New thermowell standard will encourage innovation
05 July 2011
When it comes to designing a thermowell for an oil or gas pipeline, the mechanical strength of the thermowell must be evaluated to ensure that it passes the ASME PTC 19.3 standard. The recent major overhaul of the standard may encourage buyers to now seek out alternative thermowell designs, says Chris Chant, Business Development Manager at Okazaki Manufacturing Company (OMC).
Companies that source thermowells for oil, gas and petrochemicals applications should now be consulting the latest, revised ASME PTC 19.3 (2010) standard, which recently underwent its first major revision in more than 35 years. This is likely to encourage engineers to seek out better, alternative, more innovative thermowell designs for process pipelines.
Thermowells are basically a circular cylinder installed in a similar way to a cantilever into the process pipeline. Thermowells allow a temperature sensor to be located within a process flow, whilst providing a process seal and protecting the sensor from the process fluid. As a process fluid flows around the thermowell, low pressure vortices are created on the downstream side in both laminar and turbulent flow. The combination of stresses generated by the static, inline drag forces from fluid flow and the dynamic transverse lift forces caused by the alternating vortex shedding, create the potential for fatigue-induced mechanical failures of the thermowell. Until recently, ASME PTC 19.3 (1974) has been the standard by which most thermowells are designed.
The original standard worked on a frequency ratio of f s < 0.8 f c/n but now this has changed to a more complex process whereby the cyclic stress condition of the thermowell needs to be taken into account. If the thermowell passes the cyclic stress then the ratio of f s < 0.8 f c/n is still applicable. However, if it fails, then the ratio of f s < 0.4 f c/n is applicable. Also of concern to manufacturers and end users is that the standard only applies to thermowells with a surface finish of 0.81µm (32µin.) Ra or better.
The 2010 revised standard
The new ASME PTC 19.3 standard has grown from four pages up to more than 50 pages and so engineers need to be certain that they understand the changes involved. The 2010 standard addresses a number of new design factors that were not included in the original, more simplified standard. These include in-line resonance, fatigue factors for oscillatory stress, effects of foundation compliance, sensor mass, stress intensification factors at the root of the thermowell, and fluid mass/density.
This means the new standard should lead to a greater variety of thermowell geometries and discourages the use of velocity support collars, allowing designers to achieve faster response times than ever before in applications that call for a wake frequency calculation.
Today, petrochemical plants tend to use smaller diameter pipelines but with higher fluid velocities. This means that the design of the thermowell is critical. For example, the original ASME standard did not provide guidance on liquid mass, as the standard was originally developed for steam applications. However, for oil and petrochemical pipeline applications, liquid density or mass must always be taken into account when sizing thermowells.
Many thermowell suppliers incorporate a velocity collar on a thermowell in order to move the point of vibration or resonance. But adding a velocity collar means the thermowell needs to be manufactured to a very high tolerance (on the collar OD) and that the corresponding nozzle is similarly machined to suit. This tolerance must be an interference fit so that no resonance can occur.
If supplied and fitted correctly, the collar only moves the point of resonance and does not solve the root problem. While this seems to work in practice, the extra costs incurred by the thermowell manufacturer and installation contractor are passed on to the buyer, which increases the overall cost. The addition of the collar also increases the need for stocking specific spares for a single measuring point.
So fitting a velocity collar is not always the most appropriate solution. In effect, by adding a collar, the manufacturer is simply moving the problem somewhere else. What customers need to consider are genuine alternatives to velocity collars.
One such solution is the VortexWell, a unique design of thermowell manufactured by Japanese thermowell supplier Okazaki Manufacturing Company (OMC). This groundbreaking design of thermowell incorporates a ‘helical strake’ design, rather like the helical strakes found on a car aerial or cooling tower fins. After extensive R&D using the latest Computational Fluid Dynamics (CFD) software, as well as third party, independent evaluation, OMC was able to visualise and accurately compare the flow behaviour of its new helical strake thermowell design with a standard tapered thermowell.
In these analyses, the standard tapered thermowell showed classic shedding behaviour as expected, whereas the new helical strake design demonstrated no signs of regular flow behaviour. The 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 helical strake design, this was a localised stagnation point and didn’t shed.
However, the most significant comparison made was with regard to the pressure fields. For the standard tapered well design, an oscillating pressure field was observed around the structure. The helical strake design 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 helical strake design would experience a significant improvement in practice compared to a standard thermowell design.
In further tests, this time using Finite Element Analysis (FEA) software, OMC discovered that the ASME calculations used by thermowell manufacturers could be placing significant limitations on the safety of petrochemical applications. Using the ASME calculations 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 per cent. This highlights that the ASME calculations design rules include assumptions that can, and do, lead to considerable inaccuracies when designing thermowells for petrochemical 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.
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