Selecting Instrumentation Components by Size, Temperature, Application, Media, and Pressure
Nov 22nd 2023
Instrumentation fittings and valves should be selected by the conditions of the system, not by appearance alone. The useful starting point is STAMP: Size, Temperature, Application, Media, and Pressure.
Those five checks connect the tube, fitting, valve, seal, and operating environment. If one is missed, the component may be the wrong material, wrong seal style, or wrong pressure class even when the thread size looks correct.
Size
Size starts with tube outside diameter and wall thickness. Those dimensions should be selected from the required temperature, pressure, flow rate, fitting specification, and surrounding conditions.
Tube size cannot be separated from the fitting family. A flare fitting may have a maximum wall thickness limit, while a flareless bite-type or ferrule fitting may require a minimum wall thickness to grip and seal correctly.
Temperature
The temperature range is controlled by both the fitting or valve material and the seal design. A metal-to-metal connection has different limits than an O-ring or elastomer seal.
Temperature should include the surrounding environment and the fluid or gas being transferred. A component that works at room temperature may not be suitable for hot process media, outdoor cold starts, or repeated thermal cycling.
Application
The application drives the selection. Moving hydraulic fluid is different from moving hazardous chemical vapor, fuel, diesel, gasoline, CNG, LNG, or steam. Corrosive service may require stainless steel or higher-alloy materials.
Steel fittings may use protective coatings, brass may be suitable in some corrosive or lower-pressure applications, and stainless steel may be required when corrosion resistance and strength are both needed.
Media
The fluid or gas must be compatible with the fitting body, valve body, tube, and seal material. Chemical compatibility matters because the media can attack elastomers, plating, base metal, or thread sealant.
When compatibility is uncertain, check the material data for the fitting and seal compound before installing the part. A correct thread cannot compensate for the wrong material.
Pressure
The pressure rating of an instrumentation fitting or valve should meet or exceed the system requirement. The connection design must account for both the rated strength of the component and the pressure expected in service.
Many fittings are rated with a 4:1 design factor under normal operating conditions with moderate mechanical shock, hydraulic shock, and vibration. A 3:1 factor may be used only where vibration, thermal load, and shock are not significant and the design allows it.
What Affects Pressure Capability
Pressure capability depends on tube wall thickness, tube or hose pressure rating, fitting style, material, media, seal design, and torque. Flare and flareless connections do not carry load in exactly the same way.
Final selection should follow the manufacturer's pressure rating data for the exact fitting and valve family. This is especially important in instrumentation systems where small parts may control a high-pressure process line.