Temperature Sensor Response Time

This is another area where Thermocouples excel over RTD'S, and it's a simple matter of physics to understand why. Contact temperature sensors do not indicate the temperature of the area around them, they indicate their own temperature along their own sensitive area. In order for any contact temperature sensor to indicate the temperature it is in contact with, the sensor must first come to thermal equilibrium with that environment. Let's not discuss the theoretical aspect that the two never actually attain the same temperature, but just the fact that after some time the two are approximately at thermal equilibrium.

The most basic of Thermocouples is merely a junction of the two dissimilar metal wires. This could be a beaded junction, or a butt-welded junction which turns out to be nearly the same diameter as the Thermocouple wire itself. In order to indicate the surrounding temperature, the junction must be at that temperature. That junction might only be .010" in diameter (for a 30 AWG wire thermocouple), or smaller if finer wires are used. RTD's require either a length of fine Platinum wire wound around or within a former, or a layer of platinum deposited upon a substrate. In all cases, there is an area of Platinum (which is the sensitive portion of the RTD) in contact with this inert, insulating former, and both are physically larger than a weld junction (generally speaking). Both the Platinum and the insulator have thermal mass that must come to equilibrium with the surroundings before the sensor can give an accurate reading. Since there is generally more thermal mass involved here than with the thermocouple junction, the thermocouple will respond faster when put in a similar environment.

The aforementioned statement is true only when reaching for the extremely fast response times of each type and working with bare resistance elements and exposed thermocouple junctions. If both sensors are encapsulated within metal sheaths, and the thermocouple junction is isolated from the sheath (as an RTD circuit always is), then response times will be quite similar.

Temperature Sensor Poll

http://polls.linkedin.com/p/102153/hgnwu

Tube Skin Thermocouple For Refineries

Thermometrics Corporation manufactures temperature sensors, radiant tubeskin thermocouples, petrochemical sensors and gasifer thermocouples for refining facilities around the world. Tube skin refinery thermocouples manufactured with superior metallurgy are used to measure the temperature of process such as Coker, Purge Gas Recovery, Primary Reformer, and Synthesis Converter systems. The size of the sensors are designed for each particular application by an engineer during construction or retrofit of a factory or process. The type of thermocouple sensor used is generally a Type K due to it's optimum temperature range and cost factors. Tube skin thermocouples unique design with expansion loops allows the thermocouple temperature sensor tip to move as the process vessel expands and contracts.

Given the complexities of refining and the variations in processes, Thermometrics Corporation is staffed by professional's who understand the fundamental concepts, processes, equipment and substances associated with petro chemical engineering, refining and its ancillary activities.

Contents

What are Coker Furnaces?


What is Purge Gas Recovery?

What is a C02 Removal System?

What is a Deep Hydrolyser?

What are the Energy Savings?

What are the Standard Plant Types?
 
 What are Coker Furnaces?




Coker furnaces are used during the refining process of crude oil to manufacture gas. This process involves high temperatures, vibration, corrosive atmospheres, and a volatile environment. This in turn creates a high risk to the environment, and a high cost if a process in lost or has to be burned off.



What is Purge Gas Recovery?



In order to achieve optimum conversion in a synthesis convertor, it is necessary to purge a certain quantity of gas from synthesis loop so as to as to reduce inerts concentration in the loop. This purge gas containing about 60% Hydrogen was fully utilised as primary reformer fuel. A cryogenic purge gas recovery unit is used to recover H2 from it which is recycled back convert it to Ammonia while the by - product tail gas from PGR Unit is burnt as fuel in the primary reformer.



At times, extremely tall cylinders are constructed to perform purge gas recovery operations. Typically, thermocouples are installed to monitor inlet and outlet gas temperature of each bed (bottom) of these cylinders.



What is a C02 Removal System?



