The perplexity of PVC-fitting failures
Many golf-course superintendents resign themselves to repairing broken PVC-pipe fittings. Some have humorously suggested that the task provides built-in job security. But the problem is really no laughing matter. The damage and cost of repair are generally proportional to the size of the fitting. So while the repair of smaller fittings is usually only a nuisance, the failure of larger fittings can be quite disruptive. Lack of systematic reporting, especially after the typical 1-year warranty ends, has masked the magnitude of the problem.
PVC's little-known susceptibilities The superior economics of PVC pipe and fittings have made irrigation systems available where they previously would not have been feasible. Steel pipe-the predecessor to PVC-was limited by its expense and susceptibility to corrosion. PVC pipe solved these problems, besides being easier to install.
Although PVC pipe has been quite reliable, PVC fittings can be more sensitive to the circumstances of use and installation. For instance, modern golf-course irrigation systems operate at relatively high pressures and have large and frequent surge pressures (water hammer). Although PVC pipe is well-suited to accommodate water hammer, PVC fittings have not always been up to the task.
Selecting an effective fitting repair requires an understanding of the cause of failure. This is not always straightforward with PVC fittings. Simply replacing a failed PVC fitting without addressing the underlying cause may only result in the eventual need for another repair. The first step in diagnosing the problem, then, is to understand as much as possible about the failure and any contributing factors. Doing so requires a systematic approach that accounts for all variables, as well as knowledge of the behavior of the systems in question.
First, plastic pipe fittings do not bear loads in the same way that metal fittings do. When a metal fitting experiences a load, it either successfully bears the load or it fails. A plastic fitting is different. It may successfully bear the load at the moment of initial application but, over time, it will fail as cracks initiate and slowly grow. A failure, then, can take years to happen. This is why PVC burst-pressure ratings and working-pressure ratings are so different. It is this time-delay effect that makes diagnosing plastic-fitting failures difficult.
In addition, the geometry of a pipe fitting causes concentrations of high stress in the crotch of Ts and bends that are not present in straight pipe. Being a plastic, PVC is insussceptible to the fatigueloading of water hammer in these areas of high stress. Injection-molded fittings also are typically made with plastic compounds that have somewhat lower molecular weight than that used for the pipe itself. As a result of these geometric and material characteristics, PVC fittings have inherently lower fatigue strength than PVC pipe-even though they are capable of a similar burst strength. PVC fittings also are unforgiving of mechanical loads for the same reasons. The high-stress yielding that occurs in the crotch serves as a site for a crack to start that can eventually propagate to failure under sustained loading. This is unlike metals, which generally have much higher fatigue strength and the ability to yield locally and redistribute stress concentrations in crotches in a beneficial way.
Despite the time-delay effect, the timing of a failure can yield clues to its cause. Failures generally fall into three categories. The first category is initial startup. The second category is spring startup. The third-and perhaps most aggravating-category is failures during routine operation. Failures during routine operation can be much more difficult to diagnose because the causes may lie in installation errors or operational abuses made many years before.
Diagnosing failures upon start up Failures during start up generally result from installation errors or manufacturing defects. Less often, they are from design errors if performed by a trained irrigation designer. Design errors usually manifest themselves in the longer term. Take care about being overly price conscious during the design, contracting and construction phases of your irrigation installation. Doing so can result in the temptation to cut corners.
Failures at startup are caused by forces that exceed the short-term strength of the fitting. Although manufacturing defects can occur-as with any manufactured product-it pays to look beyond that all-too-easy answer. Unfortunately, the failures caused by poor installation practices often do not happen until much later, when the cause is less obvious.
One of the most common "installation errors" that cause failures is the presence of entrapped air during initial startup. Entrapped air can result in pressure surges several times greater than a PVC fitting's capabilities. This pressure surge results in broken fittings or thrust-block failures. In the case of solvent-weld fittings, pressure surges can cause fractures to propagate into adjoining pipe. Therefore, it is essential to remove air from the system in a way that doesn't cause surges. Even if surges do not cause an immediate break, they will significantly reduce the fitting's life.
Failure detection during spring start up
Spring is a time when entrapped air is again likely to be a problem. This problem relates to the fact that accumulated fatigue damage due to water hammer-as mentioned previously-effectively reduces the burst strength of the fitting. Thus, fittings that functioned fine when you drained the system suddenly break in the spring. To avoid this "lesson," you should-with a discipline approaching a religion-perform air-venting procedures every spring before start-up.
Glued-joint failures are another common cause of breaks during spring start up. Although solvent welding is not an ideal joining method for buried piping, many still use it for pipe sizes smaller than 3 inches in diameter when short-term cost considerations dominate the contracting process. Solvent-weld joints do not permit the pipe to breathe when it experiences temperature changes, ground motion or settling. As a result, the stress concentrations in the fitting's body, or in the glue joint itself, can result in a premature failure when added to the pressure forces. In an effort to reduce some of these stresses, most solvent-weld fitting manufacturers recommend you use poured-in-place concrete thrust blocks, just as you would use for gasketed-joint fittings.
