Design Study of Venting System for Phase 3A STV’s for Dahej Terminal of Petronet LNG Limited.

Petronet LNG Limited (PLL) – Dahej Terminal at Regasified LNG (RLNG) was established in three phases. Phase 1 with north jetty got commissioned in 2004. Phase-2 got commissioned in 2009. Phase-3A commissioned in August 2016. Phase-3B1 was commissioned in June-19.

Phase-3A Stv’s four shell venting is required to vent out accumulated gases in its top part of shell and venting is being done manually. To make this process easy and semi-automatic venting system is to be installed. PLL desires engineering study for venting system installation. Petronet LNG awarded contract to iFluids Engineering to design the semi-automatic venting system.

In the PLL, Dahej LNG Regasification unit, LNG is converted to gas by the STVs. LNG is in the tube side and Water-Glycol mixture is circulated in the shell side to heat the LNG.

The Glycol water mixture system is a closed system with an ambient air heater and buffer vessel and the system is maintained at the required pressure.

In Phase-3A STVs (Four nos) shell side venting is required to be done to eliminate the accumulated gases in its top portion of the shell. To eliminate manual operation, semi-automatic venting system is proposed with routing to flare system after removing moisture to the required specification.

The new venting system consists of the following:

  • Remote operated on-off valves in the gas vent line.
  • Trap assembly to separate the Glycol water from natural gas in the vent line.
  • Knock out drum to remove any Glycol water in the natural gas. Knock out drum shall be common for 2 STVs.
  • Remote operated on-off valves in KOD drain to glycol water drain drum.
  • Moisture adsorber to remove any moisture content in the vented gas to Flare.
  • Adsorber will be common for both the knock out drums.
  • Flowmeter along with totalizer to measure the vented gas quantity.
  • Dewpoint analyzer for measuring the water dew point of the vented gas.
  • U loop shall be provided in the adsorber inlet to drain the carry over glycol water.

The following detailed engineering activities were carried out  discipline wise based on the requirement


  • Process Design Basis and Study Report
  • Process Flow Diagram for Phase – 3A STV’s Gas Venting System
  • Piping and Instrumentation Diagram for Phase – 3A STV’s Gas Venting System
  • Process Datasheets for Vent Trap Assembly and Moisture Adsorber
  • Hydraulic Study Calculation and Report
  • Knock Out Drum Sizing Calculation
  • Cause and Effect Matrix Diagram
  • Line List
  • Equipment List


  • Mechanical Datasheets for Vent Gas Knock Out Drum


  • Piping Plan for Venting System
  • Piping Layout
  • Piping Material Specification
  • Valve Material Specification
  • Pipe Support Design and Drawings
  • Isometric Drawings
  • Valve List
  • Y-Strainer Specification
  • Piping BOQ with Cost Estimation
  • As-Built Document

Civil & Structural:

  • Foundation Design Calculation
  • Analysis of Existing STV Structures
  • Civil BOQ with Cost Estimation


  • Instrument Index
  • Specifications for Flow meter and Moisture Analyzer
  • Instrument Datasheets
  • Cable Schedule
  • Hook-Up Diagram
  • Instrument BOQ Cost Estimation

Technical Safety: