Gas is a substance (elements or compound) existing in a free expanding state to fill an entire container. This implies gas will expand freely to fill up the shape of any container it is stored. Various elements and compounds including oxygen, nitrogen, hydrogen, carbon monoxide, carbon dioxide, hydrocarbon gas (Methane, Ethane, Propane…) exist in gaseous states.
Hydrocarbons are organic compounds consisting entirely of carbon and hydrogen. In this article, EPCM puts emphasis on hydrocarbon gas (Natural gas).
1.1 Gas Distribution System: Natural Gas
Natural gas is a naturally occurring hydrocarbon gas consisting mainly of methane and may contain 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 crust are exposed to intense heat and pressure over a long period of years.
Natural gas is a source of energy used for electricity generation, cooking, heating etc.
Natural gas in its processed state exists in a gaseous state, 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. Natural gas is colourless and odourless, 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) temperature. At this temperature, the gas changes into a liquid. Natural gas is transported in large quantity in its liquefied state (1/600th of its original gas volume) over long distance using specially built LNG transport Vessels. Natural gas cannot be used in its liquefied state therefore at its destination the Liquefied gas is converted into a gaseous state by warming the gas through a process called 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, the transportation of CNG using Ship is not as popular as using Ship to transport LNG due to cost implications.
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 therefore the liquefied gas is converted back into gaseous states 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 truck to end-users.
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 through short distance compared to transporting gas using Vessels/Ships and pipelines. Special purpose pressure vessels (Iso containers/special tubes) mounted on trucks are used to convey gas to end-user facilities.
When gas is transported in a liquefied state, the storage vessel should have the capability 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 pressure over 200bar therefore the storage tubes are made of high strength steel materials and thickness capable of holding the gas safely.
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 the gas is compressed, thus increasing the pressure of the gas.
The gas exits the compressor into storage cylinders, which is connected to the dispensing unit. The dispensing unit discharges the gas from the storage cylinders into storage tubes for onward transportation to customers facility.
Gas Chiller unit is incorporated into the system downstream of compressor this is primarily to ensure 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 can be handled by gas consumers. The CNG is fed into a heat exchanger unit where the temperature of the gas is gradually increased. 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 transported by trucks for onward delivery to customers’ locations (https://axxelagroup.com/operations/gas-network-services-limited/)
Figure 1: Compressed Natural Gas Tubes on Trucks
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 delivering of gas to consumers without disruption. The delivery is not affected by most environmental factors.
Pipelines can be routed to take short cuts to its destination thereby reducing the time of transportation compared to other means of transportation.
The large volume of gas is transported
Pipeline transportation is the safest and most reliable means of transporting gas
Several transmission and distribution pipelines are conveying natural gas within cities, 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 Africa 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 initially deliver a volume of 170MMscfd and in later stages a capacity of 460MMscfd to gas users (https://www.wagpco.com/the-project/wapco-pipeline)
Gaslink (GNL) a subsidiary of Axxela Group. In conjunction with the Nigerian Gas Marketing Company (NGMC) operates over 100Km length 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, American countries.
2 Gas Pipeline System
A gas pipeline system is a connection of various facilities, equipment, fittings aimed at delivering gas efficiently to end-users.
2.1 Gas Distribution System: Types Pipelines
Pipelines may be categorised into any of the following
Flowlines are used to transport fluid (gas or liquid) from wells to storage facilities or processing facilities. The Processing facility may be 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.
Figure 2: Well Head with Connected Dual Flowlines
2.1.2 Gathering Pipelines
These pipelines transport fluids from various 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 with all lines connected to a distribution manifold. The pipeline could be made of steel pipes, ductile iron pipes, plastic pipes or a combination of different materials depending on the project design philosophy.
3 Gas Distribution Pipelines System
A gas distribution system is the aggregation of facilities and equipment and fittings installed, aimed at transporting/distributing 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 is the interface between the Transmission line and the gas distribution system. The below-listed equipment may be installed in a city gate.
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 located 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 line independently from the other lines. The manifold should be appropriately sized to deliver the quantity of gas required by current end users 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 can be handled by the downstream distribution pipeline, inline components and gas consumers. There is a gas meter installed within the PRMS skid to measure the quantity of gas flowing through the equipment.
PRMS are designed to suit customer gas requirements and as per the project design philosophy i.e. designed 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. Within the network, other pressure reduction skids may be installed called “District Pressure reduction and metering skid” (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 etc. 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 lesser than 2bar in some cases as low as 0.2barg, 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 it is necessary to reduce the pressure at the city gate because a high pressure implies higher pipeline wall thickness, higher pressure class of inline components, hence higher material and installation cost. 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 customer’s location.
Figure 3: Installed Pressure Reduction and Metering Skid
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 and, may be installed at the city gate, inline the pipeline network depending on the gas characteristics.
