Multipoint Temperature Assemblies
Let’s talk about multipoint temperature assemblies. Petroleum refining needs complex yet sturdy devices to provide strict, efficient control of the processes. Refineries use catalytic reactions to get the desired quality and remove contaminants. The reactors can be up to 40 meters tall and must work nonstop 365 days a year for anywhere from 6 to 12 years. This demanding task calls for special assemblies.
Why multipoint temperature assemblies?
Because the temperatures of these reactions can reach 400-500 degrees Celsius and pressures that exceed 20 megapascals (MPa). Therefore, the refinery must minimize the number of nozzles yet maximize the number of measurements. These assemblies can provide up to 48 measuring points from a single nozzle. Also, thanks to thermocouples, they can detect hot spots on reactor beds that may affect the reaction, the reactor, or the catalyst.
Did I mention that changing a catalyst can cost more than $3 million? Oh yes! And this value doesn’t include downtime or labor. Downtime alone can cost $500,000 per hour. Gets expensive quickly!
Process licensors and internals providers define the requirements for all components, including multipoint assemblies.
Before purchasing your assembly, check the requirements from the licensor.
This list covers the major licensors and internals suppliers in the world:
- GTC TECHNOLOGY
- HALDOR TOPSOE
- KBR TECHNOLOGY
- SHELL GLOBAL SOLUTIONS
Wait, did you say 500 degrees Celsius at 200 bar?
Indeed I did. Reactions such as hydrocracking and hydrotreating occur around 450 degrees Celsius at pressures up to 200 bar.
200 Bar = 20 MPa or 2900 psi. Quite high, no?
These conditions make designing and building these assemblies challenging, but they have features that support performance in such extremes.
Multipoint assemblies can read temperatures reliably and safely for up to 12 years. Most use flanged connections, like RTJ and RF flanges, for pressure ratings up to 2500 pounds. But what challenge would engineers have if variants didn’t exist, like the EN 1092-1 or the GOST 12821-80 for the Russian market?
Licensors like Chevron Lummus Global and Axens use Hiltap® and Grayloc® modular connectors respectively, to improve installation, commissioning, and maintenance. so the variety of configurations is virtually infinite.
Besides high temperatures and pressures, these sensors must also withstand corrosive fluids. Therefore, suppliers use stainless steels like AISI 321 or AISI 347 or nickel alloys like Alloy 600 or Hastelloy. These materials have corrosion allowances up to three times higher than common stainless steels.
Most plants use type K, J or N thermocouples, built from mineral-insulated (MI) cable, with sheaths welded to the connection to avoid leaks.
I suggest sensors with single-sheath MI cables, so you’ll have no extra welds. Also, choose heavy wall MI cable, which increases the corrosion allowance and thus the life of the sensor.
As I explained earlier, the plant must shut down to replace assemblies, so they also have secondary chambers to contain leaks if the primary barriers fail. These chambers have monitors to provide status updates at any time.
The chamber helps avoid unexpected downtime by controlling leaks until the plant can plan a shutdown.
These assemblies also come with junction boxes to connect the sensors with the control system.
The junction box may have compensated terminal blocks or temperature transmitters. I suggest temperature transmitters because they use standard protocols for the signal, making it stable and unaffected by connection distance.
Few companies build assemblies for 500 degrees Celsius and 20 MPa, because these devices must meet the highest standards in the industry.
So before you buy an assembly, I suggest the following steps:
Have all retaining welds dye-tested, and test at least 20 percent of the chamber welds with a volumetric method, like X-ray or ultrasonic.
On all thermocouples, inspect continuity and resistance insulation and do three-point calibration at the process temperature.
To increase measurement reliability, inspect the hot joints of every sensor.
Test the process connection and the secondary chamber for hydrostatic pressure around 1.5 times the process pressure.
Use helium to test welds, checking for microcracks.
The length of a single sensor in an assembly can go up to 25 meters, so consider installation support when purchasing an assembly.
CAD software can find the right length, but the routing and position of the sensors need to follow certain rules:
- Avoid blocking manways and nozzles with the MI cables.
- Route the MI cable through the shadow of the internal structures to avoid condensate, especially near the distribution tray.
- For vertical sensors, add length to reduce stress from expansion during operation.
- However, don’t add too much length to avoid channeling between the catalyst layers.
- Supporting frames need to follow the requirements of the process licensor.
- Install the sensor hot joint at least 200 mm away from the support structure to avoid thermal inertia.
- Use experienced installers to speed up installation and reduce potential overcosts or damages.
Multipoint temperature assemblies are some of the most complex devices on the market. So engineers, manufacturers, and users need expertise and support for the design, production, installation, and commissioning of this kind of assemblies.
To learn more, take a look at the following vendors:
- Daily Thermetrics