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The analyser shall be selected so that the relevant ASTM Test Method on or with which the
measurement is based or correlated is the same one as that used in the site laboratory,
measuring the same property. The Principal shall be consulted if this cannot be complied
with.
Analysers that are intended for environmental/pollution control or Continuous Emission
Monitoring System (CEMS) may require approval or have to conform to the requirements
specified by the local environmental/pollution control authority.
Key characteristics that are important to the service are listed below.

Composition
Multiple-component analysis can be best carried out with PGC. Mass spectrometers have
the advantage of fast response times and are sometimes advantageous for selective trace
component analysis in gaseous products. A typical response time is 6 s per component.

The calibration matrix of a mass spectrometer is often adversely affected by components which
were not included in the initial calibration mixture or which were either under- or over-represented
in the initial calibration mixture. Calibration results may therefore have limited validity. Certain mixtures may be difficult to analyse with a mass spectrometer. For PGC, the calibration mixture for background gas should be as close as possible to the material composition of the process sample. This is particularly so for the measurement of trace contaminants.

Infrared and ultraviolet spectroscopic techniques are typically suitable for single-component
analysis of gaseous products, and have some limited application for liquid products. Infrared and ultraviolet spectroscopic techniques, more specifically Fourier Transform Infrared and Ultraviolet Diode Array techniques, are also suitable for multi-component analysis in a limited number of applications. Any cross-sensitivity to components not of principal measurement interest should always be checked when infrared and ultraviolet spectroscopic techniques are used.  Modern PGCs usually offer the advantage of calculating derived physical properties such as heating value, density, distillation properties, and even octane numbers. However, their accuracies have to be checked to ensure that they meet the specified requirements.

Density / Relative density
Density meters should be of the vibrating type. The use of radioactive types requires the
approval of the Principal.

Dissolved oxygen
Dissolved oxygen is normally measured with a polarographic cell. The output value is
dependent on the temperature due to the change in permeability of the membrane at
different temperatures. The cell should be used in accordance with the Manufacturer’s
specifications, e.g., for minimum flowing conditions. The sensor should be kept clean and
unclogged at all time. Metal-electrode-type dissolved oxygen meters often have robust self-cleaning features. The output is dependent on oxidation-reduction potentials (REDOX) of the water. Heavy metals and free oil may impede their use.

Flammable gas detection (in ambient air)
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Flashpoint analysers
Flashpoint analysers that detect the flash on explosion should be used. The response of flashpoint analysers using semi-conductors or pellistor type detectors is product (i.e. feedstock) dependent. It is also sensitive to the presence and build up of olefinic material e.g., in catalytic cracked sample. For process samples with a sulpur content > 1 %, pellistor type or flash point analysers shall not be used.

HCl measurement in process gas
Analysers using the change of conductivity of a reagent should be used. Alternatively,
analysers using the coloration of paper tape impregnated with reagent could be considered
where lower accuracy is acceptable.

H2S measurement
For measurement of H2S in ambient air, For measurement of low concentration H2S in process gas, the lead acetate paper coloration principle or PGC with flame photometric detector should be used. For higher concentrations (% range), UV spectrophotometer or PGC may be used.

KVP measurement
Where KVP is measured in order to obtain a correlation with RVP, an RVP analyser should
be used directly.

Moisture measurements in gaseous products

• Dewpoint mirror techniques are absolute measurements. They are costly and are specifically suitable for higher moisture concentrations (0.05 % and higher). The technique shall not be used to measure water in natural gas, due to the difficulty in obtaining consistent results. High hydrocarbon concentrations may interfere. Typically, the hydrocarbon dewpoint should not be higher than 10 °C above the water dewpoint.

• Karl-Fischer-type titration is an absolute measurement. Output shall be expressed in mg/m3.

• Metal-oxide-type sensor measurements are relative measurements. The output shall be calibrated against a test gas mixture or against an absolute measurement. The output calibration is dependent on temperature and pressure. For accurate measurements the flow cell shall be kept at constant pressure and temperature. The characteristics of aluminium-oxide-type sensors are, in general, not stable and regular verification of the calibration factors is therefore required (a typical requirement is once per year, but this may vary depending on the application).

