The process industry uses heating and cooling for thermal management of their core manufacturing operations. These operations involve material processing, mixing, chemical reactions, phase transitions, separation of mixtures and more: All require precise management of temperatures and pressures.
Regardless of the core manufacturing business, companies in the process industries must deal with certain essential services. These include generation of a heat and pressure source (usually steam), heat exchange, and rejection of waste heat (usually through cooling towers). Each of these is inextricably integrated with the operations.
Thermal management at process plants largely utilizes water as the primary heat transfer working fluid. Yet, it is common in many industrial sectors to consider management of the thermal circulatory system, including the circulating water system (CWS), as secondary services. As such, the operation and maintenance (O&M) of such secondary services may be delegated to third-party service providers, to less experienced operators, or as peripheral tasks to the core operation and maintenance team members. In some way, these practices lead to neglecting — and even losing sight of — one of the most critical elements of the plant. The malfunction of the circulating water system can cause problems with the core process efficiency and reliability. Conversely, an effectively operating circulating water system is a good indicator of the overall health and efficiency of a plant. For that reason, the operation and maintenance of the circulating water system can contribute significantly to energy efficiency and sustainability. In short, the circulating water system is an integral performance and health indicator for a process plant.
Using Digital Insights to Manage the Cooling Systems
Management of the circulating water system has the potential to be “low hanging fruit” to implement, benchmark and assess the benefits of digital transformation in the process industry. Such systems are ubiquitous and are operated in a similar manner irrespective of the core industry sectors served.
Digitalization or retrofit digital transformation entails connecting a data analysis layer to the process monitoring and control system of a plant. The core functions of such an upgrade are:
- Acquisition of information from a process through inline sensors or other measurements.
- Storing, collating, processing and analyzing the information.
- Providing insight to the plant operations and maintenance team regarding the health and fitness of the process.
- Assisting the O&M team with optimal operation of the plant in the form of decision support.
In some cases, these analytics results can be used directly to develop autonomous process control strategies for the plant (decision automation). In addition, digitalization of the circulating water system can:
- Improve the overall process reliability.
- Detect approaches for saving energy for the core operations.
- Estimate the health and efficiency of many process components.
- In some cases, prevent and correct many ongoing issues of critical importance that were neglected in the plant.
In this article, the benefits of a digitalization upgrade of the circulating water system are discussed in the context of wet cooling towers and the associated circulating water systems. The article then will focus on specific issues that can be addressed through retrofit digitalization of these systems.
Wet Cooling Tower Operation
Wet cooling towers are common in moderately sized industrial operations requiring integrated removal of low quality heat from a plant. The operating principle of these towers is to bring atmospheric air, and the circulating cooling water carrying the reject heat, in close contact through an appropriately designed contactor. Heat removal from the water is achieved through evaporative cooling, whereby a small fraction of the warmer water entering the cooling tower evaporates into the contacting air, and the latent heat of evaporation causes the necessary cooling effect on the remaining water. The evaporative loss implies that the cooling water needs to be replenished (as makeup water).
The open nature of these wet cooling towers determines their unique water problems. The water circulating in the cooling water loop contains dissolved and particulate entities that do not evaporate. The air can carry suspended particles and solids that get entrained in the water (air washing). The water in the cooling tower is warm, well-aerated and contains nutrients facilitating bacterial and algal growth. The packing materials of the cooling tower (fill) provide ample surface for the growth of biofilms and deposition of solids. Furthermore, if it is not purified, the makeup water carries dissolved solids into the circulating water system that increase in concentration over time.
The simplest approach to prevent the concentration buildup is to blow down some of the water from the cooled water basin (sometimes called the sump) of the cooling tower. The flow rate of the blowdown water and the corresponding makeup flow rate are determined by the concentration factor, which is ideally defined as the ratio of the dissolved solids concentration in the cooling tower water to the concentration of dissolved solids in the makeup water. Using this concentration factor, the blowdown rate and the makeup flow rate are determined. Ideally, the cooling tower system can be easily managed by setting up a target concentration factor while adjusting the makeup water and blowdown flow rates to maintain this concentration factor. Wet cooling towers are preferably operated with a concentration factor ranging between 2 to 7.
Water Quality Management Objectives
The core water treatment objectives of a wet cooling tower circulating water system are to address the following problems arising due to air washing and solids infiltration through makeup water:
- Scale formation.
- Biological fouling.
