DESIENINE. OPE]I,ATINE AI\IB I\4/AINTAININEPIPINE 5Y5TEM5 USINE

PVf FITTINES

A Handbook of Design Guidelines and Precautions

DESIGNING, OPERATING ANDMAINTAINING PIPINC SYSTEMS

USINC PVC FITTINGS

A Hanclbook of Design Guiclelinesancl Precautions

Ron D. BliesnerKeller-Bliesner Engineering

Logan, Utah

Februaryr 3, 1987

Publishecl by theIl/C Fittings Division

of the lrrigation Association191 I North Fort Myer DriveArfington, Mrginia ZZZO9

The following manufacturers of P\/C fittingsprovided the funding for this study and its publication:

Dura Plastics Pnrducts, Inc.Eslon Thermoplastics

LCP Plastics, Inc.Lasco Fittings, Philips Industries Inc.

Nibco, Inc.R & G Sloane Manufacturing Co., Inc.

Spears Manufacturing Co.

Ittj

t

I

Thble of Coffents

PREAACE

INTRODUCTION ...

PVC STRENGTH CHARACTERISTICS AND TYPICAL PVC FMTING EAILURESPVC Strenglh Characteristics .Burst FailureLong Tbrm Pressure FailureCyclic Surge FailureMechanical Failure

PVC FITTING STANDARDS-WIIAI THEY MEAN AND HOW TO USE THEM .. . . . .. . 5

PRESSURE SURGES-WTIAT CAUSES THEM AND HOW TO CONTROL THEM ....,.. 7Causes of Pressure Surges . . . . . . . . . zControlling the Magnitude of Pressure Suqges in the Design phase . . . . zThe Effects of Management on Pressure Surges .......... g

THE DANGERS OF ENTRAPPED AIRSources of Air in FipelinesWhy is Entrapped Air a Problem? . . . . ... .. 9Deal ingwi thEntrappedAir . . . . . . . 9

PRESSURE PIPE DESIGN AND SELECTION ..... 9Pressure Rat ing . . . . . . gTbmperature Considerations in Design .... 9Designing for Cycl ic Surge Capaci ty . . . . . . . . . . . . . .10

INSTALI.AIION CONSIDENAIIONS .....11H a n d l i n g . . . . . . . . . . . . 1 1Solvent Welding . . .. .11Threaded Fi t t ings . . . . . . . . . .1- tThrust Blocking ....tzTbmperature Expansion Considerations ... ....... lz

SYSTEM MAINTENANCE REQUIREMENTS . . . . . . . . . . . .13MaintainingAir Relief and Surge Control Equipment ...........18Precautions in Winterizing the System ...........13

S U M M A R Y . : . . . . . . . . . . . . . 1 3

B r B L r O G n A P H y . . . . . . . . . . . 7 4

SELECTED REFERENCES FOR FURTHER READING .......,...].4

112334

88

ilt

List of Ffgures

Frgure 7.

Figurc 2A.

FigUre 28.

Flgure 3.

F€ure 4.

Figure 5.

Figure 6.

Frgurc 7.

Frgur€ 8.

Flgure 9.

Figure L0.

Figure 11.

Tlpical stress versus time to failure regrnssion curve for PVC pipe .... 2

Examples of typicalburst failures in PVC fitt ihgs ......... 2

Examples of typical burst failures in PVC fittings 3

Examples of typical long term pressune failures in PVC fittings 3

Examples of typical cyclic pressure failures in PVC fittings .. . . . 4

Examplesof threadedPVCf i t t ingfa i luresduetoover- t ightening . . . . . . . . . . . . . . 4

Examples of failurc due to improper solvent welding of joints ......... b

Cross section of a gpical 2-inch Schedule 40 PVC tee .. . 6

Tlpical flowrate variations at the pumping plant of a golf course system 8

Cyclic fatigue life of PVC pipe when subjected to large pressure surges . .. . .. 10

Relationship betuveen hoop stress in the female thread andtums past finger tight in the makeup of 7lz-inch threaded PVC fittings ....... 11

Tlpical thrust block installations ......... 13

List of Thbles

Thble 1.

Thble 2.

Thble 3.

Thble 4.

Thble 5.

Thble 6.

PVC pipe pressune ratings-Schedule 40 and 80 ........ .-

Suggested maximum working pnessunes for Schedule 40 andSchedule 80 PVC fittings 6

Maximum pnessure surge with instantaneous valve closure forwater flowing in PVC pipe. . 7

Pressure reduction factors for specific operating temperatures above 73F ..,......,.10

Thrust developed per 100 psi of line pnessure for various pipe sizes andf i t t i ngcon f i gu ra t i ons . . . . . . , . . . . . . . l 2

Estimated bearing strenglh of typical soils . .....lZ

IV

kefacei

This handbook has been prepared for use by PVC piping system designers, installers, oper-ators and component manufacturers and suppliers to aid in understanding and eliminatingtypical pmblems that occur in today's s5rstems.

The guidelines pr€sented have been accumulated and dweloped ftom experience in eval-uating system performance and troubleshooting pmblems with H/C piping systems in com-mercial, industrial and irrigation applications over the past 10 years. Many of the pnrblemsidentified over these years are repetitious in nature, suggesting a lack of good guidelines touse in desrgn, installation and maintenance.

ln rccent years, the PVC fitting industry has become concemed that their pmducts werebetn€ used in applications without adequate design precautions and have noted considerablemisunderstanding among desiglrers, installers and operators concerning prcper applicationand use. With the support and input of a number of PVC fitting manufacturers, this handbookhas been prepared to address the problems that most ftequently occtrr and to pmvide tech-niques to avoid these problems at the design, installation and operation phases of a pmject.

I wish to express my thanks to the H/C fitting manufacturers for their support and valuabletechnical input in the preparation of this handbook and to the lrrigation Association for itspublication. I also wish to thank Dr. Jack Keller for providing valuable technical rerziew.

Ron D. BliesnerVice PresidentKeller-Bliesner EngineeringLogan, Utah

InlroductionThe piping industry was revolutionized by the intro-

duction, in the 1940s, of PolyUinyl Chloride) (PVC) pipe

and fittings. Piping system components manufactured framPVC exhibit excellent corrosion resistance (PVC is inert tomost acids, alkalies, fuels and other corrosives), are lightweight, have a high strength-to-weight ratio, are excep-tionally durable and have great resiliency. The use of PVChas grovrrn steadily since its intruduction, to the pointwhere about 100,000 miles of PVC pipe is installed eachyear in North America alone, The growth of the industryhas been due, in paft, to the availability of a wider rangeof PVC pipe sizes and compatible fittings that are inex-pensive and easy to install.

No portion of the piping industry has been affected byPVC pipe and fittings more than the irrigation industry.The large quantity of pipe and the numenrus fittings requiredmade PVC a natural choice. The relatively low cost of thematerials, the ease of installation and the corrosion resis-tant nature of PVC have made PVC irrigation systems thehealy favorite for golf course installations, home and com-mercial installations and agricultural systems.

