May 27th, 2010
Although duplex pipe flanges are widely used in the petrochemical industry, long-distance pipelines are still mainly built from carbon steel, which has a high level of corrosion. Currently, there is great interest in the oil and gas industry to implement duplex steel as the material of choice, owing to its strength and corrosion resistance.
One of the problems with using duplex flanges in pipelines is the increased skill required by the welder to achieve an acceptable root pass. This first layer of a multi-layer weld is the most critical, because it is the one most likely to crack or become contaminated. The preferable method for depositing root passes is GTAW (gas tungsten arc welding), which uses a specialised backing shield of inert gas, such as argon. It is tricky to use this method on duplex. Besides anything else, the gas itself causes problems at the welds.
Another disadvantage with duplex flanges is that it is difficult to achieve low-temperature toughness – essential for field welds. It is felt that if a gas-free arc welding technique could be developed, that somehow also attacked this problem, then duplex could replace carbon steel on long-distance pipelines. The benefits would be a radical improvement on corrosion of pipe flanges in pipelines, with a corresponding reduction in the tonnage of metal used each year.
We at Chemipetro supply both duplex and super duplex pipe flanges and fittings, all with excellent weldability. However, their unique and complex composition means that extra care must be taken over welding, in order not to compromise strength and corrosion resistance.
Tags: duplex pipe flanges, Flanges, super duplex pipe fittings, tungsten arc welding
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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
Posted in Pipe Fittings | No Comments »
May 21st, 2010
Modern technology has allowed previously uneconomical plants to be reopened for commercial gain. However, the new owners often discover they’ve inherited a lot of problems – especially when it comes to the quality of the pipe flanges and other fittings.
For example, it is often discovered that what should be full penetration, ANSI-standard butt-weld fittings are, in fact, nothing of the sort. Cheap, commercial standard pipe fittings may have been used, with only partial penetration to the inside, leading to a weak and incomplete seam.
Corrosion is known to be prevalent at the crevices of butt-weld joints. However, this is usually only a problem where crevices are accompanied by incomplete penetration at the welding joint. When ordering pipe fittings, always ensure you choose a supplier like us at Chemipetro, i.e. one who supplies only ANSI-standard industrial fittings. Where standards are not quoted, you must specify that, for butt-weld joints, full penetration and smooth ID for the root bead are essential.
To get the best quality root welds with butt-welded pipe fittings, it is recommended that GTAW (gas tungsten arc welding) is used, with an inert shielding gas such as argon to protect the welding area from atmospheric contamination. Where hand welding is employed, consumable metal inserts should be used. These are filler metal plugs which become part of the final weld.
Both methods ensure a crevice-free, flush finish, although the hand-fill method is more difficult to achieve uniformity. This means crevice formation and corrosion are more likely than with GTAW welding. To guard against this, pipe fittings should be ordered in high-resistance alloys, such as super duplex.
Tags: ANSI industrial fittings, butt welded pipe fittings, Pipe Fittings, 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 17th, 2010
Finite elemental analysis (FEA) was one of several techniques used by NACE (an international association of corrosion engineers) to investigate failure in petrochemical flanges in 2001. The study involved cracking around welded high-temperature pipe fittings. Made from high-strength austenite stainless steel alloys, these should not have failed.
The study was carried out on 24” diameter pipes leading from charge heater units. These pipes carried i-butane at an operating temperature of 650oC, under high pressure, to reactor vessels. The pipe was welded to 24″ flanges made of the same steel alloy. This was the site at which cracking occurred, despite the steel being selected for its ability to withstand temperatures in excess of 650oC.
Tests were carried out on a piece of failed pipe welded to a flange. The pipe, which had three distinct cracks of 5 – 20 cm long, was first subjected to a dye penetration test. The mechanism of damage, changes in microstructure and alterations in grain boundary chemistry were then analysed using a variety of metallurgical techniques. These included optical microscopy; scanning electron microscopy; X-Ray fluorescence, tensile tests and FEA.
