Thermowells

• Select the thermowell material according to the temperature range and the environment (corrosive, oxidizing etc.) in which it is to be used.
  • These wells can be made from different materials like SS304, SS316, HRS446, Inconel, Monel, Ceramic, etc.
• According to the construction of Thermowell (Steeped Shank, Straight Shank, Tapered Shank)
  • Steeped Shank- Provide faster response time and lower drag force.
  • Straight Shank- Extremely strong, but response time is slower and drag force on the fluid flow is high.
  • Tapered Shank- Provide good response time and strength.
• Thermowell Insertion Length
  • For best temperature measurement accuracy, the “U” dimension should be long enough to permit the entire temperature-sensitive part of the measuring instrument to project into the medium being measured.
    Liquid temperature measurement: One inch or greater.
    Gas temperature measurement:- three inches or greater.
• Resistance to vibration.
  • Fluid flowing past the well forms a turbulent wake (the Von Karman Trail), which has a definite frequency based on the diameter of the well and the velocity of the fluid.
  • The thermowell must have sufficient stiffness so that the wake frequency will never equal the natural frequency of the thermowell itself. If the natural frequency of the well were to coincide with the wake frequency, the well would vibrate to destruction and break off.
• To avoid the Thermowell failures caused by excessive pressure, drag forces, high temperature, corrosion, vibrations, it is recommended to run thermowell calculations based on your applications:
  • Maximum or operating temperature
  • Maximum or operating pressure
  • Fluid(gas or liquid) velocity
  • Fluid Density.

Thermowells provide protection for temperature probes against unfavorable operating conditions such as corrosive media, physical impact (e.g. clinker in furnaces) and high pressure gas or liquid. Their use also permits quick and easy probe interchanging without the need to “open-up” the process. The main application areas are:

• Protection tube are used in thermocouples
• Insures integrity in high pressure applications
• Smaller wells are used in low pressure applications
• Straight wells are used in corrosive and erosive environment
• For applications where a quick response to changes in temperature is required, fabricated pockets may be fitted with a reduced tip.

One-piece Thermowells are suited to the highest process loads, depending on their design. Thus in the petrochemical industry, one-piece thermocouples are now used almost exclusively.

Sheath material ranges from mild and stainless steel to refractory oxides (Ceramics) and a variety of exotic material including rare metals. Sensor inserts are fabricated unit which is comprised by a sensor and a terminal base; the sensor is housed in a stainless steel insert tube, usually of 6 or 8mm diameter and, then it is inserted into the actual protection tube. Good sheathing with physical contact between the insert tip and sheath end is essential to ensure good heat transfer. Spring contact is used in the terminal base to maintain this contact. This arrangement facilitates easy replacement of the sensor whenever required.

In general, there are two types of protection tubes:

• Metallic protection tubes
• Non-metallic protection tubes

Metallic tube, most commonly stainless steel, has mechanical advantage and higher thermal conductivity; they are generally immune to thermal shock. Metallic sheath can be used at temperature up to 1150ºC. Ceramics are superior when high purity is required to avoid sensor or product being contaminated at elevated temperature.

Ceramic sheath’s main application ranges between 1000 to 1800 ºC. They may be in direct contact with the medium or may be used as a gas-tight inner sheath to separate the thermocouple from the actual metal protection tube. They should be mounted in hanging position above 1200ºC to prevent distortion or fracture due to bending stresses. Even hairline cracks can lead to contamination of the thermocouple resulting in drift or failure. The wall thickness of the material is also important; thin walled tube is preferable to larger wall thickness. Cracks frequently developed if they are given excessively rapid temperature changes when they are quickly removed from hot furnace.

• Metallic protection tubes are composed of different types of metals like 304 S.S, 321 S.S, Inconel 600, Inconel 825, UMCO, Kanthal A1, Hastelloy B, Monel, Platinum, Titanium, Tantalum, Molybdenum etc.
• Operating Temperature range is from 250ºC to 2200ºC.
• Metallic Protection tubes are excellent heat resistant.
• Can you please let me know best image size as this one is also being cut off in screen shot attached.

• Non-metallic tubes are composed of different types of non-metals like Refractory oxide recrystallized Aluminum oxide, Silicon Carbide, Silicon Nitride, Cermet, Quartz, Tungsten Carbide etc.
• Operating range is from 300ºC to 1700ºC
• Good resistance to chemical attack. Mechanical strength is good but thermal shock should be avoided.
• Less corrosive to acids and alkalis.

According to their connection to process

• Threaded
• Socket Weld
• Flanged Welded

According to their production method

• Fabricated Thermowell
• Bar-stock thermowell
• Van-stone thermowell

Fabricated Thermowells are manufactured from tubes which are sealed by a solid welded tip at the process. It means that the stem of thermowell is manufactured from tube and tip is made separately, than both these parts are welded by utilizing suitable welding process. The flange is also joined to this assembly by welding process. Fabricated Thermowells are generally recommended for low to medium process loads. Connection to the pipe or vessel may be done by means of thread, flange or weld. Standard threads used in fabricated thermowell are NPT, BSP (Pl), BSP (Tr), API & metric thread. The thread size is dependent on the application. Standard sizes range from 1/8” to 2”. For applications where a quick response to changes in temperature is required, fabricated pockets may be fitted with a reduced tip.

