Development of sweet – CO2 containing – oil and gas fields is fast becoming a thing of the past. Depleted reserves mean that energy companies are now being forced to develop more and sourer fields, containing hydrogen sulfide (H2S). The issue is especially acute in the Middle East, where new oil and gas fields suffer from increasingly higher concentrations of H2S.
For instrumentation engineers, the presence of H2S in any fluid or gas media – or in the surrounding environment – is a game changer. Aside from being very poisonous, corrosive, flammable and explosive, it raises the specter of hydrogen embrittlement (HE). This involves the absorption of atomic hydrogen into the metal matrix – which can happen during manufacture, installation (if welding) or operational use. When hydrogen atoms diffuse into the metal, they can cause loss of ductility and load-bearing capacity. Over time HE can lead to cracking (usually at sub-microscopic levels) or catastrophic failures of a brittle nature, at applied stress levels that are well below the yield strength or design strength of a given alloy. The results can include leaks, asset damage and loss of production revenue.
Most metals are susceptible to HE, but those with an FCC (face-centered cubic) crystalline structure, such as nickel, are less prone than metals with BCC (body-centered cubic) or HCP (hexagonal close-packed) structures. The higher the hardness and ultimate tensile strength of the metal, the more likely it is to be inherently brittle.
Instrumentation engineers will be familiar with the need to select valves, tubes and fittings manufactured from specific materials to help prevent chloride-induced stress corrosion cracking (SCC) problems in offshore applications. Correct materials selection is also the key to avoiding HE. However, the issue is compounded by the fact that part of the problem can be caused by foundry processes and manufacturing methods, which are outside the control of the end user.
As one of the world’s largest manufacturers of fluid instrumentation products for offshore applications, Parker has significant expertise in corrosion resistant alloys and is well-positioned to help. It was the first company, for example, to manufacture NORSOK M-650 compliant 6Mo instrumentation products, which involved developing a new manufacturing process with special treatment stages, as well as re-qualifying the entire material supply chain.
Prevention of HE starts at the manufacturing stage, where fabrication methods, especially with regard to process temperatures, play a vital role.
The most important criterion is preventing hydrogen from becoming entrapped in the metal in the first place, requiring careful management of containment materials and temperature profiles during melting, alloying, casting and solidification. Many secondary material processing stages, such as electrochemical cleaning (pickling), electroplating and passivation, can also cause hydrogen to diffuse into the metal. Again, the choice of chemicals and temperatures for these processes is critical.
To determine which types of alloys offer the best means of preventing HE in any particular application, users need to consider a number of factors. These include the type of media, working temperatures and pressures, project timescales and costs, and whether other corrosive chemicals such as sulfur and sodium chloride also need to be taken into account. Contact between hydrogen gas in the environment and a metal surface at room temperature rarely constitutes a problem in itself. However, if the system or component reacts with other environmental contaminants such as chlorides, this can start an electrochemical process in which the corrosion byproducts can lead to HE.
Without a doubt, the best starting point is the NACE MR0175/ISO 15156 materials standard. Maintained by NACE International (formerly known as the National Association of Corrosion Engineers) and also adopted by the International Organization for Standards, this specification covers the suitability of materials for use in H2S containing environments. It details the selection, heat treatment and qualification of a wide range of carbon and low-alloy steels, and corrosion-resistant alloys.
Most of Parker’s instrumentation valves, tubes and fittings are available in a wide range of corrosion resistant alloys that comply with the metallurgical requirements of NACE MR0175/ISO 15156. The optimum choice for many applications involving H2S exposure is nickel alloy 625. This nickel-chromium-molybdenum alloy also contains niobium, which acts with the molybdenum to stiffen the material and provide high strength without requiring a strengthening heat treatment during its manufacture. The alloy is also highly resistant to a wide range of corrosive chemicals.
Parker’s instrumentation products are available in an extensive range of NACE MR0175/ISO 15156 compliant alloys.
As well as choosing the most appropriate alloy for each instrumentation application, users also need to consider suitable connection methods. Welded connections are one option, but these are notoriously expensive, demanding specialist orbital welding skills and rigorous quality inspection using X-ray or dye penetrant test techniques. It may also be necessary to pre- and post-heat components to displace entrapped hydrogen, which can be difficult, and all welding must be performed under dry conditions because water and water vapor are major sources of hydrogen. Another problem is that it can be up to 24 hours after welding before HE manifests itself, making inspection a somewhat protracted procedure.
For these reasons, many users continue to employ traditional NPT taper thread connections on instrumentation tubes. However, this early generation connection technology is not ideally suited to applications that involve exposure to H2S or other corrosive chemicals. It is very easy to over-tighten the connections, which can lead to stress corrosion cracking, especially if the environment also contains a high level of chloride. And the use of PTFE tape or fluid thread sealant can cause system contamination problems. Another disadvantage of NPT taper thread connections is that it is very difficult to rate their pressure performance because the number of engaged threads is always an unknown quantity.
Parker is committed to helping users eliminate the use of taper threads – thereby reducing the number of potential leak paths in a system – through various innovative compression style tube connection solutions. The company’s PTFree connect™ system, for example, provides a simple means of connecting impulse line to manifolds without the use of taper threads, PTFE tape or thread sealant. The manifold’s inlet/outlet and drain/test ports are fitted with parallel-threaded male adapters that are screwed into the manifold in the same manner as the valve heads, using the same type of stainless steel sealing washers. The open ends of the adapters accept standard compression style tube connectors.
Another solution is provided by Parker’s inverted A-LOK fittings. Instead of adapters, these use the manifold ports themselves to provide the female part of the connector. Each port is machined with a cone-shaped orifice and a standard parallel thread. The male part comprises the tube, two ferrules and an inverted nut with threads on its outer surface. The nut simply screws into the manifold, causing the front ferrule to form a metal-to-metal seal with both the tube and the manifold.
For direct process-to-instrument connections, Parker offers three types of flanged valve assemblies that are available in NACE MR0175/ISO 15156 compliant versions suitable for applications involving H2S, all of which are listed on Shell’s TAMAP database and carry a 2-STAR rating. Offering a choice of block-and-bleed and double-block-and-bleed configurations, these fully integrated valve assemblies provide monoflange, flange-to-flange (Pro-Bloc) and flange-to-thread connection solutions for a broad diversity of process interfacing requirements. The needle and ball valves can be supplied in versions that comply with the rigorous Class A specification of the ISO 15848 fugitive emissions standard.
Parker also offers a solution for eliminating taper threads in tube-to-tube connections, in the form of its innovative Phastite system. This provides a highly cost-effective alternative to welding and again, is suitable for applications that involve H2S. Manufactured from 316 austenitic stainless steel as standard, Phastite tube connectors are supplied as preassembled units that simply push onto the ends of tubes. They are installed using a simple compression tool and provide a permanent, leak-free alternative to threaded components.
Tareq Abdelrazek is Specification Manager - Middle-East & North Africa, Instrumentation Products Division Europe
Clara Moyano is Innovation Engineer - Material Science, Instrumentation Products Division Europe