2. Problem Background LPG No. 3 Branch Company is located at Banchang Road, Dagang District. It has long been responsible for the delivery of 80 kilometers of liquefied petroleum gas pipelines and a small amount of cylinders and cans. The first branch of Liquefied Gas is located in the south of Wangzhuangzi Village in Xiqing District, and is responsible for the gas business of some 100,000 users and some industrial and commercial households in the city and its suburbs. The measurement of the level of liquefied petroleum gas (LPG) storage tanks has always been carried out for the first and third branches of the company through the 80-km high-pressure pipeline for transporting liquefied petroleum gas. The method uses the measurement method of a magnetic flap level gauge. The magnetic flip level gauge is based on the principle of magnetic coupling. The instrument and the tank are connected to form a communicating device. The magnetic float is arranged in the instrument. To ensure the accurate display of the liquid level, the specific parameters of the magnetic float are designed according to the density of the medium in the container. A display is provided on the outer surface of the instrument connection tube, and the display is equipped with an external magnetic flap. When the magnetic float floats up and down with the change of liquid level of the tank in the connecting pipe, the magnetic flap of the display also flips and changes the color. The red display liquid phase and the white display gas phase at the junction of the red and white colors. The actual level in the tank. Such level gauges and metering methods have been used up to now, but we have tracked them over a period of time. It was found that the reading was based on the reading of the magnetic level gauge and the result was not reliable. For example, a 1000M3 liquefied petroleum gas tank, the data read at different times, the calculated results sometimes (especially summer) a difference of about 10 tons. See data 1:
date
Can number
Tonnage of tank level in the afternoon of the same day
Tank level tonnage on the following morning
Height
Tonnes
Ambient temperature
Height
Tonnes
Ambient temperature
4.24
8#
4. 9l
198
29°C
4. 8l
l 9l
24°C
5.20
l#
4.35
L62
3l °C
4.25
L55
27°C
5.28
l#
4.18
l 5l
27°C
4.12
147
24°C
5.29
L0#
4.58
176
33°C
4.50
l 7l
30°C
7.26
9#
4.89
L97
32°C
4.79
L90
28°C
10.17
11#
4.38
L64
22°C
4. 3l
L59
l 8°C
Third, the problem analysis to 1000M3 (diameter 12.3m) as an example. We know that the liquid expands when it is heated and its density becomes smaller. On the contrary, when the liquid is cooled, its volume will decrease and the density will increase. Especially when there are many liquids in the tank, when the ambient temperature is higher than the temperature inside the tank, the tank absorbs heat, and when the ambient temperature is lower than the temperature inside the tank, the tank releases heat, according to observation and observation of more than one year, In other words, the temperature inside the tank does not change as much as the ambient temperature. See data 2:
date
Ambient temperature
Tonnage of tank level in the afternoon of the same day
Tank level tonnage on the next morning
Height
Tonnes
Tank temperature
Height
Tonnes
Tank temperature
5.14
2# 25°C
3.27
98.6
23°C
3.25
97.53
22°C
5.14
l# 23°C
3.13
91.17
23°C
, 3.13
91.17
2l °C
5.16
l#l 6°C
6.37
297.8
l 5°C
6.37
297.8
l 4°C
5.17
l#l 7°C
5.77
265.4
l 6°C
5.77
265.4
l 6°C
5.17
2#l 9°C
3.16
92.75
l 8°C
3.16
92.75
l 7°C
5.18
l# 20°C
5.07
208.9
l 9°C
5.02
205.6
20°C
6.03
8# 27°C
6.25
289.5
25°C
6.19
285.4
25°C
Date Tank No. Day Afternoon Storage Tank Level Tonne Number Day Morning AM Tank Level Tonnage Tg High Tonnage Ambient Temperature Ruler High Tonnage Ambient Temperature 4.248#4.9l19829°C 4.8ll9l24°C
5.20l#4.35l623l°C4.25l5527°C
5.28l#4.18l5l27°C4.1214724°C
5.29l0#4.5817633°C4.50l7l30°C
7.269#4.89l9732°C4.79l9028°C
10.1711#4.38l6422°C4.3ll59l8°C
Date Ambient temperature Stock level Tank tonnage Day after morning Tank level Tonnage feet High tonnage Tank Temperature scale High tonnage Tank Temperature 5.142#25°C 3.2798.623°C 3.2297.5322°C