Scale Formation. Scaling is a consequence of solid precipitation when dissolved ions in the water such as calcium, iron, magnesium, sulfate, bicarbonate and silica exceed their solubility limits. Such precipitates can deposit on heat exchanger surfaces and interfere with the plant’s heat exchange efficiency. They also can block water flow in the cooling tower fill, causing dry spots or patches and reducing the air-water contact surface in the cooling tower.
Corrosion. Corrosion is the process of metal dissolution, usually by oxidation, resulting in substantial material breakdown and premature equipment degradation. This process degrades the metallic surfaces of the heat exchangers and piping and, in some extreme cases, causes pitting. Corrosion depends on the metallurgy and electrochemistry at the metal/water interfaces. It is sometimes exacerbated by bacteria.
Biological Fouling. Bacteria, fungi, algae and even protozoa can grow in aerated water when nutrients are available, forming biofilms. This growth process, and its attachment to system surfaces, is called biological fouling. This growth is considered by many to be the root of most cooling-loop water treatment problems. Those problems include reduced heat transfer efficiency, cooling tower fill fouling, water flow blockages, microbiologically induced corrosion and human health concerns.
Strategies for Cooling-Loop Water Treatment Problems
The goal of any water quality management operation is to reduce the makeup water intake rate and the disposal rate of cooling tower blowdown water without affecting the efficiency and reliability of the cooling system. Fundamentally, this is done by maintaining the system at the target concentration factor by adjusting the makeup and disposal rates. This is supplemented by additional treatment.
The most prevalent approach to supplemental water quality management is chemical treatment. Chemical treatment primarily involves the utilization of biocides to prevent biofilm formation and growth, corrosion inhibition and scale inhibition. Additionally, some plants implement side-stream treatment. Side-stream treatment can include filtration of the suspended solids and water softening (for scale inhibition). With proper operation and maintenance, a side-stream treatment system can reduce suspended solids buildup, lower the requirement of certain chemicals such as scale inhibitors and reduce blowdown volumes.
The water quality management strategy is dictated by the quality of the makeup water available at the site as well as local or regional regulatory limits on the blowdown water. Cooling tower blowdown waters are subject to strict regulatory compliance requirements for sewer or surface discharge. In many industrial locales, sewer permits for blowdown water are subject to certified laboratory testing of the water at regular intervals. Biocides and other chemicals in these waters can render them problematic as receiving waters in many U.S. publicly owned treatment works (POTW) facilities that use biological wastewater treatment.
The cooling water comes in intimate contact with critical process equipment such as heat exchangers. If this water causes problems to the heat exchangers such as reduction of heat transfer efficiency due to scaling and biofouling, or corrosion of the heat exchangers, there can be adverse consequences for the core operation.
Ideally, management of the cooling water loop should consider monitoring the efficiencies and state of the heat exchangers. Such systems should also involve frequent monitoring of the distribution system (spray nozzles) of the warm water entering the cooling tower and the water flow paths through the fill.
Finally, the cooling towers are subject to tremendous dynamic variations. The ambient conditions — for example, the temperature and humidity of the air, weather or direct sunlight exposure — can cause significant variations in the efficiency of the wet cooling process. These variations require dynamic adjustment of the water treatment system operation through continuous monitoring of key variables and adjustment of chemical dosing rates.
Implementation of a comprehensive monitoring and data-driven operations and maintenance regime for the circulating water system should consider:
- Synergistic management of the heat exchange systems involving the cooling water.
- The performance parameters of the cooling tower.
- The parameters related to water quality management.
A proper digitalization system can provide a comprehensive, reliable approach for achieving this information integrity. The implementation of a digitalization program typically involves three steps: Assessment of circulating water system health, mapping the performance of the circulating water system to a digital twin, and learning and predictive maintenance.
Assessing of Circulating Water System Health. The first step of any digital transformation effort involves an assessment of the baseline state of the plant. For wet cooling towers and the circulating water systems, this involves collecting the following operational information, mainly through historical performance data of the plant:
- Performance of all heat exchangers directly connected to the circulating water system. Comparing this to the designed performance parameters of the heat exchangers provides a clear assessment of the efficiency and health of these heat exchangers and the state of the circulating water loop.
- Performance of the cooling tower and comparison with the design and weather records to assess how the efficiency of the cooling system varies from design ratings.
- Assessment of the historical makeup water, blowdown water and the circulating water quality parameters and volumes.
