Spray Nozzles for
Exhaust Gas Cleaning Systems & Marine Scrubbers
EGCS washwater injection nozzles operate in conditions that make most industrial spray hardware the wrong choice from the start. Hot exhaust gas at 200–250°C acidified to pH 2–3 by dissolved sulfur dioxide from HSFO combustion creates an environment where 316L stainless steel pits through its full wall thickness within a single dry-dock cycle. Correct nozzle material selection — Duplex 1.4462, Super Duplex 2507, or Silicon Carbide — is not a performance upgrade. It is the difference between a scrubber that achieves MARPOL Annex VI compliance on every voyage and one that fails before the next Port State Control inspection.
Marine exhaust gas cleaning system (EGCS) scrubbers use spray nozzles to inject washwater — either seawater in open-loop systems or recirculated alkaline solution in closed-loop systems — into the exhaust gas stream. The washwater contacts the sulfur dioxide in the exhaust, absorbs it, and carries the resulting sulfurous and sulfuric acid out of the gas phase and into the washwater discharge stream. The nozzles that accomplish this operate continuously in a corrosive, high-temperature environment that is among the most demanding in any industrial spray application.
The correct nozzle specification for EGCS service depends on the position within the scrubber (primary inlet spray tower vs. venturi throat vs. secondary polishing stages), the exhaust gas temperature at that position, the pH of the washwater at that position, and whether the system is open-loop (seawater), closed-loop (alkaline solution), or hybrid. No single nozzle type or material grade is correct for all positions in an EGCS installation — this page covers the engineering basis for position-by-position specification.
Open-Loop vs. Closed-Loop Scrubbers: Different Chemistry, Different Nozzle Demands
The fundamental difference between open-loop and closed-loop EGCS systems is what the washwater contains — and that difference directly determines nozzle material grade, orifice geometry, and maintenance interval.
Open-Loop Systems
Seawater washwater — unrestricted ocean operationClosed-Loop Systems
Recirculated alkaline washwater — ECA and port operationThree Spray Positions, Three Different Engineering Problems
A marine scrubber is not a single-position spray application. The temperature, pH, gas velocity, and water quality at the primary inlet spray tower, the venturi throat, and the secondary polishing stages are all different — and the nozzle specification at each position must be derived from the conditions at that position, not from a single system-wide selection.
Primary Inlet Spray Tower
Highest temperature — highest acid exposureThe primary inlet spray tower is where exhaust gas enters the scrubber at its highest temperature — typically 200–280°C for a large two-stroke marine diesel on heavy fuel oil. The first rows of washwater injection nozzles in this zone experience the full incoming exhaust temperature on the downstream face while injecting washwater at near-ambient temperature. This creates a thermal gradient across the nozzle body that, combined with the highly acidic, chloride-bearing exhaust atmosphere, creates the most demanding material environment in the entire EGCS installation.
The washwater at this position is also most effective at SO₂ absorption because the driving force for gas absorption into liquid is highest when the washwater is fresh (lowest dissolved SO₂ content) and the gas is hottest. Getting the droplet size right at this position — fine enough to maximize the gas-liquid contact area but coarse enough to fall against the upward draft of incoming exhaust gas — has the largest impact on overall scrubber efficiency of any position in the system.
Venturi Scrubber Throat
Highest gas velocity — maximum abrasionVenturi scrubber designs accelerate the exhaust gas through a constricted throat section, typically achieving gas velocities of 40–80 m/s at the throat. At these velocities, the exhaust gas carries fine particulate matter from incomplete combustion at sufficient kinetic energy to cause direct erosion of nozzle bodies and orifice edges — the same dual-surface abrasion mechanism described for blast furnace gas scrubbers, but now combined with seawater chloride and acid exhaust gas chemistry that accelerates the corrosion component of the corrosion-erosion synergy.
Secondary Polishing & Mist Eliminator Wash
Lower temperature — fine droplet entrainment preventionAfter the primary scrubbing stage, the partially cleaned exhaust gas passes through secondary spray stages and a mist eliminator section. The secondary spray stages continue SO₂ absorption at lower gas temperatures (typically 40–80°C at this point) and lower acid concentrations. The mist eliminator wash nozzles periodically flush the mist eliminator panels to prevent salt and acid deposits from building up on the surface and restricting gas flow. These positions operate in a much less aggressive environment than the primary inlet — but they still require corrosion-resistant materials because the exhaust gas is still acidic and chloride-bearing.