The revolution in piping materials has been followedclosely in irrigation systems, especially golf course sys-tems, by a revolution in sprinklers and control equipment.The advent of the valve-in-head sprinkler and computer-ized control systems in the last 10 years has providednearly unlimited flexibility in system operation. In earliergolf course systems, the design consisted of the mainlinesystem and lateral system, with a control valve separatingthe two. The pipe and fittings down stream of the controlvalve were pressurized on$ when that block was oper-ating; they were vented to atmosphere through the sprin-klers, so high surge pressures were unlikely. Also, thecontrol systems were less sophisticated, with less flexi-bility, which meant that the flowrates in the system couldbe more easily balanced at design time, reducing thepotential for high surges.

Today's systems are usually pressurized continuously.The control points in the systems are now at the sprinkle4,subjecting all piping components in the system to maxi-mum surges. The flexibility in control systems allows anoperator to put water an5rwhere in the system every fewminutes. Without adequate precautions, very high veloc-ities can be generated, leading to high pressure surges.Also, the valve-in-head sprinklers often close quite rapidly,creating sudden changes in velocigz at the sprinkler. Pres-sure surg;es due to valve closure have been measured atover 60 psi in valve-in-head systems and the potentialexists for even higher surges.

Earlyirrigation systems were constructed mainly of steelpipe, especially in the smaller diameters. Sprinkler swingjoint assemblies were nearly always constructed with steel

components. From a pnessure capaci$r standpoint, thecomponents had strength well beyond the stresses put

on them. With plastics, although the strength-to-weightratio is fair$ high, a given fltting or pipe used in an irri-gation system is not near$ as strong as the steel coun-terpart it replaced. So at the same time that we have extrastresses placed on irrigation systems due to equipmentchanges, we have the introduction of components withless strength. This combination can lead to componentfailure unless careful consideration is given to design andoperation.

In view of above considerations, the irrigation industry

needs some guidelines in designing and operating irri-gation systems with PVC components. With care in selec-tion of components, in system design and operation, mostof the problems can be eliminated. The following sectionsdeal with aspects of PVC piping systems that are essentialto satisfactory operation.

PVC fltrengfih Characterieticsand Tlpical PVC nitting Failures

To better understand the performance of PVC fittings inpiping systems it is helpful to examine the streng[h char-acteristics of PVC and the types of fitting failure that canoccur. The types of failure fall into four main categories:1) Burst failure; 2l Long term pressure failure; 3) Cyclicsurge failure or fatigue; and 4) Mechanical failure due toexternal forces. Each of these t5,pes of failures will bediscussed separately, although failure may be caused bya combination of situations.

PVC Shengfih Characteristice

Most PVC pipe and fittings used in the irrigation industry

are manufactured from Tlrpe I, Grade I PVC compounds.Such a compound has a minimum tensile strength of 7,000psi before being stressed and a modulus of elasticity intension of 400,000 psi at 73"F. while the initial tensilestrength of the compound is listed as 7,000 psi, this isbased on a tensile test in which the loading rate is 0.20inch/minute. On the other hand, the stress rating of thematerial is developed through long-term tests (at ledst10,000 hours) on pipe under constant stress. The resultinglongterm strength is divided by a safetSr factor of 2 to arriveat a 2,000 psi design strength.

The tensile strength of PVC under stress for an extendedperiod of time is often described by a stress regressionplot as sholrn in Figure 1. For example, in Figure 1, if apipe is continuously stressed at 5,000 psi it would besubject to failing (bursting) after approximately L,000 hours.The plot is based on results from the long-terrn tests ona substantial number of pipe specimens. The outsidediameter and minimum wall thickness are deterunined for

FIGURE f . Tlpical streao versua time to faihueregpession curve for PVC pipe.

each specimen so that the hoop stress for each specimencan be calculated based on a test pressure. Each specimenis then subjected to the selected pressure (hoop stress)and the time to failure is recorded. Specimens subjectedto highest stresses fail in the shortest time and those atlower stresses last longer. By this process, a whole series

of stress regnession data points are developed. These dataare then analyzed in accordance with the ASTM D2837method to select mathematically the "best fit" stressrcgtr€ssion line. As can be seen in Flgure 1, the time requiredto bring PVC pipe specimens to failure varies accordingto the pressure (stress) being applied: e.g., individual spec-imens should withstand 6,000 psi for 10 hours; or 5,000psi for 60 days (1440 hrs); or 4,000 psi for 35 years (306,600

hrs); or 3,800 psi for 100 years (876,000 hrs).These values are for static pressure conditions. Tests

have shown that PVC pipe under continuous static pres-sure for a long period of time can be subjected to a quickburst pressure (less that 70 seconds in duration), and willperform essentially the same as pipe which has not beensubjected to static pnessure testing. It appears that, evenafter years of service, PVC pipe maintains its ability towithstand occasional high pressure surges.

Howeve4 if this pipe is subject to frequent pnessurevariations of a cyclic nature it can fail, even though thepeak pressure never exceeds the design pnessure of thepipe. The number of cycles to failure depends on themagnitude of the pressune variation. It appears that theability of PVC pipe to withstand cyclic pressure conditionsis independent of its ability to withstand constant staticpressure. PVC pipe seems to have two "funds" uponwhichto draw one labeled "static pressure life", and the other"cyclic pressure life".

It should be noted that the above conclusions are basedon limited long terrn testing of PVC pipe. Based on thesetests a design criterion considering cyclic pressure cen-ditions has been developed and will be discussed later.As more testing is done, these phenomena will becomebetter understood and the design techniques refined. Also,the rcported cyclic testing to date has been performedmainly for pipe and not for fittings. From examination of

2

H/C fittings removed ftom installations and tested, it appears

that the above observations may not hold true for fittings.

At least the number of cycles to failure maybe considerablyless, due to stress concentrations at points of directionchange in the fittings

Burst Faihrre

Burst failure in PVC pipe and fittings is usually rather

dramatic. It mdy begin at a point of stress concentration

orweakness and may continue by splitting through fittings

and pipe for some distance. T}pical examples of burst

failure are shovrrn in the photos in figures 2A and 28.

Sometimes, the failures will completely shatter a fitting

and the adjacent pipe.Burst failures usually occur during hydraulic transient

conditions that create large pressure variations in the

system. These include rapid valve closure, pumps starting

or stopping, rapidly escaping entrapped ajt or an air pocket

shifting within a pipeline. Burst failure will, sometimes,occur in a pipe or fitting that was damaged during instal-

lation or that is subject to external loads. In these cases

the failure may occur at pressunes well below the expected

burst limit of the product.

FIGURE 2A. Examples of typical buret failures in PVCfittinge.

FIGURE 23. tryemFles of bpical burst failureo in PVCfittings.

Long Tbrm Pressure Failure

Long term pressure failure occurs when the systemoperates continually at a pressure that will eventuallycause failure. The failures may occur within a short timeafter system installation or after many years. The failureswill usually appear as slits or small cracks in the pipe orfltting along the minimum wall thickness or in an area ofstress concentration. Some yielding of material will usuallybe evident.

