effective low noise valveshave trims with many parallel flow paths togenerate multiple jets.

At BHRA we modeled many flow situations

involving cavitation, principally to avoidcavitation damage and noise but also toavoid negative pressures imposing loads onstructures. Trouble shooting cavitation inoperating flow systems was usually becauseof incorrectly specified valves. Since IFS waswritten, serious damage to large flow systemshas highlighted problems in carrying outmodel studies to predict whether or notdamaging cavitation will exist. Water flowingout of the base of a deep reservoir or a verypure process liquid may have a low dissolvedgas content compared to that of water used inmodel studies. In these circumstances modelstudies are likely to underestimate the severityand the distance downstream that cavitationextends. Fortunately air can usually be bledinto cavitation zones in hydraulic structures toprevent serious damage. This option is usuallynot practical in piping systems.


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9 Compressible FlowSimulation of compressible flow in fluidsystems encompass:

• Slowly varying pressures in long astransmission lines when the gas temperatureis assumed to be constant so isothermalconditions are assumed. Alternativelycalculations take account of ambienttemperature variations along a pipeline andheat transfer to and from a pipeline.

• Rapidly varying conditions as gas flowapproaches or reaches Mach 1 and chokes.In this situation events occur so rapidly thatadiabatic flow conditions can be assumed toapply.

• Flows in which chemical reactions, or achange of phase, or high rates of heat transferor a combination of these phenomena are

taking place. These flows often involve safetyissues and need the involvement of specialists.

At BHRA we were mainly involved with type2 flows that could generally be treated asadiabatic. Back in the 1970s it could take aninordinate amount of time to find informationto answer question about compressible flow.Information was sparsely distributed in theliterature, and in those pre-internet days,difficult to find. There was also the problemthat much of the information and data was ofa contradictory nature. The difficulty of findingthe answer to compressible flow problemslead to a chapter on compressible flow being

included in the second edition of IFS.

Compressible flow is predominately the

domain of aeronautical fluid dynamists witha vast literature. The situation is very differentand more complex for flows at comparableMach numbers in fluid flow systems. Theequipment and facilities needed to carryout compressible flow tests are substantialand no university or other organisation hascarried out long term studies covering a rangeof components. In modern fluid mechanicstext books practical information related toindustrial systems is poorly or not covered.

At BHRA we carried out a number ofcompressible flow studies related to industrialproblems. These studies at least made meaware of some of the difficulties of carryingout compressible flow studies. We had a largeblow down vessel originally installed for fluidsystem noise studies. As the source pressurewas higher than needed at inlet to a testcomponent the flow had to be throttled by avalve. Often the pressure difference across thevalve was sufficient for choked flow to occur.Choked flow at a valve generates pressure

fluctuations, which in the test situations couldbe significant compared to the pressureslosses being measured. If a component undertest choked then situations arose where thelocation of choked flow alternated betweenthe throttling valve and the test component.Needless to say some interesting sounds,pressure fluctuations and forces on pipingoccurred. These experiences illustratedthe difficulties researchers face in studyingcompressible flow in fluid systems.

When it came to providing design and

calculation procedures for compressibleflow the best approach, as far as I couldsee, was to use incompressible flow losscoefficients and apply correction factors totake account of compressibility. Experimentaldata in the literature usually reported losscoefficients that increased rapidly as Mach1 was approached. However, in reanalysingexperimental data I concluded that in generalthis was a misinterpretation of the measuredstatic pressure readings. The problem withinterpreting static pressure measurements isgas density, static pressure and temperaturechange rapidly as choking conditions areapproached. Boundary layer effects meanthat the effective flow area is slightly reducedand choking occurs before it is predicted byassuming 1D flow. Allowing for boundary layereffects incompressible loss coefficient data cangenerally be used up to Mach numbers of 0.8.

When someone phoned up with a query abouta compressible flow problem I invariably askedquestions to collect relevant information

and then said I will call you back. Answeringquestions about compressible flow “off thetop of one’s head” is never advisable as oneneeds to get into a different mind-set whenthinking about compressible as against non-compressible flows. If a flow is likely to chokethen some deep thought needs to be put in,particularly if process control or safety issuesare involved.


