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Journal of Fire Protection

http://jfe.sagepub.com/content/5/4/147The online version of this article can be found at:

DOI: 10.1177/104239159300500404

1993 5: 147Journal of Fire Protection EngineeringMansour Alipour-fard, John A. Mayer, JR and Richard L.P. Custer

Gamma-Ray TechniquesDetection of Occlusion and Wall Degradation in Fire Protection Piping Systems Using Non-Invasive

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DETECTION OF OCCLUSIONAND WALL DEGRADATION INFIRE PROTECTION PIPING SYSTEMS

USING NON-INVASIVE GAMMA-RAY TECHNIQUES

MansourAlipour-fard, P.E.University of Colorado, Boulder, CO

JohnA. Mayer, Jr., P.E.Worcester Polytechnic Institute, Worcester,MA

Richard L.P. CusterWorcester Polytechnic Institute and Custer Powell Inc., Wrentham, MA

SUMMARY

Water piping occlusion categories are reviewed. Determination of sprinkler piping occlusionsand pipe wall degradation are investigated by using a non-invasive gamma-ray scanningtechnique. Theoretical and experimental investigations illustrate that the technique is capableofrevealing the thickness, location, density and shape of substantial solid occlusions without

requiring any prior knowledge about the pipe interior. Major degradation of the pipe wall can alsobe determined with this technique, provided the scanning speed and collimator thickness areselected properly.While theoretically feasible, determination of thin and porous occlusions couldnot be achieved experimentally. In theory, if the beam of gamma-ray is very thin and the scanningspeed is very small, thin and porous occlusions can be detected.

INTRODUCTION

Acceptable performance of automatic

sprinkler systems requires that the piping

be able to deliver water at or above thedesign flow rate and pressure. Over timethe interior surface of the piping becomesmore rough and the hydraulic diameter

may be reduced due to different types ofocclusion. Such occlusions reduce water

flow and consequently increase the size offire damage in sprinklered fires.

A detailed analysis of 3,134 cases of un-

satisfactory sprinkler performance revealed200 instances, approximately 7.0 percent,

of either plugged heads or obstructed pip-ing.2Another source reported that pipingobstruction represented 5.6 percent of thecauses of unsatisfactory sprinkler perfor-mance from 1970 to 1974.3A non-invasive

technique is required to determine theinterior condition of piping. Such a tech-

nique will help analyze the availabilityof water for satisfactory performance of

sprinkler systems.

To date, there is no non-invasive tech-nique to assess the amount, location, and

shape of occlusion in automatic sprinklerpiping. The authors of this paper recog-nize that many physical principles may be

employed to detect piping occlusion. The

principles include heat transfer/infra-red

radiation, fluid mechanics, ultrasonic physics,and gamma-ray absorption and scatter-ing. In fact, commercial products are cur-

rently available which use ultrasonic principlesto determine the remaining wall thicknessof corroded pipes.

The authors decided to study the feasibil-ity of one physical principle. Previous back-ground of professors Mayer and Custerdirected this research effort towards gamma-

ray techniques. Professors Mayer and Custerproposed to the National Science Founda-tion (NSF) that the feasibility of the useof a non-invasive gamma-ray technique inthe assessment of sprinkler piping occlu-sion be investigated. NSF approved the

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proposal and funded the project under grantnumber MSS-861 3561. The approval ofthe proposal by NSF was followed by aMaster of Science Thesis work at the Cen-

ter for Firesafety Studies of Worcester

Polytechnic Institute (WPI). Additionalfunding was provided by the National Fire

SprinklerAssociation (NFSA). The researchwork started in September 1986 and endedin February 1988.4 This paper presents a

summary of that work including a briefoverview of water piping occlusion and the

gamma-ray techniques explored.

OCCLUSION INAUTOMATICSPRINKLER PIPING

Occlusion, blockage, postprecipitation,and

obstruction have been used interchange- .

ably in literature to define the existence

of physical barriers to water flow in pipingsystems. For consistency, the term &dquo;occlu-sion&dquo; is used in this paper to indicate par-tial or total obstruction of water flow.

