Common faults and preventive measures of pipelines

2022-08-19
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Common faults and preventive measures of oil depot pipelines

1. What is the reason for the pressure change in the oil pipeline caused by temperature difference? What precautions should be taken

(1) the reason for the pressure change in the oil pipeline caused by temperature difference. Due to the change of temperature, sunshine and other factors, the oil temperature in the oil pipeline will also change. In the sealed pipeline filled with oil, the volume temperature expansion coefficient of the pipe and the oil is nearly 20 to 30 times different under the same temperature difference. Therefore, when the temperature rises, the expansion of the oil is greater than that of the pipe metal, causing the pressure in the pipe to rise and thermal expansion; When the temperature drops, the phenomenon of liquid column separation (or cavitation) will appear in the pipe. Both of these conditions will cause pipeline leakage (the female port is at the flange connection), damage to the gate valve, and even expansion and cracking of the pipeline, resulting in oil leakage accidents

(2) preventive measures

① for each oil pipeline, an expansion pipe should be set in front of the oil tank valve at the highest position, and attention should be paid to closing it during oil inlet and outlet operations; Open the valve on the expansion pipe when stopping the operation. Since the inlet end of the oil expansion pipe only extends into the gas space of the oil tank, if it is not closed during oil inlet, it is easy to make the oil expansion pipe enter the oil tank at the same time, and spray it into the oil tank from the gas space of the oil tank, increasing the accumulation of electrostatic charges in the oil tank and causing accidents. When sending oil, if the expansion pipe valve is not closed, the vapor in the tank will enter the sent oil due to the injection effect when the oil flows, which will affect the measurement accuracy of oil on the one hand; On the other hand, the gas entering the oil will also aggravate the generation of static electricity and release on the surface of the free liquid, increasing the risk of static electricity accidents in the oil container

② the isolating valve set in the pipeline should be kept normally open after operation, or a bypass safety valve should be added to prevent it from forming a dead pipe section without pressure relief protection

③ after receiving and distributing oil, open the vent branch pipe of the pipeline and vent some pipelines, so that the oil can expand freely without overpressure in the pipeline

④ for long pipelines, when starting the oil pump under the condition of large temperature reduction, when the indication of the pressure gauge is normal, pay attention to slowly open the outlet valve to gradually close the separated liquid column, so as to avoid severe impact and pressurization

⑤ although the temperature difference of the underground pipeline is small, because the oil inlet temperature is close to the temperature, when the temperature difference between the oil inlet temperature and the underground pipeline is large in winter and summer, it is also prone to thermal expansion or cavitation, and the expansion pipe should also be set in front of the tank or other pressure relief protection measures should be taken on the pipeline

2. Why does thermal stress occur in oil depot pipelines

most pipelines are installed under atmospheric temperature, but due to the temperature change of transmission medium and the change of season, the pipeline will expand and contract with the change of temperature. As the pipeline is installed on the fixed support, the pipeline cannot expand and contract freely, which will inevitably generate thermal stress in the pipeline. The resulting thermal stress makes the pipeline subject to longitudinal thrust, which is very large. If measures are not taken to eliminate it, it is bound to cause major damage to the pipeline and other equipment, and the accident of running and emitting oil will occur

3. What is the cause of water hammer in the pipeline

in the process of oil transportation, due to some external factors (such as sudden action of the valve or sudden stop of the pump, etc.), the liquid flow speed changes suddenly. This phenomenon of sudden change (sharp alternating rise or fall) of the pressure in the pipe caused by the change of liquid momentum is called water hammer. The main causes of water hammer in oil pipeline are as follows: ① switch the valve on the line

② the engine (pump) suddenly stops; ③ Start and stop the pump; ④ Change the speed of the pump; ⑤ The prime mover is unstable; ⑥ Air enters the pipeline and pump; ⑦ Cavitation; ⑧ The safety valve suddenly opens or closes, etc

