Natural Gas Transmission Stations: Functions, Equipment and Process Flow

Natural gas transmission pipelines transport large volumes of gas over long distances from gas fields, processing plants or other supply sources to distribution networks, industrial users and downstream facilities.

Along these pipelines, different types of gas transmission stations are required to control gas flow, maintain operating pressure, remove contaminants, measure gas quantity and support pipeline inspection and maintenance.

Depending on their location and purpose, a natural gas transmission station may perform functions such as gas receiving, filtration and separation, compression, pressure regulation, metering, gas quality monitoring, pig launching and receiving, and gas distribution.

This article introduces the main functions, typical station types, major equipment and process flow of natural gas transmission stations.

1. Main Functions of Natural Gas Transmission Stations

The configuration of a gas transmission station depends on its position in the pipeline network and its required operating functions.

Typical functions include:

• receiving natural gas from an upstream pipeline or gas processing facility;

• removing solid particles and entrained liquids;

• monitoring gas quality and operating parameters;

• increasing gas pressure where compression is required;

• reducing and controlling gas pressure for downstream users or networks;

• measuring gas flow and accumulated gas volume;

• distributing gas to different downstream pipelines or users;

• launching and receiving pipeline pigs;

• monitoring pressure, temperature, flow and equipment status; and

• providing safety shut-off and control functions.

Not every station requires all of these functions. The process configuration should be designed according to the actual pipeline conditions, gas quality, operating pressure, flow capacity and downstream requirements.

2. Main Types of Natural Gas Transmission Stations

A long-distance natural gas pipeline may include several types of process stations.

2.1 Initial or Receiving Station

The initial station is generally located near the beginning of the transmission pipeline and receives natural gas from an upstream gas field, gas processing plant or other supply facility.

Typical functions may include:

• gas receiving;

• filtration and separation;

• gas quality monitoring;

• flow metering;

• pig launching; and

• compression where required.

Gas quality parameters such as gas composition, water dew point and hydrocarbon dew point may also be monitored according to pipeline specifications.

If sufficient upstream pressure is available, compression may not initially be required. As upstream pressure decreases or pipeline operating conditions change, compression facilities may be added or adjusted according to actual requirements.

2.2 Compressor Station

Pressure losses occur as natural gas travels through a long-distance pipeline. Compressor stations are installed where necessary to increase gas pressure and maintain the required transmission capacity.

A typical compressor station may include:

• inlet separation and filtration equipment;

• compressor units;

• gas cooling equipment;

• associated valves and piping;

• instrumentation and control systems; and

• pig receiving or launching facilities where required.

Before entering a compressor, natural gas normally needs to meet specified cleanliness requirements. Filters and separators are therefore important for removing particles and entrained liquids that could affect compressor performance or reliability.

After compression, the gas may require cooling before being returned to the transmission pipeline.

2.3 Intermediate Delivery or Offtake Station

An intermediate delivery station takes part of the gas from the transmission pipeline and supplies it to a downstream distribution network, industrial facility or other user while allowing the remaining gas to continue downstream.

Typical functions include:

• gas filtration and separation;

• pressure regulation;

• gas metering;

• gas heating where required;

• flow distribution;

• safety shut-off; and

• pig receiving and launching where applicable.

Natural gas supplied to downstream users usually needs to meet specified pressure, temperature, cleanliness and metering requirements.

For this reason, pressure regulating and metering equipment is often a key part of an intermediate delivery station.

2.4 Pigging Station

Pigging stations provide facilities for launching and receiving pipeline pigs.

Pipeline pigs can be used for different purposes, including:

• removal of construction debris;

• removal of accumulated liquids and contaminants;

• pipeline cleaning;

• pipeline inspection; and

• maintenance support.

Before a new pipeline is commissioned, pigging may be performed to remove construction debris, welding residue, water and other materials remaining inside the pipeline.

During normal operation, periodic pigging may also be required to remove accumulated liquids or contaminants and maintain pipeline efficiency.

For inspection activities, specialized intelligent pigs may be used to evaluate pipeline condition.

2.5 Terminal Station

A terminal station is generally located near the downstream end of a transmission pipeline and delivers natural gas to a distribution network, city gate station, industrial facility or other downstream system.

