Gas Distribution System

1 Introduction to the Gas Distribution System

Gas is a substance (elements or compounds) that expands freely to fill an entire container. This implies that gas will expand freely to fill the shape of any stored container. Various elements and compounds exist in gaseous states, including oxygen, nitrogen, hydrogen, carbon monoxide, carbon dioxide, and hydrocarbon gas (Methane, Ethane, Propane…, etc.).

Hydrocarbons are organic compounds consisting entirely of carbon and hydrogen. In this article, EPCM emphasizes hydrocarbon gas (Natural gas).

1.1 Gas Distribution System: Natural Gas

Natural gas is a naturally occurring hydrocarbon gas mainly consisting of methane. It may contain a varying amount of other higher alkanes, a small percentage of hydrogen sulphide, carbon dioxide, and helium. Natural gas is formed when organic materials (plants and animals) buried deep in the earth’s crust are exposed to intense heat and pressure for years.

Natural gas is an energy source for electricity generation, cooking, heating, etc.

Natural gas, in its processed state, is gaseous. However, it can be conditioned to a liquid state (this is called Liquid Natural Gas or LNG).

1.1.1 Gaseous State

Natural gas exists naturally in the gaseous state unless processed and conditioned to exist in a liquid state. It is colourless and odourless and is transported in its gaseous state using pipelines, specially built tubes/storage vessels mounted on trucks and Ships.

1.1.2 Liquid State

Natural gas can be processed and conditioned to a liquid state called LNG (Liquefied Natural Gas). It can be liquefied by cooling the gas to –260° F (–162° C). At this temperature, the gas changes into a liquid. Using specially built LNG transport Vessels, natural gas is transported in large quantities in its liquefied state (1/600th of its original gas volume) over long distances. Natural gas cannot be used in its liquefied state; therefore, at its destination, it is converted into a gaseous state by warming the gas through regasification.

1.2 Gas Distribution System: Methods of Transporting Natural Gas

Natural gas, either in a gaseous or liquid state, can be transported between facilities or to consumers by Ships/Vessels, Special Trucks and pipelines.

1.2.1 Ship/Ocean Vessels

Ships can be used to transport both natural gas in its compressed state (compressed natural gas CNG) and its liquefied state (liquefied natural gas (LNG).

However, due to cost implications, transporting CNG by Ship is not as popular as transporting LNG by ship.

Ships are mostly used to transport a large volume of liquefied gas (LNG) over a long distance, especially in the absence of pipelines. Natural gas cannot be used in its liquid state, so the liquefied gas is converted back into a gaseous state at its destination. The liquefied gas is fed into a regasification facility installed on the ship or facility nearby. The regasification process converts the liquefied gas to a gaseous state before the gas is transported by pipelines or trucks to end-users.

1.2.2 Trucks

This entails transporting Natural Gas, either in the liquid or gaseous state, on roads using special trucks. This method of transportation is suitable for transporting a small volume of gas over a short distance compared to transporting gas using Vessels/Ships and pipelines. Special-purpose pressure vessels (ISO containers/special tubes) mounted on trucks convey gas to end-user facilities.

When gas is transported in a liquefied state, the storage vessel should be able to maintain the LNG temperature below the phase change temperature of natural gas.

When gas is transported by storage tubes mounted on trucks, it is transported in a compressed state (at high pressure and low temperature) called Compressed Natural Gas (CNG). CNG can be transported at pressures over 200 bar, so the storage tubes are high-strength steel capable of safely holding the gas.

Gas compression is performed in a facility that receives natural gas from pipelines. The gas is fed into a drying unit, where water vapour is removed.

From the dryer, the gas is fed into a compressor or series of compressors, where it is compressed, thus increasing its pressure.

The gas exits the compressor into storage cylinders connected to the dispensing unit. The dispensing unit discharges the gas from the storage cylinders into storage tubes for onward transportation to the customer’s facility.

A Gas Chiller unit is incorporated into the system downstream of the compressor. This primarily ensures a larger volume of gas is stored and dispensed.