Tlrpical examples of what appear to be long terrn pres-sure failures in fittings appear in Figure 3. These exampleshad been installed in golf course systems for about 2 yearswhen failure occurred. The systems were subject to rel-atively high (120 psi) operating pressures and significantcyclic pressure conditions. The failures seen may actuallybe a combination of cyclic and sustained pressure failure.It may be noted that these failures occurred at consider-ably lower pressures and fewer number of cycles than maybe required to cause failure in Schedule 40 pipe. Note that,on one of the fittings, the material is eroded away aroundthe fracture line. This is an indication that the crack wasoriginally very small and that the leak went undetectedfor a sufficiently long period of time to allow the jet of

waterto churn the sand, which produces localized erosionon the outer surface of the fitting.

Cyclic Surge Failure

Cyclic surge failure can occurin systems that are subjectto frequent changes in flow and./or pressure. Modern golfcourse systems with computer controllers are prime can-didates for cyclic failure. A typical golf course system mayexperience from 40,000 to 100,000 cycles peryear of mag-nitudes from 10 to 80 psi. H. W. Vinson (1) indicates thatit is not uncommon to see cyclic failures in golf coursesystems after 2 to 5 years of operation. T}pical examplesof cyclic fitting failures appear in Figure 4. Note the simi-larity to the photos in Figure 3. It is very difficult to dis-tinguish between cyclic failure and long terrn static failurein fittings.

Design standards have been proposed by H. W. Vinson(1) to consider cyclic surges. Howeve4, the standards applyto pipe and not fittings. It appears that fittings, due to thestress concentrations and extra forces placed on the fit-tings, will not withstand as many cycles, although theirburst strength may be equal to that of the same class pipe.Limited testing completed by Keller-Bliesner Engineering

FIGURE 3. Exarnples of typical long term preoorrrefailures in PVC fittings.

FTGURE 4. Examplee ol typicd cyclic preesure tailureein PVC fittinge.

also indicates that there may be a marked reduction inburst strength after subjection to a period of cyclic pres-sure conditions. Used tees that had been subject to cyclicconditions sufficient to cause failure in some fittings wereremoved before failure and tested for burst strength againstnew fittings. The used tees exhibited only about 567o ofthe burst strength of the new tees. These test results arenot all-inclusive due to a limited sample size and limiteddata on field conditions, but they do provide an indicationthat cyclic conditions may reduce the strength of PVCfittings.

Of all the operating conditions that can cr€ate problems,cyclic pressure appears to be the most critical, especiallyin golf course systems. Given the cyclic nature of irrigationsystem operation and their limited capacity to handlecyclic pressure conditions, fittings are the weakest systemcomponents. Serious consequences may result if this factis not adlequately considered at the design stage.

Mechanical Failure

"Mechanical failure" covers a multitude of piping fail-ures that are unrelated to but may interact with thehydraulics of the system. One of the most common qpesof failure is splitting of female threaded fittings due to

4

FIGURE 5. Examplee of tbreaded PVC fitting fallures dueto over'tightening.

over-tightening. Since PVC is visco-elastic, it yields moreeasily on thread make-up than steel. The threads are alsosmootherand create less fi:iction in make-up.It is thereforevery easy to over-tighten PVC fittings. It is possible, with

very little effort, to create circumferential stress beyond

the failure limit when assembling threaded fittings. This

is even mone pronounced when using some thread lubri-

cants, dopes orsealants. Figure 5 shows examples offailureof female threads due to overtightening. The failure usuallyappears as a split, perpendicular to the threads, beginningat the leading edge and extending into the body of the

fitting. Occasionally, a split at the base of the female threads

willappear parallel to the thread direction. This will usu.

ally occur in a fitting with a shoulder or thickened place

near the base of the threads and is mone common when

the male part bottoms against a shoulder.

A second $rpe of mechanical failure occurs when inad-equate thmstblocking is provided. This allows excessivepressure to be placed on a fitting as the line pressure tries

to displace it while the fitting is restrained by the pipe to

which it is attached.

A third type of mechanical failure occurs due to impropersolvent welding or improper fitting assembly. Figure 0shows examples of these problems. Improperpenetration

FIGURE 6. Exarnplee of failure due to irnproper eolventwolding of joints.

of pipe into socketed fittings significantly reduces thestrength of the fitting. The tee and bushings sholrm inFigure 6 were assembled with only 50yo socket penetration,significantly weakening the assembly. Improper solventwelding techniques can cause failures in the bonding,creating leaks or separation.

Another type of mechanical failure can occur due totemperature expansion. If sufficient expansior/contrac-tion allowances are not made by providing expansionloops, offsets or slip joints, severe stress can be placed onthe pipe and fittings.

PVC Fitting Standards-What They Mean and How Tb Use Them

Schedule 40 and 80 solvent weld and threaded fittingsare covered by the following ASTM Standard Specifica-tions:

D2464-Tt\Eaded Poly(Vinyl Chloride) (pVC) plastic pipeFittings, Schedule 80.

D2466-Poly(Vinyl Chloride) (pVC) plasric pipe Fiftings,Schedule 40.

D2467 -Socket-T}pe Poly(Vinyl Chloride) ( pVC ) plasticPipe Fittings, Schedule 80.

These Standards deal mainly with workmanship, mate-rials, dimensions, tolerances and testing. There are nopressune ratings for PVC fittings in these ASTM Standards.The only pressune neferences are for burst pressure. Thestandards even state that the burst pressures are used"only as an indication of qualigr" and "do not imply ratedworking pressures." The only information given that mightindicate an attempt to construct a fltting for a particularpressure rating is that the Standard requires the minimumwall thickness of the fittings to be at least 125% of the wallthickness of the same Schedule pipe except in the socketswhich are required to be 100% of the thickness of the sameSchedule pipe.Webelieve this maybe an insufficient incrEasein wall thickness to allow equal working pnessures, evenunder ideal conditions. While we recognize that literallybillions of PVC fltting are in use in a varietSz of applications,the problem with failures in some applications with noguidelines on working pnessure suggests a need for betterStandards that provide more information to the user con-cerning suitability for a particular application.

Thble 1 presents the nominal and outside diameters,burst pressure and working pressure specifications ofSchedule 40 and S0 PVC pipe. The burst pressures shownalso apply to Schedule 40 and 80 solvent weld pVC fittings.The working pressunes do not apply to fittings, however.No Standard exists that specifies a working pressure forfittings.In addition, no testinghas been done to determinethe working pressure of fittings manufactured to thesespecifications. Herein lies the problem of proper appli-cation of these fittings. Historically, most designers haveassumed that a Schedule 40 or 80 fitting would performequally with the same Schedule pipe o4, worse yet, simplyassumed that the term "schedule" meant super hearyduty and they did not bother to check the test specifica-tions.