5.14l#23°C3.1391.1723°C, 3.1391.172l°C
5.16l#l6°C6.37297.8l5°C6.37297.8l4°C
5.17l#l7°C5.77265.4l6°C5.77265.4l6°C
5.172#l9°C3.1692.75l8°C3.1692.75l7°C
5.18l#20°C5.07208.9l9°C5.02205.620°C
6.038#27°C6.25289.525°C6.19285.425°C
The table shows that the tank temperature changes more smoothly. The diameter of the connecting pipe connecting the tank gauge to the tank is small (DN50). The amount of liquid in the pipe is small, which is greatly affected by the ambient temperature. Especially when the connecting pipe is in the direct sunlight position, the temperature change is greater. That is, the density of the liquid in the tank differs from the temperature of the liquid in the communicating pipe with the liquid level gauge. It is generally considered that the liquid level in the communication pipe and the liquid level in the tank should be at the same level, but it is not necessarily true. We have done an experiment. When the outdoor temperature is 30°C, the connecting tube temperature is 30°C, and the temperature in the storage tank is 22°C. Especially because the expansion coefficient of the liquefied petroleum gas is much larger than that of water, the difference in density is Bigger. At this time, the relationship between the liquid in the storage tank and the liquid in the communication tube is the same as that of the U-type differential pressure meter. The forces of the liquids on both sides are equal to the horizontal sections at the bottom of the connection tube. Assume that the liquefied petroleum gas component in the storage tank is: 1. Propane: Butane = 50:50
2. Average temperature of liquefied petroleum gas in storage tank T°C
3, the average temperature of the liquefied petroleum gas in the connecting tube t °C
4. The liquid level in the tank is H(m)
5. When the liquid level in the connecting tube is h(m)10°C, Ïc is 10=0.516
Ï ä¸10=0.570
40 °C Ï C 40 = 0.469
D 40 = 0.532 Ï mix density: Ïш10 = ΣÏiVi-0.516 × 0.5 + 0.570 × 0.5 = 0.543Ïш40 = ΣÏiVi = 0.469 × 0.5 + 0.532 × 0.5 = temperature coefficient of the liquid density is 0.501: v = (Ïш40 -Ï 10 10)/(40-10) = (0.501 - 0.543)/(40 - 10) = -0.0014 The density at a certain temperature t °C is Ït = Ï 10 + v (t - 10) The height level of the communicating pipe When h, the pressure at the bottom of the communicating pipe is: P = P0 + Ïqh At any moment, the pressure generated in the connecting pipe and the liquid column in the tank on the horizontal section of the connecting pipe must be equal, then: P0+Ïgh=P0 +ÏvgHH=Ïth/ÏT=(1-v(Tt)/(Ï10+(T-l0))) h Formula-l1 When the ambient temperature is higher than the temperature in the storage tank, let t=30°C, T=20°C, From the formula -l can be obtained H = (1 + 0.0014 × (20-30) / (0.543-0.0014 (20-10))) h type -22 when the ambient temperature is lower than the temperature in the tank, set t = 10 °C, T=20°CH=(1+0.026)h=1.026h The above formula-3 shows that there is a gap between the liquid level reading of the liquid level gauge and the actual liquid level in the tank, and the larger the temperature difference, the greater the difference is. The bigger, so that the level meter reading h directly check the tank volume table, the calculated mass results Inevitably inaccurate, and the greater the volume of the tank, the greater the error, especially when the liquid level is just near the equatorial belt of the tank, the error is greater, thus confirming the previous data changes observed. Taking a 1000M3 spherical tank as an example, regardless of the influence of gas phase, set the reading of the level gauge to h=7m, set the liquid temperature in the level gauge to 30°C, and the average liquid temperature in the tank to be 20°C, according to the formula-2, to obtain the tank The internal liquid level is: H=0.974h=0.974×7=6.82m. The tank volume is checked and the volume of the liquid obtained is: 586.928m3 and 565.873m3. Mixing density at 20°C in the tank: Ï20=Ï10+v (T -10) = 0.531-0.0014 (20-10) = 0.529310.48 tons and 299.347 tons, then 11.13 tons more, and vice versa.
