Monitoring critical vessel temperature

Radiometric imaging can help detect and measure uneven temperature gradients quickly and reliably.

By Lenny Shaver, LumaSense Technologies, Inc. January 27, 2012

Measuring temperature accurately has always been one of the most important and difficult things to do when monitoring the safety of critical vessels in refineries and other industrial plants. Extreme temperatures and non-uniform temperature gradients make it nearly impossible for traditional measurement methods to monitor every critical point or to obtain complete data.

Without accurate and early detection of temperature changes, the likelihood of failure-related problems increases, creating safety and reliability issues. The consequences of undetected failures can be very serious and pose extreme safety risks if a vessel isn’t properly monitored. A rupture in chemical reactors, storage tanks, and piping systems can all lead to catastrophic loss of life, product, and capacity. These all require sophisticated monitoring techniques to spot irregular temperatures and trends that precede unsafe and costly problems.

For many years, thermocouple systems and fiber optic sensors have been viewed as the traditional solution for temperature measurement in vessel monitoring applications. Yet, these types of sensors can be both unreliable and cost prohibitive to install and operate. They typically utilize wired or fiber optic networks and employ point sensors which only monitor the temperature of discrete points on the outside of a vessel. This can result in inaccurate measurements due to skin temperature gradients. In addition, failures of thermocouples leave dangerous holes in overall monitoring schemes until replacement or repair can be made. Of course, missing points in the monitoring scheme put the critical vessel, plant, and staff at risk when unexpected hot spots arise.

One primary reason legacy sensors have accuracy and maintenance problems is that they must be attached or adhered to the surface of the shell or skin of the vessel. The harsh and hot environment leads to degradation of connections, failed junctions, delamination, separation from the surface, and ongoing maintenance expense and hassle. Over time, as the heat and weather elements degrade these traditional sensors, plant personnel and management lose confidence in monitoring systems that were supposed to safeguard the equipment.

Innovative thermal imaging systems, however, have demonstrated how radiometric thermography has evolved into a mature and cost-competitive alternative. The noncontact nature of infrared thermal imaging allows it to be more robust, more reliable, and easier to maintain. It is also easy to interface with modern control systems using technological advantages such as graphical visual displays, historical archiving and trending, and easy integration to plant SCADA systems. All these result in providing operators with better insight into the health of their reactors and processes, which has a major impact on safety for personnel and the overall plant.

Seeing is believing

One of the emerging trends making inroads in the chemical, power, and refining sectors is the proliferation of thermal imaging cameras for a variety of applications, including critical vessel monitoring. These devices allow operators of high-temperature and high-pressure vessels to see, in color, real-time thermal behaviors of equipment. This insight is unavailable with fiber optic systems, giving infrared thermal imaging an edge when it comes to early detection of possible failures.

Thermal imaging systems go further by providing a more complete look at the temperature profile of the vessel, highlighting where potential dangers lie. With a system of infrared cameras constantly monitoring the environment as a whole, the potential to detect an emerging problem early is much higher.

Vessel monitoring in action

A large system using 14 infrared cameras for a single gasifier has been online for over eight years, monitoring a Chevron-Texaco-designed gas separations system for a major specialty gas producer. According to maintenance personnel, the original thermocouple-grid system started to degrade from the day it was installed because it was in direct contact with the vessel shell. Over time, the internal elements began to react at different temperatures and operators lost confidence in the data. The old system gave only a general idea of where a potential problem might be developing, constraining the ability of operators to respond proactively. Furthermore, it had to be removed and reinstalled whenever work had to be done on the vessel internals, which consumed a considerable amount of extra labor and time.

After implementing the infrared imaging system, the operators quickly realized several benefits. The most convenient was the ability to connect directly with their plant’s DCS and data historian system. With the installation of a thermal imaging system, they were no longer forced to react to problems as they occur. Instead, over time they were able to document the temperature personality of the gas separation system and catalog its behaviors to properly assess, predict, and respond to potential problems.

Moreover, they could store weekly thermographs of their vessel to benchmark normal patterns and compare changes over time. After rebricking their unit with a new refractory lining, they were able to establish specific locations of hot zones and kept an eye on changes over time. This helped to understand the degradation of the refractory, particularly when zones get progressively hotter, which normally indicated some substrata refractory problem.

With the new system, operators receive alarms much further in advance, which gives ample warning to help make informed decisions on how to respond to potential dangers. Having a high level of confidence in knowing where safe limits are, operators know when to ride out an event or when to shut down. During start-up of the unit, for example, the plant operators thought they had a hot spot developing on a nozzle connection, which would not have been detected with a thermocouple-based system. However, the thermal imaging system allowed them to continue monitoring the area and the situation never progressed to an alarm. The infrared imaging system allowed them to make an informed assessment of the risk in plenty of time and determine that it was prudent to go forward while monitoring.

Enhancing safety

Better alarm response time, enhanced predictive abilities, and improved information availability have emerged as three key benefits for radiometric infrared imaging in critical vessel monitoring. Operators are now able to connect their monitoring systems to their DCS and data historians to analyze trends and obtain information necessary to make important maintenance and capital decisions that impact the bottom line of their plants.

A continuous, real-time stream of information showing exactly how a vessel is behaving allows operators to identify potential problem areas before they even arise and analyze the occurrence later. This prevents emergencies and mitigates unplanned downtime in plant areas where sufficient insight was difficult, or impossible, to obtain.

Plants continue to upgrade their monitoring systems to noncontact, infrared solutions, and greenfield designs increasingly incorporate infrared solutions into the specifications from the inception. As the manufacturing industries continue to evolve, the key to success is ensuring the right technology is in place to help them evolve as safely as possible. 

Shaver is director of business development for LumaSense Technologies, Inc.