IABLE f . PVC Pipe Pressure Ratings-Schedule 4O andaO (only burst preesures apply to fittings)*

Schedule 40 epecifrcatione

Outside Wall Bumt heeeureDiameter Thickness PresEure Rating

(psil (psi)

Schedule E0 specifrcatione

NoninalSize

WatlThicknese

BurEt Pr€ssur.ePressure nadng

(pei) (psi)

V, 0.U03/r 1.050

1 1.3151y4 1.660|y, 1.9002 2.375zyz 2.8753 3.5003y, 4.0004 4.500J 5.5t).1

o b.bz5

8 8.62510 10.75012 72.750

2720 8482200 6882020 6301660 5201510 471,1290 4041360 4251200 3751110 3451040 324930 289890 275750 246750 234730 228

0:t470.1540.1790.191

0.20.2180.276

0.30.3180.3370.3750.432

u.50.5930.687

*The same burst pressures apply to fittings but they carry no pressureratings.

0.1090.1130.1330.1400.1450.1540.2030.2160.2260.2370.2580.28003220.3650406

19101540144011801060890s70840770710620560500450420

596482450368330277304263240222195t771551.41.I t L

It may seem that, since fittings have the same burst

pFessune requirements as the equivalent pipe, the rated

wor*ing pnessunes should be equal. I-et's examine a fitting

and see why this isn't the case. Figure 7 depicts a cross

section of a t5pical 2 inch Schedule 40 tee' The straight

side of the tee is str€ssed much like an equivalent leng[h

of pipe. HoweveD the branch side of the tee is stressed

-""tt differently. In fact, a tee manufactured strictly to the

ASTM Standard will be about 257o weaker on this side than

an equivalent length of pipe if only the tee is considered

(no sirength derived from the inserted pipe). As the fitting

is configured for standard ASTM testing, the branch side

would only be about 107o weaker than equivalent pipe if

it is assumed that the stress placed on the center of the

fitting can be resisted by the fitting wall in the sockets'

This is a fair assumption under quick burst conditions

(less than 70 seconds) since the material that is over-

stressed has not had sufficient time to deform' HoweveD

under long term pressure conditions or cyclic conditions,

the unsupported portion of the fitting will carry higher

stresses and will tend to fatigue in these stress concen-

tration areas. The failure locations shovvn on the tees in

Figures 3 and 4 identi$r these stress concentration aneas'

How then, is a desi$ner to decide how to use these

fittings, if no pressure rating is given? That is the key

question, and it cannot be answered with any degree of

confidence until cyclic and long term tests are conducted

and pressure standards developed. Howeveq by analyzing

the iorces on the fitting, we can make some educated

guesses and provide some general ggidelines to use in the

absence ofpressure standards based on tests'

If the fitting received no additional strength from the

pipe cemented into the sockets, then the highest expected

p""ttnt* ratin$ would be about 75Vo of that of the equiv-

hent schedule pipe. Tests conducted on alimited number

of 2 inch Schedule 40 PVC tees that had been subjected

to conditions causing fatigue and,/or sustained pnessure

failures in similar fittings in the same system trurst at 567o

of the pressure required to burst new fittings under burst

test conditions. Using standard stress rcgression plots, the

used fittings should have been capable ofat least ToVo of

the original burst strength. The capacity of the fittings

under cyclic conditions is not knovrrn, but appears to be

significantly less than for equivalent Schedule pipe' For

these reasons it is estimated that Schedule 40 and 80 PVC

fittings should carry/ a pnessure rating of not more that

6070 ;f the rating for the equivalent Schedule PVC pipe'

Thble 2 lists these pnessune ratings. It should be noted that

these limits are based on very little hard data and should

be used with caution. Certain circumstances may require

even lower pnessune ratings. It should also be noted that

pressure tuii.tg" for Schedule 80 threaded pipe is 507o of

ih" p."ttrr." rating forunthreaded pipe' Some further de-

rating of Schedule 80 threaded fittings may be needed that

is not reflected in the values in Thble 2.

Some PVC fittingmanufacturers are now offering orplan

to offer higher str€ngth tees and elbows that are reinforced

with extra material in the stress concentration areas'These

fittings exceed the thickness requirements of the current

ASTM Standards and would normally be capable of with-

TllBLE 2. Suggested Maximum Working Pressuree forSc[edute 4O and tchedule AO PVC Fittinge'(Uee ae a $eneral gpide only' Actrral allowableworking p"e"eu"*i may vary widely with fieldconditione.)

I

Vz3/c

-1.

1Y4

1,1/z2

zvzJ

J / 2

4J

68

1012

Schedule 4O Schedule 8o

NominalSize

BuretPregaure

WorkingPreaaure

Burat WorkingPreaaure Preaaure

191015401,4401,1801060

89097084077071.O620560500450420

358285270221198166182158IM133117106

938479

27202200202016601510129013601200t-1101040

9308907SO750730

5094t33783t2282243255225zo7194473167a4at40137

FIGURE 7. Crnoes eection of a typical Z'inch Schedule 4OPVC tee.

6

standing greater pressunes. Howeveq no test results havebeen published and no new Standards set forthese fittings.Also, there is some consideration in the industry to pushfor pressure rated solvent weld fittings. This approachwould ease the difficulty of the designer in selecting theappropriate fittings.

kessure Sur(es-W'hat CanrsesThem and Hffi'Tb Conlrol Them

Causes of Preesure Sur$ee

Few piping systems are operated under "static" con-ditions for long periods of time. Hydraulic transient con-ditions or "surges" occur in every irrigation system. Apnessure surge or "water hammer" is created any time theflowrate changes in a piping system. This may be causedbyvalve operation, pumps starting or stopping, line breaks,or rapid escape of entrapped air.

14/hen a pipe contains a column of moving liquid, thereis considerable kinetic eners/ stored in the liquid byvirtueof its mass and velocigz. If the velocitSr is suddenly decreased(by the quick closing of a valve) this energy cannot beabsorbed by the liquid, since the liquid is nearly incom-pressible. It appears as an instantaneous shock to the pipewhich may lead to excessively high pressures. This effectis greater as the pipe line is longe4 the velocigz changegreater and the closing time of the valve shorter.

Instantaneous valve closure is often assumed in com-puting the maximum surge pressure per unit change invelocigz. Table 3 indicates the maximum surge pressurefor various PVC pipe sizes and SDR values for a range ofvelocity changes. These values can be used in design to

estimate the pressure rating requirements of the pipingsystem.Although the table showsvalues ofvelocity changeup to 10 feet per second (fos), it is advisable to limit veloc-ities to 5 fos to control maximum surges.

Controlling the Magpritude ofPressure Surges in the Design Phase

A number of design techniques are used to control waterhammer in piping systems. To limit surges generated bypump operation, air chambers or surge tanks that absorbsurges in conjunction with pressune regulating and pres-sure reliefvalves are often installed in the pumping station.Packaged pumping plants with all necessary controls andequipment are available from a number of manufacturersand are commonly used in golf course irrigation systems.When allvalves and controls are properlysized and adjusted,surges generatedby changes inpump flows and,/ordemandsare reduced to non-harrnful levels. If a packaged pumpingplant is not used, the pump station should include, at thevery least, a pr€ssure regulating valve to maintain constantdown stream pnessure regardless ofthe flow rate. Pressuretanks decrease the pressure fluctuation seen at the valveand increase the pressure modulation in the system.

