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Research on temperature profile in a large scaled floating roof oil tank
Case Studies in Thermal Engineering 12 (2018) 805–816
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Research on temperature profile in a large scaled floating roof oil tank
T
Lin Yang, Jian Zhao , Hang Dong, Junyang Liu, Weiqiang Zhao ⁎
Key Laboratory of Enhance Oil and Gas Recovery of Educational Ministry, Northeast Petroleum University, Daqing, PR China
ARTICLE INFO
ABSTRACT
Keywords: Oil tank Test Temperature Total heat transfer coefficient
The temperature test system for the floating roof oil tank is established. Four different kinds of test tubes are invented to test the temperature profile in the oil tank. Based on the test system, the temperature profile of a 10 × 104 double-deck floating roof oil tank is investigated. There are three low temperature regions near walls in the tank, surrounded by these regions is the high temperature region in the center of the tank. The temperature distribution in different regions is presented in detailed. With the changing of ambient temperature, there is the corresponding temperature fluctuation of oil in the surface layer. With the ambient temperature increasing, the influence range decreases. And as the variation of ambient temperature increases, the temperature fluctuation of oil increases. Moreover, the heating process of oil in the tank by the entering oil is presented by the test data. Based on the principle of conservation of energy, the total heat transfer coefficient of oil tank on different working conditions is calculated with the value from 1.09 W/m2 °C to 2.32 W/m2 °C.
1. Introduction Precisely acquiring the temperature profile in the oil tank is of great concern for the storage and transportation of crude oil. Especially in the alpine region, the extremely low ambient temperature increases the cooling rate of oil as well as the risk of gelatinization. Many scholars carried out the research on the temperature field of oil tank during the storage process. Generally, there are the numerical simulation and test method executed on this research topic. Based on the numerical simulation method, Zhao [1] investigated the heat transfer characteristic of oil during the storage. The evolution of temperature field and the natural convection were presented. Zhao [2] investigated the temperature distribution and flow characteristic of oil during the short time cooling by Fluent. Lu [3] simulated the heating process of a 10 × 104 m3 oil tank by Fluent. Based on the thermoanalysis, the rationality of heat equipment was discussed. Li [4] presented a prediction model for the temperature field of a floating roof oil tank, and the influence from the solar radiation and insulating thickness on the oil temperature was discussed in his research. Cotter and Michael [5–7] employed a control volume finite difference method to simulate the heat transfer process of oil tank. Based on this method, the flow pattern and temperature distribution were discussed. The effect of heat transfer coefficient, aspect ratio and viscosity on the heat transfer was examined. Oliveski [8] presented a numerical and experimental analysis of the heat transfer process in a small tank containing oil. The temperature profile in the simulated tank was investigated. Zhao [9] studied the temperature dropping rule of crude oil by numerical simulation. The evolution of temperature and flow field was analyzed together. Although the numerical simulation method takes great advantage for the research on the heat transfer process of oil, the acquisition of the temperature data in a real oil tank is more important for promoting this research topic. Besides, testing the oil temperature under a real working ⁎
Correspondence to: Northeast Petroleum University, Fazhan Road NO.199, Hi-tech Development Zone, Daqing 163318, PR China. E-mail address: [email protected] (J. Zhao).
https://doi.org/10.1016/j.csite.2018.10.009 Received 21 June 2018; Received in revised form 19 October 2018; Accepted 22 October 2018 Available online 27 October 2018 2214-157X/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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condition is also an important way to verify the accuracy of numerical simulation. Therefore, some researches were carried out by the test method. Yu [10,11] developed a test system to investigate the temperature distribution in a 10 × 104 m3 floating roof oil tank. Based on the test system, the temperature profile rule along the axis direction and the radial direction was discussed. However, his research mainly focused on the oil tank which was in the static storage state. In the real storage course, there are many operating states of the oil tank. Besides, limited by the install condition, most of the temperature sensors were installed in the superficial zone of the tank. There was only one group of sensors installed in the gauge hatch, so the obtained temperature data were not enough to investigate the temperature profile and variation rule in the deep layer of the oil tank. The same method was also carried out by Li [12] and Wang [13,14]. However, the same defects were also existed in their research. Considering the information in the literature on this research topic, the aim of this work is to investigate the temperature profile in the floating roof oil tank under different working conditions. As carrying out the testing on a real oil tank is an effective and direct method to acquire the real temperature distribution data, the test method was adopted in this research. In order to reveal the temperature profile more comprehensively, the temperature sensors were placed on different positions including the superficial layer and deep layer in the oil tank, so the temperature profile along the axis direction and radial direction can be obtained together. Besides, the testing condition includes the static storage and sending and receiving oil process of oil tank, so the heat transfer characteristic of oil tank under different working conditions can be revealed. 2. Test object and system The temperature test system was installed on a 10 × 104 double-deck floating roof oil tank in Daqing oil field of China. The diameter of the tank is 80 m, the total height of the sidewall is 21.97 m. And the height of the gauge hatch is 24.4 m. There is the insulating layer on the sidewall with the thickness of 80 mm. As the roof is floatable, the space which the oil occupies is changeable. There are the inlet and outlet on the sidewall with the height of 1.5 m above the foundation of the tank. The temperature test system is constituted by five module, they are respectively the “Temperature test module”, “Data transmission module”, “Explosion prevention module”, “Data collection module” and the “Power supply module”. 2.1. Temperature test module The core of the temperature test module is the specially-made test tube. The test tube is constituted by a group of temperature sensors, the signal lines, and protection structure. Platinum resistor is used as the temperature sensor which has the accuracy of 0.1 °C. As the sensors directly contact to the oil, they are the sheathed thermos resistor which has the function of anti-explosion and fireproofing. In order to obtain the temperature profile along the axis direction of the tank, more than one sensor is placed along the test tube. The number and positions of the sensors are different for different kind of test tube. The test tube is divided into two parts. The upper part of the tube is a copper pipe which is used to protect and fasten the sensors. The sensors and signal lines are installed into the copper pipe. In the lower part of tube, each sensor and the signal line is sealed by an independent flexible pipe which is made by PTFE material. The structure of flexible pipe ensures the sensors flex flexibly. Near each sensor, there is a bob-weight which is used to locate the sensor in the certain position along the tube. The cross sectional drawing of test tube is shown in Fig. 1a: Based on the number and layout strategy of sensors, there are four kinds of test tubes with different length and structure. One kind is named as A-type test tube which contains sixteen sensors, and the test range is 17 m, while the total length of the tube is 19.61 m. This kind of test tube is used to test the temperature profile along the entire axis direction of the tank. The position of each sensor is listed in Table 1, the “distance” represents the distance from the sensor to the lower wall of the floating roof. As can be seen in Table 1, the arrangement of sensors is more concentrated in the upper part, thus the temperature distribution in the region near the floating roof can be tested more precisely. The second kind of test tube is named as B-type which contains six sensors, and the test range is 1.8 m, while the total length is 4.4 m. The position of each sensor is listed in Table 2. The third kind of test tube is named as C-type which contains sixteen sensors, and the total length is 24.4 m. The C-type test tube is installed in the gauge hatch of the tank. The test range is 17 m above the base wall of the tank. The position of each sensor is listed in Table 3. The arrangement of the sensors is more concentrated in the bottom, thus the temperature distribution in the region near the base wall can be tested more precisely. The test tubes are installed through the stanchion hole on the roof and contact to the oil. There are aggregately six groups of test tubes installed in the tank. Three are the A-type test tube, and two are the B-type test tube, while one is C-type test tube. The positions of different test tubes in the oil tank are illustrated in Fig. 1b. As shown in Fig. 1a, group 1, group 2 and group 3 are the A-type test tubes, group 4 and group 5 are the B-type test tubes, and group 6 is the C-type test tube. Group 1 is placed near the inlet of the tank, with respect to test the temperature profile along the axis direction under the influence from the entered oil. Group 2 is placed in the center of the tank in order to test the temperature profile on the axis direction. Group 3 is placed far away from the inlet of the tank. By comparing the test data from group 1, group 2 and group 3, the influence from the entered oil on the oil temperature in the tank can be investigated. Group 4 and group 5 are placed on different positions along the radial direction. By comparing the test data from group 1 to 5, the temperature distribution along the radial direction can be presented. Group 6 is installed in the gauge hatch of the tank, with the aim of testing the temperature profile near the sidewall. 806
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Fig. 1. a. Cross sectional drawing of test tube. b. Lay out of test tubes and the floating roof tank. c. Lay out of the data transmission lines. d. Photograph of the test system. e. Diagram of connection.
