In a recent feature article in Trenchless Technology Canada by Dr. Mark Knight [1], reference was made to a City of Toronto pilot project in 2002 on watermain rehabilitation using Sanexen Environmental Services’ cured-in-place pipe (CIPP) Aqua-Pipe product. Unfortunately, several issues raised in the article were taken out-of-context and were based on outdated information.


Over the past 18 years, CIPP rehabilitation has become a recognized technique to renew deteriorated watermains. Along the way, in more than 350 North American cities, more than 2,090 km of watermains have been successfully lined with Aqua-Pipe, more than any other CIPP system during the same period. Among the many recognized advantages of CIPP rehabilitation, minimal impact to traffic, residences and businesses provides the highest value.


There are three types of CIPP products listed in the third edition of the AWWA’s M28 Manual on Rehabilitation of Water Mains: felt-based systems, membrane systems and so-called woven hose system. Aqua-Pipe falls into that last category, and consists of a seamless, double layer, circular woven liner that, upon epoxy resin impregnation, consolidation and curing, forms a structural composite “pipe within a pipe.”


To ensure a tight fit and compensate for variations of host pipe inner diameter and due to its seamless fabric structure, Aqua-Pipe liners are designed to be slightly oversized with respect to host pipe diameter. During its installation, Aqua-Pipe expands to form a tight fit with the host pipe and, as a result, leaves a controlled-sized longitudinal fold or wrinkle.


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Felt-based liners are a second type of CIPP liner systems used, consisting of one or more layers of flat sheets of non-structural needle-punched polyester non-woven felt containing individual embedded woven fabric layers, often made of glass fibres, at specific levels in the assembled structure. This structure is rolled and sewn longitudinally to form a tube. Contrary to Aqua-Pipe, felt-based liners, because of their assembled textile structure, are designed to be undersized with respect to host pipe diameter and meant to expand (typically less than 5 per cent) to fit the inner surface of the host pipe. Upon installation, tight fit to the host pipe is assumed from the expansion capacity of longitudinally sewn or welded liner.


The article from Knight [1] brings out that engineers have adopted ASTM F1216 design method for watermain pressure liners although it was developed for gravity or low pressure pipelines, with no consideration given for pressure surges which can occur in watermain networks. The Appendix X1 Pressure Pipe Design Considerations of ASTM F1216 however provides internal pressure ratings of pressure pipes, based on a factor of safety and a long-term factor, both of a nominal value of two.


It was therefore suggested that further confirmation of the pressure rating of watermain pressure liners be provided by means of short-term burst pressure testing according to ASTM D1599. Following this logic, the design maximum allowable operating pressure of the liner should be equal to, or below, the average burst pressure obtained following ASTM D1599 testing divided by the combined factor of safety and long-term factor.


As an example, a CIPP system having demonstrated an average short-term burst of 4.14 MPa (600 psi) would be deemed to have a pressure rating of 1.03 MPa (150 psi). For further details, refer to the AWWA Committee Report on Structural Classifications of Pressure Pipe Linings [2], which provides an overview of the industry’s consensus on AWWA M28 structural quantitative classification system. The latter Report provides quantification of pressure ratings based on hoop properties and burst pressure with appropriate safety factors, corresponding to a long-term factor and a factor accounting for ‘’geometric anomalies compromise hoop integrity, or when lining through bends and offsets.”


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To provide credible and independent results on the pressure rating of its 6-in. configuration Aqua-Pipe, Sanexen mandated the National Research Council of Canada (NRC) to perform short-term burst pressure tests according to ASTM D1599. The results obtained at NRC were included in an internal NRC report [3].


All short-term burst pressure testing complied with ASTM D1599 requirements, including controlled temperature and humidity environment of 23 ± 2 C and 50 per cent RH. The specimen, end caps, and test water were all conditioned at the test temperature for a minimum of 16 hours prior to testing. A pump was used for adding or bleeding off the system pressure before the test. An initial target pressure of 103.4 kPa (or 15 psi) was used to compress any residual air in the system and to help ensure the failure is reached. The liner system tested was pressurized up to burst failure using a water-filled 1-gallon bladder type hydraulic accumulator, with compressed nitrogen supply. Two pressure transducers were used for system pressure measurements (recorded at 50 samples per second). All tests reported here were obtained with a burst failure within 60 to 70 seconds, in accordance to ASTM D1599. A picture of an Aqua-Pipe specimen used for short-term burst pressure testing is shown in Figure 1.