In the modified system LT Shift Converter effluent is cooled in the condensate Reboilers via heat exchange with CO2 Stripper water wash condensate. Low pressure steam is generated and is utilised as motive steam for flash drum ejectors. The semilean solution is taken to a flash tank. The pressure is reduced in successive stage resulting in a move complete flashing of the stream water. A temperature drop of 121 deg C to 111 deg C result. The flashed steam is compressed by the motive steam in the ejectors & reintroduced in the stripper. This stream of flashed & motive steam is used to provide a portion of the required regeneration heat.



What is a Deep Hydrolyser?



Before incorporation of the scheme, the waste water containing about 4 - 5 % of Ammonia & 0.3 % of Urea is pumped by Distillation tower feed pump to Waste water distillation column, where the ammonia is stripper off. The stripping vapours are regenerated by boiling the purified water in a re-boiler heated with steam. From the re-boiler the treated water was cooled and sent to battery limit. After incorporation of hydrolyser the feed for hydrolyser is taken from 21st tray of the distillation column. Before entering the hydrolyser the solution is preheated in a heat exchanger by means of solution coming from hydrolyser itself. The hydrolysis heat is provided by HP Steam. The solution leaving the hydrolyser after passing through the exchanger is sent to distillation column. From the distillation column the water is cooled and then sent in the cooling tower as make.



What is the Energy Savings?



Accurate temperature measurement and a through analysis of operating conditions of a plant can improve efficiency and realize a cost savings in the hundreds if not millions of dollars yearly.



Burner Combustion Control


Boilers are often the principle steam or hot water generator system used in industrial plant or commercial heating.



Burner combustion control generally includes one or a combustion of the following methods;



Total heat control,

Regulation of excess air,

Burner cross - limiting.



Tube skin thermocouple sensors are used to control advance warming of flue gases to provide a better heat transfer rate in boilers. This can result in substantial savings on fuel.



Summary



Refinery thermocuples are commonly used in refinry equipment to measure process variable data in the following;



Absorber - Exit temperature of the absorbing liquid and exit specific gravity for the absorbing liquid.

Condenser - Outlet gas stream temperature.

Carbon absorber - temperature of carbon bed.

Thermal incinerator – Firebox temperature.

Catalytic incinerator - Temperature upstream and downstream of the catalyst bed.

Boilers or process heaters - Firebox temperature.

Flex Armor Resistance Temperature Detector (RTD), Thermistor, Semi Conductor and Thermocouple

Temperature Sensor are available in 3 configurations;

Flex Armor Sensor
Flex Armor Sensor with Transition
Flex Armor Sensor with 2 Transitions

Flex Armor Temperature Sensor are commonly used in extruder and plastic processing and bearing industries.

The samples shown are 100 ohm RTD Temperature Sensor in a 4-wire, Teflon insulated configuration. This provides an operating temperature range of -50 to 250ºC. Fiberglass insulated wire is also available.

The outside diameter of the tip is .250". Transitions are 3/8" O.D.(Additional diameters are available).

The length of the tip, sheath, transition and lead wire are specified by the customer to suit the application. The lead wire can be supplied as bare wire, terminated with plugs, jacks, spade lugs, or junction boxes. For long cable runs, stainless steel overbraid or flex armor can be added to protect the lead wires.

These temperature sensors configurations can be manufactured as RTD's, Thermistors, Thermocouples.




Common types include; 100, 500 and 1000 ohm RTD temperature sensors,



2K, 3K, 4K, 5K, 10K, 30K, 50K & 100K Thermistor temperature sensors,



Types, T, J, E, K, N, R, S, B Thermocouple temperature sensor.



We also manufacture customer specified temperature elements, including the Balco, Copper and Nickel temperature coefficients.



