Feb-2024

Refractory detection system and floating roof protection

New refractory detection system monitors skin temperatures in Claus or thermal oxidisers as well as SMRs, gasifiers, and emissions from floating roof tanks.

Bob Poteet and Andrea Biava, WIKA
Haytham Al-Barrak and Mahendran Sella, Saudi Aramco

Article Summary

There are many applications in the process industries where detecting a hotspot on the outside of an operating unit can bring safety and protection of valuable assets, such as the refractory detection system (RDS) under discussion. This technology can be used in a variety of applications, such as in the Claus section of a sulphur recovery unit (SRU).

An installation was recently completed at the Aramco gas plant. The results Aramco saw in its investigation after six months of run-time (now 12 months running) will be discussed, in addition to some other applications. The device’s potential applications are only limited by the processor’s imagination (see Figure 1). So, what exactly is this technology?

The heart of the system is a special sheath (typically 4.5mm or 3/16in) made of 316SS that only reads out the highest temperature anywhere along the sheath. To install the system on a Claus unit, first divide the unit into zones, which are areas designated for sensing. As illustrated, there can be anywhere from one to six zones in most Claus units, depending on the licensor or operator’s preference. The readout from each zone will resemble a type K thermocouple, but it indicates the hottest area in a zone. You will not know where the hotspot is in the zone, but you will be aware that one exists so that appropriate action can be taken (see Figure 2). This technology has been successfully running on a Claus unit for more than 12 years at a major refinery in Italy.

Refractory problems in Claus or thermal oxidisers
For many operators of SRU plants, detecting refractory problems in the Claus unit can keep them up at night. If the refractory starts to fail, a situation may occur where the hot gasses hit the carbon steel shell and damage or lead to failure of the wall of the reactor. Many ways to detect this have been tried, but they all have limitations. Some have tried thermal imaging, but the problem with rain shields, cowlings, and insulation can be a real barrier.

A couple of points should be noted:
• This is not a thermocouple. Modern transmitters have self-diagnostics built in, and readings below 120°C are not reliable. While we can prove the system is working at installation, you will have to start the unit without a stable reading. Older transmitters can be used that read below 0°C, but careful consideration would need to be made in the evaluation process
• Above 400°C, the readings will lose their reliability again, but they have done their job. No damage will occur to the system until it reaches 900°C.

Uncertainties
The industry standard has been to attach thermocouples, but nobody knows how many to install. As a result, many installations do not have any thermocouples at all. Thermocouples can indicate the temperature of a specific point, but temperature excursions in other areas will go undetected. If a reactor with thermocouples could be covered, costs increase substantially.

The American Petroleum Institute (API) states that an operator should have a system so that an ‘accurate shell temperature measurement system under the shroud should be included in the ETPS design.’ They also request routine thermal imaging of the external shell to spot-check the thermocouples. This can be a maintenance headache if followed as it is intended.
Aramco findings

In an effort to resolve technical challenges, Saudi Aramco (SA) piloted a newly developed RDS to monitor the skin surface temperature of SRU thermal reactors (reaction furnace).

Since 90% of SA SRU units are shrouded (weather- shielded), measuring the surface temperature online becomes challenging. Without the correct monitoring device in place, catastrophic failures could occur, affecting plant throughput. Deployment of this technology aims to detect repetitive refractory failures early by sensing hot spots on the reactor surface, even with the presence of a shroud. Maintenance and shutdown will be planned and timed accordingly.

Collaboration between the technology provider, WIKA, and SA began in March 2022 with the installation of the thermocouple on one of Saudi Aramco’s gas plant SRUs. In October 2022, the CTLS installation was completed and successfully tested.

The plant continued monitoring the unit, keeping a close eye on the measured skin temperature to confirm continuous and reliable outputs (see Table 1). Upon testing performance, the new linear thermocouple technology proved to be working well. The CTLS coils’ peak temperature readings were recorded at 10 different time stamps over the course of eight months of operation since the coils were first put into operation.

This is a great tool, but based on our first use, we believe the way forward is if we had the ability and freedom to move the thermocouple CTLS while the unit is running without the need to weld on the reactor casing. We are jointly evaluating the use of magnets to facilitate this feature.

Installation
The RDS is installed by a trained field crew. Nelson studs are applied to the reactor, and the system is held down by galvanised steel channels attached to the Nelson studs. This system ensures good contact between the RDS and the reactor shell. On a larger Claus, it took a three-man crew five to six days to completely install a six-zone unit (see Figures 3 to 5).
Industry feedback

An Italian refiner who had this bespoke system installed 12 years ago agreed to provide feedback, reporting that he knew of no other system that could provide the required coverage, particularly the need for system reliability. While two of the four zones were lost after contractors cut components, a high level of reliability was still maintained. We enquired if the system had ever alarmed during the 12-year period and he reported that it had only done so twice. In neither case did the hotspot reach the point where he had to shut down. However, during the next routine maintenance, they could see that in the zones where the alarm occurred there were indeed some refractory problems.

Other applications
Other applications include:
• Gasifier applications: Gasifier applications are very similar to the Claus but in a much larger way with more zones. The sensors are run vertically over the full unit and at the top.
• Steam methane reformers (SMRs): The outlet header in an SMR is a refractory line piping that requires constant monitoring of the outside temperature by operators. Some use thermocouples, while others just do scanning. A common question related to thermocouples is how many are needed to give complete coverage while the scanning is a real Opex cost. For this solution, we can simply wrap the pipe with the RDS and monitor for any refractory failures. This can be easily installed with a clamping system or even magnets.