Controlled valve closure speed is critical in limiting themagnitude of pressure surges in the system. A typicalvalve-in-head sprinklerwill close in 6 to 10 seconds, whichseems sufficiently slow to limit surges to acceptable levels.Howeveq, the valves are electrically controlled hydraulicvalves that operate on the principle of pressure differenceacross the valve for closure. The higher the pressure dropacross the valve, the faster it closes. Therefore, the closerthe valve is to being closed, the faster it closes, which

IABLE 3. Maximum pressure surge with inetantaneoue valve cloeure for water flowing in pv6 pipe.

PreasureWave

VelocityfUsec

Velocity Change-fps

4 5(-not recommended-)

6 a 1 0SDR

13.5'1.4

17182't252632.541,

1.5021,47413311292119310911069

954847

20.219.817.917.416.014.71.4.4t2.811.4

40.439.63s.834.832.1.29.328.725.622.8

60.659.4c J . /

52:1.48:t44.O43.1,38.534.2

80.879.271.669.564.258.7

51.345.6

101.099.189.586.980.2

77.964.1,57.O

'1.21,.2

118.9107.41o4.396.388:086.276.968.3

161.6158.5743.2139.0128.3117.3115.01.O2.691.1

201.9198.1179.O173.81,60.4146.7't43.7

128.2113.9

Pressure

Thble values based on the following equations:

a : 466o/11+k(DR-2)/dy"P : aV/2.318

Where a : pnessure wave velocity-fosP : piessurc surge-psik : bulk modulus of water : 300,000 psiDR : Dimension ratioe : modulus of elasticit5z for pVC pipe

: 400,000 psiV : change in velocitlz-fosg : acceleration due to gravity-32 .2 fusec.

significantly reduces the effective closure time of the valve'

AJ a rule oi thumb, about 75vo of the fluid flow is shut off

in the last 25Vo of the poppet movement' Therefore' a 6

second closure time becomes a 1 1/4 second effective

closure time. In general, valves with slower effective clo-

sure times generate smaller surges'

In simple systems, pressure relief valves dor'r'nstream of

the main pressure regulating valve can reduce the mag-

nitude of some pressure surges. Howeveq in complex sys-

tems, where surges can start at many places, it may be too'costlytoplacepressunereliefvalvesatsufficientinterwals

to be effective.Entrapped airis one of the most troublesome andpoten-

tially dangerous causes ofpressure surges' A later section

will deal with the entrapped air problem'

At the design stage, the safest surge control technique

is to limit velocities in the system to 5 fps and to include

computation of surge potential in determining the pres-

sure requirement of system components' Many problems

will be eliminated if the surge potential is adequately

assessed and the components of the system selected to

withstand the Potential surges.

The Effects of Management on Preseure Sur$es

As discussed earlier, given the flexibility of operation

available with today's sophisticated irrigation controllerc'

all possible conditions cannot be foreseen at the design

stage. Therefore, considerable care in management of the

,yrl"* is required. The magnitude of flowvariations' and

thereby velocifr variations and potential pressure surges'

of a typical golf course is depicted in Figure 8' Note the

fequlnt changes in flow demand at the pump' Such vari-

ation is not uncommon in today's systems' With the shorter

set times and repeated irrigations per set, the number of

potential surges during the course of an operating season

tan exceed 40,000 mentioned earlier. The stress placed on

irrigation systems by such fluctuation can reduce signif-

icantly the life of the system components, especially if the

magnitude of the fluctuations is great and the pressurc

limit of the components is approached' Other locations

in the system can have fluctuations even greater than those

shownln Figure 8. It is not uncommon to have flowveloc-

ities in given pipe segments vary from zero to over 8'0 fps

at 2Io4 minute intervals unless the operating stations arc

carefully configured to balance flows.

To limit the magnitude and fiequency of pressure surges'

system operators should use the following guidelines:

1. Operate the system to maintain pump flowrate as

uniformly as possible. This will not only reduce

hydraulic transient problems but will increase the

life of the pumPing unit'

2. Attempt to balance system flows so the sprinkler set

changes are systematic within system subunits'Avoid

changing from one main area of the system and back

again in the operating program' Maintain sub-unit

flows uniformlY, if Possible.

3. Run fewer sets for longer times' Hydraulically' it is

easier on the system to run a given set as long as

possible, provided runoffdoes not occur, orthe mois-

iure holding capacity of the soil is not exceeded' This

will allow for fewer sets and, thereby, fewer oppor-

tunities for surges to occur'

4. Avoid operating too many sprinklers in one area of

the system and elevating the operatingvelocities' Use

the design guidelines to govem the number of sprin-

klers that may operate simultaneously on a given pipe

segment or looP'

The Dangere of EntraPPed Air

Air entrapment in pressure pipelines is a much studied

and discussed topic. Most designers are concemed about

it, or should be, but many do not understand the full

implications of the problem or the pnocesses used to reduce

the dangers associated with entrapped air' The problem

of entrapped air is a complex issue. The behavior of air in

a piping system is not easy to analyze, but the effects can

be devastating.

Sources ol Air fu PiPelinee

There are many potential sources for air in pipelines

and the sources are usually multiple in any $iven system'

The most likely source is entrapment of air during fiIlin$'

either initially or when refilled after drainage' In some

systems, air re-enters each time the pumps are shut off

as the pipelines drain through low lying sprinklers or open

valves.Air is often introduced at the point where water enters

the system. This is an especially common problem with

gravity fed pipelines, but may occur with pumped systems

as well. Even water pumped from deep wells may be

subject to air entrance from cascading water in the well'

A less obvious source of air comes from the release of

dissolved air in the wateq due to changes in temperature

and/or pressure. The quantities may be small in this case'

but accumulations over time can create problems'

€$t

r !, D

2.C

2-4

2.2

2.O

1.E

t .6

1.1

1 .2

t.o

o.6

o.3

o..t

o.2

o.o1 5 1 6 1 7 1 A t 9 2 0 2 1 X 2 2 3 2 4 1 2 5 ' , t

Tima ot DoY

FIGURE a. Tlpical flowrate variations at the pumping

plant of a golf course oystGm.

8

It is also common for air to enter through air releasevalves orvacuum breakers when the pressure drops belowatmospheric pnessure. This can occur during pump shut-down or during negative surges.

Why is Entoapped Air a Problem?

Airin apiping system tends to accumulate at high pointsduring low flow or static conditions. As the flowrateincreases, the air can be forced along the pipeline by themovingwaterand maybecome lodged atthe more extremehigh points where it reduces the area available for flowThus, these pockets of air cause flow restrictions whichreduce the efficiency and performance of the system.

