Air Pressure Testing: Some Points to Consider
by Mark Cadwallader

One reason why the dual track hot wedge weld has emerged as the seaming method of choice is its ability to be air pressure tested, providing a quick and efficient way to detect leaks in the seam. Compared to the vacuum box test, there is no question which method is superior for detecting leaks.

One common misconception among engineers about the test is that it should be a measure of seam strength, raising the air pressure in an attempt to burst apart the seam. This process however becomes difficult to implement in the field due to problems working with very high pressure necessary for strength testing. Pressures sufficient to produce a film tear along the lines of the destructive test are much higher than pressures deliverable by air compressor or pump. Consider for example that the pressure at tensile yield for HDPE liner is typically 2400-2800 psi, whereas the air pressure in a car tire by comparison is typically 30-35 psi and high pressure bicycle tires reach only 100 psi.

The purpose of non-destructive testing, by definition, is not to destroy the sample during the test. In this case it is to look for leaks through which fluids could move. Therefore pressures sufficient to allow leaks to be detected are all that is required. Under the usual specification of 30-35 psi for an air pressure seam test, air can quickly escape through leaks which may be present and the pressure drop is clearly and quickly visible. Occasionally, during very sunny and hot installations, there is some pressure drop due to heating and expansion of the black plastic, under the pressure within the welded track. This type of pressure drop is minimum and very slow, especially compared with escaping air through leaks leading to complete pressure drop.

A concept promoted by certain installation companies to accommodate particular welder configurations, is that destructive testing of only one of the two hot wedge tracks is sufficient. The objective here is really to accommodate a hot wedge welder configuration which aligns the edge of the sheet with the outside weld track, making it very difficult to grip the outside edge to perform a peel test on that track. The argument that you need to test one weld track does not stand on its own to many engineers. If the installer is delivering two welded tracks "for the price of one" then we need to test both welded tracks. The philosophy here is another example of increasing the requirement so that the standard is set at the limit of the technology. Testing both welded tracks is regarded as important. Arguments that welding at the very edge of the panel allows more automated dual track welding and that it reduces wasted material are for the most part arguments directed toward a commercial industry interest, and not particularly toward the interest of quality control in general.

Another important point about dual track air pressure testing is the possibility of "squeeze-out" during welding clogging the air passage between welded tracks. The potential for this type of clogging is made worse through the use of thick and textured sheet. Eighty mil textured sheet for example is more prone to clogging the air channel than is forty mil smooth sheet. Successful welding speeds for the thick textured sheet are therefore more narrowly defined than for non-textured and thinner material.