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Table 1 Layout strategy of sensors for A-type test tube. Number
1
2
3
4
5
6
7
8
Distance Number Distance
0.1 m 9 4.8 m
0.3 m 10 7.4 m
0.6 m 11 10 m
1.0 m 12 11.4 m
1.4 m 13 12.8 m
1.8 m 14 14.2 m
2.3 m 15 15.6 m
3.3 m 16 17 m
Table 2 Layout strategy of sensors for B-type test tube. Number
1
2
3
4
5
6
Distance
0.1 m
0.3 m
0.6 m
1.0 m
1.4 m
1.8 m
Table 3 Layout strategy of sensors for C-type test tube. Number
1
2
3
4
5
6
7
8
Distance Number Distance
7.5 m 9 21.6 m
8.5 m 10 22.1 m
9.9 m 11 22.6 m
11.3 m 12 23.1 m
12.7 m 13 23.5 m
14.4 m 14 23.9 m
16.1 m 15 24.2 m
21.1 m 16 24.4 m
2.2. Data collection module There is a data acquisition unit near each test tube to collect the test data of the sensors. The data acquisition unit was produced in Wuhan of China with the accuracy is 0.1 °C. And the maximal acquisition speed is 1 Hz. The data acquisition unit is placed on the external surface of the roof. The signal lines sealed by the explosion-proof hose are used to connect the test tube and the data acquisition unit. There is a CPU installed into the data acquisition unit. Thus the collection, dispose and transmission of data can be achieved fast and precisely. Corresponding to the test tube, there are totally six data acquisition units on the roof. Moreover, there is a main-control box placed in the controlling room far away from the tank. The test data from each data acquisition unit is gathered together and transmitted to the main-control box which directly connects to the computer. 2.3. Data transmission module There is the cable used to connect the data acquisition unit in the tank. In order to reduce the total length of the cable, the data acquisition unit is connected in the form of series. And the optimal arrangement of wire is used. Fig. 1c illustrates the lay out of the data transmission lines on the tank roof. Fig. 1d is the photograph of the test system on the tank roof. 2.4. Explosion prevention module The explosion prevention module is used to ensure the safety during the temperature test. Different measures are introduced to achieve this purpose. First, the explosion-proof hose are used to seal the cables to play the role of isolating the source of danger. Besides, the explosion-proof box is used to seal the data acquisition unit. With the addition of the intrinsically safe sensors, the safety of test system can get to the maximum extent. 2.5. Power supply module The power supply module includes the main-control box of power supply which is installed in the controlling room. This maincontrol box has the function of converting the voltage from 220 V into 24 V. And then the voltage of 24 V is transmitted to the nearest data acquisition unit on the tank roof by the electric wire which is also sealed in the explosion-proof hose. After that, the power is transmitted from the nearest data acquisition unit to the farthest one. Due to the loss during the transmission, the working voltage of the data acquisition unit is less than 12 V which is also a safeguard for the safety test. The diagram of connection for the temperature test system is shown in Fig. 1e: 3. Results and discussion 3.1. Test cases The temperature test on the floating roof oil tank lasts for nearly six months. The test conditions include different environment 808
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Table 4 Basic parameters of test cases. Case
Operating state
Ambient temperature (°C)
Time (Days)
Level (m)
Initial oil temperature in the center of the tank (°C)
Inlet flow rate (m3/s)
1 2 3 4 5 6 7
Static storage Static storage Static storage Receiving oil process Receiving oil process Static storage Static storage
− 20 (average) 0 (average) 15 (average) − 22 (average) 18 (average) 9.5–19.3 11.1–27.6
8 12 9 30 h 1 1 1
12.6 11.2 10.6 13.2 9.7 11.8 13.1
38.4 38.8 40.3 40.0 40.2 39.6 40.4
– – – 0.2 0.35 – –
temperature and operating state of the oil tank. In order to investigate the temperature profile in the tank under different working conditions, seven groups of test data were collected, the basic parameters of each case are listed in Table 4. 3.2. Cooling rate of oil under different conditions As can be seen in Figs. 2a and 2b, the cooling rate on different positions along the axis direction obtained by group 1 is nearly the same except for some test positions near the roof. As the ambient temperature changes during the day, the cooing rate of oil near the roof fluctuates during the cooling. The data from test positions in the center of the tank shows a consistent and uniform variation. Moreover, the temperature in the bottom part of the tank is different from that in the center of the tank. According to the test data from H = 0 m to H = 2.6 m in Fig. 3a and the test data from H = 0 m to H = 1.2 m in Fig. 3b, there presents a thermal stratification feature in the bottom part of the tank. With the decreasing of ambient temperature, the cooling rate increases. The average cooling rate of oil is 0.44 °C/day when the ambient temperature is − 20 °C, while the average cooling rate reduces to 0.29 °C/day when the ambient temperature increases to 0 °C. However, for the oil temperature near the roof, the difference of cooling rate under different
Fig. 2. a. Temperature variation as a function of time tested by group 1 from case 1 (near wall, static operation, ambient temperature − 20 °C). b. Temperature variation as a function of time tested by group 1 from case 2 (near wall, static operation, ambient temperature 0 °C). 809
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Fig. 3. a. Temperature profile tested by group 1 from case 1 (near wall, static operation, ambient temperature − 20 °C). b. Temperature profile tested by group 3 from case 1 (near wall, static operation, ambient temperature − 20 °C). c. Temperature profile tested by group 1 from case 2 (near wall, static operation, ambient temperature 0 °C). d. Temperature profile tested by group 3 from case 2(near wall, static operation, ambient temperature 0 °C). e. Temperature profile tested by group 1 from case 3 (near wall, static operation, ambient temperature 15 °C). f. Temperature profile tested by group 3 from case 3(near wall, static operation, ambient temperature 15 °C).
