Sunday, April 27, 2014

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.

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Thursday, April 24, 2014

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.

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Monday, April 21, 2014

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.

Saturday, April 12, 2014

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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Friday, April 4, 2014

What Causes Corrosion Under Insulation (CUI)

The problem of CUI of process plant and pipework is associated mainly with steel components. Failures have occurred, with loss of containment, due to localised corrosion progressing undetected beneath insulation.

Steel corrodes when it is in contact with water and has a free supply of oxygen. When plant and pipework are insulated there is usually a space in which water can collect on the metal surface with access to air. The ingress of water into the insulation is often caused by one or more of the following:
  1. poorly designed and/or installed protective finish or cladding;
  2. cladding joint sealant breakdown;
  3. mechanical damage to the protective finish; or
  4. cladding removed and not properly replaced (common around valve boxes).
When water penetrates the insulation it tends to collect at low-lying sections of plant and pipework and around discontinuities, e.g.-
  1. the base of vessels;
  2. pipework supports;
  3. the intersection between nozzles and vessels;
  4. the underside of elbows and horizontal pipe runs; and
  5. drain legs.
CUI may be influenced by the chemical nature of the insulation material. Some cladding materials contain free chlorides which may promote CUI. Chloride stress corrosion cracking should be considered for austenitic stainless steel at temperatures of 65oC and above. This is of particular importance for coastal sites and offshore installations where chloride contamination will be enhanced. For high carbon stainless steel grades at temperatures of 50oC and above, intergranular attack and chloride stress corrosion cracking cannot be ruled out. Insulating materials applied on zinc-rich coatings should preferably have neutral pH values as the coating may deteriorate in the presence of water with low or high pH values.
Surface coatings may provide protection against corrosion if the insulation becomes wetted, though proper selection and application are particularly important. Coatings should be compatible with the insulating material and suitable for use at the anticipated temperature. Coatings applied to poorly prepared or hot surfaces are much more likely to break down. Particular attention should be given to proper application at welds
Experience indicates that many factors influence the risk of CUI including whether trace heating is installed (a high risk factor). In particular, operating temperature greatly affects the risk of CUI. The following table indicates the likely risk of CUI for carbon steel pipework, without trace heating, under various operating regimes:

At low temperatures the corrosion mechanism is suppressed and at elevated temperatures moisture in the insulation material should evaporate during start-up. However, even where the risk of CUI is generally considered low, the increased risk during periods when the plant is shut down should not be discounted. The problem may be exacerbated by frequent plant cycling between operating and shutdown conditions.
In general terms, experience has shown that the most critical temperature range for CUI is 30oC-120oC. US data for carbon-manganese steel indicates typical corrosion rates of 0.5 mm/year at 80oC under insulation. 

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