CPVC Pipe Failure Investigation
Author: Daniel Robles, PE, Coengineers, PLLC
Co-author: David Coles PE, Coles Consultants, LLC
Community Engineering Services, PLLC (d.b.a. Coengineers, PLLC) was retained to perform an investigation on the Hydronic System for a dormitory residential facility. The CPVC system was installed in 2006 as new construction, and experienced repetitive failures throughout its relatively short service life. The objective of the investigation was to determine how and why such a recent installation could become problematic and, if possible, determine root cause for systemic failures.
At the time of this CPVC Pipe Failure Investigation, the facility was in renovation and the problematic hydronic system was roughly 90% removed. A site visit was conducted on June 11, 2014. An investigation was performed on some retained components and samples taken from the removed hydronic components. A review of existing as-built drawings was performed. A known failure record was assembled from maintenance records. A sample was taken of residual water contained in the remaining CPVC system and tested for presence of hydrocarbons.
Daniel R. Robles, PE, Coengineers, PLLC performed the detailed analysis on the site samples and maintenance records. David Coles, PE, Coles Consultants, LLC assisted with the overview of system operations and best installation practices. Dr. Duane Priddy, CEO Plastic Failure Labs, was retained to validate our findings and identifying the observed CPVC failure modes.
CPVC is a reliable, robust, and inexpensive plumbing material that has gained wide acceptance in the construction industry. Like all piping materials, the most common mode of failure is improper design and poor craftsmanship. This CPVC Pipe Failure Investigation demonstrates these shortcomings.
The most common failure modes for CPVC are associated with improper installation, contamination, and incompatible chemicals. Specific examples of known failure modes found in this installation include; excessive cement, insufficient cement, inappropriate clamping, Brass threads over CPVC threads, no allowance for thermal expansion.
These deficiencies, and others, likely caused the mechanical stresses in the system, which made the CPVC increasingly vulnerable to the introduction of an incompatible chemical. This CPVC Pipe Failure Investigation demonstrates also demonstrates possible hydrocarbon degradation.
It appears more likely than not, that this system experienced two system-wide deficiencies. The first may be described as the general disregard for manufacturer installation recommendations. The second observation strongly suggests that the system was contaminated at some time in its past with a chemical or agent known to degrade CPVC.
When CPVC is mechanically stressed, it will increase absorption of hydrocarbons (contaminants) in an effort to relieve that stress. Several samples demonstrated classic “textbook” Environmental Stress Cracking (ESC) patterns and discoloration zones. Contamination may have come from one or several sources ranging from the introduction of incompatible rust inhibitor to leaking compressor lubricant to residual cutting oil on brass fittings.
Taken together, the system appears to have become fragile in its entirety, as repair records further suggest. The remainder of this report will identify multiple violations of manufacturer installation recommendations, classic failure patterns in a small sample, and additional conditions that could impact the integrity of a CPVC system.
The only failure records on hand covered a relatively short period between 2011 and the present. Little is known about the failures prior to this time. Additional records, if submitted, may be instrumental in determining if or when any system contamination occurred.
The following chart shows a box rendering of the building elevation identified by suite numbers and approximate costs of recent repairs (as a proxy for severity of the leak). The approximate location of the boiler and the chiller are also shown. This elevated view is contrasted with a typical hydronic system plan view.
There is no clear failure pattern evident with the possible exception of the B&C and symmetric F&G stack associated with dollar value. Otherwise, this appears to be a general failure pattern that, if continued, would seem random. A random failure patter suggests whole system fragility. The management decision to replace the system in its entirety is warranted.
DESIGN, INSTALLATION, OPERATION OBSERVATIONS
CPVC Not Specified Material: Mechanical specifications were observed in the mechanical drawing set. Natural Gas piping, sheet metal ductwork, and other mechanical specifications were provided. However, no specifications for hydronic piping material were called out, nor was any statement deferring the material selection to another party apparent.
No Compensation for CPVC thermal Expansion: Because of the lengths of piping involved and coefficient of expansion of CPVC piping in this design, it would seem reasonable that some indication of material expansion compensation would be required. However no mention of expansion compensation was observed on readily available mechanical drawings.
Generalized Failure to meet manufacturer installation requirements: Manufacturers requirements for supports were every 3 feet for up to one-inch diameter and every 4 feet for pipe 1 ¼ to 2 inches in diameter. From visual inspections of the remaining CPVC components, it was obvious that the installation did not meet these requirements.
Oxygen introduced to “closed” system: It was reported that the air separator and pump impeller deteriorated and were replaced. Because these items are typically cast or fabricated from mild steel the premature deterioration would indicate excess oxygen was allowed to enter the system.
Make-up water PRV set incorrectly: Having to vent air continuously from the system indicates that the make-up water PRV (pressure reducing valve) was not set correctly to prevent air entrainment. The setting of the PRV should provide approximately 5 PSIG at the highest point in the system with the circulating pump not operating.
