2007-12-21 - Thermowells must be properly designed and specified to withstand the pressure and stresses of their environment. Andrew Dunbabin, Product Manager, High Temperature Products for ABB Limited’s instrumentation business, argues that users should be aware of all the relevant standards, in order to guard against potentially disastrous failures.
A thermowell is the pressure-tight receptacle designed to protect sensitive temperature measurement instrumentation from harsh process conditions. The danger is that a hollow tube sticking into a process pipe or vessel could present a point of weakness if not properly specified. Any failure could result in damaged plant, or even injury and prosecution. So it’s vital that thermowells are manufactured and specified correctly to withstand the pressure and mechanical stresses that it may be exposed to, as well as any corrosive or erosive media they are likely to meet in a given process environment.
The first concern is obviously pressure. There is currently no thermowell equivalent to the legally binding pressure vessel regulations, but international pressure vessel and piping standards, such as ASME VIII and its European counterpart, PD 5500, set out clear standards for pressure-retaining parts. Although these standards are not enshrined in law, it would be hard to justify any deviation from them, especially following an incident.
End users specifying thermowells from reputable manufacturers can generally be confident that the correct standards have been met. But if in any doubt, it can be useful for them to know their own way around the relevant standards so that they can ask suppliers the right questions.
Pressure fittings
Thermowells are generally fixed to vessels and pipes using a flange. ASME VIII refers to ANSI B16.5 when it comes to defining the standard for flanged fittings. It is ANSI B16.5 that gives us the familiar flange rating system.
ANSI B16.5 in turn relies on other standards to specify the types of material that may be used, such as ASTM A182 for austenitic steels and ASTM A105 for carbon steels.
These materials standards are complex, but one of the main points to note is that, unlike thermowell stems, flanges must not be machined from bar stock. Instead, the parts must be manufactured from plate or from forged material that is hot worked into shape before being machined. This ensures that the flow lines of the metal follow the shape of the flange closely and so make it as strong as possible.
Material traceability starts with the mill that made the steel. A materials certificate describes the chemical and mechanical properties of any material supplied by a mill, and any manufacturer with good engineering practices and a third-party verified quality system in place should be able to trace any material back to the mill certificate.
Any flange manufactured and supplied under the ASME codes must have the flange size and rating stamped on it. Some manufacturers may also add marks indicating the cast number and the order against which it was supplied.
PD 5500 is the equivalent European standard. Although it is not legally binding as yet, it will become law when it completes its journey through the lengthy European ratification process and is adopted as a European Directive. It follows the ASME codes closely in its underlying principles, although the requirements for dimensions and markings differ.
Welding standards
Of course, materials standards are only part of the story. Thermowells manufactured from more than one part will be welded, in which case ASME VIII calls on the welding standard, ASME IX.
The situation is slightly more complicated in the European system, where there are a number of different standards that apply to welding. Once again they make largely the same demands as the American equivalent, ASME IX, but it’s a good idea to seek advice from a reputable supplier as to which particular European standard applies in a specific case.
Whether the user is working within the US or European system, there are two distinct aspects to the welding standards.
First there is the qualification of the person carrying out the work. The standards require that the quality of the individual’s workmanship will have been regularly verified by a Welding Specialist appointed by the trade’s governing body, which is The Welding Institute in the UK.
Next there is the welding procedure. Choosing the right procedure for a particular service is a highly skilled job and a competent manufacturer should be able to indicate which welding procedure they used.
Mechanical stress
Pressure is not the only physical challenge for thermowells. In pipelines, they will also experience mechanical stresses on their stem as fluid flows past them. It’s easy enough to calculate the static forces that are exerted by the mass of fluid impacting on the stem, but at higher flow rates the stem will also shed vortices, known as Von Karmen vortices. These can present a real danger if the frequency of the shedding approaches the resonant frequency of the stem.
ASME PTC 19.3 is the accepted standard for analysing the harmonic frequency of thermowells. Given the calculated harmonic frequency of the stem, the standard also provides a method for calculating the induced frequency of the vortices.
The standard recommends designing in a safety margin so that the induced frequency is no more than 80% of the harmonic frequency of the thermowell. However, many plant engineers prefer to take a more cautious approach and work to a margin of 25 or 30%, especially if the consequences of any failure are likely to be disastrous.
Some applications are more at risk of vortex-induced failure than others. Where the fluid flow rate is high and the damping effect of the fluid is low (mainly gases), it’s a good idea to check by running the calculation based on ASME PTC 19.3. If the calculation shows that a particular thermowell could be at risk, there are several possible solutions.
Geometry plays a key role, so a shorter or thicker thermowell may be more suitable. Some operators may suppose that the required length is fixed by the diameter of the pipe, but the thermowell does not have to reach the very centre. Temperature measurements should still be representative as long as the measuring element reaches the middle third of the pipe, so there is some flexibility.
Velocity collars can also change the resonant frequency of the thermowell. These devices form a tight fit where the stem meets the pipe wall, effectively shortening the unsupported length of the stem. The downside of this approach is that velocity collars can be expensive.
Again, a reputable supplier should be able to carry out the necessary calculations to ensure that a thermowell is safe to use. All they’ll need from the user is information on the fluid, including the operating temperature and pressure, the specific volume and the velocity.
Failure is not an option
They may seem like standard pieces of kit, but thermowells routinely face some of the toughest conditions encountered in industry. In many applications, particularly in the oil and gas industries, the consequences of failure would be disastrous, so it’s worth checking with your supplier that all the relevant standards have been met.