Measuring temperature in industrial processes operating under high pressure is considered one of the greatest challenges in instrumentation engineering. Whether in chemical reactors, hydraulic systems, oil extraction, or high-performance boilers, the temperature sensor faces an extremely hostile environment. If the device is not properly specified, it can suffer crushing, mechanical fatigue, or, in severe cases, cause catastrophic leaks that jeopardize the integrity of the plant. But how to choose?
Which temperature sensor should I use for high pressure?
To choose the ideal sensor, it is necessary to analyze variables that go far beyond the simple desired temperature range. Mechanical integrity, fluid velocity, and chemical compatibility become absolute priorities.
Under normal operating conditions, a temperature sensor only needs to transfer heat from the environment to its internal reading element as quickly as possible. However, when we introduce the variable of high pressure, the physics of the process changes drastically. The pressure exerts a crushing compressive force on the sensor sheath. If we use conventional sensors, the internal insulation can collapse, causing a short circuit in the lead wires or rupture of the casing.
Furthermore, pressurized fluids generally move at high speeds. This flow generates vortices and vibrations that cause mechanical fatigue at the sensor's mounting point. To solve this problem, Alutal relies on mineral compaction technology and machined external protections, ensuring that the sensor survives intact under the worst conditions of mechanical stress.
Thermocouples or Pt100 resistance thermometers?
The first technical decision involves the sensor element. Both technologies can be adapted for high pressure, but each shines in a different scenario:
Thermocouples (type K, J and N): the thermocouples They are formed by the joining of two distinct metals that generate a millivoltage proportional to the temperature. They are the champions of robustness. Because they do not have fragile components at their tip, they tolerate high levels of vibration and thermal shock. In addition, they easily reach temperatures exceeding 1000°C.
A Alutal manufactures a line of Mineral Insulated Thermocouples. (TIM Series), where the wires are encased in highly compacted magnesium oxide powder within a metallic sheath. This construction eliminates voids, preventing pressure from crushing the sensor.
Resistance thermometers (RTD / Pt100)Resistance temperature detectors (RTDs) are based on the variation of the electrical resistance of a platinum wire. They offer the highest possible precision on the industrial market, as well as excellent repeatability. They are ideal for processes that require fine control in ranges up to 600°C. To combine this precision with robustness, Alutal offers the TRP Series (RTD with Protection Well), protecting the platinum element against direct crushing.
What is a thermowell and why is it important?
In high-pressure applications, the temperature sensor almost never comes into direct contact with the fluid. It is inserted inside a metal casing known as a thermowell. The thermowell acts as a physical barrier. It remains in contact with the pressure and corrosive fluid, while the sensor is safely housed inside. This allows the sensor to be removed or calibrated without needing to depressurize the system.
For high pressure applications, Alutal emphasizes that wells must be machined from a single solid metal bar (deep drilling). Wells manufactured from welded pipes have weak points in the welds and should not be used at high pressures.
How to prevent failures caused by vibration and resonance?
A common mistake in high-pressure projects occurs when fluid velocity is ignored. As pressurized fluid flows around the thermowell, it creates vortex trails. If the frequency of these vortices matches the natural vibration frequency of the wellbore metal, resonance occurs. The well will vibrate until it ruptures due to fatigue.
To avoid this, it is essential to perform conveyor frequency calculations according to the ASME PTC 19.3 TW standard. Alutal offers engineering support to ensure that the well dimensions are within the safety zone. Furthermore, for environments with extreme vibration, the company has developed the ANTI-VIB Series, with reinforced internal damping to prevent electrical components from breaking due to constant shaking.
Checklist for choosing the correct temperature sensor.
To avoid mistakes in the specification, follow these steps recommended by Alutal experts:
- Set the temperature and accuracy: choose between the accuracy of Pt100 or the robustness of the thermocouple.
- Check the fluid pressure and velocity: Determine if you will need a machined solid rod well.
- Require the ASME PTC 19.3 TW calculation: ensure the well will not break due to resonance.
- Assess the corrosion of the environment: select the compatible metal alloy (stainless steel, Inconel, etc.).
- Opt for mineral insulation: use compact sensors to prevent internal collapse under pressure.
Investing in the correct sensor specification prevents line stoppages, excessive maintenance costs, and ensures the safety of operators in the industrial plant. Partnering with companies like Alutal ensures that every technical detail is respected, transforming measurement into confidence.
