Intro4u2u

Wherever a process is being monitored or controlled you find 4 - 20mA dc as the standard analog signal of choice. There are others such as 0 - 5V dc and of course the many digital signal standards.

As process variables: 4 - 20mA signals emerge from signal converters and transducers (= transmitters) representing variables such as temperature, pressure, power. They are routed to controllers, indicators, PLCs, DCS systems etc to be used in display, control and alarm monitoring of the process.

Transducers handling non-linear inputs - e.g. thermocouples, differential pressure flowmeters, often hand the job of linearising the signal over to the receiving device.

Some models,(called “smart”)have digital processing circuitry inside that can linearise the process signal. Even smarter models come configurable, meaning that you can select and range any one of several types of process signal as the input. It is a short step from here to incorporate a digitally coded output, typically for entry into a DCS system.

Compared with the low millivolt signals from thermocouples or RTDs the 4 - 20mA signal is robust enough to ignore electrical interference in plant-wide wiring. The signal wire can be copper, not the more expensive, dedicated thermocouple extension wire.

Why is this signal is known as a current source? It means that the receiving circuit and wiring can have any resistance between typically 0 ohms and 600 ohms without affecting the accuracy of the mA signal. Similarly, a voltage source remains stable for the whole range of different load resistances within its specification.

Loop-powered transmitters.

This is where the transmitter output, an external dc power supply and the receiving devices are strung in a series loop. Some 12V of the dc supply is dropped across the transmitter as power for the internal electronic circuit. If your receiving device resistance is too high for the power supply you can use a higher voltage dc supply up to about 90V.

Self powered transmitters. These have a second pair of terminals that take the normal 115V ac supply from which the now, internal dc supply is derived. You can now achieve electrical isolation between all transmitters on the same ac power supply as well as between inputs, outputs and ground.

This avoids the gross, unpredictable measurement errors that connection to grounded receiving devices can cause.

Input/output isolation is important too. For example, when your thermocouple tip is exposed and vulnerable to touching high heater or process voltages ( in furnaces or duct heaters). Isolation keeps hazardous voltages off the plant signal wiring and avoids common-mode measurement errors.

As a control signal the 4 - 20mA would come from say the output of a temperature controller and feed into a final control device such as an electro-pneumatic control valve or silicon controlled rectifier . Usually a high control signal calls for high output of your final control device (direct acting mode). Reverse acting is the other option, e.g. high milliamps to close your valve. So pause and decide which action you want. If your controller or wiring should break with loss of milliamps, choose the mode that you reckon is fail-safe.

Force-balance principle: The 4 - 20mA signal is about the right size for use in the force balance system found in pressure transmitters. Here the magnetic force of the milliamps in a coil performs a null-balance against the diaphragm pressure, resulting in an accurate transduction of pressure to milliamps.

The same principle is used in I/P (current to pressure) converters, often incorporated into electro-pneumatic control valves; also in valve positioners where, by servo action the valve is forced to a position proportional to the mA signal.

Signal converters. A simple example is the signal isolator with 4 - 20mA in and 4 - 20mA out. It is used to isolate an incoming signal that is grounded, from a receiving device that cannot tolerate a second ground reference. The input/output/ground isolation will usually withstand about 1500V.

There are many combinations of available signal ranges for both inputs and outputs including multiple channels, mathematical functions, user defined linearisations.

Packaging can be a DIN rail mounted box or a plant mounted pillbox shaped enclosure with two pairs of terminals, input and output. A third pair is added on models having external ac power.

The Four-to-Twenty Milliamp Circuit is the most common way to transfer an instrumentation signal from one device to another. 4 milliamps would usually translate to a zero value and 20 milliamps would be some full-scale value. For example, if we want to keep track of the level of water in a 16′ storage tank, 4 mA would mean the tank was empty, 20 mA would be full, and each milliamp in between would translate to one foot of water. A transmitter at the tank would sense the level by pressure or ultrasonics and translate that into the 4-20 mA instrumentation signal and send it to a gauge or a chart recorder in a control room.

The circuit is commonly powered by a 24 volt DC supply. When this voltage is used, up to 4 devices may be connected to a single loop to transmit and read the common signal. Typically, the transmitter will receive its operating power from the loop and may require no other electrical connection. Other devices usually receive power from another source. We might have an indicating gauge, a chart recorder and a telemetry device all reading the same signal by looping them in series as shown in the diagram below. Often, the receiving devices will have a precision 250 ohm resistor across their input terminals either externally or internal to the unit. When the 4-20 mA loop is connected across this resistor, the 4-20 milliamps is translated to 1-5 volts DC and this voltage signal is read by the instrument. The reason we don’t simply transmit the 1-5 volt signal is because some of this signal would be lost over distances due to line resistance and therefore the signal would be inaccurate. However, a characteristic of an isolated electrical circuit is that it’s amperage will be constant at every point along the circuit so therefore amperage makes a better carrier for precision signals.

