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DUCTILE IRON PIPEVS. STEEL PIPE

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ALL PIPE MATERIALS ARE NOT EQUAL:DUCTILE IRON VS. STEEL PIPE

By L. Gregg Horn, P.E., DIPRA Director of Regional Engineersand

Mark R. Breslin, P.E., DIPRA Staff Engineer

When Ductile Iron and steelpipe are bid against eachother on transmissionmain projects, owners and

engineers are often inundated withinformation relating to various aspectsof these competitive materials. Thecompetition is lively, but the facts aresometimes hard to come by. Ourpurpose is to show areas where DuctileIron pipe has distinct, irrefutableadvantages over steel pipe. Whether theconcern is engineering, installation, oroperation, an objective comparison ofthese two products shows they are notequal. Ductile Iron pipes conservativedesign and ease of installation make itdecidedly better.

Ductile Iron PipeInternal Pressure DesignIs More ConservativeThan Steel Pipe

Perhaps the most striking difference

between Ductile Iron and steel pipe is therelative way their internal pressuredesigns are modeled. Only Ductile Ironpipe has a standardized design procedure(ANSI/AWWA C150/A21.50), and theapproach in that standard is the mostconservative in the piping industry. Steelpipe design is not standardized, althoughinformation may be found in theAmerican Water Works AssociationsManual of Water Supply Practices, M-11,Steel Pipe - A Guide for Design andInstallation, as well as in Welded SteelPipe - Steel Plate Engineering Data -Volume 3 published by the AmericanIron and Steel Institute, and manu-facturers literature.

Both Ductile Iron and steel pipe useversions of the Barlow hoop stressequation to model internal pressuredesign. Ductile Iron pipe utilizes a safetyfactor of 2.0 in the design pressure,whereas, with steel pipe, the allowablestress is limited to between 50 percentand 75 percent of the minimum yieldstrength, depending on the magnitude ofthe surge pressure.

Ductile IronPressure Class Design

According to AWWA C150, ThicknessDesign of Ductile Iron Pipe, internalpressure design incorporates the workingpressure and surge pressure, which areadded together prior to applying a factor ofsafety of 2.0. This is referred to as apressure class design because the surgepressure is part of the design pressurecalculation. The standard surge pressureallowance is 100 psi, which is

approximately the surge that would occurin Ductile Iron pipe if the velocity of flowwere to abruptly change by two feet persecond. Different surge pressures arehandled by substituting them in place ofthe standard surge pressure allowance.

For Ductile Iron pipe (AWWA C150standard design), the hoop stress equation is:

(Equation 1)

t = Fs(Pw + Ps)(D)

2 S

where: t = pipe net wall thick-ness, in.

Fs = factor of safety (2.0)Pw = working pressure, psiPs = surge pressure, psiD = outside diameter, in.S = specified minimum

yield strength of DuctileIron, psi (42,000 psi)

Steel Pipe InternalPressure Design ReducesSafety Factor to asLow as 1.33

With steel pipe, design for workingpressure is based on 50% of the steel yieldstrength, a factor of safety of 2.0. It isimportant to note, however, that surgesare allowed to increase the stress in thepipe wall to a maximum of 75% of yield.Allowing the wall stress to increase to75% of yield is the same as reducing thefactor of safety in design to 1.33.

Put another way: If the surge pressureis less than or equal to one-half of theworking pressure, the pipe would bedesigned using working pressure only anda design stress of 50% of yield. In thiscase, a maximum surge would increase thewall stress to 75% of yield. On the otherhand, if the surge is greater than one-halfof the working pressure, the two pressuresare added and the design stress isincreased to 75% of yield. But the effect isthe same as discussed above: The factor ofsafety in design will vary from a maximumof 2.0 (zero surge) to as little as 1.33(working pressure plus maximum surge).In either case, steel pipe design allowssurges to compromise the factor of safety.