• P2O5 (phosphorus pentoxide) sensors are theoretically absolute measurements.
  However, they shall be treated as a relative-type measurement and calibrated with a
  test mixture or against an absolute measurement. This analyser type shall not be used
  in process streams containing double-bonded hydrocarbons or those rich in hydrogen.
  The analyser is flow-sensitive.

• LiCl (lithium chloride) type sensors are applicable for Relative Humidity
  measurements. Their main application is in buildings, for use as a tool for climatic
  conditioning.

• Hygroscopically coated vibrating crystal-type moisture meters have the best accuracy.
  This type of meter should not be used for prolonged measurements of moisture
  concentrations in excess of 2000 cm3/m3 NOTE: At moisture concentrations in excess of 2000 cm3/m3, the hygroscopic layer on the crystal may be washed off.

• Measurement by conductivity of a hygroscopic salt-glycerol solution is fairly suitable for
  natural gas application as the sensor is relatively easy to rejuvenate, although this
  requires specialist attention.

• Silicon oxide type sensors are thermally stable and less hygroscopic than aluminium
  oxide type sensors. When equipped with a feature to momentarily heat the sensor,
  they burn off any hydrocarbon contaminants and ‘left over’ moisture from high loads,
  thus giving faster response and recovery. A silicon oxide type sensor is preferred to
  metal oxide and P2O5 sensors.

• Fibre-optic type sensors provide an in situ means of measurement and measure dew
  point from - 70 °C to 10 °C at pressures up to 250 bar and temperature from - 30 °C to
  95 °C. It is a relative humidity measurement and converts results to ppmv, dew point.

Octane analyser
Cold flame front analysers and octane engine type analysers have been in use. Near
InfraRed analysers are preferred where product specifications can be satisfied.

Oil in/on water
For oil in/on water measurements the following applications should be considered:
Care must be taken with respect to correlation and cross-checking with laboratory
measurement. The intent of the measurement shall be made clear and well documented.
Wherever feasible/practicable, the analyser selected shall have the same working principle
as that of the corresponding laboratory instrument.

a) Oil on water, e.g., oil sheen in open channels - This is a qualitative detection only.
Laser Diode Infra-red reflectance-type measurements are suitable.
b) Oil emulsified/dispersed in water. Infrared-type meters, e.g., measuring hydrocarbons dissolved in an extraction fluid such as Freon, have a reduced response for naphthenic hydrocarbons and do not
respond to the aromatic hydrocarbon content of the oil. Ultraviolet-type meters do not respond to the naphthenic and the paraffinic content of the oil. Instead they rely on the co-presence of aromatics in oil. Typical solubility rates for paraffinic and aromatic hydrocarbons are about 20 mg/l and 80 mg/l
respectively.
c) Total Organic Carbon - TOC (2.5.21). Any oil or hydrocarbon entering the measuring
cell will be detected. The UV oxidation type is preferred to the “hot oxidation” type of
TOC analyser. TOC measurement is good for measuring carbon content from carbon
compounds or elemental carbon in an aqueous solution. Any oil that is insoluble in
aqueous solution may not be suitable. If sample does not contain any inorganic carbon
component, the acidification part of the analyser system need not be included. Higher
range is possible with dilution. This type of analyser requires extensive maintenance,
especially the “hot oxidation” type. The high consumption of reagent should be
considered. For low range measurement, the standard sample should not be stored for
long periods as it is not stable.