Mapping the Performance of the Circulating Water System to a Digital Twin. A dynamic mathematical model of the circulating water system — consisting of the heat transfer characteristics of all heat exchange systems, the circulating water system loop, the cooling tower system and the water treatment system — is created as a computational simulator. This is referred to as a digital twin of the circulating water system. The digital twin is parameterized using the collected data and calibrated using design data for the actual plant components. After parameterization, the digital twin acts as the mathematical model emulating the dynamic plant performance. This simulator can act as a formal basis for tracking the performance of the actual plant performance when connected to the real-time data acquired from the plant.
Learning and Predictive Maintenance. Once the digital twin model is connected to the real-time plant data acquired through its process automation and control system, it can provide a comprehensive basis for managing the overall performance of the circulating water system. Utilizing the deviations between the observed data from the plant and the model predictions from the digital twin, a special class of machine learning algorithms can be deployed that can, on one hand, continually make the digital twin model smarter, while, on the other, can provide indications of many problems in the plant. The model can assess component failure, loss of efficiency due to fouling, external condition changes (such as relative humidity) and even drifts in the inline sensors. The model provides predictions assisting the plant operators to manage the plant, maintain the sensors and plant components, and help optimize the plant performance.
The digital transformation can assist the plant operations and maintenance team along with any outsourced service providers with real-time monitoring, process visibility and operational wisdom to run the plant optimally. This can help the facility achieve energy savings, accomplish higher levels of reliability and meet regulatory compliance.
Benefits of a Digital, Integrated O&M Tool
The complexities of water treatment and variations of blowdown disposal regulatory standards have made it attractive for many process plants to outsource the operation of the circulating water system to chemical treatment and water services companies. While this approach of outsourcing the water quality management appears astute, such practices can inadvertently lead the process plant to lose sight of the circulating water system and cooling tower operational knowledge base.
When called to troubleshoot the circulating water system at multiple types of plants in the chemical processing industry, we have often observed a plethora of issues and challenges, but all originated from a single root cause. A few samples of these include:
- At one plant, the pH and the corrosion inhibitor dosage system malfunctioned, causing corrosion in the copper-based heat exchangers. The corrosion products (for instance, copper) appeared in the cooling tower blowdown and exceeded the discharge permit levels, stopping the sewer disposal. In this case, the water treatment caused issues that cascaded into a major problem requiring throttling of production at the plant. The plant owner had to resort to tanker-based waste disposal, followed by renting additional treatment to reduce the copper content in the blowdown water.
- In another instance, the chemical supplier implemented a biocide program that interfered with the already established biofilms in the side-stream filtration system, resulting in a mobilization of total suspended solids (TSS) from the side-stream filter into the main cooling tower loop. In this case, the decision was to completely bypass the side-stream treatment system and instead use chemical treatment alone. This resulted in higher sewer discharge volumes of the blowdown and, of course, overall higher chemical consumption by the plant.
- In yet another instance, following maintenance in a wooden cooling tower section, enhanced levels of copper and arsenic were observed in the blowdown water. The root cause of this problem was leaching of copper and arsenic from the freshly replaced pressure-treated wood.
In these cases, the water quality management was often outsourced to chemical treatment providers, who detached the circulating water system monitoring and automation system from the integrated main process automation and control system of the plant. The cooling water chemical management system was being monitored through a separate automation package which sent all the chemical dosing and water quality data to the cloud server of the chemical treatment provider, who then used call center operators to monitor the water treatment system performance. Consequently, the plant operations and maintenance team lost direct line of sight or real-time tracking ability of what chemicals were being added to the circulating water system loop, the circulating water system water quality and consequences of the water treatment operations on the heat exchanger performance.
The problems observed at various plants were remarkably diverse. The root cause of all of these problems, however, was the disconnect between the water quality monitoring and treatment system of the service provider and the plant automation system. An integrated decision support system can autonomously detect these variations and trigger alarms to the plant operators much more proactively, mitigating these problems at a much earlier stage.
In summary, digitalization primarily enhances real-time, system-wide visibility of the entire plant. This translates into early detection and diagnosis of anomalies and proactive management of any excursion events. Delegating the operation and maintenance to third-party service providers is not a problem, but disconnecting the integrated information management is an issue. Digitalization restores this disconnect and the benefits that accrue to a plant from this system-wide, real-time visibility and monitoring are tremendous.