An oil skimmer is chosen by the oil you expect to meet, not by the largest capacity on a datasheet. Light diesel on a calm harbour, weathered crude mixed with seaweed on a beach, and sludge in a separator tank each favour a different design. This guide uses two FOILEX equipment families — portable suction weir skimmers and TDS skimmers with an onboard screw pump — to show the questions that decide the choice.
How a weir skimmer works
A weir skimmer floats on the slick with a collecting ring set just below the oil surface. Oil flows over the weir into a central well and is pumped away. FOILEX skimmers use a three-pontoon floating frame and a flow-controlled, self-adjusting skimming ring, a design the manufacturer has used since 1991. The important difference between models is where the pump sits.
Pump outside: suction weir skimmers
FOILEX MICRO, MINI and MIDI skimmers carry no pump. An external pump draws the oil from the skimmer well through a suction hose. The manufacturer rates them at 15, 30 and 70 m³/h and describes them as suited to low to medium-viscosity spills in harbours, rivers and lakes, where a small portable unit is needed.
Each comes in two versions: a free-floating three-pontoon skimmer, and a fixed-installed well skimmer with vertical discharge, used in oil collectors, ships and industrial separation tanks. That second version is why weir skimmers also matter to waste-management and tank-cleaning operators, not only to spill responders.
Pump on board: TDS skimmers
A TDS skimmer has its own hydraulically driven pump fitted in the floating frame. Instead of relying on a suction hose to pull thick oil, the open pump screw mechanically catches, mixes and cuts oil and debris and feeds it into the pump — in the manufacturer’s words, independent of the suction capability of the pump. That is what lets the same skimmer handle anything from light diesel fuel to heavy crude mixed with debris.
| System | Max. capacity | Diesel power pack | Discharge hose |
|---|---|---|---|
| TDS 150 Coast | 35 m³/h | 20 kW | Ø3″ |
| TDS 200 Sea | 70 m³/h | 32 kW | Ø4″ |
| TDS 250 Ocean | 140 m³/h | 45 kW | Ø6″ |
Inside the Twin Disc Screw pump
The TDS pump starts from the Archimedean screw and adds two circular sealing discs, one on each side of the screw. They work alternately, so the full screw diameter displaces oil for 2 × 180° of every revolution. FOILEX expresses this as a Positive Displacement Index of 1.7, which it states is about 70% higher than traditional Archimedean screw pumps. In practice the extra displacement can be used two ways: high output in low to medium-viscosity oil, or slow, efficient pumping of high-viscosity oil with a minimum of emulsification.
Details that matter in debris
- Exposed inlet. Three 120° inlet sections leave a large rotating screw area open to the oil.
- Cutting tools. Three cutting tools and a circular knife in hardened stainless steel at the inlet cut wood, seaweed, plastics and rope; TDS 200 and 250 add four cutting knives at the outlet. All can be re-sharpened.
- Sealing discs to suit the oil. Polyurethane resists abrasive, sandy oil; nitrile rubber suits aggressive oils, high temperatures and steam. A split pump casing gives direct access for inspection.
Getting the oil to the tank: discharge size and pressure loss
With viscous oil, the hose often limits recovery more than the pump does. FOILEX’s own comparison of relative pressure drop by outlet diameter shows why it is worth keeping the discharge as large as practical:
| Discharge diameter | Relative pressure drop |
|---|---|
| Ø3″ | 100% |
| Ø4″ | 32% |
| Ø6″ | 6% |
TDS pumps take exchangeable discharge cones from Ø3″ to Ø6″. Annular injection flanges, which add water, steam or another diluting liquid around the oil, can cut pressure drop by up to 90% in certain conditions, according to the manufacturer — but a larger hose comes first.
One pump, a second job: emergency off-loading
Any FOILEX TDS skimmer converts to an off-loading pump: lift the pump out of its frame, turn it upside down and fit a vertical discharge. It can then run with a free screw for debris-laden, high-viscosity oil, with a suction pipe to empty low to medium-viscosity oil down to the tank bottom, or with an inline pipe as an inline or booster pump.
| Pump | Minimum manhole diameter |
|---|---|
| TDS 150 | 300 mm / 12″ — fits a standard tanker Butterworth hatch |
| TDS 200 | 420 mm / 17″ |
| TDS 250 | 550 mm / 22″ |
Weir skimmer or TDS skimmer? A quick comparison
| Suction weir skimmer (MICRO / MINI / MIDI) | TDS skimmer (150 / 200 / 250) | |
|---|---|---|
| Pump | External pump via suction hose | Hydraulic screw pump on board |
| Oil | Low to medium viscosity | Light diesel to heavy crude, with debris |
| Capacity | 15 / 30 / 70 m³/h | 35 / 70 / 140 m³/h |
| Typical setting | Harbours, rivers, lakes; fixed well skimmer in separators and collectors | Harbour, coastal and offshore spills; emergency tank off-loading |
| Power | Diesel or petrol pump set | Diesel power pack or ship’s hydraulics |
A checklist before you specify
- Oil type and how it will weather. If heavy crude, residual fuel or emulsion is possible, plan for a screw-pump skimmer.
- Debris. Seaweed, wood, plastics or rope in the slick favour an open screw with cutting tools.
- Where it will work. Harbour, river, coast, offshore, or a fixed tank or separator.
- A realistic rate. Recovered volume depends on slick thickness and conditions, so size against the oil you can actually encounter, not only the datasheet maximum.
- Access. Hatch and manhole sizes if the pump may be used for off-loading.
- Power and logistics. Power pack or ship’s hydraulics, hose lengths, and whether a storage-and-transport container suits your response base.
Recovery is only half of the job: storage for the recovered oil, and the route from skimmer to tank, should be planned at the same time as the skimmer.
Need help matching a skimmer to your scenario? SATU Innovative supplies FOILEX skimmers, pumps and accessories in Thailand. Send us your scenario or see the FOILEX product range.
Sources: FOILEX Engineering AB product catalogue “Oil Spill Response Products” and foilex.com product pages (checked October 2026). Capacities are manufacturer maximum continuous ratings. FOILEX and TDS are names of FOILEX Engineering AB.