pipeline coatings linings

Large-Diameter Cooling Water Pipe and Penstocks: Selecting and Applying Linings

As Seen in JPCL Magazine, August 2019

The installation of linings in large-diameter cooling pipes and penstocks is a major investment for hydroelectric and other power plants. The application should be performed in an expedient manner to minimize outage time and should last for many years—20 or more—without the need for maintenance painting. The latter is significantly important because many hydroelectric plants are located in remote areas. To achieve these goals, planning must be done long before the start of the lining process. This article provides a compilation of the necessary tasks to plan for in order to implement a successful application. Factors that must be considered when selecting a lining, the types of linings that are available and are typically used, important requirements that must be included in a specification and safety considerations are all discussed.


Planning for penstock relining work must begin months or years before the actual project start date. Major items to consider at the beginning of the project include gaining an understanding of the condition of the existing lining system, if present, and the condition of the piping itself. This is determined by performing inspections of the penstock interior. The outcome of these assessments will then be used to prepare a concise, detailed project specification. When informative specifications are developed for a relining project, overall costs are reduced, proposed schedules are more likely to be followed and change orders are minimized, or in many cases eliminated. This is because the contractors have the information necessary to prepare responsive, accurate bids, and costs are not factored into the proposals because of unknown conditions.


The scope of the inspections should include a determination of what type of lining system is present as well as its condition. The condition assessment should include lining thickness measurements, degree of rusting, adhesion and the identification of other types of lining defects. 

The existing lining should also be inspected to determine whether there is any erosion or abrasion occurring. Silt, sand and gravel can be carried through the pipe via the flow of water and abrade the surface of the pipe lining. This is usually observed on the invert of the pipe between the 4:00 and 8:00 positions. The coating can appear to be scoured or may be worn away in more severe cases. In some instances, the steel itself can be eroded. The knowledge of whether this is occurring is useful in determining what type of replacement lining to use, as some are formulated to have a greater resistance to abrasion.

If repair of the existing lining system is being considered (or even when total removal of the existing lining is necessary), samples of the lining should be removed for laboratory analysis to determine its generic type—such as epoxy, coal-tar enamel or coal-tar epoxy—to help ensure compatibility (if repair is possible) or to provide bidders with an indication of the level of difficulty they may anticipate during removal due to thickness and elasticity. Analyzing toxic metal content such as lead, cadmium and chromium is also important in determining worker- and environmental-protection requirements, as well as waste-management processes. This information is particularly important for potential bidders in preparing responsive bids that include the costs associated with controlling these hazards. That is, the contractor will be able to better estimate the means and methods necessary to safely and effectively remove an existing lining, prepare the surface of the pipe and install the new lining system. The more pertinent information that is provided to the contractor, the more accurate and complete the bids will be.

For example, if a coal-tar enamel is present in the pipe, special requirements for removal are often required. Coal-tar-enamel removal requires additional steps beyond typical pressure-washing and abrasive blast-cleaning. Ultra-high-pressure waterjetting or hand/power tool chipping is often necessary to remove the bulk of the enamel prior to the final step of abrasive blast-cleaning. Contractors will almost certainly submit a change order for performing the additional coating removal work if they are not informed up-front that a coal-tar enamel is present. Experience has shown that costs associated with the tasks included in a change order are always greater than if those costs had been included in the original bid.

Further, knowledge of toxic-metal concentrations is essential for developing a responsive bid. When lead and/or other toxic metals are present in an existing coating, special precautions must be taken to safely remove the coating without overexposing workers and/or contaminating the surrounding environment. The costs associated with managing these hazards are not incidental.

Toxic-metal concentrations should be included in the specification to inform the contractor that the metals are present in order to select appropriate controls. Requirements should direct the contractor to perform his or her own testing and base the toxic-metals-compliance plan on those results. Without this wording, change orders can be anticipated if the contractor’s test results reveal higher concentrations than those referenced in the specification. Nevertheless, it is important that the contractor be informed that toxic metals are present prior to the start of the project.

