Friday, May 23, 2014

Tools To Prevent CUI: Protective Jacketting

In terms of preventing CUI, it is worth examining what tools are available to prevent it. There are a number of different tools that can be used, some more effective than others and all with limitations.

The first rule to understanding prevention is to keep water out of the insulation. Regardless of the type of thermal insulation, keeping water out starts with the protective jacketing. 

The quality of the design, specification, procurement, installation, and maintenance of the protective jacketing system is always critical to preventing CUI. Standard 0.016-inch-thick aluminum jacketing, as well as steel sheet jacketing, installed with caulks and mastics, can effectively keep water out of the insulation system. To be effective, it is critical for everyone involved (the general contractor, insulation contractor, design engineer, and facility owner) to make certain that no shortcuts are taken in design, material specification, and installation. Both conventional aluminum jacketing and steel jacketing can be effective at keeping out the intrusion of water and preventing CUI. Hence, protective jacketing is the most important tool in the CUI prevention toolbox.

A new type of protective jacketing material that is seeing increasing interest is multilaminate, pressure-sensitive jacketing that can be purchased for either field installation or can be purchased factory applied to certain types of insulation. This family of materials essentially consists of industrial-grade tapes, available in 3 foot widths, that are weather resistant; impermeable to water or water vapor; resistant to many chemicals; and able to seal tightly with their pressure-sensitive, “peel-and-stick” surfaces. 

Some of these are available in industrial-grade weights with a thickness of almost 0.016
inches. An important accessory to making the system effective in keeping out water is a 2- to 4-inch-wide roll of tape to seal the joints and the penetrations. This type of material offers a jacketing that can be adhered to the insulation, thereby preventing moisture from accumulating between the jacket and the insulation. Its flexibility allows for easier installation and sealing at joints and penetrations as well as at termination points, making it very effective at keeping out water. Therefore, wide multi-laminate tapes should be included in the CUI prevention toolbox.

A third type of jacketing, that is durable and effective at keeping insulation beneath it dry, is a glass fiber lagging cloth with an acrylic weather coating mastic. This type of jacketing has the advantage of being able to seal penetrations effectively, to prevent water leakage. Another advantage is that is can be extremely durable, as well as being water proof. Therefore, lagging cloth with an acrylic weather coating mastic should be included as a tool in the CUI prevention toolbox.

Monday, May 12, 2014

Alternative Inspection Methods to detect CUI

The present corrosion under insulation detection methods are:
Profile Radiography
Figure 1Exposures are made of a small section of the pipe wall. A comparator block such as a Ricki T is used to calculate the remaining wall thickness of the pipe. The exposure source is usually Iridium 192, with Cobal 60 used for the pipes of heavier wall. (See Figure 1).
Profile radiography is an effective evaluation method, but becomes technically challenging in piping systems over 10 inches (25.4 cm) in diameter and only offers the limited luxury of verifying relatively small areas. This technique will not detect CISCC in stainless steels. In addition, readiation safety can be a real concern. Nobody can work within the area while the inspection is under way, this can result in downtime and manpower scheduling conflicts.
Ultrasonic Thickness Measurement
Figure 2This is an effective method, but limited to a small area (Figure 2). It is expensive to cut the insulation holes and cover the holes with caps or covers. It is not practical to cut enough holes to get a reliable result. The inspection holes cut in the insulation may compromise the integrity of the insulation and add to the corrosion under insulation problem, if they are not recovered carefully. This technique will not detect CISCC in stainless steels.
Insulation Removal
The most effective method is to remove the insulation, check the surface condition of the pipe, and replace the insulation. This approach will detect CISCC in stainless steels; may require eddy current or liquid dye penetrant inspection. This is also the most expensive method in terms of cost and time lost. The logistics on insulation removal will probably involve asbestos and its attendant complications. Process related problems may occur, if the insulation is removed while the piping is in service.
In the right conditions, infrared can be used to detect damp spots in the insulation, because there is usually a detectable temperature difference between the dry insulation and the wet insulation. Corrosion is a distinct possibility in the areas beneath the wet insulation.
Neutron Backscatter
This system is designed to detect wet insulation on pipes and vessels. A radioactive source emits high energy neutrons into the unsulation. If there is moisture in the insulation the hydrogen nuclei attenuate the energy of the neutrons. The instrument's gauge detector is only sensitive to low energy neutrons. The count displayed to the inspector is proportional to the amount of water in the insulation. Low counts per time period indicate low moisture presence.
Real-Time Radiography
Figure 3
Fluroscopy provides a clear view of the pipes outside diameter trhough the insulation, producing a silhouette of the pipe outside diameter (OD) on a TV-type monitor that is viewed during the inspection. No film is used or developed. The real-time device has a source and image intensifier/detector connected to a C-arm (see Figure 3). There are two major categories of RTR devices on the market today; one using a X-ray source and one using a radioactive source. Each has its own advantages and disadvantages, however the X-ray systems deliver far better resolution than the isotope type equipment.
The X-ray digital fluroscopy equipment operates at a maximum of 75 KV, a low level radiation source, but the voltage is adjustable to obtain the clearest image. this allows for safe operation without disruption in operating units or even confined spaces. The radiation does not penetrate the pipe wall as more powerful gamma-ray or x-ray would, instead it penetrates the insulation and images the profile of the pipe's outside wall. The radiation is generated electrically so the instrument is perfectly safe when the power is off, whereas the Iridium 192 used in wall shots produces gamma-radiation constantly, even when shielded within the camera. Therefore the gamma-ray camera always needs careful supervision and control during all operations, including transportation and shipping. The systems with the electrically generated X-rays are far more convenient for shipping.
The new systems come with a heads-up, video display. The helmet-mounted, visor-type video-display fress the system operator's hands so that he can maneuver the C-arm, while keeping the image before the operator at all times. The heads-up display also improves interpretation by shielding the screen from the sun. The video images can be printed on site using a video printer or recorded using a standard VCR for evaluation later.
Performing the Inspection
Using the sorting criteria listed above it is possible to prioritize a list of piping for inspection that is manageable in a reasonable time fram. The CUI inspection crew then inspects the pipes iso by iso.
The "C" shaped arm is the actual device used to scan the pipe. A cathode ray tube on one side generates the x-rays, shooting them across to the receiver on the other side. The operator manipulates the arm around the pipe, guiding it by the black and white heads up display on his hard-hat. A typical scan will go up the pipe while moving the arm about 45 degrees to both sides of the track. The C-arm is then rotated 180 degrees and the pipe is scanned downward in a similar fashion. After rotating 90 degrees the up and down process is repeated.
Figure 4To the untrained eye, the image in the screen would appear to indicate very serious corrosion. However, what is being imaged is the exfoliation of the rust (See Figures 4 and 5.) Performing the inspection in this manner the inspector can inspect a considerable amount of pipe in a short time.
One of the main limitations of the system is the C arm. There are a couple of sizes of C-arms available. The manufacturer has had success in checking pipes up to 24 inches in diameter. These systems were not originally designed for the field but rather for laboratory work. This limitation has been addressed and the systems available today are more robust. However, they still require a lot of care and attention. There will always be some percentage of piping where real-time X-ray can not be used. The prime example is the center lines among tightly nested pipelines with little clearance between the pipes. Finally, while the X-rays are low energy, they are still radiation, and so the system must be used with extreme caution.