ambient temperature increases apparently. The average cooling rate of oil positioned at 0.1 m from the roof is 1.03 °C/day when the ambient temperature is − 20 °C, while the corresponding value decreases to 0.3 °C/day when the ambient temperature increases to 0 °C. The changing rate of oil temperature responding to the variation of ambient temperature reduces as the capacity of oil tank increases. Based on the test data, only the oil positioned within the range of 0.3 m from the roof is affected by the ambient temperature apparently. For the oil in the tank, the influence from the ambient condition gradually decreases as the distance from the top wall increases. 3.3. Temperature profile along the axis direction under different conditions According to the test data under different working conditions, except for some irregularity data which are caused by the 810
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fluctuation of environmental condition, there is the similar rule of the temperature profile along the axis direction of the tank. The temperature profile on the axis direction can be divided into three parts. Near the top wall, due to the heat loss from oil to surroundings, there exists a low temperature region. Although the surface temperature is not tested by the test system as the nearest sensor is placed 0.1 m away from the lower surface of the roof. According to Fig. 3a and the additional test carried out by the surface thermometer, the oil temperature near the top wall is the lowest in the axis direction. However, due to natural convection, this region occupies only a small range along the axis direction. Away from the roof, the oil temperature increases and becomes more homogeneous which reflects a high temperature region in the center part of the tank. Among the test data on different environmental conditions, the temperature deviation in this region is within 0.5 °C. In addition, due to the large dimension of the oil tank, it takes long time for the thermal disturbance spreading to this region, so the high temperature region always takes over the largest range of the tank. As seen from Fig. 3a–f, below the high temperature region, there is another low temperature region near the base wall. Although heat lost from the base wall is the lowest in the tank, due to natural convection, a large amount of cooled oil transports from the upper part of the tank to the base wall. Besides, the low temperature region in the bottom part often has a larger range than that of the low temperature region in the upper part. But the temperature gradient is smaller than that in the low temperature region near the top wall. For different test conditions, as the cooling progressing, the oil temperature along the axis direction decreases together. This phenomenon reflects the influence from natural convection is more prominent than that from heat conduction during the cooling. However, as the cooling marches, the effect from heat conduction gradually appears. Although three temperature regions are always noticeable during the cooling, the range of low temperature regions continue to enlarge, and the high temperature region continues to reduce. With the decreases of ambient temperature, the cooling rate of oil increases. It is 0.5 °C per day when the ambient temperature is − 20 °C, While the cooling rate decreases to 0.23 °C per day when the ambient temperature increases to 15 °C. Especially for the oil temperature near the top wall, there is a larger temperature gradient when the ambient temperature is − 20 °C. When the ambient temperature increases, temperature gradient reduces. 3.4. Temperature profile on different radial positions Test data from five sensors on the upper part of different test tubes were collected to illustrate the temperature profile along the radial direction. As seen from Fig. 4a–c, except for the temperature at the position 0.1 m away from the roof, the temperature distinction along the radial direction is non-significant with the temperature difference less than 0.3 °C. Taking into consideration of the temperature profile along the axis direction (see Fig. 3a–f), there is also a high temperature region in the center along the radial direction of the tank. Based on the test system, the nearest test group from the sidewall is group 6 which is installed in the gauge hatch of the tank. And the gauge hatch is about 1 m from the sidewall. The test data from group 6 and group 1 are compared in Fig. 4d–f in order to investigate the temperature distribution near the sidewall. As seen from Fig. 4d–f, there is the noticeable temperature difference between group 1 and group 6. The maximal temperature difference along the axis direction can approach to 1 °C. Take into consideration of the positions of the test groups, it can be speculated that there is an obvious low temperature region near the sidewall. Besides, the low temperature region in the bottom part of the tank can also be observed in detailed by the sensors of test group 6. Relatively speaking, the temperature difference along the radial direction in the bottom is not as noticeable as that in the center part. 3.5. Evolution of oil temperature during the receiving oil process According to the test data from Fig. 5a–c, when the temperature of entered oil entering into the tank is higher than that of oil in the tank, the oil in the tank is heated during the receiving oil process. For case 5, the temperature of inlet oil is 53 °C, and the flow rate is 0.35 m3/s. When the receiving oil process begins, the entered oil rises to the upper part of the tank under the buoyancy. The maximal temperature changing happens along the track of the entered oil. When the oil reaches the lower surface of the roof, the oil near the roof is heated. After that the heated oil spread from the impact point to all around near the lower surface of the roof. Therefore, the overall tendency is that the oil temperature increases from the top to the bottom. As the distance from the top wall increases, the heating magnitude decreases. As can be seen from Fig. 5a–c, during the test process, nearly a half of oil in the tank is heated by the receiving oil process. However, the heating rate and heating magnitude is various on different radial positions. Comparing to the test data from Fig. 5a–c, it can be concluded that the nearer to the inlet of the tank, the heating rate and heating magnitude of oil is larger. The maximal temperature rising value for the oil is nearly 1.2 °C during 24 h tested by group 1 (see Fig. 5a). And the maximal temperature rising in the center of the tank is 0.8 °C (see Fig. 5b). While the corresponding value of oil far away from the inlet is nearly 0.6 °C (see Fig. 5c). Besides, there is the lag of temperature rising for the oil far away from the inlet. So the temperature rising during the receiving oil process can be concluded as follows: First, the hot oil enters into the tank and rises to the lower surface of the roof by the buoyancy. The oil near the inlet and near the lower surface of the roof is first heated by the inlet oil. After that, the hot oil spreads to other positions near the lower surface of the roof away from the inlet. The temperature of oil in the upper part gradually increases from the near to the distant. On another hand, due to the temperature difference on the axis direction, the temperature of oil below the roof gradually increases from top to bottom. When the ambient temperature is much lower than the oil temperature, the heat loss of the tank is large. Thus the heating effect from the inlet oil mainly plays the role of slowing down the cooing. For case 4, the temperature of inlet oil is 50 °C, and the flow rate is 0.2 m3/s. There is the same heat transfer rule during the receiving oil process. As can be seen in Fig. 5d, due to the inlet of hot oil, the oil temperature first increases during the cooling, and then the temperature reduces due to the heat loss. But affected by the inlet 811
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Fig. 4. a. Temperature profile on different radial positions for case 1 (static operation, ambient temperature − 20 °C). b. Temperature profile on different radial positions for case 2 (static operation, ambient temperature 0 °C). c. Temperature profile on different radial positions for case 3 (static operation, ambient temperature 15 °C). d. Comparison of test data between group 1 and group 6 from case 1 (static operation, ambient temperature − 20 °C). e. Comparison of test data between group 1 and group 6 from case 2 (static operation, ambient temperature 0 °C). f. Comparison of test data between group 1 and group 6 from case 3 (static operation, ambient temperature 15 °C).
oil, the cooling rate in the upper part of the tank is far slower than that of oil in the lower part of the tank. During the cooling, the oil temperature increases along the axis of the tank towards the roof. The similar temperature profile on the axis direction is seen in Figs. 5e and 5f. Comparing to the data from Fig. 5d–f, far away from the inlet, the heating effect from the inlet oil gradually vanishes, and the temperature difference along the axis decreases. 3.6. Influence from ambient temperature on the oil temperature Fig. 6a–c shows the evolution of oil and ambient temperature during 24 h of one day. As can be seen in the figures, with the changing of ambient temperature, there is the corresponding temperature fluctuation of oil in the surface layer of the tank. However, the effect of ambient temperature is only confined to a small region. Comparing the test data from Fig. 6a–c, when the ambient temperature increases, the influence region affected by the ambient temperature reduces. Besides, as the variation of ambient temperature increases, the temperature fluctuation of oil increases. When the ambient temperature changes from 9.5 °C to 19.3 °C, 812
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Fig. 5. a. Evolution of oil temperature tested by group 1 from case 5 (near wall, receiving oil, ambient temperature 18 °C). b. Evolution of oil temperature tested by group 2 from case 5 (center, receiving oil, ambient temperature 18 °C). c. Evolution of oil temperature tested by group 3 from case 5 (near wall, receiving oil, ambient temperature 18 °C). d Evolution of temperature tested by group 1 from case 4 (near wall, receiving oil, ambient temperature − 22 °C). e. Evolution of temperature tested by group 2 from case 4 (center, receiving oil, ambient temperature − 22 °C). f. Evolution of temperature tested by group 3 from case 4 (near wall, receiving oil, ambient temperature − 22 °C).
the maximal oil temperature fluctuation is 1 °C and the influence range in the tank is within 0.6 m from the roof. While when the ambient temperature varies from 11.1 °C to 27.6 °C, the maximal oil temperature fluctuation increases to 3 °C and the influence range in the tank is less than 0.3 m. 3.7. The total heat transfer coefficient of the tank Based on the test data during the storage process of oil, the total heat transfer coefficient of the oil tank can be calculated based on the heat balance equation. The total heat loss released by oil during the cooling process of s can be expressed as follows:
Q1 = c y Gy (TR
(2-1)
TZ ) 813
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Fig. 6. a. Evolution of oil temperature during 24 h of one day. b. Evolution of ambient temperature during 24 h of one day. c. Evolution of oil temperature during 24 h of one day.
