GB/T 17283-2014Determination of the water dew point of natural gas -- Cooled surface condensation hygrometers (English PDF)

6327.1981 "Gas analysis - Determination of the water dew point of natural gas

- Cooled surface condensation hygrometers"; AND ISO 6327 had confirmed

this in 2011.

This standard has a greater structural adjustment as compared with ISO

6327.1981, AND in Appendix A it provides the clause number comparison list

between this standard and ISO 6327.1981.

The main technical differences between this standard and ISO 6327.1981 and

the causes are as follows.

- MODIFY the scope of application of standard; based on the scope of

application of ISO 6327.1981 "The water dew point generally ranges from

-25 °C ~ 5 °C", ADD that "Under special environment, the water dew point

range may be widened"; AND expand the scope of application of this

standard for the purposes of adapting to the actual conditions of the

natural gas production in China.

This standard was proposed by China National Petroleum Corporation.

This standard shall be under the jurisdiction of the National Natural Gas

Standardization Technical Committee (SAC/TC 244).

The main drafting organizations of this standard. China National Petroleum

Corporation Southwest Oil and Gas Field Branch Natural Gas Research

Institute, Chengdu Tianke Oil and Gas Engineering Co., Ltd.

2.1 Principle of the apparatus

With this type of apparatus, which determines the water content of a gas by

measuring the corresponding dew point, a surface (generally a metallic mirror),

the temperature f which may be artificially lowered and accurately measured,

is exposed to a sample of the gas being tested. The surface is then cooled to a

temperature at which condensation occurs and is observed as dew.

Below this temperature, condensation increases with time, whilst above it,

condensation decreases or does not occur. This surface temperature is then

(for practical applications) taken as the dew point of the gas flowing through

the apparatus.

2.2 Determination of water vapor pressure

The partial water vapor pressure in the gas samples is the saturated vapor

pressure corresponding to the observed dew point, provided that the gas in the

hygrometer is at the same pressure as the gas at the time of sampling.

REFER to the relevant manual for the relationship between saturated water

vapor pressure and temperature.

It shall be noted that if methanol is present, this method determines the

methanol in addition to water. However, if the methanol content is known, the

The manual device can involve observation of condensation with the naked

eye; if a photoelectric cell is used, the mirror is illuminated by a light source

built into the test cell. The lamp and photoelectric cell can be arranged in

various ways, provided that diffusion in the direction of the light source from

the mirror is reduced by the polishing of the mirror. In any case, the mirror must

be clean before use.

In the absence of any condensation, the diffused light falling on the photocell

must be reduced. The effects of light diffused from internal surfaces of the cell

can be reduced by blackening these surfaces AND this precaution can be

supplemented by an arrangement of the optical system so that only the mirror

is illuminated AND the photocell views only the mirror.

3.4 Methods for cooling the mirror and controlling its temperature

3.4.1 General

The following methods are used for reducing and adjusting the mirror

temperature. The methods described in 3.4.2 and 3.4.3 require constant

attention from the operator AND are not suitable for automatic devices. For

automatic devices, two cooling methods are used. indirect contact with a

coolant OR cooling by the thermoelectric (Peltier) effect as described in 3.4.4

and 3.4.5. In any case, the rate of cooling of the mirror shall not exceed

3.4.2 Solvent evaporation

A volatile liquid in contact with the rear face of the mirror can be evaporated

AND cooled by an air flow. Hand bellows are generally used for this purpose,

BUT an adjustable source of low pressure compressed air or any other

suitable pressurized gas is preferable. The liquid used can be ethylene oxide,

a very efficient liquid giving cooling of the mirror of approximately 30 °C,

without effort, when hand bellows are used. However, if toxicity is a risk,

acetone can be used to obtain cooling of approximately 20 °C with hand

bellows OR even greater cooling with compressed air or other suitable

pressurized gas.

3.4.3 Gas cooling by adiabatic expansion

The mirror can be cooled by discharging onto its rear face a gas which has just

expanded through a nozzle. Compressed carbon dioxide, available from small

cylinders, is often used for this purpose, BUT other gases such as compressed

air, compressed nitrogen, propane, or halogenated hydrocarbons can also be

used. Mirror temperatures of at least 40 °C below the gas sample temperature

can be obtained.

introduced into the test cell, can also induce an observed dew point different

from the dew point which would correspond to the actual water vapor content.

4.1.2 Solid impurities

If solid impurities are absolutely insoluble in water, they do not modify the

observed condensation temperature, BUT can hinder condensation

observation. In an automatic device, without a compensation device for such

impurities, these tend to obstruct the operation of the device if the amount of

condensate is low. Defects resulting from an excess of solid impurities on the

mirror generally result in an unexpected increase of the mirror temperature for

a few minutes AND call for dismantling of the device AND cleaning of the mirror.

(It is essential for this purpose that the hygrometeric cell can be rapidly

dismounted). It may be desirable to remove solid impurities by using a

non-hygroscopic filter to avoid such difficulties.

If a filter is used, even if it is stated to be non-hygroscopic, it shall be in

equilibrium water the water vapor content of the gas; this is obtained by

allowing gas to flow through it for a period of time before the test at a rate

considerably higher than that to be used during the test.

To prevent the influence of dust particles, some automatic devices are fitted

with a "calibration sequence". This consists of an optional superheating of the

mirror, so as to remove all condensate, water and hydrocarbons, followed by a

rebalancing of the measuring bridge.

4.1.3 Impurities in vapor form

Hydrocarbons can condense on the mirror. In principle, these do no interfere

because hydrocarbon surface tension is very different from that of water. They

spread on the mirror AND form a continuous layer which does not diffuse light.

Manual detection of condensates is, nevertheless, not easy because although

the dew point is very much lower than the condensation temperature of

hydrocarbons, only a few water droplets can be detected in a large

hydrocarbon...

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