GAS PREPARATION PROCESSES FOR FURTHER PROCESSING AND
TRANSPORTATION
Natural gas coming from wells contains in the form of impurities solid particles
(sand, scale), condensate of heavy hydrocarbons, water vapors, and in some cases
hydrogen sulphide and carbon dioxide.
The presence of solid particles in the gas leads to abrasive wear of pipes, fittings
and parts of compressor equipment, clogging of measuring instruments. Heavy
hydrocarbon condensate settles in low points of gas pipelines, reducing their flow
section. The presence of water vapors in the gas leads to corrosion of pipelines and
equipment, as well as the formation of hydrates in pipelines - a snow-like substances
that can completely block the cross section of pipes.
Hydrogen sulphide is a harmful impurity. If its content is more than 0.01 mg per
1 dm3 of working zone air, it is toxic. And in the presence of moisture, hydrogen
sulphide is able to form solutions of sulphurous and sulphuric acids, which sharply
increase the corrosion of pipes, fittings and equipment.
Carbon dioxide is harmful in that it reduces the combustion value of the gas, and
also leads to corrosion of equipment. Therefore, it is advisable to separate it in the
fields.
The objectives of field gas preparation are to remove mechanical impurities,
heavy hydrocarbons, water vapors, hydrogen sulphide and carbon dioxide.
The choice of the technology for field preparation of wells products in gas and
gas condensate fields depends on a number of factors:
- field gas and gas condensate resources;
- hydrocarbon and fractional composition of well products (composition of gas
and hydrocarbon condensate);
- water content in the form of steam and drip moisture (reservoir water);
- contents of non-hydrocarbon components in the gas, such as hydrogen sulphide,
carbon dioxide, helium, etc.;
- formation pressure and temperature;
- climatic and hydrogeological conditions in the area of gas and gas condensate
production and transportation;
- technical requirements for the supply of gas and gas condensate and a number of
other factors.
The processes of complex field gas and gas condensate preparation for
transportation in gas fields, depending on the depth of the process, take place at gas pretreatment plants (GPTP) or gas complex treatment plants (GCTP). These plants are
usually supplied with well products under reservoir pressure. But during operation
(especially during the depletion phase, when the flow pressure does not exceed 5.5
MPa), this pressure is not enough to supply gas to the treatment plants. In this case,
compressing play an important role - an increase in pressure due to compression.
Compressing
Gas compressing at gas enterprises is carried out using booster compressor
stations (BCS).
Booster stations are used in facilities such as:
- wells with low gas pressure at the outlet;
- wells with a low production rate compared to those ones operating from the
same formation;
- wells that sharply reduced production rate during operation, while maintaining
intraformation pressure;
- wells with a bottom-hole zone mudded during drilling or overhaul repair;
- свердловини, що довго простоюють після буріння, капітального або
підземного ремонту;
The booster compressor station (BCS) is a complex of facilities and equipment.
You can see the appearance of the BCS in Fig. 5.
Figure 5 - The appearance of the booster compressor station
Internal view of BCS premises can be seen in Fig. 6 and 7.
Figure 6 - Internal view of low power BCS premises
Figure 7 - Internal view of high power BCS premises
The most important part of the BCS is the compressor station, which can have
several typical schemes (Fig. 8 and 9):
Figure 8 -Scheme №1
1 - inlet separator; 2 - compressor; 3 - oil collector (combined with oil tank); 4 - gas cooler;
5 - outlet separator; 6 - oil pump; 7 - oil duplex (dual) filter with the possibility of
change-over from one filter to another without stopping the operating process; 8 - oil cooler
Figure 9 - Scheme №2:
1 – inlet separator; 2 – compressor;
3 – oil collector (here the preliminary gas purification stage is also performed);
4 – filter (for fine cleaning of gas from oil); 5 – gas cooler; 6 – outlet separator; 7 – oil pump;
8 – oil duplex (dual) filter with the possibility of change-over from one filter to another
without stopping the operating process; 9 – oil cooler
The principle of operation of the station
Gas through the inlet separator enters the oil-filled compressor, where it is
compressed to a certain pressure value. Then the heated compressed gas enters the oil
collector, where, due to to a two-stage cleaning sisteme, it is preliminary and fine
cleaned of oil droplets. After that, the gas enters the heat exchanger, where it is cooled.
