Sun. Jun 20th, 2021

City gas distribution

Transporting future fuel

#15 Components of CGD Network

25 min read

The CGD network primarily comprises of medium pressure steel network which receives gas from the transmission pipeline. The subsequent low-pressure PE network takes the gas to low-pressure users in the domestic and commercial segments. From PE network the gas is supplied to the end consumers through GI pipes or a combination of GI pipes and copper tubing. The gas supply to industrial and commercial could also be directly from the medium pressure steel network.

There are several other components that make the grid operational serving all the four business segments like commercial consumer, Industrial consumer, Domestic consumer & CNG transport section. These components are essential to create reliable infrastructure which ensures the safety of the system including operational ease.

Natural gas is received at a station called the city gas station (CGS). The gas is measured after control of pressure, mostly the pressure is reduced from the transmission mains operating pressure to city mains operating pressure. Gas odorization unit and safety gadgets such as slam shuts valve, pressure safety valve, etc. form part of the network.

There are district regulating stations (DRS) some time also called as district pressure regulating station (DPRS) installed at various locations to serve low pressure consumer-like PNG domestic.

The components of CGD are better described in 2 groups – The first one is a function-based component which may comprise of various sub-components and the second one is an individual item as a component that is required for a CGD project.

1. City Gas Station (CGS): This comprises of gas delivery from the transmission pipeline by a CGD network. As per the notified regulations the transmission mains can operate at a pressure up to 98 bar and the CGD steel grid can operate up to a maximum of 49 bar.

CGS is the door to the main distribution network. CGS for the network will be located at the Tap-off point. The main purpose of the city gas station is to transfer the custody of gas from Transporter to Distribution Company. For ensuring continuity of supply more than one CGS should be planned for a CGD system. City gas station is the key to the door to a Medium Pressure Network. It assures the basic interface between the High-pressure system and the medium pressure network by operating the following facilities. CGS comprise of the following:

a) Filter separator Knock out drum: To knock out the moisture condensate and the dust from the gas coming out of transmission pipeline a knock out drum system is used. The dust, dirt, or liquid traveling with in-coming gas are separated in gravity or cyclone separator so that only clean gas goes to pressure control and metering unit.

b) Gas filtration system: To separate the dust, rust and similar other unwanted elements a filtration system is provided at the CGS. The filters could be fine mesh or candle type to filter out particles of elements up to 10 micron and above.

c) Pressure control unit: The allowable pressure in the CGD network, as per T4S regulation is limited to 49 bar and therefore the transmission pipeline pressure needs to be controlled to this limit or any other limit lower than this the designer wishes to design the system, for example at 19 bar pressure, etc. The pressure control unit may be an independent unit owned and operated by the Gas transporter or it could be a part of the metering unit or metering skid.

d) Metering skid: It is the most important part of any city gate station. It is basically a mechanical arrangement comprising of the following:

i) Filter : Whatever the precautions taken during construction and at final clean-up stage before commissioning, no pipeline is perfectly clean. Small quantities of residual dirt may always remain in the system. residual dirt will move along with gas depending on gas speed and pipeline geometry. The experience shows that for such a fully new important high-pressure system, the probability of clogging the inlet filters to city gas is very high. Filter separators shall be installed with 5 microns maximum particles passage for the specified flow range. The number of such filter depend on the consumption.

ii) Gas quantity measurement system (Meter): The measurement of gas in terms of volume is carried out with the help of flow meters. It may be either a Turbine flow or Ultrasonic flow, selection of which depends upon the usage pattern which finally governed by its investment cost. Ultrasonic meters are of higher volume and high-cost meters with respect to turbine flow meter which is comparatively economical.

iii) Isolation valves: In a metering skid, we use different types of valves of various sizes for many purposes like isolation, flow control, Bypass venting, etc. All valves should be in line with API 6D.

iv) Pressure Regulators: Regulators are used for regulating the pressure inline to operating conditions. It reduces the high inlet pressure to allowable maximum downstream pressure.

v) Heaters: It is basically a gas-fired type. When we reduce the pressure of gas, both the gas vapor and condensate formed in line due to the expansion of gas. To avoid this we pass gas through this heating process for vaporization of any liquid molecules. Heating is required when the difference between outlet and inlet pressure is quite high, otherwise, it is not required.

vi) Gas quality measurement system: Gas to be charged in CGD mains needs to conform to the specification mentioned in Appendix-I of the access code notified by the Board. Therefore, the measurement facilities for all these components need to be provided at CGS. Primarily the system comprises a chromatographic measurement of gas quality.

vii) Odorization: Natural gas is by nature, odorless that makes detection of leaks impossible without special gas detection tools. End-user connected to a natural gas distribution system is not supposed to have adequate skill for gas handling, it is mandatory to add an odorant to the gas before it enters the medium pressure distribution system. Odorization shall be based on the injection of a suitable sulphur component in adequate proportion on the medium pressure side. The flow signal is provided by a dedicated flow measurement system.