4. What harm will water hammer do to the pipeline system of the oil depot? What measures should be taken to reduce water hammer

the pressurization wave and decompression wave generated by water hammer will cause overpressure, liquid column separation and cavitation of the pipeline, resulting in equipment damage, pipe rupture, pipeline vibration and noise, and even damage to the entire oil transportation system. The pressurization wave generated by water hammer superimposes a water hammer pressure on the original pressure, which increases the dynamic pressure of the pipeline. The pressure of the oil transmission pipeline in the oil depot is generally not high. Generally, the accessories such as valves on the oil transmission pipeline are mostly low-pressure (less than 981kpa) equipment. Therefore, water hammer may cause overpressure or even rupture of the pipeline, damage to accessories such as valves, damage to the sealing parts of pumps, valves, stuffing boxes, etc., and oil leakage accidents, Even cause greater combustion and explosion accidents

the harm of the decompression wave generated by water hammer cannot be ignored. As the hydraulic pressure at the place where the decompression wave goes decreases, the flow rate slows down, but the downstream liquid flow continues to flow at the original speed under the action of inertia, and the difference between the two speeds makes the liquid sparse. When the absolute pressure of the liquid flow drops below the saturated vapor pressure of the liquid at that temperature, the liquid will vaporize and produce bubbles. In fact, this kind of situation usually occurs at the place where the pipeline is raised. Most of the bubbles gather downstream of the high point, and the liquid flow passes below it, as shown in Figure 37

once formed, its volume will continue to increase until the liquid flow velocity upstream and downstream of the bubble is equal, which is called liquid column separation. The size of bubble volume depends on the time from bubble formation to the equal velocity of upstream and downstream liquid flow and the average velocity difference during this period. After that, the upstream liquid flow will catch up with the downstream liquid flow, and the two liquid columns are joined again, which will generate greater pressurization under the effect of speed difference, which may cause equipment damage and pipeline overpressure. Moreover, the gas dispersed in the liquid flow is neither easy to be absorbed by the liquid nor easy to be eliminated, resulting in unstable oil transportation. This phenomenon is similar to cavitation of pump, so it is also called pipe cavitation

in view of the harm caused by the above water hammer to the pipeline, the following measures should be taken to weaken the water hammer

(1) pressure drop (liquid column separation) mitigation methods

① installing the flywheel increases the inertia of the pump, thereby inhibiting the sharp reduction of rotation speed, flow and pressure

② change the shape of the pipeline. Because the pipeline rises rapidly at the pump outlet, it is easy to cause liquid column separation. If the pipeline at the pump outlet is arranged smoothly, negative pressure can be avoided to ensure the safety of the pipeline

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③ set the automatic suction valve. If there is a possibility of negative pressure in the pipeline, the suction valve can be set at the place where the negative pressure in the pipeline is the largest. When negative pressure occurs, let it absorb air to reduce the water hammer pressure caused by the closure of the liquid column

④ set up a one-way balance tank. Set up a balance container at the place where the negative pressure is the largest. A one-way valve is installed between the container and the pipeline, which is cut off from the pipeline when normal. When the pressure in the pipeline is lower than the pressure that the volume change-over switch should be turned to the loading gear pressure, the check valve will be opened to replenish liquid to the pipeline

⑤ set up an air tank. Set up a closed air (or inert gas) container at the discharge end of the pump. When the power is cut off and the flow rate decreases sharply, the compressed air in the air tank will press the fluid towards the discharge pipe, thereby weakening the water hammer effect

(2) measures to prevent overpressure

① properly carry out process design and correctly select the working pressure. When selecting the working pressure, the factors that may produce water hammer such as sudden pump stop and sudden valve closure should be taken into account

② adopt equipment to prevent overpressure, such as safety valve, check valve, air tank, etc

③ it is important to operate correctly and strictly abide by the rules and actions to screen the stone chips in the sample to prevent pipeline overpressure