For steel pipe, where surge pressuresare less than or equal to one-half of theworking pressure, the hoop stressequation is:

(Equation 2)

t = (Pw)(D)

2 S

where: S = allowable stress, psi= 50% yield strength of

steel

But for steel pipe where surgepressure is greater than or equal to one-half of the working pressure, the hoopstress equation becomes:

(Equation 3)

t = (Pw + Ps)(D)

2 S

where: S = allowable stress, psi= 75% yield strength of

steel

Comparison ofDesign Approaches

The differences in these designapproaches lead to some very interestingcomparisons, all of which highlight thehighly conservative design of Ductile Iron

2

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pipe. For comparative purposes, Equation3 can be used to design Ductile Iron pipeas well as steel pipe, except that DuctileIron pipe will always limit the allowablestress to 50 percent yield strength, whichcorresponds to its factor of safety of 2.0. Asan example, if we are given a 48-inchtransmission main operating under aworking pressure of 150 psi but wheresurges are assumed to be 75 psi, the

relative wall thickness calculations shownin Table 1 would result. We will assume forthis comparison that both Ductile Iron andsteel have yield strengths of 42,000 psi.

The differences in the wall thicknessesare remarkable. This results from two facts:

Ductile Iron pipe has an outsidediameter that is typically larger thansteel pipe in the 48-inch size. Thisaccounts for a small difference in wallthickness.

Second, and more importantly, steelpipe design allows wall stress to build

to 75 percent of yield (safety factor of1.33) if surges occur. This is why inTable 1 the two steel pipe wallthicknesses are the same althoughtheir design pressures are different.

The result here is that Ductile Ironpipe has a design pressure of 225 psi at50% yield while steel pipe design uses150 psi at 50% yield or 225 psi at 75percent yield. In either case, the result is a

calculated wall thickness for steel pipethat is 35 percent thinner than for DuctileIron pipe, even though the pipes have thesame yield strength.

But what if our example assumed asurge pressure of 100 psi? The steel pipedesign now adds the working and surgepressures and allows the hoop stress toincrease to 75 percent of yield. The resultsfor these calculations are shown in Table 2.

Again, the Ductile Iron pipe designresults in a much more conservativeapproach because the Ductile Iron design

doesnt allow surge pressures tocompromise the factor of safety.

Comparison of Designat 50% Yield

If both pipes approached internapressure design using a total pressuredesign, the 100 psi surge allowance would

be added to the working pressure and thewall stress would be limited to 50% oyield. Resulting thicknesses would be asshown in Table 3.

Here, the 1.7 percent thicker wall forDuctile Iron pipe now is a function only ofthe difference in the outside diametersBoth materials have identical yieldstrengths and the same nominal factor osafety of 2.0. However, Ductile Iron pipesconservative wall thickness designdoesnt stop here.

Ductile Iron Pipe Design:Service and CastingAllowances

Ductile Iron pipe design providesadditional allowances as part of the pipewall thickness calculation. A nominaservice allowance of 0.08 inches is addedto the above calculation. Additionally, acasting allowance dependent upon pipesize (0.08 inches for 48-inch pipe) is addedThis gives the total calculated thicknessfor Ductile Iron pipe design and is the

thickness one would use to select theappropriate pressure class. Table 4summarizes the results of what happenswhen Ductile Iron pipe design includes itsstandard allowances and is then comparedwith the steel pipe examples summarizedin Tables 2 and 3.

For Ductile Iron pipe, Pressure Class150 (nominal thickness = 0.46 in.) would beselected. Steel pipe design would require0.196 in. or a standard 5 gauge (0.209 in.)wall thickness for the steel pipe designed to75 percent yield (a factor of safety of 1.33)this is the design that would normallybe the result for steel pipe. If the yieldstrength were held to 50 percent (as is donefor Ductile Iron pipe - a factor of safety of2.0) the result would be 0.295 in. or astandard plate thickness of 5/16th inch(0.313 in.) for the steel pipe alternate.

Ductile Iron pipes service allowance isreally, a traditional nominal wall thicknessaddition that dates back to Cast Iron pipedesign when it was originally called acorrosion allowance. Of course, the ideaof providing sacrificial wall thickness forcorrosion control is widely recognized as

Table 1

Internal pressure design in accordance with industry recommendations(working pressure = 150 psi, surge pressure = 75 psi)

Ductile Iron Pipe Steel Pipe Steel Pipe(50% Yield) (50% Yield) (75% Yield)

Design Pressure, psi 225 150 225

Outside Diameter1, in. 50.80 49.352 49.352

Thickness, in. 0.27 0.176 0.176

Table 2

Internal pressure design in accordance with industry recommendations(working pressure = 150 psi, surge pressure = 100 psi)