Possible piping occlusion geometries areclassified in Figure I to provide a uniformnomenclature in this report.As shown,occlusion may be full or partial. Partial

occlusion maybe

circumferential, asym-metric, segmental, or irregular. Full andpartial occlusions can be continuous orlocalized.

Figure i . Occlusion GeometryNomenclature: (a) Full Occlu-sion, (b) Circumferential, (c) Asymmetric, (d) Continuous, (e)Localized. (f) Segmental, (g) lrregular.

Water piping occlusion are traced back totwo general sources: foreign solid objectsand particles entering the piping; and solidmaterial forming within the piping, seeFigure 2.1 Foreign material entering thepiping include lump objects and materialcarried in suspension by water which

settles in stagnant or low velocity water.Solid material forming within piping sys-tem are: water frozen in piping, soft layersof slime on pipe surfaces, corrosion prod-ucts ranging from small blisters to largetubercles, and postprecipitation of previ-ously soluble chemicals. Postprecipitationproducts may remain in their place ofgeneration or break and settle elsewherein the system.

Ftgure 2. The two major sources of pp!ng occtusion andtheircategories.

Issues of interest for each category of oc-clusion include geometry, cause and for-mation process, effects of pipe material,effects of water impurities, and degree ofstructural damage imposed on piping.Anoverview of these occlusion categories is

presented on the next page.



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Occlusion Categories

Precipitation and Postprecipitation

Precipitation is a process used in watertreatment. It involves the reaction of a

soluble chemical with chemical compoundsnaturally present in water to form an in-soluble chemical compound. Solid particlesthus formed can be extracted from the

water.5

Precipitation is an intentional phase ofthe treatment process. However, not allsoluble chemicals are extracted fromwater as it leaves the treatment plant.Unintentional precipitation occurs whenchemicals existing in a water react withthe new environment in the distribution

piping.5 Consequently, precipitation ofcertain compounds can occur after waterhas been treated, thus the term postpre-cipitation.

The term postprecipitation is sometimesused to describe piping occlusion includ-

ing sedimentation and corrosion. Iron

postprecipitation, for example, is some-times used to refer to corrosion of iron

piping.

Major types of postprecipitation comprisecalcium carbonate, iron, lead, zinc,aluminum, magnesium, polyelectrolytes,manganese, .and microbiological.5 Post-

precipitation is normally irregular andcontinuous along the length of the piping.Depending on its type, postprecipitation cancause different levels of damage to the pip-ing. Table 1 illustrates some of the attributesof different types of postprecipitation.

Calcium Carbonate PostprecipitationDeposition of calcium carbonate (CaCO3)is often used to form a corrosion resistant

coating on the pipe interior. However, excessdeposition of CaC03 adversely impactswater carrying capacity of water piping 5

After the initial protective coating ofCaC03is formed, efforts should be made to pre-vent further introduction of the material.

Postprecipitation of CaCO3 is normallyirregular in shape and can cause continu-ous occlusion along a length of piping, butis unlikely to cause structural damage.

Relatively accurate predictions of postpre-cipitation rates can be made using the

Langelier Index, also known as CaCO3Saturation Index.5

. /

TABLE 1 SomeAttributes of Dififerent Types of Postprecipitation

NS= Not Studied



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When predicting postprecipitation of CaC03factors such as calcium concentration,alkalinity, dissolved solids, and watertemperatures must be taken into account 5

CorrosionMost

sprinkler pipingoccluded

bythe

processof corrosion is ferrous. While corrosion

may attack the interior and/or exterior of

piping, we have limited this overview tointerior corrosion of ferrous pipes.

The geometry of corrosion is normally ir-regular and, depending on the type of cor-rosion, may be continuous or localized.The formation process is chemical and/or

biological. Pipe material and water tem-perature, acidity, and impurities affect typeand

rate of corrosion. Corrosion,in

par-ticular pitting corrosion, can cause struc-tural damage to the pipe.66

The initial corrosion reaction is an electro-

chemical one and may be explainedas thedisplacement of a metal ion from an anodicsite on the metal surface to a site in the

solution which leaves behind a number of

excess electrons; concurrently, electronsare consumed at a nearby cathodic site?