Such thermowell bodies are machined and drilled from solid bar stock. This results in a non-welded water tight unit. In this Immersion tip is also made by same material along with stem. In such type of thermowell, no welding process is required for stem and tip production. Flange can be weld according to requirement. Bar-Stock thermowell is also known as “Solid drilled thermowell”. Single piece Thermowells are manufactured from a complete element or bar stock. One-piece Thermowells are suited to the highest process loads, depending on their design. Thus internationally or in the petrochemical industry, one-piece thermocouples are now used almost exclusively. Connection to the pipe or vessel may be done by means of thread, flange or weld. Standard threads are NPT, BSP (P), BSP (Tr), API & metric tread. The thread size is dependent on the application, but 3/4” and 1” are common.

In van stone thermowell, stem, tip and sub part of flange all these three prepared by using single bar or rod material. There is no need of welding for these three parts of thermowell. The flange sub-part serves as a gasket in such type of thermowell. On this sub-part flange is used according to requirement.

Tapered

The outside diameter decreases gradually along the immersion length. Used for high velocity applications.

Flat Tip

It one end has flat surface. Used in low pressure applications or where flow characteristics around the thermowell are not important.

Domed Tip

Such thermowell has semi-spherical tip at one end of thermowell. Used in higher pressure applications or where flow characteristics around the thermowell are important. This ensures a high degree of mechanical strength without losing the sensitivity or accuracy of the indicator

Spherical Tip

For spherical tip, special drill is used with a tip angle of 118ºC for production of thermowell. To achieve a possible uniform wall thickness, the tip is ball shaped or spherical in shape. Used in higher pressure applications or where flow characteristics around the thermowell are important. This ensures a high degree of mechanical strength without losing the sensitivity or accuracy of the indicator.

Shank Construction

• Q Dimension: The thickest part of the shank of the well on the hot side of the process connection or flange. It is dependent on the bore size and the process connection size.
• Bore Size The inside diameter of thermowell. In other words, the diameter of the internal cylindrical cavity of a thermowell or protection tube Standard bore sizes are 6.5 mm, 8.5 mm.
• Immersion (“U”) Length: The length of a thermowell or protection tube below the mounting threads, flange, bushing, etc. extending into the process area. The “U” length is measured from the bottom of the process connection to the tip of the thermowell.
• Lagging Extension (“T”) Length: The length of a thermowell, in addition to the standard head lengths, required to make the head of the thermowell accessible and this enable the probe to extend through insulation or walls.
• Internal Mounting Thread: The thread within the thermowell for attaching a temperature device of the union and nipple extension for a thermowell assembly.

Raised Face (RF)

The Raised Face type is the most applied flange type, and is easily to identify. It is referred to as a raised face because the gasket surfaces are raised above the bolting circle face.

Ring-Type Joint (RTJ)

RTJ flanges have grooves cut into their faces which steel ring gaskets. The flanges seal when tightened bolts compress the gasket between the flanges into the grooves, deforming (or Coining) the gasket to make intimate contact inside the grooves, creating a metal to metal seal. Ring Type Joint gaskets are designed to seal by “initial line contact” or wedging action between the mating flange and the gasket. Ring Type Joint gaskets* are metallic sealing rings, suitable for high-pressure and high-temperature applications.

Flat Face (FF) The flat face (full face) flange has a gasket surface in the same plane as the bolting circle face. Applications using flat face flanges are frequently those in which the mating flange or flanged fitting is made from a casting. Flat face flanges are never to be bolted to a raised face flange.

Welding is a process of joining two metals by heating the metals to a suitable temperature. It may be done with or without the application of pressure, and with or without filler.

Types:

Full Penetration Welding- This type of welding ensures a fully welded interface between the two parts and is generally the strongest joint.

Partial Penetration Welding- This type consists of a partially welded interface with filler metal being laid on the surface of the two metals.

A WPS is a document that describes how welding is to be carried out in production. They are recommended for all welding operations and many application codes and standards make them mandatory.

The purpose of the document is to guide welders to the accepted procedures so that repeatable and trusted welding techniques are used. A WPS is developed for each material alloy and for each welding type used.

A WPS is supported by a Procedure Qualification Record (PQR or WPQR). A PQR is a record of a test weld performed and tested (more rigorously) to ensure that the procedure will produce a good weld.

The variables required to be documented are typically such items as:

• Welding process used, size, type and classification of filler alloy.
• Type and thickness of base material welded.
• Type and polarity of welding current, amps and volts recorded.
• Travel speed during welding, welding position, type and dimensions of joint design.
• Preheating temperature, inter-pass temperature, post weld heat treatment details, and others.

