With the popularization and application of automation control technology in the oil and gas industry, the degree of automation of oil and gas processing stations is also increasing. The realization of automatic control technology is realized by various types of control instruments. The power sources of these control instruments mainly include electric drive, hydraulic drive and pneumatic drive. Electric drive and hydraulic drive are often expensive, complicated in structure and difficult to be explosion-proof. Pneumatic drive technology, low price, simple structure and easy to explosion-proof, so the current automatic control instrument power source on the oil and gas processing station field is mostly pneumatic control. The energy used in the pneumatic control of the automatic instrument is compressed air. The compressed air is the air that meets certain requirements such as pressure, temperature and dew point after a certain treatment process. The design of the compressed air system of the oil and gas processing station is to choose a reasonable treatment process. Convert ordinary air into qualified compressed air.
At present, although there are standards and regulations for the design of compressed air systems in China (such as GB50029-2003 compressed air station design specifications), in general, the contents are slightly more general and broad, and some details on the design of compressed air systems are not enough. Details (such as compressed air consumption, compressed air quality requirements, etc.), and these details often affect the rationality of the design of the compressed air system, so the author combined with his own practical experience, elaborated the design of the compressed air system of the oil and gas processing station .
1 The quality and consumption of compressed air requires the design of the compressed air system of the oil and gas station. The main purpose of the design is to ensure the production of qualified instrument air and factory atmosphere to meet the driving requirements of various pneumatic control valves and various pneumatic equipment downstream of the compressed air system. .
Instrumentation: Mainly to provide driving force for various control valves, emergency shut-off valves and emergency relief valves. The quality of the instrument air source is very high, which is mainly to prevent the compressed air from condensing liquid and impurities blocking the pneumatic valve drive.
Factory atmosphere: mainly for the pneumatic equipment such as air pressure hoist, pneumatic wrench, sand blasting machine, paint spraying machine, etc., but also to use the factory air to purge the oil and gas processing equipment to be repaired, only to clean the equipment to be repaired After the completion, the maintenance personnel can enter the inside for welding, cutting and other hot-fire operations. This is because if the hydrocarbon vapor is not cleaned, the hot-fire operation may cause an explosion and endanger personal safety. The quality requirements of the factory wind source are not high.
1.1 Quality requirements for compressed air 1.1.1 Quality requirements for instrument atmospheres 654-2-1995 Working conditions for industrial process measurement and control equipment Part 2: Power, IEC 60654-2-1979, operating conditions for industrial process measurement and control equipment Two parts: power and ISO8573-1-2001 generally use compressed air quality grade regulations, qualified instrument air should be free of oil droplets and other droplets, no toxic, corrosive and flammable and explosive gases, solid The particle content is less than 0.1g/m3, and the maximum solid particle diameter is not more than 3pm. For the air supply pressure regulation of the instrument wind, the working conditions of the industrial process measurement and control device of IEC GB/T17214-2005 and the industrial automation of GB/T4830-1984 The gas source pressure range of the instrument air source pressure range and quality is: the nominal pressure value of the gas source at the input of the instrument is 140, 260, 350, 550 and 700 kPa. In design, to ensure that the wind pressure of the station instrument is sufficient, often Take the maximum air source pressure, that is, the instrument air pressure design pressure is 700kPa. Some pneumatic instruments may not need such a large air source pressure. At this time, it is often necessary to install a pneumatic instrument inlet. In addition to meeting the requirements of the instrument air pressure design pressure of 700 kPa, it is also required to ensure that the residual pressure of the instrument wind to the farthest user is not less than 550 kPa, and the pressure at any point of the instrument air supply system is not allowed to be lower than 420 kPa. The pressure dew point value of the wind (dew point value under design pressure) stipulates that different specifications have different requirements. The IEC654-2, IEC60654-2, piping manual PipingHandbook and Shell Shell's instrument wind pressure dew point value are at the maximum working pressure. The lower (700 kPa) dew point value is 10 times lower than the lowest ambient temperature. However, if the compressed air system leaks or the equipment in the system is vented, the expansion cooling effect will cause condensate to precipitate in the system, and there will be ice formation. At this time, the pressure dew point ratio A minimum ambient temperature of 10 feet is easy to achieve, so many specifications consider this pressure dew point to be unsafe. The US Gas Suppliers Association's GPSA standard and the ANSI-ISA 7.0.01-1966 measured gas quality standard recommend a pressure dew point value that is -40 at the system's maximum working pressure (700 kPa), so the instrument is determined during the design process. This value can be considered when using wind pressure dew point values.