MIL-STD temperature curves are also rountinely manufactured by Thermometrics Corporation.
Specifications


Platinum 100 Ohm, .00385 Alpha

Temperature Range - 50ºC to 400ºC

DIN Class B, ± .12% ohms at 0ºC

Celsius = .3 ohms + (.005 ohms x (temperature) )



Temperature versus Resistance relationship for the range - 200ºC to 0ºC ;

RT = R0 [ 1 + At + Bt2 + C ( t - 100ºC) t3 ]



Temperature versus Resistance relationship for the range 0ºC to 850ºC ;

RT = R0 (1 + At + Bt2 )



Coefficients

A = 3.908 02 x10-3ºC-1

B = -5.802 x 10-7 ºC-2

C = -4.273 50 X 10-12ºC-4

About RTD Temperature Probes

About RTD Temperature Probes

Temperature transmitters, RTD, convert the RTD resistance measurement to a current signal, eliminating the problems inherent in RTD signal transmission via lead resistance. Errors in RTD circuits (especially two and three wire RTDs) are often caused by the added resistance of the leadwire between the sensor and the instrument. Transmitter input, specifications, user interfaces, features, sensor connections, and environment are all important parameters to consider when searching for temperature transmitters, RTD.

Transmitter input specifications to take into consideration when selecting temperature transmitters, RTD include reference materials, reference resistance, other inputs, and sensed temperature. Choices for reference material include platinum, nickel or nickel alloys, and copper. Platinum is the most common metal used for RTDs - for measurement integrity platinum is the element of choice. Nickel and nickel alloys are very commonly used metal. They are economical but not as accurate as platinum. Copper is occasionally used as an RTD element. Its low resistivity forces the element to be longer than a platinum element. Good linearity and economical. Upper temperature range typically less than 150 degrees Celsius. Gold and Silver are other options available for RTD probes - however their low resistivity and higher costs make them fairly rare, Tungsten has high resistivity but is usually reserved for high temperature work. When matching probes with instruments - the reference resistance of the RTD probe must be known. The most standard options available include 10 ohms, 100 ohms, 120 ohms, 200 ohms, 400 ohms, 500 ohms, and 1000 ohms. Other inputs include analog voltage, analog current, and resistance input. The temperature range to be sensed and transmitted is important to consider.

Important transmitter specifications to consider when searching for temperature transmitters, RTD, include mounting and output. Mounting styles include thermohead or thermowell mounting, DIN rail mounting, and board or cabinet mounting. Common outputs include analog current, analog voltage, and relay or switch output. User interface choices include analog front panel, digital front panel, and computer interface. Computer communications choices include serial and parallel interfaces. Common features for temperature transmitters, RTD, include intrinsically safe, digital or analog display, and waterproof or sealed. Sensor connections include terminal blocks, lead wires, screw clamps or lugs, and plug or quick connect. An important environmental parameter to consider when selecting temperature transmitters, RTD, is the operating temperature.

About Temperature

A Brief History of Temperature

Temperature is by far the most measured parameter. It impacts the physical, chemical and biological world in numerous ways. Yet, a full appreciation of the complexities of temperature and its measurement has been relatively slow to develop.

Intuitively, people have known about temperature for a long time: fire is hot and snow is cold. Greater knowledge was gained as man attempted to work with metals through the bronze and iron ages. Some of the technological processes required a degree of control over temperature, but to control temperature you need to be able to measure what you are controlling.


Until about 260 years ago temperature measurement was very subjective. For hot metals the colour of the glow was a good indicator. For intermediate temperatures, the impact on various materials could be determined. For example does the temperature melt sulphur, lead or wax, or boil water?
In other words a number of fixed points could be defined, but there was no scale or any way to measure the temperature between these points. It is, however possible that there is a gap in the recorded history of technology in this regard as it is difficult to believe that the Egyptians, Assyrians, Greeks, Romans or Chinese did not measure temperatures in some way.

Galileo invented the first documented thermometer in about 1592. It was an air thermometer consisting of a glass bulb with a long tube attached. The tube was dipped into a cooled liquid and the bulb was warmed, expanding the air inside. As the air continued to expand, some of it escaped. When the heat was removed, the remaining air contracted causing the liquid to rise in the tube and indicating a change in temperature. This type of thermometer is sensitive, but is affected by changes in atmospheric pressure.
The Eighteenth Century: Celsius and Fahrenheit

By the early 18th century, as many as 35 different temperature scales had been devised. In 1714, Daniel Gabriel Fahrenheit invented both the mercury and the alcohol thermometer. Fahrenheit's mercury thermometer consists of a capillary tube which after being filled with mercury is heated to expand the mercury and expel the air from the tube. The tube is then sealed, leaving the mercury free to expand and contract with temperature changes. Although the mercury thermometer is not as sensitive as the air thermometer, by being sealed it is not affected by the atmospheric pressure. Mercury freezes at -39° Celsius, so it cannot be used to measure temperature below this point. Alcohol, on the other hand, freezes at -113° Celsius, allowing much lower temperatures to be measured.

