a. Fluctuation and attenuation of inlet pipeline pressure of built natural gas vehicle filling stations

a) Fluctuation of inlet pipeline pressure

The fluctuation of inlet pipeline pressure of built natural gas vehicle filling stations is manifested in two aspects:

First, due to the large-scale consumption of natural gas in large residential areas for cooking and bathing, the pipeline pressure temporarily decreases in the morning, noon and evening. However, in most periods of the day, the pipeline pressure is normal.

Second, the continuous decrease in pipeline pressure during the cold season of the year. In the non-cold season, the inlet pipeline pressure returns to normal.

The above two aspects of inlet pipeline pressure fluctuations are normal, and they occur more often in stations connected to the city's low-pressure natural gas pipeline network.

b) Attenuation of inlet pipeline pressure

The attenuation of inlet pipeline pressure is mostly due to changes in the geological conditions of the natural gas field, and sometimes because of the parallel connection of factories with huge gas consumption upstream of the pipeline network.

11 years ago, a conventional natural gas vehicle filling station in Qingbaijiang District, Chengdu, was equipped with a 90kW-class fully air-cooled CNG high-pressure compressor with a rated intake gauge pressure of 2.0MPa. This private enterprise station was the conventional station with the highest inlet pipeline pressure in China at that time. At the beginning of the operation of the station, everything was in good condition. Unexpectedly, just one year after it was put into operation, the inlet pipeline pressure dropped to 0.6MPa. If it is operated at this low pressure, the exhaust temperature of each level of the CNG compressor will reach a level that is not allowed for safe operation, and the suspension of the station is inevitable. In addition to the influence of geological conditions, the attenuation of the inlet pipeline pressure of the station is related to the small scope of the pipeline network in the area and the lack of connection with the main pipeline network.

b. The actual need for setting up a booster compressor

The fluctuation and attenuation of the inlet pipeline pressure of the built natural gas vehicle filling station, especially the continuous and large attenuation of the inlet pipeline pressure of some stations, urgently need to set up a booster compressor in front of the high-pressure CNG compressor.

New concept of station construction: configuring a booster when building a new station

Smart gas station owners with foresight have begun to implement a new concept of station construction: configuring a natural gas booster compressor in front of the high-pressure CNG compressor when building a new station. So, why is this so?

a. It eliminates the trouble of low pressure in the pipeline entering the station during the cold season of more than four months each year.

b. It eliminates the trouble of temporary reduction in the pressure of the pipeline entering the station during the peak gas consumption period every day.

c. After the pressure of the pipeline entering the station is attenuated, it can be easily dealt with.

d. The purchase cost of the booster is only a small part of the total equipment cost of the station.

e. The overall layout, process flow, equipment placement, strong and weak power systems, station building, etc. of the station are more reasonable, neat and smooth.

Flow and pressure matching between booster and high-pressure CNG compressor, as well as selection of booster structure type and structural type

a. The booster compressor should have the ability to adapt to small changes in intake pressure under the two fundamental premises that the air supply volume (Nm3/min) of the booster compressor matches the original high-pressure CNG compressor and the exhaust pressure matches the intake pressure of the original high-pressure CNG compressor.

First, it is entirely possible that the pressure of the inlet pipeline continues to decay; second, the possibility of upstream pipeline pressure rising (such as due to grid connection) also exists.

b. The recommended structure type of booster compressor is reciprocating piston type, not rotary type.

a) Reciprocating piston compressors have lower cost and lower operating power consumption, and their pressure ratio adaptation range is much wider than that of rotary type, especially when the number of compression stages is 2 or 3. This gives the reciprocating piston compressor a strong ability to adapt to changes in intake pressure.

b) Oil-free rotary compressors are too expensive, while oil-injected rotary compressors have the possibility of natural gas medium diluting oil or causing oil to slowly deteriorate.

c. Among reciprocating piston compressors, angle-type ones should be the ideal choice for booster compressors.

a) In addition to the inherent characteristics of reciprocating piston compressors such as low cost, low operating power consumption, and wide pressure ratio adaptability, angle-type compressors also have the outstanding advantages of the most compact structure, smallest outer dimensions, and smallest floor space. Therefore, they are most suitable for ideal installation within the original site of the gas station without the need for land expansion.

b) V-type and W-type compressors among angle-type compressors are easier to change to crankcases that bear internal air pressure than L-type compressors and symmetrical balanced compressors, thus becoming "leak-free compressors" with superior energy-saving and environmental protection effects.

c) It is reported that a novel and advanced high-rigidity, extremely compact V-type/W-type general series booster will be available soon.

Basic requirements for CNG compressors

The high-pressure natural gas compressor unit, which consists of a compressor, its driving motor and related equipment, is the most critical dynamic equipment for natural gas vehicle filling stations.

