A separator for petroleum production is a large vessel designed to separate production fluids into their constituent components of oil, gas and water. A separating vessel may be referred to in the following ways: Oil and gas separator, Separator, Stage separator, Trap, Knockout vessel (Knockout drum, knockout trap, water knockout, or liquid knockout), Flash chamber (flash vessel or flash trap), Expansion separator or expansion vessel, Scrubber (gas scrubber), Filter (gas filter). These separating vessels are normally used on a producing lease or platform near the wellhead, manifold, or tank battery to separate fluids produced from oil and gas wells into oil and gas or liquid and gas. An oil/gas separator is a pressure vessel used for separating a well stream into gaseous and liquid components. They are installed either in an onshore processing station or on an offshore platform. Oil and gas separators can operate at pressures ranging from a high vacuum to 4,000 to 5,000 psi. Most oil and gas separators operate in the pressure range of 20 to 1,500 psi. Separators may be referred to as low pressure, medium pressure, or high pressure. Low-pressure separators usually operate at pressures ranging from 10 to 20 up to 180 to 225 psi. Medium-pressure separators usually operate at pressures ranging from 230 to 250 up to 600 to 700 psi. High- pressure separators generally operate in the wide pressure range from 750 to 1,500 psi. Based on the vessel configurations, the oil/gas separators can be divided into horizontal, vertical, or spherical separators. In teams of fluids to be separated, the oil/gas separators can be grouped into gas/liquid two-phase separator or oil/gas/water three-phase separator. Based on separation function, the oil/gas separators can also classified into primary phase separator, test separator, highpressure separator, low-pressure separator, deliquilizer, degasser, etc. To meet process requirements, the oil/gas separators are normally designed in stages, in which the first stage separator is used for priliminary phase separation, while the second and third stage separator are applied for further treatment of each individual phase (gas, oil and water). TYPES OF SEPERATOR There are three common type of process separator: horizontal separator, vertical separator, or spherical separator. HORIZONTAL SEPARATOR TWO-PHASE WORKING PRINCIPLE The first step separation occurs at the inlet diverter, on the inlet diverter liquid and vapor will be separated as the fluid enter vessel and hits the inlet diverter that will create sudden change in momentum. The liquid droplet will fall out from the gas stream into the bottom section. Since it will take times to collect the amount of liquid, this section will provide the retention time. Retention time is needed to vapor the entrained gas in the liquid. If there are slugs in the liquid, this retention time provides a surge volume to handle the slugs. The extracted liquid then flows through an outlet valve which regulated by a level controller. The gas part will flow over the inlet diverter into the gravity settling section which located above the liquid. The gas may entrain some liquid; this liquid will be separated out by gravity in this gravity settling section and it will fall down to the gas liquid interface. Before the gas leaves the vessel, it will pass through a coalescing section. The smaller drop which can’t be separated in the gravity settling section will be trapped in the mist extractor. The mist extractor or coalescing section consists of vane element to remove the small droplet. The pressure inside the vessel is controlled by a pressure controller. If the pressure too high, pressure controller will give a signal to open the pressure control valve so the gas can leave from the vapor space. To maximize the gas liquid interface area, horizontal separator normally operated at half full. HORIZONTAL TWO-PHASE SEPARATOR HORIZONTAL THREE-PHASE SEPARATOR WORKING PRINCIPLE In a horizontal separator, fluid enters the vessel through an inlet, and immediately hits an inlet diverter. This sudden impact provides the initial separation of liquid and vapor. In the liquid collection section of the vessel, the oil and and emulsion separate, forming a layer (or “pad”) above the free water. A weir maintains the oil level, while an interface liquid level controller maintains the water level. The oil spills over the top of the weir, and then a level controller, which operates the oil dump valve, controls its level. An interface level controller also senses the height of the oil-water interface. This controller signals another dump valve to release as much water from the vessel as is needed to maintain the oil-water interface at the pre-determined height. Meanwhile, gas rises to the top of the separator. It flows horizontally and exits thru a mist extractor to a pressure control valve, which maintains constant vessel pressure. HORIZONTAL THREE-PHASE SEPERATOR VERTICAL SEPARATOR TWO-PHASE WORKING PRINCIPLE In this vertical separator configuration the fluid enters the separator through the side. The initial separation process occurs at the inlet diverter causing the liquid droplets falls down to the collection section. The liquid will flow down to the outlet; the entrained gas bubbles in the liquid will flow upward and leaves to the vapor space as the liquid reaches equilibrium. The level controller will maintain the liquid gas interface level; if it goes to high the level controller will give an open command to the outlet valve. The gas that separated from the liquid will flows over the inlet diverter and continue upward to the gas outlet. The entrained liquid in the gas will form droplets and falls to the liquid gas interface section. Before the gas leaves the vessel, it will flow through a mist extractor to clear the gas from remaining liquids. To maintain the pressure, pressure controller will sense the pressure and control it by giving command to the gas outlet control valve. VERTICAL TWO-PHASE SEPARATOR VERTICAL THREE-PHASE SEPARATOR WORKING PRINCIPLE In a vertical three-phase separator, flow enters the vessel through a side inlet as well, and is immediately met by an inlet diverter. This impact begins the separation process. A downcomer transmits the liquid through the oil-gas interface. A chimney equalizes gas pressure between the lower section and the gas section. The “spreader,” or downcomer outlet, is located at the oil-water interface. As the oil rises from this point, any free water separates out from the oil phase. The water droplets flow down through the oil. As the water flows downward, oil droplets trapped in the water phase rise up through the water flow. VERTICAL THREE-PHASE SEPARATOR OPERATION AND MAINTENANCE CONSIDERATIONS FOR OIL AND GAS SEPATATOR Over the life of a production system, the separator is expected to process a wide range of produced fluids. With breakthrough from water flood and expanded gas lift circulation, the produced fluid water cut and gas-oil ratio is ever changing. In many instances, the separator fluid loading may exceed the original design capacity of the vessel. As a result, many operators find their separator no longer able to meet the required oil and water effluent standards, or experience high liquid carry-over in the gas according to Power et al (1990). Some operational maintenance and considerations are discussed below: Periodic Inspection In refineries and processing plants, it is normal practice to inspect all pressure vessels and piping periodically for corrosion and erosion. In the oil fields, this practice is not generally followed, and equipment is replaced only after actual failure. This policy may create hazardous conditions for operating personnel and surrounding equipment. It is recommended that periodic inspection schedules for all pressure equipment be established and followed to protect against undue failures. Installation of Safety Devices All safety relief devices should be installed as close to the vessel as possible and in such manner that the reaction force from exhausting fluids will not break off, unscrew, or otherwise dislodge the safety device. The discharge from safety devices should not endanger personnel or other equipment. Low Temperature Separators should be operated above hydrate-formation temperature. Otherwise hydrates may form in the vessel and partially or completely plug it thereby reducing the capacity of the separator. Corrosive Fluids A separator handling corrosive fluid should be checked periodically to determine whether remedial work is required. Extreme cases of corrosion may require a reduction in the rated working pressure of the vessel. Periodic hydrostatic testing is recommended, especially if the fluids being handled are corrosive. Expendable anode can be used in separators to protect them against electrolytic corrosion . Some operators determine separator shell and head thickness with ultrasonic thickness indicators and calculate the maximum allowable working pressure from the remaining metal thickness. This should be done yearly offshore and every two to four years onshore.