The first set of photographs suggests a case where the threaded brass connections throughout the installation were improper for this application. NPT tapered threads (National Pipe Thread Standard) are designed to create interference fit between similar materials as a means of creating both a high strength union and a positive fluid seal. Where one material is substantially stronger (brass) than its counterpart (CPVC), the strength of the union and the integrity of the seal may be compromised. In the case of CPVC, stressed would be introduced which accelerate the cracking and ultimately, failure.
In a proper application of CPVC threads (which this was not), the manufacturer installation recommendations for CPVC threads is to hand tighten for no more than 1-2 turns beyond finger tight using a strap wrench. It is clear that these unions where tool tightened to the point of CPVC material deformation as the brass nut bottomed out on the CPVC male connector. This introduced mechanical stresses in the material. These stresses opened leak paths, while making the material more vulnerable to chemical contamination and degradation.
ENVIRONMENTAL STRESS CRACKING
Classic environmental stress fracturing was found at the inside wall of the threaded area of pipe. This is precursor to a failure due to excess mechanical forces applied to threads and the propensity for CPVC to absorb contaminants at such stress zones; a textbook case.
THE CROSS-THREADED SAMPLE:
This cross threaded sample was found among the the demolition pile for the concurrent re-pipe. This sample provides a particularly egregious demonstration of combined deficiencies observed in this CPVC installation. A catastrophic failure at this union was imminent.
THE ANATOMY OF AN IMMINENT FAILURE:
CPVC is proven to be a robust piping material throughout the world, however, when many adverse conditions are concurrent – in this case we had poor workmanship, multiple mechanical stresses, contamination, and chemical attack – no material is resilient enough to resist such abuse.
An additional sample provided by the Heating Contractor demonstrates a condition where insufficient cement was applied allowing the CPVC tube to fall out of the connector. This is notable since the failed sample is not the original installation; rather, it appears to be a repair, which occurred at a later time. This would suggest that there might have been many hands contributing to the failure record of this facility.
DETAILED PROJECT DESCRIPTION:
This project involves failed CPVC piping for a hydronic system in a dormitory. This building was constructed in year 2006 and has been plagued by leaks in the hydronic piping system. A site visit occurred on Thursday June 11, 2014 and the following was observed;
A set of mechanical drawings was made available. The hydronic system shown appears to be a typical closed loop circulating system with fan coils that receive heated or chilled water depending on the heating or cooling requirements. Heating water is intended to circulate at 180o F. leaving the heaters. An air-cooled chiller was mounted on the roof to provide 45o F. chilled water during the cooling season. Therefore the CPVC piping system would be subjected to temperature variation of between 45o F and 180o F.
The building is three stories above grade and is approximately 250-300 feet long. The hydronic supply and return piping (from a below grade mechanical room that houses the heating boilers, expansion/compression tank, air separator, and pump) rises up through the building approximately in the center. Horizontal branch lines serve fan coil units.
Mechanical specifications were observed in the mechanical drawing set. Natural Gas piping, sheet metal ductwork, and other mechanical specifications were provided. However no specifications for hydronic piping were called out, nor was any statement deferring the material selection to another party apparent.
Because of the lengths of piping involved and coefficient of expansion of CPVC piping in this design, it would seem reasonable that some indication expansion compensation would be required. However no mention of expansion compensation was observed on the mechanical drawings.
Portions of the ceilings on floors first and second where open during the site visit. Most of the CPVC piping was removed, however samples were taken from a dumpster or preserved on the first floor of the Recreation Room. Chain of possession is uncertain. From the visual inspections, it appears no attempts were made to support the pipe per manufacturer’s requirements nor were any attempt to provide expansion compensation per the manufacturer’s requirements.
Manufacturers requirements for supports were every 3 feet for up to one-inch diameter and every 4 feet for pipe 1 ¼ to 2 inches in diameter. From visual inspections it was obvious that the installation did not meet these requirements.
CPVC pipe increases approximately 1.14 inches per 100 feet per 25oF temperature change. Assuming the horizontal branch piping on each floor is 300 feet long then the pipe will expand approximately 18.5 inches (1.14 x 3 x135/25). From visual inspections it was obvious that the installation did not include compensation for expansion or contraction that would occur during operation.
No maintenance records or operating logs were available during the site visit. The only information obtained was from the HVAC Contractor who has repaired several leaks in the past and is currently replacing sections of piping. It was reported that the air separator and pump impeller deteriorated and were replaced. Because these items are typically cast or fabricated from mild steel the premature deterioration would indicate excess oxygen was allowed to enter the system.
In a closed-loop hydronic system, only a small amount of make-up water (which may allow oxygen to enter the system) is typically required. However, as a result of several major leaks occurring over the years and the lack of isolation valves required the system to be drained down to repair leaks which would introduce more substantial oxygen amounts. It was also learned from the HVAC contractor that a flange at the chiller on the roof was loosened during operation to vent air continuously from the system. Taken together, these observations would explain how excess oxygen was allowed to enter the system.
