How to Design An Air Compressor Station in Oil Refineries

Compressed air in Oil Refineries is the most commonly used power source in domestic refineries. Compared with an electric power drive, using compressed air for power can help avoid explosion-proof issues. Therefore, the majority of self-control valves in refineries are pneumatically adjusted. Compressed air is divided into purified air and non-purified air. The main purpose of purified air is for instrumenting devices with clean air. It involves dehydrating, removing oil, and treating compressed air to eliminate dust. The quality of supply air for refinery purification should generally meet the following criteria: dust content <5 mg/m³, dust particle size <3 μm, oil content <10 mg/m³, and the pressure dew point should be at least 10℃ lower than the local extreme minimum ambient temperature. On-purified air is used as power air in refineries with no special requirements on the quality of the air source, such as pipeline blowing, power source for wind wrenches, etc. Only the pressure requirements need to be met; there are no cleanliness requirements. Therefore, the use of non-purified air costs much less than purified air. Refineries typically have specialized air pressure stations to supply purified and non-purified air throughout the entire plant, ensuring a stable air supply.

1. Air pressure station site selection: Oil Refineries

The air pressure station in Oil Refineries serves as the power supply center for the entire plant, playing a role in the refinery equivalent to that of the human heart. Theoretically, the empty pressure station should be located in the center of the entire plant. This is because it has the shortest piping for air supply in all directions and the lowest compressed air pressure, enabling an optimal equipment configuration. However, most refineries’ air pressure stations are not located in the center of the entire plant. The main principles guiding their siting are as follows:

(1) The positioning of the air pressure station should be coordinated with other utility installations in the plant (e.g., circulating water field, nitrogen station, etc.) and should be arranged together to create a unified installation for easier management whenever feasible.

(2) The arrangement of the air pressure station should be carefully planned to avoid proximity to areas where explosive, corrosive, toxic gases, dust, and other harmful substances are present. It should be situated in locations where the prevailing wind direction is least frequent throughout the year on the downwind side. Additionally, it should be close to major wind users, consider the availability of electricity and recycled water, and allow for potential expansion.

(3) The placement of the air pressure station should be integrated with the overall layout and vertical orientation of the entire plant, taking into account all aspects comprehensively.

2. Determination of air compressor station capacity: Oil Refineries

The capacity of the air pressure station needs to be determined based on the whole plant air consumption, and the amount of purified air and non-purified air is determined separately and then superimposed. Refineries generally have several units, and the continuous and intermittent dosages of purified and non-purified air should be determined separately for each unit. The more reasonable calculation methods for determining the capacity of the air pressure station include: the maximum continuous usage of purified air in the whole plant + the maximum continuous usage of non-purified air in the whole plant + 1 maximum purified air intermittent user + 1 maximum non-purified air intermittent user. From this, the amount of pure compressed air can be calculated, to which a 10 percent network loss and a 10 percent margin can be added to determine the capacity of the air pressure station. After the practice test, the capacity is determined more reasonable, that is, to ensure that the whole plant’s air supply, but not a waste of capacity.

3. Introduction to the classic process

Natural air is filtered into the air compressor, pressurized to the required pressure, and then a portion of it enters the dryer for purification to produce clean air; the remaining portion (whether it requires cooling or not depends on the process) is used directly as unpurified air for the device. Buffer tanks should be installed before the purified and non-purified air leaves the station to stabilize the air supply pressure. The specific process also needs to be determined based on the selection of each equipment.

4. Selection points for the main equipment

The main equipment of an air pressure station in Oil Refineries includes inlet filters, air compressors, and dryers. The appropriateness of selecting this equipment directly determines whether the operation and maintenance of the air pressure station will be convenient.

4.1 Inlet filter

The inlet filter is positioned at the top of the air compressor inlet pipe, serving as the essential entry point for air into the compressor. It plays a crucial role in determining the quality of the compressed air.

Commonly used inlet filters include wire mesh filters and self-cleaning cartridge filters. The filtration precision of a wire mesh filter is low, resulting in poor filtration effectiveness. It can only filter large impurities, such as leaves. It is commonly used in conjunction with reciprocating compressors. The self-cleaning cartridge filter utilizes a high-precision cartridge with excellent filtration precision and effectiveness. The air filtered by the self-cleaning cartridge filter is sufficiently clean to meet the quality standards of purified air. This filter is commonly used in combination with oil-free centrifugal compressors.