As an air pocket grows, the velocit;r past that pointincreases until eventually the air is swept on toward anoutlet. While line restrictions are problems, a more serioussituation can occur when air is rapidly vented from thesystem under pressure. Water is about 5 times more densethan air at 100 psi, so when a pocket of compressed airreaches an outlet, such as a sprinkler head, it escapes veryrapidly. As it escapes/ water rushes in to replace the void.When water reaches the opening, the velocit5r suddenlydecreases, since air escapes about 5 times faster than waterat 100 psi. The result is similar to instantaneous valveclosure, except that the velocity change can far in exceedthe normal flow velociqr in the pipeline. During tests atColorado State UniversitS,, pressure surges up to 15 timesthe operating pnessune have been recorded when entrappedair was rapidly vented under pressure. Such pressuresurges can easily exceed the strength of the system com-ponents and even at lower magnitudes, repeated surgeswill weaken the system with time.

Dealing with Entrapped Air

Obviously, the best way to reduce problems caused byentrapped air would be to prevent it from entering thesystem. Precautions should be taken to eliminate airentrance. When systems are filled, either initially or afterdraining for winterization or repai4 they should be filledslowly, at a velocit5z of 1.0 fos or less, and the air shouldbe vented from the high points before the system is pres-surized. Even with these precautions, some air can remainin the system.

To deal with this remaining air or newly admitted aiq,continuous-acting air relief valves should be installed athigh points in the line and lines should be laid to gradewherever possible. Continuous-acting valves contain a floatmechanism which allows the air to vent through a smallorifice, even when the line is pressurized. The orifice diam-eter should be about 1 percent of the diameter of the pipeon which it is installed to allow the entrapped air to beslowly released.

Several combination air ventfuacuum relief valves areavailable for control of air in systems. Air and vacuumrelease valves are designed to exhaust large volumes of airIiom pipelines during the filling pnocess and to close

positively when water reaches them. These valves operateeither by a buoyant float closing the valve as the waterrises or by the impact of the water against a plate or othervalve closure element. The valve remains closed until thepressure drops below atmospheric pressune, as wouldresult from draining the line. These S,rpes of valves closerapidly and will cause a significant change in velocit5z atclosure, thus care should be used in their sizing andplacement.

Combination valves are manufactured to perform thefunctions of both continuous-acting and ventfuacuum airrelease valves. Upon filling, a large orifice is opened. Oncewater reaches the valve, the large orifice closes and allowsair to escape only thnrugh the smaller orifice that is actuatedby a float mechanism.

Pressure Pipe Design and Selection

The selection of the proper pnessure rating of compo-nents of a piping system is a most important design con-sideration. Tfpically, the designer carefully determines thenormal operating pnessure range of the system, adds anappropriate value for expected surge, and selects com-ponents that are rated at this pressure or higher. Althoughfor many applications this is an adequate approach, con-sideration should also be given to operating temperatureand cyclic surges.

Pressure Rating

The ASTM pressune rating system for PVC pipe is basedon a hydrostatic pressure design basis using a materialstrength of 4,000 psi and a safet5r factor of 2. Therefore, thepressune rating is computed on the basis of a design hoopstress, S : 2,000 psi. The pressune rating can then becomputed by:

P _ 2SI(SDR-1)

Where P : Hydrostatic pressure rating, psiS - Design hoop stress, 2,000 psiSDR : Standard dimension ratio

: Outside pipe diameter/ wall thickness

The above procedure does not apply to PVC fittings forreasons previously discussed. In the absence of betterinformation on fitting pnessure ratings, values from Thble2 can be used as a guide for Schedule 40 and 80 pVC

fittings.

Tbrnperature Consideratione in Deeign

Most irrigation systems operate at water temperaturesbelow 73 F. HoweveA if higher fluid or ambient tempera-tures are expected, thepressure capacigzofthe pVC pipingcomponents should be reduced appropriately. Thble 4listsreduction factors for pressure ratings at elevated temper-atures for various dimension ratios. To compute the pres-sure capaci$z at an elevated temperature for both fittingsand pipe, multiply the standard pressure rating by thefactors shown for the expected temperature.

Tbmperature

TABLE 4. Pressure reduction factors for epecificoperating temperaltures above 73o F.

Thus, the peak internal pressure is:

P : 100 + 64 -- 164 psi.

Furthermore, the expected number of cycles duringthe system's life is:

C' : 60/day x 250 days/year75O,OOO cycles

Thus, the peak hoop stress should

S' : (5.0S X lO21/ZSO,O00)o+zo+

20.7

Reduction Factor*as a % of rated preseure at 73o F

up to 73oF80"F

100'F120'F140'F

7OO7o9OVo62Vo

39Vo22Vo

*Use linear interpolation for intermediate temperatures.

Deeiening for Cyclic Surge Capacity

As noted earlieD cyclic surges are not adequately

addressed by standard design techniques. In cases wherc

total pressure variation due to cyclic surges equals orexceeds SOVo of. the working pnessure, the potential forfatigue failure is significant. H.W Vinson (1) has developedan empirical technique for computing the dimension ratiorequlred to withstand cyclic surges in which the requireddesign hoop stress is computed as follows:

s' : (5.05 x1027/C',)o+204

where S' : required design peak hoop stress, psi

C' : Anticipated number of cycles in the life ofthe system.

Having determined the required design peak hoop stress,the required dimension ratio for the pipe and fittings cannow be calculated by:

DR :25, /P + 7

where S' : required design peak hoop stress, psiP - peak intemal pipe or fitting pnessure

(hydrostatic * surge),psiDR : dimension ratio

Figure 9 graphically pnesents results of the above equa-tions. This Figure may be used to select appropriate pres-sure ratings for PVC piplng systems subject to grclic surges.

To demonstrate the use of the equations, consider thefollowing example:

Given:

PVC pipe-ASTM StandardsGolf course system-anticipated cycles-60 per day

for 250 days peryearSystem life-S0 yearsNormal operating pressure-1OO psiMaximum expected veloci$r change during valve

operation-a.O fos

assume:

Instantaneous valve closure

Required:

Establish proper PVC pipe specification

Analysis:

The surge pressure from Table 3 (assume SDR 21) :

64 psi.

10

and the required dimension ratio is:

DR : 2 x1694/164

Thus, SDR 21 pipe would be required. Fittings of

equal pressur€ rating (200 psi) would also berequired. From Thble 3, fittings smaller than Z-inch

could be Schedule 40, 2-inch through 4-inch fittings

should be Schedule 80. Fittings larger than 4-inch

should be pressure rated fittings capable of 200 psi

working pnessure.

An alternative method for determining the required DR

is to use Figure 9. Entering Figure I with C' : 7.5 x 7Os

and P : 164 psi gives a DR between 27 and 26, thus SDR

21 pipe should be used.It may be noted that the Vinson equations used above

are conseryative. Howeve4 for extreme cyclic surge appli-

cations, such as goHcourses, the added safety factor obtained

by using these equations is justified.

