Temperature °Controls Pty Ltd ACN: 075 298 592 ABN: 966 501 901 83 Sydney (Head Office): 7 Yamma St, Sefton, NSW 2162 Australia Ph: 61 2 9721 8644 Fax: 61 2 9738 9339 Melbourne: 8/280 Whitehall St, Yarraville VIC 3013 Australia Ph: 61 3 9687 0000 Fax: 61 3 9687 1900 Brisbane: 28a 121 – 125 Kerry Rd, Archerfield, QLD 4108 Australi Ph: 61 7 3373 8424 Fax: 61 7 3821 3693 W: www.temperature.com.au E: [email protected]1661 ALL THERMOWELL CALCULATIONS TO THE NEW ASME PTC 19.3 TW 2016 Thermowells are used to protect Thermocouples, RTD sensors and Bimetal Thermometers when measuring temperatures in high pressure or hostile corrosive applications in pipelines. Thermowells are machined from solid barstock materials which are suitable for the process medium and temperature, and supplied with a process connection, threaded, flanged or weld– in to meet the specific installation requirements. As the flow of the medium ( Fluid or Gas ) in the pipeline passes by the thermowell it forms a turbulent wake ( known as Von Karman trail ) that generates a vibration in the thermowell proportionate to the diameter of the thermowell and velocity of the medium. The thermowell must be constructed sufficiently rigid so that it’s vibration frequency does not equal the frequency of the Von Karman trail, where the thermowell would vibrate to the point of breaking off. The American Society of Mechanical Engineers ( ASME ) have developed a Performance Test Code ASME PTC 19.3 TW-2016 for use to provide mathematical proof that the material chosen and the mechanical design will not fail given the effects of the operating conditions. The ASME PTC 19.3 TW 2016 wake frequency calculation is used for one piece thermowells in tapered, straight or stepped stem designs from solid barstock materials, process connections include, weld in, Flanged, Threaded and Van Stone styles, all with a surface finish of 0.8 um Ra or better. To ensure compliance with this latest standard, Temperature Controls have implemented a computer program to perform wake frequency calculations on barstock thermowells to ASME PTC 19.3 TW 2016 verifying that the specified thermowell dimensions are adequate to withstand the service conditions of temperature, pressure, velocity and vibration. Individual Tag number calculations can be performed as part of a quotation for supply of thermowells with the customer providing the specific process data. A certificate of compliance is issued as part of our Quality Assurance documentation with order shipment.
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THERMOWELL CALCULATIONS TO THE NEW ASME PTC ......The ASME PTC 19.3 TW 2016 wake frequency calculation is used for one piece thermowells in tapered, straight or stepped stem designs
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Support collars or velocity collars are used to support the stem inside the shielded length of a flange mount thermowell to theoretically reduce the insertion length. The ASME 19.2 2016-TW standard does not support the use of velocity collars as the collars do not assure a rigid support plane and so the thermowell stem will be subjected to a hammering effect brought on by the constant vibration. Positioning of thermowells It is generally believed that a temperature element must be centred in the pipe, however in practice so long as the temperature sensor is within the middle third of the pipe there is no reason to expect any decrease in measuring accuracy. Thermowells that fail to meet pressure, stress or frequency requirements as per ASME PTC 19.3 TW-2016 may be modified to reach a passable solution with modification to:- Material; an alternative material with increased mechanical properties. Insertion Length: Reducing the unsupported stem length will improve the force to natural frequency ratio. Tip Diameter: Increasing the tip diameter on a tapered stem thermowell will improve the thermowell strength, or change to a straight stem design. Fillet Radius: Increase the fillet radius to improve strength but consider the flange gasket face surface area.
THERMOWELL CALCULATIONS TO ASME PTC 19.3 TW 2016
Scope of ASME PTC 19.3 TW-2016
Tapered and Straight Design
Stepped Tip Design
Sample Certificate
Process Data Required for a calculation
Process Medium : Gas or Liquid Type
Velocity
Viscosity
Density
Maximum Temperature
Maximum Pressure
Shielded Length
Pipe Size / Schedule
Not within the scope of ASME PTC 19.3
Thermowells fabricated from pipe
Special surface finish e.g. Knurled
Thermowells with 2 or more piece welded stems
Stems that include flame spray ( stellite )or weld overlay
Ceramic or Non-metallic materials
Design Input Minimum Maximum Minimum Maximum
Unsupported Length U 63.5 mm 609.6 mm 127 mm 609.6 mm
The thermowell design has PASSED the wake frequency calculation“These well design calculations are based on the ASME power test code PTC 19.3- formulas. The results of these calculations
should only be used as a guide for thermowell design. Temperature Controls does not guarantee the performance of a specific
well design obtained from the use of these calculations”.
For calculation purposes When calculating sheilded length use 100mm for the Stand off plus pipe thicknessLine size 50mm = 2" STD, 100mm = 4" STD, 150mm = 6" STD, 200mm = 8" STD,