In this case, oil vapors condense in the gas. For the final purification of the cooled gas,
it passes through the outlet separator. Oil separated from the gas stream is discharged by
the oil pump into a separate circuit, where it is cleaned on a dual filter, cooled and again
supplied to the compressor.
Compressor station schemes often differ in the number and type of units
intended for cleaning gas and oil. For example, the degree of purification of gas from oil
before the cooler may consist of not one, but two purification apparatuses. This scheme
is used in high power stations.
With a large decrease in gas pressure at the inlet of the BCS due to a decrease in
reservoir pressure, 2 compressing stages can be used.
Compressors are used to increase gas pressure. Booster compressor stations are
equipped with compressors of various types - screw, centrifugal, piston. Electric motors,
gas turbines and internal combustion engines can be used as drive of compressors.
Screw (rotary) compressors (SC) have high operating parameters, ease of
operation and maintenance, reliability of design, relatively small dimensions and low
noise. In terms of their parameters, SC significantly exceed piston or centrifugal ones.
Today, BCS with SC are actively used in high-technology production processes.
There are 2 design solutions for such BCS - with a double or single screw.
The main feature of the SC is the ability to provide a fixed degree of gas
compression. The required operating pressure is provided by the geometrical parameters
of the compression chamber. Modern SC have several discrete compression ratios that
can be promptly chosen based on need. The appearance of SC can be seen in Fig. 10.
Рис. Вид винтового компрессора
Figure 10 – The appearance of screw compressor
In order to increase the efficiency of the SC and reduce gas losses, oil injection
into the working space of the unit is used. This measure allows you to maintain
tightness, ensures proper lubricating of rubbing surfaces, reduces noise, provides
cooling of the compressor motor - this is especially important in the last compression
stages. This makes it possible to effectively use SC in pneumatic systems with the
possibility of significant temperature and pressure fluctuations. The working scheme of
SC can be seen in Fig. 11.
Рис. Схема винтового компрессора
Figure 11 – The working scheme of screw compressor
1 - suction valve; 2 - filter; 3 - screw unit; 4 - electric motor; 5 - oil tank;
6 - separator (oil separator); 7 - minimum pressure valve; 8 - thermostat;
9 - gas radiator; 10 - ventilator; 11 - oil radiator; 12 - oil filter
The principle of operation of the compressor is as follows: gas is supplied to the
compressor through the suction valve, passes through the filter and is headed to the
place of its compression - a screw unit. Screw unit consists of two rotors (screw pair),
one of which is driving and the other is driven. Driving rotor is connected with electric
motor and transmits rotation to driven rotor. When the gas is headed to the screw pair, it
is mixed with the oil. Oil is necessary for lubricating during compression, seals the slots
between the system elements and removes heat. The mixture of gas and oil begins to be
pumped by rotary movements of the rotors. A gas stream with the required pressure
value is formed. When the compression process is complete, the gas should be cleaned
of oil impurities. A separator is used for this. Since the gas is heated during
compression, it should be cooled. Therefore, at the next stage, it passes through the gas
radiator with a ventilator and enters the outlet through the minimum pressure valve. A
separator is used for this. Since the gas is heated during compression, it should be
cooled. Therefore, at the next stage, it passes through the gas radiator with a ventilator
and goes to the outlet through the minimum pressure valve. This valve maintains the
pressure in the oil reservoir so that it circulates independently of the pressure in the
system. The gas is cooled to a temperature higher by 15-20 °C than the environment.
The final cooling of the gas is carried out at air cooler units. The oil goes to the
thermostat responsible for the oil temperature adjusting and is headed, passing through
the oil filter, back to the screw unit or through the oil radiator (if cooling is needed) or
directly.
SC does not require special maintenance personnel, has low operating costs, is
characterized by high reliability and durabiliS GC successfully function in both small
and very large enterprises.
Disadvantages of screw compressors:
- the using of accurate devices requires careful compliance with technical
requirements during operation.
- oil system with cooling elements is required.
- with low load of the compressor (1/5 of the rated power) in the suction section,
the efficiency is significantly reduced.
