Alternative Inspection Methods to detect CUI

The present corrosion under insulation detection methods are:

Profile Radiography

Exposures 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

This 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.

Infrared

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

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.

Results

To 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.

Limitations

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.

Source:https://inspectioneering.com/content/1996-11-01/116/inspection-techniques-for-dete

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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...

Source:https://inspectioneering.com/content/1996-11-01/116/inspection-techniques-for-dete

For more information about Corrosion and how to prevent it, contact us via our website at www.leopad.com

Newsletter: Lumut Branch April issue

Corrosion Problems Faced by the Industries in the Oil & Gas Sector

We already know that an oil or petroleum refinery is a place where natural, raw oil is processed into refined products like petroleum, gasoline, heating oil, diesel fuel, kerosene, asphalt base and liquefied petroleum gas. 

All these involve a lot of processes, and hence the refinery is somewhat similar to that of a chemical plant. All the crude oil feedstock that people talk about are got from oil production plants. An oil depot or a tank farm is usually seen near these oil plants that store liquid products that are produced. 

The management of these sectors are very important in our world today. But the real bad story about these industries is when they are affected by corrosion and the expenditure caused to rectify it. In all places around the world, the industry of oil and gas does its level best in running these refineries as efficiently and trouble free as possible to reduce costs in this sector. One cankerous factor that is eating up these industries is corrosion. Corrosion is seen lining the oil plants that occur during hydrocarbon refining process. Corrosion which is commonly said to be the break down of a metal into its atomic form due to chemical reactions with its surroundings. 

The effect of corrosion in an oil industry leads to the failure of parts which in turn leads to shut down, again to avoid this effect the plant has to be shut down to clean the facility. The amount spent in the cleaning and adverse effects of corrosion on a year amounts to US$ 3.7 billion. Well, this is a large sum and could be put to use in so many other ways, if only we could avoid the effects of corrosion. 

There are various ways and reasons for the occurrence of corrosion. By knowing the causes we could come to a conclusion on the prevention of the occurrence. 

Some reasons for the occurrence of corrosion in Oil and Gas Industries:

- are water droplets that cause pitting corrosion

- the steel becomes brittle from the exposure of  heat and impurities from hydrogen

- and sulphide attack causing stress corrosion

Some of the ways adapted today to overcome the effects of corrosion are:

- Use of certain types of metal in certain areas of the plant to facilitate the plant to be longstanding inspite of the effects of corrosion

- For example, carbon steel is used for 80% of plant requirements as it is cost efficient and withstands most forms of corrosion due to hydrocarbon impurities below a temperature of 205 degree celsius. But as it is not able to resist other chemicals and environments, it is not used every where

- Other kinds of metals used are, low alloys of steel containing chromium and molybdenum, and stainless steel containing high concentrations of chromium for excessively corrosive environments

- Tougher metals like  nickel, titanium, and copper alloys are used for the most corrosive areas of the plant which are mostly exposed to the highest of temperatures and the most corrosive of chemicals

Since, of late, with the increase in knowledge in the field gained over the years, it has become a lot easier and more effective in fighting the occurrence or corrosion. Some effective ways in preventing corrosion would be monitoring and preventing the occurrence of these effects even before they occur. "How do I do this" may be the question on your mind. 

Monitoring is done both on-line and off-line. Offline monitoring refers to the monitoring of the effects of corrosion after it has happened and is usually identified while doing maintenance. Of course this isn't the most effective way of saving the plant but, this was used in older times.  It used to be considered as a step taken that is better late than never. Today on-line system has evolved to make it more effective in  solving problem before it gets worse. 

Some of the methods of  on-line corrosion monitoring are:

- Linear polarization resistance

- Electrochemical noise

- Electrical resistance

Previously, the processing of corrosion on-line seemed to be very slow and the details

given weren't enough to fill in the criteria required besides the data collected was not always accurate. 

However, with the improvement in science we have newer technologies that give data twice as much as they used to be in the past and specific in their accuracy. This kind of assessment is known as real-time monitoring. This enables engineers to prevent and control corrosion and also enable better output by the plant. Any plants that have on-line corrosion information with accurate real time measurements help to detect and reduce high corrosion rates. These kind of technology advancements enable engineers to prevent and protect the plant by management systems called predictive management.

For more information about Corrosion and how to prevent it, contact us via our website at www.leopad.com

Source: http://www.materialsdigest.com/index.php?page=article/show&id=140

The Role of the Insulation System in CUI

It’s a common consensus among experienced people dealing with CUI that dry insulation

systems simply don't exist in the long run. They therefore tend to be regarded as a bad influence. In addition, people have believed for decades that cladding/jacketing is 100 percent weatherproof and, in combination with elevated service temperatures, that water or moisture could never get trapped. This idea resulted in many cases in which piping, other that the basic shop primer, wasn't even extra coated.

Cladding/jacketing is primarily designed as weatherproofing and not as a vapor barrier. Depending on service temperature and ambient conditions, condensation within the insulation system may not be avoidable, and therefore needs to be addressed in the engineering phase. In other situations, water enters into the insulation system through failed or broken cladding/jacketing. This can be caused by:

• Foot traffic
• Inadequate design
• Incorrect installation
• An insufficient maintenance strategy

Overlooking all possible CUI causes in relation to a consequence of the failure of piping systems or equipment, there's a justification for challenging the need for insulation.

