On a bulk carrier, cargo hold fire detection is a fixed smoke-sampling (aspirating) detection system installed as required by SOLAS Ch. II-2 and the FSS Code for ships carrying dry cargo and dangerous goods. The arrangement employs a battery of electric smoke detectors located at the bridge control cabinet linked by a network of small-bore sampling pipes (typically PVC/copper) run into each cargo hold, with regulated suction fans (sample aspirators) drawing a continuous stream of air (sample) from the holds through the sampling pipes back to the analyser cabinet.

Principle/functioning:

• A small fan continuously draws air samples from each hold through separate sampling pipes and a manifold/selector that sequentially checks each hold.
• In the cabinet the air sample passes over an ionisation or optical (obscuration/scattering) sensor. Smoke particles within the hold, taken up by the aspirated air stream, cause a change in the light passed/scattered or ionisation current, detected as an alarm condition.
• The control panel identifies which hold is involved, sounds an audible and visual alarm on the bridge and in the engine room/control room, and may activate a remote alarm.
• The system is designed so that detection is graduated: warning thresholds and alarm thresholds; the fan speed and slot/pinhole arrangement ensures balanced sampling over the hold volume.
• On detection of fire, the alarm alerts the duty officer who can immediately raise the fire alarm and coordinate response, e.g. close ventilation, alert the engine room and master, and adopt the correct (dangerous goods) response.

• Environment: cargo holds may be unoccupied, dusty, can contain flammable or self-heating cargo and are large spaces; machinery spaces are normally occupied, oily, hot and contain heavy equipment.
• Detection principle: holds rely on aspirated sampling across a large volume (point detection impractical), whereas machinery spaces use point/line heat and smoke detectors distributed through the space because personnel present and the density of equipment, and where a fire tends to produce localised heat/smoke first.
• Response: cargo hold detectors give early warning of a fire hidden in cargo so the cargo can be dealt with before it becomes catastrophic; machinery space detectors need to respond fast to protect occupied spaces, so heat detectors are often used to avoid false alarms from normal heat/smoke of engines.
• False alarm susceptibility: hot engine room, steam and combustion products call for heat-detection or flame detectors rather than smoke (which would be frequently tripped); holds use smoke sampling tuned for cargo atmospheres.
• Places of detection/alarming and maintenance: machinery space detectors need explosion-proof/increased protection for flammable atmospheres and easier local testing; cargo hold sampling systems are monitored from the bridge and are designed with explosion-proof pump and equipment considering flammables may be carried.

• For the cargo hold smoke sampling system: carry out a functional test by injecting test smoke or artificial smoke generator into a sampling point in one hold and confirming the corresponding alarm is raised at the control panel; check the suction/aspiration flow by a flow meter or by observing the flow in each sampling line; verify the alarm count/zone is correct; test the audible/visual alarms and the sequential scanning of the hold; visually inspect pipes for damage, blockage or contamination; quarterly/annual servicing per the manufacturer's instructions and FSS code.
• For machinery space heat detectors: test each point detector by applying heat (e.g. heat gun) to the sensing element and confirm the panel identifies the correct zone; for smoke detectors, test by discharging a known test aerosol replacing where necessary; check the panel indicates the zone, that the alarm operates, and re-sets; verify continuity/current of each loop, batteries and power source of the panel (normal and emergency supply).
• Records of all tests, dates and any faults and repairs are to be made in the log/Certified log and the system is to be maintained per the approved maintenance schedule recorded in the plan.