In addition to the recording of all the welding variables used during the test, in order to qualify a welding procedure, details of the inspection and test results must also be recorded. These records must show that the inspection and testing has proven that the weld samples have met or exceeded the specified standard requirement.

Different types of coating are carried out in Thermowells. Some are as follows: Metal Thermowells with Tungsten Carbide / Ceramic / PTFE / PVDF / PFA Coatings.

• Material Test
• Dimensional Test
• Hydrostatic Pressure Test
• Dye Penetrant Inspection
• Radiography

Chemical- By PMI(Positive Material Identification).
Physical- By Tensile, Elongation, Hardness, Micro & Macro examination, IGC tests.

The Thermowell must be checked according to specified dimension given in drawing.

A hydrostatic pressure test is a way in which leaks can be found in pressure vessels such as pipelines and plumbing. The test involves placing water, which is often dyed for visibility, in the thermowell at the required pressure to ensure that it will not leak or be damaged. It is the most common method employed for testing pipes and vessels. Using this test helps maintain safety standards and durability of a vessel over time. Newly manufactured pieces are initially qualified using the hydrostatic test.

Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). LPI is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.

Radiographic Testing (RT), or industrial radiography, is a nondestructive testing (NDT) method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials. irregularities and flows that can be detected which includes – cracks, voids, thickness, lack of fusion, lack of penetration, porosity and misalignment.

Materials the Longevity Factor

The selection of material is the most important factor for thermowell life.The material selected is based on the temperature of the application and the process medium.

Connection-The Installation Factor

All threaded well are made by easily welded or brazed materials.Welding and brazing is important for the installation requiring seal. The pipe thread provides mechanical strength, and the weld or braze provides the seal. Flanged wells(other than van stone type) consist of a bar stocks well which is solidly welded to a top quality flange. Standard construction uses a primary “J” groove weld and a bevel groove clean fillet. This double welded construction eliminates the possibility of crevice corrosion since no open joint are exposed from either inside or outside the installation. Socket weld well are simple to install, simply weld them into place.

Insertion Length-The Accuracy Factor

The distance from the tip of the well to the underside of the thread or other connection is defined as the insertion length (designated as “U”). For best accuracy this length should be greater enough to permit the entire temperature sensitive part of element to project into the medium being measured. A properly installed element: in liquid, the element should be immersed up to its sensitive length plus one inch, and in air or gas, the element should be immersed up to its sensitive length plus three inches.

Bore Size-The Interchangeability Factor

Almost all installation uses several type of temperature measuring sensor. The selection of a standard bore diameter can produce extreme flexibility within the plant. The same well can accommodate thermocouple, resistance thermometer, and bimetal thermometer.

Tapered or Straight Well- the Velocity Rating Factor

Tapered shank provides greater stiffness with same sensitivity. The higher strength to weight ratio give these wells higher natural frequency than the equivalent length straight shank well thus permitting operation at higher fluid velocity.

At low velocities, the risk of thermowell failure is minimal and does not usually require frequency calculations. If the following criteria are met, the designer may elect to wave calculation requirements.

• Maximum fluid velocity is less than 2.1 ft/sec. [0.46 M/s].
• Wall thickness at “A” support diameter minus “b” bore diameter ≥ 0.376″ [9.55mm].
• “L” Unsupported length ≥ 24″ [0.61 M].
• “A” support and “B” tip diameter ≥ 0.5″ [12.7 mm]
• Thermowell material satisfies “S” maximum allowable working stress ≥ 69 Mpa.
• “Sf” fatigue endurance limit, in the high-cycle limit ≥ 21 Mpa.
• Thermowell material should not subject to stress corrosion or embrittlement.

In low density gases with a Scruton Number (Nsc) of >2.5 Reynolds Number <105, the in-line resonance is suppressed and therefore the acceptable ratio will be:
fs<0.8fnc

If a thermowell passes cyclic stress conditions for operation at the in-line resonance condition, the acceptable ratio will be:
fs<0.8fnc

If a thermowell fails the cyclic stress condition for operation at the in-line resonance condition, the thermowell natural frequency will be high enough to limit excitation of the in-line resonance. Therefore, the acceptable ratio will be:
fs<0.8fnc

• Installing temperature sensor assembles into thermowell or directly into the process requires the use of some kind of brass or stainless steel fitting.
• Fitting include various threaded unions, bayonet cap (and adapters) and flange.
• Adjustable flange can similarly be used to sensor assembly into the process.
• Bayonet caps provide a method of quick fitting into suitable adapters located in the process; this technique is widely used in plastic machinery.
• Bushes and hexagon plugs are used when adjustment or removal is a lesser consideration.
• The choice of fitting may be dictated by the need for pressure integrity or by physical size constraints.
• Compression fitting and threaded bushes can be supplied with tapered threads to achieve a pressure tight connection.