Lähdekoodi sivulle Apparent Leakage in PVC Pipe

Gasketed PVC pipe is very popular today across North America for underground water mains as well as sewage force mains.  As is the case for all pipelines, regardless of material, post-construction testing is usually demanded to ensure all new lines have been installed properly.

One test that is common for gasketed pipe such as PVC is the combined 'leakage and pressure' test.  This test is intended to verify both the pressure capacity and the joint tightness of the pipe system simultaneously.

The procedure of this test is to first fill a section of the line with water.  Next, additional water is pumped into the line until the section is elevated to a desired test pressure.  The system is then closed and held for a specified test period, typically 2 hours.

During this 2 hours, it is possible that the system pressure may drop.  If the pressure drop is rapid during testing, it is likely that a significant leak is present in the system.  This could most times be attributed to an improper assembly of an appurtenance, pipe joint or service connection.

However, assuming that proper installation and assembly were performed, it is possible to have a very slight drop in pressure over the 2 hours.  To establish a limit for a permissible pressure drop, a table of 'Allowable Leakage Volumes' was created.  This table indicates the maximum volume of water permitted to be added back to the system after a pressure/leakage test has indicated a slight pressure drop.  (The actual table of values may be found in the IPEX PVC Pressure Pipe Installation Guide or in AWWA C605).

For instance, for 1000 m of 300 mm gasketed [http://www.pvc-supplier-china.com PVC film] water main, a maximum volume of 7.6 L of make-up water (theoretically, the leakage occurring during the test) is permitted to be added for a 1035  kPa test (equivalent to 2.0 US gal. for 3300 ft. of 12" pipe tested at 150 psi).  If larger volumes of water are required, the test is considered to be a failure and the installer must locate and repair any system deficiencies.

As a result of this table of  'leakage' allowances, many individuals have a perception that gasketed joint PVC is always leaking to some degree.  In actual fact, properly assembled PVC pipe will be bottle-tight.

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

(a) 	60 minutes at 2.5 times the pressure rating of pipe      (i.e. 588 psi for DR18)

(b) 	60-70 seconds at 3.2 times the pressure rating of pipe (i.e. 755 psi for DR18)

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

One example of PVC's joint tightness was illustrated on a project recently completed outside Vancouver in Delta, British Columbia.  B&amp;B Contracting of Surrey was contracted to construct 4500 m (15,000 ft.) of 400 mm (16") PVC DR18 water main as a supply main to the newly constructed DeltaPort Container Terminal.

After completing installation, Project Superintendent Rollie Knackstead showed tremendous confidence in the workmanship of his crew, as well as the pipe, by deciding to pressure-test the entire 4500 metres of pipe at once. The results were exceptional.  B&amp;B successfully tested the line at 200 psi for 2 hours with not a drop of leakage.  The gauge did not budge off of the 200 psi level.  The bottle-tight joints of PVC helped to make an already successful installation into a very successful overall project.

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

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

(a) 	entrapped air

(b) 	shifting of tees or elbows before joint restraints function

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

(d) 	radial expansion of the PVC pipe

In fact, radial expansion is such a significant factor with one other thermoplastic piping 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 one does a comparison, the actual values of allowable make-up water are found to be significantly greater with butt-fused HDPE than those for gasketed PVC pipe for equivalent pipe sections and test pressures.

In summary, the reader is advised that many times a more proper term for 'Leakage' is 'Apparent Leakage' or 'Make-Up water Allowance' when analyzing the test results of a PVC pressure pipeline.  Evidence suggests that properly assembled gasketed PVC pipe will have virtually zero leakage.  The fact that PVC will not be weakened over time from corrosion attacks will ensure that the bottle-tight joints of today will remain that way for many years to come.