4. CGD VALUE CHAIN

4.1 INTRODUCTION

CGD represents the last mile in the gas value chain and gas is supplied at low / medium pressure

to Residential, commercial consumers and small industrial consumers (PNG) and Transportation

segment as vehicle fuels (CNG).

CGD network involves movement of small volumes of gas through small diameter medium to

low pressure distribution pipelines by a local distribution company to a large number of retail

consumers. It is an integrated network of steel, poly ethylene (PE) and medium density

polyethylene (MDPE) pipelines along with CNG dispensing stations. (Chandra, 2013)

Fig 4.1 CGD Value Chain

Source: American Gas Association

4.2 VALUE CHAIN

4.2.1 Gathering Lines

Gathering lines collect crude oil and raw gas from producing areas.

4.2.2 Gathering Station

The gas produced from the wells has enough pressure to enter gathering lines directly without

compression. Sometimes compressors are added to boost the pressure to overcome other

Gathering lines pressure. These stations are called gathering station. (Dubey, 2011)

4.2.3 City Gas Station

The natural gas that is received at the city gas station is mostly passed through a cleaner to

remove liquids and dust. The primary function of the city gas station is to measure the amount of

incoming gas. It is generally measured through orifice meters. Another function is to reduce the

pressure of the gas to be sent for distribution, as the distribution system requires much lesser

pressure than that in long distance transmission. Mechanical devices called pressure regulators

lower the gas pressure and helps to control the flow rate to maintain desired pressure level

throughout the distribution system with the reduction in pressure the natural gas also becomes

cooler so sometimes it has to be heated up in regions where the temperature is below zero

degree. Last but not the least, at the city gas station the odorization of the natural gas takes place.

Different types of odorants are used so that the “smell” makes the presence of escaping unburnt

gas recognizable at very low concentration. This serves as a warning well before the gas

accumulates to hazardous levels; a mixture of air and natural gas are explosive over the range of

5% to 15% natural gas. To ensure safety, odorized natural gas is detectable at concentration of

just 1%. (Anonymous, 2011)

The main components of City Gate station are

a. Gas filtration skid

b. Pressure reduction skid

c. Flow metering skid

4.2.3.1 Gas filtration skid

This skid has been designed in such a way as to accept a single stream only. High efficiency

filter separators are used for the removal of liquid and solid particles from the incoming gas

stream over the entire operating range. The gas outlet from the processing industry is cautiously

maintained at free of impurities; the filtration skid ensures the pure gas distributed to the line .the

filter is normally designed to withstand a pressure in the range of 30-49 bar.

4.2.3.2 Pressure Reduction Skid

The pressure reduction skid is installed to reduce the pressure of the incoming gas form the

source from the pressure of 37 bar to 27 bar. Mainly creep relief valve is being installed in this

skid which maintains the downstream pressure.

4.2.3.3 Flow Metering Skid

Flow meters are installed to for a single stream. The normal flow meters used in the CGS is

orifice plates. Orifice flow meters find its use as a large pressure drop is required. The various

parameters such as temperature in the various sections of the line pressure at the inlet & outlet

joints, flow inlet & outlet are controlled by SCADA system in the control room.

4.2.4 District Regulation System

DRS is a device used to reduce the pressure from 27 bar to 4 bar. It is the interface between the

steel grid network & the medium pressure network. The locations the DRS mainly depends on

the requirements of the area i.e., the type of customers to the company.

The various components in the DRS include the following,

a. Slam shut valves for controlling the flow

b. Filtering skid

c. Pressure reduction skid

d. Odorizing Unit

Odorizing unit is highly essential to ensure the safe distribution of the both CNG and PNG. The

odorants mainly include the mercaptans. These odorant is injected in to the natural gas to detect

leaks if any. The odorants used is Ethyl Mercaptan.

e. Metering System

Pipeline customers demand to know they receive the same amount of oil or gas they put in.

pipeline companies generally use direct volume meters or inference meters. Some types are as

follows

1. Positive displacement meters

2. Turbine meters

3. Orifice meters

4. Ultrasonic meters

5. Coriolis meters (Dubey, 2011)

4.2.5 Piping system

Pipeline network consists of steel pipeline & polyethylene pipeline. A typical CGD network

should consists of the following:

(a) Primary Network

A medium pressure distribution system comprising of pipelines, gas mains or distribution mains

normally constructed using steel pipes and connects one and more transmission. Pipeline to

respective CGS or one or more CGS to one or more DRS.

