A manufacturer of temperature measurement products has launched a unique design of thermowell that can easily and safely be installed in process pipelines if a standard thermowell has failed the ASME Performance Test Code (PTC 19.3, 2004).
In petrochemicals, thermowells are used widely in temperature measurement applications. There are multiple variations of two basic types: low pressure and high-pressure versions. Thermowells are used to provide isolation between a temperature sensor and the environment, in this case oil or gas. A thermowell enables the temperature sensor to be removed and replaced without compromising either the ambient region or the process. The service life of a thermowell is therefore critical and the mechanical design needs to ensure that the unit can operate under ever increasing flow rates within process pipelines.
Fluid (oil or gas) flowing past a thermowell creates a turbulent wake as vortices form at both sides of the well. These vortices detach, first from one side and then from the other, which causes alternating lateral forces on the well perpendicular to the direction of flow. This is known as the Von Karmann effect. This wake frequency is proportional to the fluid velocity and well dimensions. Generally, if the wake frequency coincides with or comes within 20% of the natural frequency of the well, the resultant vibration could cause mechanical failure of the well. Thermowells are usually safe if the natural frequency is well below the wake frequency or if the fluid velocity continually fluctuates through the critical velocity point.
The maximum length of a thermowell for given service conditions is dependent upon both vibratory and steady state stress. The ASME calculations (PTC 19.3, 2004) are used to determine if the selected well dimensions provide a well strong enough to withstand the stresses imposed by static pressure, steady state flow and vibration.
ASME PTC 19.3 is not a design standard and should only be used by thermowell manufacturers to prove that their designs are suitable for the process conditions. Up to now, if a thermowell failed this performance test code, the manufacturer has been left with several options: either to shorten the thermowell immersion, or to increase the diameter of the thermowell, neither of which are often very practical or cost effective for the end user. Imagine the cost of having to provide manpower to replace or refit thousands of thermowells around a petrochemicals plant in order to shorten the immersion. Similarly, the cost of increasing the diameter (over-engineering) of the thermowell can also be significant if the end user needs to purchase hundreds or thousands of these units for an installation.
The other option used by the majority of thermowell suppliers is to incorporate a velocity collar on the thermowell in order to move the point of vibration or resonance. This solution is based on a power industry standard originally developed in the UK, but which was never intended for use in petrochemical applications. Adding a velocity collar means that the thermowell needs to be manufactured to a very high tolerance (on the collar outside diameter) and that the corresponding nozzle is similarly machined to suit. This ensures a tight fit so that no resonance can occur. While this solution seems to work, the extra costs incurred by the manufacturer of the thermowell usually have to be passed on to the buyer.
However, an alternative solution is now available. After extensive R&D and independent evaluation, Okazaki Manufacturing Company (OMC) has developed a unique design of thermowell, the VortexWell®, which doesn’t require a velocity collar and is cost effective for the end user in terms of purchase, installation and maintenance costs (whole lifecycle costs).
OMC’s VortexWell® incorporates an innovative helical strake design, very similar to the helical strakes seen on car aerials and cooling towers. Chris Chant, Business Development Manager at OMC (UK) comments: “By using the latest CFD [computational fluid dynamics] software to visualise the flow behaviour, we were able to accurately compare a standard tapered thermowell and our new VortexWell® design, which incorporates a helical strake. What we discovered was pretty dramatic.”
In the analyses, the standard tapered thermowell showed classic shedding behaviour as expected, whereas the VortexWell® demonstrated no signs of regular flow behaviour. The VortexWell® helical strake design disturbed the flow sufficiently to interrupt the regular formation of vortices. Whilst a small vortex was observed in the wake of the VortexWell® this was a localised stagnation point and didn’t shed.
However, the most significant comparison made was with regard to the pressure fields. As Chris Chant continues: “For the standard tapered well design, an oscillating pressure field was observed around the structure. The VortexWell® displayed a constant and stable pressure field, presenting no dynamic variations. As this pressure is the source of vortex-induced vibrations, it can be assumed that the VortexWell® would experience a significant improvement in practise compared to the standard thermowell design.”
In further tests, this time using finite element analysis (FEA) software, OMC discovered that the ASME calculations (PTC 19.3, 2004) used by thermowell manufacturers could place significant limitations upon the safety of petrochemical applications. Using the ASME calculations (PTC 19.3, 2004) gave the lowest natural frequency of vibration for the standard tapered thermowell to be 68.5Hz. However, OMC’s own FEA results showed a corresponding value of 90.3Hz, a difference of more than 30%. This highlights that the ASME calculations (PTC 19.3, 2004) design rules include assumptions that can do lead to considerable inaccuracies when designing thermowells for petrochemical (and other) applications. The risk of a thermowell failing due to under-engineering, or the extra costs incurred by the end user because of an over-engineered thermowell, can both be avoided if the buyer works with a reputable, experienced thermowell supplier such as OMC.
For more information on OMC’s range of temperature measurement products, please visit the website at www.okazaki-mfg.co.uk or contact the sales department on 01443 740777 or email: email@example.com
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