{"id":18524,"date":"2025-05-14T12:36:47","date_gmt":"2025-05-14T17:36:47","guid":{"rendered":"https:\/\/blog.wika.com\/us\/?p=18524"},"modified":"2025-12-04T21:51:01","modified_gmt":"2025-12-05T02:51:01","slug":"testing-accuracy-different-cot-sensor-designs","status":"publish","type":"post","link":"https:\/\/blog.wika.com\/us\/applications\/testing-accuracy-different-cot-sensor-designs\/","title":{"rendered":"Improving Olefin Production Efficiency: Testing the Performance of Different COT Sensor Designs"},"content":{"rendered":"

To minimize coking and maximize the conversion rate in a steam cracker, olefin producers need to closely monitor the furnace\u2019s coil outlet temperature (COT). Various types of sensors measure this key parameter<\/strong>. WIKA USA tested the common sensors designs to evaluate their performance and accuracy.<\/b><\/p>\n

This article is a continuation of \u201cImproving Olefin Production Efficiency with Coil Outlet Temperature (COT) Measurement<\/a>.\u201d<\/em><\/p>\n

Designs for COT sensors<\/h2>\n

The market currently offers three main types of sensors for measuring temperature at the coil outlet of ethylene steam crackers. Each design has its advantages and disadvantages.\u00a0<\/p>\n\n

(top)<\/i> drilled thermowell coated with erosion-resistant alloy, (bottom)<\/i> drilled bar of erosion-resistant alloy welded to thermowell stem<\/i><\/p><\/div>\n \n

1. Immersed sensor with thermowell<\/h3>\n

A replaceable thermocouple is installed within a thermowell that\u2019s inserted directly into the process stream. Since the stream in an ethylene cracker is extremely hot as well as abrasive, due to the presence of hard coke particles, manufacturers protect thermowells in two ways: by coating the outside with an erosion-resistant allow, or by fabricating the tube from a solid piece of an erosion-resistant alloy and welding it directly to a standard thermowell stem.<\/p>\n

The advantage of using an immersed temperature sensor is that the thermowell is in direct contact with the process<\/a>. This design is also easy to install if a process entry is available. The disadvantage is that despite the protective erosion-resistant alloy, immersed thermowells are still constantly buffeted by coke particles and will need to be replaced regularly, with the frequency depending on each facility\u2019s operating conditions and the quality of the feedstock. (Some plants turn their thermowells 180\u00b0 every six months or so to increase the thermowells\u2019 lifespan.)<\/p>\n

2. Permanently welded surface sensor<\/h3>\n\n

The sensor block of this tubeskin thermocouple is welded to the surface of a coil outlet pipe.<\/p><\/div>\n \n

Tubeskin thermocouples<\/a> are commonly used for these installations. With this style, the thermocouple is welded directly to the outside of the coil outlet tube<\/span>. <\/span>Thermal insulating material covers the sensor block to protect the measuring point from the influence of ambient conditions.<\/p>\n

The advantage of this design is the guarantee that the measuring point is in direct contact with the tube surface<\/a>. And since tubeskin thermocouples never come into contact with the process stream, they typically have a longer lifespan. The disadvantage of a permanently welded sensor is that installation calls for a trained technician using welding procedure specifications (WPS) tailored to the metallurgy of the coil; otherwise, sensor performance \u2013 particularly accuracy \u2013 is compromised. And when a replacement is needed, the sensor is either cut or abandoned, and a new sensor is permanently installed at another target location.<\/p>\n\n

A replaceable thermocouple is inserted into a curved guide tube whose tip has been welded onto the surface of a furnace coil.<\/p><\/div>\n \n

3. Replaceable surface sensor<\/h3>\n

In this design, a curved guide tube is mounted onto the surface of a coil outlet pipe<\/a>, with the tip of the guide tube fully welded to the coil. (An incomplete weld can create an air gap, which results in heat loss or increases the temperature influences from ambient conditions.) A thermocouple is inserted into the guide tube, making direct contact with the tip of the tube to achieve full conduction.<\/p>\n

The advantage of this design is that when a sensor needs to be replaced, operators can simply remove the failed thermocouple and insert a new one; no additional welding is needed. One disadvantage of this design is that when installing the sensor into the curved guide tube, it is difficult to know for certain whether the sensor extends to the end of the tube. If it does not, the air gap will have a negative impact on the sensor\u2019s accuracy. Similarly, accuracy is compromised if the sensor does not make contact with the tube surface.<\/p>\n

Process temperature vs. tubeskin temperature<\/h3>\n

The industry preference for COT sensor styles are diverse, as immersed-sensor systems and surface-sensor systems each have their advantages and disadvantages. Some operators prefer having data on their process temperature, while others prefer knowing the tube metal temperature.<\/p>\n

When deciding on a sensor design for a particular application, operators weigh factors such as initial cost, ease of installation and replaceability, and sensor longevity. However, the primary consideration should be accuracy, followed by a fast response time.<\/p>\n

So, how do these styles compare with one another when monitoring temperature at the coil outlet? To find the answer, a team at WIKA USA conducted a series of tests on three main designs.<\/p>\n

COT Sensor Test Design: Setup<\/h2>\n

Testing for these sensors was conducted at WIKA\u2019s R&D Center in Pasadena, Texas, which was purpose-built to be able to replicate real-world applications. Within the R&D Center is a process loop containing a 9.6M BTU fired heater and fully functioning reactor. This setup provides the ability to run repeatable, controlled tests on various products in different stages of a live refining process.<\/p>\n

Furnace installations<\/h3>\n

To run tests on the COT measurement styles, the fired heater was used to simulate COT conditions. While WIKA does not have an ethylene cracker to work with, custom fixtures were fitted onto the heater to allow radiant gases to pass through an exhaust tube. Each sensor style was fitted to its own outlet tube, which included built-in reference sensors for comparisons. The entire system was designed to withstand temperatures as high as 900\u00b0C (1650\u00b0F).<\/p>\n

Sensor styles<\/h3>\n

The tests aimed to compare common sensors that would be installed in the industry. The sensors tested include:<\/p>\n