Two main products may be considered : Tetrahydrothiophene (THT) & Tetrabutylmercapton (TBM)

TBM is often presented as being cheaper than THT. It is about 3 times more smelly and more volatile.

Odorization of the gas for the purpose of safety has been mandated in the regulations notified by the board. The media of odorization is left to the entity. Ethyl mercaptan is presently being used as an odorizer. The level of odorization has also been notified of the regulations. The characteristics of ethyl mercaptan are such that it gets observed on the steel pipe wall and therefore the quantity of odorant gets reduced downstream of dozing location. The dozing unit of the ethyl mercaptan is between 10-20 mg/m3 as per requirement.

Inter distance (mtr) between various facilities required at CGS shall be;
1Compound wall66666
2Control room / office building / store61212215
3Pressure regulation and Metering61221215
4Odorant system61221215
5Electrical sub station62121215
6Gas fired heaters615151515

2. Sub Transmission Pipeline (STPL): Design wise, it is same as transmission pipeline. It is also called as feeder line. The only notable point is that it connects the main trunk line or the transmission pipeline to the city gas distribution at city gate station and is owned by the CGD entity as against the transmission pipeline owned by the Gas transporting company. The design of STPL follows the same standards as main pipeline transporting natural gas.

3. Steel Network: This is a medium pressure steel network. The size of the network would demand on the volume of gas to be transported to various charge area, the maximum allowable pressure in this network is 49 bar (class 300 pipeline system) but 19 bar pressure (class 150 pipeline system) has been the much-used case. The design of the steel mains is recommended to be garland type or ring type so that in case of line fault at any location, the customers at best are served from another side of the network.

4. District regulation system: This system comprises of pressure reduction system mostly from 19/49 bar to 4/7 bar system. The redundancy of the pressure reduction stream varies from 33% to 50% with all safety devices such as Slam shut valve and pressure safety valve. The DRS could be an above-ground console or an underground depending on the space availability. The DRS capacity can be selected as per the demand of the downstream customers connected to specific DRS. In some nomenclature, DRS is also called District Pressure regulating device (DPRS). DRS is provided with manual isolating valves to facilitate the shutdown of the station in case of non-requirement. Functionally it is more or less the same like metering skid installed at CGS. DRS includes filtration skid, pressure reduction skid & metering skid. Generally, DRS is available in the range of 5000 SCMH to 10000 SCMH. Based on the flow rate, we select the size of the pipe, valve. fittings etc. ALl the DRS should be designed for 3 streams (2+1), two active plus one stand by basis.

5. Service Regulator: Functionally, it is the same as DRS but it reduces pressure from 4 bar coming from DRS to 100 mbar which is used by commercial and residential customers. Its flow rate is very less (25 to 100 SCMH) compared to DRS.

6. Meter Regulator: It is used mainly in a domestic segment where it reduces pressure from 100-121 mbar to 21 mbar.

7. CNG System: CNG system comprises CNG compressor, CNG station, Dispenser, Cascade, and safety system.

8. Compressor Station: Flow process of gas on CNG station starts from Mainline followed by Metering skid, compressor, priority panel, stationary cascade, LCV Cascade, and Dispenser.

9. Metering skid: The purpose of metering skid on CNG station is to measure the volume of gas consumption and to record the pressure of gas. It displays the record in digital form.

10.CNG Compressor: The purpose of the compressor is to compress the natural gas from 10-14 bar of pressure to 250 bars. This may be achieved in two or three stages of compression. In CNG station we use reciprocating compressor which may be either Motor-driven or Engine driven diesel preferably natural gas.

The CNG compressor alone covers more than half of full station costs. It may be called “Heart of any CNG Station”. So, its selection and sizing is a very important process in both ways Technical as well as Commercial. The actual sizing and number of compressor for the CNG Stations is the function of the following :

i) The actual gas filling time for the vehicle

ii) The dwell time between vehicle or turn-around time for the vehicle

iii) The storage capacity provided in the cascades for the system

Compressor are available for capacities varying from 110 SCMH to 1200 SCMH and more for suction & discharge pressure of 12 bar to 255 bar.

