Wednesday, February 18, 2015

A Primer on Sulfidation Corrosion

High temperature sulfidation is probably the most well-known corrosion mechanism in the oil refining industry because it occurs in large sections of the refinery.  Sulfidation corrosion (also known as sulfidic corrosion) is a result of naturally occurring sulfur (S) compounds found in crude oil.  In the absence of hydrogen, corrosion due to sulfur compounds in the crude is thought to occur at temperatures above 500F (260C).  Up to that temperature, corrosion rates due to sulfidation are relatively low, even for carbon steels, unless there is naphthenic acid present in the crude.  

Sulfidation corrosion results in the thinning of the pressure containment envelope, affecting such components as piping and pipe fittings, heater tubes, and pressure vessels.  Most industry incidents have occurred in piping, due to lower nominal wall thicknesses as compared to other equipment types.  Sulfidation corrosion can be localized or general in nature for a given component, though the majority of cases exhibit general thinning.   

When the damage is general and thinning occurs over a large area, ruptures are possible and can lead to the potential release of large quantities of hydrocarbon streams. Corrosive thinning of piping walls and equipment due to sulfidation depends on many factors, such as sulfur content of the oil, temperature, flow rate, and H2 concentration, making it hard to predict accurately. However, one predictor of higher sulfidation rates is low silicon content in carbon steel components. 

Susceptible Areas

According to the American Petroleum Institute (API) Recommended Practice 939-C (Guidelines for Avoiding Sulfidation Corrosion Failures in Oil Refineries), one-third of high-temperature sulfidic corrosion failures are the result of low silicon content in piping. Moreover, older pipe with low Si may corrode at rates 2 to 10 times faster than surrounding higher Si piping. Recent industry experience indicates that carbon steel components (primarily pipe) and their respective welds with low-Si (< 0.10 wt. %) content can corrode at an accelerated rate when exposed to H2-free sulfidation corrosion conditions. Piping welds, where the root passes were made with low-Si containing filler metals, have been shown to corrode at a greater rate than the base metal components when exposed to the same process conditions.


Friday, February 13, 2015

Portable Pipe Wall Thickness Measuring Technique "CUI Exposed"

One of the greatest challenges facing many of the process industries; namely the petrochemical, refining, fossil power, and pulp and paper industries is: How to effectively examine their insulated piping?

While there are a number of failure mechanisms involved in various process piping systems, piping degradation through corrosion and erosion are by far the most prevalent. This degradation can be in the form of external corrosion under insulation (CUI), internal corrosion through a variety of mechanisms, and internal erosion caused by the flow of the product through the pipe.

Refineries, chemical plants and electrical power plants have MANY thousands of miles of pipe that are insulated to prevent heat loss or heat absorption. This insulation is often made up of several materials, with calcium based material being the most dense. The insulating material is usually wrapped with an aluminum or stainless steel outer wrap.

A partial sampling of the most popularly used detection techniques are described as follows. As with anything, they have caveats, benefits, and limitations. Check with your NDE specialist to make sure you understand the availability, limitations, and benefits of the various options.

Brute forcing

The least complex way to detect CUI, brute forcing involves simply stripping the insulation off of the equipment and examining it for corrosion. This is a comparatively time-consuming, fairly expensive work process, especially if the insulation contains asbestos, so it may not be suitable for all situations.

Conventional radiography

This is the most common NDE technique used for detecting CUI without insulation removal. Conventional radiography involves a process where radioactive rays are directed at the object to be inspected, passing through it and capturing the image on a silver halide film to be examined. It has numerous advantages, including: it can be used with insulation of any thickness or type, through many types of internal product (some cases might require very high radiation sources), on pipes of varying diameters, and on both thick and thin wall pipes. Check with your NDE specialist to make sure conditions permit a valid “shot”.

Digital radiography

As opposed to conventional radiography, digital radiography relies on exposing reusable storage phosphor screens as an alternative traditional silver halide film. This allows the information to be stored digitally, saving both time and storage space. It also requires less radiation due to the phosphor film and therefore has a reduced impact on the safety compared to conventional radiography. Consult with your radiation safety specialist to make sure you fully understand the safety impact prior to establishing the Safe Zone.

Low intensity x-ray

The low-intensity x-ray imaging scope is a hand-held, totally portable fluoroscopic device utilizing a low-energy, low-intensity gamma source of Iridium. This can be a very quick way of qualitatively screening pipe for CUI. Iridum-192 is a typical radiation source for this technique.

Pulsed eddy current (PEC)

This method has been used in corrosion detection for several years and is highly useful in situations where an object’s surface is rough or inaccessible. Moreover, this method does not require surface preparation or the removal of insulation, thus it can be a quick and cost-effective solution for corrosion detection. The method works by sending out a pulsed magnetic field via probe coil, which penetrates through the non-magnetic insulation between the probe and the object being inspected. This will induce eddy currents that can be measured to determine whether or not corrosion is present.

Guided-wave ultrasonics (GWUT)

This method of testing involves sending guided waves out along the axial direction of a pipe and then measuring the reflections for echoes, which might be caused by corrosion. The main advantage of this method is that it is possible to inspect supports that are not directly accessible for visual inspection. The downside of the method though is that the actual accuracy of the results are strongly dependent on how good the inspector and the testing procedures used are; thus an inexperienced inspector can lead to inaccurate results.