The power consumption of natural gas compression system equipment accounts for more than 90% of the total power consumption of gas filling stations. The purchase cost of compression system equipment is about one-quarter of the total purchase cost of gas filling station equipment.

a. Ensure the safe operation of compressors and gas filling stations

Although the physical and chemical properties of natural gas are not "flammable and explosive", but "combustible and explosive", its density is very low, it dissipates quickly in the atmosphere, the ignition point is as high as 650℃, and the explosion limit range in the air is very narrow, which is 4.8% to 13.46% (volume ratio). However, it is necessary to take precautions before it happens, and the amount of natural gas leakage from gas filling station equipment should be minimized.

The piston rod packing at the compressor cylinder is the main part of the compressor natural gas leakage, and it is the first hidden danger for the safe operation of the gas filling station. Therefore, the amount of natural gas leakage at this location should be strictly controlled within the allowable safety range.

b. Improve the operating reliability of the compressor's mechanical and electrical components

The compressor is the most critical dynamic equipment in a gas station. Because of the large number and variety of mechanical and electrical components, it is also the most prone to failure in a gas station. Improving the operating reliability of the compressor's mechanical and electrical components and avoiding unplanned shutdowns and maintenance are of great importance.

c. Power saving, vibration reduction and noise reduction

The compressor is the largest power consumer in the gas station equipment, the main source of mechanical vibration and air flow pressure pulsation, and the main source of mechanical noise and air flow noise generated for cooling purposes.

On the premise of ensuring safe operation and improving the operating reliability of mechanical and electrical components, efforts should be made to save power, reduce vibration and reduce noise in the compressor.

Characteristics of CNG compressor operation

a. Generally speaking, the exhaust gauge pressure of CNG compressors is 25MPa, which is the case in all countries around the world. The exhaust gauge pressure of CNG compressors equipped in conventional stations and substations in NGV gas stations is always 25MPa. As for the exhaust gauge pressure of the CNG compressor in the gas filling mother station, it depends on the working pressure of the CNG high-pressure gas cylinder on the long-tube semi-trailer, which is 25 MPa or 20 MPa.

b. The suction pressure of the CNG compressor, for conventional stations/mother stations, depends on the pressure of the natural gas pipeline network entering the station. The suction pressure of the CNG compressor in the gas filling substation is a variable value, and the maximum and minimum suction pressure values ​​are determined by the process design of the NGV gas filling station. In domestic engineering practice, it is generally between 3MPa and 20MPa.

c. CNG compressors have the characteristics of intermittent operation that are different from most compressors used in process flows. The control of the start, operation or shutdown of modern CNG compressors automatically depends on the actual gas pressure of the high-pressure gas cylinder group of the gas filling station; and the operation time and shutdown time of each operation are determined by the water volume of the station's high-pressure gas cylinder group and the gas filling volume required by the vehicle (mostly the mobile gas cylinder group on the long-tube semi-trailer for the gas filling mother station).

d. Starting from the intelligent control goal of NGV filling stations, it is natural that the mechanical parts and strong and weak electrical parts of CNG compressors and units are required to have the necessary conditions to adapt to unattended and fully automatic control operation.

e. The quality of natural gas sucked by CNG compressors should meet the relevant standards. Generally, before the compressor suction port, the incoming natural gas needs to be properly processed and meet the standards.

f. The high-pressure natural gas discharged by CNG compressors should generally also be processed to meet the quality requirements for vehicle refueling. CNG compressors with very little oil lubrication in the cylinders and CNG compressors with no oil lubrication in the cylinders (the intermediate barrel between the fuselage and the cylinder is not a "double compartment" type) are difficult to get rid of the adverse effects of the lubricating oil of the transmission mechanism in the fuselage on the quality of the high-pressure natural gas discharged.

g. The exhaust temperature of each level of CNG compressor, regardless of the cooling method of the compressor (water cooling, air cooling or mixed cooling), should meet the exhaust temperature limit determined from a safety perspective, especially when the cylinder and packing are oil-lubricated (or very little oil lubricated).

h. The gas supply of the CNG compressor and the water volume value of the high-pressure gas storage cylinder group should be reasonably matched. Its core purpose is twofold: one is to prevent the CNG compressor from starting too frequently to avoid the additional load of the machine parts, electrical appliances, and power electricity costs caused by this; the other is to fully fill the cylinder group with gas during the lowest electricity price time period to reduce the operating time of the compressor during the high electricity price time period, thereby saving the power cost of the gas station.

