Wireless Battery-Free Ultrasonic Thickness Measurement Patches for CO2 Absorber

Introduction

This article investigates the use of a new type of wireless ultrasonic sensor for monitoring the thickness of equipment while on or offline. This is a case study of a CO₂ absorber tower in a remote location. The tower is vulnerable to runaway corrosion events. During these events, the corrosion rates can be high enough to threaten the integrity of the vessel. To manage this risk, regular thickness readings are taken to confirm remaining wall thickness and, if necessary, provide adequate time for corrective measures to be taken. Inductosense ultrasonic sensor technology was selected for evaluation, which allows non-specialist plant operations personnel to measure wall thickness without the need for contracting specialists. The system is made up of permanently adhered sensors and a hand-held data collector. The sensors are inductively coupled to the data collector, allowing the ultrasonic measurements to be taken wirelessly.

Fifty sensors were installed on a section of a CO₂ absorber while operating. During the one-year proof of concept period, all sensors functioned and reported thicknesses. Manual nondestructive testing (NDT) and battery-powered wireless monitoring systems were used to validate the performance of the inductively coupled sensors. The inductively coupled system showed advantages in cost and accuracy.

Case Study

This article reports on a trial of three different NDT technologies carried out on a CO₂ absorber tower in a live production environment. The tower being monitored is in a remote desert area of Central Australia where access is challenging in harsh conditions. Its function is to reduce CO₂ in the incoming hydrocarbon gas stream from 20% to 4%. The tower was constructed in 1984 of carbon steel, measuring 20m (65’ 7.4”) high, 3.0m (9’ 10”) in diameter and 70mm (2.8”) thick, operating at 6800kPa (986 psi) and 100°C (212°F).

CO₂ is removed using the UOP Benfield process which uses a recirculated hot potassium carbonate solution. Internal corrosion of the carbon steel tower is prevented through passivation treatments during start-up, and through the continual wetting of the tower walls by the potassium carbonate solution. Should the solution be prevented from contacting the vessel shell, then rapid corrosion can occur. An operating history of 36 years shows long periods of low corrosion activity, with rates as low as 0.02mm per year (0.0008 mpy) interspersed with corrosion events of short duration but high severity, where corrosion rates as high as 3mm per month (1.44 mpy) have been measured. The cause of these events is not the subject of this article but have been many and varied, with the common theme of poor (tower) wall wetting by the potassium carbonate solution.

Leading indicators, such as integrity operating windows, have been successfully used to manage solution chemistry, process parameters, and contaminant levels. Along with corrosion probes, they can also provide an indication of the “health” of the system. Extensive manual ultrasonic thickness testing supports these leading indicators and provides confirmation of tower integrity. Ultrasonic testing is an important direct measure of tower integrity, and due to the potentially high corrosion rates, it is conducted bi-monthly to allow corrective measures to be taken. The full circumference of the tower is monitored, adjacent to an internal packed bed near the top of the tower covering an area of 65m², 14m from grade. Studies and history have shown this is where the corrosion is active. As would be expected on a tower of this size, there is no direct access, therefore measurements are taken via rope access at significant cost. This ongoing expense drove the need to find an alternative means of measuring the thickness of the tower shell. The inductively coupled thickness monitoring patches/sensors provide one possible low-cost solution.

Trial Objective