Installation Considerations

One of the characteristics of PVC piping systems that

makes them attractive is the ease of installation, due to

m 30 ,l 5o l0O 2oo 3ooa 0 r w w

P E A K T N T E R N A L P R E S S U R E ( P ) , p n

FIGURE 9. Cyclic fatigue life of PvC pipe when eubjectedto large presonre surg;es.

x 50 years :

not exceed:: 1,694 psi

toi

ou

F

J

\ \ \\ \ \\

I

\T\= +

t! I + tIt L \ _l I-.L

I I- -l- tr-

251 A80VEASE PREE l

\ \ \ l \ \ \ I

ts N .\,J\,I )

their light weight and ease of fabrication. Howeve4 pre-cautions must be taken, as with other materials, in storing,handling, joining, laying, blocking, backfilling, filling andpressure testing.

Ilandling

Because of the light weight of pVC, there may be atemptation to handle it roughly. Care should be taken toavoid dropping pipe or fittings since undetected fracturescould result, causing later problems. Also, physical dam-age to the pipe or fittings in terms of scratches or gougesmay significantly reduce the long term strength of thecomponent, especially under cyclic surge conditions. Spe-cial care should be taken in cold weather to avoid impactdamage due to the increased brittleness of the material atreduced temperatures.

If pipe and fittings are going to be stored for extendedtime periods, they should be shielded from direct sunlightby some opaque covering, allowing air circulation aroundthe components to avoid over-heating. Storage at hightemperatures may cause some distortion of the compo-nents of the piping system and should be avoided.

Solvent Welding

Solvent weld joints require care during assembly. ASTMStandard D2855 should be followed, as well as any man-ufacturer's recommendations. Common problems asso-ciated with solvent cement ioints are:

1. Inadequate primer or poor priming techniques whichfail to provide sufficient glaze breaking and softeningof the joining surfaces.

2. Improper application of cement, resulting in non-uniform coverage, e.g. dry places on either of thejoining surfaces or puddling inside the joint.

3. Allowing the cement to become too drybefore assem-bly, resulting in poor bonding.

4. Incomplete insertion of the spigot into the socket onassembly, reducin$ the solvent weld contact area.

If problems such these occult, inadequate bondingbetr,veen the spigot and socket occul which may lead tofailure as shovrm in Figure 6. Although some force wasrequired to separate the two surfaces, little fusing hadtaken place, indicating insufficient solvent penetration.

Threaded Fittings

With threaded steel piping the onlyproblem pipe fittersneeded to worry about was to make sure the threadedfltting didn't leak. To make sure, the joint was tighteneda little more. With threaded PVC fittings, that techniquecan lead to fitting failure. The biggest single cause of failurein female threaded PVC fittings is over-tightening. This iseasy to do because PVC threads are much smoother thanthe threads in iron pipe fittings and, with today's lubri-cating thread sealants, it is very easy to over-tighten athreaded PVC fitting. In addition, the pVC will deform more

easily, allowing deeper thread makeup with less torque.In some cases, it is even possible to over-tighten the fittingsby hand.

Amiving at a guideline for assembling threaded pVCfitting joints is difficult. The amount of torque required toassemble two fittings will vary with the thread taper andtolerance, the tlpe of sealant used, the temperature, thesize of the fitting, etc. The number of exposed threads iseven less precise as a guide to proper assembly. The methodthat seems to work best is to first tighten the fittings "fingertight" and then add another 1 to 2 turns. Although "fingertight" is qualitative, it is a fair measure of the requiredinitial tightness, and will rarely result in overtightenedfittings. Tightening a fitting more than 2 turns past "fingertight" can exceed the design strength of the fitting. Figure10 shows a plot of the number of turns past "finger tight"and the associated hoop stress generated in the femalethread body. Two turns past finger tight for these Il/z-inchfittings produced 2,000 psi hoop stress which is the stan-dard design stress for PVC pressure pipe.

Much controversy exists over the g,pe of thread sealantto use. Some manufacturers recommend a liquid or pastesealant and some recommend Teflon tape. As long as careis taken to follow manufacturers' recommendations, eithercan be used successfully. Howevel designers and installersshould be aware of a few precautions for each type ofsealant.

With paste or liquid thread sealants/ extreme care mustbe used in selecting a sealant which is suitable for pVCpipe. Standard pipe dope and even some of the Teflonpaste compounds are not compatible with PVC, so becertain that the sealant used has been tested and approvedfor PVC fittings. For fittings that will be subjected to move-ment, such as swing joint risers, the paste or liquid sealantmust be non-setting. The higherviscosigz sealants that donot dry seem to perform better in applications requiringmovement.

o 2 , + 6

Tuh. po.t flng.r Ught

FIGURE 1O. Relationship between hoop stoess in thefemale thread and turns past linger tight inthe makeup of ryp-inch {breaded pVCfittings.

T

e ?r :* 8

Eo

Moxlmum |@mm.ndd tightn@

11

With Teflon tape, application is the primary precaution

because the thickness of the tape varies widely among

manufacturers. With very thin tape, multiple vwaps may

be required to achieve leak-free joints' Howevex' when

appl)4ng multiple wraps, the risk of thick and thin areas

is increased. Such non-uniform application can cause

additional stresses on the fittings' Uneven application can

be avoided by using a tape that is at least as wide as the

width of the threaded anea on the fitting' (Thke care to

keep the tape from hanging over the end of the male

threads as this excess tape can break loose and plugvalves

and screens.) Multiple wraps are made directly on top of

the previous wrap without spiralling, cr€ating a uniforrn

thickness. Sufficient wraps (usually 2 to 5 depending on

the tape thickness) should be used to insure that the

threads do not "gall" or "lock-up" on make-up' A few test

make-ups should be made as a trial. After make-up, the

parts shouldbe disassembled and inspected'The number

bf -tupt should be adjusted until no evidence is seen of

broken tape in the threads.

An additional precaution is necessary when handling

threaded fittings. It is a fair$ common practice to pre-

assemble sprinkler swing joint assemblies and stock-pile

them for laier installation. If this is done, the assemblies

should be kept in a shaded, ventilated storage area until

installation. Excessive heat on the assembled fittings can

cause the PVC to relax and loosen the fitting. leakage may

result, or, if the joints are re-tightened, the fittings could

become over-stressed when cooled.

Thruet Blocking

Water under pnessune exerts thrust forces in piping sys-

tems at: changes in pipe size or direction, dead ends, valves

and hydrants. The size, shape and type of thrust blocking

required depends on the maximum system pressure, pipe

size, appurtenance size, t5pe of fitting, line profile and soil

type.

The design of thrust blocking requires knowledge of the

ttuust generated and the bearing strcngth of the soil against

whichihe thrust block will be placed. Thrusts developed

per 100 psi of line pressure for various pipe sizes and

htting qpes are presented in Thble 5. These values can be

used in conjunction with the soil bearing strength data

presented in Table 6 to calculate thrust block sizes'

It is a fairly common practice to install thrust blocks at

the locations discussed above. HoweveD several pnecau-

tions are necessary to assur€ that the thrust blocks are

adequate. To be effective, a thmst block must: 1) be placed

against undisturbed or fully compacted earth; 2) contact

the fitting over a sufficiently large area so as not to cr€ate

point stresses on the fitting; and 3) have sufficient area on.

the soil side to restrain the thmst without exceeding the

bearing strcngth of the soil. Some typical examples of

thrust block installations are shown in Figure ff '

To illustrate the technique for designing thrust blocks,

consider the following examPle:

12

Given:

A 6-inch PVC 90 elbow operated at 100 psi maximum

pressute is installed in a loam soil which is between

a sand and a soft claY.