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10 Transient Analysis and otherDynamic Events

Events occur in fluid systems that causepressures to rise above or fall below the steadystate pressure. Well-known, is water hammer.Liquid flow is forced to stop abruptly and apressure rise occurs that is proportional tothe velocity of the liquid times the speed thatpressure waves travel along a pipe. As pressurewaves travel at over 1200 m/s in many pipes

carrying liquids, it is easy to understand whydamaging pressures occur. In the case of ourblood distribution system, large fluid transientpressures do not occur because our pipes(arteries) are so flexible that pressure wavesgenerated by our heart only travel at 10 m/sor so, although the speed of sound in blood is1570 m/s.

In my 30 years at BHRA numerous fluid systemswere simulated to minimise and/or alleviatefluid transients. Systems were also studied that

had failed catastrophically or were sufferingoperating problems caused by dynamic events.Dynamic events included excessive noise, flowinduced vibrations, instabilities arising fromunstable flows through parallel flow paths,and small amplitude oscillatory flows thatsynchronised with the natural frequency ofpart of a system to cause large pressures andforces.

Situations leading to rapid vapourcondensation are of particular concern as

liquids flowing into a collapsing vapour cavitycan reach high velocities resulting in water

hammer pressures causing catastrophic failure

and loss of life. At BHRA we simulated onesuch event by creating a vacuum in a sectionof pipe in which a valve similar to one that hadfailed, causing a fatality, was installed. When anisolation valve was opened to allow pressurisedwater to flow into pipe section under vacuumone of the most disturbing events to me wasthe displacement of the whole facility bythe transient event. Generating pressures ofover 100 bar, by closure of a vapour cavity,reinforced my appreciation of the forcesinvolved in fluid transients.

Due diligence is particularly important inunderstanding and analysing systems for fluidtransients and other dynamic events. Usingvalidated transient simulation software, suchas the Flowmaster transient solver, is a vitalpart of the due diligence process but it mustbe accompanied by the exercise of informedengineering judgement. In the past it couldbe claimed that particular events could nothave been foreseen but so many failures andproblems caused by rapid transients and flow

induced vibrations have been documentedthat such a claim would now be more difficultto make. Intelligent software guidance onlikely dynamic events in fluid systems issomething for the future.

Avoidance of pressure transients by design ispreferable but often not practical or possible. This is followed in order of preference bypassive protection measures. The best passiveprotection against catastrophic system failureis for the containment strength of every part

of a system to be greater than that needed tocontain the worst fluid transient events that

could occur but again this is not an option for

many fluid systems. This leaves active transientalleviation devices and operating procedures.Active transient mitigation devices have thedisadvantage that they require maintenancethroughout the life of a fluid system, withparticular attention to system modifications.Relying on operating procedures, such asthe timing of pump or valve operations relyon operators carrying out actions correctlyfor the life of fluid systems, that can be overtwenty years. Ongoing awareness needs to bemaintained by management and operatingstaff so as to avoid system changes andoperator actions that could trigger a seriousfluid transient. A significant percentage ofreports of system failures due to transients arefor systems that had been operating withoutproblems for more than ten years.

In addition to withstanding transient pressures,a piping system must withstand unbalanceloads imposed on a system by fluid transientsand the resulting pipe movements. Picturesin the literature of pipes displaced from pipe

racks bring home the need to design for theunbalanced forces transients can generate.In the case of plastic and fibreglass pipes,pressure waves that travel through pipelineswith bends give rise to the risk of leaks causedby sharp objects rubbing against pipesrepeatedly displaced by transient events.

The potential for loss of life and catastrophicdamage caused by transients is reflectedin design standards for pipelines and therequirement to take all reasonable precautions

in the design, operation and maintenanceof piping systems. The beneficial practice of


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designing pump installations for minimum

life time costs of ownership can usefully beextended to include protection against fluidtransients. In installations where transientsare generated by pump start-up appropriatecontrol of a variable speed pump might reduceboth life time costs and the risk of transientpressures and could justify the higher initialcapital costs of a variable speed drive.