Having to vent air continuously from the system indicates that the make-up water PRV (pressure reducing valve) was not set correctly to prevent air entrainment. The setting of the PRV should provide approximately 5 PSIG at the highest point in the system with the circulating pump not operating.
A site visit as attended by [customer name withheld], Dan Robles, PE, Coengineers, PLLC, David Coles, PE, Coles Consultants, LLC, and [Contractor name withheld].
Observations of the new system were made in comparison to remnants of the old system in place. A pile of samples were salvaged from the demo and laid out on the floor by the contractor for our review. Additionally, samples were selected from a dumpster located in front of the facility.
A tour of the facility was performed including entry to several suites, the basement boiler room, and the roof mounted chiller. We were looking for evidence of multiple failure scenarios offered by the owners, including a reported lightning strike event.
No evidence of lightning strike was observed and no documentation of the strike and related insurance claim were forwarded for our review and is therefore omitted from this investigation pending identification of failure modes.
A small water sample was collected from remaining old system and the results were sent to a lab looking for obvious hydrocarbon contamination. None was found, albeit on a limited sample and limited knowledge of what chemical to look for. Additional tests would need to be carried out for more specific diagnostic.
After review of the facility, collection of samples, and extended discussion, the samples were returned to Coengineers lab for analysis. The samples were photographed as collected and cleaned of extensive rust staining. Several samples were split using a manual hack saw to avoid generating heat. Samples were sanded with 100 Grit paper then 600 grit paper to produce a smooth finish for photography. Samples were labeled and reviewed at various degrees of magnification.
Extended portfolio of photographs were sent electronically to Dr. Duane Priddy, owner of Plastics Lab, a specialty chemist and consultant on CPVC and other types of plastic piping. Papers by Duane are referenced in the bibliography and Dr. Priddy’s opinion is expressed throughout this paper. Coengineers collected all data. PLLC and assembled into this document.
The Architectural building specifications did not make it clear to engineering what material would be used for the piping of the hydronic system. The engineers designed a hydronic system without providing readily obvious identification of material. It appears from the drawings that a metal system was intended (given the omission of thermal expansion loops). In the absence of this specification, the builder or sub-contractor took it upon himself or herself to use an otherwise reliable industry standard such as CPVC piping product.
Features such as thermal expansion loops and brass-inlaid connectors were not specified, so the contractor may not have known to include them. However, strict adherence to CPVC manufacturers installation requirements would have alerted the installer to seek additional information, if not to attend to industry practices. Further, the building was completed and operated without adequate regard for the make-up water pressure or constant venting for a closed system.
A closed hydronic system would not hold enough oxygen to cause the iron corrosion that was observed. Yet only oxygen in the water would corrode the pump impeller. It may have been during this time that someone introduced an incompatible corrosion inhibitor, cleaning agent, or MIC inhibitor. Any other contamination scenario is viable. The incompatible chemical was likely absorbed in high-stress areas of the system such as the brass fittings and anywhere that unchecked thermal expansion would introduce stress.
The system began to weaken as generalized fragility ensued. The repairs could not be attributed to any one cause because each occurred opportunistically at a microscopic level corresponding to invisible stress levels. Some failures were minor and some catastrophic. What is certain is that they would have continued until all components were replaced individually. Even then, it was observed, those replaced components were still vulnerable to failure.
A decision was made to replace a major part of the CPVC system with polypropylene. It was our recommendation to replace the entire CPVC system with an “Engineered System” that is specified from beginning to end to perform the function of a modern and reliable hydronic heating and cooling system.
Further, operating procedures and maintenance planning should be specified and overseen by a competent engineering firm that understands the vulnerabilities of all hydronic system components.
Finally, if the owners want to determine exactly what chemical(s) was responsible for compromising the relative integrity of this CPVC system, Further laboratory tests may be performed by Plastic Failure Labs to extract the identity of the offending hydrocarbon. However, these are fairly expensive test whose ultimate value ought to be weighed against the value of pursuing additional action in this matter.
|Dr. Duane Priddy Sr. was a leading Scientist in Dow Plastics for over 30 years, and is now a globally recognized expert in Polymer Science and Engineering.Dr. Priddy has over 100 scientific publications, over 60 US patents, and has authored several encyclopedia articles on various aspects of plastics. He has recently authored/edited a book entitled “Modern Styrenic Plastics” – Wiley 2003|
|As the founder of Plastic Failure Labs, Inc. (http://www.PlasticFailure.com), Dr. Priddy utilizes a network of world-class consultants with expertise ranging all aspects of Plastic Manufacture, Polymer Processing, Material Science, and Plastic Failure Analysis.He is currently a leading global expert in the failure of plastics, and has written several articles on the topic including “Why Do Plastics Fail?” and “Why Do PVC and CPVC Pipes Fail?”) which can be downloaded from http://www.plasticfailure.comDr. Priddy’s can be reached by email: email@example.com|