Thermowell Design

Thermowell Design Flow Sheets and Temperature Points... 

Masek, J.A. Thermowell Design for Process Piping. Hydrocarbon Processing Petroleum Refiner, Part 1 February 1964, p. 119 Part 2 March 1964, p. 119 April 1964, p. 165. 

Dynamics of Process Measurements Especially where the measurement device is incorporated into a closed loop control configuration, dynamics are important. The dynamic characteristics depend on the nature of the measurement device, and also on the nature of components associated with the measurement device (for example, thermowells and sample conditioning equipment). The term mea-.sui ement system designates the measurement device and its associated components. 

One of the most popular high-temperature sensors is the platinum thermocouples, which are usually installed inside protective thermowells or protection tubes. When installed horizontally, wells tend to droop, causing binding of the TC element, making replacement difficult. The latest designs incorporate a sheath with a flexible cable that can easily be inserted into even badly drooping wells. Ceramic wells do not suffer from droop but have other limitations such as low surface strength, brittleness, and low erosion resistance. 

Equipment. A vertical fixed-bed reactor, made of a 0.168 m I.D. and 0.5 m long 316 stainless steel tube with an axial thermowell, was used. The amounts of catalyst used for the steady state and dynamic experiments were 6.35 and 18.69 g, respectively. The reactor tube was heated by a fluidized bed sand bath. The reaction gases, O2 and CO, and the diluent, He, were metered through rotameters qnd mixed prior to their entry to the reactor. The mixing junction was designed such that either of the reaction gases or CO2 could be introduced or removed from the stream to simulate a step increase or decrease of the component in question. The effluent from the reactor was analyzed by gas chromatography in 4 minutes. 

Typically, the reactor was loaded with a preheat section in the bottom for an upflow configuration. The preheat section, which contained 1.5 L of Harshaw Tab Alumina rested on stainless steel wool. The 1-L catalyst bed was placed on top of the alumina preheat section. The small volume at the top of the reactor was filled with additional alumina topped with stainless steel wool to retain the catalyst in the reactor. The thermocouples were inserted at the designated locations in an axial thermowell which runs through the center of the catalyst bed. 

The dynamic response time of a TC or RTD sensor within a thermowell can vary over a wide range and is a function of the type of process fluid (i.e., gas or liquid), the fluid velocity past the thermowell, the separation between the sensor and inside wall of the thermowell, and material filling the thermowell (e.g., air or oil). Typical well-designed applications result in time constants of 6 to 20 s for measuring the temperature of most liquids. 

A well-designed system, including thermocouple, thermowell, and often protective sheath, generally provides more reliable measurements. 

The temperature points, with their index numbers, will be found on the process control diagrsim. These require thermowell installations in the piping. The temperature index numbers assign to each point are designated by conventional symbols such as TI, (Temperature Indicator), TH, TIC, TC, TRC, TR, TIA, TRCA, TT and TW followed by the number assigned to each of the points. 

When a 3/4-inch thermowell is to be used, the pipe designer is cautioned not to use the conventional 3/4-inch full pipe coupling for mounting it. If the 3/4-inch full coupling is welded on equipment, the narrowness of the inside of the coupling where the two pipe threads meet may prevent thermowell insertion. If, instead, a 3/4-inch, 6,000-pound thread adapter or elbow... 

The active other half of the T/C well is drilled to hold either a 1/8-inch or 1/4-inch OD pencil-type T/C assembly. The hot junction extends into the center of the process pipe flow area. This T/C assembly is shown in Figure 7-90 and is connected to the T/C head. The entire well and assembly must be designed special for each flange and ordered with the drawing from the thermowell manufacturer. 

The other advantage of this method is that it will also reduce the number of types of thermowells that have to be ordered. The types that must be carried in stock for future maintenance work are kept to a minimum. When the piping designer knows in advance the thermowell insertion lengths preferred, he will be in a better position to provide for their installation in the process piping. Time will be saved and he will be able to complete his drawing sooner. 

In Figures 7-95 to 7-108, the many piping details shown will suggest to the piping designer the various schemes that can be used. The importance of these details is that each has been provided with a scale in inches shown on the centerline run of the thermowell. This scale will enable the instrumentation engineer to select a thermowell. With it he can pinpoint the active effective flow area, preferably in the turbulence zone, where the thermal element of the instrument will be located. 

Or, can the level indicator tubes, view glasses, thermowells, gaskets, flanges, etc., with similar sizes/ratings/designs be used for different equipment such as reactors, receivers, mixers, and dissolvers in the plant ... 

Tanks should have several fixed elements of design, such as manways for cleaning and inspection, a level gauge, to indicate inventory, thermowell in heated tanks to accommodate thermometer, vents, nitrogen blanketing (recommended in most cases, prerequisite in fabrication of moisture sensitive products), pressure/vacuum relief, and filling line. 

If the foregoing is not feasible, choose a minimum-sized thermowell-detector design in which annual clearance is 0.005 inch or less. 

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