Oxygen availability, e.g., through the in-

troduction of fresh water, enhances therate of corrosion.

The corrosion process can reduce the thickness

of the pipe wall and generate tubercles,scales, and loose particles. Holler6 andRotbwe117 explain several types of corro-sion in water piping including uniform,grooving, pitting, tuberculation, exfolia-tion, water-line attack, crevice attack,dezincification, corrosion erosion, galvaniccorrosion, and graphitization. Tuberculation

can cause major occlusion of iron pipingand is explained below. Review of otherforms of corrosion is beyond the scope ofthis work. Briefly, water-line attack is a

type of corrosion that takes place at the

point where air, water, and metal inter-sect.A typical example is a drop of wateron an iron surface, e.g., interior of a drypipe sprinkler system following a trip test.

TuberculationIn 1950, Olson and Sybalski found thatthe tubercle formation initiated by bacte-rial growth on pipe walls was the criticalfactor on the internal corrosion of iron

pipes.8 Iron bacteria such as Gallionella

may add to the problem of tuberculationwithout being involved in the corrosion

process by metabolizing ferrous ions al-

ready present in the solution and layingdown large masses of ferric oxide on thesheath of the organism.7.8

Tuberculation of iron pipes is an electro-

chemical/biological process affected by waterquality. This type of piping occlusion is

normally expected to be continuous alonga length of pipe and irregular in shape.

Corrosion products formingon

the innersurface of piping often resemble tubercles,thus the term tuberculation. The mecha-

nism involves oxidation of ferrous hydrox-ide to hydrated ferric oxide. The ferricoxide forms layers and blisters often interlaidwith calcium, iron and other metallic salts.Often a hard surface coating of Fe3o3 forms,providing the main structure of growntubercle.6.8

Sedimentation

In the context of open channel hydraulics,sediment is defined as material carried in

suspension by water which would settle atthe bottom of the channel if water lost ve-

10city.9 While substantial research has beenconducted on different aspects of open chan-nel sedimentation, authors of this paperwere unable to find published correlationsfor the rate of sedimentation and settlingvelocity in the context of water piping.

The geometry of pure sedimentation is

expected to be segmental. However, whencombined with other categories of occlu-sion, the. geometry may become irregular,but thicker and more dense at the bottom

of the pipe. Sedimentation is either con-tinuous along a length of the pipe or local-ized, e.g., when pipe changes direction.Sedimentation causes occlusion to water

flow, but will not directly cause structuraldamage to piping.



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While the initial process forming the sedi-ment deposits is diverse, the settling pro-cess is a physical one.As indicated in

Figure 2, sediment in automatic sprinklerpiping comes from two major sources. Thefirst source is the suspended materialscarried by water from outside the system.Poor water treatment and penetration ofsoil into the piping, e.g., during repair,are typical causes for the presence of sus-

pended material. The second source is the

postprecipitation products generated withinthe piping which may remain in their placeof generation or break and settle elsewherein the system, e.g. due to decrease in water

velocity or change in the direction of waterflow from horizontal to upward vertical.The Fire Protection Handbook 10 reportsthat sprinkler piping occlusions usuallyconsist ofconcentrations ofthe lighter materialsuch as silt and sand in the ends of the

cross mains and in the nearby branch linesand heavier solids near the system risers.

Lump ObjectsLumps and unusual objects such as plumb-ing hardware, pieces of wood, and &dquo;pipe cou-

pons&dquo;, cut and dropped into piping duringsystem installation, are typically caused byhuman error. Their geometry is expectedto be irregular and localized. While ca-

pable of causing serious localized occlu-sion, lump objects alone are unlikely tocause structural damage to the pipe.

OBJECTIVES OF THIS STUDY

Recommended practice for maintenance of

sprinkler systems3 calls for flushing ofthe system and water flow tests on a pe-riodic basis. While these can loosen and

flush out some scale and

sediment,flush-

ing and water flow tests will also bring infresh water which will enhance corrosion.

Also, depending on its quality, fresh watercan introduce additional dissolved min-eral matter and/or suspended particles thatcould further increase sedimentation and

postprecipitation.

