May 25th, 2010
Corrosion is a constant worry for petrochemical engineers, especially in welds of flanges and pipe fittings. We at Chemipetro supply pipe fittings in corrosion-resistant alloys, such as Moly 6 and super duplex, which significantly reduce the problem. However, it can be reduced still further by good welding practice.
The best way to achieve a corrosion-resistant finish is by making sure the root-weld is as smooth as possible, with no crevices (a common fault with butt-weld joints.) The best results are achieved with tungsten inert gas (TIG) welding. Also known as gas tungsten arc welding, this has various benefits over hand-welding. However, for various reasons the latter is still widely used in the petrochemical industry.
The serviceability of hand welds have much to do with the skill and knowledge of the welder. A consumable filler metal, which becomes part of the final weld, should always be used. It is essential to the strength of the root bead that adequate amounts of filler are melted into the weld, in a uniform manner. Proprietary consumable inserts are available which make this easier.
Root-pass welding using TIG machines also requires a high level of skill. However, there are a number of automatic TIG machines available for both root-pass and single-pass welding of pipe flanges. The root-pass is made using an automatic wire feed or insert, while single-pass welds require no filler metal at all. Automatic welding systems are expensive, but allow extremely consistent ID root contours. Automatic GTAW welding is particularly useful in small diameter pipes – for which butt-weld pipe fittings are often preferred.
Tags: butt-weld joints, corrosion resistant alloys, Pipe Fittings, Pipe Flanges, Super Duplex
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May 19th, 2010
To minimise corrosion at joints, it is important to choose the right kind of pipe fittings. For example, engineers frequently find it more convenient to use socket welded, rather than butt welded pipe fittings on smaller diameter pipes. Socket welds are also frequently used on larger pipes (i.e. over 50mm diameter) with a backing ring. The problem with both these fittings is that a crevice is formed, which can lead to corrosion in hostile environments.
Pipe fittings made from alloys with good crevice corrosion resistance, such as duplex, give good service in most environments. However, in aggressive environments they may not provide sufficient resistance, if crevices are present. There is a further disadvantage on large socket welds, as the backing ring protrudes into the flow stream. This can cause unnecessary turbulence and product build-up so butt welded duplex fittings should be used where possible.
There is a further problem where commercial grade stainless steel pipes and flanges i.e. those not conforming to ASME or API standards are fitted. ASME/API pipe fittings require full penetration butt welds. However, commercial fittings don’t have to adhere to this restriction, but from the outside the two are impossible to tell apart. Owing to modern industry standards, you are most likely to come across this problem if you’ve inherited the plant from a previous owner.
If you are in any doubt about the quality of the pipe fittings on your plant, they should be replaced with fittings from a supplier like us at Chemipetro. Our pipe flanges and fittings are all ASME, ANSI and API standard, and are made from corrosion-resistant alloys like super duplex.
Tags: API pipe fittings, butt welds, corrosion resistant alloys, Pipe Fittings
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May 13th, 2010
We at Chemipetro supply flanges and pipe fittings made of extremely resistant alloys, such as super duplex, to the process industries. The development of these alloys is the result of many hundreds of hours of research. This research is on-going, because even today cracking in petrochemical pipelines, flanges and pumps is a problem – and it isn’t restricted to low-nobility alloys like carbon steel.
When selecting pipe flanges for the petrochemical industry, corrosion resistance is the thing that is usually given highest consideration. However, not all of the accidents that occur in plants today are attributable to corroded pipe fittings alone. Petrochemical pipelines operate under extremes of pressure and temperature. The volatile and corrosive chemicals they contain can leak out in a number of ways, often suddenly and with great force.
NACE (the National Association of Corrosion Engineers) is an internationally recognised organisation that actively researches corrosion control. As recently as 2001, a NACE white paper reported problems of early cracking in pipes and pipe fittings fabricated from alloys selected for their corrosion resistant properties. Failure was observed in the welded areas of stabilised austenitic stainless steel charge heater pipes, in a petrochemical plant where working temperatures in excess of 650 degrees were recorded.
Using detailed FEA (finite elemental analysis) and other techniques, the study showed that a combination of thermal pipe movement, internal pressure and bolt movement at the flanges all contributed to cracking. When choosing pipe flanges, therefore, operators should never depend on the corrosion factor alone. How the flanges are supported to minimise vibrational, thermal and pressure stresses is just as important.
Tags: corrosion resistant alloys, Pipe Fittings, Pipe Flanges, Super Duplex
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May 11th, 2010
We at Chemipetro supply pipe flanges and fittings in a wide range of corrosion-resistant alloys, which will resist most extreme conditions. However, you should still check their condition regularly, especially in highly acidic or hydrogen-rich environments.
High concentrations of hydrogen are widely found in the petrochemical industry, either through the pipe fittings of corrosive chemicals or as a side-effect of galvanic corrosion. Under these conditions, hydrogen ions can enter the metal by active or passive diffusion, causing it to become brittle. Eventually, it will crack.
In the 1970s, various experiments were carried out on the permeation rates of hydrogen into different types of steel. It was found that the cracking rate was consistent with the rate of permeation, and that hydrogen could be deposited in trap sites or at grain boundaries and other internal discontinuities.
Hydrogen traps can be reversible or irreversible. Reversible traps allow the hydrogen to diffuse back out. Irreversible traps don’t do this, and are therefore the most damaging. Fractures occur because the hydrogen lowers the surface energy of the metal.
