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Feature Temperature, humidity & moisture control


emperature is without question the most commonly measured variable in industry, being the sim- plest piece of information about the state of a process.


Well worth the effort T


Thermowells act as a barrier between a temperature sensor and the process medium, and are fundamental to process safety, so it’s important to understand how they are made


cating that it complies to ANSI B 16.5 and is made from a forging in accor- dance with ASTM A 182.


In many applications, particularly where the process medium is fast flow- ing or pressurised, the temperature sensor needs to be protected from direct contact with the medium by a metal structure known as a thermowell. The integrity of the thermowell is essential to the process, as a leak or a structural failure will expose the contents of the pipeline to the atmosphere. For this reason, very strict standards govern the manufacture of thermowells. Although the thermowell is a fairly simple piece of equipment, there is still scope for things to go wrong. The main risk comes from vortices forming in the process medium around the thermow- ell, which can cause vibrations. The subsequent stresses around the stem of the thermowell can lead to failure. Failure of the thermowell can have disastrous consequences. In 1995, a thermowell failure in the secondary coolant loop of the Monju fast breeder reactor in Japan closed the plant for the next 15 years. To ensure the safety and integrity of the process, it is important to be aware of the manufacturing standards sur- rounding thermowells and to ask the appropriate questions when designing this type of equipment. Incorrectly specifying pressure-retaining parts can have disastrous consequences, and although the industry standards do not constitute law, deviation from them would be difficult to explain in court after an incident.


The majority of thermowells are rated according to American system design standards. Good engineering practice is outlined by the ASME (American Society of Mechanical Engineers) VIII Pressure Vessel Code. This standard also refers to other American standards.


If a flange is specified as an ANSI X" 150 or greater, ANSI B16.5 is the stan- dard it has to comply with. ANSI B16.5 in turn refers to other standards regard- ing materials and their form of manu- facture. Any flanged device provided under the ASME codes will have the flange size and rating as well as the material specification stamped on it. The flange can be marked for example with 'ANSI 1" 150lb ASTM A 182', indi-


Process & Control NOVEMBER 2013


When specifying thermowells to European standards, P.D. (proposed document) 5500 (2006) is a harmonis- ing document for the European Union. It has not yet been adopted as a European Directive, but is a unifying standard. Its standards and markings differ from the American standards but the principles are the same.


Thermowells are made from single pieces of bar or forging, which pro- duces a structure that is stronger than an equivalent cast or machined part. A high-quality forged thermowell is made from a single piece which is as close to the finished shape as possible and drilled without penetrating the end of the stem. If a forged thermowell is not required the flange is made either from a hot-worked forging or in some cir- cumstances plate. Typical materials include, but are not limited to, 316, Duplex and Super Duplex stainless steels, along with specialist alloys such as Monel and Inconel. The materials used should be traceable.


Whilst the design of the thermowell is important, the manufacture and test stages are equally significant. After drilling and profiling, welding is car- ried out to detailed ‘Welding Procedure Specifications’. The procedures are qualified to ASME requirements and all weld operators should have ASME cer- tification. Bolt holes should be machined as a final step. This prevents any deformation caused by the high temperatures of the welding process. Testing of the thermowell is an inte- gral part of the manufacturing process. In addition, positive material identifi- cation (PMI) should be carried out by trained operators.


Quality control stages include dye penetrant inspection, which is fre-


The integrity of ther- mowells is essential to the process


quently used to locate surface-breaking defects, and ultrasonic weld inspec- tion. X-ray and gamma ray examination can also be carried out by third parties. Final dimensional checks are carried out by trained operators independent from the manufacturing process. All testing should be performed by opera- tors who are qualified to spot defects. Once the thermowell has been manu- factured and tested, the heart of the assembly can be fitted. Sensors are cali- brated traceable to national standards and fitted with terminal heads and transmitter extension pieces to suit requirements. It is important to take as much care during sensor selection as during thermowell manufacture, and build a device capable of measuring temperature accurately over a long period of time.


A new standard for thermowells was recently introduced to replace the exist- ing ASME PTC 19.3-1974 following some catastrophic failures. The latest revision of the ASME PTC19.3 standard makes use of new knowledge about the behaviour of thermowells, compared to the criteria laid out in 1974. In particu- lar, the standard looks at the incidence of vortex shedding, and the ratio between the vortex shedding rate and installed natural frequency. The new possibility of having a much lower fre- quency ratio limit of 0.4, compared to 0.8 in the old standard, means tighter design constraints in many cases. At a brownfield modification recently examined by ABB for new process conditions, 29 existing ther- mowells were evaluated under existing and new conditions. Only 6 passed the new standard under existing condi- tions. ABB assisted the operator by designing replacement thermowells. Bearing in mind what happened at Monju, which did conform to the 1974 standard, a review of installed ther- mowells may be well worth the effort.


Components can be specified for heavy duty process environments


ABB T: 0870 600 6122


Enter 235 E: moreinstrumentation@gb.abb.com 25


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