At the customer facility, the compressed natural gas is fed into a facility unit that depressurises the gas and increases the temperature to a value that gas consumers can handle. The CNG is fed into a heat exchanger unit where the gas temperature gradually increases. The gas exits the exchanger and enters a pressure reduction and metering unit, where the gas pressure is reduced before entering the consumer facility.

GNSL, a subsidiary of Axxela Group, currently operates a 5.2MMSCFD Compressed Natural Gas (CNG) Mother Station in Lagos, Nigeria. At the mother station, natural gas is compressed into mobile tubes and transported by trucks for onward delivery to customers’ locations. (https://axxelagroup.com/operations/gas-network-services-limited/).

1.2.3 Pipelines

This is the most efficient and safest means of transporting Natural gas.

Some of the advantages of using the pipeline to transport gas are

• Continuous delivery of gas to consumers without disruption. The delivery is not affected by most environmental factors.
• Pipelines can be routed to take shortcuts to their destinations, thereby reducing transportation time compared to other means.
• The large volume of gas transported.
• Pipeline transportation is the safest and most reliable means of transporting gas.

Several transmission and distribution pipelines convey natural gas within cities and countries to other countries or within cities.

The West Africa Gas Pipeline (WAGP), operated by West Africa Gas Pipeline Company (WAPCo), transports gas from Nigeria to three West African countries (the Benin Republic, Togo and Ghana. The pipeline receives gas from the Escravos – Lagos Pipeline at the Nigeria Gas Company’s Itoki Natural Gas Export Terminal in Nigeria. The West Africa Gas Pipeline is designed to deliver a volume of 170MMscfd initially and, in later stages, a capacity of 460MMscfd to gas users. (https://www.wagpco.com/the-project/wapco-pipeline)

Gaslink (GNL) is a subsidiary of Axxela Group. In conjunction with the Nigerian Gas Marketing Company (NGMC), it operates a 100 km-long gas distribution pipeline network in Lagos, Nigeria, with a capacity of 140MMSCFD and peak utilisation of about 70MMSCFD. (https://axxelagroup.com/operations/gaslink-nigeria-limited/).

There are several pipelines transporting gas across various European and American countries.

2 Gas Pipeline System

A gas pipeline system connects various facilities, equipment, and fittings to deliver gas efficiently to end-users.

2.1 Gas Distribution System: Types of Pipelines

Pipelines may be categorised into any of the following.

2.1.1 Flowlines

Flowlines transport fluid (gas or liquid) from wells to storage or processing facilities. The processing facility may be used for preliminary processing of the fluid. The fluid transported by the flowline is not pure. Therefore, the flowline must be designed to handle the fluid in its natural state. Flowlines are usually small in diameter (2” to 4”); however, depending on the production capacity of the well, the size may be larger.

2.1.2 Gathering Pipelines

These pipelines transport fluids from processing and storage facilities to a common trunk pipeline. The trunk lines may be larger than the flowlines.

2.1.3 Transmission Pipelines

As defined in ASME B31.8 section 803.2, A transmission line is a segment of pipeline installed in a transmission system or between storage fields. While a transmission system is one or more segments of a pipeline, usually interconnected to form a network that transports gas from a gathering system, the outlet of a gas processing plant, or a storage field to a high- or low-pressure distribution system, a large-volume customer, or another storage field.

2.1.4 Gas Distribution Pipeline System

According to ASME B31.8 section 803.3, a gas main or distribution main is a segment of pipeline in a distribution system installed to convey gas to individual service lines or other mains. Distribution pipelines convey gas to power generating facilities, factories, industries, residential apartments, gas dispensing stations, etc.

A gas pipeline distribution system may have various distribution lines originating from the city gate, all connected to a distribution manifold. Depending on the project design philosophy, the pipeline could be made of steel pipes, ductile iron pipes, plastic pipes, or a combination of different materials.

3 Gas Distribution Pipelines System

A gas distribution system aggregates facilities, equipment, and fittings installed to transport and distribute gas efficiently to end-users.

3.1 Components of Gas Distribution Pipeline System

A gas distribution system may contain any of the under-listed components.

