Thermal power plants need a continuous supply of cooling water to operate, but as the natural environment changes, more and more screen blockages are occurring at cooling water intakes. Maintaining intake equipment in good order and having a sound program to mitigate problems when events occur is important for intake reliability.
Worldwide, power generation is transitioning away from fossil fuels and toward renewable energy sources such as wind and solar. During this transition, and potentially thereafter, nuclear power remains critical for grid stability until reliable and economical energy storage options become available.
Nuclear and fossil-fueled thermal power generation require a continuous supply of large quantities of clean cooling water from natural sources. Even facilities relying on closed-cycle cooling systems require a significant amount of clean makeup water. Therefore, access to sufficient, clean cooling water is a necessity for safe, reliable, and efficient power generation.
In recent years, however, there has been increased risk of intake blockages due to environmental changes in natural cooling water sources. Problems have been caused by salp swarms in Korea, tiny shrimp masses in China, jellyfish blooms in Israel, and seaweed freed by storm activity in Russia, among others. Changes in environmental conditions such as increasing water temperature and carbon dioxide concentrations will not only have the obvious effect on thermal efficiency, but are also likely to increase the frequency of harmful algal blooms, invasive algae, and jellyfish blooms.
In addition to free-floating material (anthropogenic or biological) that can clog intakes, the nature and extent of biofouling (such as barnacles and mussels) on the submerged structures is also affected by changing environmental conditions (Figure 1). As Ali Ahmad noted in an article published July 5, 2021, in the journal Nature Energy, these sorts of intake blockages can be considered an indirect effect of increasing water temperatures—something that warrants careful consideration in the operation of existing or design of new power plants.
1. Problematic organisms for cooling water intake structures include (among others) blue mussels (left), eelgrass and kelp (center), and oysters (right). The distribution and abundance of each can be affected by changing environmental conditions. Courtesy: TWB Environmental Research and Consulting Inc.
Cooling Water Screening Systems
Cooling water is primarily used for cooling in condensers, a critical component in the steam power cycle. Cooling water is just as important for emergency and auxiliary service water systems, and it must be screened to remove debris in all cases. The screening assembly is determined by the required cooling water flow, and prevents waterborne debris and biota from entering the cooling water system. Most cooling water intakes apply several screening stages to protect downstream equipment, ensure continuous operation of the intake, and maintain the safe, continuous operation of the facility.
Intake screening typically consists of two main screening stages: one or more coarse stages (such as static trash racks, which are usually equipped with raking systems) and a fine stage (such as a rotating/traveling water screen or drum screen). The trash racks prevent large debris from damaging the downstream fine screens. The fine screens remove smaller debris that would otherwise clog the condensers and heat exchangers.
Some facilities may also incorporate a diversion structure (such as a trash boom or barrier net) upstream of the coarse and fine stages to prevent debris from reaching the intake. Environmental regulatory requirements may require that fine screens be fitted with fish protection features to protect aquatic life.
Failures sometimes occur in intake screening systems. The two primary sources of failure are poor maintenance of the screening systems and reliance on manual or scheduled screen rotation rather than equipment sensors (such as head differential sensors). It is critical that operational personnel are trained to manage intake blockage events and maintain intake screening equipment. Regardless of the root cause, each source of failure can result in screens not being operable or prematurely breaking down when called to operate during high debris loading conditions. Such events can lead to unit derates and forced outages.
Power plant operators must continually assess intake blockage risk and make modifications to maintain design and operation margins at the intake. However, the unpredictability of long-term (decadal) environmental changes makes them challenging to manage. Just as plant operators must monitor and assess the condition of intake screening equipment, owners/operators should also monitor and prepare for these changing environmental conditions.
Underpinning the civil, mechanical, and hydraulic design of an intake screening system are assumptions about the types and amounts of debris to be expected. Each debris type differs in terms of size, shape, constitution, density, and durations of events. These are strongly variable by geographic location, intake position in the source water (such as depth and onshore versus offshore arrangements), seasons, meteorological conditions, local biology, and hydraulic conditions.
Depending on the debris type and size, the material is either retained by the trash rack or the fine screens. Relying on either the coarse or fine screening stage as the primary debris collection point is a conscious design choice, though debris can be balanced between stages to prevent overtaxing one stage (Figure 2).
2. Schematic example of jellyfish ingress at a cooling water intake structure (CWIS) where various intake screening components are functioning to reduce the impact on cooling water flow: far field including a seasonally deployed, coarse barrier net (A), balanced removal at the coarse trash rack (B) and fine traveling screen stages (C), and finally disposal of the collected debris (D). Courtesy: Electric Power Research Institute and Info-graphics
Intake reliability refers to the ability of an intake structure to provide cooling water under all environmental conditions—both routine and unexpected. Reliable intake operation is a function of the suite of technologies, operational modes, personnel, training, and procedures employed by a facility.
