HAZOP studyfor CNG mother Station at Duliajan and Hazarigaon of AGCL

Assam Gas Company Ltd., a Govt. of Assam Undertaking was incorporated on March 31, 1962 in Shillong as a limited company wholly owned by the Government of Assam to carry out all kinds of business related to natural gas in India. The Company can transport over 5.5 MMSCMD of natural gas.

Assam Gas Company Limited has awarded iFluids Engineering to carry out Hazard and Operability (HAZOP) study for CNG mother station at Duliajan & Hazarigaon, Assam. This document identifies the methodology and results proposed by iFluids Engineering as well as accepted by Assam Gas Company Limited for the completion of HAZOP study.

Assam Gas Company Limited has a CNG mother stationat Hazarigon & Duliajan. HAZOP study has been conducted using the documents available for the CNG mother station.

Hazarigaon station has the following facilities,

  • METERING SKID (1 no, Capacity- 1200SCMH)
  • RECIPROCATING GAS COMPRESSOR (1 no, Compressor Capacity – 1200 SCMH)

Duliajan station has the following facilities

  • METERING SKID (1 no, Capacity- 2400SCMH)
  • RECIPROCATING GAS COMPRESSOR (2 nos, Compressor Capacity – 1200 SCMH)

The CNG station is classified into the following categories:

  • Online stations
  • Mother Stations
  • Daughter Booster Stations
  • Daughter Stations.

The CNG Stationreceives CNG through a pipeline of 19 – 22 kg/cm2g or mobile cascade from nearby mother stations above 250 Kg/cm2g. After getting compressed via compressor, the gas would be sent to Dispensing Units (DU) and Cascade in Online and Mother stations. And the compressed gas at 250 kg/cm2g would be sent to Dispensing Units (DU) to Daughter Booster stations. Also, the gas would be sent to Dispensing Units (DU) without compression in Daughter stations which is mentioned in the below figures.



Daughter Booster Stations are equipped with Booster Compressors and station cascade including Priority Panel. This arrangement helps to unload more than 75% of the incoming LCV Cascade volume for dispensing.

CNG is dispensed to vehicles based on the principle of pressure equilibrium. Once the pressure of a mobile cascade drops below 200 bar at daughter station, vehicles get less amount of gas, which is below 200 bar. With each fill thereafter, the amount of gas dispensed to vehicles starts decreasing and the filling time starts increasing, thereby leaving the customer dissatisfied.

Daughter booster stations addresses this issue. These are similar to daughter stations. However, to cater customers concerning the amount of gas dispensed as well as filling time, a booster compressor (hydraulic type with variable suction pressure) is installed in between the mobile storage and the CNG dispenser. The booster compressor increases the pressure above 200 bar as the pressure of mobile cascade falls below 200 bar. Thus, the maximum amount of gas stored in mobile cascade is dispensed to daughter booster station.

The scope of work is to carry out HAZOP study for CNG mother station at Hazarigon & Duliajan and this study covers different components and equipments associated with the facility description.

The main objective of HAZOP study is early identification of hazards that are associated with process and operation and to reduce the probability and consequences of an incident that would have a detrimental impact on Humans, Facilities, Properties, and Environment.

The Objectives of HAZOP are as follows:

  • To identify hazards associated with Process and Operations.
  • To identify credible causes such as manual/equipment/instrument failures that are likely to cause hazards.
  • To qualitatively assess the risk (L/M/H/S) and provide recommendations to prevent/overcome hazards if required which shall be assessed using qualitative judgement of likelihood and consequence ranking of an incident.
  • In addition to these issues, HAZOP occasionally identifies items that could improve unit operation and efficiency of the process.

As a first step, each facility (Hazarigon & Duliajan) has been separated into 4 nodes based on the Operation. And HAZOP study has been done for each node separately. The credible causes and consequences were identified for each parameter deviation. Then the available safeguards have been identified to mitigate the cause and consequences for each deviation.

The recommendations provided in the event of existing safeguards were found to be not sufficient. In both the facilities, insufficient safeguards in Compressor system for Pressure deviation & temperature deviation were found. Hence, it is recommended to implement a Position indicator for SSV & Running indication for Cooling fan respectively. As a general recommendation, it has been advised to maintain Instrument calibration schedule, records and to implement SOP for Hydro testing of cascade cylinders in Cascade storage.