3.1.1 City Gate

The city gate interfaces the Transmission line and the gas distribution system. The equipment listed below may be installed at a city gate.

• Gas Distribution Manifold
• Pressure Reduction and Metering System
• Pressure Indicators and Transmitters
• Temperature Indicators and Transmitters
• Gas Scrubber
• Gas Odourising Unit
• Gas Sampling Unit (Gas Chromatograph)
• Fire and Gas Detection System
• Pipeline Isolation fittings (Isolation joint, flange insulation kit)
• Emergency Shutdown Valves
• Check Valves
• Line Isolation Valves
• Cathodic Protection System

3.1.2 Gas Distribution Manifold

The distribution manifold, mostly installed above ground, splits the gas stream into different distribution lines. The city gate is a central location where different distribution lines originate; these lines may be of different sizes depending on gas demand on each axis of the distribution network. All the distribution lines are tied into a manifold at the City gate. There are isolation valves installed on each line connected to the manifold; the function of the valve is to allow for the shutdown of any of the lines independently from the other lines. The manifold should be appropriately sized to deliver the gas current end users require and envisaged future customers.

3.1.3 Pressure Reduction and Metering System

Pressure reduction units are installed on the gas distribution network to regulate the gas pressure to a level that the downstream distribution pipeline, inline components, and gas consumers can handle. A gas meter is installed within the PRMS skid to measure the quantity of gas flowing through the equipment.

PRMS is designed to suit customer gas requirements and, per the project design philosophy, to suit the pressure range it will handle. Usually, a pressure reduction skid is installed at the city gate to reduce the transmission line pressure to a set value. The reduced pressure may not be the pressure required by the customers but the pressure required to convey the gas along with the network. Other pressure reduction skids may be installed within the network, called “District Pressure Reduction and metering skids” (DPRMS). The DPRMS further reduces the pressure of the line to a level that can be handled by the “Customer Pressure Reduction and Metering Skid (CPRMS)”. The CPRMS is installed on the customer service line, preferably at the customer’s facility before the gas turbine/generator. The CPRMS reduces the pressure of the gas supplied from the main distribution line to the required pressure range required by the customer. Most gas generators’ intake pressure is less than 2 bar and, in some cases, as low as 0.2 bar. Therefore, the CPRMS installed on line-conveying gas to generators should be designed to regulate the pressure to suit the generator gas pressure requirements.

It should be noted that reducing the pressure at the city gate is necessary because high pressure implies higher pipeline wall thickness and higher pressure class of inline components, hence higher material and installation costs. It should also be noted that the reduced pressure should satisfy the hydraulic requirements to convey gas conveniently to all customers; hence, rigorous hydraulic analysis is performed to determine the pressure required along the pipeline and at the customer’s location.

3.1.4 Pressure Indicators and Transmitters

Pressure indicators should be installed upstream and downstream of the pressure reduction and metering skid at the city gate and customer’s location, all lines originating from the city gate (distribution manifold). If the system is automated, pressure transmitters may be used to transmit the measured field pressure to a control room.

3.1.5 Temperature Indicators and Transmitters

Temperature indicators should be installed inline on the distribution manifold at the city gate. The temperature data may be read on the field instrument display or transmitted to a control room in an automated system.

3.1.6 Gas Scrubbers

A Gas Scrubber removes liquid droplets or traces of liquid droplets from gas streams to protect equipment installed downstream of the scrubber from damage and failure. A natural gas scrubber system uses filters, coalescers, mesh pads, and other devices to remove pollutants from the gas stream.

Gas scrubbers are installed at the inlet of every generator/turbine to remove liquid droplets. Depending on the gas characteristics, they may also be installed at the city gate or in line with the pipeline network.

3.1.7 Gas Odourising Unit

Natural gas is odourless and very explosive, so it is important to put in place a means of detecting gas leakages. Gas odourisation is mandatory for Natural gas distribution systems, as stated in section 856.1 of ASME B31.8. A method of detecting leaks in the gas pipeline is to inject a strong-smelling substance metered appropriately into the gas. Mercaptan compounds are popularly used for odorizing natural gas.

