Method of measurement of dew point on base of water content of in natural gas

FIELD: measurement technique.

SUBSTANCE: humidity of natural gas is measured by means of dewpoint hygrometers. According to method, hydrate formation onto mirror is prevented due to introduction of vapors of fluid to gas coming for analysis into dewpoint hydrometer. Freezing point of fluid is lower than -80°C and tangent of angle of loss is small at measurement of dew point temperature by means of SHF/EHF hygrometer. Amount of vapor is measured from reduction in temperature of hydrate-formation at 8-10°C in relation to overload capacity of dew point temperature for tested gas. Methyl, ethyl, propyl alcohols or acetone are taken as fluid.

EFFECT: reduced systematic error; prevention of hydrate formation.

2 cl, 3 dwg

 

The invention relates to the field of measuring technique, namely the measurement of moisture in natural gas.

One of the most common ways of measuring the moisture is condensation [see Caliph A.L., Turevskiy E.N., Roly-poly CENTURIES, Sugars VE, the machine P.I. Instruments for determining the moisture content of natural gas): IDC Gazprom, 1995, 45 S.]. The analyzed gas is passed over the ever-cooled metal mirror. At the same time control the state of the surface of the mirror and measured its temperature So At a certain temperature T=Tpthe surface of the mirror begins to sweat it condensed droplets of water - falls the dew. Temperature Tpcalled the dew point temperature (TTR), is associated with a moisture content of gas and is a measure of its moisture.

Devices based on the described principle, called condensation hygrometers. Monitoring of the surface is visually or in more sophisticated devices use optical devices (light-emitting diodes as light sources and photocells as receivers) [A. M. Derevyagin, Mironov SURDS test and calibration equipment for measuring the moisture content of natural gas. In Proc. "On scientific and technical problems of the transition to settlements for natural gas delivered by it in the energy performance". - M: IDC Gazprom, 1997. - p.36-43], or using radio waves SHF/EHF bands [see RF patent № 2178881, IPC G01N 22/04]. When this occurs, the amount of light reflected from the mirror, and TTR is measured at the beginning of the reduction of this thread due to scattering of the condensate film for optical hygrometers or absorption of radio waves in the case of a registration TTR performed using SHF/EHF-hygrometers.

The main normative document of the gas industry in the field of gas dehydration OST 51.40-93 provides a cool zone deep drying to TTR=-10°in the summer, and to TTR=-20°in winter [OST 51.40-93. Gases flammable, natural, supplied and transported through pipelines]. On the cold surface of the pipeline in addition to water in the liquid phase can fall frost (ice) and hydrates. It is believed that during slow cooling the condensing surface in the environment of natural gas at pressures 5÷10 MPa on it hydrates are formed (that's the way they grow on the inner walls of pipelines). It is assumed that during rapid changes in surface temperature (as is the case in the condensation hygrometers) hydrates do not have time to form and the mirror remains clear until TTR achieved on the water. Then it rolled supercooled droplets of water, which is then freeze and turn into ice, and then in Hydra is you [Istomin V.A. The problem of providing quality parameters of natural gas and equilibrium hydrocarbon systems with water phases. - M: IDC Gazprom, 1999, 78 S.].

Practice, however, shows [I. Moskalev. Bloomeria natural gas: the future development. Gas industry No. 1, 2001. - s.43-46]that the measurements in gases with a high level of vapors of heavy hydrocarbons is often observed the process of hydrate formation during the measurements (5÷10 min). The water on the mirror hygrometer can simultaneously be both in liquid and in solid phases (ice, hydrates). As a result of direct and reverse phase transformations liquid ↔ ice ↔ hydrate change optical properties of the film of moisture condensed on the mirror hygrometer, resulting in oscillations of the reflection coefficient, i.e. the fluctuations of the light flux.

Especially large errors due to this effect is inherent in the condensation hygrometer using radio waves SHF/EHF bands, selected as a prototype [see RF patent № 2178881, IPC G01N 22/04]. The method of determining the dew point of water in natural gas with high content of vapors of higher hydrocarbons is based on measuring the temperature of the cooled mirror at the start of the condensation of water vapor, while monitoring the status of the mirror surface is carried out with the help of the radio waves SHF/EHF bands. The operation of the hygrometer used in this method is based on the effect of the strong absorption of radio waves centimeter (MW) or millimeter (EHF) bands with a film of liquid water. However, if the water freezes, it turns into ice or hydrates, it takes crystalline ordered structure that is highly transparent to radio waves of these ranges, i.e. done for them radiotransparent. Thus, at temperatures mirrors below TTR and below the temperature of condensation on the mirror can happen two processes - the accumulation of water (in liquid phase) by the condensation of vapor from the gas and the reduction of water (in liquid phase) by moving it into ice or hydrates. The first leads to a decrease in the signal from the detector, and the second is to grow. Hence, when such a situation is realized in practice, the real rate of condensation, measured condensation EHF/SHF-hygrometer will be underreported by an amount which is not controllable, since it depends on the ratio between the liquid and solid phases of water at the time of measurement, which is constantly changing.

Thus, the condensation rate will be measured with an undefined error. This is a shortcoming of the method.

Solved technical problem is preventing condensation on the mirror condensation hygrometer when determining TTR on the water with the go through in cases when the conditions of operation of the hygrometer is such that condensation on the mirror, the formation of ice or hydrates.

The problem is solved as follows.

In the method of determining the dew point of water in natural gas, based on the control state of the surface of the mirror using radio waves SHF/EHF bands in the region of TTR values, where the formation of ice or hydrates, what's new is that in the flow of natural gas into the hygrometer, add a pair of fluid with a low freezing point and a low value of loss tangent at microwave/millimeter-wave range, shifting the temperature of formation of ice or hydrates in the region of temperatures below TTR analyzed natural gas and practically does not affect the measured TTR.