④ in the operation and management of the pump, the action of starting and stopping the pump should be slow and stable; The pump driver should strengthen his heart and often monitor the instrument. It is strictly forbidden to start and stop the pump, open and close the valve and adjust at will

⑤ in terms of valve operation, before starting the oil pump, the valve on the pipeline should be opened and the valve well locked; When closing the valve, the first 70% of the stroke can be closed quickly, and then it should slow down, and finally it should be very slow. Considering the return time of the reflected wave, when opening the valve, it should be very slow first, and then 70% can be opened quickly; In the process of oil transportation, the valves on the pipeline shall not be opened or closed at will. When changing tanks, you should first open the valve leading to another tank, and then close the valve of this tank

5. Briefly describe the corrosion of oil pipeline and preventive measures

whether the oil transmission pipeline of the oil depot is installed in or outside the hole (room), or on the ground, underground or in the pipe trench, it will inevitably produce chemical or electrochemical corrosion due to the contact with external media, such as atmosphere, moisture, soil, oil, etc., as well as the influence of stray current. With the growth of time, the corrosion of pipelines and the aging of anti-corrosion coatings are becoming more and more serious. In addition, at the present stage, there is no reliable leak detection technology for underground oil pipelines in oil depots. The oil leakage accident of this oil pipeline due to corrosion perforation, as well as the resulting fire, explosion, environmental pollution and other problems are becoming increasingly serious, which has brought huge losses to the country, enterprises and society. Therefore, strengthen the protection and management of oil pipelines, Prolonging the service life of the pipeline is of great significance to prevent leakage accidents

at present, the protection methods of oil pipelines in oil depots mainly include coating corrosion prevention and cathodic protection

the principle of coating anti-corrosion is to apply the anti-corrosion coating evenly and densely on the surface of the derusted metal pipeline, isolate it from various metal corrosion environments, cut off the circuit of electrochemical corrosion battery, and achieve the purpose of metal pipeline anti-corrosion and insulation

cathodic protection is a method of applying cathodic polarization to the protected metal (pipeline) to reduce or prevent metal corrosion. Cathodic protection of metal pipelines mainly adopts two methods, one is sacrificial anode cathodic protection, and the other is impressed current cathodic protection

6. What damage will the earthquake cause to the oil pipeline? How to avoid these damages

the damage forms of oil pipeline caused by earthquake mainly include: the collapse of buildings (structures) or the falling of objects will smash the pipeline, and the relative displacement caused by the shaking and vibration of equipment (such as oil tanks, oil pumps, etc.) and supports connected to the oil pipeline will pull the pipeline apart; Uneven subsidence of the foundation causes relative displacement of the equipment and breaks the pipeline; The stratum is fractured, and the soil is stretched or compressed, causing the directly buried pipeline to be cut, twisted or squeezed; The pipeline breaks due to resonance with equipment and structures; Liquefaction of the foundation causes serious deformation of underground pipelines, and sometimes even floats large-diameter pipelines, breaking branch pipes; The landslide caused by the earthquake damaged the pipeline

the measures to prevent the oil pipeline from being damaged by earthquake are as follows

(1) considering the possible pipeline damage caused by earthquake, two points are mainly considered here: one is the pipeline damage caused by inertial seismic force, which is mainly to prevent the pipeline damage caused by vibration; The second is pipeline damage caused by relative displacement, which accounts for a large proportion of oil pipeline damage caused by earthquake, which should be considered in the design. For example, in the connecting parts of pipelines and oil tanks, pipelines and oil pumps, due to the relative displacement of oil tanks, oil pumps and other equipment and pipelines during the earthquake, complex stress combinations such as resistance, compression, shear, torsion and bending will be generated on the pipelines, and the vibration caused by the earthquake will cause the stress concentration of oil tanks, oil pumps and other equipment and connecting parts, resulting in the damage of the connecting parts

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