Thermowell example

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. 

Time constants. Where there is a capacity and a throughput, the measurement device will exhibit a time constant. For example, any temperature measurement device has a thermal capacity (mass times heat capacity) and a heat flow term (heat transfer coefficient and area). Both the temperature measurement device and its associated thermowell will exhibit behavior typical of time constants. 

An additional complication is that most dynamic data are stated for configurations involving reference materials such as water, air, and so on. The nature of the process material will affect the dynamic characteristics. For example, a thermowell will exhibit different characteristics when immersed in a viscous organic emulsion than when immersed in water. It is often difficult to extrapolate the available data to process conditions of interest. 

The temperature of a continuous flow of material through a steam-heated stirred tank is controlled by regulating the flow of steam. The tank temperature is measured by a thermocouple set inside a thermowell, giving a delayed temperature measurement response. This example is based on that of Robinson (1975). 

While redundancy—backing up one system with another just like it—is a logical way to protect against instrument failure, diversity may be a better choice in many cases. For example, dual thermocouples in the same well could corrode out at the same time. It might be better to have two different types of measurement, one a thermocouple in a thermowell and the other possibly a resistance thermal detector. 

When two different process lines converge into a pipe tee and then flow into one line, it is necessary to get a good representative temperature. This pipe line must extend not less than 10 pipe diameters to obtain a good mixing before the flow reaches the thermowell. An example of this is shown in Figure 7-85. 

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. 

Finally the shape of the temperature trend is very different from that of the valve position. This is caused by the inertia of the system. The heater coil will comprise a large mass of steel. Burning more fuel will cause the temperature in the firebox to rise quickly and hence raise the temperature of the external surface of the steel. But it will take longer for this to have an impact on the internal surface of the steel in contact with the fluid. Similarly the coil will contain a large quantity of fluid and it will take time for the bulk temperature to increase. The field instrumentation can add to the lag. For example the temperature is likely to be a thermocouple located in a steel thermowell. The thermowell may have thick walls which cause a lag in the detection of an increase in temperature. Lag is quite different from deadtime. Lag does not delay the start of the change in PV. Without deadtime the PV will begin changing immediately but, because of lag, takes time to reach a new steady state. We normally use the symbol r to represent lag. 

Consider next the effect of varying thermowell internal diameter while holding the external diameter constant. Again, in this example velocity was maintained at 10 ft/sec and annular fill was air. 

Process connection faults Impulse line leakage or blockage or condensation builds up. Wrong location of pressure sensors - incorrect pressure conditions. Wrong location of temperature sensors - incorrect representation of actual temperature. Thermocouple incorrectly seated in thermowell. False readings caused by changed process conditions examples ... 

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