Troubleshooting the Oily Water Separator — Ten Faults, Cause by Cause
A working diagnostic index for the oily water separator, the monitor and the bilge system that feeds it.
Key Principles at a Glance 7 points
- High p.p.m. with repeated tripping is usually overloading: the flow velocity rises, the flow stops being laminar, and oil that had already risen is dragged back into the stream.
- A separator that reads high from the moment it starts has a supply-side fault — a centrifugal pump or sheared pipework; one that performed well and has degraded has an internal or temperature fault.
- Poor separation with oil that will not rise is nearly always low temperature: open the steam heater valves and bring the vessel up to about 60 °C.
- No automatic oil discharge points at the capacitance probe, the solenoid pilot valve or the control air supply — clean the probe, check the pilot valve, and drain manually through the test cocks meanwhile.
- An alarm with the discharge diverted to the bilge tank is normal function, not a fault; the trap is treating a working safety device as a nuisance.
- A separator that drains itself overboard when stopped has a siphon in the discharge line and needs its siphon breaker fitted or checked.
- A bilge pump that loses vacuum and stops is drawing air — usually dirt under a valve lid or a failed seal at a strainer or mud box cover.
1. Master symptom table
The maker's manual and the approved procedure override. Do not defeat the monitor, the alarm or the stopping device to get a discharge away. A separator that will not hold 15 p.p.m. is a fault to be found, not an obstacle to be worked around.
Use this as the first pass; the numbered sections below give the reasoning.
| Symptom | Most likely cause | Section |
|---|---|---|
| Effluent p.p.m. high, alarm trips repeatedly | Overloading the separator — turbulent flow and re-entrainment of oil | 2.1 |
| Effluent high from the moment of start-up | Wrong pump type or mode — a centrifugal supply pump | 2.2 |
| Poor separation, oil not rising | Low temperature — viscosity high, density differential small | 2.3 |
| No automatic oil discharge | Capacitance probe fouled or faulty, or the solenoid pilot valve and control air | 2.4 |
| Water carry-over, priming-over | Interface level control lost | 2.5 |
| Alarm sounds and the discharge diverts to the bilge tank | Normal function — oil exceeded 15 p.p.m. | 2.6 |
| Separator drains itself overboard when stopped | Siphon effect | 2.7 |
| Sludge or coalescer clogged, throughput falls | Contaminant build-up | 2.8 |
| Bilge pump loses vacuum, output falls, pumping ceases | Air ingress on the suction side | 2.9 |
| Leak in the bilge piping | Corrosion, usually at the lowest point | 2.10 |
2. The faults in detail
2.1 High p.p.m., alarm trips repeatedly
Cause. The separator is being asked to handle more flow than it was designed for. The consequence is not merely that there is more oil to remove. The flow velocity through the vessel rises, the flow stops being laminar, and oil that had already risen to the surface is dragged back down into the stream and out with the discharge. The machine defeats itself.
Action. Reduce the throughput. Never run the separator at over capacity. If the throughput is already within rating and the reading is still high, look at the next two entries before assuming a mechanical fault.
2.2 Effluent high from the start
Cause. The oil is arriving at the separator already broken into fine droplets. This is a pump or pipework problem, not a separator problem. A centrifugal supply pump churns the mixture into droplets under 200 micrometres, and at that size the buoyancy force is too small for the plates to lift them.
Action. Change to a positive displacement pump, or convert the installation to suction mode if the arrangement allows it. Check the pipework for sharp bends and constrictions at the same time, since both shear the oil in the same way.
The diagnostic point is the timing. A separator that performs badly from the moment it starts has a supply-side fault. A separator that performed well and has degraded has an internal or temperature fault.
2.3 Poor separation, oil not rising
Cause. Low temperature. Cold oil is viscous, and the density differential between cold oil and water is smaller. Both effects reduce the rising velocity.
Action. Open the steam heater valves and bring the vessel up to about 60 °C. Where the ship carries high-viscosity oils, additional coils in the lower part of the vessel are fitted for exactly this reason. Running cold is the commonest avoidable cause of a tripped discharge.
2.4 No automatic oil discharge
Cause. The automatic drain depends on the capacitance probe sensing oil, and on the control air reaching the diaphragm valve through the solenoid pilot. A fouled probe reads water when it is sitting in oil; a faulty solenoid or a lost control air supply leaves the pilot valve in the wrong position.
Action. The oil sensor probe must be cleaned. Then check the solenoid pilot valve and the control air supply. In the meantime, drain manually through the test cocks — this is what they are for.
2.5 Water carry-over, priming-over
Cause. The interface level control has been lost, and water is being carried out with the oil, or the vessel has been over-primed.
Action. The third probe, fitted low down in the clean water space, is the emergency cut-out for exactly this condition. Check the probe and its circuit. Then check the interface position through the test cocks and restore it.
2.6 The alarm sounds and the discharge diverts
Cause. This is normal function, not a fault. The three-way solenoid valve has operated because the oil content exceeded 15 p.p.m., and the discharge has been diverted to the bilge tank.
Action. Open the oil drain cocks, check the oil control level, and stop the pump — the order of those actions is in Operating the Oily Water Separator, section 3. Then find out why the oil content rose — see section 2.1, section 2.2 and section 2.3.
The trap here is treating a working safety device as a nuisance. The alarm is the plant telling the truth.
2.7 The separator drains itself overboard when stopped
Cause. A siphon has formed in the discharge line. When the outside head is less than the inside head, the siphon will carry the contents overboard with the machine shut down and the overboard valve closed.
Action. Fit or check the siphon breaker — the anti-siphon valve or vented loop. Air must be able to enter the top of the discharge loop so that the water level drops away from it on both sides.
2.8 Sludge or coalescer clogged
Cause. Contaminant build-up on the plates and in the coalescer elements. A separator that is not desludged loses plate area and coalescing surface until it can no longer reach 15 p.p.m. at any throughput.
Action. Remove the internal contaminant, clean the catch plate, and renew or clean the coalescer element as applicable — whether it is renewable or disposable depends on the type fitted. Then check that the desludging interval is being observed.
2.9 The bilge pump loses vacuum and pumping ceases
Cause. Air ingress on the suction side. The two usual sources are dirt under a valve lid, preventing it from closing, and seal failure at the strainer or mud box covers.
Action. Clean the valve seating. Replace the covers carefully, preferably with a new joint or gasket — reusing a hardened or damaged gasket simply recreates the fault.
2.10 Leak in the bilge piping
Cause. Internal and external corrosion, usually at the lowest point of the line, where water and sludge collect.
Action. Isolate and blank off the affected sections and back flood with water under pressure — this is done by removing a bilge suction valve and flooding back from the sea through the valve chests. Take extreme care that the hold bilge valves are properly closed while doing it, or the flood water goes into the hold instead of the pipe.