Clamped type piping construction is today employed in a variety of applications, and growing in use. Once seen primarily at fire sprinkler service and temporary applications, grooved clamped construction is now commonly found in condenser water piping from large 24 in. diameter main risers to smaller 2 in. distribution lines. Clamped pipe is commonly installed at chill and secondary water systems, as well as for domestic water applications. In the many examples we have seen, we have found grooved clamped pipe to provide excellent service.

     Similar for any piping system, the ultimate service life is often determined by its design, operating conditions, chemical treatment protection, pipe schedule, materials used, and quality of construction. Long service life is typically available by following established maintenance guidelines, along with certain preventative measures - such as water filtration and close attention to the chemical treatment. Grooved clamped pipe will often provide service reliability equal to welded pipe, but in those examples where a moderate to severe corrosion rate is allowed to exist, will usually produce advanced failure due to the inherent weakness and vulnerability of the clamped type joint.

     The groove clamped pipe system, most commonly known by the manufacturer names Victaulic, Gruvlok, or Grinnell, offer faster installation, lower installation costs, the option of using lighter materials, tremendous flexibility, among other benefits. While in many respects viewed as equal to welded black iron pipe, there are a few critical considerations to remember. The premature failure of groove clamped piping systems can occur, and has been documented to do so, if certain precautions are not followed.

     The most important consideration for a groove clamped installation, from our perspective, is the method of producing the groove itself. The groove can be rolled or swagged into the pipe to leave a raised ridge on the interior surface, or it can be cut or machined from the outer wall.

     The below close-up photographs of a typical groove illustrates its depth into the outer pipe wall. Where cut, this represents an unnecessary and potentially dangerous loss of wall thickness material, and therefore the weakest point along that entire pipe length.

     If the groove is cold rolled or swagged, no actual wall loss at the pipe occurs, and the displaced pipe wall material still exists in the form of a raised ridge at the pipe's interior. For purposes of calculating minimum acceptable wall thickness value then, a swagged or rolled groove pipe equals that of welded pipe, and no additional wall loss is factored.

     However, if cut or milled from the pipe wall, significant thickness loss results. For 4 in. diameter piping, 0.098 in. of pipe is lost when the groove is cut; for 8 in. pipe, the loss is 0.107 in. With a specified groove depth of 0.124 in. for most larger diameter pipes, this wall loss can amount to 33% of the pipe itself, but as much as 55% of its allowable pipe loss to reaching minimum acceptable thickness limit. View a more detailed comparison of various wall thickness values for carbon steel pipe.

     From its outward appearance alone, it is almost impossible to identify the method of groove formation, although the exterior side wall of a rolled groove will sometimes appear with a slight radius. Confirming the groove construction usually requires disassembly of one or more connections or borescope inspection for the presence or absence of the internal ridge.

     In many cases, the presence of a groove is concealed by the internal deposits existing at the surface. This raises concern, from a corrosion perspective, for the opportunity of deposits and microbiological material to settle in the small gap between sections and create an accelerated corrosion condition in that area. Such threat at the gap generally does not exist for welded pipe.

     The below photos offer commonly found examples of grooved condenser water pipe having provided service over many years. The below top right photograph shows the absence of a roll groove protrusion where the iron oxide deposits have been scraped away. In this case, the loss of 0.124 in. of material cut out a 12 in. pipe having a 0.375 in. wall, combined with a 15 MPY corrosion rate over 12 years, has reduced the remaining service life of this piping system to zero. At both bottom photos, interior deposits had built up sufficiently to conceal whether a roll or cut groove existed.

     An extreme example of this threat is shown at the top left photo, where corrosion completely penetrated through to approximately 50% of the groove itself - producing a major pipe leak. Additional photos showing partial and total pipe separation at the cut groove are offered below.

     Typically, grooves are cold rolled or swagged into pipe in the overwhelming number of applications. However, cut grooving is always possible - especially where consideration of this important factor is not made, or where close attention to construction procedures has not been followed. For older properties, no construction detail may exist at all defining the method of producing the groove, and therefore a very potential though unknown threat may exist.

     The recommendation of CVI is to always establish the method of forming the groove for piping systems which already have a recognized corrosion problem, or which are know to exist in a weakened and threatened condition. In such examples, internal corrosion may not be sufficient to threaten the majority of the piping length, but sufficient to corrode through the opposing side of the cut groove to the point of failure.

     The above examples, taken from a prior testing investigation, well illustrate the threat existing where a high corrosion rate is present in combination with a cut groove. In these cases, a high but even degree of corrosion reduced the wall thickness uniformly to the point where partial or total separation of the pipe joint occurred.