Oxygen
a) Zirconium-oxide-type meters are suitable for measuring oxygen in combustion flue
gas. These meters shall not be used for gases containing flammable gas, or unburned
material, e.g., soot. The sensor, which is kept at a temperature of typically 800 °C,
should be of an Ex-d construction in accordance with IEC 60079 by means of a flame
arrestor. For very high flue gas temperature applications, a cooling extension tube
shall be used. The start-up of fired equipment may pose conditions of increased hazard due to the likelihood of unburned fuel being present in a cold environment, e.g., upon misfiring. Therefore, the design requires careful assessment of the risks involved. Equipment selection shall take into
consideration the suitability of the sensor with respect to the gas group and temperature class.
For example, the presence of a high concentration of hydrogen in fuel gas requires the sensor to
have an electrical certification allowing it to operate in such an environment, while the
temperature class shall be related to the auto-ignition temperature of the fuel.
For prolonged measurement at low levels (below 0.3 % oxygen) in an inert
atmosphere, e.g., nitrogen, zirconium-oxide-type sensors ‘fall asleep’ and require
many hours before they are again able to respond to oxygen. For this reason,
zirconium-oxide-type sensors should not be used under these circumstances.
b) Para-magnetic-type oxygen meters are suitable for oxygen measurement in a
background of high inert gas or hydrocarbon. These sensors are also preferred for the
measurement of high temperature regeneration flue gas where a high loading of
particulate matter very rapidly clogs the filter of the in-situ zirconium-oxide-type cell. If
the sample is to be returned to the process stream, the reading will be affected by any
variation of backpressure. Certain gas species present in process mixtures may affect
the measurement due to their para-magnetic properties.
c) Polarographic (electrochemical) type sensors are suitable for trace oxygen analyses,
e.g., in Liquefied Natural Gas, or for applications where a precision of 5 % relative, or
worse, is acceptable.
2.5.14 pH measurements
a) Electrode-type pH meters should be used only for pH measurements 4 < pH < 10.
Electrode-type pH meters shall not be used for pH > 12.
NOTE: Below a pH of 4, precision is reduced, and above a pH of 10, precision and electrode life are
reduced.
Dual junction reference electrodes should be considered for fouling conditions. Severe
services may require additional (automated) features to maintain clean electrodes, i.e.,
mechanical (ultrasonic, brushes, flushing) and/or chemical cleaning. Low conductivity
fluids, e.g., demineralised water, require a liquid-earthing electrode. The construction
of the reference electrode should ensure an unimpeded delivery of electrolyte from the
reference electrode to the process. Pressure compensated constructions or sleeve
(fritt) type reference electrodes may satisfy that condition.
The use of an electrode system which immobilises the reference electrolyte in an
ionically conducting, non-porous, hard polymer prevents fouling and electrode
poisoning. This is preferred for fouling service and samples containing H2S.
For low water content processes, e.g., in chemical processes, and under process
pressures of –1 bar to 15 bar and temperatures of 0 °C to 140 °C, a differential pH
sensor not using potassium chloride reference electrode should be used for pH
between 4 and 10.
b) Titrator-type analysers should be used for pH < 4 and pH > 10.
When a fast-response system is required, the installing of a titrator in combination with
an electrode-type pH meter should be considered for improved response.
c) For a fast-response pH measuring system for pH > 12, a Flow Injection Analysis
analyser may be considered.
NOTE: Systems b) and c) may require a skilled maintenance pool. The reagent consumption rate is
relatively high. The possibility of frequent breakdown should be considered in the process design.

Phenol in water
Colorimetric type water quality analysers should be used.
NOTE: PGC shall not be used for this purpose because phenol can be adsorbed and desorbed on the column material, thus causing low output values when concentrations are high and high output values when concentrations are low.

Pour Point Measurement
For pour point measurement, the detection of the motion of a controlled disturbance to the
surface of a sample in a measuring cell is used. An analyser type shall be selected
according to the measurement intent. Analysers are available which conform to different
ASTM test methods. Improved resolution is available with some analysers that follow a
different ASTM method than that adopted for finished product quality conformance.
 

RVP
Reid Vapour Pressure analysers should be used. Where oxygenated fuels are tested and where the ‘dry’ method is specified, DVPE has to be calculated and reported. An analyser selected should be able to measure and report the RVP and DVPE.

Salt in Crude
This is a batch type analysis in which the salt is released in the sample matrix by mixing it
with alcohol and xylene. The conductivity of the extract is then measured in the analyser.