Two series of 6-in. Aqua-Pipe specimens were tested. The first series consisted of two factory-produced lining specimens that were free of any longitudinal wrinkles or folds. The second series consisted of five specimens produced by lining old cast iron pipes (with an internal diameter of 148 to 150 mm,(5.83 to 5.91 in.) retrieved from various construction sites. These specimens intentionally contained longitudinal wrinkles or folds, due to the slightly oversized size selection to ensure tight fit of Aqua-Pipe to the host pipe, as mentioned previously. The selected liner had an oversize inside diameter by an average 4 mm (0.157 in.) with respect to the outside diameter of the liner. As a result, three types of fold features were noted in the specimen.


The first type of fold (Type I) corresponded to the inner and outer layers both folding together and pushing inwards to form a continuous longitudinal ridge. The second type (Type II) corresponded to the inner layer moving inward and forming a single layer longitudinal ridge. The third type (Type III) corresponded to the outer layer folding to form a longitudinal but much flatter ridge. In all cases, the space within the ridge, the space between the two fabric layers as well as the space between the double-fabric liner and host pipe (prior to removing the middle section of the latter to perform the short-term burst tests) were completely filled with epoxy.


The short-term burst results obtained by NRC are reported in Table 1. The results for the first series of tests show that the liner specimens without folds had an average burst pressure of 4.78 MPa (693 psi). Both burst pressures obtained were within less than 1 per cent of difference of each other.


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The results for the second series of tests show that the liner specimens produced from field-retrieved cast iron pipes, which contained at least one fold, had an average burst pressure of 2.65 MPa (384 psi). All burst pressures obtained were within a small range of 0.26 MPa (38 psi). These results show that the number of folds, or their type, did not affect significantly the burst pressure obtained, as no tendency could be drawn from the results.


burst pressure table

Table 1. Burst pressures of factory produced liner specimens (First series) and field-retrieved cast iron pipe liner specimens (Second series).


From the results of Table 1, and considering ASTM F1216 design recommendations for internal pressure ratings of pressure pipes based on burst pressures and applicable pressure rating, the following conclusions can be drawn:



  1. Aqua-Pipe 6-in. configuration product shows a burst pressure of 4.78 MPa (693 psi);

  2. With a pressure rating of 4 applied to the Aqua-Pipe 6-in. configuration burst pressure, the resulting maximum allowable operating pressure is 1.20 MPa (174 psi);

  3. Field-retrieved cast iron pipe Aqua-Pipe liner specimens show a burst pressure of 2.65 MPa (384 psi), corresponding to a reduction factor of 1.8 with respect to burst pressure of the Aqua-Pipe 6-in. configuration;

  4. The burst pressure of field-retrieved cast iron pipe Aqua-Pipe liner specimens is affected by the presence of a fold or folds in the liner, which correspond to a geometrical feature of the liner resulting from the specific host pipe geometry (diameter);

  5. The reduction factor of 1.8 in burst pressure of field-retrieved cast iron pipe Aqua-Pipe™ liner specimens is well within the factor of safety of 2 applied for such geometric anomalies.


Contrary to the short-term bursts reported for one CIPP liner product by Knight [1], the present results indicate that Aqua-Pipe shows low burst results variability with a pressure rating of 1.20 MPa (174 psi). The present results also indicate that the effect of the longitudinal fold, intentionally produced to ensure tight fit of Aqua-Pipe to the host pipe, lies well within the range of the recommended factor of safety of 2.


Footnotes:


1 – M. Knight, Advancing CIPP Pressure Liner Design, Trenchless Technology Canada, Volume 6, Number 3, August 29, 2019, pp. 28-30. https://trenchlesstechnology.com/advancing-cipp-pressure-liner-design/

2 – Structural Classifications of Pressure Pipe Linings, Suggested Protocol for Product Classification, 2019. AWWA Committee Report. AWWA. 44 pages

3 – H. Almansour, R. Smith, J, Margeson, O. Maadani. 2015. Burst Pressure Test According to ASTM D1599 – Test Set-up, Procedures and Results, Client Report A1-004080 (Final Report), 121 pages.



Martin N. Bureau is vice president of innovation at Sanexen Environmental Services Inc.



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