While it is often possible to clean the pip-ing, the present means for determiningwhether to clean or possibly replace thesystem is a complicated and costly processwhich involves partial disassembly of thesystem for visual inspection.Moreover, thismethod lacks the capability for measuringthe existing hydraulic diameter of the pipe,finding localized occlusions, finding local-ized pitting of the pipe wall, and deter-mining the shape of occlusion. Determina-tion of the shape of occlusion, for example,could give a clue to its nature, e.g., sedi-mentation versus tuberculation, and hencedictate the method of cleaning. In addi-

tion, finding localized occlusions and lo-calized pitting of the pipe wall could sug-gest that only partial replacement of the

system is necessary.

This study investigated the feasibility ofthe use of gamma-ray techniques to deter-mine the presence, shape, and size of pip-ing occlusion and possibly the remainingthickness of the pipe wall. In addition, theteam noted that potential health hazardsof the technique, to operators and the generalpublic, should be investigated. Followingthe establishment of the objectives, theo-retical and experimental efforts were di-rected towards securing the objectives.

i

THEORETICAL STUDIES

The theoretical feasibility studies consistedof two stages: geometrical analysis (pipe~and occlusion geometry); and gamma-raytechniques.

In practice, the process involved trial anderror between the two stages. For example,after initial

completionof

geometry stageand during the gamma-ray analysis, theteam noted that there are unknowns thatcan be further simplified by specific geo-metrical analyses. This report presentsthe final outcome and does not cover the

trial-and-error process between the two

stages.



7/17

152

GeometricalAnalysis

To simplify the analysis, this stage wasaimed at studying the dimensional rela-tionships in a circumferentially occluded

pipe. Effectively, this would be a concen-tric

three-layer cylinderwith

pipe wall,occlusion, and water forming the threelayers.As illustrated below, the thicknessof a barrier through which a beam of mono-energetic gamma-ray passes directly af-fects the attenuation of that beam. There-

fore, this section is to provide correlationsfor the calculation of the effective thick-

ness of each layer of the subject cylinderat different points.

Using basic geometric relations for cylin-

ders, itcan

be shown that Equations 1through 3 apply to the three-layer cylin-der of Figure 3. In these equations, t is theeffective thickness of each layer and is afunction of elevation, h, see Figure 3. Eleva-tion, h, as shown in Figure 3 is the verticaldistance from the surface on which the pipeis laying to the location of interest.

One may visualize that the three-layercylinder of Figure 3 is being cut along its

length with a flat horizontal cutting sur-face at elevation h. The effective thickness

of each of the three media (tl1 t21 t3) isdefined as the total thickness of that media

which is in contact with the visualizedcutting surface at the given elevation. Notethat in Equations 1-3, tlp t21 t3, h, R1, R2,and R3 all have the units of length (cm).Note: t2 and t3 change with elevation, h.

NOTE: % - t 2, and t 3 change with elevation, &dquo;h&dquo;.

Figure 3. Dimensional correlations in a three-layer cylinder(range of Equations 1. 2. and 3).

Figure 4. Plot of effective thickness vs. elevation for different layers of a three-layer cylinder.



8/17

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Using the above equations, a plot of theeffective thickness, t, vs. elevation, h, foreach layer of a three-layer cylinder and

the total thickness of all.three layers aredisplayed in Figure 4. Similar correlationsand graphs can be developed for other related

geometrical shapes such as asymmetricand segmental type occlusions.

Gamma-RayAbsorption and Scattering

As a mono-energetic beam of gamma-raypasses through an absorber, i.e. a barrier,its intensity drops as a decaying exponen-tial function of the absorber thickness andan absorber material property known asthe linear absorption coefficient, pl. Theabsorption/scattering ofgamma-rays is viathree mechanisms: pair production, Comptoneffect, and photoelectric effect. Review ofthese mechanisms is beyond the scope ofthis work. The complete effect can be sum-marized in the following equation

Where: 10= Initial Flux Intensity of theBeam (Photons/sec.)

I = Flux Intensity of Beam after

Absorption, i.e.Uncollided Photons (Photons/sec.)

lZl= Linear Attenuation Coefficientof theAbsorber (cm-1)

-t ~ Thickness ofAbsorber (cm)

For an absorber made of several different

material, e.g., a three layer cylinder, seeFigure 5, Equation 5 should replace Equa-tion 4. Subscripts 1, 2, and 3 refer to thethree media/layer.

