An RTD, resistance temperature detector, is a passive temperature sensing device that operates on the principle that the resistance of a metal changes as the temperature changes. The electrical current that passes through the element...
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This article will give more detailed information regarding thermowells on topics such as:
This chapter will discuss what thermowells are, the operational principles of thermowells, and a thermowell's importance to various industries.
A thermowell is a pressure-tight vessel that safeguards and increases the lifespan of temperature sensors in processing plants in cases where a measuring sensor is not otherwise mechanically or chemically useful in the process environment. A thermowell is installed directly into piping systems. Thermowells help in high-pressure pipelines' sensor replacement by avoiding the need to avoid disturbing the process flow, or draining the processing system, for sensor maintenance functions. In addition, the use of standardized thermowells allows the simple removal of sensors throughout a plant.
Since temperature sensors consist of bimetal thermometers, thermocouples, and thermistors, the use of thermowells shields temperature sensors from corrosion, high material velocities, and extreme pressure. Additionally, they extend the sensor's life, enable sensor replacement without emptying the system, and lower the risk of contamination. To assure integrity, thermowells for high-pressure applications are normally machined from bar stock. Smaller thermowells can be created using tubing with one end welded shut in low-pressure environments.
A thermowell separates the sensing element of a temperature measurement device and a process media (a material being processed). It guards against corrosive process media and substances that are pressure-contained or moving quickly. A thermowell also makes it possible to remove the sensing component from a particular application while keeping the system closed. A thermowell may be employed in an application as simple as a cooking pot used in the food processing industry or as complex as a boiler in a brewing industry where temperature is a critical measurement.
Thermowells are critical in industrial applications requiring accurate temperature measurement.
In order to select the best thermowell for any application, different specifications must be known and taken into consideration. The following are a few requirements to take into account when choosing a thermowell.
Thermowell Root Dimension (Q): Thermowell root dimension is the thickest portion of a thermowell that is inserted into the pipe wall. Both the process connection size and the thermowell bore diameter correlate with this thermowell dimension.
Thermowell Immersion Length (U): The thermowell immersion length is the distance between the thermowell's tip and the process connection's base.
Thermowell Bore Size (V):The thermowell bore size is the inside diameter of a thermowell. The standard bore diameters for thermowells are 0.260" and 0.385" since these dimensions accommodate sensors featuring standard sizes of either 0.25” or 0.38” diameters.
Thermowell Lagging (or Trailing) Extension Length (T): The thermowell lagging extension length refers to that portion of a thermowell extending outside the pipe wall after insertion. This length is usually an extension of the thermowell's hex length and is defined by the cold head of the process connection.
The Hex Length (P): Hex length measurement is the distance between the mounting threads' top and the stem's tip.
Thermocouple Probe: A thermocouple probe is a metallic tube containing thermocouple wire. The thermocouple probe and thermowell may typically extend through insulation or walls because of the lagging extension length. The probe's sheath is the term used to describe the tube wall. Steel and other common materials are used for sheaths.
When installing a thermowell through an insulation medium on a pipeline or piece of machinery, lagging (heat-insulating material used with pipes, etc.) is employed. When installing thermowells, the following factors must be taken into consideration in order to avoid their failure:
It is also critical that the distance between the thermowell's internal diameter (ID) and the thermocouple sheath's outer diameter (OD) be extremely small so that the sensitivity of the thermocouple does not result in potentially incorrect temperature measurement. In addition, the thermowell's bore needs to be linear and homogeneous so that it becomes easy to insert the thermocouple during installation.
Thermowells should be designed to accommodate a particular process media and different pressures, temperatures, velocities, specific gravities, etc. In addition to these factors, a properly-functioning thermowell design depends upon:
There are different types of thermowells, classified by their end connector. An end connector is the part on the thermowell which mates with a measuring sensor’s connector. The selection of a thermowell depends on the type of connector that is on the measuring sensor. The following is a list of end connector types which will be described in greater detail below:
Threaded Thermowells: As suggested by the name, these thermowells are screwed either directly into the wall of a tapped pipe. A thermowell with a threaded connection is typically the most affordable and adaptable. Normal applications for threaded thermowells involve non-corrosive processes such as measurement of molten iron temperature to protect the thermowell threads from rusting. The pipe has them screwed in. They can be brazed or welded together to provide strength because of the nature of their substance.
