A pipeline coating is a cost effective and viable solution to maintain pipelines’ integrity. This coating provides a constant protective lining that helps save pipelines from the damaging effects of corrosion. Pipeline coating is one of the most reliable corrosion prevention methods used by industries today.
Pipeline coatings are typically applied to counter the negative effects of corrosion on pipeline operation, capacity and costs.
Uncoated pipelines that are submerged in seawater are prone to corrosion. Although the corrosion debris from these pipelines can be removed, the disposal of these fragments can be detrimental to the environment. Therefore, strict regulations have been implemented, including the practice of applying pipeline coatings. This prevents the formation or reformation of corrosion. Pipe coating application is considered a standard practice and provides the following benefits:
- Improved gas flow – A smoother surface results in enhanced flow capacity. Various studies have demonstrated that flow capacity of coated pipelines is far better than uncoated ones.
- Faster inspection and commissioning – Coated pipework dries faster than uncoated pipes. This means that commissioning can be faster and easier on the line. Any type of robotic inspection is also simplified with the enhanced mobility of equipment through a coated pipeline.
- Decreased cost of energy – This is especially true in terms of compressors and pumping stations. Pipelines that are internally coated could create a vast difference in lowering the costs of compression and pumping over the pipe’s lifespan. This can possibly increase financial payback in three to five years, which means significant savings.
Other than these benefits, pipeline coating can also reduce the need for inhibitors and promote clean delivery of the product. Thus, this can serve as a cost-effective and low-maintenance option for corrosion control that offers ample and reliable protection.
Holiday Test :
Holiday detection is an inspection procedure used to locate discontinuities or “holidays” in a coating film. A holiday is a hole or void in the coating film which exposes the substrate to corrosion. The term holiday dates back to the days of wooden sailing ships. A sailor would seal the mast of the ship with tar to prevent the wood from rotting. If the sailor missed coating an area of the mast the wood would rot. It was said, the sailor must have taken a “holiday” at that point, thus not achieving a good coat of tar on the mast. Typically, holiday inspection is performed using specialized equipment divided into two main groups; low and high voltage testers. Low voltage holiday testers are used for coatings up to 20 mils DFT. High voltage testers produce up to 40,000 volts of pulsed DC current. These units are used to test thick film coatings such as those used by Raven applicators. There are several industry standards regarding holiday testing including ASTM D-4787 & D-5162 and NACE RPO 188-88. In most cases Raven coatings are applied at 40 to 250 mils in thickness. The accepted rule of thumb for a starting point test voltage is to use 100 volts per mil of coating thickness. Therefore, for 125 mils, a voltage of 12,500 volts should be used to begin with. The unit should be tested by dragging the electrode over a known or induced (a hole drilled in the coating to the substrate) holiday. If the unit does not detect the holiday, voltage and/or sensitivity should be increased until it the audible or visual alarm indicates a holiday. It is also important to ensure that the grounding cable is effectively connected to the substrate in order to complete the circuit. On concrete substrates this may be difficult depending upon the moisture content and conductivity of the concrete. The grounding cord may be connected to bare concrete using a method such as a wet sandbag with a piece of wire window screen, attaching to exposed rebar, or driving a masonry nail or screw into the concrete. If the conductivity of the concrete is not sufficient to complete the circuit, holiday testing may not be feasible. In such cases a conductive primer may be applied prior to the protective coating application. Alternatively, a close visual inspection should be performed and all possible holidays repaired. A properly calibrated test unit, having a good ground and conductive substrate, will signal holidays via the units’ audible or visual alarm. Depending upon the environment and the conditions present on the coating surface, there may be small visible electrical discharges which should not be considered holidays unless the test unit senses the completion of the circuit to the substrate as indicated by the alarm. Testing should not be performed on coatings which have been exposed to immersion service as the moisture content of the coating may provide erroneous results or possible coating damage. Testing should be performed strictly according to accepted standards and by qualified and trained personnel. Holiday testing in no way detects or confirms coating thickness and should not be used for such purposes.
Listed below are critical excerpts from the ASTM and NACE standards:
ASTM D-4787
This practice is intended for use only with new linings applied to concrete substrates. Inspecting a lining previously exposed to an immersion condition could result in damaging the lining or produce an erroneous detection of discontinuities due to permeation or moisture absorption of the lining. Deposits may also be present on the surface causing telegraphing. The use of a high voltage tester on a previously exposed lining is not recommended because of possible spark through which will damage an otherwise sound lining. A low voltage tester can be used but could produce erroneous readings.
High Voltage Spark Tester—an electrical detector with a voltage rating in excess of 800 V. The detector is to consist of an electrical energy source, an exploring electrode, a ground connection, and ground wire. The detector shall be equipped with a visual or audible indicator, or both.
Visual or Audible Indicators, or both, to signal a closed electrical circuit. Such signals shall be essential for testing the underlayment for electrical conductivity and for exposing discontinuities in the lining after it has been applied.
The surface of the applied lining shall be clean, dry, free of oil, grease, dirt, or other contaminants and be sufficiently cured in accordance with the manufacturer’s latest published instructions at the time the testing is performed.
Attach the ground wire from the instrument ground terminal to the conductive underlayment . Make contact with the exploring electrode at a known discontinuity to verify that the instrument is properly grounded. For each ground location, make contact with a known discontinuity. A discontinuity may be produced by drilling a hole through the lining with a 1⁄16-in. (1.59-mm) diameter drill bit. Conduct this test periodically during the test. With the exploring electrode in continuous contact with the lining surface, move it over the entire surface of the lining at a rate of 1 ft/s (0.3 m/s) maximum in a sweeping motion with overlapping passes to ensure that the entire surface has been subjected to the test. Identify discontinuities that require repair with a compatible marker. Completely test the lining one time only. Repair all defects found in the lining and retest only those repaired areas.
RPO 188
A high-voltage (in excess of 800 V) spark tester is an electronic device used to locate discontinuities in a nonconductive protective coating. It consists of an electrical energy source, an exploring electrode, and a ground connection from the indicator signaling current flow through a coating film discontinuity to the substrate.
The high-voltage spark tester shall be adjusted to the proper voltage for the coating thickness being tested. In selecting the inspection voltage, sufficient voltage shall be provided to break the air gap that exists at the holiday. This air gap varies depending on the total applied film thickness. Excessive voltage may produce a holiday in the coating film. The maximum voltage for the applied coating shall be obtained from the coating manufacturer.
Touch the exploring electrode to the ground-cable alligator clip. The tester signal should actuate in accordance with the manufacturer’s operating instructions.
If the tester fails to signal, it shall be considered inoperative.
The exploring electrode shall be moved over the surface of the dry coating at a rate of approximately 0.3 m/s (1 ft/s) using a single pass. Moisture on the coating surface can cause erroneous indications. If moisture exists, it shall be removed or allowed to dry before the test is conducted.