Ductile Iron Pipe Steel Pipe(50% Yield) (75% Yield)

Design Pressure, psi 250 250

Outside Diameter, in. 50.80 49.392

Thickness, in. 0.30 0.196

Table 3

Internal pressure design based on 50% yield(working pressure = 150 psi, surge pressure = 100 psi)

Ductile Iron Pipe Steel Pipe(50% Yield) (50% Yield)

Design Pressure, psi 250 250

Outside Diameter, in. 50.80 49.590

Thickness, in. 0.30 0.295

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an ineffective solution, but this thicknessallowance has been maintained by the ironindustry. It offers an additional factor ofsafety, added durability, and provides morethan an adequate allowance for scratchesand abrasions that can result from handlingor the type of superficial surface oxidationthat might occur under polyethyleneencasement.

The casting allowance accounts forvariations in the casting process. CastingDuctile Iron pipe is a dynamic process thatresults in slight variations in wallthickness along the length of the pipebeing produced. To ensure that thisvariation does not compromise the factorof safety in design it is added as anallowance. Additionally, required weighttolerances assure that effective wallthicknesses are always greater thancalculated net wall thicknesses.

Steel Pipe Allowancesfor Defects and MillTolerances

ANSI/AWWA C200, the manufacturingstandard for steel pipe, discusses twoallowances as well. However, theseallowances are normally left out of thedesign calculations. The result is thatsteel pipe design allows the wallthickness required for structuralconsiderations to be reduced, which inturn further compromises the factor ofsafety of steel pipe.

Section 1.5.1 of AWWA C200 states . . .The finished pipe shall be free fromunacceptable defects. Defects . . . will beconsidered unacceptable when the depth ofthe defect is greater than 12.5 percent ofthe nominal wall thickness.

This can be a significant thickness. Forour examples summarized in Table 4, 12.5percent of 0.196 inches is equal to 0.025inches and 12.5 percent of 0.295 is equalto 0.037 inches.

The second allowance discussed in

AWWA C200 is similar to the castingallowance found in the Ductile Iron pipemanufacturing and design standards.Section 2.2.3 states . . . For plate, themaximum allowable thickness variationshall be 0.01 in. under the orderedthickness. For sheet, the maximumallowable thickness variations shall be[0.005 to 0.009 in., depending on nominalthickness] . . .

In other words, the plate or sheet usedto make the pipe can be 0.005 to 0.01inches thinner than the nominal thicknessfor that gauge of plate or sheet. However,this variation is not accounted for indesign as it is for Ductile Iron pipe. Thisallowance doesnt seem very large, exceptwhen you compare it to the wall thicknessthat results from the steel pipe designapproach. A thin wall is made potentiallythat much thinner.

If the sum of the defect allowance and

the allowance for the mill tolerance issubtracted from the thickness calcula-tions summarized in Table 4, the wallthicknesses in our examples reduce to0.163 and 0.248 inches, respectively.These are severe reductions. Therespective factors of safety now reduce toonly 1.11 (normal steel pipe design) and1.68 (50 percent yield, no design thick-ness allowances).

Ductile Iron vs. Steel PipeDesign: ComparingApples to Apples

If the steel pipe industry designed itsproduct with the same rationale as theDuctile Iron pipe industry, it would addallowances for defects and mill tolerancesto the required design thickness and

maintain a constant nominal factor of safetywith regard to surge pressure design. Theresulting Ductile Iron and steel pipedesigns would be as shown in Table 5.

Ductile Iron pipe design would, again,result in selection of Pressure Class 150(nominal wall thickness = 0.46 inches).Steel pipe design, here, would result inselection of 0.342 in. or a nominal 3/8-inch(0.375 in.) wall thickness. Now, thedifferences between the products are onlya function of the differences in themanufacturing of the pipes, not differencesin design philosophy. Using this approach,the factor of safety for steel pipe is notcompromised.