The total heat loss through the walls of the tank during the cooling process of
Q2 = KA
0
(Tave
T0)d
s can be expressed as follows: (2-2)
Based on the principle of conservation of energy: (2-3)
Q1 = Q2 Therefore:
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Table 5 Total heat transfer coefficient under different conditions. Test number
Level (m)
Average initial oil temperature (°C)
Average final oil temperature (°C)
Average ambient temperature (°C)
Total heat transfer coefficient (W/m2 °C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
12.899 12.136 11.456 10.796 10.280 13.511 13.536 13.535 11.609 9.360 10.100 9.680 11.132 12.455 11.232 10.010 8.712 9.881 12.232 13.136 11.460 12.206 11.789 9.970 9.230 9.360 10.420 13.110 13.640 12.730 11.560
42.3 41.7 41.4 41.0 40.8 40.3 40.1 39.8 41.8 40.9 40.5 40.7 41.1 40.8 40.4 40.3 40.1 41.2 41.4 41.7 40.5 42.1 42.5 41.7 41.3 40.7 40.3 42.5 42.7 42.3 41.4
41.7 41.4 41.0 40.7 40.2 40.1 39.6 39.6 41.6 40.4 39.9 40.2 40.7 40.2 39.8 39.7 39.1 40.3 40.8 41.2 39.7 41.1 41.6 40.5 40.5 39.9 39.5 41.8 42.2 41.6 40.6
14.2 14.3 17.9 18.7 18.2 20.2 15.8 19.7 27.5 8.6 9.7 10.2 7.2 4.3 7.8 1.3 2.3 1.5 − 2.0 − 4.7 − 2.7 − 6.8 − 6.6 − 8.1 − 7.6 − 3.7 − 4.6 − 3.1 1.3 − 2.1 2.4
2.23 1.09 1.63 1.22 2.32 1.10 2.28 1.09 1.35 1.40 1.71 1.33 1.13 1.66 1.78 1.37 1.98 2.03 1.41 1.26 1.88 2.12 1.78 2.07 1.38 1.45 1.63 1.77 1.27 1.58 1.89
K=
c y Gy (TR A
0
(Tave
TZ ) (2-4)
T0)d
where, TR is the average oil temperature at the beginning of the cooling, °C; TZ is average oil temperature at the end of the cooling, °C; c y is the average specific heat capacity of oil in the tank, J/kg °C; Gy is the total mass of oil in the tank, kg; is the cooling time, s; Tave is the average oil temperature at a given time during cooling, °C; T0 is the average ambient temperature at a given time during cooling, °C; K is the total heat transfer coefficient of the tank, W/m2 °C; A is the total area of dissipation, and it can be divided into the floating roof, the sidewall contacting oil and the base wall of the tank, m2. The volume weighted method is used to calculate the average oil temperature in the oil tank. As the previous discussion, the temperature profile in the tank can be divided into the low temperature region and the high temperature region. Based on the test data, the range of different regions can be confirmed, with the addition of the temperature data in different positions, the average temperature in different regions can be calculated. On the basis of the average temperature and the corresponding volume for different regions, the average oil temperature in the entire tank can be calculated based on the volume weighted method. According to the data during the six months testing on different working conditions, the total heat transfer coefficient is shown in Table 5. And for different test cases, the cooling time is all one day. On different working conditions, the total heat transfer coefficient is within 1.09–2.32 W/m2 °C. There is no explicit relationship between the total heat transfer coefficient and the ambient temperature or the level of the tank. As the heat transfer course during the cooling of the oil tank is very complicated, the total heat transfer coefficient can be a comprehensive representation for this heat transfer course. As the fluctuation of ambient temperature, the sunlight and many factors affect this heat transfer process, the establishment of the relationship between the total heat transfer coefficient and the factors is difficult to achieve. 4. Conclusions The temperature test system for the floating roof oil tank was established which is constituted by the “Temperature test module”, “Data transmission module”, “Explosion prevention module”, “Data collection module” and the “Power supply module”. Four kinds of test tubes were invented to investigate the temperature profile in the deep and superficial layer of the oil tank. Based on this test system, different working conditions of the oil tank were tested. According to the test data under different ambient temperature, the temperature profile on the axis direction of the tank can be divided into three parts. Near the roof, there exists a low temperature region with the lowest temperature in the axis direction. Away 815
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from the roof, the oil temperature increases and becomes more homogeneous which contributes a high temperature region in the center part. Below the high temperature region, there is another low temperature region near the base wall which is generated mainly be the natural convection. For different conditions, in the initial stage of cooling, the influence from natural convection is more prominent than heat conduction. But as the cooling marches, the effect from heat conduction gradually appears. The cooling rate on different positions along the axis direction is nearly the same except for some test positions near the roof which are affected by the fluctuation of ambient temperature. Besides, the changing rate of oil temperature responding to the variation of ambient temperature reduces as the capacity of oil tank increases. The temperature distinction in the center of the radial direction is non-significant with the temperature difference less than 0.3 °C which reflects a high temperature region in the radial direction. Take into consideration of the test data in the gauge hatch, it can be speculated that there is also a low temperature region near the sidewall. When the temperature of oil entering into the tank is higher than that in the tank, the oil in the tank is heated during the receiving oil process. First, the hot oil enters into the tank and rises to the lower surface of the roof by the buoyancy. The oil near the inlet and on the upper part of the tank is heated by the inlet oil. And then, the hot oil spreads around near the lower surface of the roof away from the inlet. The temperature of oil in the upper part gradually increases from the near to the distant. On another hand, due to the temperature difference on the axis direction, the temperature of oil below the roof gradually increases from top to bottom. With the changing of ambient temperature, there is the corresponding temperature fluctuation of oil in the surface layer of the tank. With the ambient temperature increasing, the influence range decreases. Moreover, as the variation of ambient temperature increases, the temperature fluctuation of oil increases. On different working conditions, the total heat transfer coefficient of oil tank is calculated with the value from 1.09 W/m2 °C to 2.32 W/m2 °C. There is no explicit relationship between the total heat transfer coefficient and the ambient temperature or the level of the tank as the fluctuation of ambient temperature, the sunlight and many factors affects this heat transfer process. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant no. 51704077), and the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province of China (Grant no. UNPYSCT-2016125). Conflict of interest There is no conflict of interest. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Zhao Jian, Heat transfer research and process scheme optimization of oil storage in alpine cold region, Northeast Pet. Univ. (2013). Zhao Zhiming, Numerical simulation of the unsteady heat transfer problem for the floating roof oil tank in bei oil depot in Daqing, Daqing Pet. Inst. (2009). Yahong Lu, Jiangtao Wu, The optimal design for the steam coil heating instrument in the crude oil tank, Chem. Eng. 38 (10) (2010) 69–72. Wang Li, Qingyuan Wang, Ruilong Li, Numerical simulation for the temperature field of large scaled floating roof oil tank, J. Chem. Eng. 62 (S1) (2011) 108–112. M. Cotter, M. Charles, Transient cooling of petroleum by natural convection in cylindrical storage tanks—I. Development and testing the numerical simulator, Int. J. Heat Mass Transf. 36 (1993) 2165–2174. M. Cotter, M. Charles, Transient cooling of petroleum by natural convection in cylindrical storage tanks —II. Effect of heat transfer coefficient, aspect ratio and temperature-dependent viscosity, Int. J. Heat Mass Transf. 36 (1993) 2175–2182. M. Cotter, M. Charles, Transient cooling of petroleum by natural convection in cylindrical storage tanks: a simplified heat loss mode, Can. J. Chem. Eng. 70 (1992) 1090–1093. R.D.C. Oliveski, M.H. Macagnan, J.B. Copetti, Andreas de La Martiniere Petroll, Natural convection in a tank of oil: experimental validation of a numerical code with prescribed boundary condition, Exp. Therm. Fluid Sci. 29 (2005) 671–680. Zhao Bin, Numerical simulation for the temperature changing rule of the crude oil in a storage tank based on the wavelet finite element method, J. Therm. Anal. Calorim. 107 (2012) 387–393. Da Yu, Xuying Fang, Temperature drop characteristic of a large scaled floating roof oil tank, Oil Gas Storage Transp. 22 (9) (2003) 47–49. Da Yu, Development of temperature test system for a large scaled floating roof oil tank, Oil Gas Storage Transp. 24 (8) (2005) 41–43. Chao Li, Renwei Liu, Wang Li, Experiment test research for the temperature field of a large scaled floating roof oil tank, J. Eng. Thermophys. 34 (12) (2013) 2332–2334. Mingji Wang, Yong Zhang, Development of the real-time monitor and alarm system for the oil temperature in an oil tank, Chin. J. Sci. Instrum. 22 (3) (2001) 200–201. Mingji Wang, Yong Zhang, Wen Cao, Temperature distribution rule along the longitudinal of the oil tank, J. Daqing Pet. Inst. 28 (5) (2004) 74–75.
