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device. While this may provide a short-term solution, it will not solve the actual root cause of the problem, with hydrogen continuing to accumulate until the device is rendered useless. Certain types of processes carry a greater risk
of hydrogen permeation, such as those involving pressures above 1,000psi (68.95 bar) or temperatures exceeding 176°C, which can increase the dissociation of hydrogen molecules from a diatomic state into hydrogen ions because of the higher energy present under these conditions. There is also a risk in applications where pipeline equipment is made from dissimilar materials, which can increase the formation of atomic hydrogen through galvanic corrosion. This can be a common problem in water measurement applications especially, where the water in the pipe acts as the electrolyte.
ADDRESSING THE PROBLEM Tackling hydrogen permeation is not a one-size fits-all approach. Depending on the possible concentration of hydrogen ions in the process fluid, coupled with the pressure and temperature of the process itself, different solutions can be put in place to facilitate reliable measurement. These solutions have tended to utilise the following approaches:
The graphics above show the challenge of hydrogen permeation and how the fluid fill is protected from hydrogen permeation by H-Shield
diaphragm. Once through the diaphragm, these ions rejoin to form a larger hydrogen molecule making it too big to pass back out through the diaphragm. As the hydrogen accumulates, it will gradually diffuse into the pressure transmitter’s fill fluid, changing its behaviour and impairing the performance of the transmitter. Over time, the accumulation of hydrogen will cause the diaphragm to bulge, although this will not be obvious unless the transmitter is removed. For operators, the first sign of the problem will be a gradual drift in accuracy, which will then get worse as hydrogen continues to permeate through the diaphragm. However, very often, the issue is only realised when the transmitter is depressurised. It is not uncommon for the cause of the
problem to go undiagnosed, with operators instead attributing the transmitter’s deteriorating performance on other factors such as drift. In such cases, the solution is often to just re-zero the
1. For low temperature, low pressures, low concentration, and specifically, if the process fluid is not “flowing” onto the transmitter diaphragm, transmitter options are available that are suited for hydrogen without using the most expensive solutions.
2. For all the remaining installations, the most common way to protect instruments from hydrogen permeation is accomplished by preventing hydrogen ions from passing through the diaphragms. Metals should be used with a low content of nickel, avoiding if possible, metals such as tantalum or non- passivated application practise Hastelloy C coming directly into contact with process fluid that may contain or generate hydrogen ions. ABB’s portfolio includes diaphragm options manufactured from Hastelloy C and Monel that are gold plated. However, while gold can provide a good level of protection, it is important to note that it is not 100 per cent effective against hydrogen permeation. With the price of precious metals such as gold at record highs, coupled with the fact that gold does not prevent all cases of hydrogen permeation, there was interest from customers and OEMs alike to find alternative solutions to tackle hydrogen permeation while lowering material cost.
THE SEARCH FOR SOLUTIONS In recent times, pressure transmitter manufacturers have made significant advancements in helping to address this issue. ABB’s answer in the search for a solution has
Instrumentation Monthly September 2022
been the development of a titanium-based binary nano coating, known as ‘H-Shield‘. Applied using the Physical Vapour Deposition process, or PVD, this technique is commonly deployed in other industrial processes to apply high end coatings to metals. H-Shield forms a protective coating at a uniform thickness across the surface of the diaphragm. With its tight molecular structure, H-Shield provides the highest resistance against the permeation of hydrogen ions, whilst still offering the flexibility for the diaphragm to move in response to changing pressure conditions. In laboratory tests run by ABB, H-Shield was shown to offer up to 20 times greater protection against hydrogen permeation than other materials such as Hastelloy HC and gold. Considering field experiences as well, H-Shield has successfully demonstrated its ability to withstand conditions where non-ABB transmitters were failing and had to be replaced every 18 to 24 months. To prove the performance of H-Shield, the coating was put to the test at an oil refinery in Canada. The plant processes North American and overseas crude oil received via ship, rail, and pipeline. The refinery has significant storage capacity for crude oil and intermediate and refined products. The plant can also accommodate liquefied petroleum gas in its pressurised storage vessels or tanks. ABB’s channel partner was contacted by the company to address a problem with a failing transmitter due to hydrogen permeation. To solve the problem, ABB supplied an H-Shield 266 DP transmitter with remote seals. Since the installation of the transmitter in 2016, no problems have been experienced with hydrogen permeation, enabling it to offer continued high accuracy measurement without any disruption caused by failure.
TRANSMITTING A MORE ACCURATE FUTURE As a largely unrecognised problem, the issue of hydrogen permeation can have a significant impact on both measurement accuracy and the cost of pressure transmitters, with many devices often rendered useless within half of their expected operational life. Furthermore, failing to correctly trace the problem to its root cause, and ensure the necessary protection to prevent it, can condemn companies to a frustrating and expensive cycle of purchasing new units only for them to fail for the same reason. While hydrogen permeation can occur as a result of many industrial processes, the phenomenon can be managed by understanding the concentration of hydrogen ions in the process fluid, operating conditions, and adopting the appropriate preventative measures. And while precious metals such as gold may have provided a good level of protection in the past, new break- throughs have now been made to help pressure transmitters perform more effectively in the field, helping industrial companies to lower costs, reduce unscheduled downtime and increase productivity.
ABB
www.abb.com 27
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