8573-1 is an internationally recognized compressed air quality standard. The classification of compressed air quality grades is shown in Table 1. Table 1 ISO8573-1 for compressed air quality grades Compressed air quality grade 1m3 The maximum number of solid particles contained in compressed air Dew point / (0.7MPa) oil content particle size 1.1.2 Factory air quality requirements There is no special requirement for the factory air quality. After the compressor compressed air is removed by the aftercooler and the gas-water separator, the condensate is removed. It can be used as a factory atmosphere. At this time, the factory relative humidity is 100%, but the pressure of the factory atmosphere should be maintained at 600~700kPa. 1.2 Calculation of compressed air consumption 1.2.1 Calculation of instrument air consumption Generally, oil and gas processing The main equipments that consume instrument air in the station are: actuators of various control valves, emergency shut-off valves, emergency relief valves, sequence valves, online analyzers (such as gas chromatographs). The calculation of the instrument air volume under normal conditions (101.325kPa, 0) can be estimated according to the following regulations: control valve air consumption per control circuit: 1~ emergency shut-off valve and emergency relief valve (usually adopted Right-angle rotary ball valve) Instrument air consumption: 0.1m3/h. Gas chromatograph instrument air consumption: 16m3/h. If there is skid-mounted equipment in the oil and gas processing station (such as turbine-driven centrifugal compressor skid, drinking In the case of water treatment skid, torch skid, etc., and the designer can not know how many control valves and emergency shut-off valves are in the skid, consult the skid-mounted equipment manufacturer to obtain the instrument air consumption.
After calculating the instrument air consumption under normal conditions using the above regulations, a 30% margin should be taken to prevent possible fluctuations in gas consumption.
1.2.2 Calculation of factory air consumption The oil and gas processing station generally needs to use the factory atmosphere to drive various pneumatic tools, or use the factory air to purge the oil and gas processing equipment to be repaired, but the frequency of operation using the factory atmosphere is generally not high. And the probability of using a variety of pneumatic tools at the same time is very small, so the factory air consumption can be 70m3 / h without detailed regulations, this consumption allows a spray gun to be operated at the same time (gas consumption is 20m3 / h) and At least one of the following pneumatic equipment: pneumatic drill and pneumatic wrench (air consumption is 30~50m3/h).
2 Compressed Air System Design 2.1 Compressed Air System Main Process Description The main flow of the compressed air system is shown. After the air compressor is compressed, the temperature of the air rises. Therefore, the aftercooler is used to cool the high-temperature compressed air. After the cooled compressed air, water and oil droplets (injection screw compressor) are precipitated. At this time, the separator is used to condense the liquid. separate from. The compressed air treated by the separator is first stored in the factory air buffer tank. At this time, a part of the compressed air can be sent to the factory wind user as the factory atmosphere, and the rest of the compressed air flows to the instrument air treatment system: first through the pre-filter The compressed air is further filtered, and the compressed air after being purified by filtration enters the suction dryer, and the dew point of the compressed air is reduced to a required value by a suction dryer, and the compressed air sucked by the suction dryer is filtered and purified by the filter after passing through the filter. The purified compressed air after purification is stored in the instrument air buffer tank, and at this time, the qualified instrument atmosphere can be provided to the downstream users of the instrument wind.
From the process description, it can be seen that the main equipment of the compressed air system are: main backup motor driven air compressor, diesel engine driven emergency mobile air compressor, aftercooler, gas-water separator, pre-filter and post-filter , air dryer, instrument air buffer tank and factory air buffer tank, automatic drain valve and pressure reducing valve.