Compressor selection mainly depend on the following points :

i) Inlet pressure

ii) No. of stages required

iii) Types of drive

iv) Discharge volume

v) per unit per consumption etc.

The CNG compressor may have various version such as :

  1. Online Compressor: It is used to compress gas directly from the pipeline and the discharge can be used to fill the CNG vehicles at CNG retail outlet. The capacity of such a compressor is selected based on the vehicle likely to be filled in that station and with the provision for expected future expansion. It may be equipped with a 1200 SCMH capacity compressor for compressing the natural gas from 49 bar to 225 bar. All categories of vehicles are refueled i.e. Bus, Car, LMV, LCV, and autos. Cascade filling facilities will not be available at the online station as it is in Mother station.
  2. Mother Compressor: The station at which the mother compressor operates is known as the mother station. Mother compressor is a high capacity compressor used to fill cascades for transportation of gas to a Daughter station or Daughter booster station or to other industrial or commercial customers. The mother station may also have CNG filling stations in addition to filling the cascades for other applications. The capacity of the mother compressor is generally large starting from 1200 SCMH.
  3. Daughter booster compressor: This is a small compressor which takes suction from cascades filled at mother stations. The size of the compressor starts from 200 SCMH and could be as high as 400 SCMH to 600 SCMH depending on the size of the inlet cascade and demand for CNG. The preferable version for such a booster compressor is variable stroke and variable suction pressure compressor. This helps the compressor to operate from the wide range of cascade pressure from 230 bar down to 30 bar. This provides a lesser weight of a gas to be transported back to the mother station and also gives better utilization of the gas brought from other stations to the daughter station. The CNG station at which such a compressor operates is called Daughter booster station. A loaded mobile cascade is always parked at the daughter booster station for unloading.

11. CNG Cascade: The purpose of a cascade is to store the compressed natural gas whenever cascade is on higher priority thus dispenser is in off position. The cascade comprises of a number of freestanding or horizontal high-pressure seamless gas cylinders for CNG gas service suitable for a filling pressure of 250 bar at 15 degrees Celcius. The cylinder is mounted in the frame such that there is no lateral or rotational movement of the cylinder during transportation. The cylinders assembled keeping a minimum gap of 30 mm between them. Imported high-pressure cylinders are assembled inside a sturdy frame in three independent banks – Low bank, medium bank & high bank. The frame having a lifting eye to facilitate lifting with the crane or forklift with the full weight of cylinder filled with gas and have suitable tie-down clamps at the base. The cascade painted with special paint to withstand environmental conditions. All fitting and valve are of SS 316 rated for the working pressure. All tubing is of SS 316 conforming to ASTM A2269 suitable to absorb contraction, expansion, and vibration. The pressure gauge is also mounted on each bank to check the pressure of the gas inside the cylinder.

12. Priority Panel: The purpose of the priority panel is to provide the compressed Natural gas to the Dispenser/LCV Cascade on the basis of which is on a higher priority. The priority panel directs the gas flow from the compressor to the high bank, medium bank, or low bank of the storage cascade, depending on demand.

If the storage pressure is low, the discharge gas is sent directly to the vehicle to provide faster fueling. Priority panel holds the responsibility to supply the gas from the compressor unit to the dispenser, stationary cascade, and mobile cascade logically. The stationary cascade is having the following cylinder banks:

i) High bank

ii) Medium bank

iii) Low bank

The main objective of providing high bank, medium bank and low bank facility is to meet the customers with irrespective of pressure gauge. When the stationary cascades are filled, compressor will be stopped promptly. Since, frequent operation of the reciprocating compressor leads to wear and tear. Setting up of a stationary cascade bank is the excellent alternative to escape away from wear. Thus, by doing this we can intensify the compressor life.

A typical priority panel works in the following procedure;

  1. When no vehicles in the refueling stand, natural gas is being passed to a high bank, which is getting the number one priority but next to the vehicle filling. Since any vehicle coming in the middle it has to fill the gas to the vehicle first after that to high bank.
  2. When the high bank is filled, then the priority goes to the medium bank.
  3. When the medium bank gets filled, it goes to the lower bank.
  4. When all the banks are filled, gas will be diverted into the light commercial vehicle is called as LCV in general.
  5. At the time of high bank or a low bank are filled, any vehicle comes to refill the gas, Priority panel cuts the gas from filling any banks and will send to the vehicle. The order or vehicle filling from the priority panel is having different sequences. First, the gas is filled from the low bank into the vehicle cylinder, on near equalization of pressure, , the medium bank starts filling the cylinder, and again at the equalization of pressure, HP bank starts filling the vehicle cylinder till pressure equalizes. This helps in filling more gas than from a cascade without priority panel.

