3-Phase Separator or 4-phase Separator / Well Test Separator is an instrumented pressure vessel designed to efficiently separate well effluent into oil, gas and water, sand if 4 phase separator, for onshore and offshore well testing. The test separator can operate as a stand-alone unit or in combination with the surge tank, reducing the dependency on the separation process for high-quality flow measurements.

Test separator typically consists of a vessel, an oil flow-measuring system with dual meters, a flow-measuring system for gas, several sampling points for each effluent phase, and two relief valves to protect the vessel against overpressure. Most separators are also equipped to measure water flow rate. To provide accurate measurements, the test separator is fitted with pneumatic regulators that maintain a constant pressure and a constant liquid level inside the vessel by control valves on the oil and gas outlets.

The test separator is fitted with a deflector plate, coalescing plates, a foam breaker, a vortex breaker, a weir plate, and a mist extractor. These components reduce the risk of carryover (liquid in gas line) and carry under (gas in liquid line) that would affect the flow rate measurement accuracy.