Scrubber Sump & Internal Washing
Removing soot and sulfate deposits between voyagesDuring operation, carbonaceous soot and amorphous sulfate deposits accumulate on the internal surfaces of the scrubber tower, on the spray ring manifolds, and on the mist eliminator panels. These deposits reduce gas-liquid contact efficiency, restrict gas flow through the scrubber, and — if allowed to build up into thick layers — can detach as large slugs that block individual nozzle orifices. Internal washing of the scrubber tower between voyages or at scheduled maintenance intervals is necessary to maintain design scrubbing efficiency throughout the vessel's operational life.
Droplet Size Optimization for SO₂ Absorption: Balancing Contact Area Against Draft Pressure Drop
The droplet size distribution produced by EGCS washwater injection nozzles is the primary determinant of scrubbing efficiency at any given washwater flow rate. Getting it wrong in either direction — too fine, or too coarse — costs money in fuel use, capital in oversized equipment, or compliance risk from under-performance.
Why Finer Is Not Always Better for Marine Scrubber Droplets
SO₂ absorption from gas phase into liquid phase is governed by mass transfer theory — the rate of absorption is proportional to the total gas-liquid interfacial area, which is proportional to the total droplet surface area per unit volume of scrubber. For a given washwater flow rate, smaller droplets produce more total surface area than larger droplets (surface area scales as 1/d for a given volume). This creates a strong engineering incentive to produce the finest possible droplets to maximize scrubbing efficiency.
However, EGCS scrubbers operate with an upward-flowing exhaust gas stream that carries the droplets with it if they are fine enough to be entrained. The terminal settling velocity of a water droplet in air scales with the square of the droplet diameter — a 100 µm droplet has a terminal velocity of approximately 0.25 m/s, while a 500 µm droplet settles at approximately 2.5 m/s. In a scrubber tower where the upward gas velocity is 2–4 m/s, droplets below approximately 300–400 µm will be carried upward by the gas stream rather than falling downward to the sump — they either re-entrain into the cleaned gas and escape through the exhaust stack, or they load the mist eliminator section, increasing pressure drop and reducing the available gas throughput.
The Pressure Drop Penalty of Over-Fine Atomization
In a marine EGCS installation, the scrubber creates a pressure drop in the exhaust gas path that the engine must overcome — this additional back-pressure is the direct energy cost of the scrubbing operation. Fine atomization that creates excessive mist eliminator loading raises the pressure drop across the scrubber beyond the design value. On a large two-stroke marine diesel, each additional 10 mbar of exhaust back-pressure corresponds to approximately 0.3–0.5% increase in specific fuel oil consumption. For a 12,000 kW main engine operating at sea for 5,000 hours per year at $600/tonne HSFO cost, an additional 0.4% fuel consumption from excess back-pressure costs approximately $15,000–25,000 per year — more than the cost of re-specifying the nozzle array. Contact NozzlePro with your scrubber tower diameter, exhaust gas flow rate, and design gas velocity to receive a droplet size recommendation specific to your system.
The 316L Pitting Failure Mechanism — Why Standard Stainless Is Wrong for EGCS
316L austenitic stainless steel achieves its corrosion resistance through a passive chromium oxide film on its surface. In clean seawater at ambient temperature, this passive film is stable and provides adequate protection against general corrosion. In the EGCS washwater environment — chloride-bearing water at elevated temperature, in contact with acidified exhaust gas at pH 2–3 — the passive film is attacked by two simultaneous mechanisms that work synergistically.
Chloride ions destabilize the passive film at surface defects, initiating pitting corrosion at the defect sites. The acid environment (pH 2–3) prevents the passive film from re-forming over active pit sites — once a pit is initiated, the local pit chemistry becomes increasingly acidic (FeCl₃ hydrolysis), which further prevents repassivation and accelerates the pit growth rate. This autocatalytic mechanism is what causes 316L SS EGCS nozzles to fail through pitting in months of service rather than years — the pits grow progressively until they penetrate through the nozzle wall or cause mechanical failure of the orifice edge.