The power supply may be connected at any point in the loop. Its positive terminal will connect to a positive terminal of another device and its negative terminal connects to a negative terminal of still another device in the loop. All other connections in the loop are positive to negative. Shielded cables are used to guard the low level signals against outside interference. Shields of each cable run are grounded at one end only, preferably at a common point. This is to prevent current from traveling down the shield and causing interference. Ungrounded ends should be insulated to prevent accidental grounding or shorting.

I/p converters are used to convert electrical input signals into pneumatic outputs.

Traditionally, i/p converters were used to convert 4-20 mA signals into 3-15 psi outputs. Nowadays, the Fairchild range of i/p converters can accept both voltage and current input signals from zero up to 50 mA or 12Vdc. Extended range i/p converters can give output pressure of up to 150 psi.
All Fairchild i/p converters can accept mains air pressure without the need for a separate regulator.

The lead acetate tape method for the detection of hydrogen sulfide (H2S) and Total Sulfur in gaseous streams is based on the established principle that H2S reacts specifically with lead acetate to form a brown lead sulfide stain.
The concentration of H2S is directly proportional to the rate of change of staining on the lead acetate tape. This principle is the basis for a number of ASTM methods, and is by far the most reliable and simplest way to measure H2S in process.
The analyzer moves the treated paper tape one section at a time. Depending on the sample concentration, the tape will begin to darken at a rate proportional to the concentration of H2S in the sample stream. The analyzer exposes a fresh section of tape to the sample inside the sample chamber every 3 minutes. GASI Analyzers are designed with the following in mind:
The lead acetate series of analyzers are third generation systems designed to meet increasing demands for low-level measurement in pipelines and process. The 6 models to choose from are the 801, 801W, 801WTS, 902 H2S, 902 TS and 902 H2S/TS.
bullet

1. Low maintenance
2. Extended tape life
3. Fast speed of response (under 20 sec. to alarm)
4. Low power consumption
5. Optional total sulfur measurement
6. Multiple streams
7. Auto calibration
8. Remote communications

1. Corrosion protection of pipe lines.
2. Catalyst protection in refinery/polymer reactor operations.
3. Environmental protection agency compliance.
4. Gas detection for personnel safety.

Common Sulphur Compounds

Sulphides                                Mercaptans
1.    Hydrogen Sulphide H2S                Methyl Mercaptan MeSH
2. Carbonyl Sulphide  COS            Ethyl Mercaptan EtSH
3. Dimethyl sulphide   DMS            Propyl Mercaptan PrSH
4. Methylethyl Sulphide MES            Iso-Propyl Mercaptan i - PrSH
5. Diethyl Sulphide     DES            N-Butyl Mercaptan       n-BuSH
6. Diallyl Sulphide      DAS            I-Butyl Mercaptan       i-BuSH
7. Dipropyl Sulphide   DPS
8. Dibutyl Sulphide     DBS

Common Sulphur Compounds

Disulphides
1.  Carbon Disulphide            CS2
2.  Dimethyl Disulphide        DMDS
3.    Methylethyl Disulphide     MEDS
4.    Diethyl Disulphide            DEDS
5. Dipropyl Disulphide            DPDS
6. Tetrahydro Thiophene        THT
Methods of Measurement

Lead Acetate Tape type analysers
Chemiluminescence analysers
Ultraviolet radiation absorption spectrometers
Gas Chromatograph with FPD
Electrochemical cells for gas detection
X’ray Flourescence for % level detection.

Although sulphur is in the same group in the periodic table as oxygen, there are more differences in the chemical characteristics of these elements than there are similarities. Thus, while oxygen always displays a valency of two, sulphur displays valences of four (in sulphur dioxide, SO2), six (in thionyl chloride, socl2) and eight (in sulphur hexafluoride, SF8).

In the elemental state, sulphur exists in polymeric forms.

Sulphur is an essential element for living organisms, and is present in some amino-acids.
Discovery

* Sulphur has been known since the beginning of history, and is described in the Bible.

* Sulphur has been used by the Greeks and Romans as a fumigant and disinfectant.

Occurrence

* Sulphur is widely distributed as the free element and combined in compounds.
* Sulphur is found near volcanoes, where it is formed by the reaction of sulphur dioxide and hydrogen sulphide which are associated with the volcanoes.
* Sulphur also occurs in many metal ores, including
o gelena, PbS,
o zinc blende, ZnS,
o cinnebar, HgS,
o stibnite, Sb2S3,
o copper pyrites, Cu2S.Fe2S3, and
o iron pyrites, Fe2S.
* The important sulphate ores include
o gypsum, CaSO4, and
o heavy spar, BaSO4.