Stack gas composition analysers
The analysers selected shall satisfy the requirements of the local Environmental Regulatory
Authority. In the absence of local requirements, preference shall be given to analysers that
meet the requirements of Authorities such as the EPA and TÜV. The selection of the
analysers is generally dependent on whether the measurement is required to be done on
the wet or dry basis. Typically, the following measurements can be performed by stack gas composition analysers (in order of most common measurements):
- Oxygen (ZrO2-type oxygen meter) by:
· in-situ type measurements, or
· extractive with analyser mounted against duct/stack wall.
- Opacity
· across stack with visible light optical type instrument.
- SO2, NOx
· across stack with infrared or UV light type instrument
· extractive with pre-dilution and chemo-luminescence type analysers
· extractive with infrared or UV light type instrument.
- CO
· across stack with infrared or UV light type instrument
· extractive with infrared or UV light type instrument.
- Dust/Particulates
· across stack with visible light optical type instrument.
Electronic equipment, other than that forming part of an in-situ assembly associated with
stack measurements, should be located at ground level.
A complete Continuous Emission Monitoring System (CEMS) typically consists of a few of
the above memtioned analysers together with a central data acquisition system.
NOTE: The analysers shall be selected together with the sample take-off and sample conditioning system as
a total package by the selected analyser Vendor.

Sulphur
For measurement of sulphur in distillate fuels down to 25 ppm, an x-ray flourescence
principle should be used. For measurement of sulphur below 25 ppm, an ultraviolet
flourescence technique shall be used. The use of PGC with FPD technique for measurement of sulphur below 25 ppm

TOC in water
A distinction shall be made between the measurement of TOC (Total Organic Carbon) to
meet environmental regulatory requirements and the measurement to detect oil leakage
into steam condensate and/or steam system. For environmental considerations, the
requirements are to be followed as specified. For measurement considerations, the
technique selected shall be determined by the sensitivity and range of measurement
required. TOC should be measured by the UV oxidation technique as a first choice. The ‘hot catalytic
type’ shall be selected if the UV oxidation technique cannot assure the completeness of
carbon detected and the sensitivity required. Because of the high combustion temperature and the relatively slow cool-down of the oven section, hot catalytic type analysers shall not be used in hazardous areas, and should not be used in analyser houses (even if they are ventilated).
The typical usable range for TOC measurements is 0 mg/l to 50 mg/l or less. The lowest
practicable reading is limited to the background signal, which could be typically 2 mg/l for
tap water, or even higher in systems where corrosion inhibitors are applied.
NOTE: Compounds that have reduced oxidation efficiency with the UV oxidation technique do not normally occur in petrochemical processes. They would normally occur in minor concentrations compared to the bulk of hydrocarbons. Typical reduced efficiencies are in general better than 80 %. The overall error in measuring organic carbon is normally better than 2 % of the measured value.
The use of a variant of the PGC approach, stripping the hydrocarbons and detecting them
by means of FID, could be considered for the ppb range of measurement of light
hydrocarbon (Carbon < 12) contamination in water or steam samples. It shall not be used
for residual fuel contaminated samples.

Viscosity
For optimum precision, capillary-type viscometers shall be used.
Other measuring principles, e.g., the tuning fork principle, may be considered for
measurements on binary mixtures, or for applications requiring only a rough precision (5 %
of the range or worse) such as in assessing the pumpability of heavy fuel oil.

Water Analysis (Other properties)

Chlorine in fresh water or seawater
Residual or free chlorine should be stated. Different reagents are used for each species of
chlorine. The same polarographic principle of releasing ions from reagent reaction with
chlorine and measuring the current by electrodes in a flow cell should be used.

Ammonium ion in sour water
Instruments of the on-line filtration type should be used. Cross sensitivity to interferences
should be checked with vendors.

Alkalinity (m and p alkalinity)
On-line titration type analysers should be used.

COD analyser
On-line titration type analysers should be used.

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