Ultrasonic thickness measurements

This process can be used to determine the external condition of vessels and remaining thickness of piping components. It works by sending ultrasonic waves into the surface of the object and measuring the time taken by the wave to return to the surface. It is a fairly simple technique. However, it does require adequate contact with the material, so it is not viable for every situation.

Other techniques, such as neutron back scatter and infrared thermography, can help to find moisture under insulation, which may then help detect where CUI is occurring as well. These methods infer that there is potential CUI activity, as opposed to directly detecting metal loss or cracking


Monday, February 9, 2015

New Inspection Tool - Corrosion Under Insulation (CUI)

In May 1995, Omega International Technology, Inc., began testing a new system to measure pipe wall thickness using digital radiography (RT) scanning. This new system has the potential for being faster, less labor intensive, and shown improved accuracy over traditional ultrasound testing, and at a lower cost. Perhaphs best of all, scanning can be performed while the pipe is in service, insulation in place.
Digital radiography refers to the process of producing and analyzing X-Ray images using electronic devices instead of traditional film. For twenty years difital radiography, in the form of Computed Tomography (CT) scanners have been used to peer inside the human body. In the past decade, digital readiography has been used to inspect critical components for military and industrial applications. Even more recently, systems employing digital radiography are being used on processing lines in the food industry as an integral part of their quality assurance programs.
The concept behind digital radiography is similar to taking an X-Ray at a doctors office. A source, such as an X-Ray tube, sends a beam of high energy photons through the object of interest (see Figure 1). Some of the photons are absorbed by the object and the rest pass through it. The relative number of photons that are absorbed is directly related to the amount of material in the path of the photons. On the other side of the object, an array of defectors measure the number of photons passing through (the signal), producing an image on the inside of the object.
Figure 1
Traditionally, digital radiography has employed X-Rays to pass trhough objects, as opposed to other types of photons, such as Gamma Rays. X-Ray generating systems are bulky, require electricity, and are not very portable. Readioactive isotopes, however, are small, inexpensive, and easy to maintain. Unfortunately, readioactive isotopoes also tend to release much less energy than X-Ray systems. Most conventional detectors do not have the sensitivity to use radioactive isotopes to produce images. A new generation of detectors developed by Omega, however, are able to use very low intensity Gamma ray signals to measure pipe wall thicknesses.
The system employs an Iridium[+192] Gamma ray source and the new scintillator-based detectors instead of an X-Ray system and traditional detectors (see Figure 2). Coupled with a portable computer system, running data analysis and storage software, the entire system is mounted onto a pipeline and allowed to travel under its own power along the length of the pipe. The system automatically scans the pipe and computes the pipe wall thickness.
Figure 2

Monday, February 2, 2015

Corrosion Under Insulation: I Wonder What's Going On Under There?

Corrosion is one of those "equal opportunity" hazards that affects all industries indiscriminately, to the tune of billions of dollars annually in repair and replacement costs. Some types of corrosion are readily apparent, such as rusting of unprotected plain carbon steel tanks and piping. But when corrosion occurs in hidden places as it so often does, it may go undetected, sometimes with catastrophic results. Two common and costly examples, hidden from view under insulation, are non-uniform attack of plain carbon steels and stress corrosion cracking of 300 series stainless steels.
Because corrosion under insulation occurs out of sight, it frequently is out of mind until a leak occurs, producing a release. Personnel, the environment, plant process uptime and system integrity all may be impacted adversely, as a result.

CUI happens when water is allowed to enter an insulated system or component and contact the underlying surface. This water can originate as rain (CUI is predominant in high humidity, high rainfall coastal areas), run-off from equipment washdowns, deluge systems tests, condensation from temperature cycling and leakage from aqueous process systems. A minute low pressure stream leak from a steam tracer tube fitting or valve stem packing under insulation can cause all sorts of problems. Furthermore, corrosion and cracking are accelerated by corrosive salts such as chlorides which are leached from insulation materials by intruded water.

We know how to prevent CUI. It requires a fundamental systems approach, starting with a good protective coating suitable for aqueous immersion service, such as a catalyzed epoxy-coal tar or epoxy-phenolic, on the bare metal substrate. This is followed by installation of dry insulation under dry conditions, all wrapped with protective metal or non-metallic jacketing designed to exclude water, not collect it. Finally, the system must be monitored and maintained to keep it dry, corriosion-free and thermally efficient. Don't expect caulked seams to stay tight and resilient year after year. Don't expect to keep water out if you drill access holes in insulation for ultrasonic wall thickness measurements, and forget to seal the holes. I think you'll agree, the need for monitoring and maintenance can't be over emphasized.

Unfortunately, there's a lot of insulated process piping and equipment in our chemical pants and refineries which never received immersion grade protective coatings before insulation was applied, and the unsulation system was poorly designed/installed/maintained, and trouble in the form of corrosion/cracking is brewing. So it behooves us to find the trouble spots before process fluid leaks and spills and releases occur, especially the hazardous ones. After all, we are charged with keeping the processes inside the pipes, tanks and pressure vessels and not out in the environment. Here's where NDE and awareness training can rise to the occasion.