ADNOC LNG (Train 3) processes High Pressure gases from ADNOC Off shore and the gas goes through a series of processes to remove Hydrogen Sulphide & other impurities and gets dehydrated before the gases are processed to separate the various components as products. The Acid Gas Removal Unit is a hybrid arrangement of the basic Benfield Carbonate unit and Amine technologies. The Carbonate Unit, using Potassium Carbonate as the solvent, does the bulk of the Acid Gas removal from the feed gases.
Subsequent to restreaming of Train-3 after 2018 T/A, in the Acid Gas removal unit, Lean Carbonate Flash Drum level control valve 2C-LV-61 got stuck and became inoperable from DCS. Since then the level of Lean F/D 2C301 was controlled by operation of manual bypass valve in field along with frequent adjustment of Makeup water flow to Carbonate Reflux Drum 2C313 by DCS controller 2CFC0082. The valve 2C-LV-0061 could not be released for maintenance due to passing Isolation valves. To enable complete draining of the system and replacement / repair of the defective valves a mini-SD of Train 3 (minimum of 7 days) was planned in June’19. Seven days’ shut down for the sake of this repair would result in loss of production resulting in a revenue loss of US dollors35 million.
A specialist review meeting was conducted on 12th Feb and on 9th March 2019 with Operations, Process Engineering & Control Maintenance to discuss the alternative option of automating makeup water flow control (2CFC0082), with the objective of reducing daily field operations required to control carbonate flash drum level during steady state conditions, whilst also improving level control and retaining plant stability.
The new control scheme to cascade the level of Lean F/D to existing flow control 2CFC0082 was implemented vide minor DCS change MDCSC/008/19 on 21st March 2019. The Performance has been satisfactory during normal steady state operating conditions.
Based on the observations, it was recommended to, continue with Auto operation of FC0082 to control level of Lean Flash Drum during normal operation with field operator support to operate the LV61 bypass valve, in case of a programmed or unexpected upset in level of Carbonate flash drum.
Thus the Shut down was deferred successfully till the next planned turnaround opportunity resulting in an opportunity to save US dollors 35 million.
Statement of Issue: Soon after restreaming the train after Turnaround, an important control valve in the carbonate circuit of the LNG Train 3, namely 2-C-LV-0061, became inoperable while the Train was running. Attempts to isolate the valve failed due to passing isolation valves which made online repair impossible. A complete shut down of the Train to repair the defective valve would mean a revenue loss of US dollors 35 million due to loss of production (in addition to the impact of flaring).