Threaded PVC joints are another common source of failure and, like solvent-weld joints, are notoriously susceptible to installation errors. Threaded joints are easily over-torqued. You are lucky if you over-torque the joint enough to cause an immediate failure. More likely, your fitting will fail later, after you've pressurized the system and after high stress eventually causes a crack to initiate and propagate. Under no circumstances, then, should you subject threaded-PVC joints to loads caused by deflecting the adjacent pipe or fitting. Like solvent-weld joints, make threaded joints in strict accordance with the manufacturer's instructions. Female plastic threads on male metal threads are the most vulnerable to over-torqueing, so avoid them if possible.
Inadequate thrust-blocking also can lead to premature failure. During start up, thrust blocks that are too small for the soil's bearing strength will move. If this movement is large enough, the pipe can blow off the gasketed fittings. If the thrust-block displacement is less, mechanical loads from pipe deflection in the fitting's bells or sockets will result. These loads can result in immediate failure or can occur later as the resulting cracks develop and propagate more slowly. Splits, which appear to originate in the end of the fitting's bells or sockets, often characterize these failures. As mentioned previously, you should thrust-block all buried PVC fittings-whether gasketed or solvent-welded. Often, installers don't block smaller-diameter solvent-weld fittings. Doing so is not a conservative practice. Always properly size thrust blocks and use poured-in-place concrete, fully embracing the fitting and extending to undisturbed soil. Do not use cinder blocks, bricks, rocks or boards as thrust blocks. Doing so always will result in the application of mechanical loads that will reduce the life of the fitting.
Another cause of premature fitting failure is deflecting the pipe in the fitting bell or socket during installation to try and make a turn. While this is an acceptable practice with ductile-iron fittings, you should never do so with PVC fittings. Failure may not necessarily occur on startup. (Again, it depends on the magnitude of the deflection-induced stresses.) Nevertheless, once the failure occurs, it usually looks like one that results from a displaced thrust block, as described previously. Pipe manufacturers do typically allow some deflection of their pipe to allow you to make turns. But you should never make these turns in any way that forces mechanical loads on PVC fittings. For example, using a narrow trench often can lead to such loads. At a bend, the intersecting narrow trenches are not wide enough to allow the fitting to precisely sit in the turn. Even so, installers force the pipe and fitting into the trench. The trench walls then serve as a form to force deflection of the pipe into the fitting, thus imposing the offending loads.
The most difficult failures to spot: During routine operation Elevated temperatures can cause fitting failures during routine operation. Fittings can be exposed to elevated temperatures either from the water they carry or from the surrounding soil when burial is shallow in hot locales. This can lead to reductions in burst strength, long-term pressure strength and fatigue strength. The damage done during limited high-temperature operation may not lead to an immediate failure. It, instead, may lead to a premature failure at some distant time as the fitting experiences more service. This makes it difficult to determine the true cause, as well as to implement the proper repair. Failures from this cause don't leave obvious evidence.
Though unlikely, it is conceivable that fittings can be exposed to temperatures as high as 80 degrees F to 90 degrees F. In regular PVC pipe, this would cause a derating of 12 to 25 percent from a 73 degrees F rating. No published data is available that quantifies the effect of this on fitting strength, although most authorities presume a fitting will behave similarly to a pipe. Given the stress-concentration effects in the crotches of bends and Ts, it is possible, however, that the derating might be somewhat higher for fittings. In fact, the effect on fatigue strength is probably more exaggerated. In practice, then, you can assume the effective derating is much larger. This is because the system usually operates at cooler temperatures, in the range of 45 degrees F to 55 degrees F, where components have higher-than-rated strength. In solvent-weld piping, thermal expansion increases the general state of stress in pipe and fittings, which would be relieved in a gasketed system.
Knowledge of the temperatures to which your system is exposed, then, is useful information. If you don't account for elevated temperatures in the design phase, then your only means of offsetting high-temperature effects is to operate your irrigation system in a more benign way during such periods. Of course, this is precisely when you are likely to be tempted to operate your system more aggressively.
In this situation, repairing a failure using the same fitting probably will lead to another failure if you make no changes in operation. The obvious solution, of course, is to change to a more temperature-resistant material, such as ductile iron.
A special word needs to be said here on water hammer. The benefits of automatic controllers and valve-in-head sprinklers come with the price of water hammer. While water hammer does not adversely affect most system components, it can destroy PVC fittings over time. Diagnosing a fatigue failure from water hammer can be quite simple. Break open the fitting so that you reveal the fracture surface. If you see concentric tree-ring-like structures, then the failure was caused by water-hammer-induced fatigue. However, the absence of such a feature does not necessarily mean the failure was not from fatigue.
If it is a fatigue failure, you can be sure that repairing with another PVC fitting will lead to another failure. If the materials are available, it is best to repair such a failure with a ductile-iron fitting.
PVC-fitting failures are expensive and frustrating. Unfortunately, many installers plant the seeds of long-term failures during installation, and hydrotesting does not reveal these faults. Even a manufacturer's typical 1-year warranty-generally adequate to detect manufacturing defects-can be woefully inadequate in protecting you from design or installation errors. The keys to maximum performance of your irrigation system, then, is to operate the system within the prescribed parameters, monitor failure patterns to aid in diagnosis and take the time to understand the root cause of failures so you can make a repair for the last time. Doing so will enhance the performance and reputation of your organization and your course.
John Riordan is product manager with the Harrington Corp. (Lynchburg, Va.).
Want to use this article? Click here for options!
© 2015 Penton Media Inc.