Fri. Sep 24th, 2021

City gas distribution

Transporting future fuel

#80 Holiday Test

6 min read

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.

A holiday test is an inspection method used to detect discontinuities in painted/coated surfaces using specialized tools and equipment. These tools, called holiday detectors, are portable devices that are swept across the coated surface.

Holidays tests work on the concept of electrical conductivity. Metal substrates are excellent conductors of electricity, and therefore allow current to flow through them. On the other hand, many coatings are poor conductors of electricity and resist the flow of electricity. Using this principle, holiday tests use instruments to locate flaws in anticorrosive paints and coatings.

During holiday testing, a ground wire and probing electrode are attached to the same power source. The ground wire is clamped to the specimen being tested while the probe is swept across the surface of the metal substrate. If the probe comes into contact with a coating discontinuity, the exposed metal completes the electrical circuit between the electrode and the grounding wire, resulting in a flow of electricity. This electricity shows up on an indicator, alerting the equipment operator of the defect.

While holiday tests are effective, they do possess limitations. Because these tests depend on the coating being non-conductive, they are not effective on conductive metallic coatings, such as zinc-rich primers.

A holiday test is also known as a continuity test.

Types of Holiday Testing Devices

There are two main types of holiday testing devices: low voltage and high voltage holiday detectors.

Low Voltage Holiday Detectors

Low voltage holiday detection (uses 5 to 90 volts direct current (DC)), also known as the wet sponge technique, is used to test relatively thinly coated substrates. They are effective for coating thicknesses of 500 microns or less. This detection method is suitable for identifying pinhole defects only.

During low voltage holiday detection, the electrode is connected to a damp sponge, which is swept over the tested surface. The ground wire, which is connected to the same power source as the electrode, is clamped to an uncoated part of the substrate.

The pinholes act as capillary tubes that pull moisture from the damp sponge down to the bare metal surface. Because water is a conductor of electricity, the circuit is completed, and the indicator alerts the operator of the defect.

High Voltage Holiday Detectors

High-voltage holiday detectors, as their name implies, operate using higher voltages (greater than 800 volts) than their low-voltage counterparts. This higher voltage allows them to detect holidays in thicker coatings (greater than 500 microns). The electrodes of these testing devices are composed of copper wires or carbon-embedded runners as opposed to sponges.

Like low-voltage detectors, the high-voltage electrodes are swept over the coated surface. The presence of holidays completes the electrical circuit, which triggers the alarm on the indicator. These devices also feature adjustable voltages that can be set depending on the coating’s thickness. The thicker the coating, the higher the voltage needs to be to detect the defect.

Caution needs to be exercised when testing holidays with high-voltage devices because excessive voltages can damage the coating. The voltage used is determined by the coating’s type and thickness. A high-voltage holiday detector can be destructive because the high voltage pulses used to detect holidays can burn or destroy some of the thinly coated areas. Furthermore, high voltages can cause a current to flow even through properly coated areas, resulting in false readings.

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