Socket-Welded Thermowells: Due to their sturdy connection, socket-welded thermowells can be used as a permanent connection for most applications. Socket-welded thermowell connections are used in applications with extremely high temperatures and pressure because the socket-welded join provides a firm contact that resists pressure and is not affected by high temperatures. In addition, when thread impurities must be avoided, like in the food and pharmaceutical industries, welded-in thermowell connections are recommended because a welded socket joint does not allow any impurities or air to pass through, which may affect the food or medicine being manufactured.
Van Stone Thermowells: Van Stone thermowells are the best option for high-pressure applications because they are strong enough to withstand these forces when exerted on them. A Van Stone thermowell is typically sandwiched between a cover flange and nozzle and are machined from solid bars. A phonographic (or continuous) spiral serration is present on the Van Stone flange surface. This spiral serration enables the availability of several grades of surface polish.
Scruton Thermowells: To prevent damage brought on by mechanical load or when considering process criticality (the probability of failure based on a company’s risk matrix study), a Scruton thermowell is constructed using the ScrutonWell® design, where the helix is incorporated onto the outside of the thermowell. This design enables a simple and quick thermowell installation without requiring a support collar, avoiding costly and time-consuming site rework. It also reduces oscillation amplitude by more than 90%. The ScrutonWell® design also enables the availability of several grades of surface polish. Time and money are saved on any site rework because of the Scruton thermowell.
Sanitary Thermowell: The sensing element of any temperature sensor is isolated and safeguarded using a sanitary thermowell. Sanitary thermowells are constructed with hygienic connections to prevent the growth of microorganisms.
There are many different processes used to connect thermowells to a piping system and these process connections can also be used as a means to distinguish them. Below are a few common types of thermowell connections:
Flanged Thermowells: An extended “collar” is present on a flanged thermowell and is used to provide better support for the thermowell when connected to a pipe.
Weld-in thermowells: are immediately welded into a pipe or a process vessel. The direct welding of weld-in thermowells to a pipe or tank creates the highest quality of any thermowell connection method. Because they are designed to be permanently welded, weld-in thermowells should only be used when their access is unnecessary and where corrosion is not a problem. They are frequently installed in high-temperature and high-pressure applications where non-corrosive media are to be measured in a piping system.
Threaded Thermowells: Threaded thermowells are typically utilized in smaller pipelines containing non-corrosive fluid's since a threaded thermowell is screwed into the pipe and must not be replaced. The following list contains some applications where threaded thermowells are regularly found.
Threaded thermowells are used in these cases because they provide a tight fit, guaranteeing that no air or water gets insides to create corrosion inside pipes or equipment or which could contaminate a manufacturing process.
There are four different types of stem design shapes of thermowells available depending on the thermowell’s application:
The diameter of a straight thermowell is constant throughout its insertion length. As a result, they are easy to construct, have good stiffness, and provide defense against erosion and corrosion. Stepped thermowells have stepped diameters; they normally have a larger diameter at the top (often 3/4"), and they have a smaller diameter at their tip (commonly 1/2"). These thermowells offer smoother velocity and react to temperature changes faster than their straight counterparts due to less thermal inertia at the process end.
A tapered thermowell has variable diameters (resulting from a smooth continuous taper) throughout its whole insertion length. Heavy-duty applications requiring fast response time can benefit from the tapered thermowell's quick response time since the tapered thermowell has a small wall thickness. Built-up thermowells are suited for long process insertion lengths. There are numerous types of built-up thermowells, but what they have in common is a pipe that is welded between the tip and process connection to achieve their long insertion length. This feature is critical for thermocouples requiring insertion into longer, deeper vessels holding larger liquid volumes.