IT IS EASIER AND LESSEXPENSIVE TO CONTROLCORROSION ON DUCTILEIRON PIPE THAN IT ISON STEEL PIPE

Steel Pipe RequiresBonded Coatings

Steel pipe requires a bonded coatingfor corrosion control. The type of coatingrecommended by the steel pipe industryvaries with the manufacturer and market.These coatings are typically either acement-mortar coating found on steel

Table 4

Total Calculated Thickness: Comparison of standard Ductile Iron pipedesign, normal steel pipe design, and steel pipe design at 50% yield

Ductile Iron Pipe Steel Pipe Steel Pipe(50% Yield) (75% Yield) (50% Yield)

Thickness, in. 0.30 0.196 0.295

Service Allowance, in. 0.08

Casting Allowance, in. 0.08

Total Calculated Thickness, in. 0.46 0.196 0.295

Table 5

Total Calculated Thickness incorporating allowances

Ductile Iron Pipe Steel Pipe(50% Yield) (50% Yield)

Thickness, in. 0.30 0.295

Service Allowance, in. 0.08

Defect Allowance, in. 0.037

Casting Allowance, in. 0.08

Allowance for Mill Tolerance, in. 0.010

Total Calculated Thickness, in. 0.46 0.342

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pipe products in the western UnitedStates or a tape-wrap coating found in theeastern part of the country. AWWAstandards exist for providing coatingssuch as coal-tar enamels and tapes, liquidepoxies, fusion bonded epoxies,polyethylene tape-wraps, extrudedpolyolefins for pipe, and more coatingsfor specials and fittings.

Cement-mortar coatings are porous

coatings that protect ferrous materialsthrough chemical passivation. Unfor-tunately, some environments areaggressive to cement-mortar coatings.Any damage or degradation of thecoating that exposes the steel can set upa corrosion cell that utilizes thepotentially large variance in pH betweenthe exposed steel and the steel incontact with the cement as the drivingforce. This can result in accelerated localcorrosion cells. Low pH environments orsoils with sulfates are examples ofenvironments that are aggressive to

cement-mortar coatings. Soils with highchloride content can be detrimental tocement-mortar coated steel. Addi-tionally, cement-mortar coatings do notoffer resistance to potential stray currentcorrosion. Cement-mortar coatings alsoreduce the flexibility of the pipe. As aresult, the pipe cannot be allowed todeflect as much under external load, norcan internal pressures be allowed toexpand the pipe and cause cracking ofthe coating.

Steel Pipe CoatingImperfections Require

Supplemental CathodicProtection

Tape-wrap coatings (and the otherslisted previously) are barrier coatingsthat protect the steel by isolating the pipefrom the corrosive environment. Theyare perhaps more resistant to deterio-ration and more flexible than cement-mortar coatings. Unfortunately, as istypical with bonded coatings, they tend torequire cathodic protection as a sup-plement. This is because bonded coatingsare practically impossible to installwithout the type of damage that resultsfrom shipping and handling from thecoating applicators plant to the job site.The pipe is, therefore, subject toaccelerated corrosion cells at local coatingimperfections. As a result, steel pipenormally requires an expensive andmaintenance-intensive system ofcorrosion control.

Ductile Iron PipeCorrosion Control isAccomplished withPolyethylene Encasement

Ductile Iron pipe and its predecessor grayCast Iron have the same inherent corrosionresistance that makes corrosive environ-

ments much less of a concern. And, if thereis a concern, the soils along the route of aproposed pipeline can be tested forcorrosivity using the 10-point systemdescribed in Appendix A of ANSI/AWWAC105/A21.5. If corrosive soils are en-countered, an unbonded film of polyethyleneencasement in accordance with this standardis the coating that the iron pipe industrygenerally recommends for corrosion control.It also is the only standardized corrosioncontrol method for Ductile Iron pipe. Thissimple and inexpensive method of corrosioncontrol is applied at the job site, eliminating

the problems associated with coating damageen route from the applicator. Beingunbonded, it also eliminates the con-centration corrosion cells that are a potentialproblem for bonded coatings. It also makesfield repairs very simple to accomplishbecause no special surface preparations arerequired. The monitoring and maintenanceassociated with cathodic protection are notrequired for this passive system. Sinceelectrical currents are not introduced into thesoil, the potential for stray current corrosiondamage to nearby structures is avoided. Andpolyethylene encasement has an effective

dielectric strength that protects against mostpotential stray current environments,including the majority of those resulting fromcathodic protection systems on other pipelines.

DUCTILE IRON PIPE ISEASIER TO INSTALLTHAN STEEL PIPE

The largest practical advantage ofDuctile Iron pipe compared with steel pipeis that Ductile Iron pipe is much easier to

install properly. Handling, assembling,backfilling, and adapting to field conditionsall are areas in which Ductile Iron pipeoffers distinct benefits.