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Case Studies in Thermal Engineering journal homepage: www.elsevier.com/locate/csite
Research on temperature profile in a large scaled floating roof oil tank
T
Lin Yang, Jian Zhao , Hang Dong, Junyang Liu, Weiqiang Zhao ⁎
Key Laboratory of Enhance Oil and Gas Recovery of Educational Ministry, Northeast Petroleum University, Daqing, PR China
ARTICLE INFO
ABSTRACT
Keywords: Oil tank Test Temperature Total heat transfer coefficient
The temperature test system for the floating roof oil tank is established. Four different kinds of test tubes are invented to test the temperature profile in the oil tank. Based on the test system, the temperature profile of a 10 × 104 double-deck floating roof oil tank is investigated. There are three low temperature regions near walls in the tank, surrounded by these regions is the high temperature region in the center of the tank. The temperature distribution in different regions is presented in detailed. With the changing of ambient temperature, there is the corresponding temperature fluctuation of oil in the surface layer. With the ambient temperature increasing, the influence range decreases. And as the variation of ambient temperature increases, the temperature fluctuation of oil increases. Moreover, the heating process of oil in the tank by the entering oil is presented by the test data. Based on the principle of conservation of energy, the total heat transfer coefficient of oil tank on different working conditions is calculated with the value from 1.09 W/m2 °C to 2.32 W/m2 °C.
1. Introduction Precisely acquiring the temperature profile in the oil tank is of great concern for the storage and transportation of crude oil. Especially in the alpine region, the extremely low ambient temperature increases the cooling rate of oil as well as the risk of gelatinization. Many scholars carried out the research on the temperature field of oil tank during the storage process. Generally, there are the numerical simulation and test method executed on this research topic. Based on the numerical simulation method, Zhao [1] investigated the heat transfer characteristic of oil during the storage. The evolution of temperature field and the natural convection were presented. Zhao [2] investigated the temperature distribution and flow characteristic of oil during the short time cooling by Fluent. Lu [3] simulated the heating process of a 10 × 104 m3 oil tank by Fluent. Based on the thermoanalysis, the rationality of heat equipment was discussed. Li [4] presented a prediction model for the temperature field of a floating roof oil tank, and the influence from the solar radiation and insulating thickness on the oil temperature was discussed in his research. Cotter and Michael [5–7] employed a control volume finite difference method to simulate the heat transfer process of oil tank. Based on this method, the flow pattern and temperature distribution were discussed. The effect of heat transfer coefficient, aspect ratio and viscosity on the heat transfer was examined. Oliveski [8] presented a numerical and experimental analysis of the heat transfer process in a small tank containing oil. The temperature profile in the simulated tank was investigated. Zhao [9] studied the temperature dropping rule of crude oil by numerical simulation. The evolution of temperature and flow field was analyzed together. Although the numerical simulation method takes great advantage for the research on the heat transfer process of oil, the acquisition of the temperature data in a real oil tank is more important for promoting this research topic. Besides, testing the oil temperature under a real working ⁎
Correspondence to: Northeast Petroleum University, Fazhan Road NO.199, Hi-tech Development Zone, Daqing 163318, PR China. E-mail address: [email protected] (J. Zhao).
https://doi.org/10.1016/j.csite.2018.10.009 Received 21 June 2018; Received in revised form 19 October 2018; Accepted 22 October 2018 Available online 27 October 2018 2214-157X/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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condition is also an important way to verify the accuracy of numerical simulation. Therefore, some researches were carried out by the test method. Yu [10,11] developed a test system to investigate the temperature distribution in a 10 × 104 m3 floating roof oil tank. Based on the test system, the temperature profile rule along the axis direction and the radial direction was discussed. However, his research mainly focused on the oil tank which was in the static storage state. In the real storage course, there are many operating states of the oil tank. Besides, limited by the install condition, most of the temperature sensors were installed in the superficial zone of the tank. There was only one group of sensors installed in the gauge hatch, so the obtained temperature data were not enough to investigate the temperature profile and variation rule in the deep layer of the oil tank. The same method was also carried out by Li [12] and Wang [13,14]. However, the same defects were also existed in their research. Considering the information in the literature on this research topic, the aim of this work is to investigate the temperature profile in the floating roof oil tank under different working conditions. As carrying out the testing on a real oil tank is an effective and direct method to acquire the real temperature distribution data, the test method was adopted in this research. In order to reveal the temperature profile more comprehensively, the temperature sensors were placed on different positions including the superficial layer and deep layer in the oil tank, so the temperature profile along the axis direction and radial direction can be obtained together. Besides, the testing condition includes the static storage and sending and receiving oil process of oil tank, so the heat transfer characteristic of oil tank under different working conditions can be revealed. 2. Test object and system The temperature test system was installed on a 10 × 104 double-deck floating roof oil tank in Daqing oil field of China. The diameter of the tank is 80 m, the total height of the sidewall is 21.97 m. And the height of the gauge hatch is 24.4 m. There is the insulating layer on the sidewall with the thickness of 80 mm. As the roof is floatable, the space which the oil occupies is changeable. There are the inlet and outlet on the sidewall with the height of 1.5 m above the foundation of the tank. The temperature test system is constituted by five module, they are respectively the “Temperature test module”, “Data transmission module”, “Explosion prevention module”, “Data collection module” and the “Power supply module”. 2.1. Temperature test module The core of the temperature test module is the specially-made test tube. The test tube is constituted by a group of temperature sensors, the signal lines, and protection structure. Platinum resistor is used as the temperature sensor which has the accuracy of 0.1 °C. As the sensors directly contact to the oil, they are the sheathed thermos resistor which has the function of anti-explosion and fireproofing. In order to obtain the temperature profile along the axis direction of the tank, more than one sensor is placed along the test tube. The number and positions of the sensors are different for different kind of test tube. The test tube is divided into two parts. The upper part of the tube is a copper pipe which is used to protect and fasten the sensors. The sensors and signal lines are installed into the copper pipe. In the lower part of tube, each sensor and the signal line is sealed by an independent flexible pipe which is made by PTFE material. The structure of flexible pipe ensures the sensors flex flexibly. Near each sensor, there is a bob-weight which is used to locate the sensor in the certain position along the tube. The cross sectional drawing of test tube is shown in Fig. 1a: Based on the number and layout strategy of sensors, there are four kinds of test tubes with different length and structure. One kind is named as A-type test tube which contains sixteen sensors, and the test range is 17 m, while the total length of the tube is 19.61 m. This kind of test tube is used to test the temperature profile along the entire axis direction of the tank. The position of each sensor is listed in Table 1, the “distance” represents the distance from the sensor to the lower wall of the floating roof. As can be seen in Table 1, the arrangement of sensors is more concentrated in the upper part, thus the temperature distribution in the region near the floating roof can be tested more precisely. The second kind of test tube is named as B-type which contains six sensors, and the test range is 1.8 m, while the total length is 4.4 m. The position of each sensor is listed in Table 2. The third kind of test tube is named as C-type which contains sixteen sensors, and the total length is 24.4 m. The C-type test tube is installed in the gauge hatch of the tank. The test range is 17 m above the base wall of the tank. The position of each sensor is listed in Table 3. The arrangement of the sensors is more concentrated in the bottom, thus the temperature distribution in the region near the base wall can be tested more precisely. The test tubes are installed through the stanchion hole on the roof and contact to the oil. There are aggregately six groups of test tubes installed in the tank. Three are the A-type test tube, and two are the B-type test tube, while one is C-type test tube. The positions of different test tubes in the oil tank are illustrated in Fig. 1b. As shown in Fig. 1a, group 1, group 2 and group 3 are the A-type test tubes, group 4 and group 5 are the B-type test tubes, and group 6 is the C-type test tube. Group 1 is placed near the inlet of the tank, with respect to test the temperature profile along the axis direction under the influence from the entered oil. Group 2 is placed in the center of the tank in order to test the temperature profile on the axis direction. Group 3 is placed far away from the inlet of the tank. By comparing the test data from group 1, group 2 and group 3, the influence from the entered oil on the oil temperature in the tank can be investigated. Group 4 and group 5 are placed on different positions along the radial direction. By comparing the test data from group 1 to 5, the temperature distribution along the radial direction can be presented. Group 6 is installed in the gauge hatch of the tank, with the aim of testing the temperature profile near the sidewall. 806
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Fig. 1. a. Cross sectional drawing of test tube. b. Lay out of test tubes and the floating roof tank. c. Lay out of the data transmission lines. d. Photograph of the test system. e. Diagram of connection.
