Continuous wall-thickness monitoring via installed sensors – practical challenges
A number of new Corrosion Monitoring techniques and methods are emerging allowing in-service inspection or corrosion monitoring with installed sensors looking at wall thickness. They are destined to play an important role in the development of corrosion management in the refinery environment.
Practical challenges of high temperature in-service monitoring
With the drive towards new Corrosion Monitoring techniques and methods via installing high temperature in-service monitoring systems underway, plant operators are now facing the practical challenges which must be overcome to realise the full performance and benefits of the technology.
Three of the practical challenges that are emerging in implementing robust long-term permanent installations for thickness monitoring are as follows:
- The selection of Sensors and Hardware, and the optimum system arrangement
- Coupling of sensors to plant assets
- Calibration with fluctuating temperatures
Sensors & Hardware
Specialist hardware is required to operate in extreme environments and comply with hazardous environment certification. Only a limited number of sensors are available which can truly survive the harsh conditions of long term permanent installation on plant.
Ionix has invented a series of proprietary high performance piezoelectric materials which enable it to produce sensors that can serve in a range of demanding environment applications.
Ultrasonic transducers have been designed to operate continuously in extreme environments. Powered by Ionix’s novel piezoelectric technology, HotSense™ transducers produce reliable, stable and cost effective ultrasonic measurements at temperatures from –40 °C up to +380 °C.
A further critical aspect in selection of a permanently installed sensor and hardware is the Intrinsic Safety aspects. Ideally sensors are certified for use in hazardous locations meeting the requirements of systems such as ATEX, IECEx, FM and NEPSI. Sensors selected for various applications should meet the zonal classifications required by the use environments.
Transducers need to be designed to retrofit to high temperature plant, aiming for system coverage including elbows, bends & T-sections, all areas where corrosion and erosion issues may occur.
An important consideration in the probe and installation design is for a sensor that will sit under insulation, thereby minimising the risks of water ingress as a result of the installation process. Minimising the footprint of sensors while maximising area coverage is a key goal of new technology.
Ionix design its probes to be compatible with standard system hardware and instrumentation and this allows current service providers to create system and service solutions which build on conventional service-led approaches, or increasingly move to the exploitation of autonomous, wireless monitoring systems, with the benefits of cable-free installation
Reliable and repeatable thickness measurement data requires stable ultrasonic coupling between the sensor and the inspection part across a wide temperature range. High-temperature coupling is particularly challenging, as any air gap between the sensor and part can destroy the signal intensity due to the acoustic mismatch between stainless steel and air. Typically, a coupling medium is used, which include options such as dry coupling, fluid coupling and the use of solid couplants.
For high temperature applications, the use of dry coupling is generally ineffective unless the asset is extremely well prepared, which is difficult to achieve under normal plant environments. The use of liquid coupling while relatively common for inspection and scanning applications at high temperature, is impractical for use in permanent installations. As a result, the use of solid couplant is often the preferred option. This requires the selection of an appropriate material, often a metal foil, with appropriate sizing to achieve the best results for a given pipe or asset dimension. The preparation of a metal surface for use with a solid couplant is important.
Dynamic conditions of operation and process require dynamic compensation for temperature to achieve accurate in-service thickness monitoring in materials at high temperatures.
Compensation allows for the accurate absolute wall thickness measurement and determination of metal loss rate in a dynamic environment. Temperature compensation is required to meet the demands of the ASTM E797 test method. This method does however require the addition of a temperature measurement system working in parallel with the transducer.
Ionix has been able to develop a new approach to thickness compensation based on the change in speed of sound of the delay line in the Ionix probe, as a function of temperature. This has been demonstrated in a case study, which shows that a dynamic temperature compensation using an integrated delay line can allow live or off-line calibration from A-scan data.
Compensation results in a stable measurement during thermal cycling allowing repeatable high temperature thickness measurements to be made for determination of the rate of wall loss. In the case study, only 73 µm variation in thickness was measured across 40 thermal cycles between 50°C and 350°C, which with cross correlation and other mathematical solutions increase resolution to within 1 mil.
Using the Ionix integrated delay line compensation method, corrosion rates can be quickly determined to monitor the effect of corrosion management interventions and process changes, and agrees well with current ASTM standards.
The method is suitable for use in dynamic thermal environments such as refineries where absolute thickness measurements can be made which allow rapid corrosion rate analysis, and assessment of inhibitor effectiveness and monitoring of the effects of TAN / Sand content.
New Corrosion Monitoring techniques and methods are under constant development and the use of ultrasonic permanently-installed thickness monitoring is growing and provides an important tool for refining operators for the management of corrosion,. The approach has limitations but where it can be applied effectively it can provide major benefits in terms of increased intelligence, risk minimisation, productivity enhancement and safety improvements.
A number of practical issues are being faced in the use of permanently installed sensors on site, but the introduction of new technology and approaches are addressing these challenges. These approaches are enabling the broader implementation of continuous monitoring which allows a lower-entry cost to what in the future will be a critical capability for any refinery.