In addition to information about the existing lining system, the condition of the substrate is also essential. The presence and characterization of pitting, more severe corrosion and erosion should be included in the specification. This information is important because some pits and rough surfaces caused by corrosion and erosion should be repaired prior to abrasive blast-cleaning and filling. Pits and corroded or eroded areas that have sharp edges should be smoothed to facilitate acceptable coverage of the lining. Many liquid coatings do not have edge-retentive properties. Similarly, pits that have wider bases than mouths will have to be widened to permit lining material to fully cover the surface. Deep pits and degraded areas are likely to require the application of a paste-grade surfacer (pit filler) to smooth out the surface prior to lining installation. If the pits are deeper, it may be necessary to fill them with weld material.

Details regarding the design of the penstock should also be included in the specification, including the dimensions (length and diameter) of the portions of the penstock to be lined, the slopes that are encountered and available access points. Penstock dimensions are used by the contractor to calculate the area to be lined and the volume of material required. Some penstocks are located on the side of a mountain with steep slopes, so special access will be required to prepare and line the surfaces. On steep slopes, movable platforms on pulleys are frequently used to access all pipe elevations. When slopes vary, platforms are often movable to create a level surface on which to place equipment and personnel as it moves from one slope to the other. 

Information on the location and size of the current access points is essential to the contractor to plan his or her work. High-solids (100%) materials, the common type of lining used in penstocks today, can only be pumped about 1,000 feet. The specification should require the contractor to identify whether additional access points are necessary and where they should to be located. The responsibility of establishing additional access-point locations (and sizes) is placed on the contractor so that the owner is not responsible if difficulties are encountered during the surface-preparation and lining processes.

Also, the method by which the individual pipe spools are joined—welded, bolted, riveted or by Dresser couplings—must be identified so that appropriate measures can be taken to coat these areas properly. Welds may need to be ground smooth and coated with paste-grade material, while bolted connections and Dresser couplings may need to be caulked or sealed in some other way. Because these are time-consuming operations and require special materials, full knowledge of the scope is essential to assuring that the project proceeds on time and on budget.

Other information that is useful to include in a specification are details about the penstock isolation. Isolation valves are usually located at the top of the penstock so that it can be isolated from the water source. These valves inherently leak. Dams usually constructed using plastic sheeting and sand bags are used to collect the water and isolate the leakage from the rest of the pipe. Submersible pumps are placed in the collected water and pumped out of the penstock so that it doesn’t flow down prepared surfaces, requiring the contractor to perform rework. The specification should include language that places the responsibility of rework resulting from water intrusion on the contractor unless there is a major failure of the valve. The author has seen more than one project where the contractor inadvertently shut off the submersible pump, causing the dam to overflow and ruin entire sections of pipe that had just been abrasive blast-cleaned or coated.

Finally, a specification should address logistics including available utilities such as power, water and area access; areas for equipment and material storage; and in some cases, even locations for personnel housing. Many penstocks are remotely located and a clear indication of what roadways, utilities and housing are available is important information. Schedules can be drastically impacted if the necessary utilities and work areas are not available.


Once this information is gathered, the preparation of a technical specification can be initiated. In addition to the general information described earlier, the items that should be included in the specification encompass contractor qualifications, safety requirements, surface-preparation requirements, lining materials and installation requirements and quality control/assurance-inspection requirements.

On major projects such as penstock relining, it is essential to hire a qualified contractor. A convenient way to achieve this is to require bidders to be certified to the applicable quality procedure (QP) of the SSPC Painting Contractor Certification Program (PCCP). For penstock relining work, contractors should hold SSPC-QP 1 certification and if toxic metals are present in the existing coatings, then QP 2 certification should be required in addition to QP 1. These certifications provide reasonable assurance that the contractor has established the necessary work practices and quality-control procedures to perform field painting and to safely remove coatings containing toxic metals. If modifications are included in the project that require the installation of new shop-coated piping or components, then the shop should hold SSPC-QP 3 certification as well. While these certifications do not guarantee project success, they provide the owner with peace of mind that the contractor has the appropriate procedures in place to safely perform quality work.

The selected contractor should not only be qualified but should also have experience in penstock lining. These requirements typically include having experience on at least three similar projects in the past five years. This provides reasonable assurance that the contractor is able to deal with the challenges of preparing and lining several thousand feet of steel in a confined space.