In addition to the various end connections used to classify a thermowell, there are additional methods used to classify thermowells. As described below, thermowells can be classified based on the type of material used to construct them. Or, they may be grouped based on their shank.
Bar Stock Thermowells: These are thermowells that are made from a bar stock (raw, purified metal) that is machined and drilled; this process does not involve welding.
Thermowell Classification by Type of Shank: The shank is the portion of a thermowell that is inserted into a process piping system or vessel. Straight, stepped, and tapered shanks are the most prevalent forms of thermowells. Along its immersion length, a straight-shank thermowell has the same dimension. To react more quickly, step-shank thermowells feature a varied outer diameter at the immersion length. Along the immersion length, the outer diameter of a tapered thermowell gradually decreases. Due to an outdated definition of tapered thermowells found in the ASME PTC 19.3 standard (1974) used to classify thermowells machined from bar stock, the heavy-duty tapered thermowell was developed in response. The heavy-duty tapered thermowell was designed specifically for, and is typically employed in, heavy-duty applications. These applications include those where a thermowell is threaded into or welded to flanges and welded into pipes or process vessels with or without weld adaptors. However, the straight shank thermowell type is frequently the best option for resistance to velocity-induced resonance if the inner nozzle diameter is a design constraint.
Lagging extension is used as another category of shank thermowells. As discussed in chapter two, lagging extension refers to the remaining length of a thermowell after insertion into a piping system. Lagging extension thermowells are used when utilizing insulation to cover the vessel or piping system.
To demonstrate that a thermowell has the strength to manage the hydrostatic pressure limit and dynamic static stress in connection to process circumstances, wake frequency calculation (the ratio of the Strouhal frequency to the natural frequency "must not exceed" 0.8 Hz) as defined by ASME PTC 19.3 standard is performed. These calculations are performed before thermowell manufacture to ensure that the thermowell can withstand the stress and strain that any process media can create and that the thermowell design is valid. Strouhal frequency of a body at rest is used to describe oscillating flow mechanisms and is defined as St = fstD/U (where ST=Strouhal number, fst=vortex shedding frequency, D refers to the diameter of the circular cylinder, and U is the ambient flow velocity). The thermowell length is assessed in the final phase using the steady-state stress.
As fluid flows past a thermowell, a turbulent wake is left behind it due to fluid momentum changes. In this wake, vortices form and shed from different sides of the well. The vortex shedding frequency (also known as the wake frequency) is inversely related to the thermowell tip width and is linearly correlated with the flow velocity. These shedding vortices exert a periodic force on the thermowell that consists of two components: (i), a lift force that is parallel to the flow direction and oscillates at twice the wake frequency, and (ii) a lesser drag force that is normal to the flow direction. The magnitude of the vibrations is typically insignificant due to the modest vortex-induced forces that generate thermowell vibration. However, once the wake frequency (FW) gets close to the thermowell's natural frequency (fn) (within 20%), it may shift and lock into that frequency. The thermowell enters resonance, and vibrating forces quickly rise when fw = fn. The well's mechanical failure may be a result of the resulting vibrations. The only factor considered by ASME PTC 19.3 standard for thermowell vibration is the oscillating lift force. To prevent resonance, the wake-to-natural frequency ratio is limited to a maximum of 0.8 Hz. Although the oscillating drag force is minimal, because it happens at twice the wake frequency, it can drive the thermowell into resonance at lower speeds. The Murdock analysis falls short for high-density fluids (liquids and high-pressure steam). The velocity rating can be decreased by up to 50% when the oscillating drag component is considered. Only a guide should be used to choose the right thermowell based on the results of these calculations.