Piston compressors (PC) are quite common in industrial and production
enterprises. The PC operate on the principle of pressurizing at the cylinder with a piston
reciprocating by means of the crank mechanism (Fig. 12).
Figure 12 - Crank mechanism
1 — engine shaft; 2 — crank; 3 — connecting rod; 4 — piston.
In these mechanisms, the crank shaft receives rotational motion from the engine
and transmits it through the connecting rod to the piston, which performs the necessary
work - compresses the gas.
The compressor unit consists of thus elements: cylinder, piston, two valves,
located in the cylinder cover - first for gas suction, second for discharge.
During operation of the compressor unit, the connecting rod directly connected to
the rotating shaft transmits limited movements along the compression chamber to the
piston. During the process, the volume between the valves and the lower part of the
piston increases, as a result of which vacuum occurs. Exceeding the resistance of the
plate that closes the suction valve, the gas opens it and goes to the cylinder through the
suction tube. In the process of reverse movement of the piston, the gas is compressed
and its pressure increases. The valve through which the gas goes out and which also
closed by the plate is opened by a high pressure gas flow. Then compressed gas goes to
discharge tube. (Fig. 13).
Figure 13 – The principle of the piston compressor operation
In practice, single cylinder compressors are ineffective. Multi-cylinder
compressors are usually used, at the same time one part of cylinders is in the
compression phase and the second part of them is in the vacuum phase. An example of a
two-cylinder compressor is shown in Fig. 14
Figure 14 – The principle of the two-cylinder compressor operation
Multistage compressor units are used to increase the pressure of the gas going
out from the compressor. The operation scheme of the two-stage piston compressor is
shown in Fig. 15.
The gas compression process to a certain pressure value occurs in the cylinder of
the first stage. Then the gas goes to the interstage cooler. Then, going to the second
stage cylinder, the gas is additionally compressed. This allows for a more degree of gas
pressure.
Figure 15 – The principle of the two-stage piston compressor operation
The advantages of PC are:
- simple design, improving reliability and, as a result, ease of maintenance. Any
part can be replaced if repair is needed quickly enough, which reduces downtime
compared to other types of compressors;
- PC are mobile and can produce compressed gas with very high pressure
values;
- PC modifications operate without oil injection, which leads to a high degree of
gas purity at the outlet;
- PC cost is lower at other equal parameters compared to other types of
compressors.
Piston BCS, unlike screw BCS, in most cases are able to create the necessary
working pressure only by 2-stage compression.
PC disadvantages - PC noise level is quite high. To reduce the noise level a
special casing is used in the PC design.
Centrifugal compressors (CC) operate based on the principle of compression of
gases under the influence of centrifugal forces. CC can work in 2 and even 4
compression stages. 6-stage CC is divided into 3 compartments. CC are used mainly
when it is necessary to obtain large amount of compressed gas.
The CC design consists of a rotor with working symmetrical impellers (wheels)
and a casing. During the operation of the CC, gas moves under influence of centrifugal
forces. The gas is displaced to the periphery of the impeller, compressed and, at the
same time, acquires a certain speed of movement. Further, the gas goes to the annular
diffuser. A diffuser is a pressure pipe that expands along the stream. The gas movement
in the diffuser is accompanied by a decrease in speed and an increase in pressure. The
process of converting the kinetic energy of the gas into potential pressure energy takes
place. After that, the gas goes to the next stage of the unit. CC scheme is shown in Fig.
16.
Рис. Центробежный компрессор
Figure 16 – The centrifugal compressor design
a - longitudinal section; b - sections of blade diffuser and impeller;
1 - shaft; 2 - diffuser; 3,7 - blades; 4,5 - seals; 6 - impeller; 8 - compressor casing
The maximum pressure that can be achieved in one stage is determined by the
strength of the impellers capable of allowing a speed of up to 280 m/s. Power, pressure,
and efficiency are directly related to CC capacity.
You can adjust the operation of the CС using a throttle (flow control valve) on
the suction line or changing the speed of the shaft rotation.
When choosing compressor stations, it is necessary to carefully analyze the
operating conditions, properties and characteristics of use.
Air cooling
During the compression of the gas at the compressor station, its temperature rises,
so the gas should be cooled to prevent damage to the equipment.
