Joined: June 20, 2006
Posts: 180
Location: Singapore
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Posted:
Aug 04, 2006 - 10:41 PM
Post subject: Thermowell - Temperature probe/elements
Thermowell Design Factors
Material of Construction
Thermowell material must be chemically compatible with the process system and the temperature sensor. In most cases, thermowell selection is based on the corrosive conditions in the well environment. Sometimes The selection may be based solely on the mechanical strength needed to withstand operating pressure and process flow. Often a combination of factors must be considered. In addition to selecting the proper base material, coatings may be used to improve a thermowell's resistance to abrasion or the chemical process.
The thermowell wall must be thin enough to minimize sensor error caused by thermal conduction and slow sensor response, but thick enough to withstand collapse from process pressure, erosion from abrasive media and bending from the process flow.
Spring-load mounting styles are recommended to ensure positive contact to maximize thermal transfer and minimize sensor vibration within a thermowell.
Insertion Length
The insertion length or 'U' length is the distance from the end of the well to the underside of the thermowell thread or other connection device. For maximum accuracy, this length must be long enough to permit the temperature sensor to be fully immersed in the media to be measured and minimize sensor error caused by thermal conduction, But short enough to withstand damage caused by process flow vibration. As a general rule of thumb, the thermowell should extend into the process a minimum of 10 times the sensor diameter or, in the case of RTDs, 10 times the sensor diameter plus one inch. This should extend the sensor into the process between 1/3 and 1/2 the diameter of the process pipe. The insertion length must also take into consideration any dead length required to pass through walls, pipe fittings and insulation.
Velocity
The most common cause of well failure is the vibration effect caused by fluid forming a turbulent wake as it flows past the well. This turbulence has a definite vibration frequency based on the diameter of the well and the velocity of the fluid. The well must have sufficient stiffness to ensure that the wake frequency will never equal the natural frequency of the well. If the natural frequency of the well coincides with the wake frequency, the well will potentially vibrate to destruction. To be in compliance with the ASME Performance Test Code, the thermowell should have a natural frequency a minimum of 125% of the wake frequency. Tapered shank wells (heavy duty - Type H) have a high strength-to-weight ratio with a resultant higher natural resonant frequency than the equivalent length straight shank well. Tapered shank wells are preferred for operation at higher fluid velocities.
Process Connection
Most of vendors provide standardized wells in most of the common connection types, including threaded, flanged and socket weld types with standard bore sizes. Threaded wells are available in materials that can be readily welded. Flanged wells are manufactured by welding a bar stock well to the specified flange style. Doubled-welded construction reduces crevice corrosion and stress problems by ensuring that no open joints are exposed inside or outside the installation.
Bore Size
Selection of a standard bore size throughout the plant permits the use of several types of temperature measuring instruments in the same wells. Standard bore sizes fit most commonly used temperature sensing devices. Most applications use 0.260" or 0.385" diameter bores. This number represents the inside diameter of the well, expressed in thousandths of an inch.
fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
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Posted:
Aug 04, 2006 - 10:48 PM
Temperature Probe Design Consideration
The term "temperature probe" is used to describe a temperature sensor assembly that enables a temperature sensing element to be located at a particular position and to be electrically connected to an instrumentation system. A typical temperature probe consists of the following constituent parts: the sensor element ; the probe housing and components for mechanical and environmental protection - including potting compounds and special coatings; and the electrical cabling and connectors. Probe design always starts with a consideration of the final application which may bring many factors to bear.
In many cases, these factors can be conflicting and so good probe design often requires that reasonable compromises in performance requirements are reached.
The most relevant considerations in temperature probe design for an application are outlined below.
First is the temperature range including minimum and maximum temperatures that the probe may experience.
Second comes the temperature cycling that the probe will experience.
In some applications (such as industrial refrigeration) the temperature will cycle in a freeze-thaw environment.
In this example, a robust probe design will be required Measurement temperature range and critical temperature points.
Although the probe may be exposed to a wide temperature range (eg +50 to +100C) the critical temperature measurement range may be across a narrower range (eg +70 to +80C) or at a single temperature point (eg +75C).
Third comes the accuracy required in the application.
This can be expressed in terms of temperature accuracy, for example +/- 0.1C, or in terms of resistance accuracy, for example +/-5%.
Accuracy can also be specified at a single temperature point or over a temperature range.
Next comes the distance from location where temperature is being sensed to control instrumentation.
In many electronic temperature-sensing applications it is necessary to measure temperature at a location that is remote from a control unit or data logging equipment.
Another factor is the time response required for the temperature probe (ie how quickly should the sensor be able to respond to a change in temperature?).
Keep in mind that the definition of thermal time constant is the amount of time required for a sensor to change by 63.2% when subjected to a step function change in temperature.
For example, if a sensor is at 0C and is then plunged in a 100C.
bath, one time constant is the amount of time it takes the sensor to reach 63.2C.
The thermal mass of temperature probe relative to the system that is being measured is also important, as is the thermal contact or thermal coupling between the temperature probe and the system that is being measured.
Environmental conditions that the temperature probe and connecting wires will be exposed to or environmental ratings that it must conform to must be taken into account.
This can involve a variety of environments including immersion (partial/full), liquids, air/gas etc.
Also to be considered are the mechanical requirements for the temperature probe in terms of shock or vibration performance, and any electrical requirements in terms of noise performance, insulation or shielding.
Other factors include: the lifetime of product design (fir how long will the probe need to last?); compatibility with existing or future instrumentation systems; product qualification requirements (eg CE marking, ESA etc); price - the budgeted cost of the probe is often an issue which will affect final design; and the choice of materials.
Although every probe design does not involve such a complex consideration of issues, there are always multiple variables which determine the final construction and materials used.
fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
Status: Offline
Posted:
Aug 04, 2006 - 11:07 PM
This is a excellent software to see the material compatibility with different 14000 types of fluids.
It shows which material of thermowell is suitable for which area of fluid/medium.
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