As fluids flow past a thermowell, they swirl around and form low-pressure eddies downstream in the wake of the well. These mini whirlpools then release from different sides of the well producing two periodic forces on a thermowell:
Vortex shedding can happen between 50Hz and 1500Hz. The force rises with the square of the fluid velocity. The vortex shedding frequency (Strouhal frequency) grows linearly with fluid velocity. When the Strouhal frequency becomes close to the thermowell's inherent frequency, it may lock in and cause resonance, resulting in significantly amplified forces. The thermowell's inherent frequency needs to be greater than either the in-line resonance condition or the transverse resonance condition to avoid lock-in. An operation through the in-line resonance is only acceptable if the cyclic stresses at the resonance state are acceptably small. A crucial velocity to consider is the fluid velocity at which resonance occurs. For each thermowell natural frequency, two velocities are critical: one defining the transverse response and the other characterizing the in-line response. The corresponding critical velocity is roughly one-half of what is needed for transverse resonance because the in-line force fluctuates at a frequency that is twice that of the transverse force. A significant resonant increase in vibration amplitude can take place if the thermowell's inherent frequency overlaps fs or 2fs. Resonance fatigue is the primary factor in thermowell failure. A thermowell can function at the in-line or even transverse resonance frequencies if sufficient dampening exists.
The lifespan of a thermowell depends on the choice of material used to construct it. When choosing a material, it is important to consider the temperature, the type of chemical a thermowell may interact with, and the flow rate the thermowell will be subjected to. Chemicals have a greater tendency to corrode materials at higher temperatures and concentrations. Additionally, erosion can be brought on by particles suspended in a fluid. The following list comprises some of the most popular materials used for thermowell construction:
Carbon steels can only be used in low-temperature, low-pressure applications because of their low resistance to corrosive chemicals. The most-common material used to create thermowells is stainless steel. Thermowells made from stainless steel are affordable and have high heat and corrosion resistance. One high-strength steel used for pressurized vessels is chromium/molybdenum steel. Molybdenum increases corrosion resistance when added to chromium. The elements cobalt, nickel, chromium, and tungsten make up the Haynes alloy®. It is most frequently utilized during carburizing heat treatment, and in chlorine or sulfide conditions.
Insertion Length: The distance from the thermowell's connection point to the tip is known as the insertion length. The insertion length should be long enough to allow the entire temperature-sensitive section of the measuring device to extend into the measured medium in order to attain the highest level of accuracy. A minimum of one inch should be added to the length of the temperature-sensitive part when using a temperature sensor to measure the temperature of liquids. When the length of the temperature-sensitive segment will be submerged in gas or air, an additional three inches should be added. A thermocouple or thermistor can use a thermowell with a lower insertion length since its temperature-sensitive region is shorter. For acceptable accuracy, the temperature-sensitive portion of bimetal thermometers, RTDs, and liquid-in-glass thermometers must be submerged at least 2½ " in liquids.
The petrochemical, food processing, refinery, cosmetics, chemicals, power generation, pharmaceutical industries are only a few of the industrial fields where thermowells are used. Thermowells protect their equipment from external forces and disturbances, including pressure, abrasion, vibration, and corrosion, that the medium being processed may bring. In addition, thermowell-protected sensors can be taken out and replaced without damaging the manufacturing environment in any other way.
Thermowells provide an invaluable service by enhancing the process of measuring temperature in several ways. They provide protection from corrosion, high material velocities, and extreme pressure damage. Thermowells allow for the replacement of temperature sensors without draining the system. In short, thermowells exist in order to save money in industries where applied. Below, is a summary of some factors to consider regarding a thermowell’s cost savings.
Longevity: Thermocouples and RTDs can be produced in various ways. However, they are frequently housed in 14" (diameter) stainless steel probes. If the process is comparatively inert, this wrapping alone will provide the sensor with great protection. Nothing further is required. Some environments where sensor probes are employed, on the other hand, are blatantly hostile. The stainless steel structure will get stressed at over 1000°C (1,832°F) and a sensor probe will become warped. Even the process medium itself can be taxing for a probe. These corroding substances might range from solid particles in gas to corrosive liquids like acid or seawater. High pressures in a process operate as a force multiplier, amplifying the negative effects of high temperatures and corrosive conditions. The temperature sensor will live significantly longer with thermowells than without them since they bear the brunt of these deteriorating impacts.
