A skim tank is the first vessel in most produced water plants, and its job is deceptively simple: give the water enough quiet residence time for the small amount of oil still carried over from upstream separators to float to the surface, where it can be skimmed off. It is not a bulk separator like a gun barrel or a free water knockout - by the time water reaches the skim tank, most oil has already been removed, and the skim tank is recovering the low-concentration remainder before the water moves on to flotation, filtration, or disposal. This guide explains what a skim tank does, how its level and oil-water interface are controlled, and how skim-oil recovery is monitored.
Skim Tank in one line: A skim tank is a horizontal or vertical gravity-separation vessel that removes residual free oil from already-separated produced water by letting the oil rise and collect at the surface, where a weir, skim nozzle, or oil box captures it for recovery while cleaner water is drawn off lower down.
Produced water entering a skim tank still contains a few hundred to a few thousand ppm of dispersed and free oil that the upstream three-phase separator or FWKO did not fully remove. The skim tank slows the flow dramatically, giving buoyancy time to work: because oil is less dense than water, droplets rise, coalesce with one another at the surface, and build a distinct oil layer. The design variable that governs this is retention time - the number of minutes a given barrel of water spends in the vessel - and it is set by tank volume divided by throughput. More retention time lets smaller droplets reach the surface, which is why skim tanks are sized generously and why sudden flow surges hurt performance.
The recovered oil layer floats above a water leg and is removed through an overflow weir into a skim-oil box or through an adjustable skim nozzle set just below the oil surface. That skim oil is usually pumped back to the front of the process to be re-treated with the incoming production, so nothing is wasted. Clean water leaves through an outlet positioned well below the oil-water interface, often behind an internal baffle or weir that keeps the floating oil from short-circuiting to the water outlet.
A skim tank has to hold two things steady at once: the total liquid level and the position of the oil-water interface. If the water level rises too high, oil can carry over the water outlet weir and contaminate the downstream flotation or filter units; if it drops too low, the water pumps can lose suction. Level is typically held by an outlet control valve or a level-controlled transfer pump, driven by a level transmitter reading the water leg. Many atmospheric skim tanks run at a simple overflow weir height instead, letting gravity fix the level mechanically.
The oil-water interface is the harder measurement. Because the oil layer is thin and the fluids are similar in density, operators use interface level transmitters - capacitance, displacer, or guided-wave radar with dual measurement - or a series of manual sample taps up the side of the tank to find where oil ends and water begins. Skimming is triggered when the oil layer grows to a target thickness, either continuously through a fixed weir or intermittently by opening a skim valve. Getting this wrong is expensive: skim too aggressively and you send water back into the oil recovery loop, skim too little and oil escapes downstream.
In a monitored water plant, the skim tank's level transmitter, interface transmitter, and inlet flow feed a PLC or RTU that manages the outlet valve, the transfer pump, and the intermittent skim valve. The controller digitizes those readings into tags - water level, interface position, skim-oil recovered - and a cloud SCADA platform polls them so operators can see at a glance whether the tank is holding its band or drifting. Trending inlet flow against interface position over a shift quickly reveals when a slug of oil has arrived and the skim cycle needs attention.
Merobix, as a cloud-native SCADA, reads those already-digitized skim tank tags from the site PLC, RTU, or flow computer over a protocol such as Modbus or DNP3 - it does not wire into the raw transmitter loops. From that data it can alarm on a rising interface, a high water level that threatens carryover, or a stalled skim cycle, and it can trend skim-oil recovery so a remote operator confirms the tank is doing its job without standing on the catwalk. That visibility matters because the skim tank protects every polishing step downstream of it, from flotation cells to injection filters.
A gun barrel is a wash tank that separates bulk oil and water on the production side, treating a stream that is still largely oil. A skim tank sits in the water plant and treats a stream that is already mostly water, recovering only the residual oil that carried over. In short, the gun barrel makes clean oil, and the skim tank makes clean water.
Retention time is how long a barrel of water stays in the tank, and it directly controls how small an oil droplet can rise to the surface before the water leaves. Longer retention time removes smaller droplets and lowers the oil concentration in the outlet water. If flow surges and retention time falls, oil that would have floated out instead escapes to the next process step.
The recovered oil layer is usually pumped or gravity-drained back to the front of the facility so it can be re-treated with fresh production and eventually sold. Some operations route it to a slop or reclaimed-oil tank first. Either way the goal is to keep the recovered hydrocarbons in the process rather than losing them to the water system.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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