Thermowell insertion

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 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. 

A2.5.1.2 Set up the melting point bath containing a suitable metal chosen from Table A2.1 and ensure that there is about 0.5 mL of suitable silicone oil in the bottom of the thermowell. Insert the temperature sensor(s) into the well making certain that the sensing tips are touching the bottom. Connect the associated instruments). 

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. 

The catalyst diluted with inert quartz was placed in a tubular stainless steel reactor and the temperature in the bed was measured by a movable thermocouple inserted in a thermowell at the center of the reactor... 

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 reactor consisted of a quartz tube (12 in. long, 1/2 in diameter) and was heated by a 1200 watt furnace with a feed back temperature controller. The catalyst was placed in powder form on a porous frit, and its temperature was measured by a chrome1 Alumel thermocouple inserted into a thermowell. The reactor inlet and outlet compositions were measured by an on line gas chromatograph (G.C.) equipped with an 18 ft long, 1/8 in O.D column filled with 80/100 Porapak S and operated at 115°C using He as a carrier gas. An ice bath, placed between the reactor exit and the G.C. sampling valve, condensed the steam and removed the water. 

Apparatus and Procedure. A simplified flow diagram of the apparatus used for the gasification study is shown in Figure 1. The reactor was a vertical 2-9/16-in.-i.d. by 32-in.-long stainless steel vessel it was modified for these experiments by a stainless steel sleeve that was inserted to reduce the internal diameter to 1 in. This modification improved temperature control. The reactor contained 18 in. of alundum granules at the top to serve as a preheater, 12.5 in. of catalyst (150 cc.), and 1.5 in. of alundum granules at the bottom. Catalyst temperatures were determined by five thermocouples, spaced at 2.25-in. intervals in a central thermowell in the catalyst bed. 

In one application, the TRC/TRC cascade control was used with four thermocouples installed, in parallel, in the convection section of the heater which was installed in thermowells. The thermowells were inserted in the heater wall between the tubes. Because the thermocouples receive the same heat as the convection section tubes, an instantaneous detection of furnace temperature was made. 

Thermocouples are the most commonly used temperature-sensing devices. They are typically inserted into a thermowell, which is welded into the wall of a vessel or pipe. The two dissimilar wires produce a millivolt signal that varies with the hot junction temperature. Iron-constantan thermocouples are commonly used over the 0 to 1300°F temperature range. 

All reactions were carried out in 700-mL stainless steel, high pressure reaction vessels. The reaction solution was added, along with a Teflon-coated stirring bar, to a vessel that was flushed and loaded with CO to the desired pressure. The vessel was heated in an insulated oven, which rests on a magnetic stirring motor. Temperature control ( 1°C after the desired reaction temperature was reached) was maintained using a proportional temperature controller with a thermocouple inserted in a thermowell,. which extended below the solution level of the reaction vessel as a sensor. Heating the reaction vessel from room temperature to 160°C typically required from 40 to 45 minutes. 

Mount probes and flow meters in vertical lines to prevent soUds accumulation. Provide insertion lengths at least ten times the diameter for thermowells. 

Local measurements of wet chlorine temperature use tantalum-sheathed flanged thermowells with bimetallic dial thermometers. The insertion length should be selected to place the tip about one third of the diameter into the piping or into an elbow. 

The illustration shows different methods of installation of thermowells (with thermocouples inside) in a refractory brick-lined furnace. This is based on practical experience. It was seen that the thermowell projecting right inside up to the hot process material was getting damaged frequently. Hence, it was inserted only up to the back surface of the irmermost reftactory brick. Though it was not able to indicate the correct temperatirre of the hot process material, it could orrly irrdicate the trend for rise or fall of temperatiue and had more life. 

The ability to run a complete performance test requires experience and preparation. For a first performance test, you should simply obtain a complete unit pressure profile. This is done with a single pressure gauge, which you move downstream from point to point. Obtain pressure drops across heat exchangers and control valves. Then, obtain a complete temperature survey. Where no thermowells exist, use a glass thermometer inserted under the pipe insulation. Next, record all flow rates. You will probably find many flow inconsistencies. Finally, obtain a complete set of samples. Learn how and where the samples are taken (see Chapter 27). 

Temperatures indicated in the control room are normally very reliable. If necessary, they can be checked at the thermowell with a portable temperature indicator. Dial thermometers are not to be trusted. Simply unscrew the dial thermometer and insert a glass thermometer into the thermowell. Note that the thermowell is sealed off from the process fluid. 

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