In order to ensure the gas transmission efficiency of the long-distance natural gas pipeline in normal operation, we should carry out timely and effective pigging of the natural gas pipelines. In particular, the hydrogen sulfide corrosion products, condensate water and sundries brought into the pipeline are gathered in the gas transmission pipeline due to long-term use, so as to prevent the blocking of gas transmission pipelines and equipment failure, it is necessary to carry out regular pigging work on the pipeline. In order to ensure the stability and safety of pigging, the pig can be recovered after it is sent out. So, we must adjust the working process of the pipeline in the working area according to the actual operation condition of the pipe network, and control the reasonable running speed of the pig without affecting the users below. Therefore, it is very important to analyze and discuss the pigging technology of natural gas pipeline.
I. In pigging technology for long-distance natural gas pipelines, there are three types of pigging devices
(1) Pigging balls
The pigging balls used in gas pipelines are usually made of rubber material, the middle is hollow, and the thickness of the pipe wall is in the range of 3-5 cm. The upper part of the pigging ball has a sealed water injection vent, and a one-way valve for pressurizing at the water injection port, which has the function of controlling the water in the ball and controlling the interference of the gas transmission pipeline. The pigging ball has the function of removing the liquid and separating the medium in the pipeline, but it is relatively poor in cleaning the lumps.
(2) Straight pig
The main structure of a straight pipe pig is basically similar to that of a bowl pig. Straight pig it is divided into guide plate and sealing plate two, its shape is disc, guide plate diameter slightly less than the gas pipeline diameter; The sealing plate has a certain amount of interference compared to the inside diameter of the duct. The pig can move in both directions. It is recommended to use the straight-plate pig before putting into operation because if there is a blockage, the pig can be used for backblowing operation to remove the blockage.
(3) Bowl pig
This type of pig is simple in structure and flexible in installation. According to its shape, the bowl is usually divided into taper, plane and spherical. The pig is composed of a rigid frame and a leather bowl with two or more sections at the front and back. When it runs in the long-distance pipeline, it can run in a fixed direction, so the pipe-bowl pig can carry the instruments and equipment to detect the pipeline. In order to ensure that the bowl pig can easily pass through the branch pipe of large diameter, there should be a shortest interval between the two bowls of the pig. It is determined that the distance between the front and the back of the pig should be greater than or equal to its pipe diameter D. The distance of the total length of the pipe-bowl should be between 1D and 1.5D according to the number of the sections and the pipe diameter. The interference of the lip of the pig bowl to the inner diameter of the pipeline should be in the range of 2% -5%; There are many kinds of seals in the pig, and the sealing performance is particularly good, and its cleaning tool is a steel brush.
II. Pigging methods and selection methods for the use of long-distance natural gas pipelines
(I) Pipeline pigging scheme for waxy crude oil
The use of waxy crude oil pipelines in a certain period of time, oil wax will be deposited in the wall of the gas pipeline, When the wax deposition reaches a certain thickness, it is necessary to pigging the gas transmission pipeline in order to reduce the operating expenses of the pipeline and increase the transportation capacity. When carrying out wax pigging operation, we should choose suitable soft pigging and mechanical pigging with tracker. When pigging, the first step is to use a soft pig to understand the specific situation in the pipeline, when to ensure that there is no problem, then use mechanical pig to do pigging. The mechanical pigging device can pigging the impurities and foreign objects in the gas pipeline, and can also use its steel brush to remove the wax and corrosion rust in the pipeline wall.
(2) Pigging methods for gas pipeline applications
The gas in the natural gas pipeline usually contains carbon dioxide and hydrogen sulfide. These gases combine with water to form hydrate, which causes the blockage of natural gas transmission pipeline and equipment. It is a hidden danger threat to the use of the gas pipeline. For the gas transmission pipeline put into operation, the traditional pigging device is selected for pigging. For example, the use of a bowl pig, for the transmission of moisture gas pipeline, its main job is to remove the water in the pipeline, then we will choose a very good sealing straight pig to remove, If it is from the ground reservoir or gas field out of the pipeline, the pipeline will contain light hydrocarbons and other condensate, we directly use the straight pipe cleaner to remove treatment.
(3) Pigging method of natural gas pipeline for oil-gas-water mixed transportation
In the long-term use of long distance natural gas pipeline, two-phase flow pipeline in long-term operation, There will be the corrosion of the liquid inside the pipeline and the inner wall, the liquid accumulates at the low point of the pipeline, leading to the pressure drop of the line, reducing the flow area in the pipeline. The efficiency of transporting gas and natural gas will also be reduced. The factors that influence the pigging effect of two-phase flow pipeline include: the flow velocity and pressure drop of gas-liquid mixture in the pipeline; The ratio of gas to liquid in the pipeline, the shape of the vertical section of the pipeline; The amount of fluid accumulated in the pipeline; Size of slug trapping instrument; In an offshore pipeline system, the height between the bottom of the riser and the slug trap. The common pigging method for two-phase flow pipelines is to use the general pigging balls and pigging, and also to use pigging with bypass.
III. Key Contents of Pigging Technology for Long-distance Natural Gas Pipelines
(1) Relevant procedures for arranging pigging work
There is a certain risk when pigging technology is used in natural gas pipeline. If the staff carelessly because the pig pipe stuck in the gas pipeline, then the pipeline must be cut to take out the pig, so it is easy to make gas pipeline leakage in the course of transportation. Drawing lessons from past pigging operations in China, staff in the process of pigging, The first step we should do is to use the straight pigging device to explore, clear the internal situation of the gas pipeline, and then use reasonable methods to pigging gas pipeline according to the specific situation; In carrying out the pigging process, particular attention should be paid to the gradual, not just speed.
(2) Formulating specific pigging standards for pigging requirements
The pigging of long-distance natural gas pipeline is complicated and risky. Before pigging, careful planning and detailed pigging standards should be made. The staff should first search the relevant drawings of the pipeline, in order to clearly understand the detailed parameters and actual situation of the gas pipeline; Then, the staff will work out a reasonable and detailed programme of cleaning measures, in accordance with the established operating procedures to carry out, step by step. First of all, the pigging personnel use the foam pig to inspect the gas pipeline. If there is no serious damage to the pipeline, That can use the erfly bowl pig to do its processing work, such as pipe wall debris not more than 20 kg, can use diameter suitable aluminum plate to deal with. If there is no major damage or serious curvature of the gas pipeline after inspection, the pig can use the straight pig for two-way transmission. Generally speaking, the pigging must be carried out according to the standard procedure, and not too hastily.
(3) Efficient supervision and administration shall be carried out in the service of pigging of long-distance natural gas pipelines
In the service operation of pigging for long-distance natural gas transmission pipelines, pigging operators must first do the relevant preparatory work. In the pipeline before pigging on the inlet valve, outlet valve and other parts of the valve should be carefully maintained and checked. In addition, it is necessary to strengthen the drilling of pigging technology to prevent the use of collision, friction and other sparks; Special attention should also be paid to the use of nitrogen to reduce the pressure differential when the ball is retracted from the cylinder. There is also a reducer in the cylinder joints but also to install a balance valve, in the slow process of boosting, in order to open the cylinder in and out of the valve, then close its balance valve so that service operations can be normal and effective.
