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Design and Realization of liquid level detection in super heavy oil storage tank

Time: 2012-10-08 08:54:48 source: Author:

Abstract: super heavy oil medium has unique physical properties, and its viscosity is high at room temperature, and its production process needs to be operated at higher temperature. Because the operating temperature is high and the density of super heavy oil is close to that of water, reliable and practical measurement method should be adopted in liquid level measurement. This method usually includes microwave method, ray method, radio frequency admittance type, differential pressure type, etc Radar level gauge Etc.


1. Preface

At present, the detection methods of super heavy oil level in storage tank include microwave method, ray method, capacitance radio frequency admittance type, differential pressure type, radar level gauge, etc. Why not talk about it here Magnetic level gauge This is because the magnetic flap level gauge is not suitable for the medium with high viscosity. The price of X-ray measurement instrument is relatively high, because its measurement principle is suspected of radioactivity, there are certain hidden dangers in operation and use; the price of microwave method measurement instrument is relatively high, and it is easy to be interfered by "water mist" (for example, radar level gauge, Ultrasonic level gauge )It is easy to cause "false liquid level" phenomenon, which brings inconvenience to production operation; the price of capacitance RF admittance measurement instrument is relatively high; and the differential pressure measuring instrument which is not affected by price and production conditions is more suitable for the practical application of ultra heavy oil production.

2. Design basis of differential pressure measuring instrument

The principle of liquid level measurement by differential pressure method is to detect the pressure difference formed by the static pressure of liquid at two points in the longitudinal direction. The differential pressure measurement in engineering is to sense the sensing diaphragm by detecting the missing, and then convert the mechanical action into electrical signal to the amplification components of the instrument to form the electrical signal and transmit it to form the differential pressure measuring instrument. The measurement accuracy (accuracy) of the current differential pressure measuring instrument is usually no less than 1.5 level, or even higher.

The accuracy is as follows: 1. Datum, standard; 2. First class standard; 3. Second class standard. The accuracy grade used in gongchengguan production is generally no less than the first class standard, which can meet the stability of production and operation. Of course, the higher the measurement accuracy, the more accurate the maintenance parameter display of the project, except for the specific and engineering requirements belonging to the measurement level. For the production of super heavy oil medium in stations and plants, it belongs to the engineering production operation of non metering level. The measurement accuracy of liquid level detection is not lower than the second class standard, which can meet the needs of engineering production operation management.

Due to the special physical characteristics of super heavy oil, the operation temperature of super heavy oil production is between 130-200 degrees; another special physical property is that its density is very close to that of water. According to the principle of the same static pressure produced by the liquid at the same height in the longitudinal direction, because the density of super heavy oil is close to that of water, the hydrostatic pressure generated is relatively close. For the medium of super heavy oil with water cut, the density of the whole liquid has no change. The differential pressure measurement is used as the design basis for the ultra heavy oil level detection in the container.

3. Design method of differential pressure measurement

According to the principle of the same pressure formed by pressure acting on the same height, it is used as the benchmark for measuring the liquid level of super heavy oil medium in the oil tank. The measurement method is shown in Fig. 1. The "low pressure point" in the figure is the lower liquid level or below that the production separator can undertake production, which is the benchmark requirement of differential pressure measurement; The "high pressure point" refers to the higher liquid level or above that the production separator can undertake the production. The high-level point is for Fig. B. the high-level pressure taking positions in Fig. A and Fig. B are different, which are mainly determined according to the actual situation of the equipment and process. The pressure tapping position in Fig. A is a suitable position on the top of the three-phase separator, which does not affect the installation and operation of process specialty and other specialties, It also has space for installation and maintenance.


There are two installation methods of differential pressure measuring instrument: one is flange type with pressure pipe connection, which can be directly connected with prefabricated flange and flange stop valve on equipment; the other is with threaded connection hole, which is connected with prefabricated pressure joint and threaded stop valve on equipment through pressure pipe.
Using differential pressure to measure the liquid level of super heavy oil (water bearing) in production separator, due to the difference of oil and water density, the influence of density difference on measurement accuracy is: the absolute density error (1.036.3-1) = 0.0363 (g / cm3)
Generally, the oil-water ratio of three-phase separator (or two-phase separator) in super heavy oil production is 5:1 (large water content). Under this condition, for differential pressure measuring instrument, even if the measurement accuracy is not very high and the accuracy is 1.0, the larger measurement error caused by density difference is as follows:
0363 / 6 x 100% = 0.605%
It can be seen that the influence of density is far lower than the measurement error [accuracy] of the instrument itself, which is completely acceptable compared with the measurement accuracy of level 1.5; for the production operation at station level, it will not affect the operation requirements of process production and equipment operation.
Compared with other kinds of measuring instruments, the differential pressure method has the following advantages: 1
(1) it is not affected by the phenomenon of "water mist";
(2) it can adapt to the production medium with higher temperature;
(3) the one-time investment is low and the maintenance is very common.

4. Application suggestions
(1) the influence of ambient temperature should be considered in instrument installation.
(2) the low pressure point must be deep below the lower production liquid level of the medium.
(3) the installation position of the instrument shall be suitable for convenient operation, maintenance and repair, and the migration after installation shall be considered.
(4) the instrument should not be too large.

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