Classification of oil pipelines in pipeline cleaning has two categories:
The first is the internal oil pipeline of the enterprise. For example, the gathering and transmission pipeline connecting the oil well with the metering station or the combined station in the oil field. Or pipelines inside refineries and oil depots. Its length is generally short, and it is not an independent business system.
The other is a long-distance oil pipeline. For example, the pipeline transporting qualified crude oil from the oil field to the refinery or railway transfer station. Its pipe diameter is generally large, and there are various auxiliary projects. This kind of pipeline is an independent system. They have another name “trunk pipelines”.
According to the type of medium transported, oil pipelines include crude oil pipelines and product oil pipelines. To ensure the service life of the pipeline, both of them need pipeline cleaning.
1. Group Cost of A Long-Distance Oil Pipeline
A long-distance oil pipeline includes two parts: an oil station and a pipeline.
The main function of the oil transportation station is to pressurize and heat the oil products. The oil transportation station at the starting point of the pipeline’s other name is the initial station. Which receives oil products from oil fields, Refineries, or ports. Then, it is transmitted to the next station after metering. During the transportation process, due to dirt or terrain height, the oil pressure drops continuously. Therefore, in the long-distance pipeline, it is necessary to set up an intermediate oil transfer pump station to pressurize the oil.
In addition, users also need to conduct regular pipe cleaning. For the pipeline heated for transmission, the temperature of oil products gradually drops during the transmission process. This requires an intermediate heating station to heat the oil. The oil pump station and heating station are set together, they are called heat pump stations. The oil transmission station at the end of the pipeline is” the terminal station”. Which receives the oil and gas from the pipeline. Then it is transferred to oil refineries, railways, and waterways. The terminal station has equipment such as many oil tanks and an accurate metering system.
The pipeline part of a long-distance oil pipeline includes the pipeline itself, valve chambers along the line, crossing structures through rivers, highways, and valleys, cathodic protection facilities, communication, automatic control lines, etc. Long-distance oil pipelines are welded by steel pipes and generally laid underground. This part is also the main part of pipeline cleaning.
To prevent the soil from corroding the steel pipes, the pipes are covered with an anti-corrosion insulation layer and protected by the electric method. Block valve chambers are set at intervals at both ends of long-distance oil pipelines and large crossing structures. Once an accident occurs, cutting off the oil in time in the pipeline can prevent the expansion of the accident and facilitate emergency repair. The communication system is an important facility for a long-distance oil pipeline. Which is used for the production scheduling of the whole line and the transmission of system monitoring information. The main communication modes include microwave, optical fiber, and satellite communication.
2. Characteristics of the long-distance oil pipeline
Compared with railway, highway, and waterway transportation of oil products, pipeline transportation has the following advantages.
The Advantages of The Pipeline Transportation
(1) Large traffic volume.
(2) Low freight, less energy consumption, and the larger the caliber, the lower the unit freight.
(3) We usually bury oil pipelines underground, which are relatively safe and reliable. They are less affected by the environment and climate and have less pollution to the environment. The loss rate of oil transported is lower than that of railway, highway, and waterway transportation.
(4) The construction investment is small and the floor area is small.
Although pipeline transportation has many advantages, it also has its limitations:
The Limitation of The Pipeline Transportation
(1) It is mainly applicable to large-scale, one-way, and fixed-point transportation. Which is not as flexible and diverse as vehicle and ship transportation.
(2) For a certain diameter pipeline, there is an economic and reasonable range of transmission capacity.
(3) There is a limit on the throughput. It requires regular maintenance. Lack of pipeline cleaning will reduce the efficiency of pipeline transportation
3. Operation And Control of Oil Pipeline
(1) Connection Mode of Oil Transfer Pump Station
Another name for the connection mode of a long-distance oil pipeline pump is pipeline transportation mode. It mainly has two types, namely, the “pump to pump” transportation mode and the “side connected oil tank” transportation mode.
The “pump to pump” transmission mode`s other name is the secure transmission. Which connects the oil transmission trunk line from the previous station to the suction pipeline of the oil transmission pump of the next station. During normal operation, there is no side-connected oil tank for regulation. (Most pump stations have small accident tanks). Its characteristic is that the transmission capacity of each station must be equal, the inlet and outlet pressure of each station has a direct impact, and the whole line forms a unified hydraulic system. This oil transportation mode is convenient for the unified management of the whole line, but it should have reliable automatic control and protection measures.
The “side connected oil tank” transportation mode is that the oil transmission trunk line from the previous station has a connection with the suction pipeline of the oil transmission pump of the next station
. And the side-connected oil tank connected to the atmosphere is connected in parallel to the suction pipeline. The side-connected oil tank plays a role in regulating the difference between the two stations. Because of its existence, the long-distance pipeline has several independent hydraulic systems. The pipeline operated in this way is convenient for manual control, and the automation level of the pipeline is not high. However, it is not conducive to the full utilization of energy, and there is a volatilization loss of oil products in the bypass tank.
(2) Water Hammer And Its Control in Oil Pipeline
One of the keys to the closed transportation of oil pipelines is to solve the problem of the “water hammer”. “Water hammer” refers to the impact on the pipeline caused by the sudden change of liquid flow velocity in the pipeline due to a sudden pump stop (power failure or failure) or wrong valve closure, and the sudden sharp rise or fall of pressure in the pipeline due to the inertia of the liquid in the tank.