Developments leading toFlowmaster’s transient analysis

capabilities

The comprehensive transient analysiscapabilities in Flowmaster have their originsin software originally developed, used andvalidated by BHRA. Through the 1950s andearly 1960s BHRA was providing transientanalysis services to industry using graphicaland analogue computer methods. Thetransition was made to digital computers inthe early 1960s and by 1975 large and complexfluid systems in the power, oil and gas andwater industries were being analysed. BHRA’ssimulation software had grown over the yearsto deal with the increasingly complex systemsthat were being built but the software wasbecoming difficult for new users to understandand to modify.

In 1975 the decision was made to developfluid transient analysis software that wasnot constrained by assumptions about fluidsystem layout. Requirements for the software

included its use by engineers who were

knowledgeable about fluid systems but werenot computer programmers. An aim, whichwas achieved, was to develop the software andits documentation to the standard required tosell the software for use by fluid specialists inindustry.

The design and programming of BHRA’stransient software, called HYPSMOP, wascarried out by a team led by Mike Papworth. The approach adopted for the structure ofthe software and the solution methods is

recognisable in all modern transient and steadystate simulation software.

A fluid system is represented as modulesconnected at nodes. Modules included:

• Dynamic components - pumps, turbines,control valves;

• Passive components such as pipes and bends;

• Physical transitory events such as the growthand decay of vapour cavities or air pockets;

• Prescribed flow, pressure and/or time history;• Passive and active devices and structures fortransient pressure and surge control; and

• User defined modules coded using tools builtinto the software.

Nodes represented connection points to whichany number of modules could be connected.A system could be assembled by the user byselecting modules from a library and inputtingappropriate data for the module. During an

analysis information about time dependent

events, such as a pump tripping or a valveclosing, was accessed from information thatthe user entered into the software.

Validation of the HYPSMOPsoftware

Validation studies were carried out to confirmthat transient pressures measured on pipingsystems were realistically predicted byHYPSMOP. Some systems used for validation

were analysed prior to field measurements. Forsystems that had failed due to a transient eventthe cause of failure was reproduced, remedialmeasures implemented and measurementscarried out to confirm the effectiveness of theremedial measures.

Conclusion from comparison of predicted andmeasured pressures and timing of events were:

• In systems where pressure wave velocitiesthrough a system could be accuratelypredicted the calculated pressure variations

and their phasing were in close agreement;and

• In systems where gas bubbles, gas pocketsor flexible tubing or hoses were involvedmaximum pressures where usually overpredicted and the phasing of maximum andminimum pressures displaced in time. Bychanging the wave speed close agreementcould be achieved for pressures and eventtimings.


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The presence of gas bubbles in liquid flows,

such as in rising sewage mains, substantiallyreduces the speed at which pressure wavespropagate. In such circumstances one shouldalways search for values of wave speed thatgive rise to the maximum pressures. It mustbe stressed that great care should always beexercised in the analysis of liquid systems inwhich pockets of gas exist. Venting of gas

from a system can generate severe transients

and the presence of an air pocket may allowhigh liquid velocities to be reached on pumpstart up or on opening a valve. If a liquid isflammable compression of air pockets can,and has, caused temperatures to exceedthe ignition temperature with catastrophicconsequences.

Since in many cases it is not known what

amount of gas is present, simulations shouldcover a range of pressure wave propagationspeeds and gas pocket volumes. The samecomment applies to pipe friction where frictionvalues that could exist over the lifetime of apiping system should be simulated.

Flowmaster at Amazon Computers Ltd. circa 1970s. Flowmaster V7 running on Windows computers 2016.


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11 The Origins of FlowmasterWhen I was writing Internal Flow Systems(IFS) in the 1970s, I was also investigatingproblems with large flow systems in theprocess industries (chemical, oil and gas,water treatment, etc.). The most commonproblem was oversized pumps. One of theworst was 50MW of installed cooling waterpumping power when 30MW would have beenadequate.

Oversized pumps mean plant owners arebuying electricity to cause flow recirculationwithin pumps. Flow recirculation results incavitation, noise and vibration. The life of pumpseals and bearings is reduced, maintenancecosts are increased and less product isproduced because of plant downtime. Not avery sensible scenario.