Sensor Replacement Cost Analysis: An RTD, thermocouple, or thermistor may be inexpensive or expensive depending on the cost of the temperature sensor housed and the potential rate of degradation the sensor may experience due to harmful conditions. The protection provided by a thermowell would obviously be more beneficial if the sensors are pricey or their procedure of use is demanding. Thermowells will incur a cost, but eventually, this expense will be more than offset. The savings is from not having to keep replacing the temperature sensors. A cost-benefit analysis can assist in determining the point at which a thermowell transitions from being a luxury to an absolute requirement, just like when faced with any other manufacturing decision.
Labor Costs: A thermowell might still be a practical addition even if the temperature sensor costs little. It will cost labor to replace temperature sensors continually. Remember that the sensors will ultimately need to be replaced. Sensor replacement is easy when thermocouples and RTDs are inserted into a thermowell. Additionally, because the thermowell seals the process, no outside contamination may enter when a temperature sensor changes and a worker doesn't need extra protection or equipment.
No Shut-Downs:The operational advantage of having the temperature-monitoring process entirely independent from the temperature sensor is one of the thermowells' benefits that is hard to ignore. A temperature sensor will eventually need to be taken out and replaced. The thermowell will contain the process if this occurs. Therefore, other than a process temperature not being detected during the swap, removing the sensor has no impact on an operation. While a temperature sensor is being replaced, there is a chance that the process will be completely or partially disrupted without a thermowell. When determining when to utilize a thermowell, one should consider the cost or intrusiveness of such a break. Thermowells can be coated with FEP (fluorinated ethylene propylene).
Benefits of FEP (Fluorinated Ethylene Propylene) Coated Thermowells: A thermowell can be made from a variety of materials. Still, to ensure the best protection, the following must be taken into account to aid in the decision-making process, such as:
As listed below, FEP (fluorinated ethylene propylene) possesses several advantageous qualities and is considered the perfect material for encasing a thermowell.
Being practical in all contexts, FDA-approved FEP-coated thermowells are offered in any length with diameters ranging from 1mm to 50mm. Spray coatings, available in PVDF (polyvinylidene fluoride) and PTFE (polytetrafluoroethylene), are useful for applications requiring a quicker temperature response and more complicated forms. PTFE or PVDF works well as a diffuser for instruments exposed to the weather: Such a coating has a small thermal expansion coefficient (0.000122mm per degree Celsius). As a result, it absorbs less moisture (less than 0.01%). In addition, a coated thermowell is resistant to ultraviolet radiation damage. The sensor element incorporated within the thermowell then benefits from these protections as well.
Thermowells require precise engineering to work and that is one of thermowells' most notable and costliest drawbacks. Thermowells are employed in processes that use aggressive media and this material is potentially harmful to both person and product if there is a leak or if anything escapes a pipe. Thermowells are designed following acknowledged standards that correspond to established process conditions. However, such processes are frequently subject to unanticipated alterations. When working with gas or oil, for example, assumptions are made about what, exactly, is coming from the source material. However, these assumptions might alter over time owing to various external circumstances. For example, organizations might need to be made aware that sand is being accidentally taken into a piping system, which would begin degrading the thermowell because it's happening beneath the surface of the pipe. Also, the sensor being housed by a thermowell can’t always determine or measure “what” material is flowing through. This might lead to catastrophic collapse. Even with careful planning and engineering, these process changes can only sometimes be accommodated by thermowells. The other disadvantage of thermowells is that certain types of thermowells only pair with certain sensors.
All temperature-monitoring equipment is prone to degradation from flow, heat, and pressure exposure. A severe processing environment will impact a sensor's structural integrity and performance over time. For instance, the metals used to construct thermocouple probes are susceptible to the corrosive conditions where they are typically employed. Under such harmful conditions sensors often begin to deteriorate quickly and, also, may report false readings while they begin to fail. Thermowells protect a sensor’s lifespan and, by doing so, lower overall production costs for the industries they are employed by eliminating the need for regular sensor replacements and the associated material (sensor replacement), labor, and operational costs.
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