Gauge/thermocouple (resistive)/bimetallic thermometer/pressure transmitter gasket specifications and materials

Thread Content

This post was last edited by Yunmu on 2019-4-8 at 14:56. Pressure gauges, pressure transmitters, thermocouples, and bimetallic thermometers that are installed using threaded connections all require gaskets. Instrument technicians must be familiar with the specifications of these gaskets. Changhui Instruments has compiled, based on HG/T 21581-2010 Automation Installation Manual and the technical data provided by manufacturers, the specifications for gaskets suitable for commonly used pressure gauges, pressure transmitters, thermocouples, and bimetallic thermometers for reference. The gasket specifications discussed in this article are all intended for pressure gauges, pressure transmitters, thermocouples, and bimetallic thermometers that have threaded process connections; they do not cover the selection of gaskets for instruments with flanged process connections. Points to consider when selecting gauge gaskets: Choosing the right gauge gaskets is key to ensuring no leaks in the equipment. The selection of gauge gaskets must take into account the conditions of the medium (such as its physical properties, pressure, temperature, operating conditions, and duration of continuous operation), ensuring that the sealed medium does not leak or get corroded. The gaskets must be able to withstand factors such as temperature, pressure, and corrosion over a long period of time. Over time, gauge gaskets are significantly affected by temperature and pressure: they all have a maximum operating temperature; as the temperature rises, the material of the gasket softens, undergoes creep, and stress relaxation occurs, resulting in a decrease in the gasket’s mechanical strength and sealing capacity. The same is true in reverse. Instrument gasket materials all have specified limits for operating temperature and pressure; using them under these extreme conditions poses no safety guarantee. Specifications and materials of pressure gauge gaskets ① Thread specifications for pressure gauges ◆ In accordance with GB/T 1226-2001 \"General Pressure Gauges\", the thread specifications for pressure gauges include three types: 60° fine-thread ordinary threads, namely M10×1, M14×1.5, and M20×1.5. Gaskets for pressure gauges are required when installing metric-threaded pressure transmitters. http://yunrun.com.cn/upload/201904/07/201904070036486087.png ◆ For process connections, pressure gauges use 55° non-self-sealing cylindrical thread connections; the available thread specifications for such pressure gauges are G1/8″, G1/4″, G3/8″, G1/2″, and G3/4″. http://yunrun.com.cn/upload/201904/07/201904070056215764.png ◆ For process connections, pressure gauges use 55° sealing tapered pipe thread connections; the available thread specifications for such pressure gauges are ZG1/8″, ZG1/4″, ZG3/8″, ZG1/2″, and ZG3/4″. http://yunrun.com.cn/upload/201904/07/201904070109105730.png ◆ For process connections, pressure gauges use 60° American-style sealing tapered pipe thread connections; the available thread specifications for such pressure gauges are NPT1/8, NPT1/4, NPT3/8, NPT1/2T, and NPT3/4. http://yunrun.com.cn/upload/201904/07/201904070117578433.png ② Specifications and materials of pressure gauge gaskets ◆ When the thread connection of the pressure gauge is M20×1.5, the appropriate gasket specification is φ18×8×3 (outer diameter × inner diameter × thickness). Polytetrafluoroethylene gauge gaskets are used in the vast majority of applications ； For the high-pressure sections of the pressure gauge gaskets in hydrogenation units, aluminum gaskets are used exclusively (as is common in the design drawings of SEI, LOPEC, Chengda, Tianchen, and Shanghai First Institute of Industry) ； For pressure gauges used in thermal power plants, annealed copper gaskets with specifications of φ18×8×3 (outer diameter×inner diameter×thickness) are selected. Note: Red copper gaskets cannot be used as gaskets for gas pipes. ◆The specifications of the gauge gaskets to be used when the pressure gauge thread connections are M14×1.5 and M10×10 need to be specified. Specifications and materials for fixed-thread-mounted thermocouple gaskets: For thermocouples (thermoresistors) connected via threads, common thread specifications include M27×2, M33×2, M16×1.5, M12×1.5, G1/2″, G3/4″, and G1″ external threads. The corresponding specifications for thermocouple gaskets are as follows: ◆ When the thermocouple has an M27×2 external thread, the appropriate gasket specification is φ43×30×3 (outer diameter × inner diameter × thickness). ◆ When the thermocouple has an M33×2 external thread, the appropriate gasket specification is φ50×34×2 (outer diameter × inner diameter × thickness). ◆ When the thermocouple has an M16×1.5 external thread, the appropriate gasket specification is φ32×18×2 (outer diameter × inner diameter × thickness). ◆ When the thermocouple has an M12×1.5 external thread, the appropriate gasket specification is φ28×14×2 (outer diameter × inner diameter × thickness). ◆ When the thermocouple has a G1/2″ external thread, the appropriate gasket specification is φ35×22×2 (outer diameter × inner diameter × thickness). ◆ When the thermocouple has a G3/4″ external thread, the appropriate gasket specification is φ43×30×2 (outer diameter × inner diameter × thickness). ◆ When the thermocouple has a G1″ external thread, the appropriate gasket specification is φ51×38×2 (outer diameter × inner diameter × thickness). ◆ For armored thermocouples connected using clamp flanges, the appropriate gasket specification is φ25×10×2 (outer diameter × inner diameter × thickness). ◆ When installing wear-resistant thermocouple flanges equipped with shut-off valves, the appropriate gasket specification is φ102×49×3.2 (outer diameter × inner diameter × thickness). The material of the thermocouple gasket is usually selected