In the 1970s, papers were being publishedon a wide range of problems in the processand power plants caused by oversize

fluid machines. One of the reasons foroversizing was that flow system head losseswere calculated using component lossdata generated in the 1930s. These 1930’smeasurement of loss coefficients mainly relateto steel pipes and components in unknownstates of corrosion. Details of the proceduresused to measure pressure losses and calculateloss coefficients are mainly unknown.

Hand calculation of fluid system head losses isa tedious and time consuming process. Great

ingenuity was exercised through the 1930s to

1950s in devising “aids” to speed up the processof calculating fluid system head losses. Thisresulted in design guides that had a few pagesof friction and component loss coefficient datasurrounded by many pages of nomographs,charts, tables and formulae. A young engineergiven the task of calculating head losses forthe piping in a process plant soon becameproficient at using one of these design guides.Not unnaturally when engineers reached aposition of responsibility they required juniorengineers to use the same design guide. Go

on the web today and lo and behold designguides recommended by senior engineersand writers on process plant design are thosethat contributed to pumps being oversized inthe 1970s. You can guess the outcome, withanecdotal evidence that today up to seventyfive percent of pumps in the process industriesare oversized.

Only simple formula are needed to applythe validated data in IFS but there is still theproblem of calculating fluid system head

losses for a process plant with numerous fluidsystems. It was very obvious at the time ofwriting IFS that widespread use of the data itcontained was only going to occur if it couldbe readily applied and that meant it had to be“computerised”. Putting the data into softwareis only one condition for its widespread use.Legislative, safety, and economic pressurescombined with the desire by engineers to usevalidated data being the primary drivers.

Concerns over safety and fluid system

failures had already brought about (forced)the investment to be made into software tocalculate fluid system transient behaviour.At BHRA in the 1970s, our transient softwarealready represented fluid systems in thecorrect form for steady state analysis; thefirst calculation carried out before simulatingtransients had to be the steady state. Thebig change from a program to calculate fluidtransients to a general purpose softwarefor fluid system analysis was that the losscoefficient data from IFS had to be contained

within the software, rather than fed in as it wasfor the transient analysis software. From theoutset the aim in developing the software thatwas to become Flowmaster was:

• It had to be easy to use;

• It would have a core that contained IFS dataand the calculation procedures that appliedthe data;

• That it would be validated against operatingfluid systems;

• It had to have a modular structuretoallow modules customised for particularindustrial sectors – process, power, aerospace,automobile – and for particular types of flow –compressible, transient, two phase, etc.; and

• That the loss coefficient data in the corewould be represented, where possible, in theform of performance charts.


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This latter requirement of representing data

in the form of digitised performance charts isvery important but a requirement that is notreally understood by developers of other fluidsystem software and by researchers measuringloss coefficients. The ideal is for a chart torepresent loss coefficients for a particular typeof component, such as circular cross-sectiondiffusers, on which lines of constant losscoefficient are mapped. In the case of diffusersthe chart axes are the area ratio and the non-dimensional length. Such charts, first plottedby Kline’s group at Stanford University, contain

a lot of information other than loss coefficients.If you have some knowledge of fluid flowbehaviour, then looking at a chart you candeduce whether the flow through your diffuserwill be stable or whether it will operate withmildly transient or violent stall. You can make areasoned guess as to how distorted the outletflow will be into the downstream componentor process – the consequential costs ofdesigners specify diffusers with unstable flowconditions and with poor outlet conditions

runs into many billions of dollars with in somecases plant commissioning being delayed bymany months.

A researcher only needs to test a limitednumber of components in a particular familyto produce a performance chart for thewhole family. If a researcher’s measured losscoefficient does not sit comfortably relativeto lines on a performance chart, eitherthe measurement is wrong or some flowphenomena is occurring that needs furtherinvestigation.

I have been asked a number of times by writers

of fluid system software “could I give themthe formula on which a chart in IFS is based”. They knew the Moody friction factor chart wasmainly plotted using an equation so why notcomponent loss coefficient charts. Ideally theMoody chart should be replace by a chart thatmore accurately represents experimental dataas it is only accurate in some areas to about15%. The Moody chart, which is a replotting ofa chart by Hunter Rouse, which was itself partlygenerated using equations by Colebrook whichwere an approximation to… and so it goes on.

