Air Testing

 

ASTM D7177 - Air Channel Testing PVC Geomembrane Thermal Welds

Years of R&D by EPI have resulted in a NEW Testing Method for PVC geomembranes. In June 2005 a new Air Channel Testing Specification was adopted by ASTM. ASTM D 7177-05 Standard Specification for Air Channel Evaluation of Polyvinyl Chloride (PVC) Dual Track Seamed Geomembranes. 


EPI first began hot wedge welding PVC in 1990 and Dual Track Thermal Welding of PVC field seams in 1992.  After years of experimenting, field trials, equipment modifications & improvements, and numerous consultations with equipment manufacturers, EPI made a wholesale change to hot air welders for welding PVC. We recieved considerable help from Bruno Zurmuhle of Leister and J.B. Budny from Heely-Brown Company. Developing the procedures for using hot air welders on thinner, more flexible PVC materials was a challenging task. But the problems were conquered and technicians began to develop the skills to professionally weld PVC geomembrane in any thickness in almost any weather condition.

What evolved from these problem solving sessions and the reams of test data developed, was the absolute belief that the air channel test could be used to verify the physical strength of a PVC weld. In 2001, TRI Environmental agreed to do some testing and research on PVC thermally welded seams. Rick Thomas also became intrigued that PVC seams could be tested for peel strength using an air channel test. In 2002 burst testing research was initiated at TRI on hot air and hot wedge welded PVC seams in 30 and 40 mil PVC. The result of this testing and other research was a graph of pressure vs. sheet temperature for air channel testing PVC geomembranes that verifies a minimum of 15 lb/in peel strength for the full length of the test section. This temperature and pressure correlation is necessary to correct the test to the same conditions required in the laboratory when peel testing flexible PVC seams (72 degrees Fahrenheit).

In 2002 Mark Wolschon, Quality Control Manager of EPI, introduced to ASTM the idea of a standard air channel test for PVC. ASTM Committee D35 established an ASTM Task Group to develop a new standard.  After over two years of extensive discussions, ASTM D7177 Standard Specification for Air Channel Evaluation of Polyvinyl Chloride (PVC) Dual Track Seamed Geomembranes was adopted by ASTM in 2005. ASTM D7177  is now the recognized standard for air channel testing of PVC field seams. This test method does not apply for HDPE welds due to the rigid nature of that material.

  

Simply stated, poorly made thermal welds peel open when subjected to air channel testing according to ASTM D7177 . This test stresses the entire length of the seam, so any weak areas, no matter how small, will be immediately located. Any failing seam should be replaced. EPI has also experienced welds with passing destructive samples removed, failing an air channel test in another area of the same seam. Any seam that fails must be rewelded to insure the customer recieves the best possible product.

Sheet Temperature ºC

Air Pressure KPa

Sheet Temperature ºF

Air Pressure PSI

Hold Time

4.5

345

40

50

30 Seconds

7

324

45

47

30 Seconds

10

310

50

45

30 Seconds

13

290

55

42

30 Seconds

15.5

276

60

40

30 Seconds

18

262

65

38

30 Seconds

21

241

70

35

30 Seconds

24

228

75

33

30 Seconds

26.5

214

80

31

30 Seconds

29.5

193

85

28

30 Seconds

32

179

90

26

30 Seconds

35

165

95

24

30 Seconds

37.5

152

100

22

30 Seconds

40.5

138

105

20

30 Seconds

43.5

131

110

19

30 Seconds

46

117

115

17

30 Seconds

48.5

103

120

15

30 Seconds

51.5

90

125

13

30 Seconds

54.5

83

>130

12

30 Seconds

The chart above lists the air pressure required to verify 15 lb/in Seam Peel Strength for PVC dual track welds at various sheet temperatures, per ASTM D7177

 

Using a hot air welder with a dual track nozzle, EPI's installation crew can weld seams leaving an air channel  between the welds. EPI has the expertise to complete air channel testing of dual track thermal welded PVC field seams. Testing is accomplished by sealing both ends of the seam and introducing air pressure into the channel between the two parallel welds. The seam is pressurized to the minimum required pressure to verify the minimum of 15 lb/in peel strength, based on the ambient sheet temperature of the PVC geomembrane material. 

The pressure is monitored to insure weld integrity throughout the seam.  As a matter of practice, a passing seam will hold pressure immediately, whereas a poor seam will continue to loose pressure as the weld gradually peels open. The minimum pressure is determined according to the graph and chart shown below.

Air channel testing a dual track weld is an improved non-destructive testing method for field seams. Air channel testing will find flaws and weak areas that would otherwise be missed by an air lance.  Air lance testing of single welds, whether made by heat or chemical, does not insure the quality of the weld.

Pressure used in each test varies with the temperature of the PVC geomembrane. Air channel testing is performed according to EPI QC Standards based ASTM D7177 .
 

"T" Seams:

ALL field seams must be tested and T-seams can be difficult to air channel test if not welded properly.  T-seams are defined as a point in the seam where three layers of material overlap each other.  This occurs at the point that a dual track field weld crosses a factory seam, usually at a 90 degree angle.  (sometimes refered to as butt seams or end seams)   We have had hundreds of questions from engineers and customers regarding the air channel testing of "T" seams, the seam created along the end of a PVC geomembrane panel.  EPI air channel tests ALL field seams, including the seams along the ends of factory panels.


                              

Specimen of EPI "T" Seam in 30 mil PVC


There is a potential at each "T" to have a very tiny hole at the junction of the three layers of material. This is another key reason why air channel testing of every seam is critical to the integrity of the liner system, finding and eliminating these holes. Special care is taken by the welding technicians when setting up the welder to make sure this type of overlap is completely sealed, so the air channel test can be used to verify strength and continuity of these seams also.

EPI factory seams have no loose edge, so the process for welding T-seams is relatively easy. Slowing the welding machine's rate of travel will allow the melted PVC material to flow together at the junction of the three layers of material, providing the necessary seal and weld strength.  For fabricators who leave a loose edge on the factory seams, then each loose edge will need to be trimmed, similar to the process used on field welds which intersect other seams. 
 

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