224 5 Natural Gas Measurement Systems

5.4.5 Odorimetry and Leak Detection Measurement

This section has been split into two parts: odorimetry and leak detection. Firstly, odorimetry explains why odour is added to the gas and why accurate measure-ment of odour concentration is essential. Secondly leak detection, leading on from odorimetry, explains why measurements of the leak are carried out.

5.4.5.1 Odorimetry

Odorosity is an extremely important criterion of gas quality. The smell in gas is man-made since raw natural gas is odourless. It is added during the gas-manufacturing/gas-refining process so as to be able to detect it. The chemical is a mercaptan. It is important that the concentration levels of the odorant are correct and consistent across the country. If the odour is too weak, then gas leaks may go undetected, and if too strong, then even the minutest of leaks will be detected miles away. One drop of mercaptan can be detected up to three miles away. For this reason, it is crucial that accurate measurements of odori-sation are made. Figure 5.52 shows the schematic features of an odorisation chambers.

Odour can be measured using either subjective or analytical methods. Analytical methods work by tracing the sulphur compounds contained in odorants. They use the instruments and techniques designed for measuring total sulphur and hydrogen sulphide to assess odorant concentration.

Subjective measurement of odour is carried out by trained personnel known as rhino-analysts. Rhino-analysts smell the gas and assess the odour concentration depending on the intensity of the smell and the odour assessment scale which they are using. There are various scales of odour assessment which are used within the global gas industry: the odorisity scale, the odour level and the odour intensity scale.

The instruments used for the analytical and subjective measurement of odour are detailed in the Odorimetry section of Table 5.23. Figure 5.53 also shows the typical British Gas odorimetry.

5.4.5.2 Leak Detection

Subsequent to the detection of a gas leak, measurements of the intensity of the gas leak must be made. Broadly speaking, this is carried out in order to track down the exact location of the leak and to assess the immediate danger pre-sented by the leak. Leak detection instruments are listed and detailed within the Odorimetry section in Table 5.23. Figure 5.53 also shows the Gasco Seeker, the typical flame ionisation and electron capture detection for gas leak detection (Fig. 5.54).

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5.4.6 Sulphur and Hydrogen Sulphide Concentration Measurement

In addition to water and oil removal, one of the most important parts of gas pro-cessing involves the removal of sulphur. Natural gas from some wells contains sig-nificant amounts of sulphur. Natural gas with a high sulphur content is commonly called sour gas because of the rotten smell caused by the sulphur.

Sulphur exists in natural gas as hydrogen sulphide (H2S), and the gas is usually considered sour if the hydrogen sulphide content exceeds 5.7 mg of H2S per cubic metre of natural gas. The process for removing hydrogen sulphide from sour gas is commonly referred to as sweetening the gas. Sour gas is undesirable because the sulphur compounds it contains can be extremely harmful, even lethal, to breathe. Sour gas can also be extremely corrosive. It is therefore essential that it is meas-ured and controlled. Please refer to the relevant section in Table 5.23 for details on sulphur and hydrogen sulphide concentration measurement devices. Figure 5.55

To silencer and extractor fan

To separate extractor fan

Slide valve inside chamber

C

A

B

Light

Light

E

F

D

Fig. 5.52 Ordorisation chamber. A Test room. B Air lock. C 4 mixing fans. D Carbon air filter. E Sample injection chamber. F Electric fan switches. G Odorant injection point

5.4 Gas Quality

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226 5 Natural Gas Measurement Systems

Table 5.23 Various gas quality instruments

Instrument Description, range and accuracy Advantages (A) and disadvantages (D)

Application

The Watson House Odorimeter (Odorimetry)

This is a subjective method whereby the measurements are made by rhino-analysts. Gas is introduced to a stream of pure air, which then passes through a funnel into the atmosphere. The observer smells the effluent from the funnel and then records his/her analysis which is recorded in accordance with the odorisity scale. Range: Flow range of 0.0021.5 ft/h and pressure range of 0.56 in water gauge

Cost, static nature of the system and training required for rhino-analysts

Gas industry and laboratory

British Gas Odorant Chromatograph (Odorometry)

This method is an analytical method. Sulphur contents are separated and passed into a hydrogen/oxygen flame, burning in a hydrogen atmosphere at the tip of a burner in a silica tube, any sulphur compound present forms species which emit a characteristic radiation in the violet and UV regions. Can determine individual sulphur compounds down to con-centrations of 0.01 ppm

Extremely accurate costly

Used in offtakes and pressure reduc-tion stations (PRS)

Gasco seeker (leak detection)

Portable device for measuring methane concentrations in gas /air mixture atmospheres. It uses a pellister (VQ3 Pellister) and Thermal conductivity detector as the second filament. A hand-operated aspirator is used to draw the test atmosphere. Range: +1 % LEL scale +1 % Gas scale +3 % reading

Gas detector is a reliable means of detecting flammable gas, and of measur-ing the percentage of lower explosive limit and the percentage by volume

Emergency gas engineers use this device on a daily basis to detect gas leaks

Must be regularly calibratedDoes not pinpoint the exact location of the leak

Flame ionisa-tion monitor (leak detection)

Effluent gas is passed into hydrogen/air flame in which two electrodes are placed with a poten-tial difference between them. The detector current passes through the high resistance creating a potential difference which appears across the output of the impedance converter and after suitable attenu-ation is measured by the recorder. Max leak sensitivity of 6

Extremely sensitive. Must handle with care

Used to detect high-risk main leakage in win-ter surveys

(continued)

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also typifies a gas pipe exposed to excessive sulphide gas concentration which led to drastic deterioration of the corresponding pipe over a period of time.

5.4.7 Component Analysis (Chromatography)

The field of component analysis is also very significant as the analyses have a bearing on the pricing structure for the natural gas. Component analysis or gas chromatography as it is often referred to is a physical method of separating the

Instrument Description, range and accuracy Advantages (A) and disadvantages (D)

Application

Electron cap-ture detector (leak detection)

Consists of an irradiation chamber and a pair of electrode. Eluted molecules with a strong electron affinity will capture electrons to form heavy, slow moving negative ions. The resulting decrease in the current flowing between the electrodes is measured. Range in sniffing mode is 1 10*-8 mls. when sampling 1 10*-9 mls

High sensitivity Used for leak testing new mains on air and pressure testing of vessels

The British Gas Sulphur Selective Detector

(Same as BG Gas chromatograph) sulphur contents are separated and passed into a hydrogen/oxy-gen flame, burning in a hydrogen atmosphere at the tip of a burner in a silica tube, any sulphur compound present forms species which emit a characteristic radia-tion in the violet and UV regions

A: good and accurate Gas industry and laboratoryD: Sensitive

The Barton sulphur titrator

Consists of three basic units, a titration cell, an electronic control circuit and a recorder. Bromine is generated by passing an electri-cal current through a suitable electrolyte solution. Compounds bubbled through the solution react with the reagent. The net generating current is thus directly proportional to the concentration of the compounds titrated and is continuously recorded. Range: 01,000 in 7 steps, Hydrogen sulphide (0.08 ppm H2S by vol), organic sulphides (0.16 ppm) and sulphur dioxide (0.16 ppm)

Sensitive Gas industry and laboratory

Table 5.23 (continued)

5.4 Gas Quality

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