Although the flanges and pipe fittings were made of the same alloy, there were marked differences between the grains of the flange and pipe metal. Cracks near the weld went deep into the flange, and carbide precipitation, grain sliding, and brittle granular precipitation could be seen.
Most telling were the FEA results. These confirmed that pipe flanges undergo severe thermal, internal pressure and bolt-loading stresses which, when combined, can lead to catastrophic failure. We at Chemipetro supply super duplex flanges, which are heat, pressure and chemical resistant. However, the importance of correct fitting must never be underestimated.
Tags: Flanges, Pipe Fittings, steel alloys, Super Duplex
Posted in Pipe Flanges | No Comments »
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
Posted in Pipe Flanges | No Comments »
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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May 5th, 2010
Studies into the effects of seawater corrosion on pipe flanges go back at least as far as the 1980s. Many of these concentrated on the effect of “sweet brine,” i.e. pure seawater. However, in the 1990s experiments were done using “sour” brine, i.e. seawater contaminated with hydrogen sulphite and other acidic solutions.
Sour brine is, unfortunately, commonly found around petrochemical installations, owing to leakage of processing fluids from poorly maintained pipe fittings and valves. Highly acidic seawater is not only harmful to the environment – it is harmful to pipe flanges, too. Galvanic corrosion is already a problem in seawater pumping systems, owing to the electro-conductive nature of seawater. By increasing acidity, you supposedly increase the rate of corrosion.
However, in both sweet and sour brine environments, the largest amount of corrosion occurs when both metals are galvanically active, i.e. when both are non-corrosion resistant. It is for this reason that companies like us at Chemipetro specialise in supplying pipe fittings in noble alloys, like super duplex, to the petrochemical industry.
This can bring its own problems, though. Experiments showed that in both sweet and sour brine conditions, the rate of corrosion of active alloys was increased when they were matched to inactive metals like duplex. In this case, a galvanic cell is established in which the duplex acts as the cathode.
Although corrosion occurs at the anode, i.e. the pipeline side, duplex flanges can be damaged via hydrogen embrittlement, as hydrogen is produced by the cathode during the galvanic process. Hydrogen embrittlement of duplex pipe flanges has been reported in down-hole systems, where duplex pipe fittings were used with carbon steel well casings.
Tags: duplex pipe fittings, Flanges, Pipe Flanges, super duplex alloys
Posted in Pipe Fittings | No Comments »
May 3rd, 2010
There has been much concern over environmental cracking in pipe fittings used in gas and oil plants, leading manufacturers to develop new alloys that are resistant to galvanic and chemical corrosion. We at Chemipetro supply pipe flanges and other fittings in a range of alloys, including Super Duplex, to minimise the risk of cracking. Provided these pipe fittings are correctly installed, and the usual maintenance and checking procedures are in place, corrosion is not usually a problem.
However, corrosion can be a problem in older petrochemical plants, where outdated metals are married to new, noble alloys. Then, galvanic or bimetallic corrosion can occur, owing to the electro conductive nature of the process fluids. The increasing interest in marginal oil fields i.e. fields with uneconomical reserves has made the problem more acute.
Marginal fields are of little interest to major oil companies, but are attractive to entrepreneurs because of their low operating costs and overheads. These low costs are often down to the poor quality of the flanges used in the original plant design. In addition, the process fluids used in marginal fields are often highly corrosive. If these are tied in to older pipelines, with pipe fittings that were designed for more mildly corrosive processing fluids, corrosion can occur at the joints.
Seawater (or “sweet” brine) still poses a big threat to pipe flanges, and many studies have been conducted over the years on its corrosive effects. The effects of “sour” brine, i.e. seawater contaminated with acidic chemicals, are especially of concern.
Tags: alloys, Flanges, Pipe Fittings, Pipe Flanges, Super Duplex
Posted in Flanges | No Comments »