Clearly I have a bias towards performancecharts – since loss coefficients are determinedexperimentally why not use computers to workwith experimental values not some formulathat approximates to the values. A designercan call up the chart from IFS to study it on thescreen not have to re-compute the chart. Inthe next Part I touch on some of the recentwork to extend friction coefficient studies toReynolds numbers above ten million and to tryto better understand the effects of roughness

on friction coefficients.

Putting IFS into easy-to-use software wasno easy task. It was clear it would only beachieved by securing substantial risk capitaland this took five years to secure. Fortunatelythe funders soon realised that what theywere funding did not have an effective routeto market. At the time, early 1980s, majorengineering software companies tended toaddress specific industrial sectors whereas fluidflow software addresses the needs of many

industries. The answer was the setting up of acompany to market the software which laterbecame Flowmaster Ltd.

My main objective for fluid system analysis

software was the power and process industries.What I did not appreciate at the time, is thatstakeholders involved in the design, buildingand operating power and process plants hadconflicting requirements as regards the designof fluid flow systems. Fluid flow softwareon its own did not address these conflictingrequirements because it addresses a smallpart of the overall plant design and did notintegrated with the CAD and process software.

The first users of Flowmaster were companies

who had in effect written a specification for thefluid system simulation software they neededand then went looking for such software.Invariably the fluid systems being simulatedwhere in self-powered moving artefacts– aircraft, submarines, ships, automobiles,commercial vehicles, etc. A far cry from themarket I had originally set out to addresswith IFS. The good news is Flowmaster nowintegrates with other process and powerplant design software which brings manytechnical and economic benefits. One of the

most important is, it removes the incentive fordifferent disciplines to add margins on marginswhen specifying fluid machines.

Going with the flow: Life costing for industrial pumping systems. This paper is based on “A guideto life cycle cost analysis for pumping systemsby the Hydraulic Institute (US) and Europump(Europe) with US Department of Energyinvolvement.


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12 Developments since theBHRA studies and the needfor Standards

Since the publication of the 2nd edition of IFS(1990) there have been substantial advances influid system simulation and the understandingof internal flows. The most important is thematuring of computational fluid dynamics(CFD) software for 1D simulation of thermal/fluid systems and 3D simulation of thermal/

fluid flows within system components.

Areas where advances in fluid systemknowledge have been made throughexperimental studies include:

• Insights into the development of organisedstructures within turbulent flows and thereeffects on fluid systems

• Measurement of pipe friction factors at highReynolds numbers (>106).

These experimental advances are discussed

below but first I will try to explain why thestore of validatable loss coefficients has notincreased even though there have beennumerous experimental studies of componentloss coefficients over the past 25 years. This isworrying because the knowledge and skills forgenerating validatable loss coefficients are alsothose required to generate experimental datato validate 3D simulations of fluid system flowsand also to set up and to interpret the outputof CFD simulations. The CFD community

should be concerned about the quality of theexperimental data that is being quoted in

support of the output from 3D simulation for

parts of fluid systems. The discussion in thePostscript is relevant to this problem.

In Part 3 it was stressed that a due diligencestudy is required before starting experimentsto measure component loss coefficients. Norecent experimental studies I have found havebrought to bear the necessary resources, interms of fluid system knowledge, to carryout due diligence to generate validatableloss coefficients. In most cases the literaturewas superficially look at, misinterpreted and

conclusions drawn that have resulted inflawed justifications for experiments. That losscoefficients in the literature disagree should“set warning bells ringing” triggering the needto investigate why they disagree, particularlywhen this has been going on for over 80 years.One of the fundamental canons of engineeringethics is “Engineers shall perform services onlyin the areas of their competence”. Clearly thisis not happening in regards to measuring losscoefficients. I think the main reason individualsthink they have the knowledge and skills

needed is that they have not been exposed tosituations that would indicate otherwise.

Tacit Knowledge about FluidSystems

Over 90% of our knowledge is tacit that is tosay it has to be mined to be brought to theconscious level. An experience engineer hastens of thousands of chunks of tacit knowledgeto mine when faced with design, operational

and other decisions. This knowledge is gainedover time by practicing ones art with much of

it learnt from mentors and by “trial and error”.

