Wake Up Frequency

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Thermowellsare a necessary component in many industries. Wake frequency calculations are critical for getting the right thermowell. A proper wake frequency calculation formula will ensure the thermowell will work correctly for your application.

Our thermowell calculations ensure thermowells provide optimal operational performance. They follow the latest ASME PTC 19.3 TW-2016 standard for vibrations, which is a critical wake frequency for thermowelll design.

Our wake frequency calculations account for variables in fluid properties and pipe specifications. They consider data related to the thermowell itself, like its connection and mounting type. Engineers also need to know the bore, root and tip diameters.

Wake calculations help design engineers evaluate the construction and installation of thermowells. These calculations measure the data needed to evaluate thermowells under real-world operating conditions. They determine or prove the required dimensions and suitability of a thermowell.

A wake is the area of recirculating flow right behind a solid body. It is also known as a Von Karman trail. When fluid flowing past a thermowell has a change in momentum, it creates a wake. Each wake has a specific frequency, which is a function of the diameter of the thermowell and the fluid velocity. This frequency is also called the vortex shedding frequency. Vortices form in the wake behind the thermowell, shedding from alternate sides. This creates two forces on the thermowell:

The vibrations usually have a small magnitude. As the wake frequency approaches the natural frequency of the thermowell, vibrations increase. The thermowell goes into resonance when the wake frequency matches its natural frequency.

Wake frequency calculations improve safety for plant operations. If the thermowell stem shears off, it could damage other equipment in the pipeline. Pipeline pressure could push the temperature measurement instrument out of the thermowell. This could cause a loss of pipeline fluid containment.

Thermowell wake frequency calculations are generally conducted before the thermowell is manufactured. They ensure that the thermowell design can handle stresses from the process media. The wake frequency calculation data is available with the following basic parameters:

A wake frequency calculator takes four types of stresses into account. Each type of stress could cause the thermowell to fail. The thermowell must pass in all four areas to be acceptable for use in your application.

Temp-Pro follows ASME PTC 19.3 TW-2016, which is the latest standard for thermowell wake frequency calculations. The new standard replaces two earlier ones: ASME PTC 19.3 TW-1974 and ASME PTC 19.3 TW-2010.

Thermowell dimensions, material, and process mounting are the responsibility of the designer. A thermowell should have a wake frequency ratio of 0.8 or less. If a thermowell fails to meet the frequency, pressure, or stress requirements, changes to the dimensions or material may help. Possible alterations include:

Wake frequency calculations for thermowells, and frequency ratio formulas are critically important to the safety of your facilities and personnel. They help ensure your operations are at their optimal levels.

Nicole Chotain is a passionate marketing and sales specialist in the temperature sensor industry who finds it incredibly fulfilling to be involved in marketing and selling crucial components used in power generation and renewable energy. She takes great joy in creating remarkable campaigns, forging meaningful connections between Temp-Pro and its customers, and driving the growth of the brand.

The Excel version of the wake frequency calculation in accordance with ASME PTC 19.3 TW-2016, which has proven itself for over 20 years, has been made fit for the future. The increased security requirements of external users make it more difficult to use third-party software with VBA macros for fear of malware. The new online version of the successful WIKA software solves this problem.

Basically, the online program is divided into two parts. On the one hand, there is a freely available version with a functionality limited to the essentials for the calculation of a single thermowell (single calculation). On the other hand, after registration and verification of the user, the full version of the wake frequency calculation is available, which enables the simultaneous calculation of any number of thermowells (multiple calculation).

Thermowell wakes, and the resulting thermowell wake frequency, is a physical characteristic resulting from the interaction of a thermowell with the fluid flowing past it. The thermowell dimensions and geometry, and the properties and process conditions of the fluid flowing past the thermowell all determine the effects and frequency of the wake vortices. In some applications, the wake frequency can result in the design of the thermowell being unsuitable for the application, and use of such a thermowell being a potential safety hazard.