Extraction

* Sulphur is separated from the minerals in its ores by heating, when the liquid sulphur drains from the ore body. Normally, part of the sulphur in the ore is burned and the heat used to melt the remaining sulphur so that it leaches from the hot mass. This extraction procedure called the Gill Process, and it is widely used in Sicily.

* Sulphur is extracted from underground ore bodies in the United States using the Frasch Process, where superheated water is pumped underground to melt the sulphur which is then forces to the surface.

Preparation
Properties
Sulphur occurs in a large number of allotropic forms in the solid state. Several liquid forms are also known.

Rhombic Sulphur
Rhombic Sulphur which is also known as Octahedral Sulphur or alpha-Sulphur, is a yellow crystalline solid, which crystallises from a solution in carbon disulphide. X-ray investigation shows this form to consist of eight sulphur atoms in the molecule in a ring structure.

Monoclinic Sulphur
Monoclinic Sulphur which is also known as Prismatic Sulphur or beta-Sulphur, is a yellow crystalline solid, which is obtained by allowing molten sulphur to solidify, when long prismatic needles form on the walls of the container that can be separated from the still molten liquid by pouring off the latter. X-ray investigation shows this form to consist of eight sulphur atoms in the molecule in a ring structure, but arranged within the crystal in a different manner to that in rhombic sulphur.

Plastic Sulphur
Plastic Sulphur which is also known as gamma-Sulphur, is a tough elastic substance that is formed when molten sulphur is poured into cold water. On standing it slowly crystallises.

Amorphous Sulphur
Amorphous Sulphur is the insoluble white amorphous solid that remains when flowers of sulphur are extracted with carbon disulphide.

Colloidal Sulphur
Colloidal Sulphur which is also known as delta-Sulphur, is a yellow crystalline solid.
Reactions

* Sulphur burns readily in air forming sulphur dioxide.

S    +    O2    ==>    SO2

*

Sulphur reacts with hydrogen gas when the latter is bubbles
through molten sulphur near its boiling point.

S    +    H2    ==>    H2S

* Sulphur dissolves in caustic alkali solutions forming sulphides and thiosulphates.

4 S  +  6 NaOH  ==>  Na2S2O3  +  2 Na2S  +  3 H2O

Reaction of sulphur with air

Sulphur burns in air to form the gaseous dioxide sulphur(IV) oxide, SO2.

S8(s) + 8O2(g) → 8SO2(g)
Reaction of sulphur with water

Sulphur does not react with water under normal conditions.
Reaction of sulphur with the halogens

Sulphur racts with all the halogens upon heating.

Sulphur reacts with fluorine, F2, and burns to form the hexafluoride sulphur(VI) fluoride.

S8(s) + 24F2(g) → 8SF6(l) [orange]

Molten sulphur reacts with molten sulphur to form disulphur dichloride, S2Cl2. This apparently smells dreadfully. With excess chlorine and in the presence of a catalyst, such as FeCl3, Snl4, etc., it is possible to make a mixture containing an equilibrium mixture of red sulphur(II) chloride, SCl2, and disulphur dichloride, S2Cl2

S8 + 4Cl2 → 4S2Cl2(l) [orange]

S2Cl2(l) + Cl2 2SCl2(l) [dark red]
Reaction of sulphur with acids

Sulphur does not react with dilute non-oxidizing acids.
Reaction of sulphur with bases

Sulphur reacts with hot aqueous potassium hydroxide, KOH, to form sulphide and thiosulphate species.

S8(s) + 6KOH(aq) → 2K2S3 + K2S2O3 + 3H2O(l)

Health effects caused by exposure to high levels of SO2 include breathing problems, respiratory illness, changes in the lung’s defences, and worsening respiratory and cardiovascular disease. People with asthma or chronic lung or heart disease are the most sensitive to SO2. It also damages trees and crops. SO2, along with nitrogen oxides, are the main precursors of acid rain. This contributes to the acidification of lakes and streams, accelerated corrosion of buildings and reduced visibility. SO2 also causes formation of microscopic acid aerosols, which have serious health implications as well as contributing to climate change.

The following table shows the health effects of different Air Quality Index levels caused by sulphur dioxide.

At room temperature, sulfur is a soft bright yellow solid. Elemental sulfur has only a faint odor similar to that of matches. The odor associated with rotten eggs is from hydrogen sulfide (H2S) and organic sulfur compounds. Sulfur burns with a blue flame that emits sulfur dioxide, notable for its peculiar suffocating odor. Sulfur is insoluble in water but soluble in carbon disulfide and to a lesser extent in other nonpolar organic solvents such as benzene and toluene. Common oxidation states of sulfur include −2, +2, +4 and +6. Sulfur forms stable compounds with all elements except the noble gases.

