Transient Analysis for Cooling Water System Modification

Industrial cooling water system with large pipes and cooling towers, representing transient analysis for system modification
Transient Analysis: Modifying an industrial cooling water system with interconnected pipes and cooling towers

Overview

iFluids Engineering and Consultancy WLL conducted a comprehensive Transient Analysis for the cooling water system modification at the ORYXGTL facility in Ras Laffan Industrial City, Qatar. The study aimed to evaluate the impact of transient conditions and ensure the cooling water network’s pressure remained within the system’s maximum allowable limits. This assessment is critical for enhancing system reliability and preventing facility shutdowns caused by pressure surges.

Valid Scenarios for Transient Analysis:

  • Cooling Water Transfer Pump Sudden Trip
  • Cooling Water Transfer Pump Trip and Immediate Restart
  • Closure of Inlet Valves at Consumer Points (Battery Limits)
  • Cooling Water Transfer Pump Changeover to Standby Pump

Objectives of the Transient Analysis

A diagram showing the objectives of a process, represented in a central red circle labeled "Objectives" with five surrounding red boxes connected to it. Each box contains a numbered objective: 1) Identify Maximum and Minimum Pressures, 2) Ensure Material Compliance, 3) Prevent Pressure Surges, 4) Stress Analysis Input, and 5) Develop Preventive Measures
Key objectives: pressure limits, material compliance, surge prevention, stress analysis, and prevention
  • Identify Maximum and Minimum Pressures: Analyze transient conditions to ascertain pressure limits.
  • Ensure Material Compliance: Confirm that the pipeline material specifications meet design pressure requirements.
  • Prevent Pressure Surges: Propose solutions to mitigate pressure surges beyond allowable limits.
  • Stress Analysis Input: Provide peak force/load data for stress analysis to ensure pipeline support adequacy.
  • Develop Preventive Measures: Recommend strategies to handle transient effects and prevent system damage.

System Background and History:

The current cooling water system at the ORYXGTL Plant has experienced repeated failures due to leaks in the underground portion of the system, resulting in five total facility shutdowns since 2011. This issue has been addressed by repairing leaks and reinforcing critical joints. However, with multiple system failures in recent years, the need for a system overhaul was identified. As per the Management of Change (MOC) process, it was recommended that the cooling water system be rebuilt to enhance its reliability and prevent further shutdowns caused by leaks.

Cooling Water System Configuration:

The cooling water system utilizes an open cooling tower, which is divided into three cells. This configuration allows one cell to be taken offline for maintenance during the winter while maintaining the cooling tower’s thermal duty. Each of the three cooling water pumps is rated for 55% of the cooling tower’s design capacity. Two pumps run continuously, while the third serves as a backup. The system also utilizes effluent-treated water as the main water source, supplemented by a backup raw water supply in case of process water loss.

Transient Analysis Process:

The transient analysis focused on assessing the system’s response to rapid changes in pressure, caused by events such as pump trips, startup, and valve closures. The Pipenet model was used to simulate the system’s response under different transient scenarios. The analysis incorporated the Coulson and Richardson pressure drop model, adjusted to match real-world conditions.

Assumptions:

  • The system’s geometry and specifications, including pipe diameter, length, elevations, and the number of fittings, were modeled based on piping isometrics and material specifications.
  • Pressure loss calculations were performed across each unit, adjusting for back pressure and maintaining a flowrate to meet the cooling tower’s operational requirements.
  • The model assumed straight valve closures and did not consider transient heat transfer with the environment, as it was determined to have minimal effect on the system’s fluid properties.
  • Rapid pump startups and isolating valve operations were considered as sources of pressure transients, where quick changes in flow can cause pressure surges.

Results and Recommendations:

  • The maximum and minimum pressures in the system were assessed during pump trips, valve closures, and pump changeovers. The analysis confirmed that these pressures remain within safe limits, with key recommendations made to control transient pressures.
  • A Combination Air/Vacuum Relief Valve was recommended for use in identified system loops to prevent cavity formation and ensure pressure does not exceed the system’s maximum allowable pressure of 9.31 barg during pump trip events.
  • The minimum valve closure time was suggested to limit transient pressures during valve closure operations, ensuring that pressure surges stay within allowable limits.

Key Insights:

  1. Pressure Surge Control: The study highlighted the importance of controlling pressure surges to prevent damage to pipes, joints, and equipment. When pressure changes too rapidly, the system can experience both positive and negative pressure spikes, which may lead to pipe failure or implosion at vulnerable points.
  2. Pipeline Stress Analysis: Uneven pressure forces were identified, requiring stress analysis to evaluate the strength and reliability of pipeline supports.
  3. System Optimization: Based on transient analysis results, the cooling water system can be optimized with additional air relief valves and adjustments to valve operation times to ensure the system remains within safe pressure limits during all operating conditions.

Conclusion:

The transient analysis for the ORYXGTL cooling water system modification provided critical data on system behavior during transient operations. By simulating scenarios such as pump trips, valve closures, and system startups, iFluids Engineering was able to recommend design changes that will improve system reliability, prevent future facility shutdowns, and ensure the cooling water system meets operational safety standards.