Fgure 5.A portion of a three-layer cylinder through w hicha beam of gamma-ray is passing.

In Equation 5 the intensity of incident

beam, &dquo;Io&dquo;, and that of the beam after going

throughthe

pipe, &dquo;I&dquo;,can be measured.

Also, the linear attenuation coefficient ofwater and pipe material are known. How-

ever, recalling that the technique is to benon-invasive and the pipe is to remain

closed, the occlusion thickness (t2), thelinear attenuation coefficient of occlusion

(Pl.2) and the remaining thickness of pipewall ttl) are unknown. Therefore, contain-ing three unknowns, Equation 5 alone cannotprovide us with the occlusion thickness.Note that the thickness of water layer wasnot listed

amongunknowns since the ex-

terior diameter of the pipe is known. Oncethe thickness of two layers (tl and t2) andthe external diameter of the pipe (tl+t2+t3)are known, t3 can be obtained by substraction.

If Equation 5 could be solved, the theoreti-cal studies would have reached a resolu-

tion.As explained in the previous para-graph, this resolution was not to be. How-



9/17

154

ever, at this stage, a question was raisedas to whether there is a need to solve this

equation. Instead, the team decided to lookinto the use of Equation 5 to construct atopographical map showing the combinedimpact of the effective thickness and the

attenuation coefficient across the pipe.

The factors affecting the linear attenua-tion coefficient are the mass density (re-ferred to as &dquo;density&dquo; throughout thispaper), and the atomic number of the ab-

sorbing media. However, it can be shownthat, for photon energies of approximately0.4 to 4.0.Mev, J.11 is nearly independent ofthe atomic number of the intervening mediaand is predominantly dependent on the

density (g/cm3) of the intervening media.12

Based on this view of the linear attenua-tion coefficient, the construction of the

subject topographic map appeared viable.

Using Equations 1-3, and multiplying thethickness of each layer by the density ofthat layer, a graph of (Thickness x Den-sity) Vs. (Elevation) was constructed (seeFigure 6 and compare with Figure 4). Thisplot is for a steel pipe, circumferentiallyoccluded with a homogeneous materialwith a density of 2.0 g/cm3, and water, 1.0

g/cm3, inside the pipe. This procedure andthe resulting graph, Figure 6, formed thebasis of the &dquo;Parallel Beam Gamma-RayScanning.&dquo;

Figures6. Plot of &dquo;Thicknessx Density&dquo; vs. Elevation* (h) fora three-layer cylinder.

Parallel Beam Gamma-Ray Scanning

The principles used in the construction of

Figure 6 were also used to hypothesize the

scanning technique. In this technique, acollimated mono-energetic beam of gamma-ray of a known intensity is passed throughthe pipe. The uncollided gamma-rays, i.e.,photons with the same energy as the inci-dent beam, are counted as they departfrom the other side of the pipe. Up to this

point, the process is similar to the simpleuse of Equation 5 but in this case the

process is repeated by making a traversenormal to the pipe centerline from bottomof the pipe to the top, i.e. parallel scan-

ning across the pipe. Figure 7 illustratesthis scanning procedure. The resulting data,from the detector, can be used to constructa plot of the beam position (1~) versus de-

parting flux intensity of the gamma-ray. beam (I).

Rgure 7. Gamma-ray traces through pipe.

This technique takes advantage of two facts:that the amount of attenuation of a beam

of gamma-rays depends on the densitiesand thicknesses of the intervening media;and that the thicknesses of different lay-ers of a multilayered cylinder change withelevation h.

As noted earlier, the linear attenuationcoefficient is largely a function of densityin the range of energies proposed. There-fore, based on Equation 5 the intensity ofthe departing beam (I) has a decreasing



10/17

155

exponential relation with the product ofthickness and density of the absorbingmedia.The decaying function suggests that a plotof I Vs. h should resemble the mirror imageof Figure 6.

