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The metering stations of oilfields in Xinjiang Uygur Autonomous Region of China mainly use screw pumps as heavy oil export equipment. The wide use of screw pumps ensures the smooth flow of crude oil sales. The physical properties of heavy oil are mainly characterized by high viscosity and high density.
The screw pump is a positive displacement pump, which runs smoothly and has high efficiency when transporting high-viscosity oil. The previous oil pumps used packing seals, which caused a large amount of crude oil leakage during use, were easily damaged, were frequently replaced, and caused great environmental pollution. With the renewal of oil field equipment, the SMH screw pump has played a great role in the production of crude oil metering stations, and the screw pump uses a mechanical seal, which is a sealing device installed on the pump shaft and the pump body . Due to the continuous increase of mining life, the decline of oil quality in this block directly affects the service life of mechanical seals, increasing the number of times to replace mechanical seals, directly resulting in an increase in maintenance costs.
In order to prolong the service life of the mechanical seal of the screw pump, a detailed investigation has been made through the analysis of the on-site oil products and the operating environment of the screw pump: the mechanical seal of the screw pump is composed of a rotary face & a stationary seat；The rotary face is made of silicon carbide which is plated with a layer of hard alloy；The stationary seat is graphite structure, and the material of the "O" type sealing ring is rubber. The working principle of the mechanical seal is as follows: the mechanical seal is a pair or several pairs of end faces perpendicular to the axis of rotation, under the action of the fluid pressure and the elastic force of the compensation mechanism, and the cooperation of the auxiliary seal, it is a device that prevents fluid leakage and is formed by keeping fit and sliding relative to each other.The types of pumps currently used in that unit’s metering station are as follows: 27 screw pumps with mechanical seals, and 17 centrifugal pumps with packing seals.
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The main reasons for the leakage of mechanical seals are factors such as irregular installation, deterioration of oil quality, and high operating temperature.
1.1 Reasons for mechanical seal leakage caused by improper installation of mechanical seal
1.1.1 The sealing surface is not filled with lubricating oil. During the installation process, the sealing surfaces of the rotary face and the stationary seat are not coated with lubricating oil, which will cause the mechanical seal to be in a dry grinding state when the screw pump is started, causing the temperature of the sealing surface of the mechanical seal to rise rapidly instantaneously and burning the sealing surface, making the mechanical seal lose its sealing performance, and oil leakage occurs.
1.1.2 Deformation of rotary face and stationary seat of mechanical seal. The end faces of the rotary face and stationary seat were not carefully checked during installation, and the sealing effect cannot be fully achieved due to deformation. In addition, after long-term operation of the mechanical seal, problems such as deformation of the sealing surface or aging of the O-ring seal occured, and the seal was not carefully checked after the maintenance of the screw pump, and seal leakage occured after installation.
1.1.3 The sealing surface of the mechanical seal is damaged. The sealing surface is damaged due to improper operation during installation, and leakage occurs when the pump is running.
11.4 The sealing surface of the mechanical seal is not cleaned up. Foreign matter enters the sealing surface during installation, but this point has not been discovered or the foreign matter has not been completely cleaned up, causing sealing surface wear and leakage during operation.
1.1.5 "O" type sealing ring is damaged. During the installation process, the "O" sealing ring in the base is accidentally damaged or an aging "O" sealing ring is used.
1.2 Cause analysis of mechanical seal leakage caused by oil quality
The main cause of mechanical seal leakage due to oil quality is that the crude oil contains fine sand particles, which enter the mechanical sealing surface during operation, causing the sealing surface to wear and leak.
1.3 Cause analysis of mechanical seal leakage caused by operating temperature
1.3.1 The main reason for the leakage of the mechanical seal caused by the oil temperature is that the temperature is too high, which causes the accelerated aging and deformation of the "O" rubber sealing ring, resulting in the leakage of the mechanical seal. In addition, because the operating temperature of the mechanical seal is too high, the carbon substances in the crude oil will accumulate on the mechanical seal and constitute crystal particles, which will speed up the grinding of the sealing surface of the mechanical seal.
1.3.2 When the pump is in operation, pumping out should be avoided to avoid dry friction on the sealing surface of the mechanical seal and damage to the seal.
1.1 Reasons for mechanical seal leakage caused by improper installation of mechanical seal
1.1.1 The sealing surface is not filled with lubricating oil. During the installation process, the sealing surfaces of the rotary face and the stationary seat are not coated with lubricating oil, which will cause the mechanical seal to be in a dry grinding state when the screw pump is started, causing the temperature of the sealing surface of the mechanical seal to rise rapidly instantaneously and burning the sealing surface, making the mechanical seal lose its sealing performance, and oil leakage occurs.
1.1.2 Deformation of rotary face and stationary seat of mechanical seal. The end faces of the rotary face and stationary seat were not carefully checked during installation, and the sealing effect cannot be fully achieved due to deformation. In addition, after long-term operation of the mechanical seal, problems such as deformation of the sealing surface or aging of the O-ring seal occured, and the seal was not carefully checked after the maintenance of the screw pump, and seal leakage occured after installation.
1.1.3 The sealing surface of the mechanical seal is damaged. The sealing surface is damaged due to improper operation during installation, and leakage occurs when the pump is running.
11.4 The sealing surface of the mechanical seal is not cleaned up. Foreign matter enters the sealing surface during installation, but this point has not been discovered or the foreign matter has not been completely cleaned up, causing sealing surface wear and leakage during operation.
1.1.5 "O" type sealing ring is damaged. During the installation process, the "O" sealing ring in the base is accidentally damaged or an aging "O" sealing ring is used.
1.2 Cause analysis of mechanical seal leakage caused by oil quality
The main cause of mechanical seal leakage due to oil quality is that the crude oil contains fine sand particles, which enter the mechanical sealing surface during operation, causing the sealing surface to wear and leak.
1.3 Cause analysis of mechanical seal leakage caused by operating temperature
1.3.1 The main reason for the leakage of the mechanical seal caused by the oil temperature is that the temperature is too high, which causes the accelerated aging and deformation of the "O" rubber sealing ring, resulting in the leakage of the mechanical seal. In addition, because the operating temperature of the mechanical seal is too high, the carbon substances in the crude oil will accumulate on the mechanical seal and constitute crystal particles, which will speed up the grinding of the sealing surface of the mechanical seal.
1.3.2 When the pump is in operation, pumping out should be avoided to avoid dry friction on the sealing surface of the mechanical seal and damage to the seal.
