Thermowells

What Is a Thermowell?

In industrial processes, temperature sensors are rarely immersed directly in the fluid. Instead, the sensor sits inside a closed-end metal tube mounted in the process — the thermowell, sometimes also called a protection tube. The thermowell isolates the sensor from the forces generated by the flow, from high pressure and from corrosive chemical attack. At the same time, it becomes a sealed, pressure-retaining part of the line or tank to which it is welded, screwed or flanged.

The main operational benefit of a thermowell is that the sensor can be removed for calibration or replacement without shutting down the process or draining the line. In return, the well places extra mass and thermal resistance between the sensor and the fluid, which slows the measurement response. Thermowell design therefore requires a deliberate balance between mechanical strength and thermal response.

Because it is a wetted part, the thermowell must be compatible with the design pressure, temperature and material of the pipe or tank it is connected to, as well as with the applicable pressure equipment rules. An unsuitable design can lead to fatigue failure from flow-induced vibration and to a process leak; thermowell selection is therefore a critical engineering decision for plant safety as much as for measurement accuracy.

Engineering Principles of Thermowell Design

Whether a thermowell suits an application is decided by evaluating four basic design topics together:

1.      Process connection and pressure boundary: The connection type is usually chosen to match the mating connection specified by the process designer. Threaded connections are easy to install but have a lower pressure rating than flanged connections; welded connections provide the highest pressure capability and tightness.

2.      Stem profile and flow forces: The stem (shank) that projects into the process is exposed to the drag force of the fluid. For the same root diameter, straight, tapered and stepped profiles behave differently in terms of drag force, natural frequency and response time.

3.      Vortex shedding and resonance: The flow creates periodic vortices behind the thermowell stem, and the vortex shedding frequency rises with flow velocity. If this frequency approaches the natural frequency of the thermowell, resonance occurs and the stem can fail through fatigue. The wake frequency calculation assesses this risk.

4.      Thermal response and sensor fit: The standard thermowell has a 6.6 mm bore and a 6.4 mm tip thickness. The smaller the gap between sensor and bore and the smaller the tip diameter, the faster the thermal response; the sensor length is chosen equal to the sum of the head length (H) and immersion length (U).

ASME PTC 19.3 TW Acceptance Criteria

ASME PTC 19.3 TW is the internationally recognized standard for the mechanical design of thermowells machined from bar stock. To be accepted for a given set of process conditions, a thermowell must satisfy four quantitative criteria: the frequency limit (a natural frequency high enough to stay clear of destructive flow-induced oscillation), the dynamic stress limit (primary dynamic stress has to remain within the allowable fatigue limit, and an extra fatigue check applies if resonance is crossed while the process ramps up to operating conditions), the static stress limit (steady-state stress stays within the allowable value according to the von Mises criterion) and the hydrostatic pressure limit (external process pressure stays within the ratings of the tip, the stem and the flange or thread). Corrosion, erosion and the effect of process conditions on material properties must also be assessed separately.

How Is Immersion Length Determined?

There is no single standard formula for immersion length, but there are established practices. In turbulent flow the tip should sit near the pipe centerline, where the most representative temperature is found. As a general guide, immersion length is taken as 10 times the root diameter for gases and 5 times for liquids; another approach is to immerse at least one third of the pipe diameter. The API recommendation is to add 50 mm to the length of the sensing element. Because long immersion lengths are unfavorable for wake frequency, the length must always be chosen together with the mechanical calculation.

Increasing the root diameter raises strength; reducing the tip diameter improves response time. Both dimensions can be used as design parameters to arrive at a design that passes the wake frequency calculation.

Key Features

·         One-piece stem machined from bar stock: Thermowells are machined from solid bar for strength and integrity; the bore is drilled to a standard 6.6 mm diameter.

·         Four connection families: Threaded (tapered and parallel threads), flanged (ASME B16.5 and EN 1092-1), Van Stone (lap joint) and welded (socket weld or DIN 43772 Type 4 weld-in) options.

·         Three stem profiles: Straight, tapered and stepped profiles, chosen to balance drag force, natural frequency and response time.

·         Flexible design modifications: Root diameters of 10–80 mm and tip diameters of 10–46 mm, adjustable in 0.5 mm steps; the bore (3.5–11.0 mm) and a tip thickness of 5.0 or 6.0 mm can also be customized.

·         Three manufacturing methods for flanged wells: Partial penetration welded, full penetration welded or weld-free forged one-piece flanged thermowells; the flange-to-stem weld is made in accordance with ASME Section IX.