Two Situations of Pipeline Overpressure:
First, the pressure surge of the water hammer (pressure wave higher than the normal operating pressure) may cause the pipeline pressure to exceed the allowable maximum working pressure and cause pipeline rupture;
The second is that the decompression wave (pressure wave lower than the normal operating pressure) may reduce the pressure of the pipe section with low pressure to the saturated vapor pressure of the liquid during steady-state operation, resulting in liquid flow separation. For long-distance pipelines with intermediate pump stations, the decompression wave may also cause the inlet pressure of downstream pump stations to be too low. This also affects the normal suction of the downstream pump unit.
Two methods usually work to solve the problem of water hammers, namely, discharge protection and advance protection. Relief protection is to install an automatic pressure relief valve system on the pipeline. When the water hammer pressurizes the waveguide to make the pressure in the tube reach a certain value. The valve will discharge a certain amount of oil. This can reduce the pressure wave and prevent the damage caused by the water hammer. Advance protection means that when a water hammer occurs, the pipeline control center quickly sends instructions to relevant pump stations. Then each pump station shall take corresponding protective measures to avoid the harm caused by the water hammer.
4. Sequential Pipeline Transportation of Different Oil Products
In a unified pipeline, the pipeline continuously transports multiple products in a certain order. This transportation method is sequential transportation. Long-distance pipelines transporting product oil generally use this transportation method to transport oil and gas. This is because there are many kinds of oil products, and the batch of each oil product is finite. When the transportation distance is long, it is uneconomical or even impossible to lay a small diameter pipeline for each oil product. However, we adopt sequential transportation, add the transportation volume of various oil products, and lay a large-diameter pipeline. Which will greatly reduce the oil transportation cost. When several crude oils of different quality are transported by the same pipeline, to avoid the “degradation” of high-quality crude oil caused by the mixing of different crude oils, or to ensure the quality of finished oil, sequential transportation is also adopted.
Trunk Gas Transmission
A long-distance gas transmission pipeline has another name “a trunk gas transmission pipeline”. It is a transportation channel connecting the natural gas origin and low consumption. The medium transmitted is generally treated commercial natural gas that meets the gas quality requirements of pipeline transportation. Long-distance trunk lines have a large diameter and high pressure. The distance can reach several kilometers. The annual gas transmission volume of large diameter trunk lines can reach tens of billions of cubic meters. Long-distance gas transmission pipeline mainly includes gas transmission pipeline section, initial station, compressor station (also known as compressor station), intermediate gas receiving station, intermediate gas off load station, terminal station, pigging station, trunk line block valve station, etc. The structure and process of a gas transmission pipeline depend on the specific conditions of the pipeline, and it does not necessarily include all these parts.
It has a connection with the oil pipeline. Intermediate block valves shall be set at certain intervals along the pipeline connection to facilitate shutdown in case of accident or maintenance. There are cathodic protection stations and other auxiliary facilities along the pipeline to protect the underground pipeline from corrosion. The other two subsystems that usually need to be constructed synchronously with long-distance gas transmission pipelines are the communication system and instrument automation system. These two systems are the basis of the supervisory control and data acquisition system. Their function is to conduct real-time monitoring, control, and remote operation of the pipeline operation process. To ensure the safe, reliable, efficient, and economic operation of the pipeline.s
Compressor Unit And Compressor Station:
The compressor and its supporting prime mover are collectively referred to as compressor units. The compressor unit is the main process equipment of the main gas transmission pipeline. And it is also the core part of the compressor station. The basic function of the compressor station is to use the compressor to increase the gas pressure to overcome the pipeline resistance and meet the gas supply volume and pressure requirements along the pipeline.
At present, it uses two types of gas transmission compressors in the main gas transmission pipeline, namely, reciprocating compressor and centrifugal compressor.
A reciprocating compressor has another name “piston compressor”. Its basic working principle is to realize gas compression by using the reciprocating motion of the piston in the cylinder and the opening and closing of the coordinated suction valve and discharge valve. Many reciprocating compressor units used on the main gas transmission pipeline are integrated, that is. The compressor has a connection with the matching prime mover. Reciprocating compressors are mainly applicable to occasions with the medium and small flow and high-pressure ratios. Such as gas field gathering and transmission pipe networks, ground gas injection systems of underground gas storage, etc.
The basic working principle of the centrifugal compressor is to use the high-speed rotation of the impeller to improve the kinetic energy of the gas. And then convert the kinetic energy into pressure energy through the diffuser. To make the gas discharged from the compressor reach a higher pressure. According to the required pressure ratio, the centrifugal compressor can be single-stage or multi-stage. The centrifugal compressor is mainly applicable to occasions with large and medium displacement but low pressure. Such as the trunk gas transmission pipeline.
(2) Prime mover
The prime movers used to drive the gas compressor on the main gas transmission pipeline include a gas turbine, steam turbine, gas engine, diesel engine, and motor. Among them, gas turbines and gas engines are the most used. Their main advantage is that they directly use the natural gas transmitted in the pipeline as fuel. So the energy supply cannot restrict them. The motor can work to drive both reciprocating compressors and centrifugal compressors. It is still a choice worth considering where there is a reliable power supply and the electricity price is cheap.
(3) Compressor station
The station equipped with a gas transmission compressor on the gas transmission pipeline is the compressor station or compressor station. The main process facilities in the compressor station are compressor units. In addition, the compressor station usually includes gas dust collectors, gas cooler, pig trap, and other process facilities. If the compressor station also has the function of a gas receiving or distributing. It should have gas flow metering and pressure regulating devices. One compressor station generally has multiple compressors as equipment. Its specific number is related to the connection mode, the flow and pressure ratio of the compressor station, and the specification and performance of the selected compressor. In a compressor station, the compressors can be connected in series, parallel, and hybrid. However, centrifugal compressor and reciprocating compressor should not work at the same time.