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Measurement and Testing
Why it Pays to be in the Know about Thermowells
Steve Gorvett, ABB Measurement Products Tel number: +44 0870 600 6122 Email:
moreinstrumentation@gb.abb.com • Web:
www.abb.com/measurement
Te thermowell may seem a fairly simple piece of equipment, designed to act as a barrier between a temperature sensor and the process medium. Yet it is fundamental to the safety of the process; a faulty thermowell can jeopardise the whole operation. Exacting standards, which the user needs to be aware of, govern the manufacture of thermowells.
Temperature is without question the most commonly measured variable in industry, being the simplest piece of information about the state of a process.
In many applications, particularly where the process medium is fast flowing 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 thermowell, which can cause vibrations. The stresses caused around the stem of the thermowell by these vibrations can, over time, lead to failure.
Incorrectly specifying pressure-retaining parts can have disastrous consequences, in the worst cases potentially leading to loss of life, loss of the plant and possible prosecution. Although the industry standards do not constitute law, deviation from them would be difficult to explain in court after an incident.
Follow the standards
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 refers to other American standards from bodies such as ANSI (American National Standards Institute) or ASTM (American Society for Testing and Materials).
If a flange is specified as an ANSI X" 150 or greater, ANSI B16.5 is the standard it has to comply with. ANSI B16.5 in turn refers to other standards regarding materials and their form of manufacture, for example 'ASTM A182 Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges' and 'ASTM A105 Standard Specification for Carbon Steel forgings for Piping Applications'.
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', indicating that it complies to ANSI B 16.5 and is made from a forging in accordance with ASTM A 182. Additional markings from reputable manufacturers may also indicate the cast number of the material, tag number and the order against which it is supplied.
When specifying thermowells to European standards, P.D. (proposed document) 5500 (2006) is a harmonising document for the European Union. Although this has not yet been adopted as a European Directive, it is used as a unifying standard. Its standards and markings differ from the American standards but the principles remain the same.
Solid steel
Thermowells are made from single pieces of bar or forging. When forging a piece of metal, the internal grain of the metal deforms or ‘upsets’, aligning it with the shape of the part and producing a structure that is much stronger than an equivalent cast or machined part would be.
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 circumstances plate. Except for the weld seam that attaches the flange to the stem, the structure is made from single pieces of metal. 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 should be traceable all the way back to the mill and any reputable manufacturer should be able to produce verification upon request. The manufacturer needs to be in possession of supporting documentation in order to mark the equipment as compliant. Any raw materials used in the manufacturing must be inspected and measures taken to ensure traceability.
OCTOBER / NOVEMBER 2013 •
WWW.PETRO-ONLINE.COM
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