Such reliability is not ensured via a simple checklist; it requires a culture of awareness and responsibility that hinges on the participation of various environmental, engineering, and operational personnel. It is also a framework of procedures that allows operators to avoid uncertainties, take short-term corrective actions, and implement adequate long-term management. Intake operators should consider the following critical items to ensure reliability of the cooling water intake system.
How Do I Know When an Intake Blockage Event Is Imminent? Some intake blockage events are episodic (such as massive jellyfish ingress) and, historically, have been difficult to predict. Other intake blocking phenomena may result from years of steady (but unnoticed or ignored) biological change, primarily though shifts in species community and their seasonality in the source waterbody. Therefore, long-term changes in the source waterbody should be monitored and result in modified approaches to managing potential blockage events.
Short-Term Forecasting. Most approaches to mitigating the effect of an intake blockage event are reactive (such as increasing trash rack raking frequency or increasing rotational speed on traveling water screens). By contrast, forecasting potential debris blockage events is proactive.
Current and emerging monitoring tools enable close and detailed observation of biota and materials in the far-field, mid-field, and near-field, as well as monitoring in-plant. Integration of data from combined monitoring tools allows estimation of the likelihood that an event will take place and (potentially) the event magnitude. Such tools may include traditional sonar, optical or high-frequency acoustic cameras, meteorological and oceanographic data buoys, hydraulic current monitoring, numerical hydrodynamic modeling, and satellite and hyperspectral imagery. Data from these various tools can be integrated and evaluated online by a dedicated software platform, though such technologies address only near-term threats.
Long-Term Monitoring of Environmental Change. An important component of intake reliability is preparing for intake threats that have yet to materialize due to their slow rate of change. Preparation for intake blockage events that may not occur for 10 or 20 years requires more than maintaining the operability of intake screening equipment and ensuring that procedures are in place to manage intake blockage events when they occur. It requires that dedicated personnel identify, collect, and analyze relevant long-term data sets.
Moreover, when data indicate a change is happening, these observations should be shared and discussed among the wider plant operations team. At such inflection points, decision-makers should identify modifications (structural and/or operational) to the intake system in anticipation of new threats.
Long-term environmental changes can cause unnoticed ecological shifts until a tipping point is reached. Whether the changes are caused by rising seawater temperatures, shifts in sand/silt transport/deposition, introduction of invasive nuisance species (such as zebra and quagga mussels), or another source, each may raise new challenges for which a plant is no longer designed, equipped, or operated to manage.
What’s the Condition of the Intake Screening Equipment? Major problems resulting from intake blockage events are typically caused by equipment failure due to inadequate preventative maintenance (PM), postponed corrective maintenance, or incorrect operational settings. Examples include out of calibration water differential level alarms, corrosion of mesh panels and chains, missing or broken seals, blocked or misaligned spray-wash systems, or improper chain alignment. Such factors may go unnoticed during the normal operation of the intake and equipment; however, they become painfully evident when the equipment is challenged during a debris event. Inadequate preventative maintenance thus leads to a reduced equipment reliability and increased risk of intake blockage.
Condition assessment of intake equipment is comprised of making sure that equipment issues are identified through observation and that intake operations personnel keep up with regular PM and, where required, corrective maintenance. PM tasks depend on the equipment type and components that are subject to wear and tear (such as traditional traveling water screen chains, rotating components, or material type) as well as water quality conditions (including saline water and abrasive content).
Observation/Inspections/Walkdowns. Information collection and trend analyses are critical to understanding equipment status and its readiness to handle debris events. Information is collected by monitoring through in-plant sensors (such as operational status and head differential trends) and visual observation (including in person or remotely via video camera) of equipment/material integrity (such as drive mechanisms, corrosion, biofouling, or wear and tear of sprockets and chains). Such information should be documented and clearly reported to other station team members for evaluation.
Station team members should actively review collected information to quickly recognize suboptimal conditions and respond with additional appropriate PM or, in some cases, immediate corrective actions. Assessing equipment reliability may include:
- ■ Documenting differences between design drawings and as-builts (particularly in cases where the differences are visually obvious). Current conditions can vary from as-built because long-term operation and frequent maintenance can affect tolerances/clearances. In addition, repairs may further alter components from their original design condition. Updated documentation can assist in troubleshooting intake equipment reliability questions when issues arise.
- ■ Adhering to vendor recommendations about operation and PM. Often, vendors have considerable experience at other similar plants and their recommendations are based on operational experience and equipment performance data.
- ■ Encouraging intake operations personnel to frequently observe and report potential issues with the intake equipment and its critical components before it affects intake availability during a debris event.
Preventive Maintenance. PM is the cornerstone of an intake reliability program. PM templates should be based on best practices, taking into consideration vendor recommendations and site-specific conditions. PM recommendations are available from each component vendor, from industry groups, and from peer-to-peer exchange. Generally, vendor recommendations should be relied upon as a baseline; however, site-specific conditions (such as silt-laden or high-salinity cooling water, and screen operation frequency—intermittent or continuous) may require that the vendor-recommended PM schedule be modified.
