How does a flash evaporator fresh water generator work?

Seawater is fed into a low-pressure (vacuum) shell where the reduced boiling point (around 40–60 °C at 0.06–0.10 bar absolute) allows the heating medium — usually jacket cooling water from the main engine at 70–90 °C, or steam — to evaporate the seawater. The vapour rises, passes through a demister to remove salt droplets, and condenses on the condenser tubes cooled by seawater. The condensate (fresh water) is pumped to the domestic tank. Brine reject is discharged overboard.

What is the salinometer cut-off and why is it set at 5–10 ppm?

The salinometer continuously measures the salinity of the produced water. If the conductivity rises above the set-point (typically 5–10 ppm NaCl equivalent, per the manufacturer's manual), a solenoid valve diverts the product back to the sea chest rather than the fresh water tank. Seawater contains around 35,000 ppm; even 10 ppm is well below human health thresholds, but the cut-off is set low to protect against foaming or carry-over events when the product can momentarily contain much higher salt.

What does MLC Regulation 3.2 require for potable water?

MLC 2006 Regulation 3.2 and Standard A3.2 require that ships provide seafarers with free access to sufficient quantities of potable water of adequate quality. The water must meet WHO drinking water quality standards. Ships must carry enough capacity or generating ability for the entire voyage. Both the supply and the quality must be recorded; ships are inspected by port State MLC inspectors who may sample the water.

How is the water chlorinated and tested?

Chlorine (as sodium hypochlorite solution) is dosed into the fresh water line to achieve a residual of 0.2 mg/L (ppm) free chlorine at the point of use. The residual is measured with a colorimetric DPD test kit. Too little risks bacterial growth including Legionella; too much causes taste complaints and can corrode copper pipework. WHO Guide to Ship Sanitation (3rd edition) recommends maintaining 0.2–0.5 mg/L free chlorine residual throughout the system.

Fresh water generator

Ships making extended ocean voyages typically produce potable water underway using either a flash evaporator (vacuum evaporator) or a reverse osmosis (RO) unit. Both must produce water meeting the WHO Drinking-water Quality Guidelines, enforced by MLC 2006 Regulation 3.2. Operation, salinometer management, and chlorination are critical deck and engine department duties.

Flash evaporator — operating principles

· Heating medium: main engine jacket cooling water (70–90 °C) or low-pressure steam circulates through the evaporator tubes.
· Shell is held at vacuum (0.06–0.10 bar absolute) so seawater boils at 40–60 °C, preventing scale formation at the temperature of conventional boiling.
· Seawater feed rate and brine level are controlled by float valves; brine reject concentration typically 1.5–2 × ambient seawater salinity.
· Demister (wire mesh or chevron pack) removes entrained droplets from the vapour before it reaches the condenser.
· Production stops when entering port or coastal waters (within 12 nm in most operational guidance) to avoid contaminated seawater entering the product.
· Cannot operate efficiently when the main engine is below ~75% MCR — insufficient jacket water temperature to maintain vacuum.

Reverse osmosis — operating principles

· Seawater is pumped to 55–70 bar against a semi-permeable membrane; fresh water permeates; salt-concentrated brine is rejected at ~2 × seawater salinity.
· RO membranes require seawater pre-filtration (cartridge filters, 5 µm typical) and dosing with anti-scalant to prevent mineral precipitation.
· RO is not dependent on engine load and can operate in port on electrical supply; suitable for ships with low waste heat (diesel-electric, gas turbine).
· Membrane life 3–5 years; chlorine in feed water destroys polyamide membranes — de-chlorinate before the membrane.

Salinometer operation and cut-off

· Salinometer measures conductivity and converts to ppm NaCl; calibrate daily with standard solution (1,000 µS/cm = approximately 500 ppm).
· Cut-off set-point: typically 5–10 ppm; three-way valve diverts substandard product overboard until reading drops below set-point.
· At start-up, allow 15–30 minutes before product valve opens — early product is always high-salinity due to retained brine in the system.
· Log daily: product volume (m³), salinity reading, running hours; entries reviewed at port State MLC inspection.

Chlorination and bacteriological testing

· Dose sodium hypochlorite to achieve free chlorine residual of 0.2–0.5 mg/L throughout the distribution system (WHO Guide to Ship Sanitation, 3rd ed.).
· Test residual with DPD colorimetric test kit at the most distant outlet daily; record in the potable water log.
· Bacteriological sampling (total coliforms, E. coli, heterotrophic plate count) every six months or after any repair to the distribution system; samples sent to a shore-based laboratory.
· Tanks should be cleaned, inspected, and disinfected (500 mg/L chlorine shock dose, 30 min contact time, then flush) at each annual class survey.
· MEPC.107(49) prohibits discharge of potable water chlorinated above 0.2 mg/L into restricted areas (some port limits); switch to shore supply when in such ports.

Sources: MLC 2006 Regulation 3.2 and Standard A3.2 — Food and catering; WHO Guide to Ship Sanitation 3rd edition (2011); IMO Resolution MEPC.107(49) — Guidelines for addressing water pollution from ships. See also MLC 2006, potable water, MARPOL Annex V.

Last verified 4 months ago