Handling Steel Pipeis a Factor in WallThickness Design

Handling steel pipe can actually be afactor in design. The design approach canresult in a wall thickness calculation that

leaves a pipe not stiff enough or withoutsufficient beam strength to stand aloneduring installation. In fact, handlingconsiderations can potentially govern walthickness design. One may go through thewall thickness design procedure andcalculate a required wall thickness based oninternal pressure and external load but findthat the walls are still too thin to handle thepipe. Therefore, after accomplishing

design, a check must be made to ensurethat a minimum wall thickness (as afunction of pipe diameter) is present.

The relationship between wall thicknessand diameter can be used to describe thestiffness of a cylinder. In steel pipe, thestiffness is typically such that stulling, or theplacement of braces inside the pipe, must beprovided to ensure that the pipe maintains itsshape up to the time that the backfill isplaced, at which time the stulling can causeunwanted stress concentrations. WithDuctile Iron pipe, no such stulling is required

Ductile Iron PipePush-on Joints

The most popular joint for Ductile Ironpipe underground applications is the pushon joint. This is a compression ringgasketed joint that is easily assembled andprovides outstanding pressure holdingcapacity. It has been tested to 1,000 psinternal pressure, 430 psi externapressure and 14 psi negative air pressurewith no leakage or infiltration. The pushon and mechanical joints are standardized

under ANSI/AWWA C111/A21.11American National Standard for RubberGasket Joints for Ductile Iron PressurePipe and Fittings. Steel pipe has nocomparable standard.

There are two types of push-on jointsavailable, the Fastite and Tyton jointsThey differ somewhat in configuration, butboth feature a gasket recess that isintegrally cast into the bell of the pipeThe compression of the standard dual-hardness gasket results from the pushinghome of the spigot. The result is a flexiblejoint that is easy to assemble and difficult

to dislodge or roll during installation.

Fastite Joint

Tyton Joint

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Field Cutting Steel PipeGasketed Joints isImpractical

In steel pipe a push-on type of joint isalso available. However, rather than usingthe dual hardness gasket that inserts intothe bell, an O-ring type gasket is placedaround the spigot. The gasket recess is

either fabricated or formed onto the spigotend of the pipe. This type of assemblymakes field cutting steel pipe impractical.Further, steel pipe gasketed joints haveminimal deflection capacity to assist inrouting the pipeline. That is why linedrawings and laying schedules arerequired for steel pipelines. In many caseswelded joints are required. This, too,increases installation costs as skilledworkers must be used.

Ductile Iron PipelinesAdapt to Field Conditions

in InstallationDuctile Iron pipes push-on and

mechanical joints are deflectable. Thismeans that the ample deflectioncapabilities of the joints may be used tohelp reroute the pipe along curves or

around existing underground obstructions,such as existing utilities. Also, since thegasket fits into the bell, the spigot may becut in the field to make spool pieces. Thisnormally gives the contractor moreflexibility in the field than steel pipe canoffer. It eliminates the common need forspecial lengths of pipe to be manufacturedand shipped. It allows for a small variety ofstandard bends to be utilized in con-

struction. It makes laying schedules orline drawings unnecessary. If an existingutility is encountered during construction,special orders for fittings are often notnecessary. The joints can simply bedeflected to route the pipeline around,over or under the obstruction.

Furthermore, Ductile Iron pipe isfurnished in 18- or 20-foot nominal layinglengths. Steel pipe can be furnished in 20-foot lengths, but is more commonlyoffered in 36- to 50-foot lengths. Thisfurther limits the deflection capabilitiesand exacerbates handling concerns.

Table 6 shows the minimum rateddeflectability of Ductile Iron pipe push-onjoints for selected diameters (otherdiameters and larger deflections areavailable) and the maximum deflectionavailable for steel gasketed joints(according to manufacturers literature).

