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Sewage tank magnetic flap level gauge troubleshooting and solutions

Jan 10, 2019

Sewage tank magnetic flap level gauge troubleshooting and solutions

Abstract: The magnetic flap level gauge is a commonly used liquid level gauge in chemical plants. It is mainly used for on-site liquid level display. It can be used with sensors to transmit liquid level signals to the control room only, which is convenient for employees to operate. In a natural gas purification device, a sewage tank is used for separating light hydrocarbons and free water, discharging water into a sewage tank for sedimentation, and light hydrocarbons are pressurized by a pump, and the container is provided with two magnetic flap level gauges, which are in operation. During the process, the water level gauge display is inaccurate. Through analysis and on-site inspection, the cause of the fault was found and the problem was solved.

        In an oil and gas processing plant, the sewage tank is used to separate sewage and heavy hydrocarbons. The staff discharges the water into the sewage tank for sedimentation, and the light hydrocarbons are pressurized by the pump. During the operation process, the water level display is inaccurate, and the liquid level can not be automatically adjusted, which brings difficulties to the operation of the staff. It not only increases the labor intensity, but also causes the liquid level to be too low, resulting in the loss of light hydrocarbons. In the sewage tank, it pollutes the environment and creates a large safety hazard.

1 Principle and characteristics of magnetic flap level gauge

           Principle: The magnetic flap level gauge works according to the principle of buoyancy and the principle of magnetic coupling. When the liquid level in the measured container rises and falls, the float in the main conduit of the magnetic flap level gauge also rises and falls, and the permanent magnet in the float is transmitted to the field indicator through magnetic coupling, driving the red and white flip column to rotate 180°. When the liquid level rises, the column turns from white to red. When the liquid level drops, the column turns from red to white, and the red and white boundaries of the indicator are the actual height of the medium level in the container. Bit indication. Features: Suitable for level measurement of corrosive media. In addition to the on-site instructions, remote transmitters and alarms (control switches) can be equipped with complete detection functions. The instructions are novel, the readings are intuitive and eye-catching, and the direction of the indicator can be changed according to the user's needs. The measuring range is large, and is not limited by the groove height. The indicating mechanism is completely isolated from the measured medium, so the sealing is good, the reliability is high, and the use is safe. The structure is simple, the installation is convenient, and the maintenance cost is low (Fig. 1).

Sewage tank magnetic flap level gauge troubleshooting and solutions

2 Failure cause analysis

        The main causes of the malfunction of the level gauge are: sensor damage, float magnetic failure, float chamber jamming, float damage or inaccurate float density.

2.1 The sensor is damaged.

        By checking the magnetic flip column of the on-site level gauge and flipping the magnetic flip column through the magnetic block, the display value in the control room also changes uniformly, thus eliminating the damage of the sensor.

2.2 The magnetic failure of the float.

        By checking the magnetic properties of the float of the on-site level gauge, the magnetic block is slid up and down at the float, and the float is lifted and lowered to prove that the float has a magnetic force to eliminate this cause.

2.3 There is impurity or float damage in the float chamber.

        The magnetic flap level gauge was removed for inspection and the float chamber was cleaned and found to be free of impurities in the float chamber. When checking the float itself, it was found that the inside breakage hole was leaked into the water and slowly sank below. Contact the manufacturer to re-create a float and replace it, then restore the installation (Figure 2).

Sewage tank magnetic flap level gauge troubleshooting and solutions

After installation, the liquid level display still shows inaccuracy: the sewage level is controlled at 20% (the site is 28cm), one person is on site, one person is in the control room, one person is in the sewage pool, and when the liquid level reaches 28cm, there is no in the sewage tank. Light hydrocarbons are discharged. When the liquid level reaches 60cm, there is a slight light hydrocarbon outflow in the drain tank. We immediately close the regulating valve manually. It is suspected that the float is stuck, so that the real liquid level cannot be measured. The regulating valve is always in the open position, but the magnetic block is magnetic. The flap level gauge slides up and down, and the float follows the movement, which proves that the float does not live.

2.4 Float density is not accurate

        A transparent glass container was found, and the container was filled with light hydrocarbons sampled in half of the sewage tank, and half of the water was sampled into free water sampled by the water level gauge, and after being settled, the layers were floated, and the float was placed in a glass container. The position of the float is located at the hydrocarbon part above the boundary. We judge that the density of the float is too small, resulting in a high water level measurement. After the regulator is fully opened, the light hydrocarbons are lost, the density of the water is large, and the settling time is short. The density of water will be smaller.

3 Solutions

        The manufacturer measured the original float ball and estimated the density value by calculation (m=ρV). The density is about 0.90g/cm3, which is the same as the design value. After the communication research by the manufacturer, the guess may be due to the natural gas gas group. The fractions were changed. The density of the free water and light hydrocarbons after sedimentation for a period of time were measured by a density meter, and the densities were 0.98 g/cm3 and 0.88 g/cm3, respectively, which were 0.05 g/cm3 higher than the design value. The density of the medium is changed. The density of the float is not applicable to the two media. The float should be remade. The density is taken as the middle value of the two measured values. The final density value is 0.93 g/cm3. Make a new float recovery installation based on the results. After checking again, the liquid level is normal and the water level in the sewage tank can be adjusted automatically.

4 Conclusion 

        Through analysis and investigation, the cause of the problem was found to be the change of natural gas composition, which changed the density of hydrocarbons and water in the sewage tank, and finally the density of the float was not applicable to the two media. The magnetic flap level gauge showed Inaccurate. In order to ensure the true and accurate measurement values, the oil and gas processing equipment should sample and test the medium components every year. Once the composition changes greatly, the density of the float should be adjusted at any time.