Measurement and Testing 41 Manufacturing the thermowell


The design of the thermowell is important, but equally significant are the manufacture and test stages. Each part of the process interacts with the others to ensure a tough, reliable product suitable for demanding duty.


When drilling, the first step is to drill the barstock or forging with a specialised ‘gun drill’. This ensures a straight hole through the material up to 1200mm long. This work piece is then marked for its centre and the rest of the process works from the drilled hole and not the external shape.


Profiling is then carried out to shape the thermowell stem to the measurements that it has been designed to, using state of the art CNC machines. At this point, a concentricity check is carried out.


All welding is carried out to detailed ‘Welding Procedure Specifications’. The procedures are qualified to ASME requirements and all weld operators should have ASME certification. In addition, many operating companies have their own requirements and standards which the operators need to follow during manufacture.


Bolt holes should be machined as a final step. This prevents any deformation caused by the high temperatures generated in the welding process.


Quality control


Testing of the thermowell is an integral part of the manufacturing process. In addition, positive material identification (PMI) should be carried out by trained operators. Third party PMI is also commonly requested.


Dye penetrant inspection is frequently used to locate surface-breaking defects. This helps to detect casting, forging and welding surface defects such as hairline cracks. Penetrant fluid is applied to the piece and a developer is added, making invisible flaws detectable under ultraviolet or white light.


Ultrasonic weld inspection can also be carried out. This uses the principle that a gap in the weld changes the propagation of ultrasound through the metal. Bore concentricity is tested using an ultrasonic device. X-ray and gamma ray examination can also be carried out by accredited third parties.


Final dimensional checks are carried out by trained operators independent from the manufacturing process. All testing should be performed by operators who are qualified to spot defects.


Choose your sensor


Once the thermowell has been manufactured and tested, the heart of the assembly can be fitted. Sensors are calibrated traceable to national standards and fitted with terminal heads and transmitter extension pieces to suit requirements. As great care has been taken throughout the manufacturing of the thermowell, it is important to take equally great care of the sensor selection. The goal is to build a device capable of measuring temperature accurately over a long period of time, so high quality component parts should always be fitted.


The revised ASME PTC 19.3 TW-2010 standard A new standard for thermowells was recently introduced to replace the existing ASME PTC 19.3-1974.


The latest revision of the ASME PTC19.3 standard makes use of significant new knowledge about the behaviour of thermowells, compared to the criteria laid out in 1974. The standard evaluates thermowell suitability with new and improved calculations for various thermowell designs and material properties. It also takes some detailed information about the process into account.


In particular, the standard looks at the incidence of vortex shedding. This is the pheonomenon where vortices formed in the wake of the thermowell move from side to side; this is what causes vibrations in the thermowell. If this vortex shedding rate matches the natural frequency of the thermowell, resonance occurs, and dynamic bending stress on the thermowell increases.


Tighter design constraints


The frequency ratio is the ratio between the vortex shedding rate and the installed natural frequency. In the old standard, the frequency ratio limit was set to 0.8. The new standard stipulates that in some cases, the limit should be set to 0.4. The new possibility of having a much lower frequency ratio limit of 0.4 means tighter design constraints in many cases. As the majority of existing assets will have been designed to the 1974 standard, the new 0.4 frequency ratio means a lot of thermowells will not pass the new standard.


Re-evaluation and re-certification services are available. Operators will need to consider the implications when an existing thermowell fails the new calculation. If process conditions change, for example increased throughput for a part of plant, this should also be evaluated.


At a brownfield modification recently examined by ABB for new process conditions, 29 existing thermowells were evaluated under existing and new conditions. Only 6 passed the new standard under existing conditions. ABB subsequently assisted the operator by designing replacement thermowells.


Summary


ABB has a strong pedigree in the manufacture of temperature and primary flow elements, particularly for use in oil and gas applications. Its factory in Workington, Cumbria, is ABB’s Centre of Excellence for temperature measurement in the oil and gas industry and specialises in the design, manufacture and testing of both standard and bespoke temperature instruments.


For more information about thermowells or ABB’s temperature measurement capabilities, email moreinstrumentation@gb.abb.com or call 0870 600 6122 ref. ‘Thermowells’.


Self-Contained Digital pH and ORP Measurement


Yokogawa (Netherlands) has combined its proven expertise in pH and ORP sensor technology with integrated digital communications and signal- processing circuitry to produce the SENCOM (SENsor COMmunication) range of self-contained smart sensing solutions.


The SENCOM smart sensor design enables improved maintenance management and diagnostics, further enhancing the simplicity and reliability that Yokogawa is renowned for. In addition, SENCOM has its own permanent calibration memory, allowing it to be calibrated in the laboratory before returning it to the field, resulting in improved accuracy with reduced operating costs, shorter downtime, and improved safety.


SENCOM sensors maintain specific measurement and calibration data and this information can be exchanged between the sensor and a transmitter such as Yokogawa’s FLEXA family. Dedicated SENCOM SPS24 management software is also available for use with a Windows PC. Utilising historical measurement, calibration and diagnostic data from the sensor, the SPS24 data management system provides users with the tools necessary to predict maintenance and calibration frequency and estimate sensor life. The calibration information can be managed, analysed and conveniently documented by the SENCOM SPS24 software.


This new sensing approach enables simplified maintenance by the replacement of 'pre-calibrated' sensors. Furthermore, it is possible to monitor sensor aging in real time using a sensor 'wellness' feature, as well as carrying out reference impedance measurements for the early detection of diaphragm clogging.


The new SPS24 SENCOM PC software makes it possible to optimise the performance of pH/ORP sensors for enhanced reliability and process safety. The software allows the operator to continuously monitor pH/ORP sensor measurements, perform calibrations and configure the various parameters, keeping track of up to 100 different SENCOM sensors.


The first SENCOM sensor to be introduced in Yokogawa’s new family is a pH/ORP sensor, targeting applications requiring easy, accurate and reliable measurements. Future sensors will target the full spectrum of pH and ORP applications in the power, utilities, petro-chemical, chemical and pulp & paper industries. The new SENCOM sensors are designed to deliver new levels of accuracy and reliability even in the harshest of industrial environments.


For More Info, email: 26859pr@reply-direct.com


OCTOBER / NOVEMBER 2013 • WWW.PETRO-ONLINE.COM


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