Most, if not all, production meters require conditioning of the process stream prior to measurement. The conditioning process allows for solids to drop out of the stream, entrained gas to break out of the stream or oil/water/gas separation. One tool for this is the three phase separator.
Three phase separators are typically horizontal vessels that separate production fluids into separate measurable streams. Vessel internals
Production fluids enter the vessel and are separated using residence time in conjunction with vessel internals. Gas is taken across the top of the vessel. Oil and water are separated and measured at the outlet of the vessel. The fluids may or may not be recombined.
Gas / Vapor Space
Typically there will be a layer of gas at the top of the vessel. The gas can be from a VRU, make up gas or instrument air system. The pressure held on the process prevents certain light ends from breaking out of the production stream prior to measurement.
Gas may or may not be metered on the outlet of the vessel. Gas can be metered with a turbine meter, orifice meter or coriolis meter.
Oil and Water separate and are measured separately. For separation, the driving factor is residence time. The liquids must sit long enough for separation to complete. A good example of this is oil and water in salad dressing. Shake it up and wait ten seconds, a minute and an hour and you will see the separation take place. Note that most of the separation occurs within the first minute or so. Oil /Water Separators can be optimized in the same way.
To ensure good separation, water is taken off the bottom of the vessel and oil 'waterfalls' over a weir located near the front of the vessel. The image at the right depicts a weir box. Oil & Water emulsion fill the vessel. Since the level of fluid will not be higher than the weir, the lighter of the two fluids (in this case the oil) will float over the weir. The size of the oil pad is controlled by the location of the interface.
There are several types of level control in three phase separators. First is the fluid level. This can be accomplished with something as simple as a float level switch or something like a capacitance probe or radar probe.
Second is the level of the oil/water interface. This is important to ensure oil isn't carried down the water leg and water isn't going down the oil leg.