Sulfur in the solid state ordinarily exists as cyclic crown-shaped S8 molecules. Sulfur has many allotropes besides S8. Removing one atom from the crown gives S7, which is responsible for sulfur’s distinctive yellow color. Many other rings have been prepared, including S12 and S18. By contrast, its lighter neighbor oxygen only exists in two states of allotropic significance: O2 and O3. Selenium, the heavier analogue of sulfur can form rings but is more often found as a polymer chain.
The structure of the cyclooctasulfur molecule, S8

The crystallography of sulfur is complex. Depending on the specific conditions, the sulfur allotropes form several distinct crystal structures, with rhombic and monoclinic S8 best known.

A noteworthy property of sulfur is that its viscosity in its molten state, unlike most other liquids, increases above temperatures of 200°C due to the formation of polymer chains. The molten sulfur also becomes dark red in colour above this temperature due to the presence of free valences on terminal atoms of the polymer chains. However, after a specific temperature is reached, the viscosity is reduced because there is enough energy to break the chains.

Amorphous or “plastic” sulfur can be produced through the rapid cooling of molten sulfur. X-ray crystallography studies show that the amorphous form may have a helical structure with eight atoms per turn. This form is metastable at room temperature and gradually reverts back to crystalline form. This process happens within a matter of hours to days but can be rapidly catalyzed.

Sulfur has many industrial uses. Through its major derivative, sulfuric acid (H2SO4), sulfur ranks as one of the most important industrial raw materials. It is of prime importance to every sector of the world’s economies.

Sulfuric acid production is the major end use for sulfur, and consumption of sulfuric acid has been regarded as one of the best indices of a nation’s industrial development. More sulfuric acid is produced in the United States every year than any other industrial chemical.

Sulfur is also used in batteries, detergents, the vulcanization of rubber, fungicides, and in the manufacture of phosphate fertilizers. Sulfites are used to bleach paper and as a preservative in wine and dried fruit. Because of its flammable nature, sulfur also finds use in matches, gunpowder, and fireworks. Sodium or ammonium thiosulfate is used as photographic fixing agents. Magnesium sulfate, better known as Epsom salts, can be used as a laxative, a bath additive, an exfoliant, or a magnesium supplement for plants. Sulfur is used as the light-generating medium in the rare lighting fixtures known as sulfur lamps. Elemental sulfur crystals are commonly sought after by rock collectors for their brightly colored polyhedron shapes.

In the late 1700s, furniture makers used molten sulfur to produce decorative inlays in their craft. Because of the sulfur dioxide produced during the process of melting sulfur, the craft of sulfur inlays was soon abandoned.

Sulfur is an essential component of all living cells.

Sulfur may also serve as chemical food source for some primitive organisms: some forms of bacteria use hydrogen sulfide (H2S) in the place of water as the electron donor in a primitive photosynthesis-like process. Inorganic sulfur forms a part of iron-sulfur clusters, and sulfur is the bridging ligand in the CuA site of cytochrome c oxidase, a basic substance involved in utilization of oxygen by all aerobic life.

Sulfur is absorbed by plants via the roots from soil as the sulfate ion and reduced to sulfide before it is incorporated into cysteine and other organic sulfur compounds (sulfur assimilation).

In plants and animals the amino acids cysteine and methionine contain sulfur, as do all polypeptides, proteins, and enzymes which contain these amino acids. Homocysteine and taurine are other sulfur-containing acids which are similar in structure, but which are not coded for by DNA, and are not part of the primary structure of proteins. Glutathione is an important sulfur-containing tripeptide which plays a role in cells as a source of chemical reduction potential in the cell, through its sulfhydryl (-SH) moiety. Many important cellular enzymes use prosthetic groups ending with -SH moieties to handle reactions involving acyl-containing biochemicals: two common examples from basic metabolism are coenzyme A and alpha-lipoic acid.

Disulfide bonds (S-S bonds) formed between cysteine residues in peptide chains are very important in protein assembly and structure. These strong covalent bonds between peptide chains give proteins a great deal of extra toughness and resiliancy. For example, the high strength of feathers and hair is in part due to their high content of S-S bonds and their high content of cysteine and sulfur (eggs are high in sulfur because large amounts of the element are necessary for feather formation). The high disulfide content of hair and feathers also contributes to their indigestibility, and also their disagreeable odor when burned.
The burning of coal and petroleum by industry and power plants creates massive amounts of sulfur dioxide (SO2) which reacts with atmospheric water and oxygen to produce sulfuric acid. This sulfuric acid is a component of acid rain, which lowers the pH of soil and freshwater bodies, resulting in substantial damage to the natural environment and chemical weathering of statues and structures. Fuel standards increasingly require sulfur to be extracted from fossil fuels to prevent the formation of acid rain. This extracted sulfur is then refined and represents a large portion of sulfur production.