In mining my knowledge store I bear in mindthat when I joined BHRA a very experiencefluid dynamists told me “You will find thatmany of you preconceived ideas about howfluid flows behave will turn out to be wrong”. This advice proved to be well founded ason numerous occasions I needed to revisemy ideas about particular flow situations.Industrial fluid system problems caused byothers preconceived ideas about fluid flowsbeing wrong accounted for a significant part ofmy colleagues and my work at BHRA. Studies

were also made for others who knew that theirpreconceived ideas could be wrong.

By the end of the 1960s academic experimentalstudies related to fluid systems virtually ceasedas it was no longer supported by researchfunding bodies. Research groups dispersedbefore much of the knowledge gained by thesegroups diffused into engineering curricula. Atthe same time universities started chargingindividual departments for floor space. Thisresulted in large scale fluid flow rigs, with pipe

diameters comparable to industrial fluid flowsystems, being replaced by small scale test rigs.Some of latest rigs being promoted are batterypowered so they can be carried into the lectureroom – not the best introduction to industrialpiping systems that typically account for 30%of the cost of process plants and consumeover 25% of the electrical energy generatedworldwide.


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For the past 40 years university lectures in fluid

mechanics have not had a strong groundingin industrial fluid systems. Today’s teachingmaterial and fluid mechanics text booksare mainly based on pre 1960s informationand data and do not reflect the importanceof fluid system to industrial society. It is notsurprising, therefore, that given a project toexperimentally determine loss coefficientsinvestigators do not have the knowledgeto appreciate what is involved to generatevalidatable loss coefficients. Why don’t similarproblems occur in other areas of engineering?

The answers is Standards.

The effects of not having Standardsand Codes of Practice for FluidSystems

In engineering there are few absolutes inthe data and procedures used for design.Confusion is avoided by the developmentof Standards and Codes of Practice, some ofwhich become mandatory when enshrinedin law. Once a Standard is in place feedbackoccurs that focuses research efforts onimproving the accuracy of data and methodsof its application – ending the needless andwasteful practice of yet more confusingmeasurements being made, formulasproposed and papers published.

Fluid systems are vital to all industries, tonational and local infrastructures, to vehicles,ships, aircraft and satellites and in commercial,public and private buildings. Standards comefrom co-operation and agreement by the

stakeholders involved but great diversity in

the use of fluid systems means no governmentdepartment, professional institution ortrade organisation has “ownership” of fluidsystems. Without “ownership” the environmentto generate Standards for fluid systemperformance has not existed.

Paradoxically, the simplest fluid systemcomponent of all – a plate with a hole in it – hasextensive Standards associated with it, someof which are mandatory. The reason is everyday hundreds of millions of dollars of fluids are

metered, for custody transfer purposes, usingorifice plates. Extensive collaboration betweennational flow meter test facilities aroundthe world helped in establishing Standardsfor metering with orifice plates. Periodicallystainless steel orifice plates with their attachedinlet and outlet stainless steel pipes, whichare not allowed to be disassembled becausealignment may be disturbed, are shipped tonational flow metering facilities in Europe, theUS and Japan to check agreement betweennational laboratories.

When measuring loss coefficients we donot need the extreme accuracy requiredfor custody transfer but it is necessary tounderstand why orifice plate Standardsrequire up to 145 diameters of straight pipebetween some components and an orificeplate – evidence is accumulating that at highReynolds numbers (>106) pipe lengths ofover 200 diameters may be necessary to avoidmetering errors. Investigators too often thinkthey are providing defined flow conditions

into components and measuring all the effects

on pressure losses after components when

in reality their experimental equipment andmethods do not do so.

