Surge analysis study was carried out by IFluids Engineering, Qatar, for the Feed Condensate Line at Laffan Refinery with an objective to analyze various scenarios and ascertain the maximum pressure in the Feed Condensate Line which does not exceed the Maximum Allowable Surge Pressure of the system.
The following scenarios are identified as valid and applicable cases to perform Surge analysis.
- Pump Startup / Trips / Change over
- Emergency Shutdowns
Surge Analysis, involves investigating the abrupt pressure fluctuations that arise within a pipeline system because of sudden alterations in fluid flow velocity. To know more about What is Surge analysis? Click Here. As part of this Surge analysis maximum pressure, minimum pressure in the system were studied, checked and found the max. pressure was observed within the allowable pressure limits
Qatar gas is operating two Refineries namely Laffan Refinery 1 and Laffan refinery 2 each having a design capacity of 146,000 BPSD of Untreated Field Condensate from QG(N) and QG(S) Inlet receiving(IR) facilities. Each Refinery is equipped with various Process units – Condensate Fractionation Unit, Naphtha Hydrotreating Unit, Saturated Gas Plant, LPG Trim Treating and Splitting Unit, Kerosene Hydrotreating Unit, Hydrogen Production, Amine Regeneration Unit, Sour Water Stripping Unit, Utilities, and Effluent Treatment plant. The units common for LR1 and LR2 are Diesel Hydrotreater and Zero Liquid Discharge
Based on Qatar gas Internal study, LR1 is operated at 110% of design capacity at 160,600 BPSD. LR2 is operated at design capacity of 146,000 BPSD. The feasibility of operating LR2 at 110% of design capacity needs to be studied.
The current Feed Condensate is lighter than the original design specification. The operating capacity of NHT reaches 100% while Condensate Fractionation unit is below 100%. Qatar gas intention is to identify modifications to operate LR2 U12 at 110% of design feed rate.
The main objective of the Surge analysis study is to identify the impact and design the systems to handle the impact occurring during transient operations which is listed below,
- To ascertain the maximum and minimum pressure prevailing in the Feed Condensate Line during transient upset conditions for various scenarios.
- To recommend the minimum requirements to avoid surge pressure of the Feed Condensate Line if required based on the outcomes from surge analysis.
- To provide surge mitigation measures if required when the surge pressure exceeds the maximum allowable surge/transient pressure.
In the course of the study, the following presumptions were taken into account:
- The actual geometry and configuration of feed condensate line piping i.e. pipe diameter, length, elevations, number of pipe bends and fittings are modelled from piping isometrics and piping wall thickness from piping material specification as shared by the Company. Tie in point location considered as per available input documents.
- K-factor for pipe fittings is considered as calculated by the model.
- The pressure drop model used in the simulation is Coulson and Richardson.
- The fluid phase considered in the piping system is single phase.
- Boundary Condition is considered from Transfer Pumps Suction to Condensate Tower Inlet.
- Control valve Cv’s considered as per valve datasheet
- Valve closures considered as linear and closure time as per datasheet & P&ID notes.
Simulation cases such as Steady State and Transient simulation cases are performed.
- Steady state simulation has been performed for the Feed Condensate Line for the boundary conditions.
- Transient analysis is performed for Feed Condensate Line. The following transient scenarios has been considered as a part of study in transient simulation. These scenarios will determine the maximum pressure developed in the piping due to sudden valve closure and pump trip / startup / Change over.
Following are the steps taken to perform Surge analysis:
To check the adequacy of the Feed Condensate Line when surge occurs, the following scenario has been considered as a part of study in transient simulation. These cases will determine the maximum/minimum pressure developed in the Feed Condensate Line due to sudden valve closure, sudden pump trip, pump startup and pump change over.
For the cases of Transient simulation, an initial steady state run of 20 seconds is considered before initiation of transient operations, for example valve closure (i.e.) valve closure shall start from 20sec and close completely according to their closing times.
Based on the steady state and transient cases analyzed for Feed Condensate line, the conclusions and recommendations are summarized below:
- For Sudden trip of Pump, the observed peak pressure found to be below the piping Maximum Allowable Transient/Surge pressure. Hence no mitigation is required.
- For Sudden trip and startup of Pump, the observed peak pressure found to be below the piping Maximum Allowable Transient/Surge pressure. Hence no mitigation is required.
- For Pump Change over from Pump, the observed peak pressure found to be below the piping Maximum Allowable Transient/Surge pressure. Hence no mitigation is required.
- For Closure of ESDV, the observed peak pressure found to be below the piping Maximum Allowable Transient/Surge pressure. Hence no mitigation is required.
For Closure of ESDV, the observed peak pressure found to be below the piping Maximum Allowable Transient/Surge pressure. Hence no mitigation is required.