Steel Pipe JointsRequire Pointing andDiapering

Of course, welded joints are alsoavailable and, as noted earlier, are often

used. However, regardless of the type ofjoint, the jointing procedures are morecomplicated for steel pipe. First,whether the coating is cement-mortaror a flexible coating, the inside of thepipe joint has a significant area ofexposed steel. This requires that thejoint be pointed, or covered withcement-mortar after assembly. On theoutside, the joints must also be coated

either by diapering (cement-mortarcoating) or by the application of abonded coating. Experience has shownthat it is unnecessary and undesirable topoint Ductile Iron pipe joints. This isdue to differences in materialcomposition and joint design. As aresult, there is no need to point theinside of Ductile Iron pipeline jointswith cement mortar. If external

corrosion is a concern, the use of poly-ethylene encasement makes thecoverage at joints much more easilyaccomplished than diapering or applying

a bonded coating.Further, pointing and diapering of

steel pipe joints make the jointsinflexible. This allows any subsequentdifferential soil movements to stressthe joints, possibly loosening orcracking the mortar and exposing thesteel to possible corrosion. Becausepointing and diapering are not requiredfor Ductile Iron pipe, it retains itsflexibility throughout the life of thepipeline. This flexibility, in addition tothe shorter pipe lengths, makes DuctileIron pipelines less susceptible to

damage resulting from normal groundmovements, and this advantage extendsto more violent conditions such asthose associated with earthquakes.

Backfilling is Easier withDuctile Iron Pipe

Pipe stiffness is a function of thepipe materials modulus of elasticity andthe moment of inertia of the pipe. SinceDuctile Iron pipe design results in a

Table 6

Sample Comparisons ofJoint Deflection Capacities.

Nominal DuctilePipe Iron Steel

Diameter Pipe2 Pipe3

12 5 3.49

16 3 2.6420 3 2.80

24 3 2.34

30 3 1.88

36 3 1.57

42 3 1.35

48 3 1.18

54 3 1.05

60 3 0.94

Steel Pipe FabricatedRubber Gasket Joint

Steel Pipe Rolled-GrooveRubber Gasket Joint

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thicker wall for a given set ofparameters, Ductile Iron pipe is a stifferproduct than steel pipe.

Further, in calculating stiffness forsteel pipe, one accounts for the stiffnessof the cement-mortar lining and, ifpresent, the cement-mortar coating.Ductile Iron pipes cement-mortarlining is not considered to be astructural component of the product and

is not included in the pipe stiffnesscalculation.Since Ductile Iron pipe is a stiffer

product, it relies less heavily on thesidefill soils to help support theexternal load. This means that typicalinstallations do not require select and/or highly compacted material in order toprovide adequate support.

Steel pipe, however, requires moredesign with regard to laying conditions.Where the maximum standard modulusof soil reaction, E, for Ductile Iron pipeis 700 psi (per ANSI/AWWA C150/

A21.50), this is more nearly a minimumor ordinary condition for steel pipe.According to steel pipe manufacturersliterature, an E as high as 3,000 psimay be needed to prevent excessivedeflection. This translates to a selectgranular material compacted to 95percent Standard Proctor density tohelp support a steel pipe alternate andthe inspection and diligence to enforcesuch a specification.

That is not to say that an E of 3,000is not possible. If a granular material isplaced at 95 percent Standard Proctor

density the assumption of an E of 3,000may be appropriate. However, DuctileIron pipe design is not so optimisticabout the reality of obtaining such uni-form compaction in general installation.An E of 700 psi is a very conservative,yet realistic, expectation.

Ductile Iron Pipelines

Are More EnergyEfficient than Standard

Steel PipelinesAnother aspect of comparing Ductile

Iron pipe with steel pipe are the costsassociated with operating systems.Cathodic protection systems, often arequirement for steel pipelines, involvehigher design and installation costs. Theyrequire monitoring and maintenance overthe lifetime of the pipeline. There arealso costs associated with pumping waterthrough a pipeline and these costs aredirectly related to pipe inside diameters.

Ductile Iron Pipe Inside

Diameters are NormallyLarger than Steel

In all normally specified pipe sizes,cement-mortar lined Ductile Iron pipehas an inside diameter that is larger thanthe nominal pipe size. On the otherhand, steel pipe inside diameters areusually equal to the nominal pipe size.Therefore, for a given flow, the velocitywill be greater in steel than in DuctileIron pipe. Higher velocities translate tohigher head losses and, therefore,greater pumping costs in a steel pipealternate. When this difference is takeninto account, the use of Ductile Iron pipecan generate significant savings throughlower pipeline head losses.