The ASHRAE has funded substantial testingof commercial pipe components/fittings inrecent years. Tests involved a wide range ofcomponents – ells, reducers, expansions, Tees, contracting and expanding bends -with typically four fittings from differentmanufactures. Welded, screwed and PVCfittings were tested. Standards to whichcommercial components are manufactured

allow for significant variation in flow passagegeometries and flow surface roughness. Anaim of tests was to arrive at a recommendedloss coefficient for a particular type andsize of component. Unfortunately test rigconfigurations were such that in many casesdeveloped flow conditions did not exist atinlet to components and all the pressurelosses caused by components in outlet pipeswere not measured – in the case of the largestcomponents tested upwards of 40% of thepressure loss was not recorded. Standards for

testing fluid system components are desirable

The reports on the ASHRAE tests are factualwith little interpretation as regards howthe variation in internal geometry betweennominally similar components effected losscoefficients. Manufacturing techniques todayare such that manufactures could essentiallyproduce identical components – if this was arequirement in an industry Standard.

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At some stage it needs to be accepted that

there should be a core data base of validatedloss coefficients for defined componentgeometries and procedures to estimatedloss coefficients for components that differin geometry from the defined geometries.Instead of investigators just generating moreand more loss coefficients they need theknowledge to explain why a componenthas a loss coefficients that differs from thatfor a defined geometry component. Designguidance could then be provided to predictthe effects on loss coefficients of departures

from defined geometries

Advances in Turbulence Research

Fluid dynamics greatest challenge remainsgaining better insights into the physics ofturbulence and its role in the transfer ofmomentum, heat, and mass in engineeringapplications. During the experimental work atBHRA we recorded flow events which we couldnot explain but which now could probably beshown to be due to organised flow structuresin turbulent flows. Our instruments allowedlocal measurements of turbulence whereasone needs to understand what is happeningin a large flow volume and have the resultspresented in pictorial or video form thatis easy to comprehend. This is now beingachieved using techniques such as ParticleImage Velocimetry and flow visualisation usinganisotropic particles that are orientated by theflow and reflect incident light differently from

location to location thereby revealing flow

structure. The picture that is emerging is of coherentstructures within boundary layer flows withfamilies of vortices generated and arranged toform a hierarchy of coherent flow structures. The starting structures are vortices with theirends momentarily anchored close to a walland their middle carried downstream andaway from the wall to form hairpin shapes. These vortices appear in packets and are sweptdownstream to create a so called large scale

motion which may then join with other largescale motions to form very large scale motions.

Very large scale motions can have lengths of10 or so pipe diameters which is longer thanthe flow path through many components. Inpassing through a component a very largescale motion can change the flow behaviourwithin and downstream of a component.An example of the effect of very large scalemotions is on the two secondary circulationcells after a bend. The cells lose their symmetryas very large scale motions pass through abend resulting in one secondary flow cell beingmore dominate in the outlet pipe. This is clearlyshown by flow visualisation of flow through abend and the velocity profiles into and after abend .

If a component has areas of unstable flowseparation and attachment, that effects acomponent’s loss coefficient, the frequencyof very large structure generation couldbe expected to have an effect on the loss

coefficient and possibly on the structural

behaviour of a fluid system. Usually the mostimportant structural requirement for fluidsystems is that fluid must be contained withina system under all foreseeable circumstance– some of the worst manmade disasters werecaused by failure to achieve this. Designingto piping Standards will usually result in fluidsystems in which fluid forces, due to unstableflow within components, are small comparedto pressure and other forces. However, thereare circumstances where the existence of verylarge scale motions in turbulent flow may give

rise to structural problems. This in most likelyfor systems operating at near atmosphericpressure where flow induced forces could besubstantial relative to a systems structural and/or fatigue strength. A possible example of thisis discussed in the Postscript.

Friction Factors

The most widely used formulae for calculatingfriction factors are known to be inaccurate.However the formulae, and the figures basedon them, are so entrenched in the engineeringcommunity that only an internationallydeveloped Standard is going to lead to theadoption of more accurate methods of arrivingat friction factors.

The development of methods for calculatingpipe friction head losses were driven by civilengineers who built the infrastructure thatbrought clean water to and removed sewagefrom towns and cities in the 18th century –

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saving countless millions of lives. They had

a job to do and they knew that corrosion,deposits and growths were going to reducecapacity substantially so factors withinformulae for calculating head losses had tobe based on engineering judgement. In 1931Nikuradse in German

confidence in fluid system design_mentor graphics

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