2.2 Air Compressor The compressors used in the compressed air system are mainly divided into two categories: the first type is the main and standby compressors. In normal operation, the compressed air system mainly uses the main and standby compressors for compression. The machine is mainly driven by electric motor, and has dual power supply system, that is, mains and emergency diesel generator power supply, and the main power is the first power supply (in the remote and remote areas without mains, gas generator can be used as the first power supply). ), the emergency diesel generator is powered by the second power source.
The second category is emergency mobile compressors, which are only used in emergency situations. These compressors are powered by diesel engines. Emergency mobile compressors are not required and are recommended only if the safety requirements of the compressed air system are relatively high. Currently, Shell's standard ShellDEP, Malaysian Petronas standard and the US Veco Design Company standard recommend the use of emergency mobile compressors in compressed air systems.
At present, most types of air compressors are mainly screw compressors, reciprocating piston compressors and centrifugal compressors. When the calculated compressed air volume Q矣2500m3/h, a screw compressor or a reciprocating piston can be used. Compressor, and when the calculated amount of compressed air Q > 2500m3 / h, the centrifugal compressor is economical.
Since the demand for compressed air in oil and gas stations is often not very large, screw compressors and reciprocating piston compressors are the most widely used in compressed air systems. Compared with the screw compressor, the reciprocating piston compressor has many wearing parts, high noise, low efficiency and unstable gas supply. Therefore, the air compressor currently used in the compressed air system of the oil and gas station (main use, standby) And emergency mobile compressors) mostly use screw compressors.
Screw compressors are generally classified into oil-injected screw compressors and dry screw compressors by cooling. The oil-injected screw compressor absorbs and takes away the heat generated by the compression process by injecting a certain amount of lubricating oil into the cylinder, thereby improving the heat exchange during the compression process, reducing the exhaust temperature, etc.; on the other hand, lubricating, sealing and Due to the effects of noise reduction, the fuel injection compressor has a faster development than the dry screw compressor due to its low exhaust gas temperature, high pressure ratio, low leakage, low noise, simple structure and reliable operation. Accounted for 80%. See the injection screw compressor process.
2.3 Lead-Lag control of air compressors The Lead-Lag control can be translated as “lead-lag control of compressorsâ€, which means that one air compressor is used as a Lead compressor and the other is used as a backup compressor. Lag compressor. The Lead-Lag control can automatically start and stop the main and standby air compressors to prevent the instrument from being over- or under-pressured. General volumetric air compressors require Lead-Lag control, while centrifugal compressors do not have this control mode.
The Lead-Lag control of the volumetric air compressor is generally measured by the four pressure switches measuring the working pressure fluctuation of the instrument wind buffer tank, and automatically starting and stopping the main backup compressor by detecting the pressure of the instrument wind buffer tank. Since the compressors in the compressed air system that require the Lead-Lag control mode have two main and standby compressors, in order to complete the start and stop operations of each compressor, four pressure switches must be used.
There are two main ways to select the installation position of the pressure switch: one is to install directly on the instrument air buffer tank; the other is to install on the inlet pipe of the instrument wind buffer tank.