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Table 1 Layout strategy of sensors for A-type test tube. Number
1
2
3
4
5
6
7
8
Distance Number Distance
0.1 m 9 4.8 m
0.3 m 10 7.4 m
0.6 m 11 10 m
1.0 m 12 11.4 m
1.4 m 13 12.8 m
1.8 m 14 14.2 m
2.3 m 15 15.6 m
3.3 m 16 17 m
Table 2 Layout strategy of sensors for B-type test tube. Number
1
2
3
4
5
6
Distance
0.1 m
0.3 m
0.6 m
1.0 m
1.4 m
1.8 m
Table 3 Layout strategy of sensors for C-type test tube. Number
1
2
3
4
5
6
7
8
Distance Number Distance
7.5 m 9 21.6 m
8.5 m 10 22.1 m
9.9 m 11 22.6 m
11.3 m 12 23.1 m
12.7 m 13 23.5 m
14.4 m 14 23.9 m
16.1 m 15 24.2 m
21.1 m 16 24.4 m
2.2. Data collection module There is a data acquisition unit near each test tube to collect the test data of the sensors. The data acquisition unit was produced in Wuhan of China with the accuracy is 0.1 °C. And the maximal acquisition speed is 1 Hz. The data acquisition unit is placed on the external surface of the roof. The signal lines sealed by the explosion-proof hose are used to connect the test tube and the data acquisition unit. There is a CPU installed into the data acquisition unit. Thus the collection, dispose and transmission of data can be achieved fast and precisely. Corresponding to the test tube, there are totally six data acquisition units on the roof. Moreover, there is a main-control box placed in the controlling room far away from the tank. The test data from each data acquisition unit is gathered together and transmitted to the main-control box which directly connects to the computer. 2.3. Data transmission module There is the cable used to connect the data acquisition unit in the tank. In order to reduce the total length of the cable, the data acquisition unit is connected in the form of series. And the optimal arrangement of wire is used. Fig. 1c illustrates the lay out of the data transmission lines on the tank roof. Fig. 1d is the photograph of the test system on the tank roof. 2.4. Explosion prevention module The explosion prevention module is used to ensure the safety during the temperature test. Different measures are introduced to achieve this purpose. First, the explosion-proof hose are used to seal the cables to play the role of isolating the source of danger. Besides, the explosion-proof box is used to seal the data acquisition unit. With the addition of the intrinsically safe sensors, the safety of test system can get to the maximum extent. 2.5. Power supply module The power supply module includes the main-control box of power supply which is installed in the controlling room. This maincontrol box has the function of converting the voltage from 220 V into 24 V. And then the voltage of 24 V is transmitted to the nearest data acquisition unit on the tank roof by the electric wire which is also sealed in the explosion-proof hose. After that, the power is transmitted from the nearest data acquisition unit to the farthest one. Due to the loss during the transmission, the working voltage of the data acquisition unit is less than 12 V which is also a safeguard for the safety test. The diagram of connection for the temperature test system is shown in Fig. 1e: 3. Results and discussion 3.1. Test cases The temperature test on the floating roof oil tank lasts for nearly six months. The test conditions include different environment 808
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Table 4 Basic parameters of test cases. Case
Operating state
Ambient temperature (°C)
Time (Days)
Level (m)
Initial oil temperature in the center of the tank (°C)
Inlet flow rate (m3/s)
1 2 3 4 5 6 7
Static storage Static storage Static storage Receiving oil process Receiving oil process Static storage Static storage
− 20 (average) 0 (average) 15 (average) − 22 (average) 18 (average) 9.5–19.3 11.1–27.6
8 12 9 30 h 1 1 1
12.6 11.2 10.6 13.2 9.7 11.8 13.1
38.4 38.8 40.3 40.0 40.2 39.6 40.4
– – – 0.2 0.35 – –
temperature and operating state of the oil tank. In order to investigate the temperature profile in the tank under different working conditions, seven groups of test data were collected, the basic parameters of each case are listed in Table 4. 3.2. Cooling rate of oil under different conditions As can be seen in Figs. 2a and 2b, the cooling rate on different positions along the axis direction obtained by group 1 is nearly the same except for some test positions near the roof. As the ambient temperature changes during the day, the cooing rate of oil near the roof fluctuates during the cooling. The data from test positions in the center of the tank shows a consistent and uniform variation. Moreover, the temperature in the bottom part of the tank is different from that in the center of the tank. According to the test data from H = 0 m to H = 2.6 m in Fig. 3a and the test data from H = 0 m to H = 1.2 m in Fig. 3b, there presents a thermal stratification feature in the bottom part of the tank. With the decreasing of ambient temperature, the cooling rate increases. The average cooling rate of oil is 0.44 °C/day when the ambient temperature is − 20 °C, while the average cooling rate reduces to 0.29 °C/day when the ambient temperature increases to 0 °C. However, for the oil temperature near the roof, the difference of cooling rate under different
Fig. 2. a. Temperature variation as a function of time tested by group 1 from case 1 (near wall, static operation, ambient temperature − 20 °C). b. Temperature variation as a function of time tested by group 1 from case 2 (near wall, static operation, ambient temperature 0 °C). 809
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Fig. 3. a. Temperature profile tested by group 1 from case 1 (near wall, static operation, ambient temperature − 20 °C). b. Temperature profile tested by group 3 from case 1 (near wall, static operation, ambient temperature − 20 °C). c. Temperature profile tested by group 1 from case 2 (near wall, static operation, ambient temperature 0 °C). d. Temperature profile tested by group 3 from case 2(near wall, static operation, ambient temperature 0 °C). e. Temperature profile tested by group 1 from case 3 (near wall, static operation, ambient temperature 15 °C). f. Temperature profile tested by group 3 from case 3(near wall, static operation, ambient temperature 15 °C).
