1. PVC Piping Systems. Helpful Tips for Avoiding Problems. Published by the. Plastic Pipe and Fittings Association. Authored by. Larry Workman. 800 Roosevelt
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© PPFA 2011 All Rights Reserved 2 PVC Piping Systems Helpful Tips for A voiding Problems Preface The information in this book is the culmination and cooperation of manufacturer representatives who unselfishly work to improve their products for greater user experience with PVC piping. As the author, I was able to combine inf ormation the PVC pipe and fitting industry has assembled, gained and developed over the past decades. Many of the topics included have been shared, sometimes preached, by those who have preceded this document. Some topics may be new enough that this may be first time you have read about them. I wish to express my thanks to the Plastic Pipe and Fittings Association, Terry McPherson, Jack Roach, IPS Corp., and LASCO Fittings for their assistance and indulgence during the development of this book. Larr y Workman www.Expert4PVC .com Trabuco Canyon, CA
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© PPFA 2011 All Rights Reserved 3 Table of Contents Preface .2 Failures .4 Short Term (Burst) Failure 5 Air 5 Long Term Failure 6 Surge (Water Hammer) ..6 Cyclic Fatigue .8 Freeze ..8 Mechanical 9 Split Female Threaded parts 10 Solvent Welding Failures ..11 Materials .12 Prevention and Cures .13 Cyclic Failures .13 Controlling Cyclic Surges ..16 Freeze 17 Entrapped Air 17 Bending and Mechanical Loads ..18 Threads .19 External Threads .20 Inter nal Threads ..21 Solvent Welding ..24 Exposure to Sunlight & Ultraviolet Radiation .28 Thrust Blocking ..28 System Repairs .31 Transition Fittings and Adapters .32 Reference .35 Index ..36
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© PPFA 2011 All Rights Reserved 4 Failures When things go wrong it is important to fix the failure, but it is even more import ant to find the cause. The cause most times is not simply a defective part, but more often the effect of another problem. Good design practice is to incorporate piping and fittings with a reasonable service factor for system reliability. As an example, pipe with a pressure rating of 200 psi, should not be used in a system with a 175 psi working pressure . System pressure needs to include both static and expected surge pressure. When systems have repeated failures it may take some extensive investigative analysis of the entire installation to find the cause. Many times the closeness of the failures to each other could show the existence of pressu re surges, or water hammer . It could be the consequence of the system installation, revision or repair at a different time than the rest of the site. In above ground piping, exposure to the ambient conditions could encroach on the life or service factor of the system. The recent changes to a system, such as expansion or revision, have many times had a direct connection with system failures. Experience has shown that the increased flow rates, ent rapped air from system restart have been the culprit in more than a few installations. These failures like those mentioned above tend to be clustered, but not necessarily in the new part of the system. Often the older piping suffers the damage because of these revisions. A comprehensive check of the system operational modes, including cycle time, pressure surge frequency and amplitude can provide valuable information in finding the cause of piping failures. The use of most pressure recorders although helpful, cannot provide precise data on the damaging effects of pressure surges, air slugs or water hammer . In these cases the unusually high pressure spike the system experiences is of such short duration, milliseconds, that the recording mechanism cannot react quickly enough to record its peak. The pressure pea k travels through a PVC system at roughly 1400 ft/sec. so the recorder only sees it for 0.7 milliseconds. Before and after the pressure spike , the system pressure would be close to normal and only a small fib lipfl would have been recorded, with the amplitude many times removed from the true value. A simple but often overlooked piece of evidence in tracking the source of a failure is to go over all the failed components to look for similarities. On more than o ne occasion a manufacturer has been accused of a defective product only to have a group of components of differing brands or configurations on the same system failing. The common denominator would likely be the system and not the manufacturer. Any failur e that is the result of external stress or loads will reoccur if the stress is not removed. If you need to install extra supports or restraints to remove the external stress, make sure you do it before making repairs . With external failur es, a crack or break will progress from the outside of the pipe or fitting toward the interior.