Typical functions may include:

• gas receiving;

• filtration and separation;

• pressure regulation;

• gas metering;

• gas heating where required;

• pig receiving;

• safety control; and

• gas distribution.

The outlet gas pressure and flow must be controlled according to downstream requirements.

Depending on the gas supply network and peak-shaving strategy, terminal or distribution systems may also be connected with gas storage or LNG peak-shaving facilities.

3. Typical Equipment Used in Natural Gas Transmission Stations

Although station configurations vary, several types of equipment are commonly used throughout natural gas transmission systems.

3.1 Gas Filters and Separators

Natural gas entering a station may contain solid particles, liquid droplets, condensate or other contaminants.

Filters and separators are used to protect downstream equipment such as regulators, meters, compressors, valves and heat exchangers.

Typical equipment may include:

• gas filters;

• filter separators;

• cyclone separators; and

• liquid separation equipment.

Equipment selection depends on gas composition, flow rate, operating pressure, contaminant characteristics and required separation efficiency.

3.2 Gas Pressure Regulating Equipment

Pressure regulation is required when downstream operating pressure is lower than pipeline pressure or when a stable outlet pressure must be maintained despite variations in inlet pressure and gas demand.

A pressure regulating system may include:

• pressure regulators;

• control valves;

• monitor regulators;

• safety shut-off valves;

• pressure transmitters;

• bypass lines; and

• associated control equipment.

Depending on the pressure reduction ratio and project requirements, pressure reduction may be performed in one or multiple stages.

HUAYI provides gas pressure regulating and metering equipment for natural gas transmission, distribution and industrial gas applications.

[Internal link: Gas Pressure Regulating & Metering Equipment]

3.3 Gas Metering Equipment

Gas metering is required for operational monitoring, network management and commercial accounting.

Different metering technologies may be selected according to:

• gas flow range;

• pipeline size;

• operating pressure;

• required accuracy;

• gas composition; and

• applicable standards.

A metering system may also include pressure and temperature measurement, flow computers and communication interfaces for corrected or standard gas volume calculation.

3.4 Gas Heating Equipment

When high-pressure natural gas undergoes significant pressure reduction, its temperature may decrease.

Where low gas temperature could affect regulators, valves, metering equipment or downstream operation, gas heating may be required.

Gas heaters or heat exchangers can therefore be installed upstream of or between pressure regulating stages according to process requirements.

3.5 Pig Launchers and Pig Receivers

Pig launchers and pig receivers provide controlled access to the pipeline for inserting and removing pigs.

These systems normally include:

• launcher or receiver barrel;

• quick-opening closure;

• isolation valves;

• kicker or bypass piping;

• vent and drain connections; and

• associated instrumentation.

Their design must consider pipeline diameter, operating pressure, pig dimensions and applicable pipeline requirements.

3.6 Instrumentation and Control Systems

Modern transmission stations use instrumentation and automation to monitor and control process conditions.

Typical monitored parameters include:

• inlet and outlet pressure;

• gas temperature;

• instantaneous flow;

• accumulated gas volume;

• filter differential pressure;

• valve status; and

• equipment alarms.

Depending on project requirements, field instruments may be connected to PLC, RTU, HMI, SCADA or DCS systems for local or remote monitoring and control.

4. Typical Natural Gas Transmission Station Process Flow

The exact process flow depends on the station type, but a typical gas handling sequence can be represented as:

Gas Inlet → Separation / Filtration → Heating (if required) → Pressure Regulation → Metering → Gas Distribution / Pipeline Outlet

Compression may be added where the gas pressure needs to be increased:

Gas Inlet → Separation / Filtration → Compression → Cooling → Metering → Pipeline Outlet

Pig launching and receiving facilities are normally connected to the main pipeline according to the required pigging arrangement.

Different functions can therefore be combined according to the purpose of the station.

For example, an intermediate delivery station may primarily require filtration, heating, pressure regulation and metering, while a compressor station focuses on gas pretreatment, compression and cooling.

5. Why Filtration and Separation Are Important

Gas cleanliness affects the reliability of many downstream components.

Solid particles can cause wear or blockage in valves and regulators, while entrained liquids may affect metering accuracy, control performance and downstream equipment.