Set minimum flow of vapor of the liquid, lowering the temperature of hydrate formation on 8-10°below With a valid TTR for the test gas. As the liquid can be used in different alcohols: methyl, ethyl, propyl, etc. and acetone, diethyl ether and other organic liquid, soluble in water and having a freezing temperature t<80°C, and the tangent of the loss angle tgδ<10-2in the working frequency range.

The method is illustrated in the drawings, figure 1-3.

Figure 1 shows the device, implement the its way and creates a pair of fluid, which prevents hydrate formation, as well as the scheme of supply of the vapor in hygrometer. Figure 1 shows: 1 - the pressure vessel; 2 - liquid to prevent condensation; 3 - heater; 4 - temperature sensor wall of the vessel 1; 5, 6 - needle valve; 7 - supply tube of the test gas from the main pipeline; 8 - hygrometer; 9 - rotameter.

Figure 2 shows: a curve of 10 - temporary temperature variations (T,° (C) condensing surface hygrometer in curve 11 - temporary speed signal detector hygrometer (Uq), t - time.

Figure 3 shows a curve 12 - temporary speed signal detector hygrometer at a temperature T2below TTR on 8÷10°in the absence of hydrate formation; g - curve 13 is the same, but when in the moment of time t=t2begins to freeze water film or to form hydrates; d - curve 14 is the same, but when in the analyzed gas is added pair of fluid 2.

Traditionally, the determination of TTP in the absence of ice or hydrates as follows. With the help of valve 6 installed gas flow through the hygrometer 8; put the hygrometer in a mode in which the temperature of the condensing surface, starting from time t=0, starts slowly decrease, as shown in figa curve 10. The gas flow measured by the flowmeter 9. Observe the change in the signal from the detector hygrometer - curve 11 (figb). When the temperature drops below the dew point - Tpon the condensing surface of the hygrometer will be deposited droplets of water. The signal from the detector Uq, celebrating the control device of the hygrometer will start to diminish curve 11 (figb). Processing mathematically curve Uq(t), it is possible with good accuracy to find the start of the change (break) features Uq (T), i.e. the dew point temperature of water Tp.

The proposed method is designed to determine TTR in the conditions of formation of ice or hydrates and is implemented as follows. Pre-determine the presence of a solid phase condensation on the mirror. For this purpose, the purified condensation from the surface of the deposited moisture, warming it up to 40÷60°and install on it the value of temperature T=T2below expected (valid) Tpon 8÷10°: T2≈Tp-10°C. Supporting further selected constant temperature (T2=const), again observe the progress of the signal from the detector hygrometer in time (figure 3). Since T2lying on 10°below Tpthen on the condensation surface abundantly sits down dew. If water remains in the liquid phase, then over time its thickness of the film increases, and the signal Uq falls monotonically curve 12 (pigv). If it starts to form ice or hydrates, the turn signal is and the detector becomes non-monotonic (curve 13, Figg).

In case of detection of the phenomenon of transition from water to ice or hydrates, the tube 7, the feed gas for analysis, add a pair of liquid 2. The amount of added steam can be set using the needle valve 5 and the heater 3. The temperature control fluid is conducted using a temperature sensor 4. These regulations establish minimum flow of vapor of the liquid, reducing the temperature of hydrate formation on 8÷10°below With a valid TTP. For this purpose, keeping the temperature of the vessel 1 constant, slowly open the valve 5 and observe the behavior of the signal from the detector hygrometer Uq.

As soon as Uq will again fall (time t3on high), the valve 5 left in this position. If the fully open valve 5 hydrates are not destroyed, then the valve 5 is closed, raise the temperature of the heater. After some time, repeat the procedure described above, achieving break-up of ice and hydrates.

After the temperature of the vessel 1 and the valve positions are selected, they are fixed. Then the condensation sensor clean and carry out control measurements and get a monotonic decline with time (curve 14 on figd), which shows that the hydrates or ice on the sensor are not formed. Selected thus the flow rate of vapor warrants hydratite higher temperatures, ie allows you to work confidently in the temperature range 2÷6°With lower TTR, where and are the main measuring the speed of condensation in order to compute Tp[see RF patent № 2189582, IPC G01N 25/66].

Since the film of the condensed vapors gidroresursami liquid 2, in principle, also can absorb light or radio waves, so it will also introduce error in the measurement of TTR (the difference between the curve 12 and the curve 14).

However, this effect is, first, small, and secondly, can be determined by preliminary measurements with a "dry" gas and taken into account in the final calculations TTP.

The method of lowering the temperature of hydrate formation when working SHF/EHF-hygrometer has been realized in laboratory conditions, and then tested when measuring TTR on underground gas storage in the village. steppe (Saratov region). As gidroresursami fluid was used ethyl spirtkombinate devices if this was a regular condensation hygrometer "Kharkiv IM" and the latest fiber optic device "Cong-prima 4" Russian company NPP "Vympel".

1. The method of determining the dew point of water in natural gas, including the measurement of the temperature of the cooled mirror at the start of the condensation of water vapor through condensation UHF/microwave hygrometer according to the second judge on the search parameter, characterized in that the natural gas flowing in hygrometer, add a pair of fluid with a freezing temperature t<-80°C and the tangent of the loss angle tgδ<10-2and the amount of vapor of the liquid determined to reduce the hydrate formation temperature for 8-10°With relative limit values for the TTR for the test gas.

2. The method according to claim 1, characterized in that as the liquid is chosen alcohols: methyl, ethyl, propyl or acetone.



 

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