according to the following criteria: when the temperature is less than 350°C and the pressure is less than 4.0 MPa, asbestos rubber gaskets are used for thermocouples ； When the temperature is < 600°C and the pressure is < 6.3 MPa, a graphite composite gasket should be used for the thermocouple ； Copper gaskets (used after annealing) are generally selected for thermocouples in thermal power plants. Specifications and materials for fixed-thread-mounted thermoresistive gaskets: For thermoresistors connected via threads, common thread specifications include M27×2, M33×2, M16×1.5, M12×1.5, G1/2″, G3/4″, and G1″ external threads. The corresponding specifications for the thermoresistive gaskets are as follows: ◆ When the thermoresistor has an M27×2 external thread, the appropriate gasket specification is φ43×30×2 (outer diameter × inner diameter × thickness). ◆ When the thermoresistor has an M33×2 external thread, the appropriate gasket specification is φ50×34×2 (outer diameter × inner diameter × thickness). ◆ When the thermoresistor has an M16×1.5 external thread, the appropriate gasket specification is φ32×18×2 (outer diameter × inner diameter × thickness). ◆ When the thermoresistor has an M12×1.5 external thread, the appropriate gasket specification is φ28×14×2 (outer diameter × inner diameter × thickness). ◆ When the thermoresistor has a G1/2″ external thread, the appropriate gasket specification is φ35×22×2 (outer diameter × inner diameter × thickness). ◆ When the thermoresistor has a G3/4″ external thread, the appropriate gasket specification is φ43×30×2 (outer diameter × inner diameter × thickness). ◆ When the thermoresistor has a G1″ external thread, the appropriate gasket specification is φ51×38×2 (outer diameter × inner diameter × thickness). ◆ For armored thermoresistors connected using slip-on flanges, the appropriate gasket specification is φ25×10×2 (outer diameter × inner diameter × thickness). The material of the thermoresistive gasket is usually chosen according to the following criteria: when the temperature is less than 200°C, the pressure is less than 4.0 MPa, and there is a corrosive medium present, polytetrafluoroethylene gaskets are used for such thermoresistors ； When the temperature is <350°C and the pressure is <4.0 MPa, an asbestos rubber gasket should be used for the thermal resistor ； When the temperature is <600°C and the pressure is <6.3 MPa, a graphite composite gasket should be used for the thermal resistor ； For thermal resistors used in thermal power plants, copper gaskets (used after annealing) are generally selected. Specifications and materials of bimetallic thermometer gaskets: Although bimetallic thermometers have the same mounting threads as thermocouples (thermoresistors), the position of their threaded sealing surfaces differs from that of the thermocouple (thermoresistor) threaded sealing surfaces. Therefore, the gaskets/installation protrusions for bimetallic thermometers cannot be used interchangeably with those for thermocouples (thermoresistors), and mixing them is not allowed; strict distinction must be maintained when using them. ◆When the bimetallic temperature corresponds to an M27×2 external thread, the recommended gasket specification is φ22×14×2 (outer diameter×inner diameter×thickness). ◆ When the bimetallic temperature corresponds to an M27×2 internal thread, the recommended gasket specification is also φ22×14×2 (outer diameter×inner diameter×thickness). ◆ For bimetallic temperatures with other types of threads, the appropriate gasket specification needs to be determined. The material selection for gaskets in bimetallic thermometers is generally based on the following requirements: when the temperature is less than 200°C, the pressure is less than 4.0 MPa, and there is a corrosive medium present, polytetrafluoroethylene gaskets are used for bimetallic thermometers ； When the temperature is < 350°C and the pressure is < 4.0 MPa, an asbestos rubber gasket should be used for bimetallic thermometers ； When the temperature is < 600°C and the pressure is < 6.3 MPa, a graphite composite gasket should be used for bimetallic thermometers ； For bimetallic thermometers used in thermal power plants, copper gaskets (used after annealing) are generally selected. Specifications and materials for gaskets of direct-connection pressure transmitters: Common installation threads for directly-mounted pressure transmitters include M20×1.5 external threads and NPT1/2 threads, etc. Gaskets are not required for NPT threads, while gaskets are necessary for all directly-connected pressure transmitters that have non-self-sealing cylindrical threads. ◆When the threaded connection of the pressure transmitter is M20×1.5, the pressure gauge gasket to be used is a polytetrafluoroethylene gasket with specifications of φ18×8×3 (outer diameter×inner diameter×thickness). Note: Red copper gaskets cannot be used as gaskets for gas pipes. Specifications and materials of gaskets for the three-valve assembly of differential pressure transmitters: The gaskets at the wafer flanges of the three-valve assembly used with differential pressure transmitters have specifications of φ24×17×3 (outer dimension×inner diameter×thickness), and they are made of polytetrafluoroethylene. Specifications and materials of gaskets used inside instrument fittings ◆ The specifications for gaskets used inside 1/2″ fittings are φ26×18×3 (outer dimension×inner diameter×thickness), made of polytetrafluoroethylene. ◆The gasket used inside the 3/4″ threaded connection has specifications of φ33×24×3 (outer dimension×inner diameter×thickness), and it is a polytetrafluoroethylene gasket. The selection of materials for gasket inserts to be used in certain specific applications requires a great deal of practical experience. If readers have such experience, they are kindly asked to share it in the comment section, so as to enrich the content of this article and benefit everyone.