i. Compressors are generally driven by electric motors, while CNG compressors are not lacking in natural gas engines. Due to the constraints of the power and electricity conditions of the operating site, only natural gas engines can be used, which is one of the situations. In order to pursue low power costs for gas stations, all compressors in the station are driven by natural gas engines, which is the second situation. The third situation is that the driving machine of the compressor in the station is a combination of electric motors and natural gas engines, in order to achieve flexible control and save power costs: when the industrial electricity price is the lowest at night, the electric CNG compressor is used to fill the high-pressure gas storage cylinder group with large water volume; when the compressor needs to be started during the day when the electricity price is high, the natural gas engine-CNG compressor group is the main force.

j. CNG compressors also have the characteristics of extremely wide operating areas and extremely different meteorological conditions: severe cold, high altitude, water shortage, humidity and heat. There are obvious differences between CNG compressors used in fixed NGV filling stations and outdoor mobile NGV filling stations: fixed users must meet the environmental noise limit regulations of the operating site, which are sometimes very harsh; outdoor mobile users need to adapt to the conditions of no cooling water source, high cold climate conditions and even no power electricity.

k. CNG compressors should be able to adapt to the important factor of pressure changes in the natural gas pipeline network entering the station, including the possible pressure changes due to cooking and bathing of urban residents within a day, as well as possible pressure increases due to pipeline transformation or natural gas attenuation and pressure reduction.

l. The specifications and models of CNG compressors should not only be consistent with the actual gas filling capacity required when building a gas station, but also take into account the need for possible capacity expansion of the gas station. It is advisable to increase the number of compressors after the expansion while keeping the specifications and models the same, that is, to adopt a "modular" capacity expansion method.

m. Considering the flammable and explosive characteristics of natural gas and the actual operating conditions of CNG compressors in NGV gas stations, the compressor room should be a large open type, and it is more appropriate to adopt a skid-mounted compressor unit under the awning. If it is the latter, the fully air-cooled CNG compressor is the most suitable in the area north of the Yangtze River in my country.

n. For CNG compressor units equipped with a circulating cooling water system, a closed system is more suitable than an open system. Direct discharge of cooling water leads to a huge waste of precious water resources and should be abandoned. A closed system refers to a cooling water circulation system that uses an air-cooled radiator so that the water does not come into contact with the ambient air during the heat exchange process. An open system refers to a cooling water circulation system that is aerated and cooled in the ambient air. A closed system using softened water containing antifreeze has strong comprehensive advantages in preventing the formation of scale and metal corrosion.

o. The safety of CNG compressor operation should be guaranteed, which is certain. NGV gas stations require CNG compressors to ensure safety, reliable operation, energy saving and longevity. Therefore, the treatment method of trace leakage gas at the filler (lead venting, gas recovery in the sewage tank, leakage gas stored in the fuselage under internal air pressure, etc.), exhaust temperature limits at all levels, replacement period of wearing parts, pressure ratio of moving parts bearing surface, air flow velocity and pressure drop, etc., all need to be controlled.

Several important concepts about CNG compressors for gas stations

When confirming the natural gas compression system equipment of the gas station, the CNG compressor should be further explored under the premise of safe operation, improving the operating reliability of components, saving electricity, reducing vibration and noise. This is very helpful for how to select the compressor.

a. Flow rate and pressure unit of the compressor and its conversion

The further clarification of the general term "compressor flow rate" can be decomposed into two terms: the volume flow rate of the compressor and the air supply of the compressor.

a) Explanation of the volume flow rate of the compressor

The volume flow rate of the compressor refers to the volume value of the gas discharged by the compressor per unit time at the rated exhaust pressure. This value is measured at the exhaust end and converted to the compressor inlet state, that is, the volume value at the pressure and temperature at the first stage of the compressor. This value should also include the volume of steam converted from the water separated between the stages, and the influence of gas compressibility.

b) Explanation of compressor air supply

The air supply of a compressor refers to the value of the gas volume discharged by the compressor per unit time converted to the standard state, and does not include the moisture separated between stages and the air extraction volume (when the process flow extracts air from the compressor between stages).

The standard state of air supply is: absolute pressure 0.1013MPa, temperature 0℃, dry gas.

c) Commonly used volume flow and pressure units of compressors and their conversion

According to the International System of Units, the commonly used volume flow units of compressors are m3/min (meter 3/minute) and m3/h (meter 3/hour), and the commonly used air supply units of compressors are nm3/min (standard meter 3/minute) and nm3/h (standard meter 3/hour).

However, in domestic and foreign technical and business exchanges related to compressors, British units still appear frequently. For the convenience of communication, the author will list the simplified version of the commonly used volume flow units of compressors, the commonly used pressure (pressure) units of compressors and their conversions in Table 1 and Table 2. The numbers in the two tables are rounded off to meet the needs of the engineering level.