unit 1 gas compression systems
DESCRIPTION
Gas HandlingTRANSCRIPT
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UNITS IN THIS COURSE
UNIT 1 GAS COMPRESSION SYSTEMS
UNIT 2 AMINE GAS SWEETENING UNIT
UNIT 3 NATURAL GAS LIQUIDS (NGL) RECOVERY UNIT
UNIT 4 GAS FRACTIONATION PLANT
UNIT 5 SULPHUR RECOVERY UNIT
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TABLE OF CONTENTS
Para Page
1.0 COURSE OBJECTIVE 3
1.1 GAS OIL SEPARATION PLANT PROCESS - SOURCE OF THE FEED GAS 4
1.2 GAS COMPRESSION PROCESS - GENERAL INTRODUCTION 5
1.3 PROCESS EQUIPMENT 6
1.4 HIGH PRESSURE AND INTERMEDIATE PRESSURE GAS STREAMS 6
1.5 LOW PRESSURE GAS COMPRESSION SYSTEM 8
1.6 INTERMEDIATE PRESSURE GAS COMPRESSION PROCESS 11
1.7 HIGH PRESSURE GAS SYSTEM 12
1.8 GAS LIQUID CONDENSATE STRIPPER 13
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1.0 COURSE OBJECTIVE
On completion of this unit the trainee will be able to describe the gas compression systems.
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1.1 GAS OIL SEPARATION PLANT PROCESS - SOURCE OF THE FEED GAS
We will use the onshore gas gathering system as our example. The gas must be separated from the crude oil before it can be compressed. This is done in the Gas/Oil Separation Plants (GOSPs). The following is a simplified description of a gas/oil separation process.
The crude oil input to the GOSP comes from many wells at high pressure. The GOSP has three stages in the gas / oil separation process for separating the gas from the crude oil. See Figure 1-1.
Figure 1-1
There are 3 trains of staged separation. Each train has the same 3 stages.
At the GOSP facilities the crude oil (oil and gas mixture) from the oil field flows into the first stage production separators. These HP separators operate at approximately 15 Barg pressure.
The liquid accumulated in these separators flows on level control to the second stage IP separators that operate at approximately 2.5 Barg pressure.
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The liquid from these IP separators flows on level control to the three boot gas LP separators that operate at approximately 0.1 Barg pressure. After separation the oil flows on level control to the oil production storage tanks.
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1.2 GAS COMPRESSION PROCESS - GENERAL INTRODUCTION
We will use the onshore gas gathering system as our example.
The gas for the gas compression units is supplied from the Gas Oil Separation Plants (GOSPs) as shown in Fig 1-1. The gas comes from the GOSP's at three different pressures through three separate pipelines.
These pipelines are insulated and are electrically heat traced to prevent the gas and any water content from condensing. If the water was allowed to condense this would cause corrosion.
The three pipelines to the gas compression units are: High Pressure (HP), Intermediate Pressure (IP) and Low Pressure (LP).
The gases from the IP and LP pipelines must be compressed to raise their pressure to the same pressure as the gas from the high pressure pipeline. Then all the gases can be combined into one high pressure gas feed line for further processing.
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1.3 PROCESS EQUIPMENT
The following is a list of the main equipment used for a typical gas processing / compression system.
EQUIPMENT PURPOSE
High Pressure & Inline Separators (K.O. Drums)
To collect and remove any liquids before the gas goes through the pipelines..
Suction DrumsTo collect and remove any liquids to prevent any damage to the compressor
Booster CompressorTo compress Low Pressure Gas so it will flow to the compression unit.
Low Pressure Gas CompressorTo compress Low Pressure Gas in two stages of compression so it can be combined with the High Pressure Gas.
Intermediate Pressure Gas Compressor To compress the Intermediate Pressure Gas
in two stages of compression so it can be combined with the High Pressure Gas.
Interstage Suction DrumsTo collect and remove any liquids before the second stage of compression so as to prevent any damage to the compressor.
IntercoolersTo cool the gas after leaving the first stage of compression.
AftercoolersTo cool the gas after leaving the second stage of compression.
Gas SeparatorsTo collect and remove any liquids that have condensed after the second stage of compression.
1.4 HIGH PRESSURE AND INTERMEDIATE PRESSURE GAS STREAMS
Gases from the three second stage production separators are combined to form the Intermediate Pressure (IP) gas stream. The high pressure (HP) gas stream is made up of the gas from the three first stage production separators as well as gas from the three test separators.
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High Pressure and Intermediate Pressure Inline Separators
The high pressure and intermediate pressure gases are further separated in a two stage gas / liquid separation process. This separation ensures that no liquid is carried over into the pipelines to the NGL centre. The high pressure and intermediate pressure units are identical in principle. The first stage separators are vertical centrifugal separators. The vapour leaving the separators then passes to another stage consisting of horizontal filter separators.
The liquids collected in either separator are returned to the boots under their own pressure and on level control. Gases from the separators flow to the pipelines for transport to the NGL centre. To prevent condensation of water, with the resultant increase in corrosion, both lines are insulated.
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1.5 LOW PRESSURE GAS COMPRESSION SYSTEM
The low pressure gas is supplied to the gas compression units from the GOSPs. The gas is separated from the oil in the third stage production separators at the GOSP. These three streams are combined into one pipeline. The pressure is increased by the booster compressors at the GOSP so that there is enough pressure in the pipeline for the gas to flow to the compression unit.
