Tube is measured with outer diameter and wall thickness; pipe is measured in its inside diameter. Tube is normally bent, giving curves; pipe uses joints and screwed connections. Tubing is up to 1-2 inches in diameter; pipe usually starts at that size. Small-bore tubing applications include air distribution manifolds; hookups – the connection from piping through a transmitter to a gauge; hydraulic applications at high pressure; sampling and analyser systems.
The main thing we are looking for in oil and gas applications is safety. The weak point should always be the tube, not the fitting. If the tubing bursts, it’s a failure, but that’s classed as a safe failure. We design systems based on a 4:1 safety factor of the working pressure and tube pressure rating from manufacturers’ tubing charts. A failure of the fitting could lead to a great risk of injury, particularly on high-pressure systems where the fitting blows off of the tubing. Also, the media being released might have an environmental or health impact.
Then there are the process consequences of a failure of the system; a compressor might get shut down, or a safety system associated with that tubing system might trip. An air system might be linked to an emergency shutdown system; loss of pressure could cause that to shut down, which might have a major business impact. After a shutdown, technicians will carry out safety tests and look at the problems associated with that and why it happened.
Unfortunately, the oil and gas industry never works in wonderful conditions. Usually it is working offshore or near shore, experiencing high or low ambient temperatures in a humidity, salt-laden environment. The sea contains tonnes of microbes. It works with challenging media such as sour gas. Plant has lots of crevices. Core product selection can add to these issues.
The critical component in this system is the ferrules (pictured above); they are what hold the system together. You have the front ferrule compressing on to the tube and the back ferrule biting and securing the system together. Then there is a nut at the back that is securing the system all together providing a back stop (see diagrams, p39).
The external media – air, seawater – will go into the fitting and affect the back ferrule; whereas the internal media, process media, will be in contact with the first seal, the front ferrule.
With respect to surveys Parker has carried out, pitting corrosion seems to be the key issue in our industry (see also pie chart, p38). It usually occurs because surface of the material, particularly stainless steel, has become damaged. The passive surface layer on stainless steel is very thin. Bad handling, bad workmanship, paint splatter and weld splatter can damage that surface. Then the chlorine in the salt-laden atmosphere oil & gas plant is exposed to starts to go into a pit. As soon as that happens, it releases positive ions in the pit, which are basically part of the structure of the metal, and start to cause that pit to grow. Once it starts, there’s very little to stop it from continuing.
Crevice corrosion is corrosion in a gap or crevice between two surfaces. One of the key areas where crevice corrosion occurs is under tubing clamps, where there is salt water sitting. Making sure that buyers are making the right choice of tubing clamps is quite critical to prevent buildup of residual water.
In a case where corrosion appears at the back of the fitting, that is usually because the back ferrule has been in contact with salt-laden environment, and is becoming degraded. That is one of the key components in a twin-ferrule system to keep it together to retain that pressure. In something like this, it could be just that rust holding the whole system together.
Microbiologically-influenced corrosion is basically the same. Microbes on an existing pit or crevice form a film on surface imperfections, such as pits or crevices, on weld lines, paint or debris. That prevents oxygen reaching the metal surface by microorganisms in a bio film. The most active temperature is 20-35°C. There are many types of bacteria; some are sulphate-reducing, some are iron-oxidising, some are acid-producing; those products add to the damage.
Stress corrosion cracking occurs under tensile stress in the presence of specific corrosives, including chloride – the most common. Stress corrosion cracking can also occur in the presence of caustic materials, polythionic acid, high-temperature water and sulfides. Often stress corrosion cracking occurs on the nuts of a fitting where the choice of nut or fitting is incorrect.
There are three major causes of galvanic corrosion. First, a difference in electrical potential of two metals in contact with one another. Metals can be classified on the galvanic scale, from the most anodic to the most cathodic. Galvanic corrosion occurs between two dissimilar materials that are far apart on the scale. Second is an electrical path between the two metals. Third is that a fluid such as atmospheric humidity or salt fog exists that can break down the metals. Offshore rigs normally have a sacrificial anode on system to balance out this galvanic circuit, which will need to be replaced every so often. That’s because linking everything together creates a system. But in small bore tubing, that isn’t a possibility, so what will happen is that a rust area starts to build. The challenge here is pinpointing the source of the rust, because the system could be losing material a way back.
Erosion is the last degradation mechanism. When high pressures are used, there will be a fast-flowing media. And particularly on a cone-and-thread fitting system, if that cone has some sort of damage because of incorrect turning – and it’s quite difficult to get the cone very smooth and not put indentation into it during production – that will be a point where the pressure will cut through material. First comes erosion, then a failure point, and then leakage. An initial, very small leak at the cone can lead to erosion as the high pressures will cut through the metal.
Erosion can be caused by poor tube preparation, poor machining of tube, cone and thread materials, or a high-vibration application. To prevent corrosion by vibration, standard industry installation procedures should be followed. Other options include using an anti-vibration option and selecting the right material to resist the initial corrosion attack.
EXAMPLES OF CORROSION
A typical case of pitting corrosion was experienced on a pipe fitted to a desalination plant by the seashore in the Middle East. The choice was made to use stainless steel 316 on seawater. Ambient temperatures in the region are quite high: 19-40°C. At a connection between the flange and installation bolt, a pit has occurred, and there are deposits of rust and corrosion. The system had reached a point where the tube thinned so much that it caused a breakage.
What should have been considered was not just the temperature of the media but also the temperature of the environment. That is quite critical as both work together, so the overall effective temperature is much greater, and that affects the corrosion.
With respect to corrosion, engineers must consider the whole system: not just the tube, not just the fittings. Compression fittings can give a misleading impression of being thicker than the tube, and therefore less susceptible to corrosion. Also, these metals look the same, but specifiers need to ensure that they are not accidentally mixing materials.
