Sensors thermowells

Burner flame is impinging directly on temperature sensor. Thermowell is damaged due to flame. Check burner installation, tertiary air distributor, and atomisation of fuel oil which can cause this problem. Provide additional view glasses on furnace shell to observe the flame. Clean strainer in fuel oil pipe. 

The dynamic response of most sensors is usually much faster than the dynamics of the process itself. Temperature sensors are a notable and sometimes troublesome exception. The time constant of a thermocouple and a heavy thermowell can be 30 seconds or more. If the thermowell is coated with polymer or other goo, the response time can be several minutes. This can significantly degrade control performance. 

As noted in the previous section, the dynamics of the thermowell-thermocouple sensor are often not negligible and should be included in the dynamic analysis. 

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. 

Thermowells typically are cylindrical metal tubes that are capped on one end and protrude into a process line or vessel to bring the TC or RTD into thermal contact with the process fluid. Thermowells provide a rugged, corrosion-resistant barrier between the process fluid and the sensor that allows for removal of the sensor while the process is stiU in operation. Thermowells that are coated with polymer or another adhering material can significantly increase the lag associated with the temperature measurement, i.e., sigiuficantly increase the response time of the sensor. 

TCs typically have a repeatability of 1°C, whereas RTDs have a repeatability of 0.1°C. Accuracy is a much more complex issue. Errors in the temperature reading can result from heat loss along the length of the thermowell, electronic error, sensor error, error from nonlinearity, calibration errors, and other sources. ... 

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. 

Determining the time constant of the sensor is usually more difficult than estimating the repeatability. To determine the time constant of the sensor system, one needs to know the actual process measurement. Consider a temperature measurement. A measurement of the actual process temperature is required to estimate the time constant of a sensor. Instead, the thermal resistance, which causes the excessive thermal lag of the temperature sensor, can be evaluated. The location of the thermowell should be checked to ensure that it extends far enough into the line that the fluid velocity past the thermowell is sufficient the possibility of buildup of insulating material on the outside of the thermowell should be assessed, and the thermal contact between the end of the temperature probe and the thermowell walls should be evaluated. In this manner, an indirect estimate of the responsiveness of the temperature sensor can be developed. The velocity of a sample in the line, which delivers a sample from a process line to a GC, can indicate the transport delay associated with the sample system. A low velocity in the sample line from the process stream to a GC can result in excessive transport delay, which can greatly reduce controller effectiveness. 

Temperature control loops can be applied to control the temperature of a stream exiting a heat exchanger, of a tray in a distillation column, or of a CSTR. Figure 15.32 shows a schematic of a temperature controller applied to control the temperature of a process stream leaving a gas-fired heater. The sensor is an RTD element placed in a thermowell located in the line leaving the heater. 

A common vessel used in the batch production of fine chemicals and pharmaceutical is the jacketed glass-lined reactor-still as shown in Figure 9.18. It is available in 5 to 4,000 gallon capacities and is usually fitted with a retreat curve agitator and single baffle or thermowell/baffle assembly cantilevered from the vessel lid, a distillation column, and a condenser. The same temperature sensor... 

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. 

Low-pressure steam is the source of heat for the brine entering the reactor. The temperature transmitter is an RTD-type sensor in a tantalum-shea.thed flanged thermowell. The drawing shows a steam exchanger on a pumped feed line. The brine flow control valve is globe-style and fails closed. Section 7.5.9.4 described the use of a steam eductor as the feed mechanism. This eliminates the need for the heat exchanger. The eductor itsdf can be titanium or PTFE-lined. 

The process temperature is measured in the uncondensed gas as it leaves the chlorine knockout pot or separator. This procedure is more accurate than measurement of the temperature of the liquefied chlorine, which may be subcooled. The tenqierature sensor in Fig. 11.29 is an RTD in a flanged Monel thermowell. The reverse-acting proportional-plus-integral controller adjusts the set point of the refrigeration chiller pressure controller. 

Signal transmitter Signal converter Final control element Sensors Thermocouple Flow, pressure, level Analysers 1-5 s (pneumatic) Instantaneous (electrical) 0.5-1. Os (electronic to pneumatic) l s (0-100% valve open) Almost instantaneous (bare) 5-20 s (in thermowell) Several seconds 5-30 minutes or longer (usually discrete)... 

In this section, the dynamic measurement error associated with a temperature sensor is analyzed. Figure 9.16 shows a thermocouple placed in a metal thermowell with mass m and specific heat C. The dynamic lag introduced by the thermowell/thermocouple combination can be easily estimated if several simplifying assumptions are made. In particular, assume that the well and thermocouple are always at the same temperature which can be different from the surrounding fluid temperature r. Further assume that heat is transferred only between the fluid and the well (there are no ambient... 

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 ... 

Place a few drops of silicone oil in the bottom of the thermowell of the flask and insert the temperature sensor to the bottom. The sensor can be secured with a wad of glass wool at the top of the thermowell. 

A 1.5.1 For sensors that are mounted loosely in a thermowell, place enough silicone oil or other inert liquid in the bottom of the well so as to make good physical contact between the sensor and the tip of the well. Those sensors that are fused into good contact with the tip of the well may be... 

A2.3.4 Remove the sensor and insert it into the thermowell in the beaker of water. After the sensor has reached a temperature of 80 C, remove it and immediately insert it into the hole in the box. 

A9.6.5 Insert the temperature sensor into the thermowell of the flask. 

A6.6.1.1 Ensure that approximately 0.5 mL of silicone oil or other inert liquid is in the bottom of the thermowell and insert one or more thermocouples or other sensors connected to their respective measuring instruments. 

A 1.3.4 Remove the sensor and insert it into the heated thermowell in the beaker of water. After the sensor has reached a temperature of 70 C (158 F), remove it and immediately insert it into the hole in the box. Note with a stopwatch, or record on the strip chart, the time interval while the sensor cools from 30 C (54 F) above to 5 C (9 F) above the temperature recorded in A 1.3.3. 

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). 

A2.S.2.3 Insert the test sensor and apply heat at a rate that will maintain active steady boiling so that the tip of the thermowell or sensor is covered with bubbling liquid. 

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