3.1.8 Gas Sampling and Analysis Unit (Gas Chromatograph)

A gas chromatograph is installed at the city gate to analyse the components of the natural gas. This validates the gas composition in the Gas Sales and Purchase Agreement (GSPA). The gas chromatograph is connected to the gas pipeline via small diameter stainless or other material pipes. The Chromatograph takes gas from the pipeline, analyses the gas, and separates the gas into various components by sending the gas through a chromatographic channel. The devices calculate the composition of each component and send the report to a display system.

3.1.9 Fire and Gas Detection System

A fire and gas detection system shall be installed at the city gate. The field instruments detect gas leakages by measuring the gas concentration in the atmosphere. Installed temperature sensors shall detect any possible fire. When facilities are automated, the fire detection system should trigger the shutdown of the gas network and activate the firefighting system.

3.1.10 Gas Service Lines or Spur Lines

Spur lines are branches from the main distribution line transporting gas to each consumer. These lines may be made of carbon steel, Plastic materials, Ductile iron, etc. Spur lines are low-pressure because the pressure required by gas consumers is significantly lower than the pressure on the distribution main. When the pressure delivered via the spur line to the gas consumption is higher than required, a pressure regulator shall be installed at the inlet connection to the consumer facility.

3.1.11 Gas Compressor Station

A gas compressor facility shall be installed along the pipeline if the pipeline pressure cannot transport the gas to the customer’s location, satisfying the pressure required by the customer. The actual location of the compressor station shall be specified based on the hydraulic analysis. A gas compression unit can also be installed at the city gate to increase the gas pressure in the pipeline.

A typical compressor station contains:

• Gas scrubbers and filters that remove liquid droplets or traces of liquid droplets from the gas and other impurities
• Valve assembly upstream and downstream of the gas compression unit for isolation and maintenance purposes
• Depending on design requirements, a compressor unit may contain one or more compressors.
• Emergency Shutdown system

Compressors are categorised into two groups: positive displacement compressors and dynamic compressors.

In positive displacement compressors, the inlet volume of natural gas is confined in a given space (cylinder) and compressed by reducing this confined space or gas volume. The compressed gas is discharged into the pipeline at higher pressure. The most common examples of positive displacement compressors are reciprocating or piston compressors and screw compressors.

Dynamic compressors increase gas momentum as it flows through them and convert the energy into pressure. The major types of dynamic compressors are centrifugal and axial compressors.

3.1.12 Valves

For adequate isolation, maintenance or repair purposes, blowdown/venting and purging processes and valves are provided to operate a gas distribution pipeline system. Valves may be welded, flanged or threaded depending on pressure class. However, welded valves provide a better leak-tight system. All valves installed in the gas distribution system shall conform to any of the codes stated in section 831.1 of ASME B31.8 or as per other applicable codes and standards. All valves shall be installed in an easily accessible location and accordance with codes and standards such as ASME B31.8. Any of the following valves may be installed on the gas distribution networks.

3.1.12.1 Emergency Shutdown Valves

Depending on the project design philosophy, emergency shut-down valves (ESDV)/block valves shall be installed on the transmission line conveying gas to the city gate or inlet distribution lines. The valve may be installed above or below ground depending on the location of the valve and design requirements. The ESDV provide positive isolation of the city gate from unforeseen operating conditions.

The use of an automatic shutdown valve is not mandatory, as stated in section 846.2 of ASME B31.8; however, when automatic shutdown valves are used, the valves should be fitted with appropriate instrumentation and control system to trigger the valve to close when:

• There is a pressure surge beyond the set value.
• There is a temperature rise beyond the set value.
• Fire is detected by the fire and gas system installed in the facility.

3.1.12.2 Sectionalisation/Isolation Valves

Sectionalisation or isolation valves should be installed along the pipeline length in the region where major concerns (populated areas, etc.) have been identified, at spur lines branch off from the distribution main, before pressure reduction and metering equipment. Sectionalisation valves shall be installed as per the requirement of section 846 of ASME B31.8 or other applicable codes and standards. This arrangement is useful in limiting gas losses during pipeline leakage or rupture and maintenance of any section of the pipeline. The arrangement should be such that a few customers are cut off from the gas supply during maintenance activities.