Pumping Costs are

Lower for Ductile IronPipelines

For example, consider a 24-inchpipeline 30,000 feet long with a flow rateof 5,000 gpm. The actual inside diameterof Pressure Class 200 cement-mortarlined Ductile Iron pipe would be 24.95inches compared with 24.00 inches forthe steel pipe alternate. The cor-responding velocities would be 3.28 fpsfor the Ductile Iron pipeline and 3.55 fpsfor the steel pipeline. The head loss for

the entire length of the pipeline wouldbe 36.9 feet for Ductile Iron pipe and44.7 feet for the steel pipe alternate.This means that pumping costs wouldbe 17 percent lower for the Ductile Ironpipeline. This reduction in pumpingcosts will save the system ownersignificantly over the life of the pipeline.

Equivalent Head Loss

PipelinesAnother way to look at this example

would be to determine the equivalentsteel pipeline alternate that would berequired to meet the head loss of theDuctile Iron pipeline. This equivalentsteel pipeline would require some pipeof a larger size to reduce its overall headloss. The next larger standard nominalsize of steel pipe is 26 inches with acorresponding inside diameter of 26.00inches. The equivalent steel pipelinewould consist of 13,954 feet of 24-inchpipe and 16,046 feet of 26-inch pipe.More than half the pipeline would have

to be up-sized just to handle the sameflow as the Ductile Iron pipeline.

To offset the increased pumping costsover the life of a steel pipe line, severalalternatives may be considered. Theincreased pumping costs can beannualized and used to determine apresent worth value in an economicanalysis. This present worth of addedpumping costs should be considered an

added cost to purchase the steel pipealternate. Alternatively, equivalentpipeline theories as discussed abovecould be used or the head losses can bemade equivalent by specifying the insidediameter of the steel pipe alternate beequal to that of Ductile Iron pipe.

All Pipe Materials areNot Equal Conclusion

For the engineer, the owner and thecontractor, the advantages of DuctileIron pipe abound. The conservativedesign approach gives the engineer aneffective design that is easilyaccomplished while offering animpressive factor of safety. Thecontractor experiences the ease oassembly and adaptability of a pipeproduct that allows field adjustments tominimize costly delays in constructionThe owner receives a long-lasting pipethat is easy to operate and maintainincluding a simple corrosion controsystem that requires no cathodicprotection and no monitoring ormaintenance. And he has the knowledgethat unforeseen changes in operatingconditions arent likely to compromisethe ability of Ductile Iron pipe toperform. When comparing Ductile Ironand steel pipe it becomes apparent all pipe materials are not equal.

Notes1 Ductile Iron pipe outside diameters arestandardized in ANSI/AWWA C151/A21.51. Steel pipe outside diameters

are typically based on clear insidediameters equal to the nominal pipediameter. The outside diameter forsteel pipe is, therefore, calculatedby accounting for the thickness of thecement-mortar lining (0.5 inches for 48inch pipe) and the steel cylinderthickness.

2 Deflections listed are minimums setforth in ANSI/AWWA C600.

3 Deflections listed are maximums foundin manufacturers literature.

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DVS/8-01/5M

Published 12-96

Revised 8-01

Manufactured from recycled materials.

DIPRA

MEMBER COMPANIES

American Cast Iron Pipe CompanyP.O. Box 2727Birmingham, Alabama 35202-2727

Atlantic States Cast Iron Pipe Company183 Sitgreaves StreetPhillipsburg, New Jersey 08865-3000

Canada Pipe Company, Ltd.1757 Burlington Street EastHamilton, Ontario L8N 3R5 Canada

Clow Water Systems CompanyP.O. Box 6001Coshocton, Ohio 43812-6001

McWane Cast Iron Pipe Company1201 Vanderbilt RoadBirmingham, Alabama 35234

Pacific States Cast Iron Pipe CompanyP.O. Box 1219Provo, Utah 84603-1219

United States Pipe and Foundry CompanyP.O. Box 10406Birmingham, Alabama 35202-0406

An association of quality producers dedicated to highest pipe

standards through a program of continuing research.

245 Riverchase Parkway East, Suite O

Birmingham, Alabama 35244-1856Telephone 205 402-8700 FAX 205 402-8730

http://www.dipra.org

Copyright 2001, 1996 by Ductile Iron Pipe Research Association.

This publication, or parts thereof, may not be reproduced in any form without

permission of the publishers.