The D-26 liquid level measuring instrument LT-13 is an E+H coaxial rod guided wave radar level meter, the model is FMP40-1LL2CRJB21CA, the measuring range is 0~1700mm, the measuring medium is C5- distillation. Medium density 546.4kg/m3, operating pressure 1.0MPa(G), operating temperature 44℃. One day, the operator found that the LT-13 displayed 0mm while operating in the DCS system, called the historical trend curve of the table, and the liquid level measurement value of the LT-13 suddenly decreased from the normal value of 1102mm to a vertical state to near 0mm level.
Iii. Reason analysis
(A) Process inspection and analysis Process operators to the site to check the D-26 tank glass plate level meter measured value of about 1100mm. The instrument maintenance worker goes to the site to check that the display of the table transmission unit is normal and there is no fault alarm, and then the table is restored to normal after low emission treatment. Guided wave radar liquid level meter is measured according to the principle of reflected wave received after the transmitted radar wave reaches the liquid level. Because the dielectric constant of different liquids is different, the intensity of the reflected radar waves is also different. Because there is water at the bottom of D-26 tank, and the dielectric constant of water is larger than that of C5-distillate, that is, the reflected wave intensity of water is greater than that of C5-distillate, LT-13 directly measures the liquid level of water and ignores the liquid level to be measured. 1. Direct cause: the technical personnel did not dehydrate the D-26 tank in time, resulting in water at the bottom of the tank, and the process medium formed two phases of oil and water. The guided wave radar level meter directly detected the reflected wave of water, and the LT-13 showed 0mm. Indirect cause: the probe rod of guided wave radar level gauge is too long. D-26 tank level meter LT-13 outer cylinder flange spacing is 1700mm, the length of the probe rod of the liquid level meter is 1900mm, the bottom end of the probe rod extends beyond the lower flange into the bottom area of the outer cylinder. When the bottom of the tank is filled with water, the bottom of the outer cylinder is also filled with water, and it cannot be discharged through the lower flange. Management reason: the technical personnel did not drain the D-26 tank regularly according to the operating procedures. Instrument personnel installed guided wave radar level meter is not standard, did not choose the length of the probe rod according to the actual measurement range.
4. Rectification measures
1. The technical personnel should strengthen the monitoring of the liquid level of the D-26 tank water bag and drain the D-26 tank regularly. 2. Instrument personnel shorten the length of the LT-13 probe rod and ensure that the end of the probe rod is at the center of the lower flange of the outer cylinder.
1. Guided wave radar level meter is an advanced level meter commonly used in recent years. Compared with traditional float level meter and double flange level meter, it has the advantages of high measurement accuracy, small maintenance and large application range. At the same time, we can see from this case that any advanced measurement technology has certain limitations. 2, the use of any measuring instrument is closely related to the operating conditions of the process, when the process conditions or parameters change, the instrument may no longer be applicable to the current condition. 3, instrument installation status directly affects the accuracy of measurement, standard installation is crucial to the use of instruments.