Fourth, the solution to the problem According to the above analysis of the situation, the magnetic flap level gauge measurement errors do exist in the measurement, to solve the problem can be used:
1 Install the magnetic flip plate liquid level meter to install sun protection device, choose a good reading time, can overcome the impact of environmental factors on the reading of the liquid level gauge to a certain extent, choose when the temperature is close to the internal temperature of the tank, such as summer in the day Before reading out, reading in the winter around 9:00 am, but this method has a certain impact on the production and operation;
2 Before reading, the liquid in the liquid level gauge shall be completely discharged, and the new liquefied petroleum gas shall be changed into the tank. The liquid level data shall be read as soon as possible. This method is not safe.
3 The float level meter is used to measure the level gauge. According to the principle of force balance, when the liquid level rises, the tension of the steel belt decreases and the tension of the steel belt decreases. The system balance is destroyed, and the torque of the coil spring as the source of force balance is reduced. Rewinding is done accordingly, and the loosened steel strip is wrapped around the steel pulley to tension the steel strip so that the system is balanced and the liquid level is lowered. A very uniform hole is made in the steel strip. When the strip moves up and down, its hole just engages with the teeth on the sprocket and drives the gear and pointer on the meter head to display the liquid level. Due to the problems in manufacturing technology and installation quality, at present, most of the domestic steel belt level gauges are ineffective. When the liquid level in the tank is very low, the high-speed liquefied petroleum gas can easily make the belt liquid. The buoy and its guide wire of the gauge system fluctuate greatly. Moreover, different components of the liquefied petroleum gas and different temperatures, the density is different, resulting in liquid buoyancy of the buoy different, there is an error, affect the accuracy of the level gauge display.
4 In order to better and scientifically configure the metering device and safeguard the interests of the company, we also installed a US-made DS300S/56SU mass flow meter on a branch company based on the US-based mass flowmeter manufactured by the Sankei Company. The flow meter is composed of three parts: sensor, transmitter and display. The device has functions of pressure, temperature and density compensation. The accuracy of the flow meter can reach ±0.2% and the accuracy of the system can reach ±0.35%. It is through the transmission of liquid flow by The sensor transmits the collected data signal to the transmitter, and the transmitter analyzes and adjusts the collected data to obtain a high-precision mass flow signal, and displays the flow rate and the tonnage through the display. After a period of operation, the errors of the two units of measurement data are basically controlled below 0.6%. See Data 3: During operation, the mass flow meter zero drift occurs, the tracking data collected by the observation, the density value at 0.54 t / m3 or less, a flow meter began to zero drift, analyzed to verify the existence of the pipeline liquefied petroleum gas gas. How can we solve this problem, we consider the location of the flowmeter play a decisive role, meter installation must be lower than the height of the highest point of the process pipe, the only way to ensure that when the meter is full of non-working, avoid liquid pipeline Vaporization occurs and the zero drift of the flowmeter occurs.
In summary, mass flowmeters are used to avoid errors due to the effects of temperature, pressure, and density. At the same time, the safe operation of storage tanks is still inseparable from the float level magnetic flap level gauges. Ensure safe operation and ensure accurate measurement.
date
Three company flow meters
A company flow meter
Actual error
System error
04.5.1
L52.57
L51.72
0.85
±1.07
04.5.2
173.98
175.69
-1.7l
±1.22
04.5.9
L46.3
L45.65
0.65
±1.02
04.6.16
301.47
299.4l
2.06
± 2.1l
04.5.2 l
310.4l
3l 0 .7l
-0.3
±2.17
(This article is published by Runzhong Instrument Technology. If you encounter any problem in the process of using the instrument, you can call our national sales service hotline: we will have professional staff to answer questions for you.)
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