Transient Analysis for Cooling Water System Modification

Transient analysis Cooling Water System Modification

iFluids Engineering and Consultancy WLL carried out Transient study for ORYXGTL Total cooling water system modification for the Cooling water supply from existing cooling water pump. The transient analysis was carried out with an objective to analyze the worst-case scenario and ascertain the maximum pressure in the cooling water network does not exceed the maximum allowable pressure of the system. Click here to know more about, What is Transient Analysis?

The following cases are identified as valid and applicable scenarios to perform Transient Analysis for the systems.

  • Cooling Water transfer pump sudden Trip
  • Cooling Water transfer pump trip and immediate start up
  • Closure of inlet valves at the consumer points battery limits
  • Cooling Water transfer pump change over to standby pump

As part of this Transient analysis, the system’s maximum and minimum pressures were looked at, checked, and any changes needed to meet the pressure limits were made. The pipenet model also pulls out unbalanced forces caused by the rate of change in pressure in both directions during different transients. These forces are then sent to a stress analysis to see if the pipeline support is strong enough.

The underground part of the current cooling water system at the ORYXGTL Plant leaks, which has led to five Total Facility Shutdowns since 2011. It will take at least ten days to fix the problem and get the drinking water system back to normal.

Based on what happened in the past, there have been nine cooling water breakdowns since 2011, and five of them caused the facility to shut down completely. The leaky connection has been fixed, and key joints that can be reached have been strengthened to improve the reliability of the cooling water system that is already in place.

Taking into account how well the current cooling water system works and the fact that it has already failed several times, it is clear that the current cooling water system needs to be improved or changed. So, under the MOC, it is suggested that the cooling water system be rebuilt so that it meets the standards. This will make the cooling water system more reliable and prevent any leaks that could cause a Total Facility Shutdown in the future.

The cooling water system is an open system that uses a Cooling Tower that pulls air through evaporation. The cooling tower is divided into three separate cells, which lets one cell be taken offline for cleaning or repair in the winter (Design Dry Bulb Temperature: 27°C, Design Wet Bulb Temperature: 21°C) while keeping the design thermal duty. The Cooling Tower Basin is where the water from the cooling tower pools.

From each basin section, the water flows through sluice gates into a main basin section, where it runs through suction screens and into a common pump suction bay. There are three Cooling Water Pumps, each of which is rated for 55% of the Cooling Tower Design Capacity. Two of the pumps are running at all times, and the third is on backup. The pumps get their air from the cooling water basin. The screens in the cooling water basin keep big particles from getting into the cooling water pumps. The suction screen is made to keep all particles larger than 11mm.

The water that is added to the cooling water system comes from the Effluent Treatment Plant. This water has been cleaned. This is put into the basin of cooling water when the amount is adjusted with a control valve. In case Process Water make-up is lost, there is also a back-up source of raw water. The Cooling Water Side Stream Filter package filters a side stream flow of cooling water (2.5% of the original circulating rate) to get rid of suspended solids.

The planned facility will replace the current cooling water system with a stronger one that won’t change how well the cooling water system works. The new installation will take place at the ORYX GTL plant in Ras Laffan Industrial City in the State of Qatar.

The main goal of the Transient analysis study is to figure out the effects and build the system so that it can handle the effects that happen during transient operations.

  • Find out what the highest and lowest pressures are in the suggested cooling water system during transient upset conditions for different scenarios.
  • To make sure that the chosen pipe material specification for the pipeline systems meets the design pressure.
  • Based on the results of the Transient analysis, suggest the minimum requirements to avoid pressure surge in the cooling water system.
  • To give the pipes a peak force or load for a Stress Analysis.
  • To offer ways to stop Transient when the surge pressure is higher than the maximum transient pressure.

In the study, the following assumptions are taken into account:

  • The actual geometry and configuration of cooling water system piping, such as pipe diameter, length, elevations, number of pipe bends and fittings, are modelled from piping isometrics and piping wall thickness from piping material specification shared by Company.
  • The model is used to figure out the K-factor for pipe joints.
  • The Coulson and Richardson pressure drop model is used in the exercise.
  • The way a valve closes is thought to be straight.
  • Each unit of Pipenet comes with the pressure loss tool. Consumer Unit pressure drop across each unit is calculated by adjusting pressure drop to get the desired flowrate and assuming back pressure to unit at cooling water inlet return line is 0.1 barg at the top of the cooling tower at 15.1 m elevation.
  • The model has the restriction opening that can be used in isometrics.
  • The transient heat transfer with the environment isn’t taken into account because it doesn’t have a big effect on the properties of the fluid.

Due to the quick movements of the isolation valve and pump start-up and trip, the piping system could have a flow that isn’t steady and has a lot of pressure and force Transient. When flow rates change quickly in pumps and pipeline systems, this is called a hydraulic transient. When speed drops quickly or stops completely, the kinetic energy of the moving liquid column is briefly taken up by the elastic deformation of the pipe and the fact that the liquid can be pushed down. Then, a pressure wave is made that moves back and forth inside the pipe

Flow control is an important part of how a liquid flow system works, like when valves are opened and closed or when pumps are started and stopped. When operations are done quickly, they can cause pressure transients in the system. If the pressure transients are not controlled within the limits, the system could be damaged or fail if the transients are not controlled.

If the pressure change is too high, hydraulic transients can cause pipes and other equipment to break. If the pressure is higher than what the pipe can handle, the joint, bend, elbow, or pipe could break. On the other hand, too much negative pressure (low pressure) can cause pipe joints to buckle, implode, or leak during sub-atmospheric stages. Due to uneven forces, the rate of change in pressure during different transients can also cause pipes to break. Because of this, uneven forces are being taken from different transients and will be used for stress analysis.

Based on the results of the Transient analysis study, the following suggestions are made: Air/Vacuum relief valve (Combination type) to be provided at identified loop as proposed in earlier section, to prevent cavity formation and keep the maximum pressure within the maximum allowable transient pressure of 9.31 barg during both pump trip cases.

The suggested minimum valve closure time to limit the maximum Transient pressure falls within the maximum allowable transient pressure of 9.31 barg during the closing of the Units battery limit valve.