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Wednesday, May 7, 2014

When Does Corrosion Under Insulation Occur?

The problem occurs on carbon steels and 300 series stainless steels. On the carbon steels it manifests as generalized or localized wall loss. With the stainless pipes it is often pitting and corrosion induced stress corrosion cracking (CISCC). Though failure can occur in a broad band of temperatures, corrosion becomes a significant concern in steel at temperatures between 32 F (0 C) and 300 F(149C). Corrosion under insulation is caused by the ingress of water into the insulation, which traps the water like a sponge in contact with the metal surface. The water can come from rain water, leakage, deluge system water, wash water, or swearing from temperature cycling or low temperature operation such as refrigeration units.
Systems Susceptible to CUI
API 570 specifies to the following areas as susceptible to CUI:
  • Areas exposed to mist overspray from cooling water towers.
  • Areas exposed to steam vents
  • Areas exposed to deulge systems.
  • Areas subject to process spills, ingress of moisture, or acid vapors.
  • Carbon steel piping systems, including those insulated for personnel protection operating between 25 F and 250 F. CUI is particularly aggressive where operating temperatures cause frequent condensation and re-evaporation of atmospheric moisture.
  • Carbon steel piping systems that normally operate in-service above 250 F (120 C) but are in intermittent service.
  • Deadlegs and attachments that protrude from insulated piping and operate at a temperature different than the active line.
  • Austenitic stainless steel piping systems that operate between 150 F and 400 F (60 C and 204C). These systems are susceptible to chloride stress corrosion cracking.
  • Vibrating piping systems that have a tendency to inflict damage to insulation jacketing providing a path for water ingress.
  • Steam traced piping systems that may experience tracing leaks, especially at the tubing fittings beneath the insulation
  • Piping systems with deteriorated coatings and/or wrappings.
  • Locations were insulation plugs have been removed to permit thickness measurements on insulated piping should receive particular attention
All equipment will be shut down at some time or other. The length of time and the frequency of the downtime spent at ambient temperature may well contribute to the amount of corrosion under insulation that occurs in the equipment. It would be a daunting task to muster the resoures needed to tackle this extensive list of piping with the traditional inspection methods. This is where real-time X-ray offers a real advantage. Once the damaged areas are indentified, follow-up X-rays and ultrasonics can measure the loss by external corrosion. These techniques will not detect CISCC in stainless steels.
Alternative inspection methods will be discussed in our next blog...
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Happy Deepavali

The festival of light is here! May you be the happiest and may love be always with you. Happy Deepavali!