The city gas distribution (CGD) network comprises city gate stations (CGSs), where gas is received from the supplier after the filtration, regulation and metering processes. The natural gas thus received at CGSs is then further distributed to the domestic, commercial, industrial and compressed natural gas (CNG) sectors through a network of carbon steel and polyethylene (PE) pipelines.
The basic steel grid pipeline network is laid for transporting and distributing gas from the CGSs to various installations such as district regulating stations (DRS), metering and regulating stations (MRS) and CNG compressors. This network operates at a pressure of 18 bar (g). The steel grid network, along with isolation valve assemblies and above-ground installations such as DRS and MRS, are designed, constructed and operated in conformance with national/international standards such as ASME B31.8 and the Petroleum and Natural Gas Regulatory Board (PNGRB) guidelines.
In a CNG station, gas is compressed and dispensed into the NG-fueled vehicles. Here, the gas received from the 18 bar (g) steel pipeline is further pressurised in stages, using compressors, up to a pressure of around 250 bar(g), required for dispensing into vehicles. A typical CNG station comprises reciprocating compressor(s) with a capacity of 750/975/1200 SCMH, CNG storage cascade(s) and dispensing unit(s). Depending upon the location and the expected load, the CNG station is designed to the appropriate size, with the required number of compressors and dispensers.
DRSs are installed along the steel grid network at appropriate locations to cater to the loads of domestic, commercial and industrial customers in the area. The DRS reduces and regulates the pressure of gas from the steel grid network and supplies it to the medium pressure PE network at 4 bar (g).
The medium pressure network starts from the downstream isolation valve of the DRS and is constructed with poly-ethylene pipes varying from 32 mm to 180 mm in diameter. It operates at a pressure of 4 bar (g) and supplies gas to piped natural gas (PNG) customers. The medium pressure PE is connected to the service regulator (SR) to reduce the pressure from 4 bar(g) to 100 mbar(g). Using galvanised iron pipes and domestic meter regulators, the gas is supplied at 21 mbar for household uses.
The maintenance philosophy is adopted to ensure that the gas supply network is fit for purpose during its operational life. It comprises various actions that are undertaken right from the commissioning of the network and throughout its operational life cycle to ensure that appropriate maintenance is provided to all gas handling assets, in line with applicable codes and standards and industry best practices.
The objectives of the maintenance philosophy are to ensure that the gas supply assets:
- Operate in a safe and environmentally sound manner;
- Offer sufficient reliability for the operating conditions within which they are used and will continue to operate, at the appropriate set points;
- Are installed correctly and remain in sound mechanical condition;
- Are subjected to optimum and cost-effective maintenance activities.
Maintenance is undertaken either as part of a routine preventive planned process, or as a reaction to breakdown situation. All equipment on the gas distribution network is included in the routine preventive scheduled maintenance plan. The frequency of these routine preventive maintenance activities is determined as per original equipment manufacturer (OEM) guidelines, applicable codes and standards, and industry best practices. Accordingly, a maintenance schedule plan is generated from systems, applications and products in data processing (SAP) software. The details of all the assets are recorded in the SAP, and a unique number is assigned to the asset. Maintenance scheduling is done as per the defined frequency.
In the CNG sector, CGDs carry out preventive, breakdown and corrective maintenance of the compressor package and its allied equipment at CNG stations through technically competent and experienced manpower. The maintenance of equipment is carried out as per OEM guidelines and the running hours of the equipment. The testing of the equipment is executed as per Petroleum and Explosives Safety Organisation guidelines (presently once in three years for pressurised gas cylinders). Calibration of the instruments is executed by a National Accreditation Board for Testing and Calibration Laboratories-accredited laboratory, as per OEM guidelines and internal company policy.
In the PNG sector, the maintenance of the DRS/MRS is executed on a half-yearly/yearly basis, as per the maintenance schedule generated from the SAP. SRs serving gas to domestic as well as commercial consumers are installed in the medium pressure PE network. Preventive maintenance of the SR is executed as per company policy. All the SRs are checked for leakage with soap solution. If a leakage is encountered at any joint of an SR assembly, it is rectified. Maintenance and inspection of domestic installations is carried out as per company policy and regulatory guidelines.
The maintenance of a pipeline network is ensured by patrolling the entire pipeline, periodic leak surveys, and lock pressure tests. Cathodic protection (CP) is provided to the entire underground pipeline network to protect it from external corrosion, as per National Association of Corrosion Engineers standards. An impressed current cathodic protection system, comprising of conventional anode ground beds with transformer rectifier (TR) units, powered by an alternating current supply, is used. These automatic TR units manage the pipe to soil potential in the pipeline section to be protected. Monitoring of the CP system is carried out as per schedule. It is ensured that proper CP is provided without interruption to both the primary and the secondary steel pipeline networks. Steel valve assemblies are provided along the entire network for sectionalising and isolation purposes, located at above-ground locations, as well as underground chambers, on the roads and footpaths. The steel valve chambers are cleaned on a monthly basis. The periodic maintenance of these steel valve assemblies is executed as per company procedure.