Duplex 1.4462's higher pitting resistance — quantified by the PREN formula (PREN = %Cr + 3.3×%Mo + 16×%N) — reflects a passive film that is more resistant to chloride-induced depassivation. At PREN 35–38, Duplex 1.4462 maintains passive film stability at chloride concentrations and temperatures where 316L SS (PREN 24–26) has already entered active pitting. For the primary inlet spray position where temperature and acid loading are highest, Super Duplex 2507 (PREN above 40) provides the additional margin required for reliable long-term service between dry-dock intervals.
Once pitting initiates in a 316L SS EGCS nozzle, the autocatalytic pit growth mechanism means the failure rate accelerates rather than remaining constant. A nozzle that shows no visible corrosion after six months may show through-wall pitting within the following three months because the local pit chemistry has become self-sustaining. Port State Control inspections that find EGCS nozzle failures can result in operational restrictions that are far more expensive than the cost of correct Duplex specification at initial installation. The incremental cost between 316L SS and Duplex 1.4462 EGCS nozzles is recoverable within a single avoided repair incident.
EGCS Nozzle Material by Scrubber Position
Contact NozzlePro with your scrubber OEM, tower geometry, exhaust gas temperature profile, and washwater chemistry. Position-specific material selection is essential — a single material grade across the entire scrubber is either over-specified at some positions or under-specified at others.
| Scrubber Position | Temp. Range | pH at Position | Recommended Material | Key Constraint | Replace At |
|---|---|---|---|---|---|
| Primary inlet spray rings — closest to exhaust gas entry | 200–280°C | pH 2–3 | Super Duplex 2507 body or SiC ceramic | Highest temp + highest acid loading — Duplex 1.4462 at its service limit; SiC preferred for infinite corrosion resistance | Inspect every dry-dock; replace at any sign of pitting or orifice distortion |
| Primary spray rings — upper rows (gas partially cooled) | 120–200°C | pH 2.5–4 | Duplex 1.4462 or Super Duplex | Still within corrosion-acceleration range for 316L SS; Duplex 1.4462 provides adequate service; upgrade to Super Duplex if gas temperature stays above 150°C at this position | Inspect every 12 months; replace at 10% flow deviation |
| Venturi throat injection | 150–250°C | pH 2–3 | Duplex or Super Duplex body + SiC inserts | 40–80 m/s gas velocity causes rapid abrasion of metallic orifices — SiC inserts mandatory at venturi throat regardless of corrosion grade | Inspect every dry-dock; measure orifice diameter; replace at >10% enlargement |
| Secondary polishing spray stages | 40–80°C | pH 4–6 | Duplex 1.4462 | Adequate corrosion resistance at lower temperature and pH; scale monitoring required; do not specify 316L SS even at this position | Inspect every 12 months; acid-clean if scale detected |
| Mist eliminator wash nozzles | 40–80°C | pH 5–7 | Duplex 1.4462 | Low-pressure periodic flushing — moderate chemical environment; full-cone pattern for panel coverage; 1–3 bar supply pressure | Inspect every 12 months |
| Scrubber internal soot wash | Ambient–60°C | Variable pH | Duplex 1.4462 | Variable pH during washing as acid deposits dissolve; high-impact flat-fan or solid-stream; Duplex handles the full range | Inspect annually |
EGCS Scrubber Nozzle Specification at a Glance
Key Parameters for EGCS Washwater Injection Nozzles
EGCS-Grade Materials Supplied by NozzlePro
All nozzles manufactured under ISO 9001. Material grade documentation available on request. Customers are responsible for classification society submission using NozzlePro-supplied hardware.
EGCS Nozzle Failures Are Operational Disruptions, Not Maintenance Events.
A scrubber that cannot achieve washwater pH targets because its primary inlet nozzles have pitted through fails to demonstrate MARPOL Annex VI compliance at the next Port State Control inspection. Specify Duplex or SiC from installation — not after the first failure. Contact NozzlePro with your scrubber OEM, exhaust gas temperature profile, and washwater chemistry.