Hydrogen embrittlement is related to the degree of strength that is already present in the alloy matrix. Metals that are in a state of higher strength, i.e. age hardened or cold-worked, are actually at a higher risk of cracking than those of a lower strength. Hydrogen diffusion coefficients are another factor. Titanium pipe fittings, for example, generally have higher diffusion rates than those with high aluminium content.
Modern alloys are constantly being refined. Buying your pipe flanges from us at Chemipetro will minimise the likelihood of hydrogen-induced cracking. It can be minimised further by good plant practice.
Tags: corrosion resistant alloys, Pipe Fittings, Pipe Flanges, titanium pipe fittings
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May 7th, 2010
Although modern flanges are made from highly corrosion-resistant alloys, such as duplex, they are quite often fitted to pipelines made of lower nobility metals such as carbon steel. This means there is a risk of corrosion at flange joints and other vulnerable areas.
If galvanic corrosion is allowed to take hold, it can weaken flanges, even those made from corrosion-resistant alloys. This is because, in a galvanic reaction, the higher nobility steel acts as a cathode. Although corrosion occurs on the anodic side, part of the galvanic effect is the evolution of hydrogen on the cathode side. This can result in hydrogen embrittlement of the metal.
Hydrogen embrittlement occurs when hydrogen ions enter the metal, weakening its structure. It has been reported in petrochemical components such as K500 drill collars and duplex flanges – in other words, alloys which are normally known for their strength. Therefore it’s important that pipelines are monitored efficiently, especially where corrosion-resistant pipe fittings are in contact with lower nobility metals, such as carbon steel.
Pipeline corrosion monitoring starts at the point of installation, and should then be ongoing. Pipe flanges create air-tight or liquid-tight seals. Once corrosion begins, that seal is broken. It then becomes a point of weakness, with the potential for catastrophic failure. This is especially true of buried pipelines – soil is a very corrosive environment.
Where pipe fittings are buried, a soil corrosion rate monitor should be used. Modern regulations demand that pipeline operators have measures in force to maintain integrity of pipelines, flanges and pipe fittings. A corrosion rate monitor will enable this to be done at relatively little cost.
We at Chemipetro would also recommend you use our super duplex flanges in corrosive environments.
Tags: corrosion resistant alloys, duplex flanges, Pipe Fittings, super duplex flanges
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April 27th, 2010
We at Chemipetro make flanges in a wide range of corrosion-resistant alloys for offshore use. Titanium is a popular choice for pipe fittings, owing to its high resistance to corrosive elements. However, this can cause unexpected problems if the pipeline itself isn’t of similar quality – in essence, galvanic corrosion.
Galvanic corrosion occurs when two widely dissimilar metals are coupled together in the presence of a strong electrolyte, such as seawater, and with good electrical contact. In the absence of any of these factors, galvanic corrosion will not occur. Pipe flanges and other joints represent an area of weakness, and thus corrosion-resistant alloys such as titanium and super duplex are widely used in seawater environments. However, it’s unfeasible to use these metals along the entire pipeline, and so galvanic corrosion can potentially occur.
In galvanic corrosion, the less noble (i.e. less resistant) metal acts as an anode, and the more noble alloy acts as the cathode. The two form a galvanic cell, behaving like a zinc battery. Titanium pipe fittings are highly noble, and so will behave as cathodes where there is an electrolyte and good electrical contact.
The rate of corrosion is dependent upon several factors. However, if the nobility of the connecting metal is low, and the area of exposure large, titanium flanges can cause more rapid damage to a pipeline than if a less resistant alloy was used. The answer isn’t to install lower quality flanges, but to remove any possibility of the metals coming into contact and forming a galvanic cell.
Tags: corrosion resistant alloys, Pipe Fittings, Pipe Flanges, Super Duplex, titanium pipe fittings
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April 15th, 2010
As a supplier and manufacturer of high-quality pipe flanges for the petrochemical industry, we at Chemipetro understand the importance of caring for our flanges and pipe fittings once they’re installed.
All flanges need regular maintenance, or you will start to get decreased performance from other components on your site. It’s essential you eliminate problems at their source, as soon as they arise. Not only will this improve plant efficiency, it will create a safer working environment and save on running and repair costs. A large number of fires and explosions start as a result of leaking or poorly welded pipe fittings.
No matter how high the quality of the alloys, it is essential you protect flanges, gaskets and other components from their environment as much as possible. This starts by reducing corrosion to an absolute minimum. Using corrosion resistant alloys, such as super duplex, has reduced corrosion substantially – but the annular seal on the outer diameter of flanges can still be vulnerable to moisture and dirt. Once these creep in, eventually it will lead to corrosion and degradation around the gasket area.
You should also remember that welded joints are more vulnerable to corrosion than other areas of pipe fittings, even if tough stainless steel alloys are used. Excessive corrosion of flanges is dangerous, and if they have to be replaced it can lead to plant shut-down. To reduce this risk, use flange protectors to stop dirt and moisture attacking the joints.
While strong, corrosion-resistant alloys like duplex steel will ensure you get long life from the pipe flanges you buy from us atChemipetro, they will last even longer if you remember these few basic rules.
Tags: corrosion resistant alloys, Pipe Fittings, Pipe Flanges, Super Duplex, weld neck flanges, welded joints
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