Domestic maintenance includes riser inspection, riser leak tests, replacement of corroded risers, and inspection and leak tests of domestic installations inside kitchens. The preventive maintenance of domestic and commercial installations is carried out as per schedule and company procedure.
The condition monitoring system is adopted to ensure the safe and efficient operation of equipment and installations. The monitoring is done by analysing the parameters of the equipment, recorded on site, with respect to benchmarks set by OEM and changes in the trend. Trend monitoring is a continuous process, which includes interpretation of data collected during the operations and analysis of the variations in the condition of the equipment or its components, for safe and economical operations.
Hazard identification and risk assessment (HIRA) is completed on site, with controls put in place for carrying out the activity. The HIRA record is maintained on site during the activity.
Emergency handling and management
A team of fully equipped emergency vans are available 24×7, throughout the year. These emergency teams are deployed in strategic locations in each region of CGD operations, headed by an engineer in charge. CGDs have an emergency response plan and a disaster management plan in place, as per PNGRB guidelines.
The emergency control room is operational 24×7 all year round and is supervised by a control room shift in-charge. The emergency control room receives all the emergency complaints of PNG customers, as well as emergency complaints regarding the equipment at CNG stations.
The emergency control room logs each complaint in the SAP and informs the relevant engineer-in-charge (EIC). The EIC reviews the complaint and deploys the emergency technician of the respective area. In the event of a more serious incident, such as a large number of people affected by an emergency, the information is also to be passed to the zonal-in-charge/head of department by the emergency control room as well as the respective EIC. The technician goes to the site as soon as possible, within a target response time of 60 minutes from the receipt of the call.
The emergency crew attending to the site is equipped with the necessary safety and emergency equipment, which are always available in the emergency vehicles and replenished as and when necessary. Upon reaching the site, the crew contacts the person reporting the gas escape to confirm the details of the emergency.
The priorities when dealing with any gas escape are always:
- Safeguard life
- Safeguard property
- Evacuate if needed
- Find and secure the escape
- Carry out a final site investigation
In the event of a major incident, there are three distinct phases that must be managed to limit the impact on the business. The different plans for each of the three phases are:
- Emergency response plan
- Incident management plan
- Business continuity plan
These procedures deal with such subjects as the necessary actions to control an incident, and the response to breakdown of maintenance requirements.
Adoption of technology in the CGD business
A supervisory control and data acquisition (SCADA) system has been installed for the safe and efficient operation of the CGD network and pipeline facilities geographically spread around the city of Mumbai and its suburbs, Navi Mumbai, Taloja, and Ambernath. The SCADA master control centre (MCC) has been set up at centralised location and is manned round the clock by SCADA operation engineers. The SCADA system at the MCC provides cohesive monitoring and control of the network and pipeline facilities such as CGS, an odourisation system, a sectionalising steel valve station and CNG compressor stations. A sub-master control centre (SMCC) has been set up for monitoring and control of the remote stations in case of an emergency at the MCC.
Automatic meter reading (AMR) is the technology of automatically collecting consumption, diagnostic, and status data from a base mechanical meter, and transferring that data to a central database for billing, troubleshooting, and analysis. AMR modules are installed at commercial customer locations, and readings are obtained and monitored daily at a centralised server.
A geographic information system is implemented to map a gas network and their installations, and view them readily on web-based or mobile applications.
An In-vehicle monitoring system (IVMS) is implemented to monitor the fleet of emergency vans and CNG transport vehicles. The main objective of an IVMS is to monitor and track the vehicles and driver behaviour, reduce road safety violations, and improve efficiency by tracking real-time information about the vehicles’ position and speed, as well as stoppages, detours, etc.
Infrared leak detection camera
Gas detection infrared cameras for optical gas imaging safely detect and pinpoint gas leaks, such as that of methane gas. With the help of such a camera, it is easy to identify and rectify almost all minor gas emissions from CGSs, CNG stations, and other regulating stations.
By Srinivasan Murali
Srinivasan Murali is senior vice-president, operations and maintenance, at Mahanagar Gas Limited (MGL). He has 35 years of experience in design and engineering, project execution, and operations and maintenance. At Mahanagar Gas Limited, he is responsible for providing leadership management and direction for all activities related to the O&M of MGL’s assets, with maximum uptime. He is also responsible for providing online connections to domestic customers, CNG projects, upgradation, and metering. Prior to working with MGL, he has worked with Cabot India Limited, BILT Chemicals Limited, Indian Aluminium Company Limited, and Cement Corporation of India Limited.