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270˚ versus 360˚ PVC Liners for Pipelines
By Jose L. Villalobos, P.E.
and Derek M. Wurst, P.E.

V&A has conducted several investigations on the interior surfaces of large diameter pipelines that are protected with a PVC liner and has found that the integrity of the liner and the condition of the concrete surfaces depends on several factors. Those factors include: installation methods, flow level at off-peak hours, turbulence, and hydrogen sulfide (H₂S) exposure. PVC liners have been installed for more than 30 years and have proven to be one of the best ways to protect concrete from H₂S corrosion. A key decision for the design engineer will be to specify 270˚ or 360˚ of PVC liner around the interior surfaces of the pipe. Based on V&A’s experience with PVC-lined pipelines, this article should provide some insight for the design decision.

PVC Liner After Several Years

V&A has extensive experience with confined space entries into wastewater pipelines to inspect the existing condition of PVC-Lined, large diameter pipe surfaces. The following photos and figure show some of the conditions encountered during those inspections.

Photo 1. One-inch concrete degredation below the termination edge of a 270˚-lined pipeline.	Figure 1. Schematic showing the wastewater level below the PVC liner termination edge. Click on the figure for a larger image.

Photo 1 shows the concrete surfaces of a reinforced concrete pipeline after 10 years of service. During off-peak hours the flow level in the pipeline dropped below the termination edge of the PVC liner. This flow level drop resulted in the H₂S degradation of the concrete caused by anaerobic bacteria on the surfaces of the pipe (Figure 1). The low flow levels during off-peak hours were not anticipated during the pipeline design. The concrete would continue to corrode during those low flow conditions and eventually expose the reinforcing steel. Photos 2 and 3 show more situations where the PVC liner’s integrity has been compromised.

Photo 2. Detached PVC liner from the embankment of a flow diversion channel.	Photo 3. Delamination of the PVC liner from the ceiling of a structure exposes corroded concrete underneath.

The situations shown in Photos 2 and 3 may have resulted from missing joint strips covering termination points and overlapping sections of the liner. When concrete is exposed to H₂S gas it loses its structural integrity, becomes soft, and begins to expand underneath the PVC liner (as shown in Photo 3).

Forecasting for the Future

When designing a new wastewater pipeline or structure, engineers should consider several factors when specifying a PVC-lined pipe.

1. Flow levels should always be higher than the PVC liner termination edge for both the off-peak and peak flow.
2. Dissolved Oxygen less than 1 mg/L will promote the growth of anaerobic bacteria which will corrode concrete and metal.
3. Dissolved sulfide levels greater than 1 mg/L indicate high levels of anaerobic bacteria present in the wastewater which produces a corrosive environment.
4. Localized turbulence caused by abrupt changes in the pipeline’s profile or hydraulic grade line, such as splash zones, may produce H₂S off-gassing.

It may be difficult to accurately predict the future peak and off-peak flows, so V&A recommends that a pipeline be lined 360˚ on the interior surfaces if flows are not expected to be at full capacity during the first years of its design life. Otherwise the lining may detach from the pipe wall as shown in the photos above. To avoid the case shown in Photo 2, PVC-lining contractors must be given instructions on how to properly terminate the PVC liner by embedding it into the concrete. Continual and repetitive exposure to cyclic wet/dry atmosphere may detach the PVC liner from the concrete or degrade the concrete enough to expose the reinforcing steel. Photo 3 shows a joint welding strip that either fell off the surface or was never installed. In either case, a third-party inspector is recommended for all PVC liner installations to avoid similar situations.

V&A’s Experience with PVC Liner

V&A is a proponent of using a PVC liner for large diameter (greater than 48-inch) pipelines for wastewater. Even if flow levels are assured to be above the PVC liner termination edge, future conditions may change. V&A recommends that a cost benefit analysis be conducted to determine if the savings on material cost for 240˚ or 270˚ of PVC liner is worth the potential concrete damage if the flow level were to drop below the PVC Liner termination edge. When flow levels are assured to be above the PVC liner termination edge, V&A recommends 240˚ or 270˚ PVC liner installation. Contact V&A at the number below if any further information or guidance is needed with respect to these services.

V&A Consulting Engineers
1999 Harrison Street, Suite 975, Oakland, CA 94612
Phone 510-903-6600     Fax 510-903-6601

One Riverway, Suite 1700, Houston, TX 94612
Phone 713-840-6490

www.vaengr.com
Copyright 2005