Figure 4: Gas Scrubber Installed at the Inlet of a Generator
Figure 5: Gas Scrubber Installed Inline a Pipeline
3.1.7 Gas Odourising Unit
Natural gas is odourless and very explosive therefore it is important to put in place a means of detecting gas leakages. Gas odourisation is mandatory for Natural gas distribution system as stated in section 856.1 of ASME B31.8. A method of detecting leaks in the gas pipeline is to inject 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 is used to validate the gas composition stated 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, analyse the gas, 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.
Figure 6: Installed Rosemount Gas Chromatograph
3.1.9 Fire and Gas Detection System
Fire and gas detection system shall be installed at the city gate, the field instruments detect gas leakages by measuring the concentration of gas 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 material etc. Spur lines are low-pressure lines because the required pressure by gas consumers is significantly low compared to the pressure on the distribution main. When the pressure delivered via the spur line to the gas consumption is higher than the pressure required, a pressure regulator shall be installed at the inlet connection to the consumer facility.
3.1.11 Gas Compressor Station.
Gas compressor facility shall be installed along the pipeline if the pipeline pressure cannot transport the gas to customer’s location satisfying the pressure required by the customers. 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 purpose
Compressor Unit which may contain one or more compressors depending on design requirements.
Emergency Shutdown system
Compressors are categories into two groups: positive displacement compressors and dynamic compressors.
In positive displacement compressors, 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 operation is based on increasing gas momentum as it flows through the compressors and converting the energy into pressure. Centrifugal and axial compressors are the major types of dynamic compressors.
For adequate isolation, maintenance or repair purposes, blowdown/venting and purging processes, provision of valves is made in the operation of 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 to be 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 to codes and standard such as ASME B31.8. Any of the following valves may be installed on the gas distribution networks.
22.214.171.124 Emergency Shutdown Valves
Emergency shut down valve (ESDV)/Block valves shall be installed on the transmission line conveying gas to the city gate or on the inlet distribution lines depending on the project design philosophy. The valve may be installed above or below ground depending on the location of the valve and design requirement. The ESDV provide positive isolation of the city gate from unforeseen operating conditions.
The use of 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 pressure surge beyond set value.
There is temperature rise beyond set value.
Fire is detected by the fire and gas system installed in the facility.
Figure 7: Emergency Shutdown Valve
126.96.36.199 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 standard. This arrangement is useful in limiting gas losses during pipeline leakage or rupture and during maintenance of any section of the pipeline. The arrangement should be such that a minimal number of customers are cut off from 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 there may be provision for automatic actuation.
The sectionalisation valves arrangement may be installed below ground, above ground or in a vault. In any of the above mention installation position, all valves operators must be easily accessible for operation 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 have the same size as the main distribution pipeline, the valves may be directly welded to the pipeline to minimize the possibilities of leakage in the pipeline system or flanged. For an automated system, the valve shall be equipped with appropriate instrumentation to facilitate remote operation.
The bypass arrangement may include two ball valves (for shutting – off purpose) and two globe valves (for throttling purpose). These shall be put to use when there is a need for maintenance or repairs in any section of the pipeline. They are located for depressurizing the pipeline segments and also to bring the pipeline segments online during the start-up process after maintenance or repair of a section of the pipeline.
Vent lines are primarily used for venting and blowdown purposes in the operation of the pipeline. This line is used to depressurise 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 for tight shut-off. A permanent vent system may be installed or tie-in provision for temporary mobile vent system may be provided.
188.8.131.52 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 there exist lower pressure upstream of the PRMS due to pipeline failure or any other event.
3.1.13Gas Distribution System: Cathodic Protection System
It is required that all steel pipes buried underground are 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 up against the corrosion process thereby protecting the pipeline. Typical components of a cathodic protection system include Transformer Rectifier Unit, anode ground bed, cathodic protection cables and cathodic protection test points installed along with the entire steel pipeline network.
Figure 8: Installed Transformer Rectifier Unit
3.1.14 Pipeline Isolation Fittings (Isolation joint or Flange Insulation Kit)
The main function of an isolation joint or flange insulation kit is for electrical isolation of various sections of a gas distribution system. The requirement for electrical isolation is well spelt out in section 861.1.3 of ASME B31.8
The above-ground facilities e.g. city gate, customer connections are externally painted 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 kit 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 Overhead electrical transmission line.
It should be noted that current leakages only occur in the pipeline section made of metals, non-metallic sections of the gas distribution system do not require the installation of Isolation joint.
Figure 9: Installed Pipeline Isolation Joints
3.1.15 Control Station.
Depending on the level of automation desired, a gas distribution system may be fully automated or semi-automated.
The function of the control station is to oversee the entire pipeline network and all connected equipment. The control station receives signals from field instruments such as pressure indicators and transmitters, temperature indicators and transmitters, flow measurement instruments, etc. The control station may also have the capability to close any valve on the gas distribution pipeline.
