Oct-2005

Optimising cooling water systems

An integrated approach to chemical treatment can improve reliability, reduce costs and extend the life of cooling systems. Cooling system problems can be overcome by combining cooling water programs with analytical tools.

Daniel A Meier
Nalco Energy Services

Article Summary

Refineries and petrochemical plants represent the most challenging water-treatment environments. High heat fluxes, difficult water conditions, process leaks and intense pressure on capital and consumable budgets require water-treatment programs to operate under maximum stress. This can result in scale, corrosion and fouling that jeopardise asset integrity and increase total cost of operation.

It has become clear that a new approach is required to address cooling tower problems systematically. Such an approach combines an analysis of the entire system, including mechanical, operational and chemical factors, with a new integrated treatment program comprised of stress-resistant chemistry, active-based control and real-time performance monitoring.

Mechanical analysis develops a profile of the cooling water heat exchangers, focusing on the skin temperature, water velocity and heat flux through each exchanger. Metallurgies and exchanger designs are also identified and used in the reliability assessment. A detailed drawing is made to describe water flow through the entire unit that can be used in problem identification, as well as in the determination of specific upgrade solutions.

The mechanical profile is used to identify system stresses that affect the treatment program. The analysis evaluates low flow and high temperatures that increase scale and corrosion potential. Analysis also provides cost quantification of high stress factors that can be used for mechanical upgrade justification.

One of the most common causes of reliability issues in cooling water relates to poor operation, specifically wide variations in key control parameters (cycles, pH, phosphate and FAH). The operational audit component statistically analyses key parameter data and calculates the current control capabilities of the system; how these compare with the current specification limits; and what is required from an operational standpoint to meet the control requirements of the treatment program and its reliability objectives.

Chemical analysis examines water and treatment chemistry, with special attention given to variations that can occur in the water. This data, along with operational and mechanical information, is used to model scale and corrosion conditions that exist throughout the cooling water system, and the exchangers in particular. With this technology, sensitivity analyses may be conducted to quantify the impact of mechanical, operational or chemical changes on system costs and reliability.

In addition to these analytical tools, an effective system for addressing cooling system problems includes an integrated suite of new stress-resistant chemistry, active-based control and real-time performance monitoring that provides unique capability.

Trial application
A field application of the new integrated cooling water program and cooling system analysis techniques began in August 2003 at a supermajor chemical plant in Beaumont, Texas (BMCP). This field application indicates that such a system can produce substantial long-term cost savings and greatly improve reliability in refining and petrochemical cooling systems. The subject of the trial, #5 Paraxylene Cooling System, had problems typical of many cooling systems:
— Highly corrosive make-up water
— Very long holding time
— Uncontrolled supplemental make-up with steam condensate
— Poor key parameter control.

Several heat exchangers serviced by this cooling tower had to be retubed after only five years in service because of heavy localised corrosion associated with an under-deposit mechanism. To overcome these problems, a new total cooling water program called 3D Trasar was deployed.
To date, the following trial results using 3D Trasar have been measured:
— 71% reduction in phosphate variation
— 86% reduction in active polymer variation
— 50% reduction in calcium phosphate fouling potential
— Instantaneous response to system upsets (chemical addition/control)
— 21% reduction in general corrosion rates from coupons
— 70% reduction in general corrosion rates from online corrosion probes
— 81% reduction in localised corrosion rates
— Calculated 625% increase in mild steel exchanger life
— Calculated $165 784/yr savings in retubing costs.

These improvements are significant and will increase cooling water reliability for all systems within the BMCP and PE complexes.

Cooling tower conditions before trial
The #5 Paraxylene Cooling System was analysed to obtain mechanical, operational and chemical data required to evaluate causes and provide solutions to this localised corrosion problem.

Continuous introduction of condensate into the cooling tower was a unique mechanical factor. This has the significant effect of increasing the holding time index (HTI), a reflection of how long minerals and chemicals remain in the system. Nalco’s measurements of HTI are shown in Figure 1.