13. Dispenser: The purpose of a dispenser is to dispense the CNG to vehicles and recording the volume of gas dispensed. The CNG dispenser is a stand-alone unit that consists of an enclosure where the measurement and regulation devices are located. The refueling procedure can be done by receiving the natural gas directly from the compression unit or through the cascade. The four hose dispenser could be used for simultaneously filling bus/truck on one side and car on the other side. Depending on which type of nozzle switch is pressed the corresponding valve will be switched to the right hose. The dispenser is available in the single hose, double hose, four hose, and automatic dispenser. The dispensing pressure can’t be higher than 220 bar at the refueling stations in which the compression is realized at a pressure higher than 220 bar, the pipeline to the dispenser is equipped with a device that limits the pressure to a valve of 220 bars. It also has to be insured with proper equipment, that the maximum established pressure does not exceed the set pressure. For this purpose, after the compression unit at the beginning of the pipelines that deliver the natural gas to the dispensing units, besides the principal equipment of reduction of the pressure, a safety device is installed. This acts before the pressure overcome the maximum established value of set pressure. At the refueling stations in which the compression is realized at pressures higher than 220 bar, the pipeline to the dispenser is equipped with devices for the automatic cut off of the compressors as the pressure becomes higher than 220 bar. The emergency shutdown system is activated by an emergency button, manually operated, and located near the local panel of the compressor or near the refueling area and at the station main office. This system performs: i) The total isolation of the pipeline to the dispensing units through the valve. ii) The total isolation of the low-pressure pipeline from the inlet of the compressor. iii) The total deactivation of all the electrical circuits, with the exception of the ones necessary for the safety system.

14. CNG Retail Outlets (CNG RO): The retail outlet for CNG serves the CNG vehicles for filling their cylinder. The dispensers are the mass flow meters with filling nozzles compatible with the CNG station. The retail outlet means to meet the safety distance requirement around the station as defined in T4S Regulations encompassing Gas cylinder rules and OSID standard 179. The number of dispensers in a RO will depend upon the requirements of the vehicles to be filled at that outlet. The RO operator needs to ensure that the cylinder in which CNG is being filled must have been tested within the last five years failing which he can refuse to fill cylinder since as per cylinder rules the responsibility of filling gas in a tested cylinder lies with him. It is desirable that the retail outlet has a provision for public conveniences such as toilets, drinking water, and PCO. It should also have an air filling facility and even utility store & PUC certification facility.

15. Service Regulator: To regulate the gas pressure from 4/7 bar to 121 mbar or so depending upon the design of the PE network. This regulator is mostly located in a building complex so that the downstream gas can be supplied to domestic purposes to the building.

16. Transition fitting: This fitting is for connecting underground PE pipe to GI pipe serving the domestic network. These fitting are also called risers since they constitute a transition underground PE pipeline to the aboveground GI pipeline. The original fitting has been with the crimpling process and now the modified version has been produced by many vendors which are having a threadolet or hexagonal nut embedded inside a PE elbow. The PE portion can be fusion welded with the underground PE pipe and the GI pipe can be threaded with threadolet or nut.

17.Meter regulator: This pressure regulator brings down the gas pressure from 121 mbar to 21mbar which is the pressure of natural gas being used for kitchen burners. The same pressure is also used for geyser for water heating.

18. Domestic gas meter: These are primarily positive displacement type meter employing two diaphragms type pockets for gas measurement. While one pocket is an underfilling operation, the other one is in emptying position serving the domestic gas consumption. The size of this meter depends on the expected consumption of the household. A typical size varies from 2.5 SCMH to 6.5 SCMH. The domestic gas meter can be of the manual type where meter reading is manually recorded by meter reader visiting from the entity. The meter could be AMR compliant which can be read by a vehicle moving in an area. The other version of the meter is a pre-paid smart card operated. This type of meter is preferred in the market because the house owner is free from the hassle of attending to meter reading, receiving, and making payment of bills. The entities are also happy because they get paid for gas in advance which can help them to mobilize the working capital.

19. CNG kit: The CNG kit comprises of CNG cylinder, pressure regulator cum safety valve, gas distributor, and tubing. The CNG kit is either fitted in the original version in the new vehicle or retrofitted in the old vehicle through the authorized workshop.

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