4 Gas Distribution System Design and Operation Considerations
During various design stages of a gas distribution project (conceptualisation to detailed design), various considerations including gas consumers, gas volume, material selection, pipeline routes, line size shall be analysed.
4.1.1 Gas Consumers
Gas consumers are the key determinant factor in planning a gas distribution pipeline system
Before conceptualizing a gas pipeline, there have to be consumers available to utilize the gas to be transported. The number of customers, location of the customers, quantity of gas to be purchased by all customers, envisaged future customers has to be analysed to determine the feasibility of the project.
4.1.2 Gas Volume
This is an important factor to be considered while planning a gas distribution network. The gas volume has to be checked from both 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 it is better to have one high volume gas consumer (e.g. power generating plants) than having many customers with low volume consumption.
4.1.3 Gas Supply Requirements
This is a key factor that should be analysed when planning a gas distribution network. Customers gas requirements such as delivery pressure, temperature varies because the gas will be utilised for various purpose. The pressure required by gas engines varies therefore the pressure reduction unit to be installed on each service line may be different. 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 progress 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 along with the network. The simulated pressure at the customer location will be used to 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 such 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
Line sizing is performed by a process engineer. Line sizing entails determining optimum pipe size that can convey the desired volume of gas to the customers. Line size shall take into consideration future expansion of the gas distribution network. It is critical to properly size the line especially when future customers are envisaged. This is to ensure that the line can convey the quantity of gas required by all customers.
4.1.7 City Gate Location
The location of the city gate is very key to any gas distribution network. The city gate should be situated such that the length of the transmission line and distribution network is significantly reduced. While siting the city gate the location of gas sources such as processing facility, or export pipeline should be properly analysed. The 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 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 highly susceptible to corrosion compared to ductile iron pipes 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 this necessitates different joining methods. Therefore, in selecting materials for a gas distribution network, the advantages and the limitations of any material selected should be carefully examined.
Steel pipes manufactured as per the following standards API 5L, ASTM A53/A53M, ASTM A106/A106M, 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, titled Ductile iron pipes, Centrifugally Cast, for Gas, may be used to 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 manufacture as per D2513 standard (Polyethylene (PE) Gas Pressure Pipe, Tubing, and Fittings), may be used.
Polyamide-11 (PA-11) pipes manufactured as per ASTM D2517 (Polyethylene (PE) Gas Pressure Pipe, Tubing, and Fittings), ASTM D2517 Reinforced Epoxy Resin Gas Pressure Pipe and Fittings may be used on 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 such that gas can be economically and efficiently transported to gas consumers. The pipeline shall be routed such that the pipeline is close to consumers. Also, the routing shall take into consideration future customers.
4.1.10 Permitting and Regulations
Permitting is an essential factor while planning a gas distribution pipeline. Permitting procedures varies across countries. The permit is the certifying document showing that a pipeline can be installed in the designed route as per specified standard. For example, in Nigeria, pipeline permitting is performed by the Department of Petroleum Resources (DPR). The permitting procedure is performed in adherence to the Oil Pipeline Act.
Before construction activities all permits from government agencies including Ministry of transportation (for pipeline road crossing and railway crossings), waterways (for pipeline river crossing), etc. shall be obtained.
4.1.11 Pipeline Fittings
When planning a gas distribution network, a critical analysis of the fittings to be used shall be performed. Some of the fittings’ requirement is explained below:
Specified pipeline bends shall meet the pressure, temperature, thickness and bend requirements
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 for easy passage of the tool, therefore the bends must satisfy the 5D bend radius
Barred tees are used on piggable main lines. The barred tee specified shall meet the temperature, pressure and pigging requirements. The barred tee shall be installed at all branches from the mainline this is to ensure that pigging tool is not stock at branch connections.
Flanges are key connecting elements used in a gas distribution network. When flanges are used in a gas distribution system, they are the weakest link where gas leakages can occur. The specified flange class shall meet the pressure and temperature requirement of line.
4.1.12 Pipeline Burial Depth
Pipeline transporting gas should be buried at appropriate depth as per design codes and standard such as section 841.1.11 of ASME B31.8. Also, local guidelines requirements are mandatory and supersede any international standard requirement. Actual pipeline burial depth shall be decided after critical examination of the pipeline route taking into consideration pipeline safety and other considerations.
4.1.13 Pipeline Integrity Assessment
Pipeline integrity assessment ensures that the pipeline is operated safely. During the conceptual stage of the project, the envisaged method of assessment shall be analysed. There are several integrity assessment methods 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 feasibility of the project. 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 design stage of the project to the commissioning stage while operating expenditure cover the cost that will be incurred in running the gas distribution system.
Natural gas can be transported in its gaseous state (NG or CNG) or liquid state (LNG). Natural gas can be transported by ocean vessels/Ship, 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, the total CAPEX and OPEX of the project.
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 – Part1: Polyolefin coatings (3-layer PE and 3-layer PP).