Particularly on higher pressures with sour gas service, operators working to NACE MR 0175 should choose a material applicable to that. Because on high-pressure systems, strain-hardened 316 is even more susceptible to stress corrosion cracking. In such applications, pressure limits on that system could be reduced by up to 60% for safety.
Material contamination can also damage tubing. Construction debris from grinding will damage the surface and cause increased issues with respect to corrosion. Those responsible for overseeing such processes should make sure that installers are using the right tools, not mixing materials, and storing tube correctly.
NPT threads are a key point of cracking, because in the motion of screwing that fitting in, the installer is adding stress. An alternative is to choose a solution with integral fittings.
For example, Parker offers a manifold with a parallel threaded connection with a fitting incorporated. That system has been put together pressure-tested, and all the fitter needs to do is install the tube into the fitting with one and a quarter turns after finger-tight. With an NPT thread, there are no torque settings or guides. There, the final arrangement depends on the strength of the fitters, their experience, what materials they are using: tape or Loctite thread locker, and how many times they have to do it to get the fitting in the right position.
Make sure you’re using high-quality fittings. Look at material composition on the 3.1 certificate and make sure there is heat code traceability so you can go back to the original specifications. Use of nitride hardening makes a back ferrule more susceptible to corrosion; choose a back ferrule with Parker’s Suparcase, a proprietary chemical treatment of 316 stainless steel, or equivalent.
Factors affecting material selection include cost and availability, corrosion resistance, working pressures and temperatures. It’s about making the right choice of material, particularly where looking for an application where you want longevity. If you don’t mind changing the system out regularly – you have the maintenance people and the time to swap it out, go for a cost-effective solution. But if you want to have negligible maintenance, no changeovers, and want it to last 10, 15, or 30 years, then you need to make the right selections. Make sure that you are meeting not only the design considerations but also pressure, temperature, media and environmental. What’s the delivery time (will it impact on price and delivery)? For example, Parker’s A-LOK tube fitting materials are: 316 stainless steel, Monel 400, 254 SMo (6Mo) Norsok M650, Alloy 825, Alloy 625, Alloy C276, Titanium grade 2 (for more information, see www.is.gd/ihipek).
Discuss the specification with the manufacturer; it needs to align with availability, certification and testing requirements. Look at the tubing charts; some sizes are not available. If you choose them, they will be specials. Make sure you’re choosing the right material, in right size, at the right pressure. It’s no use standardising on one certain size because the pressure ratings at those sizes will be different.
For example, a 12mm x 1.5mm tube in different materials has a different pressure rating. Tubing charts show that for Alloy 600, it’s 290bar; for 316/316L stainless, it’s 310; for 6Mo, it’s 380, for Alloy 625, it’s 420. In addition, in temperatures above ambient, use the temperature derating factor multiplied by the pressure to add an additional safety factor to compensate for its stress on materials.
Do not mix tube and fitting materials, even if the solution is mechanically acceptable. No mixed materials means a uniform corrosion rate and no risk of galvanic corrosion. Consider using one material per tube OD size to avoid mix on site; consider using one to three materials on one site maximum.
If you go up to six materials in seven or eight sizes, cost-effectiveness really reduces. You end up buying only short quantities. That’s acceptable for the bulk of the project, but a problem then arises for the small skid builders, the OEMs, who only need a few metres; getting hold of that material can become a real challenge for them. Unless, that is, you have spoken to your manufacturer and buy a bulk consignment that suppliers can pull off.
Consider the ease of installation, because time is money. If it’s easier to install, it is going to be quicker. An installation with lots of bends requires a bending rig. 6Mo ½-inch tubing can be bent with a hand tube bender. But if you have a queue of installers behind one bending machine because you are using ½-inch Super Duplex, that’s going to waste a lot of time, and those machines are cumbersome and require air supply.
Make sure all of that information is available to the designers, the buyers, the manufacturers and the OEMs: standardise on the project specification. Have your specifications for tube storage, clamps, installation, materials, sizing, and make sure that the installers have been trained. Finally, seek manufacturer support. Can they review and comment on that specification to make sure that it’s best-in-practice?
This article is based on Parker’s webinar ‘Corrosion issues within small bore tubing systems’ first presented on 26 November 2020.
BOX: Product news
Parker Hannifin has launched a new range of small-bore subsea ball valves and subsea needle valves for leak-free and reliable, long-term performance in subsea applications up to 10,000 PSI (689 bar) and depths down to 3,050 m (10,000 ft). Parker’s small-bore subsea quarter turn ball valve, featuring floating ball design, utilises the same design technology as the standard Hi-Pro series ball valve. This valve design additionally incorporates shaft and end adaptor O-ring ingress seals, which prevent contact with sea water to any threaded or rotating parts of the valve. The two-way small-bore subsea ball valve design features PEEK self-relieving seat for resistance to chemicals, heat, and wear/abrasion and PTFE packing. The non-rising, non-rotating stem subsea needle valve features a metal-to-metal seat. The valves are available in a range of corrosion resistant alloys and a range of connections including cone-and-thread, butt-weld and NPT.
Phastite is a push-fit permanent instrumentation connection that is said to eliminates the need for welding tube up to 1 inch, providing permanent leak-free connections. Parker has extended the range to include a number of corrosion-resistant alloys (CRAs), including Alloy 825, 625 and Super Duplex. To make a joint, all that is required is to slide the fitting on to the tube end and then push the collar along the fitting body until it reaches a dead stop. With the help of a hand tool, the connection can be installed in under one minute, while assuring users of right-first-time connections. The system is said to require neither hot work permits nor non-destructive testing.