The service intensity is reduced by 0.48 MPa. The decrease between 30 and 0C is slower. With the popularization of domestic central heating, the application area of ​​prefabricated direct-buried insulation pipes is wider and wider, and the demand in cold regions has been increasing year by year. Increase, this puts higher requirements on the performance index of the insulation pipe, especially the HDPE outer protection pipe. Since the HDPE outer protection pipe is a plastic pipe, its performance index varies greatly with the climate change, occasionally at low temperature. The phenomenon of cracking of the outer protective tube occurs. Therefore, some people think that it is not suitable to use HDPE as an outer protective tube in cold regions. Through cooperation with a university, the performance indexes of polyethylene outer protective tube and polyurethane insulation layer at different temperatures were tested, and the test data were analyzed and studied to determine the prefabricated direct buried insulation with HDPE as the outer protective tube. Whether the tube is suitable for use in cold regions (the polyethylene material used is HHMTR480) 2 The relationship between the tensile yield strength of the polyethylene tube and the temperature change. Because the polyethylene tube is plastically deformed, when the temperature changes, its tensile yield strength and elongation The rate of change changes accordingly. The tensile behavior of the polyethylene pipe at different temperatures is as shown.
It can be seen from the tensile yield strength-temperature curve that the tensile yield strength of the polyethylene pipe is almost unchanged between 0 and 23 C, and the yield strength is almost linear with the temperature between 23 and 50 C. For every 1C tensile elongation, the tensile strength decreases with each increase of 1C. The growth is obviously accelerated after 30C, and the tensile yield strength increases by 0.6MPa for every 1C decrease. It can be seen that the lower the temperature, the lower the temperature. The higher the tensile yield strength, the higher the increase.
The elongation of PE pipe at different temperatures can be seen. When the polyethylene pipe reaches yield, its elongation increases with the increase of temperature, and becomes smaller with the decrease of temperature, and the higher the temperature, the more the increase. Big.
It can be seen that when the temperature is high, the external force required for the deformation of the polyethylene pipe is small, and the deformation amount is relatively high when the yield is generated, so that the fracture is not easily generated by the external force. When the temperature is low, the elongation at yield is relatively low, and cracking is easy. However, the lower the temperature, the higher the tensile yield strength, and the higher the external force must be at low temperature to make the polyethylene tube The energy of the crack will only crack the polyethylene outer tube.
3 outer casing outer diameter increase rate performance research Because HDPE plastic pipe has certain plastic deformation, when the polyurethane foams, the outer diameter of the outer sheath of HDPE increases, so that it is in a state of stress for a long time; when the temperature is lowered Due to the restraint of the foam, the polyethylene outer protective tube cannot be shrunk, causing stress. The stress and elongation of the polyethylene outer tube are determined by decreasing the temperature after a certain elongation, and it is found that when a given initial elongation is maintained and constant, the temperature is gradually lowered, at a certain temperature range. Within the stress increase, the elongation increases with the increase of the stress, but when the elongation increases to a certain extent, it will not increase with the decrease of temperature. At this point, the stress increase will suddenly increase, and the stress growth will increase as the temperature decreases. The C/T114-2000 standard stipulates that the outer diameter increase rate of the polyethylene outer protective pipe should not exceed 2%. In order to prove the influence of the outer diameter increase rate on the low temperature of the HDPE pipe, the initial elongation of the HDPE is intentionally increased. Rate,) The elongation at 40C is 4%. The change in temperature-extension can be seen from the following: After 4% of the initial shape variable is given, the plastic deformation begins to decrease with the decrease of temperature, at 10C. When the maximum value is reached, it will no longer increase with the decrease of temperature, thus causing an increase in the amount of stress change after 10C. The stress increase per 1 C elbow below 10 C is about 1 C more than twice the stress increase per 1 C.
It can be seen that although the stress increases with decreasing temperature, the tensile yield strength increases with the decrease of temperature. For example, at 35C, the tensile yield strength of HDPE pipe is 35 MPa and the initial elongation is 35 MPa. At 4%, the stress at a 35C is only 0.028 MPa, which shows that the stress increase due to the increase in the outer diameter is much lower than the tensile yield strength that should be achieved when the tube is cracked. It will not cause cracking of the outer sheath of HDPE at low temperatures.
In order to verify the relationship between the increase rate of the outer diameter of the HDPE tube and the stress, a set of comparative tests was carried out, that is, an external force was applied at 40 C to make the elongation of the HDPE tube sample reach 2%, and the state was maintained, and the stress was measured. The temperature changes as shown.
