Thermowell installation

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. 

Figure 7-87. Three types of 1-inch connections for thermowells installed on welding pipe elbows (a) regular pipe coupling that should not be used, (b) improvised connection made with a boss, (c) 6,000 lb. forged steel elbow adapter provides best thermowell connection at lowest installed cost.

The sleeve forces the bulb to one side of the well insuring a metal-to-metal contact throughout the length of the sensitive thermobulb. Sometimes, two sleeves are used on both sides of the bulb. The corrugations in the sleeve itself provide a metallic path between the well and the bulb on the opposite side. The advantage of using a sleeve is that it can be applied to thermowells installed in a vertical, horizontal, or in an upside-down position. 

Fig. 7-106—1" Thermowell installed in threaded pipe fittings and pipe flange fittings...

Figure 7-105. Thermowells installed at outlet of 1-1/2 to 4-inch welding pipe tees with either 1-inch thread adapter or 1-inch half coupling, 6,000 lb. rating.

Thermowell installation in liquor spaces in two-phase lines... 

In order to make maintenance activities easier, thermowells are welded in the pipe (or tank) where the temperature is to be measured. Then the thermometer is installed in the thermowell, using some thermal material to make good contact between the thermowell and the thermometer. It is important to notice that the use of the thermowell increases the response time of the thermometer to changes in the temperature of the process being monitored. If this information is important for safety, one should consider the possibility to avoid the use of the thermowell, installing the thermometer directly in contact with the process. 

Decide and show those ancillary instruments needed for the monitoring of the plant operation by the operators and for trouble-shooting and plant development. It is well worthwhile including additional connections for instruments which may be needed for future trouble-shooting and development, even if the instruments are not installed permanently. This would include extra thermowells, pressure tappings, orifice flanges, and extra sample points. 

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. 

To eliminate the errors caused by lead wire resistance changes, it is recommended to use transmitters that accept 4-wire RTD inputs as a standard feature. In order to eliminate errors by self-heating, the measuring current should be minimized. For platinum elements, it should not exceed 250 pA. The best installation practice is to place all electronics directly on top of the thermowell and thereby eliminate lead wire and noise effects. If this cannot be done, the lead wires should be twisted and shielded the wires should also not be stressed, strained, or made to go through steep gradients. The... 

Layout The piping must be run in the space provided, without interfering with structural steel platforms and other piping. Sufficient space must be provided for the valves, flanges, instrument connections and for the installation of thermowells. 

Thermowells can be installed with ease in process pipe lines 3 inches and larger. Every effort should be made to install the thermowell connection in a welding pipe elbow. The connection should by made by welding a 1-inch, type 6,000 pound or other size forged steel elbow adapter in the heel of the pipe elbow (see Figure 7-8 5f). If... 

Figure 7-85. For process piping 3-inch and larger, typical thermowell locations. Installation details thermobulb (g), thermocouple (f) (h) (i) and dial thermometers (j) (k) (I).

In Figure 7-86, four typical piping arrangements are shown for installing thermowells. When the enlarged sections must be provided with a 2-1/2-inch spool piece, the minimum length should be at least 12 inches. Note that the Figure 7-86 details are accessible so the thermowells can be taken out... 

In Figure 7-86b, the 2-1/2-inch section is installed with two 1-inch elbow adapters in the horizontal with one connection for the indicating thermometer and the other for a duplex thermowell containing a thermocouple and a 3/8-inch diameter thermobulb for a temperature transmitter. If vibration is likely to be transmitted in the piping, the transmitter should not be supported from the process piping but instead mounted on a building column, wall or from a floor pedestal. 

In Figure 7-86c, two 2-1/2-inch pipe elbows are installed together with a pipe spool piece at the lower end to increase its len h for the installation of a thermowell with a thermobulb. The other elbow adapter holds a dial thermometer in a 6-inch long thermowell. In Figure 7-86d, a single 2-1/2-inch pipe elbow is installed with suitable pipe reducers to fit the small size process line. The 1-inch elbow adapter holds a 6-inch-long well with a thermocouple (T/C). The T/C head is connected with an electrical conduit that carries the circuit to the temperature recorder on the control panel. 

The best and cheapest installation for a 1-inch thermowell is to weld a 1-inch 6,000 pound F.S.S.E. elbow adapter in the heel of a welding pipe elbow as shown in Figure 7-87c. The elbow adapter is shaped for welding and gives a streamline flow service and appearance. It will prevent unnecessary flow turbulence in comparison to a boss installation, mentioned above. During some... 

When thermowells are installed, in a vertical position in piping and equipment, various substances are used to increase the heat transmission and temperature response — one is mercury (see Figure 7-89c). Although mercury is probably the best substance from a thermal consideration, it must not be used when the process operating temperature approaches its boiling point (375°C or 674°F). Both the thermowell and bulb should be made of steel or ferrous alloy. A brass thermowell or bulb immersed in mercury would be destroyed by amalgamation. 

When temperature control instruments are provided with very long thermobulbs they must be installed in suitable thermowells. An example of such is shown in Figure 7-91. Thermowells 15 inches to 24 inches long, if installed and held in place only by the pipe thread connection, could cause the well to vibrate with the flow velocity. Eventually the well could bend and possibly fracture. 

In some process pipe lines the flowing product may carry a mixture of entrained solids. Such an installation is shown in Figure 7-92 which also requires a long thermowell. This process line will... 

The process flow sheet will often show a pressure gage and thermowell connection that must oe installed in the same piping. It is considered good practice to install the pressure gage connection ahead of the thermowell, as shown in Figure 7-94. Because the thermowell will create a Turbulence in the direction of the flow, this could have an effect on the pressure gage connection. 

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. 

When the thermowell is installed perpendicular or at a 45° angle to the pipe wall, the thermowell tip should be at or near the centerline of the pipe. Thermowells and their connections should point counter to the flow in the process line whether installed on pipe elbows or on a 45° angle. The well perpendicular to the pipe will naturally be in the turbulent flow section of the pipe. Such an arrangement will not always be possible because of the piping arrangement and direction of the process flow. 

Figure 7-95. (above) Thermowell connections made with 1-inch, 6.000 lb. elbow adapter installed on 2-1/2 to 12-inch pipe weld elbows. 

T.g. 7-96—1-in., 6000 lb, elbow adapter installed at welding line of 2" to 10" pipe elbow for thermowell... 

Select another piping arrangement so a longer thermowell with more immersion can be installed. 

In Figure 7-99 the 2-1/2-inch welding pipe elbows are used for swaging-up a section of the piping. This applies to 3/4-inch to 2-inch process line thermowells. By installing a pipe spool piece between the pipe elbows and welding 1-inch elbow adapters on both elbows, two temperature instruments can be handled for the same service. 

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