The sectionalisation valves may be welded directly to the pipeline to minimise the possibility of gas leakage from flange connections or threaded joints. They may be manually operated, or automatic actuation may be provided.

The sectionalisation valve arrangement may be installed below, above, or in a vault. In any of the above-mentioned installation positions, all valve operators must be easily accessible and protected from damage. When the valve arrangements are installed in underground vaults, the vaults shall be designed under section 847 of ASME B31.8.

Sectionalisation point arrangement may consist of the following:

• Mainline sectionalisation valve

These valves shall be the same size as the main distribution pipeline. They may be directly welded to the pipeline to minimize the possibility of leakage in the pipeline system or flanged. The valve shall be equipped with appropriate instrumentation for an automated system to facilitate remote operation.

• Bypass Lines

The bypass arrangement may include two ball valves (for shutting off) and two globe valves (for throttling). These shall be used when maintenance or repairs are needed in any section of the pipeline. They are located to depressurize the pipeline segments and bring them online during the start-up process after maintenance or repair of a section of the pipeline.

• Vent Lines

Vent lines are primarily used for venting and blowdown purposes in the pipeline operation. They are also used to depressurize any section of the line where maintenance is required. Vent lines shall be appropriately located away from public places. A ball valve should be installed at the tail of the line to enable tight shut-off. A permanent vent system may be installed, or a tie-in provision for a temporary mobile vent system may be provided.

3.1.12.3 Check Valves

Check valves shall be installed downstream of pressure reduction and the metering system, as stated in ASME B31.8 standard section 848.3. The check valves protect the PRMS from back pressure if lower pressure exists upstream due to pipeline failure or any other event.

3.1.13Gas Distribution System: Cathodic Protection System

All steel pipes buried underground must be externally coated to prevent external corrosion. Buried gas distribution lines may be externally coated with 3 Layer Polyethylene Coating (3LPE), any other material following ISO 21809-1 or other codes and standards. However, in many cases, damaged external coating leads to severe pipeline corrosion. In conjunction with 3LPE protection, all underground steel pipelines shall be cathodically protected. The essence of the protection is to eliminate corrosion. Impressed current is the preferred means of protecting buried underground pipelines.

Impressed current entails feeding current generated by a Transformer Rectifier Unit (TRU) connected to an anode ground bed into the buried pipeline. The current is used against the corrosion process, thereby protecting the pipeline. Typical components of a cathodic protection system include a Transformer Rectifier Unit, anode ground bed, cathodic protection cables, cathodic protection test points installed, and the entire steel pipeline network.

3.1.14 Pipeline Isolation Fittings (Isolation joint or Flange Insulation Kit)

The main function of an isolation joint or flange insulation kit is to isolate various sections of a gas distribution system electrically. The requirement for electrical isolation is well spelt out in section 861.1.3 of ASME B31.8.

The above-ground facilities, e.g., the city gate and customer connections, are externally painted and hence externally protected from corrosion. It is imperative to isolate the underground section protected by CP (Cathodic Protection) and external coating from the above-ground section. This is achieved by installing Isolation joints or flange insulation kits at the transition point from above ground to below ground.

Pipeline isolation joints/ flanges shall also be installed at the identified location to minimise or eliminate current leakages to third-party facilities that may be directly or indirectly connected to the distribution network. Isolation joints may be installed at locations where the pipeline is installed parallel to the Overhead electrical transmission line.

It should be noted that current leakages only occur in the metal pipeline section; non-metallic sections of the gas distribution system do not require the installation of an Isolation joint.

3.1.15 Control Stations

Depending on the level of automation desired, a gas distribution system may be fully automated or semi-automated.

The control station’s function is to oversee the entire pipeline network and all connected equipment. It receives signals from field instruments such as pressure indicators and transmitters, temperature indicators and transmitters, flow measurement instruments, etc. The control station may also close any valve on the gas distribution pipeline.

