Leaks are common on compressed air lines in maintenance facilities and are often left unrepaired until they negatively impact downstream operations. A common response to a substantially leaky compressed air system is to provide additional compressor capacity to meet end user flow and pressure requirements, which increases energy consumption. Leaking compressed air lines represent wasted energy that should be addressed to reduce energy costs at Department of Defense (DoD) installations.

Engineers from the Naval Facilities Engineering and Expeditionary Warfare Center (NAVFAC EXWC) investigated the use of epoxy coatings to seal leaks on compressed air lines. The team focused on pipelines found in inaccessible areas such as those underground that are costly to repair with conventional techniques (requiring demolition, excavation and reconstruction). In particular, the team demonstrated the epoxy technology on two recently abandoned below grade steel pipelines (1-1/2 and 4-inch diameter) located at Naval Base Ventura County in California. The lines served three maintenance buildings, but were recently abandoned due to extensive leakage, requiring that each building operate its own respective air compressor. If corrected with the epoxy technology, the lines could be recommissioned, which would allow one air compressor to service three maintenance buildings in lieu of the three air compressors currently being used.

Specific project objectives were established to measure the epoxy coating’s ability to seal leaks in-situ, to reduce energy losses, and to improve operational performance. The two primary performance objectives included: 1) Static pressure tests requiring > 90% reduction in pressure loss over a baseline measurement with minimal negative side effects on line performance; and 2) Coating installation period of less than 72 hours to avoid operational disruption.

Technology Description

The technology consists of a two-part epoxy coating that is introduced into the pipeline using low pressure compressed air to uniformly coat the inner wall of the leaking pipe. It was originally developed to extend the service life of new wastewater pipelines installed on Navy ships. The following steps are used in the deployment of the technology on compressed air systems:

  1. Conduct leak audit to identify large pipe breaches
  2. Repair large pipe breaches and remove sensitive equipment (i.e., flow meters, filters),
  3. Create access points to inspect pipe condition for follow-on application of the epoxy,
  4. Remove moisture with heated air and use abrasive media to remove surface rust, which creates an interior surface texture for proper coating adhesion,
  5. Apply epoxy using low-pressure heated air, and
  6. Allow epoxy to cure and return pipeline back to service.

The return on investment (ROI) of the technology was evaluated for the following three scenarios:

  1. Represents military activities that simply add more compressors to ensure adequate air supply is available to end users.
  2. Represents military activities that do nothing to address leaks, operate at high leakage rate, and are marginally able to provide adequate air supply to end users.
  3. Represents military activities comparing the cost of installing new pipelines through traditional methods to address substantially leaking in underground compressed air pipelines

Demonstration Results

The results of the pressure variation tests were varied. The 90% reduction in air loss was not achieved on the 1-1/2-inch diameter pipe due to significant wall degradation caused by corrosion (major leaks were found) that likely worsened during the abrasive surface preparation step. The 90% reduction in air loss was achieved on the 4-inch diameter pipe (over 50 years of prior service life). Due to the extra time used to resolve the major leaks in the 1-1/2-inch pipeline, the 72 hour time limit for both pipelines was not met.

The return on investment (ROI) of the scenarios were:

  1. The ROI for scenario 1 was calculated at 2 years. The major benefits include: 1) Annual energy savings realized by reducing the number of air compressor used; 2) Annual energy savings from reduced air leakage; and 3) Improved service life of compressors with deferred capital cost by avoiding acquisition of additional compressors.
  2. The ROI for scenario 2 was calculated at 12 years. The major cost benefits are derived by: 1) Annual energy saving from reduced air leakage; and 2) Improved compressor efficiency and service life.
  3. The ROI for scenario 3 was calculated at 17 years. Assuming the annual energy savings and the ability to operate with one compressor as equitable, the cost benefit is derived from the difference in capital cost.

Implementation Issues

The application of the epoxy coating technology at Naval Base Ventura County provided mixed results. The full scale demonstration on two separate underground pipelines exposed the inherent risk associated with the technology, as the pipe wall thickness (maximum limit of < 40% degraded) cannot be accurately assessed with existing inspection technologies prior to applying the epoxy coating. Large breaches in the pipe wall greater than a 1/16-inch wide cannot be repaired using the epoxy coating process.

The demonstration revealed that pipe conditions can vary considerably within a length of pipe, and when significant corrosion is encountered, it may likely point to multiple and significant breaches, which result in a higher risk for epoxy failure at normal operating pressure. Technology shortfalls were also identified with pipeline camera inspection equipment, as the smallest camera head was unable to navigate through multiple pipe fittings in series, and the resultant visual inspection was unable to identify large breaches that were possibly masked by extensive corrosion. Additional research should be conducted to identify cost effective alternatives to existing camera pipeline inspection equipment.

Static pressure and pressure drop tests may be the most pragmatic approach to assessing relative pipe conditions and whether a pipeline is a good candidate for epoxy repair applications. NAVFAC EXWC recommends that the epoxy coating technology be used primarily as a preventative maintenance measure to extend the service life of aging compressed air systems that do not have significant indicators of corrosion as indicated by static pressure leak tests.