The initial stress is 23 times of the initial stress of 2% elongation. The initial shape variable is different, and the stress increase value is also different. When the initial elongation is 4%, the stress increase from 40C to 40C is about 0. When the rate is 2%, the stress increase value from 40C to 40C is about 0.013MPa. When the 40C elongation is 2%, the temperature-stress curve can be seen, and the initial elongation (that is, the outer diameter increase rate) is higher. Large, the greater the initial stress, the lower the temperature, the faster the stress rises. Although the stress caused by the increase in the outer diameter is insufficient to damage the outer protective tube, in order to reduce the initial stress and prolong the service life of the pipe, the outer diameter increase rate should be minimized when the production of the heat insulating pipe is performed.
4 Drop hammer impact test In the case of the same outer diameter increase rate and the same amount of injection, according to the C/T114-2000 standard requirements, respectively, the polyethylene outer protective tube with different elongation at break was subjected to the drop weight impact test. Proof: Even the outer casing with elongation at break below 350% (standard value) does not show cracks. At the same time, the drop impact test under different temperatures (20, 30, 40, 50C) was performed, and no crack occurred. This proves that as long as the transportation and hoisting process are strictly in accordance with the operating procedures, the prefabricated direct buried insulated pipe with polyethylene as the outer protective pipe is completely suitable for district heating in cold regions, and the polyethylene outer protective pipe will not be stored. Cracking during transportation and construction.
5 Polyurethane performance study In order to verify the effect of the change of strength of polyurethane on the outer sheath of polyethylene with the increase of temperature, the compressive strength of polyurethane foam was tested with temperature. The compressive strength of the polyurethane foam (free foaming density of 59.6 kg/m3) as a function of temperature is as shown. It can be seen that at 20C, its compressive strength is the lowest, and as the temperature increases or decreases, the compressive strength gradually increases.
When compressing 2mm, the compressive strength changes with temperature. It can be seen that the compressive strength of polyurethane foam is about 0.7MPa at 30C. It can be seen that when the initial outer diameter increase rate of polyethylene outer tube is 2%, The tensile stress of the outer sheath of ethylene at 0.01C is 0.018 MPa. Therefore, the strength of the polyethylene outer tube and the polyurethane foam is simultaneously increased due to the decrease of temperature, but the compressive strength of the polyurethane is higher than the tensile stress of the polyethylene at this time. , polyurethane foam will not be used in Table 5 Lu Ning line different operating mode electricity consumption consumption mode electricity consumption mode operation mode electricity consumption 6 stations 2 pumps 12 stations 2 pumps 6 stations 3 pumps 12 stations 3 pumps 12 stations 1 pump 5 optimization program Under the premise of benefit analysis 106t, the best operation mode is the combination of 12 stations, 2 pumps and 6 stations and 3 pumps, that is, 12 stations, 2 pumps, 21d6 stations, 3 pumps, and 9d. If the average electricity price is calculated according to the current Luning line, the optimization is adopted. Operating mode is 12 stations 2 pumps and 12 stations 1 pump, 12 stations 2 pumps and 6 stations 2 pumps, 12 stations 3 pumps and 12 stations 1 pump, 12 stations 3 pumps and 6 stations 2 pumps, 12 stations 3 pumps and 6 stations 3 pumps save 290,700 yuan, 224,400 yuan, 355.88 million yuan, 222.72 million yuan, 1,367,900 yuan each month. From the above analysis, we can know that:
Table 6 Different operation modes Combined power consumption analysis table Operation mode Operation days/d Total power consumption/(kWh) 12 stations 2 pumps 12 stations 1 pump 12 stations 2 pumps 6 stations 2 pumps 12 stations 2 pumps 6 stations 3 pumps operation mode Running days Total power consumption 12 stations 3 pumps 12 stations 1 pump 12 stations 3 pumps 6 stations 2 pumps 12 stations 3 pumps 6 stations 3 pumps Chen Jiayu. Hydraulics. Beijing: Petroleum Industry Press, 1985: Welcome to the instrument technology and sensor network http://~ Please leave valuable comments (on page 29), every 3 points) is 206HB; the maximum value is 239HB , the upper limit is not more than 250HB; the microstructure of the microstructure is martensite orientation, the tempered sorbite grain size is 6~7, and the structure is normal.
Table 3 P91 material thermal processing and heat treatment process project heating rate constant temperature temperature holding time terminal temperature heating normalizing tempering 7 trial system summary This design uses the US TRD US steam boiler monitoring gauge 97 finite element analysis program for calculation, can meet the strength needs After the manufacturing process is improved, the height of the branch pipe is obviously increased by 20% without increasing the outer diameter and wall thickness of the blank material. The wall thickness of the branch pipe is 90% ~ 93% of the thickness of the blank material. The thickness of the blank material is increased by 10%~125%. Therefore, when the blank is selected, the material whose wall thickness is lower than 10% of the product design wall thickness can be used. The test products have passed the mechanical performance test, microstructure inspection, ultrasonic and hardness test, and all the indicators have reached the relevant standards of raw materials and products.
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