At the time, thermometers were calibrated between the freezing point of salted water and the human body temperature. (Salt added to crushed wet ice produced the lowest artificially created temperatures at the time). The common Flemish thermometers of the day divided this range into twelve points. Fahrenheit further subdivided this range into ninety-six points, giving his thermometers more resolution and a temperature scale very close to today's Fahrenheit scale. (In fact there appeared to have been between 15 and 20 different temperature scales at this time, determined by nationality and application.)

Later in the 18th century, Anders Celsius realised that it would be advantageous to use more common calibration references and to divide the scale into 100 increments instead of 96. He chose to use one hundred degrees as the freezing point and zero degrees as the boiling point of water. Sensibly the scale was later reversed and the Centigrade scale was born. See Olof Beckman's short History of the Celsius Temperature Scale.

The Nineteenth Century: A productive era
The early 1800's were very productive in the area of temperature measurement and understanding.
William Thomson (later Lord Kelvin) postulated the existence of an absolute zero. Sir William Hershel, discovered that when sunlight was spread into a colour swath using a prism, he could detect an increase in temperature when moving a blackened thermometer across the spectrum of colours. Hershel found that the heating effect increased toward and beyond the red in the region we now call 'infrared'. He measured radiation effects from fires, candles and stoves, and deduced the similarity of light and radiant heat. However it was not until well into the following century that this knowledge was exploited to measure temperature.
In 1821 T J Seebeck discovered that a current could be produced by unequally heating two junctions of two dissimilar metals, the thermocouple effect. Seebeck assigned constants to each type of metal and used these constants to compute total amount of current flowing. Also in 1821, Sir Humphrey Davy discovered that all metals have a positive temperature coefficient of resistance and that platinum could be used as an excellent temperature detector (RTD). These two discoveries marked the beginning of serious electrical sensors.

Gradually the scientific community learnt how to measure temperature with greater precision. For example it was realised by Thomas Stevenson (civil engineer and father of Robert Louis Stevenson) that air temperature measurement needed to occur in a space shielded from the sun's radiation and rain. For this purpose he developed what is now known as the Stevenson Screen. It is still in wide use.
The late 19th century saw the introduction of bimetallic temperature sensor. These thermometers contain no liquid but operate on the principle of unequal expansion between two metals. Since different metals expand at different rates, one metal that is bonded to another , will bend in one direction when heated and will bend in the opposite direction when cooled (hence the term Bimetallic Thermometer or BiMets). This bending motion is transmitted, by a suitable mechanical linkage, to a pointer that moves across a calibrated scale. Although not as accurate as liquid in glass thermometers, BiMets are more hardy, easy to read and have a wider span, making them ideal for many industrial applications.

The 20th Century: Further discovery, refinement and recognition
The 20th century has seen the discovery of semiconductor devices, such as: the thermistor, the integrated circuit sensor, a range of non-contact sensors and also fibre-optic temperature sensors. Also, Lord Kelvin was finally rewarded for his early work in temperature measurement. The increments of the Kelvin scale were changed from degrees to Kelvins. Now we no longer say "one-hundred degrees Kelvin;" we instead say "one-hundred Kelvins". The "Centigrade" scale was changed to the "Celsius" scale, in honour of Anders Celsius.

The 20th century also saw the refinement of the temperature scale. Temperatures can now be measured to within about 0.001°C over a wide range, although it is not a simple task. The most recent change occurred with the updating of the International Temperature Scale in 1990 to the International Temperature Scale of 1990 (ITS-90). This document also covers the recent history of temperature standards.