Effective gas treatment can therefore help:

• protect pressure regulators and control valves;

• reduce contamination of metering equipment;

• protect compressors and other rotating equipment;

• reduce maintenance requirements; and

• improve overall station reliability.

HUAYI provides gas treatment and auxiliary equipment, including filtration, separation, heating and related equipment for natural gas applications.

[Internal link: Gas Treatment & Auxiliary Equipment]

6. Pressure Regulation and Metering at Delivery Stations

For many downstream users, pipeline gas cannot be supplied directly at transmission pressure.

The gas must first be conditioned to meet the required delivery conditions.

A typical pressure regulating and metering section may therefore include:

Filtration → Gas Heating → Pressure Reduction → Flow Metering → Safety Control → Gas Outlet

The actual sequence and equipment configuration depend on the project.

For example, heating may be positioned according to the pressure reduction stages and required outlet temperature, while redundant regulating lines may be provided where continuous gas supply is critical.

Accurate engineering therefore requires more than simply selecting a regulator according to pipe diameter.

7. Key Design Parameters for Gas Transmission Station Equipment

Before selecting or designing station equipment, the actual operating conditions should be clearly defined.

Important information normally includes:

Gas Properties

• gas composition;

• gas density;

• water content;

• hydrocarbon content;

• contaminants; and

• corrosive components where applicable.

Flow Conditions

• minimum gas flow;

• normal gas flow;

• maximum gas flow; and

• expected future capacity.

Pressure Conditions

• minimum inlet pressure;

• normal inlet pressure;

• maximum inlet pressure;

• required outlet pressure; and

• allowable pressure variation.

Temperature Conditions

• minimum and maximum gas temperature;

• ambient temperature;

• required outlet gas temperature; and

• possible temperature reduction during pressure regulation.

Pipeline Information

• pipeline diameter;

• connection size;

• pipeline design pressure;

• pigging requirements; and

• station layout constraints.

Metering Requirements

• required measurement accuracy;

• flow range;

• gas volume correction requirements;

• flow computer requirements; and

• communication requirements.

Automation and Safety Requirements

• local or remote control;

• PLC or RTU requirements;

• SCADA or DCS interface;

• alarm requirements;

• emergency shutdown requirements;

• hazardous area classification; and

• applicable electrical and explosion-protection requirements.

Codes and Standards

Applicable design, manufacturing, inspection and testing standards should be confirmed according to the project location and customer requirements.

8. Integrated Engineering of Station Equipment

A natural gas transmission station should be considered as an integrated process system rather than a collection of independent pieces of equipment.

For example, filter sizing affects pressure drop. Pressure reduction affects gas temperature. Gas temperature can influence regulator performance and downstream operation. Meter selection depends on flow range, pressure and required accuracy.

Instrumentation and control requirements must also match the mechanical process configuration.

For this reason, the relationship between filtration, separation, heating, pressure regulation, metering, safety control and automation should be evaluated during the engineering stage.

Skid-mounted and modular configurations can integrate several process functions into a compact package, helping simplify site installation and interface management where appropriate.

9. Selecting Equipment for a Natural Gas Transmission Station

There is no single standard equipment configuration suitable for every station.

Equipment selection should be based on:

• the station's function within the pipeline network;

• gas composition and cleanliness;

• required flow capacity;

• inlet and outlet pressures;

• pressure reduction ratio;

• temperature conditions;

• metering requirements;

• required system redundancy;

• pigging requirements;

• automation philosophy; and

• applicable project standards.

A receiving station, compressor station, intermediate delivery station and terminal station may therefore use significantly different equipment even when they are part of the same transmission pipeline.

Understanding the actual operating conditions is the first step toward developing an appropriate technical solution.

Conclusion

Natural gas transmission stations are essential for the safe and reliable operation of long-distance gas pipeline systems.

Depending on their function, these stations may perform gas receiving, filtration, separation, compression, pressure regulation, metering, heating, pig launching and receiving, gas distribution and process monitoring.

The correct equipment configuration depends on the role of the station and the actual gas flow, pressure, temperature, gas quality and downstream requirements.

HUAYI provides natural gas equipment for transmission, distribution and industrial gas applications, including gas pressure regulating and metering equipment and gas treatment and auxiliary equipment. Equipment and system configurations can be engineered according to different operating conditions and project requirements.