Equipment Replacement/Upgrade. Changing debris types and/or loads may increase demand on intake screening. In some cases, water quality changes may demand application of other materials. Also, aging equipment and the increasing availability of novel screening system types may make retrofit or modifications desirable. However, modifications to existing screens (such as adding larger debris troughs, or reducing bar spacing or mesh size) may result in undesirable hydraulic conditions that increase head loss and can result in pumps operating close to their hydraulic limits. If operational settings for a modified system are not able to compensate for the hydraulic losses (such as through faster screen rotation or raking), retrofitting with new equipment may be the next best alternative.
Is the Operational Staff Prepared? In addition to equipment-related considerations, the preparedness of intake operational personnel (that is, the human component) is critical for confidently managing debris events. In response to debris events, it is crucial for environmental compliance personnel, intake system engineers, and operational staff to coordinate to ensure that there is no potential for loss of cooling water flow.
Debris Management Plan
Managing debris events requires a site-specific plan. Some debris events are seasonal (such as ecological seasonality), others are occasional (such as storm-related events) or may be induced by gradual changing climatic conditions and appear unexpectedly (such as jellyfish blooms). To adequately prepare for all event types, it is important to understand the local aquatic environment and changes to it. Also, lessons learned from past events and events that have occurred at nearby facilities with similar water intake systems should be evaluated. This could aid in improvement to the design, operation, and maintenance of intake equipment.
To incorporate all of the above, a site-specific Debris Management Plan (DMP) should be developed to provide protocols for actions during a debris event (including step-by-step procedures), and outline preparation and maintenance of intake equipment to handle potential/expected events. A DMP will include a Standard Operating Procedure (SOP), an Early Warning Plan, and other debris management information.
Standard Operating Procedures. SOPs are written protocols designed to provide explicit instructions to station personnel on how to implement intake event response tasks. The SOPs ensure that every team member is informed, trained, and understands how to complete each task safely, efficiently, and accurately. Developing and implementing SOPs ensures consistent, efficient response to intake challenges and minimizes the risk of avoidable mistakes. SOPs are intended to be reviewed and updated as needed, but at a minimum on a yearly basis.
SOPs vary in the level of instructional detail provided, but generally provide step-by-step procedures to follow in reaction to observed or forecasted events. In some cases, logic flow charts are implemented to assist the operating staff in determining the existence and magnitude of an event and may also provide critical actions dovetailing into the SOPs.
SOPs may be different for different debris types. SOPs may specify, for instance, increased monitoring or staffing in preparation for potential inundation, manual operation of trash rack raking and fine screens, or inspection of the waterfront for debris buildup during potential events. Some facilities may call for a different screen operation parameter (such as increased raking frequency or rotation speed) during a typical debris event/season (including after storms), or implement flow reduction under some conditions.
Event Response Team. As part of the SOPs, specific tasks must be accomplished to efficiently manage a debris event and maintain the readiness and proper functioning of the intake screening equipment. Where staffing allows, a dedicated Event Response Team should be assembled and trained to execute the SOPs.
Each SOP task must be based on a protocol describing the frequency of particular actions and a log of visual observations, measurements, status reporting, and manual equipment operation. Each task must also have a dedicated person/role who acts as the lead coordinator, and is responsible for integrating feedback and specifying actions. Each debris event provides practical experience and learning points that should be used to refine the SOPs, modify/optimize equipment, and adjust operational settings.
An Integrated Effort Is Required for Success
Reliance on thermal power generation (including nuclear) remains strong given its role as a baseload power source in the current period of energy transition toward renewables. However, operating facilities are becoming increasingly challenged by intake blockages. While some blockages are episodic in nature, others are a result of slow environmental change.
Mitigation of potential debris problems at water intakes requires that facility operators remain vigilant about current debris conditions and also monitor the long-term environmental changes that can affect them. In some cases, the original assumptions used as the design basis for a facility may have changed.
However, detecting when conditions have changed sufficiently to warrant a response (screening or operational modification) is challenging. It requires dedicated personnel who are tasked with collecting and analyzing long-term data sets that allow them to understand ongoing trends and anticipate future challenges to intake reliability. Failing to account for these long-term threats can adversely affect cooling water intakes and facility availability including loss of operating margins or loss of production.
Maintaining intake reliability is an integrated effort that includes proactive monitoring of external debris conditions (items beyond the control of the facility), as well as monitoring of intake screening equipment condition and the readiness of operational staff to respond to an intake blockage event (items within the control of the facility). Operators that keep tabs on items both within and outside of their control will be best equipped to maintain availability of the intake, and avoid the potential safety and economic effects created by intake blockage events.
—Timothy Hogan is principal consultant with TWB Environmental Research and Consulting Inc., Maarten Bruijs is principal consultant with Pecten Aquatic, and Jonathan Black is a program manager overseeing research in both the aquatic and terrestrial environment with the Electric Power Research Institute (EPRI).