The oil crisis during the 1970s brought new insights on energy savings and resulted in other design criteria for thermal insulation for the (petro)chemical industry. In some cases, this resulted in excessive insulating, which was not always economically feasible. However, recent geopolitical CO2 reduction goals persuaded many asset-owners to re-evaluate these old goals and translate them into new company policies.

Below is a flow diagram that provides a few logical steps to understand whether insulation is necessary or could be replaced.

For more information, visit our website at www.leopad.com

Source:http://www.corrosionpedia.com/2/1372/corrosion/corrosion-under-insulation-the-challenge-and-need-for-insulation

How Corrosion Under Insulation Could Be Destroying Your Pipes

Thermal insulation is used to reduce heat loss from equipment operating at elevated temperatures. There are three reasons for this:

• To keep a product at a desired temperature
• To reduce energy costs
• For personnel safety

Protected equipment is frequently exposed to adverse weather, so insulation must be waterproof.

Corrosion Under Insulation

Measures taken to keep moisture out of mechanical insulation products like fiber glass and rockwool are only partially successful, in part due to continual movement caused by expansion and contraction of the equipment.

In exposed locations where humidity is high, it’s almost inevitable that moisture will seep through to the protected surface and, once there, it rarely evaporates, especially when the equipment being protected is exposed to adverse weather.

These conditions are ideal for corrosion under insulation (CUI) to take place. Even where an anti-corrosion paint layer has been applied, it is not long before localized pitting corrosion can start.

Consequences of Corrosion Under Insulation

Pitting corrosion is insidious, difficult to detect, and usually continues until the vessel being protected is holed. Pitting corrosion happens quickly and serious damage can occur within a relatively short time.

A National Board of Boiler and Pressure Vessel Inspectors article on preventing corrosion under insulation points out that pitting corrosion leads to significant losses due to product spillage and fires.

Methods of Preventing Corrosion under Insulation

Traditional methods of preventing CUI focus on regular maintenance of the outer skin of the mechanical insulation, supplemented by regular physical inspection.

Source:http://www.ncmiinc.com/corrosion-under-insulation

How to Detect Corrosion Under Insulation (CUI)

The inspection of plant and pipework for CUI can be extremely expensive if all

insulation material has to be removed. 

Often windows are cut into the insulation for localised inspection. However, where selected areas of plant are exposed, confidence in the effectiveness of the examination depends on the ability of the inspector to identify the critical sections of pipework or plant.

In an attempt to improve the selection process, a number of non-destructive testing (NDT) techniques can be employed to detect corrosion under insulation or to identify areas that may be susceptible to corrosion without the need to remove insulation. Each technique has advantages and disadvantages and they have different capabilities. The inspector should understand the capability and limitations of any technique applied. 

The results of the NDT examination can then help to target problem areas where further examination may be necessary. However for critical plant/pipework where no leaks are acceptable full insulation removal may be required to secure proper inspection.

Examples of some NDT techniques available for detecting corrosion under insulation include:

1. Pulsed Eddy Current

The decay of an eddy current pulse is monitored within a ferritic pipe or vessel and the signal is used to calculate the remaining wall thickness beneath a coil unit. This technique can indicate areas of localised corrosion averaged over the area of the sensor. There are limitations on the thickness and type of insulation through which the eddy currents can penetrate.

2. Guided Wave Ultrasonics

This remote screening technique can be used to look for degradation of internal or external pipe surfaces. It can be used where access is restricted and is suitable for long pipe runs. It can detect losses of cross-sectional area of 10% and upwards and is useful to identify areas for more detailed examination.

3. Flash radiography*

This is an established technique which produces high energy X-rays in very short pulses of about 50 nano-seconds duration. When used for detecting CUI, a tangential exposure is made using very fast film. A variation of the system was developed using an unshielded hand-held 'gun' that allowed unacceptable levels of radiation exposure to operators. HSE radiation specialists have considered that this adaptation of the technique should not be used.

4. Real-time imaging radiography*

Developments in real-time radiography have been rapid in recent years and mobile systems are now available to carry out on-site monitoring with a direct visual display of the image. A specially built, shielded hand-held device is also available with this system that, it is claimed, gives negligible exposure to the operator.

Examples of some NDT techniques available for detecting moisture under insulation include:

1. Thermal imaging (thermography)

Detects temperature variations and has been widely used to detect breakdowns of thermal insulation of cryogenic storage vessels and thermal linings of furnaces etc. Areas of damaged and waterlogged insulation can be detected using this technique. Thermography does not give a definitive indication of corrosion, but highlights areas where corrosion may develop in the future. The possibility of intermittent wetting and drying-out of insulation, especially on hot plant and pipework, leading to a misleading result needs to be kept in mind.

2. Neutron Backscatter*

Neutron backscatter devices (hydrodetectors) can be used to locate areas of wet insulation on vessels and pipework, which are potential CUI sites. The system comprises a neutron source and detector assembly on the end of a telescopic pole allowing access to hard to reach areas. Typical screening rates are around 300m of insulated pipework per day.

*Activities subject to the Ionising Radiations Regulations 1999.

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source:http://www.hse.gov.uk/foi/internalops/hid_circs/technical_general/spc_tech_gen_18.htm#detection