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The team at Western Engineered Containment (WEC) worked closely with one of their clients to solve a problem that they were facing – they discovered many leaks on their above-ground lined tanks.
While the current lining method involves the fabrication of polyethylene sheet liners into the cylindrical shape of the tank, these methods weren’t proven successful for our clients. As a result of the current lining method, ineffective seams at the junction of the wall and the circular base lead to the leaks that we were seeing in the client’s above-ground tanks.
The current industry practice has been lacking due to the sheer fact that a reliable wedge weld can’t be done in an arc pattern, resulting in less reliable extrusion welding. To accomplish the existing industry practices, the wall liner section extends a few feet onto the circular baseline to provide a flat surface for extrusion welding; however, it also results in excess material-gathering from the wall section liner as a consequence. The material is further pleated to complete the seam, and a complex shape needs to be extrusion welded as opposed to a standard single line. These complex welds are subsequently the weakest construction points of the systems.
Results From Testing
After testing various welding styles in order to reduce the leakages that our clients had, we had the following results:
*PPI is the lbs force per inch width
Single Line Weld
| Sample | Peel Strength (PPI)* | Extension at Break (mm) |
| 1 | 75 | 460 |
| 2 | 77 | 120 |
| 3 | 75 | 105 |
| 4 | 66 | 75 |
| 5 | 58 | 50 |
| 6 | 71 | 150 |
| Average | 71 | 160 |
Pleated Weld, First Weld Bottom Layer
Note: this is where the original 2 sheets are joined; the pleated section is folded a little behind and a little on top of the seam, resulting in 2 seams that are staggered a short distance from one another with overlap).
| Sample | Peel Strength (PPI)* | Extension at Break (mm) |
| 1 | 38 | 80 |
| 2 | 74 | 80 |
| 3 | 41 | 110 |
| 4 | 41 | 120 |
| 5 | 52 | 75 |
| 6 | 48 | 135 |
| Average | 49 | 100 |
Pleated Weld, First Weld Top Layer
Note: At this point, we are peeling the top sheet of the first weld from the rest of the mass.
| Sample | Peel Strength (PPI)* | Extension at Break (mm) |
| 1 | 70 | 110 |
| 2 | 60 | 130 |
| 3 | 76 | 110 |
| 4 | 64 | 200 |
| 5 | 82 | 270 |
| 6 | 74 | 130 |
| Average | 71 | 150 |
Pleated Weld
Note: This weld includes peeling pleat apart.
| Sample | Peel Strength (PPI)* | Extension at Break (mm) |
| 1 | 84 | 500 |
| 2 | 80 | 120 |
| 3 | 59 | 150 |
| 4 | 69 | 380 |
| 5 | 76 | 90 |
| 6 | 55 | 120 |
| Average | 70 | 227 |
By taking a look at these results, you see that the weak point of the pleated seams was peeling the very bottom layer of the liner off the initial weld. The other layers were consistent with the standard extrusion weld; however, the bottom peeling results in an average of 49 PPI in peel strength.
While there was one higher value at 74 in the dataset from peeling the bottom layer, the other values averaged much lower – ranging from 38 to 52. Additionally, the minimum published field extrusion weld peel strength is 44 PPI, and 3 of the 6 samples in this sample set failed to meet the standard.
There is a possibility that a weakening of the layer is due to the expansion & contraction stress on this seam, resulting from the heat & cooling of the pleat seam being placed beside it. Ultimately, expansion and contraction stress on polyethylene seams are an unavoidable consequence of welding seams of polyethylene lining systems.
Conclusion
Their experimentation shows that the extrusion welds – particularly the complex pleated welds – used in above-ground tank application are the weakest point of the lining system.
WEC is an experienced polyethylene installer; however, their team has found that the most beneficial way to eliminate leaks in the systems you’re using is by replacing the polyethylene lining system and extrusion welds with spray-applied seamless systems.
To produce extrusion welds in this configuration that meets the acceptable industry minimum requirements consistently is seemingly unachievable. However, the solutions that WEC provides by replacing the polyethylene lining system and extrusion welds with a spray-applied seamless system have proven to be successful alternatives that minimize the leaks experienced by our clients.
Are you interested in knowing more about their solutions? Visit their website: https://www.wecontain.com/.
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