Temperature
The following types of instruments are used for temperature measurement.
- Thermocouple
- Resistance temperature detector
- Non contact type
- Temperature gages
- Temperature calibrators
Thermocouple
Thermocouples are the most common type of temperature measuring instrument used in the refinery. Its principle is as follows:
Any two wires of different materials can be used as a thermocouple if connected together as in Figure 1. The AB connection is called the "junction". When the junction temperature, TJct, is different from the reference temperature, TRef, a low-level DC voltage, E, will be available at the +/- terminals. The value of E depends on the materials A and B, on the reference temperature, and on the junction temperature.
Different types of thermocouples are used. A comprehensive list of the different types of thermocouples is shown below. Since there can be a large distance between the sensor and the connected electronics (for cold junction compensation and further processing for displaying in engineering units), instead of the thermocouple cable, compensating cables are used.
Thermo well are used along with thermocouple as thermocouples are seldom placed in process stream line. The use of thermo wells make replacement of thermocouple easier without affecting the process. Generally flanged SS thermo wells are used but there are threaded ones also. Sheathed thermocouples are available as grounded and ungrounded. Grounded thermocouple provide faster response but they are not electrically isolated form the sheath. Since all the temperature loops are connected to DCS through isolating barriers, grounded thermocouple are mainly used in the refinery. The thermocouple is physically insulated from the sheath by MgO powder (soft).
In case of process where the velocity of the fluid is very high that may erode the well and so satellite coating may be done as in FCCU reactor- regenerator and orifice chamber.
In certain tank gauging systems MTT s are used to get the average temperature for the products. The use thermocouples for their function.
Currently temperature transmitters are being used rather then, connecting the thermocouple to the electronics at the control room through compensating cable. This eliminates the requirement of compensating cable. Since the transmitter output is standard 4- 20mA, no special cards in the control room is required and it can be treated just like any other transmitter signal. Moreover, these temperature transmitters can be used for any type of temperature sensors like thermocouple & RTD.
Resistance temperature detector:
A basic physical property of a metal is that its electrical resistivity changes with temperature. All RTD's are based on this principle. The heart of the RTD is the resistance element.
Some metals have a very predictable change of resistance for a given change of temperature; these are the metals that are most commonly chosen for fabricating an RTD. A precision resistor is made from one of these metals to a nominal ohmic value at a specified temperature. By measuring its resistance at some unknown temperature and comparing this value to the resistor's nominal value, the change in resistance is determined. Because the temperature vs. resistance characteristics are also known, the change in temperature from the point initially specified can be calculated. We now have a practical temperature sensor, which in its bare form (the resistor) is commonly referred to as a resistance element.
Through years of experience, the characteristics of various metals and their alloys have been learned, and their temperature vs. resistance relationships is available in look-up tables. For some types of RTD's, there are also equations that give you the temperature from a given resistance. This information has made it possible for instrument manufacturers to provide standard readout and control devices that are compatible with some of the more widely accepted types of RTD's.
Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it.
Common Resistance Materials for RTDs:
· Platinum (most popular and accurate)
· Nickel
· Copper
· Balco (rare)
· Tungsten (rare)
The RTD is a more linear device than the thermocouple, but it still requires curve fitting. The Callendar-Van Dusen equation has been used for years to approximate the RTD curve: 9
Where:
RT = Resistance at Temperature T
R0 = Resistance at T = O0C
a = Temperature Coefficient at T = O0C
(Typically +0.00392W/W/0C)
d = 1.49(typically value for .00392 platinum)
b = 0 T>0
0.11(typical) T<0
The exact values of coefficients a, b, and d are determined by testing the RTD at four temperatures and solving the resultant equations. This familiar equation was replaced in 1968 by a 20th order polynomial in order to provide a more accurate curve fit.
The plot of this equation shows the RTD to be a more linear device than the thermocouple.
An RTD is inherently a 2-wire device, but lead wire resistance can drastically reduce the accuracy of the measurement by adding additional, uncompensated resistance into the system. Most applications therefore add a third wire to help the circuit compensate for lead wire resistance, and thus provide a truer indication of the measured temperature.
Four-wire RTD's provide slightly better compensation, but are generally found only in laboratory equipment and other areas where high accuracy is required. When used in conjunction with a 3-wire instrument, a 4-wire RTD will not provide any better accuracy. If the fourth wire is not connected, the device is only as good as the 3-wire RTD, if the fourth wire is connected, new errors will be introduced. Connecting a 3-wire RTD to a 4-wire instrument can cause serious errors or simply not work at all, depending on the instrument circuitry. A 2-wire RTD can be used with either a 3 or a 4 -wire instrument by jumping the appropriate terminals, although this defeats the purpose and reintroduces the uncompensated resistance of the leads.