The operation of booster compressor stations at gas fields over the life of the field
is characterized by varying pressure values from 0.5 to 7.35 MPa and well product
temperature from 5 to 40 °C at the inlet to the BCS. At the same time, the main factor is
a decrease in gas pressure at the inlet of the BCS due to a decrease in reservoir pressure.
Therefore, an increase in the compression ratio of gas compressor units (GCU) is
required to ensure optimal operation of the process lines of the GСTP and the interfield
gas pipeline, which leads to an increase in the gas temperature at the GCU outlet.
The need to cool the crude gas at the BCS is caused by such factors:
- providing the possibility of gas drying to the required dew point in technological
processes, i.e. cooling before GCTP;
- maintenance of the required operating parameters of gas compressor units
(GCU) , the so-called interstage cooling.
To cool the gas flow at booster compressor stations, air cooling apparatuses
(ACA) have become widespread. They have a number of advantages over other types of
heat exchangers: they are reliable in operation, environmentally friendly, have simple
connection schemes, and do not require preliminary preparation of coolants.
The gas temperature at the outlet of the ACA of the gas flow interstage cooling is
determined, on the one hand, by the conditions of its further arrival at the next
compression stages in the BCS, at which the condensation of water vapor contained in
the gas will not occur, and on the other hand, by the maximum permissible gas flow
temperature according to the GCU operating parameters, which should be of 20-35 °С.
The final degree of gas cooling using ACA at the BCS is defined by the need of
maximum releasing of equilibrium droplet moisture from the gas in the separators at the
inlet to the GCTP in order to ensure the required temperature regime of the gas drying
process.
The operation of air cooling devices is based on the principle of natural cooling of
gas by atmospheric air. The gas, moving under pressure through the tubes inside the
apparatus, is cooled by a forcibly injected stream of atmospheric air.
ACA can be attributed to surface-type devices, where atmospheric air is used as a
coolant. These devices are designed to operate in a wide range of operating pressures.
The pressure of the apparatus is determined by the cooled medium and its temperature.
A schematic view of the apparatus is shown in Fig. 17.
Рис. Аппарат АВО (1)
Figure 17 – The schematic view of the air cooling apparatus
ACA consists of the following main parts:
- sections of heat exchange finned tubes;
- air supply systems;
- supporting steel structures.
ACA sections are a bundle of finned tubes assembled in a pipe grid. The tube grid
is connected to the manifold, which is connected to pipelines supplying or discharging
cooled gas. Finning of the tubes is carried out by the method of rolling or winding.
More often, rolled fins are used, obtained by extruding ribs from an aluminum tube
placed onto steel one. Such tubes have an increased heat transfer coefficient compared
to smooth tubes, which makes it possible to compensate for low air heat transfer (Fig.
18).
Рис. Секции АВО
Figure 18 – ACA section
Air supply system (Fig. 19) includes:
- fan impeller;
- electric motor;
- diffuser of electric fan;
- safety wire-netting.
Рис. Система подачи воздуха в АВО
Figure 19 – The air supply system of АCA
Sections of the apparatus are installed on supporting steel structures (Fig. 20).
The air supply system is attached to the bottom.
Рис. Аппарат АВО (2)
Figure 20 – The air cooling apparatus
The principle of operation of the ACA:
Air is supplied into outside of tubes by blades of fan impeller . Blades of fan
impeller are located in cylindrical manifold intended for direction of air flow. Manifold
is connected to heat exchange section by means of diffuser. The diffuser is an inverted
quadrangular pyramid and helps to equalize the air flow rates before entering the
section. Fan manifold diffuser is attached to the frame. Heat exchange sections are
attached to this frame. The fan with the engine is located on a special frame. The air,
passing through the section, is heated, and the gas in the tubes is cooled. The degree of
gas cooling depends on the air flow rate. To change the air flow rate in the ACA
section, a blade speed controller or frequency converter is installed on the fan. Also, for
this purpose, metal shutters are installed on the surface of the tube sections, the change
in the position of the plates of which changes the air flow rate.
Several ACA are installed on powerful BCS (Fig. 21).
. Рис. Общий вид АВО
Figure 21 – Unit of several air cooling apparatuses