ambient temperature increases apparently. The average cooling rate of oil positioned at 0.1 m from the roof is 1.03 °C/day when the ambient temperature is − 20 °C, while the corresponding value decreases to 0.3 °C/day when the ambient temperature increases to 0 °C. The changing rate of oil temperature responding to the variation of ambient temperature reduces as the capacity of oil tank increases. Based on the test data, only the oil positioned within the range of 0.3 m from the roof is affected by the ambient temperature apparently. For the oil in the tank, the influence from the ambient condition gradually decreases as the distance from the top wall increases. 3.3. Temperature profile along the axis direction under different conditions According to the test data under different working conditions, except for some irregularity data which are caused by the 810
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fluctuation of environmental condition, there is the similar rule of the temperature profile along the axis direction of the tank. The temperature profile on the axis direction can be divided into three parts. Near the top wall, due to the heat loss from oil to surroundings, there exists a low temperature region. Although the surface temperature is not tested by the test system as the nearest sensor is placed 0.1 m away from the lower surface of the roof. According to Fig. 3a and the additional test carried out by the surface thermometer, the oil temperature near the top wall is the lowest in the axis direction. However, due to natural convection, this region occupies only a small range along the axis direction. Away from the roof, the oil temperature increases and becomes more homogeneous which reflects a high temperature region in the center part of the tank. Among the test data on different environmental conditions, the temperature deviation in this region is within 0.5 °C. In addition, due to the large dimension of the oil tank, it takes long time for the thermal disturbance spreading to this region, so the high temperature region always takes over the largest range of the tank. As seen from Fig. 3a–f, below the high temperature region, there is another low temperature region near the base wall. Although heat lost from the base wall is the lowest in the tank, due to natural convection, a large amount of cooled oil transports from the upper part of the tank to the base wall. Besides, the low temperature region in the bottom part often has a larger range than that of the low temperature region in the upper part. But the temperature gradient is smaller than that in the low temperature region near the top wall. For different test conditions, as the cooling progressing, the oil temperature along the axis direction decreases together. This phenomenon reflects the influence from natural convection is more prominent than that from heat conduction during the cooling. However, as the cooling marches, the effect from heat conduction gradually appears. Although three temperature regions are always noticeable during the cooling, the range of low temperature regions continue to enlarge, and the high temperature region continues to reduce. With the decreases of ambient temperature, the cooling rate of oil increases. It is 0.5 °C per day when the ambient temperature is − 20 °C, While the cooling rate decreases to 0.23 °C per day when the ambient temperature increases to 15 °C. Especially for the oil temperature near the top wall, there is a larger temperature gradient when the ambient temperature is − 20 °C. When the ambient temperature increases, temperature gradient reduces. 3.4. Temperature profile on different radial positions Test data from five sensors on the upper part of different test tubes were collected to illustrate the temperature profile along the radial direction. As seen from Fig. 4a–c, except for the temperature at the position 0.1 m away from the roof, the temperature distinction along the radial direction is non-significant with the temperature difference less than 0.3 °C. Taking into consideration of the temperature profile along the axis direction (see Fig. 3a–f), there is also a high temperature region in the center along the radial direction of the tank. Based on the test system, the nearest test group from the sidewall is group 6 which is installed in the gauge hatch of the tank. And the gauge hatch is about 1 m from the sidewall. The test data from group 6 and group 1 are compared in Fig. 4d–f in order to investigate the temperature distribution near the sidewall. As seen from Fig. 4d–f, there is the noticeable temperature difference between group 1 and group 6. The maximal temperature difference along the axis direction can approach to 1 °C. Take into consideration of the positions of the test groups, it can be speculated that there is an obvious low temperature region near the sidewall. Besides, the low temperature region in the bottom part of the tank can also be observed in detailed by the sensors of test group 6. Relatively speaking, the temperature difference along the radial direction in the bottom is not as noticeable as that in the center part. 3.5. Evolution of oil temperature during the receiving oil process According to the test data from Fig. 5a–c, when the temperature of entered oil entering into the tank is higher than that of oil in the tank, the oil in the tank is heated during the receiving oil process. For case 5, the temperature of inlet oil is 53 °C, and the flow rate is 0.35 m3/s. When the receiving oil process begins, the entered oil rises to the upper part of the tank under the buoyancy. The maximal temperature changing happens along the track of the entered oil. When the oil reaches the lower surface of the roof, the oil near the roof is heated. After that the heated oil spread from the impact point to all around near the lower surface of the roof. Therefore, the overall tendency is that the oil temperature increases from the top to the bottom. As the distance from the top wall increases, the heating magnitude decreases. As can be seen from Fig. 5a–c, during the test process, nearly a half of oil in the tank is heated by the receiving oil process. However, the heating rate and heating magnitude is various on different radial positions. Comparing to the test data from Fig. 5a–c, it can be concluded that the nearer to the inlet of the tank, the heating rate and heating magnitude of oil is larger. The maximal temperature rising value for the oil is nearly 1.2 °C during 24 h tested by group 1 (see Fig. 5a). And the maximal temperature rising in the center of the tank is 0.8 °C (see Fig. 5b). While the corresponding value of oil far away from the inlet is nearly 0.6 °C (see Fig. 5c). Besides, there is the lag of temperature rising for the oil far away from the inlet. So the temperature rising during the receiving oil process can be concluded as follows: First, the hot oil enters into the tank and rises to the lower surface of the roof by the buoyancy. The oil near the inlet and near the lower surface of the roof is first heated by the inlet oil. After that, the hot oil spreads to other positions near the lower surface of the roof away from the inlet. The temperature of oil in the upper part gradually increases from the near to the distant. On another hand, due to the temperature difference on the axis direction, the temperature of oil below the roof gradually increases from top to bottom. When the ambient temperature is much lower than the oil temperature, the heat loss of the tank is large. Thus the heating effect from the inlet oil mainly plays the role of slowing down the cooing. For case 4, the temperature of inlet oil is 50 °C, and the flow rate is 0.2 m3/s. There is the same heat transfer rule during the receiving oil process. As can be seen in Fig. 5d, due to the inlet of hot oil, the oil temperature first increases during the cooling, and then the temperature reduces due to the heat loss. But affected by the inlet 811
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Fig. 4. a. Temperature profile on different radial positions for case 1 (static operation, ambient temperature − 20 °C). b. Temperature profile on different radial positions for case 2 (static operation, ambient temperature 0 °C). c. Temperature profile on different radial positions for case 3 (static operation, ambient temperature 15 °C). d. Comparison of test data between group 1 and group 6 from case 1 (static operation, ambient temperature − 20 °C). e. Comparison of test data between group 1 and group 6 from case 2 (static operation, ambient temperature 0 °C). f. Comparison of test data between group 1 and group 6 from case 3 (static operation, ambient temperature 15 °C).
oil, the cooling rate in the upper part of the tank is far slower than that of oil in the lower part of the tank. During the cooling, the oil temperature increases along the axis of the tank towards the roof. The similar temperature profile on the axis direction is seen in Figs. 5e and 5f. Comparing to the data from Fig. 5d–f, far away from the inlet, the heating effect from the inlet oil gradually vanishes, and the temperature difference along the axis decreases. 3.6. Influence from ambient temperature on the oil temperature Fig. 6a–c shows the evolution of oil and ambient temperature during 24 h of one day. As can be seen in the figures, with the changing of ambient temperature, there is the corresponding temperature fluctuation of oil in the surface layer of the tank. However, the effect of ambient temperature is only confined to a small region. Comparing the test data from Fig. 6a–c, when the ambient temperature increases, the influence region affected by the ambient temperature reduces. Besides, as the variation of ambient temperature increases, the temperature fluctuation of oil increases. When the ambient temperature changes from 9.5 °C to 19.3 °C, 812
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Fig. 5. a. Evolution of oil temperature tested by group 1 from case 5 (near wall, receiving oil, ambient temperature 18 °C). b. Evolution of oil temperature tested by group 2 from case 5 (center, receiving oil, ambient temperature 18 °C). c. Evolution of oil temperature tested by group 3 from case 5 (near wall, receiving oil, ambient temperature 18 °C). d Evolution of temperature tested by group 1 from case 4 (near wall, receiving oil, ambient temperature − 22 °C). e. Evolution of temperature tested by group 2 from case 4 (center, receiving oil, ambient temperature − 22 °C). f. Evolution of temperature tested by group 3 from case 4 (near wall, receiving oil, ambient temperature − 22 °C).