Safety requirements are equally important. Because many penstocks are located on the sides of mountains and are steeply sloped, care must be taken to assure that workers do not cascade down the pipe. Fall protection should be mandatory when working in these configurations. Also, penstocks are considered to be confined spaces and pose a unique set of safety concerns. Perhaps one of the worst coating-industry-related accidents was the Cabin Creek tragedy that occurred in a penstock in Colorado in 2007. Five workers were killed, and three others injured as a result of a fire that started when flammable solvent was introduced to a plural-component spray rig fitted with an electric heater. The fire was fed by numerous open five-gallon drums of solvent. It occurred between the workers and the egress point of the penstock and consumed the oxygen in the pipe, causing suffocation. 

Once the general and safety information is assembled, the technical requirements associated with surface preparation and lining installation can be established. As with many surface-preparation activities, one of the first operations is to pressure-wash the pipe. Pressure-washing at 5,000–10,000 psi is typically done to remove the silt and sediment that may have accumulated in the pipe, as well as loose lining materials. The specification may also include the option to perform high-pressure waterjetting at pressures between 10,000 psi and 30,000 psi. In addition to the silt, sediment and loose paint, high-pressure waterjetting (at the 30,000-psi level) will typically remove all existing lining and corrosion products.

Waterjetting and pressure-washing operations must be followed by abrasive blast-cleaning. Most lining manufacturers will require a minimum of SSPC-SP 10/NACE No. 2, “Near White Blast Cleaning,” yielding a nominal surface profile of 3.0–5.0 mils (75–125 µm). Also, testing for non-visible surface contaminants such as chlorides, sulfates and nitrates should be performed on the blast-cleaned steel. If permitted to remain on the surface, these ions can shorten the service life of the lining by drawing water through the coating via osmosis, resulting in blistering and/or premature rusting. 

Some specifications have included more restrictive surface-preparation requirements to extend the service life of the lining by increasing the cleanliness requirement to SSPC-SP 5/NACE No. 1, “White Metal Blast Cleaning.” In addition, a final blast may be performed with #20 aluminum oxide to create a very dense, sharp anchor pattern in the steel. The aluminum oxide is only used in the final blast since it is typically more expensive than the other abrasives. However, the contractor may elect to perform all abrasive blast-cleaning with the aluminum-oxide abrasive.

It is important to note that if new pipe spools are installed, the joints will need to be prepared and coated after the spools are joined. While it is easiest to power-tool clean the joints prior to coating, experience has shown that when power-tool cleaning is performed, these areas are frequently the first to fail. Therefore, it is highly recommended that the joints be abrasive blast-cleaned.

The most commonly specified lining materials for penstocks include 100%-solids epoxies and elastomeric urethanes or urethane hybrids. The epoxy linings have over 40 years of history of successful use and some of the first applications in the United States in the early 1980s are still in service today with little need for maintenance. These early epoxy lining installations consisted of two to three coats to achieve a total dry-film thickness of 25–30 mils (625–750 µm). In more recent times, high-build coatings have been developed where 50 mils (1,250 µm), and in some cases more, can be applied in a single coat. The epoxy linings have low permeability, enabling them to perform longer; however, the standard 100%-solids epoxy linings are not known for their abrasion resistance. There are special epoxy coatings that can be used that will resist the abrasion caused by sand and sediment, or in high-flow areas. Pitted areas and other rough surfaces and sharp edges will require the use of a paste-grade material to prevent pinholes or thin spots, and the epoxy linings often take several days to cure prior to putting them back into immersion service, so cure time must be factored into the schedule.

Urethane lining materials have a slightly shorter historical track record than the epoxy linings. Urethane linings were first used in the late 1980s and like the epoxy materials, some of the original applications are still in service. These linings have the advantage of a relatively quick cure; generally, polyurethanes can be put back into service 24 hours after installation. The urethanes are usually applied in one coat with multiple passes. The typical specified dry-film thickness is 60–80 mils (1,500–2,000 µm). The thickness of the elastomeric urethanes is higher because in many cases, they have higher permeance than the epoxy linings. The increase in thickness makes them very useful over pitted and rough surfaces, in some cases eliminating the need for additional surfacers. 