In a pipeline, a thermowell extends into a flowing fluid which exerts forces upon the stem of the thermowell. Static forces are exerted by the mass of the fluid impacting on the thermowell. Additionally, as the fluid flows past the thermowell, the change in fluid momentum creates a turbulent wake behind the well. Vortices, known as "Von Karman vortices", form in this wake and shed from alternate sides of the well. The vortex shedding frequency (or wake frequency) is linear with fluid flow velocity and inversely proportional to thermowell tip diameter.

The vortex induced forces cause the thermowell to vibrate. The magnitude of these thermowell vibrations are generally negligible. However, as the wake frequency approaches the natural frequency of the thermowell the vibrating forces increase, and when the wake frequency matches the natural frquency of the thermowell it goes into resonance. At resonance the vibrating forces increase rapidly, and the resultant vibrations can cause thermowell failure.

If the thermowell stem shears then it could travel at high velocity down the pipeline causing damage to downstream equipment e.g. pumps, valves etc. Additionally, the temperature measurment instrument within the thermowell (e.g. thermocouple or RTD) is now exposed to pipeline presure. This could cause the measurement instrument to be pushed out of the thermowell head leading to a loss of pipeline fluid containment.

Thermowell wake frequency calculations should be conducted during thermowell specification and definately prior to thermowell manufacture. These thermowell calculations ensure that the thermowell design is robust enough to cope with the forces produced by the process media.

It is not strictly necessary to carry out a wake frequency calculation for every thermowell. Many instrument engineers only carry out wake frequency calculations when the fluid flow rate is high (typically in gas flows) and the damping effect of the fluid is low, or when the thermowell insertion length is long compared to the pipe internal diameter.

Reputable suppliers of thermowells are only too happy to provide an individual calculation of the stresses induced upon and the relative strength of their thermowells - this provides a useful check on any calculations you may perform.

ASME PTC19.3TW-2016 is a newer standard, released in 2016 to replace the 2010 standard. It uses more advanced methods for evaluating the suitability of a thermowell for a specific application, and is applicable to tapered, straight and reduced-tip profiles. It is not applicable for fabricated (welded tube) thermowells.

It references work carried out by Murdock, which is now the accepted standard for harmonic frequency analysis of thermowells. Given the calculated harmonic frequency of the thermowell stem, the standard provides a method of calculating the induced frequency from the vortices. The standard then requires a safety margin to be applied such that the induced frequency is no more than 80% of the thermowell harmonic frequency.

Budenberg, who make themowells in the UK provide an MS Excel spreadsheet with thermowell wake frequency calculations based on ASME PTC19.3TW-2010. This spreadsheet should be used as a guide only - it lets you, the instrument engineer determine whether further detailed calculations need to be performed on the thermowell you are specifying.

Emerson, the multinational instrument supplier provide an online thermowell wake frequency calculator based on ASME PTC 19.3TW-2016. This online calculator is intended to be an aide in selecting a thermowell, and will indicate if further more detailed calculations are required.

If the calculated wake frequency is too close to the natural frequency of the thermowell i.e. the ratio is greater than 0.8, the following structural changes to the design of the well may be a solution:

The use of velocity collars (also known as support collars) is not generally recommended. ASME PTC 19.3 TW-2016 points 6-7-(e) points out that rigid support can be obtained only with an interference fit between the velocity collar and the installed piping, and anyone who has tried to install a thermowell with support collar will know that obtaining an interference fit can sometimes be tricky. In fact velocity collars or other means of support are outside the scope of the ASME PTC 19.3 TW-2016 standard.

Despite the above many manufacturers will, on customer request, provide thermowells with support collars. In this case the thermowell will be designed in accordance with the design and calculation criteria of ASME PTC 19.3 TW-2016, but crucially will fall outside the scope of ASME PTC 19.3 TW-2016 and a guarantee for support collar solutions is generally not given.

A helical vane, or more correctly a strake, on the stem of the thermowell deliberately introduces turbulence resulting in the resonant load frequencies having negligible amplitudes. The effectiveness of helical strakes for reducing vortex induced vibrations was discovered by Christopher Scruton so these vanes are often described as Scruton strakes. For maximum effectiveness in suppression of vortices caused by gas flow, each vane or strake should have a height of about 10 percent of the cylinder diameter, and its length should be approximately 5 times the cylinder diameter.

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