When wires A &B are perfectly matched in length, their impedance effect will cancel because each is an opposite length of the bridge. The third wire C acts as a sense lead and carries no current.
Generally in the refinery RTD’s are used in temperature probes for the conditioning monitoring of compressors. They are also used in certain process applications also where the temperature encountered is comparatively low as in the case of truck loading and PIBU unit. In certain tank gauging systems MRT s are used to get the average temperature for the products. The use RTDs for their function
Non-contact type:
The Pulsar 11 model 7000SR from E2 Technology Corporation is used in SRU unit for reactor temperature measurement. The basic principle is described below. Similar type of principle is used for DHDS flare monitoring also.
All objects above absolute zero emit infrared energy. The amount of energy emitted is proportional to the body temperature. The PULSAR II collects this energy by means of a focusing optical system concentrating the energy from a body onto a sensitive infrared detector. Specialized amplification circuitry converts the signal generated by the detector into linear output signals corresponding to 1mV/O and 4 -20mA.
The efficiency of energy emission from different objects varies significantly. A perfect energy emitter is known as a blackbody radiator and is assigned an emissivity value of one. Any object that emits with less than perfect efficiency is assigned an emissivity value between zero and one, and a perfect reflector is assigned an emissivity value of zero.
The PULSAR II calibrated against nearly perfect blackbody radiators in the laboratory. However, the emissivity of objects and processes which you are measuring with typically fall somewhere between zero and one. This results in the need for an adjustment of the PULSAR II’s emissivity setting to obtain a match against a known reference temperature. Once the emissivity setting has been adjusted for a particular installation, the PULSAR II will accurately track temperatures as they rise and fall.
Temperature gauges:
Temperature gauges are of different types namely Mercury in steel, Gas filled and Bimetallic type. All these type of gages are available in different ranges, dial size, stem length and connections
Mercury-In-Steel Thermometers deploy Mercury as an expansion liquid filled into a closed system comprising of a bulb (of Steel or Chrome – Moly Steel), a microbore capillary (of Steel or Stainless Steel) and a spiral or ‘C’ shaped Bourdon Tube (of MS or SS). This system when heated at bulb end the mercury in the bulb expands and a pressure is generated within which moves the spiral / ‘C’ Shaped bourden as it is the only the elastic element. This movement is transmitted to a rack and pinion movement, which drives a pointer thus showing temperature on a calibrated dial. Rigid stem (horizontal, vertical and any angle) and capillary type are available. They have an inherent time delay in transmission.
Gas Filled Thermometers deploy Nitrogen Gas at high pressure as an expansion gas filled into a closed system comprising of a bulb (of Steel or Chrome – Moly Steel), a microbore capillary (of Steel or Stainless Steel) and a spiral or ‘C’ shaped Bourdon Tube (of MS or SS). This system when heated at bulb end the Gas in the bulb expands and a pressure is generated within which moves the spiral / ‘C’ Shaped bourden as it is the only the elastic element. This movement is transmitted to a rack and pinion movement, which drives a pointer thus showing temperature on a calibrated dial.
Bimetal thermometers are the most common type used in refinery. Strips of two different metals, one with high coefficient of expansion and the other with lower coefficient of expansion are welded together by a special process to form bimetallic strip. Such strips are coiled to form a helix, which is fastened at one end, and the other end is free. Such coils when heated tend to decoil resulting into a rotational movement of the free end. This free end is attached to a shaft connected a pointer which rotates pointer on a calibrated dial indicating the temperature. They are not available in capillary tube. The most commonly used material is an invar-nickel-crome alloy.
Temperature Calibrators:
For actual temperature simulation and checking, temperature bath can be used. They can be set at the desired temperature, and the reading of the instrument to be calibrated can be compared to the bath reading. One of the temperature bath available is of the dry type, which can be used directly without pouring any oil into the well.
For field checking of thermocouples, portable temperature calibrators can be used which when connected to the thermocouple senses the mill volt generated and display in terms of the temperature. The type of the thermocouple has to be selected in the calibrator. RTD calibrators are also there which can be used to measure the temperature form an RTD source wherein they provide the bridge circuit and other necessary electronics for RTD measurement. Many of these calibrators have the facility to simulate Thermocouple and RTD sensors so that the complete field wire and connected electronics (DCS or other devices) excluding the sensor can be checked.
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