·         Wide range of materials: In addition to standard 316/316L, 304/304L and A105 carbon steel: duplex, super duplex, 6Mo, chromium-molybdenum steels, nickel alloys, Nickel 200 and Titanium Grade 2.

·         Coating and sheath options: Anti-stick PFA or PTFE coating, a 0.38 mm tantalum sheath and a cobalt-chromium-based hardfacing of at least 40 HRC to resist erosion.

·         Choice of flange facings: Spiral serrated raised face is standard; concentric serrated, flat face, RTJ, tongue/groove and male/female (spigot/recess) facings can be selected.

·         Improved surface finish: Beyond the standard 0.8 µm Ra finish, machining to below 0.3 µm Ra and electropolishing are available to increase corrosion resistance and cleanability.

·         Vent hole: An optional vent hole prevents gas build-up, and fluid escaping from it indicates that the integrity of the thermowell has been lost.

Technical Specifications

The values below are typical design ranges for the bar-stock thermowell family covered on this page. Final dimensions and pressure rating should be determined at project stage according to the connection type, material and the result of the ASME PTC 19.3 TW calculation.

Thermowells longer than 1,067 mm require an internal pressure test; with the tapered profile, the total of immersion and head length must remain below 1,067 mm. Hardfacing is applied to straight and tapered profiles only.

Key Advantages

·         Sensor maintenance without a shutdown: The sensor can be removed for calibration or replacement without draining or depressurizing the line, so maintenance planning no longer depends on the process schedule.

·         Mechanical safety verified by calculation: A calculation to ASME PTC 19.3 TW documents that the thermowell is acceptable in terms of resonance, fatigue, static stress and external pressure.

·         Process-specific geometry: Adjustable root and tip diameters, bore and tip thickness make it possible to reach a design that both passes the calculation and has an optimized thermal response.

·         Materials for corrosive media: Material options from stainless steels to nickel alloys and from duplex steels to titanium, plus coatings, allow a solution matched to the process chemistry.

·         No weld-joint corrosion: Van Stone and forged one-piece designs have no weld between stem and flange, removing both weld-zone corrosion and the need for weld qualification.

·         Traceable quality documentation: Material certificate, PMI, NDT and pressure test options allow the equipment to be fully integrated into quality and pressure-system documentation.

·         Complete assembly matched to the sensor: The thermowell can be supplied assembled with the sensor, hand-tight or torqued and ready for the process, reducing the risk of length mismatches on site.

Application Areas

Thermowells are used at almost every temperature measuring point where the sensor must be isolated from the process. The choice of connection type, profile and material varies with the application conditions below.

High-Velocity and High-Pressure Lines

On lines with high flow velocity, high temperature or very high pressure, welded thermowells and the tapered profile are preferred; an RTJ facing can be used on high-pressure flanged connections. Temperature compensation points for steam and gas flow belong to this group.

Corrosive Processes

Because it contains no weld, the Van Stone design eliminates weld-zone corrosion and allows the flange material to differ from the stem material. Nickel alloys, a tantalum sheath or titanium are evaluated according to the process chemistry.

Sour Service (H2S-Containing Media)

A NACE MR0175/ISO 15156 and MR0103 compliance certificate documents thermowell materials intended for media containing hydrogen sulfide.

Offshore Projects

316/316L, duplex and super duplex materials qualified to NORSOK M-630 and M-650 are supplied together with ferrite content, low-temperature Charpy and corrosion tests.

Erosive and Sticky Fluids

Hardfacing on the stem in erosive service, and PFA or PTFE coating for fluids that tend to stick, limit wear and build-up.

Oxygen Service and Hygienic Applications

Special cleaning to ASTM G93 for oxygen-enriched service; for hygienic applications, a surface finish below 0.3 µm Ra and electropolishing improve cleanability.

How to Select a Thermowell

Thermowell selection affects both measurement performance and the safety of the pressure system. The following information is needed for a technical assessment and mechanical calculation:

Process Data

·         Fluid type (liquid, gas or steam) and any mixed-phase flow

·         Minimum/normal/maximum flow velocity or flow rate, and pipe inside diameter

·         Fluid density and viscosity

·         Design and operating pressure and temperature

·         Corrosion and erosion risk; presence of H2S

Connection and Dimensions

·         Mating connection: thread type and size, flange standard/size/class and facing, socket weld or weld-in

·         Immersion length (U) and head length (H); insulation thickness and nozzle height

·         Stem profile, root and tip diameter

·         Bore matched to the sensor diameter, and instrument connection thread

Material and Surface

·         Compatibility with the process chemistry, temperature limits of the material and weldability with the pipe material