Figure 1-2 Booster Compressor Process Flow Diagram
Using figure 1-2 we can describe the low pressure gas booster compression process as follows:
The low pressure gas from the GOSP enters the suction drum for the booster compressor. The purpose of the suction drum (sometimes called a gas scrubber) is to separate any entrained liquids from the incoming gas. If the liquids are not removed from the gas they will cause serious damage to the booster compressor.
The gas leaves the suction drum and flows into the booster compressor.
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The booster compressor increases the pressure of the gas. When the pressure of the gas increases the temperature of the gas also increases.
The gas leaves the booster compressor and flows to a fin fan cooler where the temperature of the gas is reduced. (See figure 1 3). As the gas cools in the fin fan cooler some of the gas will condense to liquid. From the outlet of the fin fan cooler the gas flows to the inlet of the low pressure suction drum. The condensed liquids are separated from the gas in this drum.
Figure 1-3 LP Gas Compressor Process Flow Diagram
The separated condensate passes from the bottom of the suction drum to the condensate pump. The gas leaves the top of the suction drum and flows to the first stage of the low pressure gas compressor.
The gas is compressed in the first stage compressor and its temperature goes up. Then, the gas flows from the first stage of the LP gas compressor to the inter stage cooler which cools the gas. When the gas is cooled some of the gas will condense.
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1. The gas and condensate flow from the inter cooler to the low pressure interstage drum. Here, any condensed liquids are separated from the gas. The oil condensate that is separated from the gas in the interstage drum is pumped from the bottom of the interstage drum to the condensate stripper. The water goes to the water blowdown system.
1. The gas flows from the top of the interstage drum to the suction of the second
stage of the LP gas compressor. The gas is compressed in the second stage of the LP gas compressor.
1. The gas flows from the second stage LP gas compressor to an after cooler. The
gas is cooled here. When the gas is cooled some of the gas will condense. The cooled gas flows to the LP gas separator. Any condensate is separated from the gas here. The condensate is pumped from the bottom of the LP gas separator to the condensate stripper.
The gas flows out of the top of the L.P. gas separator. It goes to the high pressure gas line and combines with the high pressure gas from the GOSP's. This gas then flows to the gas treating unit.
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1.6 INTERMEDIATE PRESSURE GAS COMPRESSION PROCESS
The intermediate pressure ([P) gas is supplied to the gas compression units from the GOSP. The gas is separated from the oil in the three second stage production separators at the GOSP. The gas flows through one pipeline. Figure 1-4 shows the intermediate pressure gas compression process.
Figure 1-4
Intermediate Pressure Gas Compression Process Flow Diagram
The process is as follows.
1. The gas flows from the GOSP facilities to the Knock Out Drum (see figure 1-3). The purpose of the K.O. Drum is to separate any entrained liquids that are in the gas.
1. The gas flows from the outlet of the K.O. Drum to the IP compressor suction
drum where any entrained liquids are separated from the gas. (See figure 1-4).1. The gas flows from the top of the IP compressor suction drum to the first stage
of the intermediate pressure gas compressor. The gas is compressed in the first stage compressor and its temperature goes up
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1. The gas flows from the first stage of the (IP) gas compressor to the intercooler which cools the gas. When the gas is cooled some of the gas will condense. The gas and condensate flow from the intercooler to the interstage suction drum. The liquids that have been condensed in the interstage cooler separate from the gas in the interstage drum.
1. The condensate leaves the bottom of the interstage drum. The condensate
from the interstage suction drum flows back to the LP compressor inter-stage drum. The gas flows from the top of the interstage drum to the second stage IP compressor where it is compressed. The gas flows from the second stage IP compressor to the aftercooler where it is cooled.
1. The cooled gas flows to the IP gas separator. The condensed liquids are
separated from the gas. The liquids are pumped to the condensate stripper.1. The gas leaves the top of the IP gas separator. It goes to the high pressure gas
line and combines with the gas in that line. It then flows to the gas treating unit.
1.7 HIGH PRESSURE GAS SYSTEM
1. The high pressure gas flows from the GOSP facilities to the HP K.O. Drum (see figure 1-3). The purpose of the K.O. Drum is to separate any entrained liquids from the gas. The gas leaves the top of the K.O. Drum and flows to the gas treating plant.
1. The gases from the low and intermediate pressure compressor trains combine
with the high pressure gas.1. The combined gases make up the feed gas for the gas treating plant.
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1.8 GAS LIQUID CONDENSATE STRIPPER
See Figure 1-4.
1. The liquid hydrocarbons collected from the LP/IP compression drums are combined in a main header.
1. This header feeds into the top tray of the condensate stripper column. (Tray
20).1. Hydrogen sulphide gas (H2S) is stripped (removed) from the liquid
hydrocarbons by heating.1. The increase in temperature vaporizes some of the lighter hydrocarbon
condensate.1. This vapour passes up the column. It is counter flowing against the condensate
liquid which is flowing down the column.1. Close contact between the vapour and the liquid occurs in the bubble cap trays.1. The H2S exits the top of the column with the gas vapour. This gas is spiked into
the H.P. gas line.1. The heating is by a hot oil reboiler.1. The stripped, sweet condensate is pumped through a fin fan cooler.1. The sweet condensate is sprayed in-to the NGL feed gas stream downstream
of the NGL plant feed gas compressor after cooler. See Unit 3.
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