Sun. Jun 20th, 2021

City gas distribution

Transporting future fuel

#70 PNG Infrastructure

8 min read
  1. Natural Gas is transported to City Gas Station where custody transfer of natural gas from transmission line to CGD entities takes place.
  2. Natural gas at high pressure of 40-100 bar is reduced at CGS to acceptable levels of 15-19 bars for distribution networks.
  3. In the domestic & commercial segment, gas is transferred from CGS to District Regulating Station (DRS) and in the industrial segment; gas is transferred from CGS to Common Pressure Reduction Skid (CPRS).
  1. District Pressure Regulating Station (DPRS) or District Regulating Station (DRS) is a station located within the authorized area for CGD network and has isolation, pressure regulation skids & overpressure protection devices.
  2. Steel pipelines from CGS transfer gas to DRS at a pressure of 15-19 bars. The pressure of gas is then reduced to 4 bar and gas is transferred to Service Regulators through polyethylene pipelines.
  3. The major component of a DRS are slam-shut valves, pressure regulating valves, creep relief valves, isolation valves, non-return valves & pressure gauges.
  4. In the DRS, gas is passed through a filter for removal of suspended impurities if any. The pressure of gas is then reduced by passing through pressure regulating valve.
  5. The line is installed with Slam-shut valves & Creep Relief valves to combat pressure fluctuation (if any).
  6. Two regulating devices viz. Monitor & Active Regulators are installed in series (with 50% redundancy).
  7. The pressure settings of these regulator are stepped so as to allow one (Active) normally to control the outlet pressure & the other (Monitor) to assume control in the event of failure of the active device. The active regulator normally assures pressure reduction to the setting value. The monitor is installed to assure continuous gas supply (in case of failure of the active regulator). The monitor regulator is normally wide open & in case of pressure fluctuation, the monitor regulator will take over as control valve.
  8. Usually two stream, one line is working while the other line is in stand-by are installed. A creep valve is installed in the line to release the excess pressure. In case both active & monitor fails, CRV releases excess pressure. A Slam-shut valve installed in the line protects the system from overpressure or under pressure, in case active regulator, monitor regulator & creep relief valve fail to control the pressure.
  1. Domestic PNG Segment: Medium Pressure Gas is transferred from CGS to DRS via. Steel Pipeline Network. The pressure of gas is reduced from 15-19 bars to 4 bar at the DRS.
  2. From DRS, gas is utilized for PNG distribution through polyethylene pipelines (underground network).
  3. The size range of main PE lines from DRS is between 125 mm to 180 mm, & size range of pipes providing inter connection between main pipes is 63 mm to 90 mm. Pipes with size range 20 mm to 32 mm are used near domestic installations.
  4. In the domestic PNG segment, natural gas is sent to a service regulator installed near the domestic households which further reduces the gas pressure for domestic use.
  1. Service Regulators: SR are installed before tertiary PE lines, generally located at customer premises for maintaining supply pressure & designed to maintain safe condition even in the event of rupture in the regulating downstream section.
  2. It reduces the pressure from 4 bar to 110-100 mbar to the service device. Service regulators control the gas pressure before it enters the customer’s residence or building.
  1. Domestic PNG Installation: Gas at 4 bar pressure is distributed as domestic PNG to residential complexes & individual households.
  2. Gas at 4 bar passes through a Service Regulator (SR), where gas pressure is reduced to 110 mbar. SR is enclosed in a box & installed at the entrance of the society. One SR caters to up to 100 or more houses. In case of sudden & large leakage, SR automatically cuts off the supply.
  3. An RCC guard is installed after the SR. PE pipe coming out of ground is protected from external impact by RCC guard.
  4. The riser is then connected to GI pipe riser through a transition fitting. The GI riser is attached to the premise wall with the help of a metal clamp.
  5. A meter control valve is fitted in each house at the entry point. The valve valve has to be shut-off immediately in case of suspicion of leakage. If the premises is closed for a log time or gas is not being used for a long time, it is advisable to shut-off this valve.
  6. A gas meter is installed outside the customer’s premises, & it measure the consumption of gas in cubic meters. The last three digits of meter displayed are in red & indicate 1000th of a cubic meter. About 16 cubic meter of natural gas is equivalent to one LPG cylinder of 14.2 kg.
  7. A Meter regulator is installed downstream of the gas meter to reduce the pressure of gas from 110 mbar to 21 mbar. It is capable of shutting down the supply in case of any leakage.
  8. Gas at 21 mbar pressure is then carried towards home appliance by a GI line. A Copper/GI pipe is used to connect the GI line to the gas tap. Gas tap is installed just before the suraksha hose. This tap stop gas supply to stove/gas appliance. It is advisable to shut this valve when any gas operated appliance is not in use. The suraksha hose which is a wire braided flexible hose (with rubber covering) carries gas to the burner, stove or home appliance. The length of the rubber hose shall not exceed 1.5 mtrs. Both the end of the rubber hose are clamped by metallic clamps on the nozzle.
  1. Suraksha hose: Suraksha hose is used to connect the household appliance to the copper/GI pipe carrying gas at a pressure of 21 mbar.
  2. Suraksha hose is manufactured as per IS-9573:1998 Type 4.
  3. It is an 8 mm NB flexible and steel wire braided hose and the length of rubber hose is generally 1.2-1.5 m.
  4. It has three layers i.e. inner & outer layers are made of special quality rubber and the middle layer is wire reinforced in braided from in between the lining and the cover.
  5. Wire braided construction eliminates various deficiencies of rubber tube and provides a superior alternative to the flexible rubber tube & is guaranteed for 5 years.
  6. The outer layer is fire resistant, ozone, weather & abrasion resistant. The lining material for inner & outer layer is made from synthetic rubbers like Nitrile Butadiene Rubber or Chloroprene Rubber. The middle layer is made of braided copper or brass coated high carbon steel wire mesh. The outer rubber covering is consolidated by wrapping & is uniformly vulcanized to give good adhesion.
  7. The minimum tensile strength for lining & cover is 10 MPa & elongation at break is minimum 200% for lining & 250% for cover. Suraksha hose has a minimum burst pressure of 0.5 MPa.

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