·         Need for coating, sheath or hardfacing

·         Surface finish and electropolishing requirements

·         Lap flange material for Van Stone designs

Documentation and Tests

·         ASME PTC 19.3 TW calculation report

·         NACE, EN 10204 3.1 material certificate and PMI

·         Hydrostatic external/internal pressure test, liquid penetrant, UT/RT wall thickness check

·         Welding procedure and welder qualification records (WPS, PQR, WPQ) for the flange weld, and phased-array UT or radiography for full penetration welds

For full penetration welded flanged thermowells, liquid penetrant examination should also be requested so that the requirements of ASME PTC 19.3 TW are met.

Design Standards, Tests and Certificates

The design, manufacture and quality verification of thermowells are defined by the following standards and test options:

Conformity assessment under pressure equipment legislation and regional registration requirements should be evaluated for each project together with the TLY Enerji engineering team.

Thermowell Solutions from TLY Enerji

At TLY Enerji we treat the thermowell not as an accessory but as a pressure-retaining component that determines the safety of the measuring point. Our engineers collect flow, pressure, temperature and fluid property data to define the connection type, stem profile, material and immersion length, and we manage the process of having the ASME PTC 19.3 TW calculation performed and evaluating its results.

Where the calculation does not pass, we compare alternatives technically — a change in root or tip diameter, a shorter immersion length, a different profile or non-intrusive measurement. We help align documentation requirements such as material certificates, NACE, PMI and pressure testing with the project specification, supply the thermowell together with the sensor and transmitter as a complete measuring point, and support site installation and commissioning.

Frequently Asked Questions

What is a thermowell used for?

A thermowell is a closed-end protective tube that shields the temperature sensor from flow forces, high pressure and corrosive media. It becomes a sealed, pressure-retaining part of the pipe or tank and allows the sensor to be removed for calibration or replacement without shutting down the process or draining the line. The trade-off is a somewhat slower measurement response.

Why is a wake frequency calculation necessary?

The flow generates periodic vortices behind the thermowell. If their shedding frequency approaches the thermowell's natural frequency, resonance occurs and the stem can break through fatigue. A calculation to ASME PTC 19.3 TW evaluates the frequency, dynamic stress, static stress and hydrostatic pressure criteria. Performing this calculation is recommended for every thermowell.

What is the difference between straight, tapered and stepped thermowells?

A straight profile has the same diameter along its whole length; it has the highest drag force and the slowest response. A tapered profile narrows from root to tip; as a balance between strength and response, it is a common choice for high-velocity flows. A stepped profile ends in a narrower tip section; compared at equal root diameter, it offers a higher natural frequency and the fastest response.

Which connection type should be selected?

Threaded thermowells are easy to install and remove but have lower pressure ratings than flanged types. Flanged thermowells can be removed for maintenance and suit high pressure classes. Welded thermowells offer the highest pressure capability and tightness and are preferred at high velocity, temperature and pressure. The Van Stone type stands out in corrosive service because of its weld-free design.

How is thermowell immersion length determined?

In turbulent flow the aim is to place the tip near the pipe centerline. A general guide is an immersion length of 10 times the root diameter for gases and 5 times for liquids; immersing at least one third of the pipe diameter and the API recommendation of adding 50 mm to the sensing element length are also used. Because long wells increase the resonance risk, the final length must be verified by mechanical calculation.

What is the advantage of a Van Stone thermowell?

A Van Stone (lap joint) thermowell is clamped between the mating flange and a loose lap flange. As there is no weld between stem and flange, weld-zone corrosion cannot occur. The wetted stem can be made from an expensive alloy while the flange above it is chosen in a more economical material such as carbon steel, and it can be replaced easily when needed.

What can be done if a thermowell calculation fails?

Increasing the root diameter raises strength and natural frequency; changing the tip diameter and shortening the immersion length also help the calculation pass. Changing the profile — for example a tapered instead of a stepped stem — is another option. If no geometry is adequate, non-intrusive measurement from the pipe surface can be considered as an alternative.

Which tests and certificates can be requested for thermowells?

The main options are the ASME PTC 19.3 TW calculation report, NACE MR0175/ISO 15156 and MR0103 compliance, an EN 10204 3.1 material certificate, PMI, hydrostatic external and internal pressure tests, liquid penetrant examination, ultrasonic or radiographic wall thickness checks, low-temperature Charpy testing and oxygen service cleaning. For flanged types, welding procedure and welder qualification records (WPS, PQR, WPQ) can also be requested.