Temperature Measurement Guardians in Corrosive Environments: How to Select the Material for Resistance Temperature Detector Tubes

In daily industrial production, we often need to measure temperature in various corrosive media. As a core component of temperature measuring instruments, the choice of material for the protective tube of a resistance temperature detector (RTD) directly affects the accuracy of the measurement and the service life of the equipment. Today, we will discuss, from the customer's perspective, the special requirements for the protective tube material when using RTDs in corrosive media.

Challenges of Corrosive Environments for RTDs

In special environments such as chemical plants, smelters, and thermal power plants, ordinary thermocouples and RTDs are easily damaged. The corrosive media in these environments can severely corrode temperature measuring instruments, leading to decreased measurement accuracy, shortened equipment life, and even safety accidents. Therefore, RTDs with special materials and structures must be used in these environments.

Corrosive media are diverse, including acidic, alkaline, and organic solvents, each with different corrosion mechanisms and degrees. This requires us to select appropriate protective tube materials based on the specific characteristics of the medium.

Corrosion Resistance Characteristics of Different Protective Pipe Materials

Stainless Steel Protective Pipe: Stainless steel (0Cr18Ni12Mo2Ti) protective pipes are suitable for weakly corrosive media such as industrial water, domestic water, and sewage. They are widely used in petroleum, chemical, and steel industries, as well as municipal and environmental protection fields. This is the most economical and practical choice, suitable for general corrosive environments.

Hastelloy Protective Pipe: Hastelloy comes in two types, B and C. Hastelloy B has good corrosion resistance to hydrochloric acid of all concentrations below its boiling point, and is also resistant to corrosion from non-chlorinated acids, alkalis, and non-oxidizing salt solutions such as sulfuric acid, phosphoric acid, and organic acids. Hastelloy C is resistant to corrosion from non-oxidizing acids, such as nitric acid, mixed acids, or mixtures of chromic acid and sulfuric acid, and is also resistant to corrosion from oxidizing salts. Hastelloy is an ideal choice for highly corrosive environments.

Titanium Protective Pipe: Titanium protective pipes can withstand corrosion from seawater, various chlorides and hypochlorites, oxidizing acids (including fuming sulfuric acid), organic acids, and alkalis. However, it is not resistant to corrosion from relatively pure reducing acids (such as hydrochloric acid). However, if the acid contains an oxidizing agent (such as nitric acid), its corrosiveness will be greatly reduced.

PTFE Protective Tube: PTFE is one of the most chemically stable plastics, resistant to boiling hydrochloric acid, sulfuric acid, nitric acid, and aqua regia, as well as concentrated alkalis and various organic solvents. However, it is not resistant to corrosion from chlorine trifluoride, high-temperature chlorine trifluoride, high-speed liquid fluorine, liquid oxygen, and ozone. PTFE protective tubes can be used for extended periods within the temperature range of 0-250℃, making them a common choice for various highly corrosive environments.

Key Considerations for Selecting Protective Tube Materials

As users, when selecting the material for a resistance temperature detector (RTD) protective tube, we need to consider the following factors:

Medium Properties: First, the chemical composition, concentration, temperature, and other parameters of the medium must be clearly defined. Different materials have significantly different corrosion resistance to different media; no single material can withstand all media. For example, PTFE is resistant to almost all chemical media, except for molten lithium, potassium, sodium, chlorine trifluoride, and high-flow-rate liquid fluorine trifluoride.

Temperature Range: Different protective tube materials have their applicable temperature ranges. For example, PTFE protective tubes are generally used long-term within the range of 0-250℃, while metal protective tubes are suitable for higher temperature environments.

Pressure Conditions: Operating pressure is also an important consideration. The maximum operating pressure for flanged installations can reach 4.0 MPa, while the maximum operating pressure for threaded connections is generally 0.6 MPa.

Expected Service Life: Different materials have different service lives, which directly affects the maintenance cycle and total cost of the equipment. In environments with both corrosion and erosion, additional wear-resistant layers can extend the life of corrosion-resistant components by 8-10 times.

Cost Budget: The manufacturing costs of different materials vary greatly, and a balance needs to be found between performance and cost.