Analysis:

From Table 5, the thrust which must be supported

by the soil equals 4,000 lbs.

From Table 6, the bearing strength of the soil is about

750lbs/ft'z.

Therefore, the required thrust block contact area must

be:

contact area : 4,OOo /750 : 5.3 square feet

Tbmperature Expaneion Consideratione

All pipe materials expand and contract with changes in

temperiture and this dimensional change must be con-

sidered in the design and installation of piping systems'

As a general rule, a 10oF change in temperature will cause

WC lipe to expand or contract 3/8 in for every 100 ft of

tengtft. f'o. example, a 1,000-foot pipeline installed in the

,rrrirn", when the ambient temperature is 90"F would

shrink about 20 inches if the soil cooled to 40oF in the

winter. This change in length must be accommodated or

severe damage to the pipe and fittings will result'

There are several methods for dealing with thermal

expansion and contraction. The most common today'

especially for 4-inch or larger diameter pipe, is the use of

gurt "t

ioints. A 3-inch or smaller diameter pipe can be

TA"BLE 5. Thruet developed per loo pei ol line preesurefor various PiPe eizes and fittingconliguratione.

Valvee,Ttses

Dead Ende(lbe. force)

PipeSize

(inch)

Fittinggoo Elbow(lbe. force)

Fitting45o Elbow(lbe. force)

200300

7tA2.

3468

1012

300500

1,0001,8004,0007,2OO

t7,20016,000

1,1002,3OO4,10O6,3009,100

200400800

1,3002p005,1007,SOO

11,300

TA"BLE 6. Estimated bearing sfrength of gpical eoils

Bearing StrengflhSoil TVPe Lbs/ftP

Much, Feat, etc.Soft ClaySilt loamSandSand and gravelSand and Gravel with ClaYSand and Gravel Cemented with ClaY

Hard Pan

0500750

1,0001,5002,OOO4,0005,000

l f th rus ts , due to h igh pressure , a re expec ted , anchoa va lves asbe iow. A t ver t i ca l bends , anchor to res is t ou tward th rus ts .

Thru line cotlnectioil, tceThru line cotvectiott, crost

Diection chdkge, elbowChoilge line size, re.luccrDiectioil change, tee usad

Direction cheilge, crott usul

Dircction changeThru line connlctioil, \|.r(

Dirccion chdfige vettical,

FIGURE f r. Tfilical thruet block inetallatione.

snaked in the trench to accommodate the thermal expan-sion. For larger diameter pipe with solvent weld ioints,periodic expansion joints, offsets or expansion loops mustbe used to accommodate the length change.

System Maintenance Requirements

MaintainingAir Relief and Eurge Conhol EquipmentAutomatic air relief and surge control equipment are

on$ effective in limiting the magnitude of pressune surgeswhen it is operating properly. In practice it is not uncom-mon to find an irrigation system with a number of the airvents closed or inoperable. Air vents, pressune relief valves,pnessune regulation valves and surge tanks should beinspected and serviced at least annually to assure thatthey are operating properly.The small orifices on contin-uous-acting air relief valves can become plugged and mayneed foequent periodic cleaning. As with any mechanicaldevice, periodic maintenance is necessary to maintainreliable operation.

hecautione in Winterizing the System

Systems that are not installed below the frost line mustbe drained during the winter. The most effective methodof draining the system is to install the pipelines on gradewith drain valves at the low points. Howeve4 with manygolf course systems, the common method of removing thewater fiom the lines is with high pressurp-high volurng

air compressors. The water is literally blolrrn from thepipes. This method is used since it is far less expensivethan laying the many thousands of feet of pipe to gradeand installing drains.

There are some inherent dangers in using compressedair to "blow out" pipelines. Remembering the discussionof air-induced pressure surges in pipe lines, the risk ofhigh surge pressunes is great if the water and air becomemixed, or air pockets form within areas that are not totallydrained. The velocities created can be very high and thesurge potential is equally high

If compressed air must be used to evacuate pipelines,considerable care should be exercised. A high volumecompr€ssor shonld be used, but the output pnessune shouldbe limited to less than 507o of the system operating pres-sure. If a sufficient volume of air can be developed at lowerpressure/ so much the better. Valves should first be openedin the low points and at distal ends of lines to drain thalargerdiameterpipes and remove the majority ofthewater.Pressure should be limited to about 25 to 307o of the normaloperating pressure during this phase. Once the major linesar€ evacuated, the close-invalves shouldbe closedto allowevacuation of more distant segments. It will probably taketwo to three passes through the system, working fromupstream to downstream to completely evacuate the waterfrom all lines. A pressune gauge should be installed on thepipeline near the compnessor and monitored continu-ously during the operation. pressure should be built upslowly to allow the water columns to begin moving grad-ually, avoiding any sudden pnessure surges. If the linepnessune gauge fluctuates dramatically, the air pnessureshould be reduced to lower the risk of pipe damage.

SummaryPVC piping offers many advantages over other g,pes of

piping, especially to the irrigation industry. If the systemsare carefully designed, installed and maintained, the pip-ing will give years of satisfactory service. Howeveq, inade-quate consideration of potential hydraulic situations, faulgzinstallation, or improper operation can lead to significantproblems, if not immediately, then at some time in thefuture. The cost of system failure is too great to ignorethese potential problems, especially since moderateadjustrhents in system design can eliminate many of theseproblems. We hope that using the guidelines presentedherein will aid designers and operators in meeting therigorous demands placed on them. We also hope thatadditional per:formance testing can be accomplished onSchedule 40 and 80 fittings and that, ultimately, all pVCfittings can be pressure rated to eliminate the confusionthat now exists in regard to their appropriate applicationt.

L2.

3.4.J.

6.

7.8.9.

10.

IL-

13

Bibliography'1. Vinson, H. W., "Response of PVC Pipe to Large, Repetitive

Pressure Surges," Proceedings of the International Con-

ference on Underground Plastic Pipe, American Sociegr of

Civil Engineers, New York, N.Y. (March 1981)

Selected Referencesf;or Flrr{her Reading

Handbook of PVC Pipe, Design and Construction

Uni-Bell Plastic Pipe Association2655 Villa Creek Drive, Suite 150

Dallas, Texas 75234

Pipeline Design for Water and Wastewater

American Society of Civil Engineers

345 East 47th StreetNew York, New York 10017

Plastics Piping ManualPlastics Pipe InstituteSocietSr of the Plastics Industry355 t€xington Ave.New York, New York 10017

1,4