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© PPFA 2011 All Rights Reserved 5 A simple method can help you decide if a failure is external. Try to open the crack or make it wider. This will show where the external loads were applied. It is also important to know that cracks and splits are perpendicular to the producing load. For example when paper is torn, pulling horizontally causes it to tear vertically. Each has a set of characte ristics that can be used to help find the source of the problem. Causes of failures in PVC systems: Short Term (Burst) Hydrostatic (Liquids) Pneumatic (Air/Gas) Long Term Failure Surge (Water Hammer) Air Slugs/Entrapped Air Cyclic (Fatigue) Freeze Mechan ical Bending Flange Installation Over tightened threads Solvent Welding Short Term (Burst) Failure The failure of a pipe or fitting from exceedingly high pressure over a short period, usually defi ned as less than a minute, would be classified as a burst or short term failure. The more common evidence for these failures is sharp edged cracks and fragments, similar to glass. If these fragments are not contained or entrapped during th e failure they can be dangerous. This is the foremost reason that PVC piping and fittings are NOT to be used to transport or to be tested with compressed air .i Figure 1: Typical burst or short term failur e A short term or brittle failure shows no visible, to the naked eye, material deformation, stretching, elongation or necking down close to the break .Air ii Except under very special circumstances , PVC pipi ng is not to be tested or used with compressed air or gases. iii The catastrophic failure of a PVC air assembly, with its sharp shrapnel pieces is dangerous, and can be deadly. Water and most liquids are not compressible, but air and g ases
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© PPFA 2011 All Rights Reserved 6 are. The potential energy stored in a compressed air piping system at about 100 -psi has propelled sharp edged fragments hundreds of feet in all directions. Because a compressed air failure is almost instantaneous, the failure s urfaces are reminiscent of brittle and freeze type failures. Cracks or breaks are somewhat straight with tributaries that merge and create sharp edges. Long Term Failure A fitting or pipe that has failed from exposure to high pressure over a long time will not generally shatter. The failure will show evidence of stretching or deformation especially at the extremes of any crack or split. Generally, a magnified examinati on of the failure will show material that has tapered off -shoots, whitened surface or fistretch marksfl (a necked down cross section) past the end of the crack. (See figure 2.) Long term failures are most commonly found in the high stressed areas of a fitt ing, such as the inside corner or ficrotch fl of an Elbow or Tee. A long term failure shows material deformation, stretching, elongation or necking down, along the edges of the crack . Surge (Water Hammer) iv There are many similarities between surge pressure and short term failures. In most instances the part breaks down after repeated exposures to high and short duration pressure applications. The repetitive surges tend to significantly degrade directional fittings considerably more than pipe or couplings. The failures tend to appear first in the high stress areas or crotch of Tees and Elbows. A simple vector an alysis shown in Fig. 3 suggests that the stress in the crotch of an elbow (or tee) is 1.4 times that found in the body of the fitting. This simple vector analysis is conservative and does not consider the extra stress of outward bulging that is clear in 3 D vector analysis, Finite Element Analysis (FEA) and laboratory testing. Some industry testing has shown that the Figure 2: Classic long term failure
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© PPFA 2011 All Rights Reserved 8 instantaneously as it seeks unsuccessfully to move thru the same orifice. This rapid change in fluid velocity creates a large pressure surge or water hammer condition. Cyclic Fatigue Similar to surge, cyclic failures are a result of material fatigue from a high number of repeated pressure cycles . Each time a PVC fitting is pressurized or the system pressure increases the high stressed areas stretch slightly. This causes tensile stress , or stretch, in the crotch of tees and elbows. Piping and fittings are designed to withstand pressure fluctuations if they are limited, both in strength and quantity. But, high amplitude and/or quantity can cause the pipe o r fittings to fail prematurely. The fistretch marksfl found along the edge of the break along with fi clamshell marksfl or fibeach marksfl on the internal fracture surface are evidence of cyclic failure propagation. v Figure 6: Stretch mark at end of a crack. Freeze Water expands when it freezes. PVC, as do most materials, becomes brittle as the temperature is lowered. So, when PVC fails from entrapped frozen water; the break surfaces and cracks are brittle in appearance and give an appearance much like shattered glass. Figure 5: Beach or clamshell marks.
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© PPFA 2011 All Rights Reserved 9 Figure 7: Breaks due to freezing water Mechanical There are two common causes of mechanical breakage in PVC piping systems. The bending , or vibration, and the over tightened threaded connection. The bending or vibration failure starts on the outer surface of the part, and progresses inward. This distinguishes it from the other forms. In the hands of a plastics failure expert, the fracture surface will have clues not only of the origin site of the failure but will also tell the expert the fihowfl or fiwhatfl caused the failure. Many times it is possible t o find the orientation of external load by carefully fitting the pieces together and observing the fit and distortion of the failure area. This should not be attempted by a novice, as surface features unique to the failure may be lost due to the relativel y soft surfaces coming together and smearing the fracture surfaces. Figure 8: Break due to mechanical load.
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© PPFA 2011 All Rights Reserved 10 Because many of the mechanical failures are caused by vibration, the crack and fracture surface is similar to that of a cyclic fatigue, except its orientation. Cyclic fatigue with a hyd raulic source will start on the interior of the pipe or part. The mechanical failure will start on an exterior surface. Male pipe threads, such as found in the male adapter , are the most common examples of a vibration fatigue failure. Because pipe threads are tapered, the material is the thinnest under the male threaded section at the first thread past the female threads. The trimmed down wall thickness within the thread segment of the PVC male threaded part become s the focus of all bending loads induced from any system vibration or misalignment . Split Female Threaded Parts When tapered threaded parts are assembled and the PVC female threa ded part splits, the most likely cause is from over -tightening. The failure of the female part with a crack that is parallel to the axis of the fitting identifies the cause as over tightening. Many times the crack has progressed through the pipe and other parts, but its origin was in the threads. A crack or split is always oriented perpendicular to the causing load. In these failures the male threads induced stress (stretch) to the female threads. Most materials, including PVC, are more tolerant to compressive, than tensile loads, leading to a split and a leaking joint. Figure 9: Vibration failure of male threads. Figure 10: Over tightened connection caused the female adapter to split.
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© PPFA 2011 All Rights Reserved 11 Solvent Welding Failures The solvent weld joint creates a chemical welding of the components resulting with a leak free connecti on. The use of correct equipment, procedure and chemicals are mandatory! Solvent weld failures fall into three general categories. The dry joint , is the result of solvent cement that has partly dried, either in the can or during the installation. The lack of sufficient solvent to create the fusion bond will have the consequence of a weak and/or leaking joint. If a solvent joint is disturbed during the cure period then the fusion bond is broken and a leak is imminent. The cure time depends on the temperature, piping size, humidity and solvent cements used. It is important to follow the manufacturer™s recommendations about the time necessary to allow the joint to set long enough for handling. Figure 11: Insufficient cement resulted with a leak path. Figure 12: Assembly was moved during cure time.
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