One elastomeric urethane manufacturer has developed a process where carbon dioxide is injected into the liquid coating, causing it to expand. This enables very pitted and rough surfaces to be coated uniformly. The system is finished with the application of a layer of unexpanded material.

No matter which lining system is used, it should have a proven track record. It is the author’s opinion that no lining should be used that does not have a proven service life of at least 15 years. A record of successful use in a similar environment is one of the most important pieces of information an owner can use to select a lining. If the owner has established a qualified products list (QPL), the contractor should be required to provide references for at least three applications where the lining they plan to use (from the QPL) has been in service for at least 15 years.

Candidate lining materials should also be subjected to a number of tests in order to provide reasonable evidence of successful performance. Recommended tests and acceptance criteria are shown in Table 1 for epoxy linings and Table 2 for urethane linings.

Table 1: Recommended Testing and Acceptance Criteria for Epoxies.

Table 2: Recommended Testing and Acceptance Criteria for Urethanes.

While the tests in these tables are important, the performance of the lining in the Atlas cell test and the cathodic-disbondment test are the most critical. For the Atlas cell test, coated test panels are exposed to warm, demineralized water in the presence of a temperature gradient across the test panel. The Atlas cell test is intended to simulate the conditions in an uninsulated steel tank or vessel that has an internal lining and handles hot process fluids. The temperature gradient that occurs across the coating produces a cold-wall effect that tends to draw water into the coating, thereby promoting blistering and disbonding. While the temperature gradient is relatively small between the penstock contents and the wall of the pipe, the test is still very useful for assessing how well the coating will perform in service.

Even though cathodic protection is not usually installed on the interior of a penstock, the cathodic-disbondment test is useful for evaluating a coating’s inherent ability to resist undercutting. This test is conducted by creating a defect in the coating applied to a test panel. The coated panel is immersed and subjected to electrical current. The coating’s resistance to undercutting is evaluated based on the increase in the size of the affected area surrounding the defect after exposure. 


Once the specification is prepared and the project is advertised, the bidding process can begin. The request for proposals should include owner-specific requirements for preparing the cost proposals. Lump-sum costs are acceptable for performing the routine work associated with the project, such as initial pressure-washing, abrasive blast-cleaning, worker and environmental protection, and the lining installation. However, the unique nonquantifiable operations should be bid on a unit basis. For example, additional pressure-washing to reduce nonvisible surface contamination should be bid on a square-foot basis. Similarly, operations such as pit filling and caulking should also be bid on a unit basis (cost per square foot or cost per linear foot).

It is highly recommended that a pre-bid meeting be held, which may or may not be mandatory depending on owner preference, and that bidders be given the opportunity to visit the site and walk down the penstock and power plant prior to preparing their bid. In addition, the bidders can consider material- and equipment-storage areas. Responses to questions that arise during the pre-bid meeting should be distributed to all plan holders. The questions are usually very useful as they often originate from a contractor perspective, specifically focusing on the means and methods of performing the work. Conducting a pre-bid meeting is another step that will assist in minimizing change orders.


Finally, in-process quality assurance (QA) and technical support is essential for achieving project success. If third-party QA inspection is desired (versus using owner staff), the QA personnel should be hired under a separate contract from the coating work; however, it is important that the QA inspectors work in cooperation with the contractor’s QC inspector. Technical support should come from the coating manufacturer and in some cases a coatings engineering firm. No matter what the source, technical support should have broad-base knowledge of the application of linings to large-diameter pipes. 


When the installation of linings to large-diameter cooling pipes and penstocks is performed in an expedient manner, outage time is minimized. When the application is performed correctly it should last at least 20 years without the need for significant maintenance. Several key elements must be considered in achieving a successful lining installation such as proper planning, inspection of the existing lining condition, gathering detailed information about the structure, preparing a project specification, selecting an appropriate lining material, and verifying that surfaces are prepared properly and that the lining system is properly installed.


Ray Tombaugh is a Senior Consultant for KTA-Tator, Inc. He is an SSPC-certified Protective Coatings Specialist, a NACE-certified Coating Inspector (Level III) and an ASTM D4537 Level III Inspector with almost 30 years of experience in the protective coatings industry. He holds a B.S. degree in chemical engineering from Lehigh University.

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