Using Equation 5 as the governing gamma-ray absorption equation and Equations 1-3as the governing geometric relations, a com-puter program was written to investigatethe theoretical feasibility of the technique. 13Using that computer program, extensivetheoretical feasibility and sensitivitystudies were conducted. The studies in-

cluded computer modeling of occluded andnon-occluded pipes of different size,circumferentially occluded with variousthickness and density occlusion material.The computer models included air or wa-

ter inside the pipe. These studies illus-trated the theoretical feasibility of theapproach.A sample of the computer out-put plot is given in Figure 8; compare withFigure 6. Visual examination of such plotsreveals the occlusion.

The theoretical feasibility studies werelimited to circumferentially occluded pipeswith homogeneous occlusion material.However, experimental studies covered awide range of pipe and occlusion geom-etries.

Fgure 8. Computer simulations of gamma-ray scanningacross a circumferentially occluded pipe and an occlusions-fres pipe.

Longitudinal Gamma-ray Scanning

Unlike the case of moving across a cylin-der (see &dquo;GeometricalAnalysis&dquo;), the thicknessof each layer of a three-layer cylinder re-mains constant as a point moves along the

cylinder axis. Therefore, based on Equa-tion 5, with the thicknesses, attenuationcoefficients and incident beam remainingconstant, the value of &dquo;I&dquo; remains con-stant as the source and detector move alongthe cylinder. In other words, if a pipe issymmetrically occluded with a material ofuniform density, its longitudinal scan plotwill be a horizontal line.Any change inthe uniformity of thickness of layers ortheir densities will affect the shape of theplot. For example, if the cylinder or pipeis occluded by a lump object, the longitu-dinal scan plot will show marked decreaseat that point.

EXPERIMENTAL DESIGN

The TestApparatus and ExperimentalProcedure

A schematic diagram of the test apparatusis shown in Figure 9. This apparatus was

designed for scanning across the pipe. Withthe setup shown in Figure 9 the detectorand the source are stationary, while the

pipe (sample) is moving at a constant speed.The difference between this setup and anypractical application is that in this setupthe pipe moves while the source and detec-tor remain fixed. In field investigations,the pipe is fixed and the source/detector

assembly must move. Both methods willresult in the same outcome since in bothcases the pipe moves relative to the appa-ratus.

The source emits a collimated beam of

gamma-rays towards the detector while

the pipe moves across the beam (see Fig-ure 4). The signal received by the detectoris then sent to a Multi Channel Scaler(MCS). The MCS is provided with meansto filter photon energies above and belowa certain range, i.e., by means of upper



11/17

156

Figure 9. Schematic diagram of the experimental

setup.

and lower level discriminators (ULD and

LLD). ULD and LLD should be set suchthat only uncollided gamma-rays will be

counted. Using the number of detectedgamma-rays at each scan position, the MCSmakes a plot of the number of photons vs.the scan position (h). To create a hard copyof the scan, a plotter is attached to theMCS. Several examples of the plotter out-put are given later in this paper.

A computer program was written so thatthe MCS could communicate with a per-sonal computer. The number of counts foreach channel, representing (I) in Equation

4, along with the channel numbers, repre-senting h in Equations 1-3, are transferredto the PC. The data was restored on diskfor later analysis.

The apparatus consisted of the followingcomponents:

A Pipe Moving Mechanism whichconsisted of a chuck, a slider mecha-

nism, and an electric motor. Scanningspeeds used in this study ranged from

0.6 to 15.0 cm/min.

. The Shielded Gamma-ray Source wasa 24.0 mCi-Csl3~.A second source, 0.6

mCi-Cos, was also used in some

experiments to investigate the effectof source strength.

~ A Source Collimator, made of lead,was used to direct a thin beam of

gamma-ray towards the detector.

Geometrically, two types of collima-tors were used: circular and rectangu-lar/slit shaped.

. The Detector Window, in the lead

shield around the detector,was

usedto prevent the collided gamma-raysfrom getting into the detector. Thiswindow is geometrically similar to,but proportionally larger than, thesource collimator. Note that with the

proper setting of ULD and LLD, and

adequate separation of the detectorfrom the pipe, the provision of the

journal of fire protection engineering 1993 alipour fard 147 62

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