Selection of process isolation diaphragm and filling fluid when selecting Rosemount pressure sensors
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I work in pump testing and often purchase pressure transmitters, with the 3051 model being the most commonly chosen. As for the materials used for the process isolation diaphragms in these devices, there are various options such as stainless steel, Hastelloy, Monel, and tantalum; the filling fluids can be silicone oil or inert liquids. Are there any colleagues who are familiar with these materials and could explain the differences between them and advise on when to use each one? Thank you
Materials such as stainless steel, Hastelloy, Monel, and tantalum have different corrosion resistance levels; the choice of material depends on the specific medium involved. The difference between ordinary silicone oil and inert liquids is that one is used at medium and low temperatures, while the other is used at high temperatures
Keep in mind that the material is chosen based on the medium, while the filling fluid is selected based on temperature
The medium I use is plain water, with a maximum temperature of 250 degrees. I usually opt for stainless steel diaphragms, along with silicone-based filling fluids
I called Rossmont’s customer service line at 400 today to ask questions about the filling fluids. The technician explained that the choice of filling fluid indeed depends on temperature, but silicone oil can withstand higher temperatures; he checked the data and said that the maximum operating temperature for silicone oil is 121 degrees, while that for inert liquids is only 85 degrees. He said that a long enough pressure tap is necessary; although the temperature of the medium being measured is high, the long length of the pressure tap ensures that the medium inside it hardly moves, so the temperature of the medium in contact with the sensor is not high. Therefore, silicone oil is usually sufficient. If there is still concern, it is possible to measure the temperature of the medium at the point where the pressure tap meets the sensor. I’m not sure either if the explanations given by their technical staff are accurate; I’m sharing this for reference. If you have similar questions as I do, it’s quite convenient to call them.
Stainless steel: Due to its high chromium content, stainless steel exhibits good corrosion resistance and oxidation resistance; however, its mechanical properties and workability are poor. It is commonly used in acid-resistant structures that are not subject to heavy stresses, as well as as an oxidation-resistant steel. This type of steel can resist corrosion from the atmosphere, *ao acid, and saline solutions. It also features good oxidation resistance at high temperatures and a low coefficient of thermal expansion. It is used in equipment for *ao acid and food processing industries, and can also be used to manufacture components that operate at high temperatures, such as parts for gas turbines. Stainless steel corrodes very rapidly in environments with chloride ions, even faster than ordinary low-carbon steel. Hastelloy: Hastelloy is suitable for various chemical industries that involve oxidizing and reducing media. The higher contents of molybdenum and chromium enable the alloy to resist chloride ion corrosion, while tungsten further enhances its corrosion resistance. At the same time, C-276 Hastelloy tubing is one of the few materials resistant to corrosion by humid chlorine gas, hypochlorite, and chlorine dioxide solutions; it also exhibits significant corrosion resistance to high-concentration chloride solutions such as ferric chloride and copper chloride. Application areas: heat exchangers, bellows compensators, chemical processing equipment, flue gas desulfurization and denitration, paper industry, aerospace applications. In acidic environments, Monel: Monel 400 is a corrosion-resistant alloy that is widely used, has numerous applications, and exhibits excellent overall performance. This alloy exhibits excellent corrosion resistance in hydrofluoric acid and fluorine gas environments, as well as good resistance to hot concentrated alkaline solutions. It is also resistant to corrosion by neutral solutions, water, seawater, the atmosphere, organic compounds, and more. An important feature of this alloy is that it generally does not develop stress corrosion cracks, and it has good machinability. The microstructure of Monel 400 alloy is a high-strength single-phase solid solution. Product applications: Monel 400 alloy is a versatile material that can be used in many industrial fields: 1. Seamless water and steam pipes in power plants; 2. Sea water exchangers and evaporators; 3. Environments involving sulfuric acid and saline acids; 4. Crude oil distillation; 5. Pump shafts and propellers for equipment used in deep-sea environments; 6. The nuclear industry, where it is used to manufacture equipment for uranium processing and isotope separation; 7. Pumps and valves used in equipment for producing saline acids. Tantalum: The properties of tantalum enable it to be used in a wide range of applications. In equipment for producing various inorganic acids, tantalum can be used as a substitute for stainless steel, offering a service life that is dozens of times longer than that of stainless steel. Furthermore, in industries such as chemicals, electronics, and electrical engineering, tantalum can replace the role previously played by the precious metal platinum, thereby **reducing** costs. Tantalum is used to manufacture capacitors for military equipment.
The filling fluid is chosen primarily based on temperature, and it should be of food-grade quality so that no harm will occur even in the event of a leak. When selecting a material, factors such as the medium, temperature resistance, corrosion resistance, wear resistance, and the viscosity of the medium must be taken into account. However, Rosemount offers model selection services; you just need to specify your specific application conditions, and they will choose the right model for you.