4 Gas Distribution System Design and Operation Considerations

During the various design stages of a gas distribution project (conceptualisation to detailed design), various considerations, including gas consumers, gas volume, material selection, pipeline routes, and line size, shall be analysed.

4.1.1 Gas Consumers

Gas consumers are the key factor in planning a gas distribution pipeline system.

Before conceptualizing a gas pipeline, consumers must be available to use the gas to transport. The number of customers, their locations, the quantity of gas to be purchased by all customers, and envisaged future customers must be analysed to determine the project’s feasibility.

4.1.2 Gas Volume

This is an important factor to consider when planning a gas distribution network. The gas volume has to be checked from both the demand and supply end. After identifying all the customers, the volume of gas required by all customers is summed. The total gas available is checked against the total gas demand. It should be noted that having one high-volume gas consumer (e.g., power generating plants) is better than having many customers with low-volume consumption.

4.1.3 Gas Supply Requirements

This key factor should be analysed when planning a gas distribution network. Customers’ gas requirements, such as delivery pressure and temperature, vary because the gas will be utilised for various purposes. The pressure required by gas engines varies; therefore, the pressure reduction unit installed on each service line may differ. This implies that the cost of PRMS on the distribution network will vary amongst customers.

4.1.4 Process Simulations

This entails determining process parameters along with the distribution network. The preliminary analysis utilises assumed data, including route elevation, fittings, etc.; however, as the project progresses from concept studies to detailed design, actual survey data shall be used for hydraulic analysis. Process hydraulic simulation is critical because the simulation results show the gas characteristics (pressure and temperature) at each customer location and the network. The simulated pressure at the customer location will specify the PRMS to be installed if required.

4.1.5 Pipeline Isolation Philosophy

This is critical when planning a gas distribution network. Usually, the network should be designed so that a minimal number of customers are affected during maintenance activities. This is usually achieved by correctly locating isolation valves at strategic locations.

4.1.6 Line Sizing

A process engineer performs line sizing. Line sizing entails determining the optimum pipe size that can convey the desired volume of gas to the customers. Line size shall be considered in the future expansion of the gas distribution network. It is critical to properly size the line, especially when future customers are envisaged. This ensures that the line can convey the quantity of gas required by all customers.

4.1.7 City Gate Location

The city gate’s location is key to any gas distribution network. The city gate should be situated so the transmission line and distribution network length are significantly reduced. While siting the city gate, the location of gas sources such as processing facilities or export pipelines should be properly analysed. A detailed hydraulic analysis is required to check gas properties (pressure, temperature) upstream and downstream of the city gate.

4.1.8 Pipe Materials

The pipeline is made up of joined pipes and other inline components. As stated in section 812 of ASME B31.8, the pipeline can be made of steel materials, ductile iron materials, plastic materials or a combination of materials. However, most of the gas distribution pipelines are made of steel pipes.

Material selection is very key when planning a gas distribution network. Steel pipes are more susceptible to corrosion than ductile iron and plastic pipes. Plastic pipes do not corrode; however, they have the lowest strength compared to steel and ductile iron pipes. Ductile iron has low weldability compared to steel pipes, necessitating different joining methods. Therefore, when selecting materials for a gas distribution network, the advantages and limitations of any material should be examined carefully.

Steel pipes manufactured according to the following standards—API 5L, ASTM A53/A53M, ASTM A106/A106M, and ASTM A134—and other standards stated in section 814.1.1 may be used for the pipelines.

As stated in section 14.1.2 of ASME B31.8, ductile iron pipes manufactured following ANSI A21.52, such as those centrifugally cast for gas, may be used in a gas pipeline.

Section 814.3 section of ASME B31.8 permits the use of plastic pipes. Plastic pipes and components manufactured following any of the under-listed standards may be used.

Polyethene Pipes manufactured according to the D2513 standard (Polyethylene (PE) Gas Pressure Pipe, Tubing, and Fittings) may be used.