the maximal oil temperature fluctuation is 1 °C and the influence range in the tank is within 0.6 m from the roof. While when the ambient temperature varies from 11.1 °C to 27.6 °C, the maximal oil temperature fluctuation increases to 3 °C and the influence range in the tank is less than 0.3 m. 3.7. The total heat transfer coefficient of the tank Based on the test data during the storage process of oil, the total heat transfer coefficient of the oil tank can be calculated based on the heat balance equation. The total heat loss released by oil during the cooling process of s can be expressed as follows:
Q1 = c y Gy (TR
(2-1)
TZ ) 813
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Fig. 6. a. Evolution of oil temperature during 24 h of one day. b. Evolution of ambient temperature during 24 h of one day. c. Evolution of oil temperature during 24 h of one day.
The total heat loss through the walls of the tank during the cooling process of
Q2 = KA
0
(Tave
T0)d
s can be expressed as follows: (2-2)
Based on the principle of conservation of energy: (2-3)
Q1 = Q2 Therefore:
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Table 5 Total heat transfer coefficient under different conditions. Test number
Level (m)
Average initial oil temperature (°C)
Average final oil temperature (°C)
Average ambient temperature (°C)
Total heat transfer coefficient (W/m2 °C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
12.899 12.136 11.456 10.796 10.280 13.511 13.536 13.535 11.609 9.360 10.100 9.680 11.132 12.455 11.232 10.010 8.712 9.881 12.232 13.136 11.460 12.206 11.789 9.970 9.230 9.360 10.420 13.110 13.640 12.730 11.560
42.3 41.7 41.4 41.0 40.8 40.3 40.1 39.8 41.8 40.9 40.5 40.7 41.1 40.8 40.4 40.3 40.1 41.2 41.4 41.7 40.5 42.1 42.5 41.7 41.3 40.7 40.3 42.5 42.7 42.3 41.4
41.7 41.4 41.0 40.7 40.2 40.1 39.6 39.6 41.6 40.4 39.9 40.2 40.7 40.2 39.8 39.7 39.1 40.3 40.8 41.2 39.7 41.1 41.6 40.5 40.5 39.9 39.5 41.8 42.2 41.6 40.6
14.2 14.3 17.9 18.7 18.2 20.2 15.8 19.7 27.5 8.6 9.7 10.2 7.2 4.3 7.8 1.3 2.3 1.5 − 2.0 − 4.7 − 2.7 − 6.8 − 6.6 − 8.1 − 7.6 − 3.7 − 4.6 − 3.1 1.3 − 2.1 2.4
2.23 1.09 1.63 1.22 2.32 1.10 2.28 1.09 1.35 1.40 1.71 1.33 1.13 1.66 1.78 1.37 1.98 2.03 1.41 1.26 1.88 2.12 1.78 2.07 1.38 1.45 1.63 1.77 1.27 1.58 1.89
K=
c y Gy (TR A
0
(Tave
TZ ) (2-4)
T0)d
where, TR is the average oil temperature at the beginning of the cooling, °C; TZ is average oil temperature at the end of the cooling, °C; c y is the average specific heat capacity of oil in the tank, J/kg °C; Gy is the total mass of oil in the tank, kg; is the cooling time, s; Tave is the average oil temperature at a given time during cooling, °C; T0 is the average ambient temperature at a given time during cooling, °C; K is the total heat transfer coefficient of the tank, W/m2 °C; A is the total area of dissipation, and it can be divided into the floating roof, the sidewall contacting oil and the base wall of the tank, m2. The volume weighted method is used to calculate the average oil temperature in the oil tank. As the previous discussion, the temperature profile in the tank can be divided into the low temperature region and the high temperature region. Based on the test data, the range of different regions can be confirmed, with the addition of the temperature data in different positions, the average temperature in different regions can be calculated. On the basis of the average temperature and the corresponding volume for different regions, the average oil temperature in the entire tank can be calculated based on the volume weighted method. According to the data during the six months testing on different working conditions, the total heat transfer coefficient is shown in Table 5. And for different test cases, the cooling time is all one day. On different working conditions, the total heat transfer coefficient is within 1.09–2.32 W/m2 °C. There is no explicit relationship between the total heat transfer coefficient and the ambient temperature or the level of the tank. As the heat transfer course during the cooling of the oil tank is very complicated, the total heat transfer coefficient can be a comprehensive representation for this heat transfer course. As the fluctuation of ambient temperature, the sunlight and many factors affect this heat transfer process, the establishment of the relationship between the total heat transfer coefficient and the factors is difficult to achieve. 4. Conclusions The temperature test system for the floating roof oil tank was established which is constituted by the “Temperature test module”, “Data transmission module”, “Explosion prevention module”, “Data collection module” and the “Power supply module”. Four kinds of test tubes were invented to investigate the temperature profile in the deep and superficial layer of the oil tank. Based on this test system, different working conditions of the oil tank were tested. According to the test data under different ambient temperature, the temperature profile on the axis direction of the tank can be divided into three parts. Near the roof, there exists a low temperature region with the lowest temperature in the axis direction. Away 815
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from the roof, the oil temperature increases and becomes more homogeneous which contributes a high temperature region in the center part. Below the high temperature region, there is another low temperature region near the base wall which is generated mainly be the natural convection. For different conditions, in the initial stage of cooling, the influence from natural convection is more prominent than heat conduction. But as the cooling marches, the effect from heat conduction gradually appears. The cooling rate on different positions along the axis direction is nearly the same except for some test positions near the roof which are affected by the fluctuation of ambient temperature. Besides, the changing rate of oil temperature responding to the variation of ambient temperature reduces as the capacity of oil tank increases. The temperature distinction in the center of the radial direction is non-significant with the temperature difference less than 0.3 °C which reflects a high temperature region in the radial direction. Take into consideration of the test data in the gauge hatch, it can be speculated that there is also a low temperature region near the sidewall. When the temperature of oil entering into the tank is higher than that in the tank, the oil in the tank is heated during the receiving oil process. First, the hot oil enters into the tank and rises to the lower surface of the roof by the buoyancy. The oil near the inlet and on the upper part of the tank is heated by the inlet oil. And then, the hot oil spreads around near the lower surface of the roof away from the inlet. The temperature of oil in the upper part gradually increases from the near to the distant. On another hand, due to the temperature difference on the axis direction, the temperature of oil below the roof gradually increases from top to bottom. With the changing of ambient temperature, there is the corresponding temperature fluctuation of oil in the surface layer of the tank. With the ambient temperature increasing, the influence range decreases. Moreover, as the variation of ambient temperature increases, the temperature fluctuation of oil increases. On different working conditions, the total heat transfer coefficient of oil tank is calculated with the value from 1.09 W/m2 °C to 2.32 W/m2 °C. There is no explicit relationship between the total heat transfer coefficient and the ambient temperature or the level of the tank as the fluctuation of ambient temperature, the sunlight and many factors affects this heat transfer process. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant no. 51704077), and the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province of China (Grant no. UNPYSCT-2016125). Conflict of interest There is no conflict of interest. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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