Jointed PVC pipes are very popular today in North America for underground water and sewer pipes. As is the case with all pipes, regardless of material, post-construction testing is generally requested to ensure that all new lines have been installed correctly.
One test that is common for gasketed pipes like PVC is the combined ‘leak and pressure’ test. This test is intended to simultaneously verify the pressure capacity and the tightness of the joints of the piping system.
The procedure for this test is to first fill a section of the line with water. Additional water is then pumped into the line until the section rises to the desired test pressure. The system is then shut down and held for a specific test period, usually 2 hours.
During these 2 hours, the system pressure may drop. If the pressure drop is rapid during testing, there is likely a significant leak in the system. Most of the time, this could be attributed to improper mounting of a fitting, pipe joint, or service connection.
However, assuming proper installation and mounting, a very slight drop in pressure may occur during the 2 hours. To set a limit for an allowable pressure drop, a table of ‘Allowable Leak Volumes’ was created. This table indicates the maximum volume of water that is allowed to be added back to the system after a pressure / leak test has indicated a slight pressure drop. (The actual table of values can be found in the IPEX PVC Pressure Pipe Installation Guide or in AWWA C605).
For example, for 1000 m of PVC water main with 300 mm joint, it is allowed to add a maximum volume of 7.6 L of make-up water (theoretically, the leakage that occurs during the test) for a test of 1035 kPa (equivalent to 2.0 US gal. For 3300 feet of 12 “pipe tested at 150 psi). If higher volumes of water are required, the test is considered a failure and any deficiencies in the system must be located and repaired by the installer .
As a result of this table of ‘leakage’ allowances, many people have the perception that gasket-to-gasket PVC always leaks to some degree. In fact, properly assembled PVC pipe will be bottle-tight.
This claim is verified by rigorous testing performed on the packed joint in the manufacturing plant. Packed PVC pressure joints, to be certified to CSA B137.3 and to meet ASTM D 3139, must undergo the following 2 tests:
(a) 60 minutes at 2.5 times nominal pipe pressure (i.e. 588 psi for DR18)
(b) 60-70 seconds at 3.2 times the pressure rating of the pipe (i.e. 755 psi for DR18)
Both tests are carried out while the joint is traveling to its maximum recommended angle to simulate a worst-case field condition. Zero leakage is allowed at the joints during these tests. The consistency and precision found in PVC pipe extrusion, along with very tight dimensional tolerances, ensure that the sealed joint is as reliable in the field as it has been proven in the plant.
An example of the tightness of PVC joints was illustrated in a project that was recently completed outside of Vancouver in Delta, British Columbia. B&B Contracting of Surrey was contracted to construct 4500m (15,000ft) of 400mm (16 “) PVC DR18 water main as the main supply line to the newly constructed DeltaPort Container Terminal.
After completing the installation, Project Superintendent Rollie Knackstead showed tremendous confidence in his equipment’s workmanship, as well as the tubing, by deciding to pressure test the entire 4,500 meters of tubing at a time. The results were exceptional. B&B successfully tested the line at 200 psi for 2 hours without a leak. The gauge did not move outside the 200 psi level. PVC gaskets helped turn an already successful installation into a very successful overall project.
If during a pressure / leak test on PVC, the pressure drops slightly (within the allowable make-up water volumes), does this mean that there have been leaks? Perhaps, but plant testing of PVC pipe, as well as numerous case studies, such as the B&B Contracting project in Delta, seem to indicate that properly assembled PVC pipe will not leak.
So why is it sometimes necessary to have small volumes of make-up water after a pressure test? Here are 4 possible reasons:
(a) trapped air
(b) displacement of elbows or tees before joint restraints work
(c) faulty connections for valves, fittings, hydrants, or services
(d) radial expansion of PVC pipe
In fact, radial expansion is such an important factor with another thermoplastic pipe material (HDPE) that its test limit is specifically identified as an ‘Expansion Permit’, rather than a ‘Leak Permit’. It is also interesting to note that if a comparison is made, the actual allowable make-up water values are found to be significantly higher with butt-cast HDPE than PVC pipe with gaskets for pipe sections and equivalent test pressures.
In summary, the reader is cautioned that many times a more appropriate term for ‘leak’ is ‘apparent leak’ or ‘make-up water tolerance’ when analyzing the test results of a PVC pressure pipe. Evidence suggests that properly assembled gasketed PVC pipes will have virtually zero leaks. The fact that PVC does not weaken over time due to corrosion attacks will ensure that today’s gaskets will remain that way for many years.