Polyamide-11 (PA-11) pipes manufactured according to ASTM D2517 (Polyethylene (PE) Gas Pressure Pipe, Tubing, and Fittings) and ASTM D2517 Reinforced Epoxy Resin Gas Pressure Pipe and Fittings may be used in the gas distribution network.

Thermoplastic pipe, tubing, fittings, and cement conforming to ASTM D2513 may be used. However, they shall be produced following the in-plant quality control program recommended in Annex A3 of the specification.

4.1.9 Pipeline Route

The distribution network shall be routed so that gas can be economically and efficiently transported to gas consumers. The pipeline shall be routed so that it is close to consumers. The routing shall also take into consideration future customers.

4.1.10 Permitting and Regulations

Permitting is an essential factor while planning a gas distribution pipeline. Permitting procedures vary across countries. The permit is the certifying document showing that a pipeline can be installed in the designed route as per the specified standard. For example, in Nigeria, pipeline permitting is performed by the Department of Petroleum Resources (DPR). The procedure is performed in adherence to the Oil Pipeline Act.

Before construction activities, all permits from government agencies, including the Ministry of Transportation (for pipeline road crossings and railway crossings), waterways (for pipeline river crossings), etc., shall be obtained.

4.1.11 Pipeline Fittings

When planning a gas distribution network, a critical analysis of the fittings will be performed. Some of the fittings’ requirement is explained below:

Bends

Specified pipeline bends shall meet the pressure, temperature, thickness and bend requirements.

The main distribution line may be pigged depending on client and code requirements. When lines are to be pigged, all bends shall meet the requirement of the proposed pigging tools. Some pigging tools require 5D bends to pass the tool easily; therefore, the bends must satisfy the 5D bend radius.

Barred Tee

Barred tees are used on peggable main lines. The specified barred tee shall meet the temperature, pressure, and pigging requirements. The barred tee shall be installed at all branches from the mainline to ensure that the pigging tool is not in stock at branch connections.

Flanges

Flanges are key connecting elements in a gas distribution network. They are the weakest link where gas leakages can occur. The specified flange class shall meet the line’s pressure and temperature requirements.

4.1.12 Pipeline Burial Depth

Pipelines transporting gas should be buried at an appropriate depth per design codes and standards, such as section 841.1.11 of ASME B31.8. Also, local guidelines are mandatory and supersede any international standard requirement. The actual pipeline burial depth shall be decided after critically examining the pipeline route, considering pipeline safety, and other considerations.

4.1.13 Pipeline Integrity Assessment

Pipeline integrity assessment ensures that the pipeline is operated safely. The envisaged assessment method shall be analysed during the project’s conceptual stage. Several integrity assessment methods exist, such as ultrasonic testing, pigging with intelligent tools, etc.

Pipelines planned for pigging shall have all fittings and valves satisfying the pigging requirements.

4.1.14 Cost of Building the Gas Distribution Network

A proper cost estimate shall be performed during the design of a gas distribution system to determine the project’s feasibility. The capital expenditure (CAPEX) and operating expenditure (OPEX) shall be analysed to determine if the project is feasible. CAPEX is a category of expenses that results from the project’s design stage to the commissioning stage. At the same time, operating expenditures cover the cost incurred in running the gas distribution system.

5 Conclusion

Natural gas can be transported in its gaseous state (NG or CNG) or liquid state (LNG) by ocean vessels, Ships, Special Tubes, Pressure Vessels mounted on Trucks, and pipelines.

Selecting a means of transporting gas and the state of transporting the gas require critical evaluation of available transportation infrastructure, the quantity of gas required by end-users, and the total CAPEX and OPEX of the project.

6 References

ASME B31.8 – 2016: Gas Transmission and Distribution Piping Systems

ISO 21809-1: Petroleum and natural gas industries – External coatings for buried or submerged pipelines used in pipeline transportation systems – Part 1: Polyolefin coatings (3-layer PE and 3-layer PP).

West Africa Gas Pipeline: https://www.wagpco.com/the-project/wapco-pipeline

Axxela Group: https://axxelagroup.com/operations/gaslink-nigeria-limited/

Axxela Group: https://axxelagroup.com/operations/gas-network-services-limited/