Apparent leak in PVC pipe

Jointed PVC pipe is very popular today in North America for groundwater mains as well as sewer mains. As is the case with all pipes, regardless of material, post-construction testing is generally required to ensure that all new lines have been installed correctly.

One test that is common for gasketed pipes, such as PVC, is the combined “leak and pressure” test. This test is intended to simultaneously check the pressure capacity and tightness of the joints of the pipeline 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 reaches the desired test pressure. The system is then shut down and held for a specified testing period, typically 2 hours.

During these 2 hours, it is possible for the system pressure to drop. If the pressure drop is rapid during the test, it is likely that there is a significant leak in the system. In most cases, this could be attributed to improper assembly of a fitting, pipe joint, or service connection.

However, assuming proper installation and mounting has been done, a slight drop in pressure is possible over the 2 hours. To set a limit for an allowable pressure drop, a table of ‘Allowable Leakage 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 drop in pressure. (The actual rating table can be found in the IPEX PVC Pressure Pipe Installation Guide or AWWA C605).

For example, for 1000 m of PVC water main with 300 mm joints, 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 larger volumes of water are required, the test is considered failed and the installer must locate and repair any deficiencies in the system.

As a result of this table of ‘leakage’ allowances, many people have the perception that gasketed PVC joints always leak to some degree. In fact, a properly assembled PVC pipe will be airtight.

This claim is verified by rigorous testing of the gasketed joint at the manufacturing plant. Gasketed PVC pressure joints, to be certified to CSA B137.3 and to comply with ASTM D 3139, must undergo the following 2 tests:

(a) 60 minutes at 2.5 times the nominal line pressure (ie 588 psi for DR18)

(b) 60-70 seconds at 3.2 times the nominal line pressure (ie 755 psi for DR18)

Both tests are conducted while the joint is traveling at its maximum recommended angle to simulate a worst-case field condition. Zero joint leakage is allowed during these tests. The consistency and precision found in the extrusion of PVC pipe, along with the very tight dimensional tolerances, ensure that the gasketed joint will be as reliable in the field as it is proven in the plant.

An example of the waterproofness of PVC joints was illustrated on a recently completed project outside of Vancouver in Delta, British Columbia. B&B Contracting of Surrey has been contracted to construct 4,500 m (15,000 ft) of 400 mm (16″) DR18 PVC water pipe as the main supply line to the newly built DeltaPort container terminal.

After completing the installation, project superintendent Rollie Knackstead showed great confidence in his team’s workmanship, as well as the pipeline, by deciding to pressure test the entire 4,500 meters of pipeline at once. The results were exceptional. B&B successfully tested the line at 200 psi for 2 hours without a drop of a leak. The pressure gauge did not go above the 200 psi level. PVC watertight joints helped turn an already successful installation into a very successful overall project.

If during a PVC pressure/leak test the pressure drops slightly (within allowable make-up water volumes), does this mean there has been a leak? Perhaps, but tests at the PVC pipe plant, as well as numerous case histories, such as the B&B Contracting project in Delta, seem to indicate that properly assembled PVC pipe will not leak.

So why is there occasionally a need for small volumes of make-up water after a pressure test? Here are 4 possible reasons:

(a) trapped air

(b) displacement of tees or elbows before joint restraint function

(c) faulty connections for valves, fittings, hydrants, or services

(d) radial expansion of PVC pipe

In fact, radial expansion is such a significant factor with other thermoplastic pipe material (HDPE) that its test limit is specifically identified as an “expansion allowance”, rather than a “leakage allowance”. 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-fused HDPE than those for jointed PVC pipe for equivalent pipe sections and test pressures.

In summary, the reader is cautioned that many times a more appropriate term for ‘Leak’ is ‘Apparent Leak’ or ‘Make-up Water Allowance’ when discussing test results for PVC pressure pipe. Evidence suggests that properly assembled PVC pipe with gaskets will have virtually zero leaks. The fact that PVC does not weaken over time due to corrosion attacks will ensure that today’s watertight joints will remain so for many years.