Self-cleaning cartridge filters utilize high-precision cartridges; however, they are prone to clogging easily with dust. This setup requires a filter with a filter element positioned both before and after the differential pressure monitoring device and blowback function. When the filter element becomes sufficiently clogged, the differential pressure monitoring device will emit a blowback signal. During this process, a portion of the filter element undergoes blowback, and this section regains its filtration capacity once the blowback process is completed. As the self-cleaning filter always has a part of the cartridge in a blowback state, the flow range selection requirements for the self-cleaning filter should be 2 to 3 times the air compressor flow rate to ensure a sufficient circulation area.

 In terms of material selection, it is necessary to use stainless steel where the air flows through the channel to prevent metal parts from rusting. If rust enters the system, it can adversely affect the air quality or cause damage to the impeller of the centrifugal compressor, leading to significant economic losses. In addition, the filter in the arrangement of the inhalation port should be positioned as high as possible, at least 4 meters above the ground, to prevent the inhalation of debris in the vicinity. Simultaneously, an aperture of approximately 10 mm should be set at the entrance with a metal mesh to filter out leaves and other debris.

4.2 Air compressor

The air compressor is the central equipment of the air pressure station, and it is also the most precise, complex, and costly equipment in the station.

Commonly used air compressors include screw, piston, and centrifugal types. The screw compressor is generally smaller, for the occasion below 20m3/min; the piston compressor structure is complex, covers a large area, and the volume of gas can generally reach more than 80m3/min, the application is more widespread; the centrifugal compressor mechanism is precise, smooth operation, low failure rate, in recent years has been widely used. The vast majority of refineries use centrifugal compressors, with the general lower limit of the gas volume range of 60m3 / min, the gas volume and then lower investment is not as economical as the piston compressor, the upper limit of the gas volume of up to 500 ~ 600m3 / min, the larger the gas volume in the investment in equipment, the more economical, but the larger the model, the larger the footprint, the noise level is also higher. A more reasonable range of gas volume in the 150 ~ 400m3 / min.

The centrifugal compressor shell has two splits: horizontal split and vertical split. Vertical split is mostly used for high-pressure compressors, while horizontal split is mostly used for low and medium-pressure compressors. As the vertical split model is not suitable for removing the large cover when dismantling and inspecting the gearbox, there should not be any obstacles in front of the machine. The horizontal split structure type requires only a crane for dismantling and inspection. The large cover can be lifted without any obstacles above it. Therefore, it is convenient to overhaul and easy to assemble, and the majority of manufacturers’ models are horizontally split.

Air compressors are divided into two categories based on lubrication: oil lubrication and oil-free lubrication. This is based on whether the machine lubricant will contaminate the compressed air. Early compressors are mostly oil-lubricated. Sometimes, there will be a trace of oil droplets in the compressed air at the outlet of the compressor. To address this, you need to install a de-oil filter before the compressed air enters the dryer. However, due to the instability of the de-oil filter, it often occurs that oil-containing gas enters the drier and contaminates the desiccant. In recent years, with the increasing precision of machine processing in China, oil-free lubrication compressors have become the trend. This model is not only easy to operate and maintain, but more importantly, it can ensure compressed air quality.

The outlet pressure of the air compressor is typically determined based on the requirements of the process gas. The general calculation method involves adding the normal gas pressure of the farthest device to the pressure loss of the conveying process and the resistance loss of the dryer. Under normal circumstances, the outlet pressure of the air compressor is 0.6-0.8 MPa.

Air compressors are typically equipped with two general types: screw machines and piston machines. It is recommended to have 1 to 2 spare machines for each type of model, which can be used for temporary adjustments in flow rate. In total, 3 to 6 spare machines are usually required. Centrifuges, due to their high operational stability, generally do not require a spare machine. The total number typically ranges from 2 to 5 units, and it is advisable to use the same model.

Air compressors are generally multi-stage compression, with coolers between stages. The outlet temperature of the last stage of the compressor is determined based on the selection of the dryer.

4.3 Dryer: Oil Refineries

Compressed air still contains a certain amount of moisture after post-cooling, and its moisture content depends on the temperature and pressure of the air. Commonly used gas drying methods in the industry include the freezing method, adsorption method, and mixing method. The freezing method involves using refrigeration equipment to cool compressed air to a specific temperature. This process causes the precipitation of supersaturated water, thereby achieving the drying objective. Typically, the dew point temperature of compressed air processed by the freezing method is 2～7℃. To achieve drier compressed air, the adsorption method can be utilized. The adsorption method of the dryer is the main equipment of the air pressure station to produce purified air, each dryer with two drying towers, drying towers filled with adsorbent (generally alumina and molecular sieve). The basic principle is saturated compressed air through the adsorbent, most of the moisture in the compressed air is adsorbed, to become dry purified wind, this process is called adsorption. When the adsorbent is saturated switch to another tower for adsorption, this tower needs to be warmed up so that the adsorbed water is analyzed, and then the pressure is reduced with a dry finished gas to bring out the water, this process is called regeneration. Due to the adsorbent regeneration needs the adsorbent temperature to be 130 ~ 200 ℃ (theoretically the higher the regeneration temperature regeneration is more complete), the heating of the heat source is different, in the dryer is divided into no heat regeneration, micro-heat regeneration, and residual heat regeneration and so on several kinds of type:

(1) Heatless regeneration: the regeneration process of desiccant does not need to warm up the adsorbent but only needs to lower the pressure and use a large number of finished products to purify the wind to bring out the moisture, because high temperature and low pressure is the key to desorption, heatless regeneration of desiccant is only operated under low pressure, it will lose a large number of finished products to purify the wind, the regeneration of the gas consumption of about 15-20% of the processing capacity.

(2) Micro-thermal regeneration: the desiccator is electrically heated adsorbent, creating favorable conditions of high temperature and low pressure, and its regeneration gas consumption is about 4% to 8% of the processed volume, and although the gas consumption has been reduced, the electricity consumption has increased.

(3) Waste heat regeneration: the drier is improved based on micro-heat regeneration drier, which makes full use of the compression heat generated by the compressor when compressing air to heat the adsorbent. If this kind of drier is used, it is necessary to cancel the cooler of the last stage of the air compressor, so that the outlet temperature of the air compressor reaches more than 120℃. Compressed air for adsorption before first entering the regeneration tower to heat the adsorbent, while taking out some of the water, and then through the water cooler to reduce the temperature to about 40 ℃ into the adsorption tower for drying, when the regeneration of adsorbent regeneration tower is completed, a small part of the finished gas out of the regeneration tower to reduce the temperature of adsorbent, and at the same time take away a part of the water, to further strengthen the effect of regeneration. The gas consumption is only 1-2% of the processing capacity.

Waste heat regeneration dryer because of its low energy consumption. In recent years, it has become more and more widely used. The air pressure station equipped with waste heat regeneration drier also needs to set up compressed air water cooler in parallel with the drier, which is used to cool the unpurified compressed air to below 40℃, as a non-purified supply device.

Regardless of the type of desiccator, a filter needs to be provided at its outlet to prevent broken sorbent from being blown out and carried into the system. The filter should be set up to monitor the differential pressure, and the cartridge should be cleaned as soon as the differential pressure exceeds a certain threshold.

The processing capacity of the dryer should be determined based on the demand for purified air throughout the entire plant. In northern cities, to prevent water in the non-purified air from precipitating due to cooling and freezing in winter, most refineries replace the non-purified air with purified air. This requires selecting the processing capacity of the dryer based on the air volume of the air compressor.

5. Piping design points: Oil Refineries

Piping design in the air pressure station is relatively simple, because the process of air pressure station is simple, single medium. However, piping design details should not be ignored, several details are as follows：

(1) The air compressor inlet pipe should be short and straight, and the number of elbows should be reduced as much as possible. The inlet filter should be arranged as close as possible to the inlet of the air compressor. The flow rate of air in the reciprocating compressor inlet pipe should be 10-20m/s, and its pressure drop should be in accordance with the requirements of the air compressor; the flow rate of centrifugal air compressor suction port pipe should be 10-40m/s, and its pressure drop should be in accordance with the requirements of the air compressor;

(2) Reciprocating compressor import and export pipelines need to do vibration analysis, and should make the pipeline’s intrinsic frequency to avoid the pipeline’s gas column intrinsic frequency and the machine’s excitation frequency.

(3) The flow rate of the compressor outlet pipeline is generally 8 to 20 m/s, and the pressure drop in 100 meters does not exceed 1% of the outlet pressure.

(4) The material of centrifugal air compressor inlet piping is generally stainless steel, and stainless steel is recommended for other types of air compressor inlet piping.

(5) Stainless steel is recommended for the purified air piping in the air pressure station. If the system uses galvanized piping, galvanized piping may also be used in the station with threaded connections, not welded.

(6) The inner walls of purified air ducts, valves, and accessories should be cleaned, degreased, or passivated before installation.

(7) Non-purified air ducts in cold areas shall be accompanied by heat in winter.

6. Conclusion

At present, China is at a crucial moment for energy conservation and emission reduction. Most of the refinery units have undergone process optimization, rational energy grading, and utilization, with limited room for further optimization. However, the air pressure station, as one of the main auxiliary devices in the refinery, directly affects the stability of the compressed air supply. This, in turn, is crucial for the smooth operation of the entire refinery. There is still a lot of room for process optimization, equipment selection optimization, and equipment matching optimization. With the continuous introduction of various new equipment, the design of air pressure stations can be more optimized and reasonable. Click here.