     Often used as an alternative to threading small diameter pipe and the losses which occur, cutting a groove into pipe can produce equal, if not worse result. Similarly, where extra heavy or schedule 80 pipe is used rather than schedule 40, the heavier pipe wall often requires cutting the groove rather than rolling or swagging.

     In fact, cutting a 0.124 in. groove into extra strong 12 in. pipe stock of 0.500 in. wall can actually result in pipe which has a remaining thickness value at the groove which is equal to lighter standard pipe of 0.375 in. and having a rolled groove. In such cases, the added expense, labor and anticipated benefit of using extra heavy pipe is negated immediately by employing cut groove installation.

     Generally, a look inside the pipe is necessary in order to determine whether the groove has been cut or rolled. A protruding ridge on the pipe interior shows that the groove has been rolled. Conversely, the absence of any ridge shows that it has been cut. While many pipe construction specifications will define the method of grooving, some do not - thereby leaving it up to the piping contractor.

     From discussions with various steam fitters and piping specialists, we have been advised that cutting the pipe is an easier and cheaper installation method - especially for larger diameter stock. This may possibly explain why some of our clients have been surprised to find the groove cut when the piping specification clearly called for a rolled groove. Such information is typically revealed only after one or more leaks or piping failures have occurred.

     Another potential threat involving the use of cut grooved piping has to do with it being unevenly cut. We have identified some installations where the groove was cut eccentrically into the pipe - with the 0.124 in. specified depth of the groove actually measured at 0.050 in. on one side, and 0.200 in. at its opposite wall. This not only removes an extreme amount of pipe material in one area, but produces a weakened joint due to inadequate and uneven clamping.

     Such a condition can only be attributed to poor workmanship by the contractor, and is all but impossible to detect once the pipe is in place. Careful attention to all all aspects of any piping installation is always justified. Any opportunity to closely inspect groove clamped pipe prior to assembly is always advised.

     A further threat exists at the small gap between pipe sections and between common fixtures such as elbows, tees, strainers, etc. Here, particulates and especially microbiological growths often accumulate to produce a very localized but accelerated corrosion condition. Corrosion rates of 50 mils per year and greater are known to occur. This gap at the joints, in addition to allowing foreign deposits to collect, also shields them from chemical treatment products, biocides, and cleaning agents.

     Sufficient depth of those deposits then favors the development of MIC, or microbiologically influenced corrosion against the pipe as well as the clamping gasket. Corrosion then has the opportunity to attack the pipe from both interior and ends, and upon reaching the rubber gasket, will produce a leak condition. See Technical Bulletin # C-5 for more about the threat of MIC.

     Since this end gap is concealed by the clamp, its wall loss cannot be measured, and therefore always exists as an unknown for any form of nondestructive testing. A high wall loss measured elsewhere at the same piping by ultrasound, or biological testing which has confirmed the presence of elevated cell cultures, should always be viewed in terms of the potential for high wall losses in this critical area.

     This gap presents further difficulty for any pipe cleaning operation. Chemical cleaning agents will certainly have difficulty reaching the innermost depth of the gap to either sterilize the MIC or remove deposits. Water jet cleaning, while extremely effective at removing deposits at the pipe wall itself, may not be able to reach within the small end gap, nor may it be advised.

     The above photographs, taken from the failure point of a grooved clamped section of 5 in. ASTM A53 pipe, well illustrate the aggressive action possible due to deposits at the end gap between sections.

     Clamped pipe construction places far more emphasis on maintaining a top quality water treatment program. For open water condenser systems especially, this means maintaining excellent chemical inhibitor control and closely monitoring for corrosion rate and microbiological content. See Technical Bulletin C-2 for recommendations on maintaining good corrosion control.

     Filtration is critically important in order to lower the particulate content and therefore minimize the potential for under deposit corrosion or MIC to establish itself between the end gaps of the piping system. See Technical Bulletin W-4 regarding various types of filters available.

     Unlike welded pipe, crevices exist at each clamped joint where dirt and bacteria can accumulate. A piping system found with pitting activity in any area can be expected to show even greater threat at the end gap of each grooved clamp pipe section.

    While the chemical resistance of the rubber gasket is high, the use of oxidizing biocides and cleaning agents always introduces the potential to cause deterioration and cracking over extended time. A review of proposed chemical agents with the clamping system manufacturer, therefore, is always highly recommend prior to any aggressive cleaning program.

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