The slightest droplet of water or salt escapes into the steam, the turbine can slow down, crust over and then collapse. The industry responded with separator-lined steam drum designs and 24/7 online monitoring.
The numbers make engineers hold their breath: a 30 MW turbine lost more than 5% of capacity and eventually tripped in three months due to carryover fouling(www.watertechnologies.com), a revenue loss equivalent to more than $5 million per year when assuming ~$30/MWh(www.scribd.com). National surveys even show the problem is widespread: up to 40% of utility turbines in the US and 60% of industrial turbines operate with excessive levels of steam impurities(www.scribd.com).
Not surprisingly, steam purity limits are now super-tight. The IAPWS (International Association for the Properties of Water and Steam) guidelines set dissolved silica in superheated steam <10 µg/kg and sodium <2 µg/kg (µg/kg ~ ppb)(studylib.net), as below these thresholds heterogeneities in steam remain below solubility and do not precipitate. Practically, many plants keep the vapor conductivity (after cation exchange) below ~0.3 µS/cm; exceeding this specification immediately triggers an alarm(studylib.net). Any deviations "must be identified and eliminated as quickly as possible" to avoid turbine corrosion(studylib.net; www.watertechnologies.com).
Impact of impurities and vapor specification limits
Even the carryover of trace droplets or dissolved solids can build up in turbine blades, cutting flow capacity and efficiency(www.watertechnologies.com; www.researchgate.net). Independent studies concur: in geothermal analysis, Na >2 µg/L or SiO₂ >10 µg/L in the steam "will settle" on the turbine surface(www.researchgate.net). Operational targets therefore often aim for a cationic conductivity of around 0.3 µS/cm or lower, with IAPWS limits as a reference.
Internal steam drum and multi-stage separation
The steam drum in a HRSG (heat recovery steam generator, heat recovery steam generator) is the first line of defense for steam purity. Its internal design incorporates several droplet separation principles. Gravity baffles and flow equalizers in the form of perforated plates slow and flatten the upflow so that water falls back into the drum pool(www.researchgate.net). In high-pressure boilers, this demands very uniform flow distribution and adequate residence time; droplets are swept away as the steam rises "at a steady low velocity," aided by submerged perforated plates that catch the droplets(www.researchgate.net).
After that, the mechanical separator takes over for the fine mist. Inertial cyclones create a powerful vortex; centrifugal force throws droplets against the wall to gather and flow down(www.researchgate.net). A chevronvane/demister package forces a sharp change in velocity so that droplets are dislodged from the flow(www.researchgate.net; www.researchgate.net). Towards the outlet, steam typically passes through a wire mesh demister pad to capture the smallest droplets, ensuring very high dryness; vendor notes mention HRSGs utilizing horizontal/vertical chevron modules or mesh pads at the outlet drum(www.beggcousland.com). Critical: installation requires even flow distribution and correct drainage gaps-any channeling can trigger carryover(www.beggcousland.com).
In practice, multi-stage internals (e.g. primary cyclone plus secondary chevron/pad) are designed to remove >99% of droplets down to the ~20-50 µm range, so the vapor exits the drum almost dry. Recent designers even use CFD; a 2011 study on an industrial HRSG mapped the velocity field in the drum and optimized the demister position for performance assurance(koreascience.or.kr). Without these internals, the boiler droplet trail would carry silica/salt volatiles into the turbine and form heavy deposits(www.watertechnologies.com; www.researchgate.net).
Steam/water online monitoring (SWAS)
As steam purity is crucial, continuous monitoring is standard. SWAS (steam/water analysis system) samples condensate, feed water, sometimes steam, and sends it to an online analyzer. Typically installed are specific conductivity and pH meters (for ammonia inference), cationic conductivity - i.e. conductivity after the sample has been passed through a hydrogen form ion exchange resin column - as well as specialized ion analyzers for sodium, silica, chloride. Frequently recommended targets: condensate/feedwater cationic conductivity ≤0.2 µS/cm and sodium <2 ppb(www.power-eng.com; the same link also features Fe limits <2 ppb: www.power-eng.com).
Degassing cationic conductivity is monitored to detect condenser leaks or deaerator faults; increases above ~0.2-0.3 µS/cm already violate turbine specifications(studylib.net). Practical experience shows instrument correlation is important: if cationic conductivity rises but Na is ~0, calibration drift is likely; if both rise, it is an indication of real contamination(www.power-eng.com). To produce this cationic conductivity reading, many systems pass the sample through a column of ion exchange resin (hydrogen-form)-similar media can be found in ion exchange resins.
Major vendors consider this monitoring irreplaceable. Endress+Hauser states SWAS is "proven in more than 100 countries," and at plants "continuous monitoring of water quality is the only way to ensure specifications are met, avoiding costly repairs"(www.fi.endress.com; www.fi.endress.com).
Field cases and conditioning options
At Saudi Aramco, low-cost analyzers for silica, sodium, and conductivity were installed in the HRSG feed line. The results were revealing: steam cationic conductivity of ~0.5-1.0 µS/cm (exceeding the specification of 0.3), traced to chlorides in the makeup water. Engineering recommendations: add mixed-bed ion-exchanger and continuous silica/conductivity monitoring; projected impact to reduce boiler blowdown and raw water usage by ~10-15% and avoid turbine fouling, "thus maintaining long-term efficiency"(gasprocessingnews.com; gasprocessingnews.com). The implementation of mixed-bed is in line with the use of units such as mixed bed systems for chemical polishing. Instrumentation costs are considered "relatively low," as modern analyzers can run without an operator; resin columns can be automatically regenerated or replaced every 1-2 months(gasprocessingnews.com).
Operational conclusions
The combination of effective droplet separation drum internals and real-time chemical monitoring enables HRSGs to produce the "ultrapure steam" that turbines demand. Strict targets - such as SiO₂ < 10 µg/kg, cationic conductivity < 0.3 µS/cm, and Na < 2 µg/kg/ppb(studylib.net; studylib.net) - are maintained with alarms and diagnostics. Any deviation (e.g. increase in conductivity, silica or sodium) triggers action: purge/crashout of contaminated water or repair of leaks before damage occurs. The end result is concrete: fewer forced outages, longer turbine overhaul intervals, and higher net output.
Key reference sources and industry studies: IAPWS Steam Purity guidelines(studylib.net; studylib.net), US industry Water Handbook(www.watertechnologies.com), internal drum technical literature( www.researchgate.net; www.researchgate.net; koreascience.or.kr), and monitoring practices in the generation industry(www.power-eng.com; gasprocessingnews . com; www.fi.endress.com).
