Raw-water pretreatment is a small cost—often ≲2% of operating spend—but its impact on HRSG uptime is outsize. Data and field practice point to two levers that keep plants online: rigorous preventive maintenance and N+1 design.
Raw make‑up water for HRSGs (heat recovery steam generators) looks like a utility; in reality, it’s a reliability gate. Pretreatment—clarification and filtration—costs little to run (often ≲2% of operating costs), but the availability impact is disproportionately large (studylib.net).
When raw water carries high turbidity or suspended solids, filters foul, heat exchangers scale, and piping corrodes—classic precursors to unplanned plant outages. Industry guidance warns that excessive solids “lead to filter fouling and scale buildup, which decreases plant efficiency and increases required maintenance as well as any resulting downtime” (powermag.com).
The downstream effects are tangible in combined‑cycle fleets: untreated‑water problems are tied to “delayed start‑ups, subsequent poor plant heat rates, and…unscheduled downtime” (studylib.net). Uninterrupted plant operation hinges on a reliable pretreatment line.
Preventive maintenance program architecture
A structured PM program underpins reliability in clarifiers/sedimentation units, filters, pumps, valves, and controls. Sites running a clarifier train document daily/weekly inspections and specific care routines.
Clarifier and sedimentation maintenance tasks
Operators drain condensate and any standing water from rotating drives at least weekly to prevent gear and bearing corrosion—cited as “the single most important aspect of drive maintenance,” because neglect can trigger immediate gearbox failure (netsolwater.com).
During outages, plants clean sludge scrapers, weirs, and launders of algae and debris; exposed surfaces are repainted or coated to resist fouling (westechwater.com; netsolwater.com). Accumulated sludge is removed on a regular schedule—monthly or as indicated by sludge depth—to preserve tank capacity (id.scribd.com).
Even simple housekeeping matters: clearing vegetation and debris from inlets/outlets prevents clarifier flow “short‑circuiting” that can degrade performance (westechwater.com; id.scribd.com).
Filter backwash, media care, and monitoring
Automatic backwash cycles are set—often daily—to flush trapped solids; manual scrubbing of filter walls or media is periodically performed to scour green biofilm or mineral encrustation before an automatic wash (id.scribd.com). Multi‑media beds routinely use sand/silica media to remove suspended particles in the 5–10 micron range.
Some plants add a top layer of anthracite media to increase dirt‑holding capacity in multi‑layer filters. After each backwash, any lost media is replenished to maintain design depth and uniformity (id.scribd.com).
Underdrains and nozzles are inspected when offline; clogged, broken, or leaking elements are replaced (id.scribd.com). Performance is tracked with turbidity—targeting less than 5 NTU (Nephelometric Turbidity Units)—and differential pressure; if turbidity rises or head loss grows, backwash frequency is adjusted or the filter is serviced to prevent breakthrough (id.scribd.com).
As a polishing step ahead of demineralization, a cartridge filter is often used to capture fine particulates. Industrial units that face higher pressures typically deploy housings such as a steel filter design.
Pumps, dosing, and instrumentation routines
Duty and standby pumps are maintained on routine intervals: alignment checks, bearing lubrication, and seal verification are standard. Spare units are exercised periodically so they are ready to assume duty when needed.
Chemical feed systems—coagulants, flocculants, and pH control—run monthly functional tests, with accurate metering through a dosing pump. Control valves, flow meters, and critical sensors are kept clean and calibrated; poor instrumentation can mask problems until failure.
Supporting skids and utilities, including mixers, air scour blowers, and backwash gear, are included within a plant’s water-treatment ancillaries program for inspection and upkeep.
Digital monitoring and predictive actions
Where available, sites add real‑time turbidity monitoring, pressure transducers, and vibration sensors, then trend data in a digital PMS or predictive maintenance tool. This approach “reduces the breakdowns of machines…to the minimum limits,” improving overall equipment effectiveness and minimizing downtime costs (carewater.solutions).