(b) Secondary Network

A low pressure distribution system comprising of gas mains or distributions mains usually

constructed using thermoplastic piping (MDPE) and connects DRS to various service regulators

at commercial, industrial and domestic consumers.

Fig 4.2 Main Gas Transmission Pipeline

Source: Chandra, 2013

(c) Tertiary Network

A service pressure distribution system comprising of service lines, service regulators and

customer/consumer meter set an assembly constructed using a combination of thermoplastic

(MDPE) piping and GI/copper tubing components. The connection between consumer meter set

assembly and gas appliance (provided by consumer) may be made by GI pipes or copper tubing

or steel reinforced rubber hose. (Anonymous, 2011)

Fig 4.3 CGD Pressure Regimes

4.2.6 Piped Natural Gas (PNG)

District regulation station are installed where the distribution is to be done like in the industrial

area and commercial segment. Gas to the various consumers is transferred after being maintained

at a pressure of about 4-5 bar. Then the gas is transmitted to single stream regulator through 4

bar medium pressure PE pipelines. SR further reduces the pressure from 4 bar to 100 mbar. From

SR the gas is supplied through a 10 mbar low pressure PE pipeline to a G.I. riser isolation wall.

From this valve the gas is carried through a G.I. (Galvanized Iron) 10 mbar pipelines to end user.

The control valve is placed at the height of 5 ft which controls the flow. Then a regulator are

installed which rings down the pressure to 21 mbar for home users. A meter is installed which

tells the amount of gas being used depending on which they are charged. (Prasad, 2011)

Fig 4.4 Gas Transmission and Distribution system

Source: Mahanagar Gas

4.2.7 CNG Infrastructure

4.2.7.1 Mother Station

Mother stations are connected to the pipeline & have high compression capacity. These stations

supply CNG to both vehicles & daughter stations (through mobile cascades). The Mother station

requires heavy investment towards compressor, dispensers, cascades, pipelines, tubing etc.

4.2.7.2 Online Station

CNG vehicles storage cylinders need to be filled at a pressure of 200 bars. “Online Stations” are

equipped with a compressor of relatively small capacity, which compresses low pressure

pipelines to the pressure of 250 bar for dispensing CNG to the vehicle cylinder. The investment

in an online station is midway between daughter station & mother station.

Fig 4.5 CNG Stations

4.2.7.3 Daughter Station

The “Daughter Stations” dispense CNG using mobile cascades. These mobile cascades at

daughter stations are replaces when pressure falls & a depleted mobile cascade is refilled at the

“Mother Station”. The investment in a daughter station is least among all types of CNG stations.

There is reduction in storage pressure at daughter stations with each successive filling. Once the

storage pressure drops, the refueling time increases, while the quantity of CNG dispensed to

vehicle also decreases.

4.2.7.4 Daughter-Booster Station

Installing a booster compressor can eliminate drawbacks of daughter stations. Daughter booster

(compressor) is designed to take variable suction pressure & discharge at constant pressure of

200 bars to the vehicles being filled with CNG. The investment in daughter booster station is

higher than that of daughter station. (Das, 2011)

References

American Gas Association. (2014) How Does the Natural Gas Delivery System Work? Available

from: http://www.aga.org/Kc/aboutnaturalgas/consumerinfo/Pages/NGDeliverySystem.aspx

[Accessed: 12th Nov 2014]

Anonymous. (2011) CGD – Introduction report.

Chandra P. (2013) City Gas Distribution: Coming Out of the Woods. Dolat Capital. Jan 2013

Das A.T. (2011) Report on CGD Business in India. School of Petroleum Management,

Gandhinagar

Dubey S. (2011) City Gas Distribution. School of Petroleum Management, Gandhinagar

Gas transmission and distribution system. (March 2010) Mahanagar Gas Limited, Mumbai.

Available from: http://www.mahanagargas.com [Accessed: 15th Nov 2014]

Operation and Maintenance Manual, 3rd edition, Gandhinagar: GSPC GAS

Prasad S. (2011) Overview of CGD Business in India. School of Petroleum Management,

Gandhinagar