Due to the large number of pressure points, most of the current pressure switch installation positions are placed on the inlet pipe of the instrument air buffer tank, as shown: four pressure switches are installed on the inlet pipe of the instrument air buffer tank, and these four are used. The pressure switch senses the pressure change of the instrument wind buffer tank and controls the start and stop of the main backup compressor. The set values ​​of the four pressure switches are PSHH (pressure high one high), PSH (high pressure), PSLL (pressure low one low), and PSL (low pressure). 2.3.1 The pressure fluctuation range of the instrument wind buffer tank needs to know the pressure fluctuation range of the instrument wind buffer tank. 2.3.1.1 For the condition that there is no pressure reducing valve downstream of the instrument wind buffer tank, the pressure fluctuation range of the instrument wind buffer tank should be the pressure fluctuation range of the instrument wind user plus the pressure drop along the gas line, ie Pmin=550kPa+ Piss, Pmax=700kPa+Pi generally takes Piss=100kPa (this is the estimated value), then it can be determined that the pressure fluctuation range of the instrument wind buffer tank is (650kPa, 2.3.1.2 for the pressure relief valve downstream of the instrument wind buffer tank) In general, there should be a pressure reducing valve downstream of the instrument air buffer tank. As shown in the figure: two PCV pressure reducing valves are installed side by side downstream of the instrument air tank. The instrument air pressure is high before the pressure reducing valve, and the pressure is reduced by the pressure reducing valve. The pressure of the instrument wind is reduced to the pressure required by the user. The purpose of the pressure reducing valve is mainly to reduce the size of the instrument air buffer tank after the operating pressure is increased, otherwise the size of the buffer tank may be reduced if there is no pressure reducing valve. It will be very large. At this time, Pmin=the pressure drop value when the valve is fully open, generally can take P=200kPa (this is the estimated value). At this time, the pressure fluctuation range of the instrument wind buffer tank is 850~1000kPa. This pressure fluctuation range is not specified. rigid Different design companies may have different regulations on the pressure fluctuation range, such as. 2.3.2 Switch design value With the pressure fluctuation range of the instrument wind buffer tank, PSHH, PSH, PSLL 100kPa can be set. For example, There is no pressure reducing valve downstream of the wind buffer tank. The Lead-Lag control setting value is shown in Table 2. Table 2 The setting value of the pressure switch when there is no pressure reducing valve downstream of the instrument air tank Compressor starting pressure / kPa deactivation pressure / kPa The main compressor (Lead) standby compressor (Lag) has a pressure reducing valve downstream of the instrument air buffer tank. The typical pressure switch setting values ​​are shown in Table 3. Table 3 Pressure of the pressure reducing valve downstream of the instrument air tank Switch setting value Compressor starting pressure / kPa deactivation pressure / kPa main compressor backup compressor 2.4 factory wind buffer tank size determination Factory wind buffer tank is close to the air compressor and is located downstream of the aftercooler, at this time the factory The compressed air received by the wind buffer tank is saturated humid air which is cooled by the aftercooler without being treated by the dryer, that is, the relative humidity of the compressed air is 100%, so the factory wind buffer tank is also called a wet air buffer tank; And because the factory wind buffer tank supplies air to the instrument air treatment system, it is in the upstream position relative to the instrument air buffer tank, so the factory wind buffer tank is also called the first stage buffer tank. The volumetric air compressor (reciprocating, screw and sliding) The compressed air at the exit of the chip type is connected to the factory air buffer tank after passing through the aftercooler and the gas-water separator. This is mainly due to the following considerations: balancing the pressure fluctuation of the gas source; further separating the condensed water and oil. ; store part of the gas volume to balance the changes between user gas consumption and gas demand.
There is currently no universally applicable method or formula to calculate the volume of the buffer tank, but the designer uses the following formula to calculate the volume of the buffer tank: a buffer tank volume / m3; compressed air temperature / K; compressed air flow / ( M3 / h); a buffer time / h; a buffer tank maximum pressure / kPa; a buffer tank minimum pressure / kPa. With buffer time and working pressure fluctuation range, you can follow the adsorption capacity of the public water and adsorption occurs The partial pressure of water vapor in compressed air is related to this principle. When the partial pressure of water vapor in compressed air is low, the adsorption capacity of the adsorbent to water is small, and when the partial pressure of water vapor in compressed air is high, the adsorbent is on water. The adsorption capacity is large. Using the characteristics of the adsorbent, the pressure in the dryer is changed while the dryer is running. At high pressure, the adsorbent starts to adsorb the moisture in the compressed air, and the moisture absorbed in the adsorbent starts to evaporate and is dried under low pressure or under vacuum. The purge gas is taken away to complete the regeneration of the adsorbent bed.
3 Conclusion Compressed air (instrument wind and factory wind) is widely used in oil and gas stations, but the relevant standards and regulations for compressed air stations in China are relatively broad and not detailed enough. Therefore, this paper combines the experience and relevant standards in the work. The various points and details that need to be paid attention to in the design of compressed air station are expounded in detail. The principle, selection and calculation method of some important equipments are introduced, which has certain practical significance for designers to design compressed air stations better. : Sun Hai (1980-), male, Dalian, Liaoning, engineer, graduated from the University of Petroleum (East China) in 2006, master's degree, is now mainly engaged in the process design work of the compressor station project.
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