Proactive replacement of worn parts avoids forced outages, and the spend on labor and spares is small relative to the savings from avoiding unplanned shutdowns (carewater.solutions). For inventory continuity, operators typically stock a core set of water-treatment parts and consumables.
In practice, scheduled filter backwashing combined with hands‑on cleaning can cut labor‑intensive cleanings in half. Weekly clarifier drive checks and condensate drains, in particular, are singled out for their outsized impact on failure avoidance (netsolwater.com).
Redundant design (N+1) in pretreatment
Redundancy ensures that no single fault halts production. Plants adopt “N+1” design—one extra unit beyond base demand—so maintenance doesn’t cost throughput (xylem.com; water.co.id). In clarifiers, at least two parallel tanks are sized so one can be cleaned while the other(s) handle 100% of flow, an approach explicitly noted in local guidance where dual trains enable washdown without full shutdown (water.co.id).
To meet peak loads or storm surges, surface area is uprated such that overflow rates remain within design even with one train offline. Compact modules like a lamella settler can be considered where footprint is constrained.
Parallel filter banks are equally standard so one unit can be backwashed while others stay online (water.co.id). Upstream debris control frequently includes an automatic screen to protect the filter beds and extend run time.
Higher‑surface‑area, multi‑tube designs used as pre‑strainers add inherent redundancy; these can filter 3–4× the area of a single basket strainer, cutting backwash frequency and clogging risk (powermag.com). For guard filtration, a strainer is typically paired with the primary filters.
Pumping systems mirror this approach—2 duty + 1 standby on critical flows (raw intake, treated‑water transfer, chemical injection)—with “run–shutdown” sequencing to periodically test standbys. Controls add duplicate PLC (programmable logic controller) channels or fail‑safe outputs in critical loops, and backup units are kept for sensitive steps such as RO (reverse osmosis) modules and UV (ultraviolet) disinfection, which are described as “highly valuable” to switch on if the main unit fails (water.co.id). Where UV disinfection is part of pretreatment, plants standardize on a ultraviolet unit that offers a 99.99% pathogen kill rate without chemicals.
Storage adds resilience: more than one raw‑water tank or equalization basin allows alternating cleaning and acts as a buffer if intake supply is interrupted, a practice highlighted in local best‑practice guidance (water.co.id). Treatment capacity is often designed above average demand—e.g., 110–120%—so a single train can handle peak conditions.
Redundancy does not always inflate footprint. One seawater electrolytic treatment example—Chloropac—uses N+1 electrolyzers sized for full duty with one extra, reducing total electrode volume by ~30% compared to conventional parallel trains (xylem.com).
Operational outcomes and business impact
Combining rigorous PM with N+1 design changes the uptime equation. A double‑clarifier/two‑filter configuration can approach ~100% pretreatment uptime, whereas a single line might have several forced outages annually. Even a 1–2% uptime improvement in pretreatment can pay back in megawatt‑hours at fleet scale, and unplanned water‑system outages often cost millions per event in lost generation and repair (studylib.net; powermag.com).
Quantitatively, swapping conventional strainers for high‑area self‑cleaning units reduced filter backwash events by 3–4× in one analysis, saving backwash water and man‑hours (powermag.com). Plants often specify robust housings—such as a fiberglass filter shell—where corrosion resistance helps sustain those gains.
On the mechanical side, enforcing a strict open/short‑circuit test schedule (for example, weekly drive checks and condensate drains) has been shown to eliminate most clarifier mechanical failures (netsolwater.com). Over time, plants report lower fixed O&M and fewer expensive emergency repairs when these practices are embedded (carewater.solutions).
The through‑line is consistent across sources: plants with redundant filters and clarifiers and a diligent PM program consistently report higher water quality and reduced downtime (studylib.net; id.scribd.com). In pretreatment, small, repeatable actions—paired with design headroom—are what keep the megawatts flowing.
