Discussion on separator knowledge

Screening: Screens. Sedimentation: Dust collection chambers, sedimentation tanks, cyclone separators, hydrocyclones. Filtration: Pressure filters, leaf filters, rotary drum filters. Centrifugal separation: Drum centrifuges, disc centrifuges, tubular centrifuges

We often design oil-water separators, which are mainly used for separating oil and water from gases.

Air flotation could probably be considered one as well, right?

To maintain good separation performance, the separator requires control of its liquid level and pressure. The control of liquid level and pressure in traditional separators employs constant pressure control technology. In the variable-pressure level control of the separator, a float level controller is used to drive the oil and gas control valves, enabling them to operate together in order to regulate the volume of crude oil and natural gas and thus control the liquid level within the separator, without affecting the pressure in the separator. The variable-pressure level control method can minimize throttling at the oil-gas outlet valve, reduce the pressure in the separator, and improve separation efficiency. Oil-gas separators and oil-gas-water three-phase separators are widely used in oilfield transfer stations and combined stations. To maintain good separation performance, the separator requires control of its liquid level and pressure. From the perspectives of reducing throttling losses in the process flow, saving energy and reducing consumption, as well as improving separation efficiency, this paper analyzes the control methods for liquid level and pressure in traditional separators, and proposes a simple, reliable pressure-variable liquid level control method for separators that helps to reduce energy consumption. (1) Control of liquid level and pressure in traditional separators 1.1 Oil-gas two-phase separator The oil-gas two-phase separator separates the oil-gas mixture into crude oil and natural gas, each in a single phase. The pressure is controlled by a pressure control valve located at the natural gas outlet, while the liquid level is adjusted by an oil outlet valve controlled by a controller. The pressure control valve at the natural gas outlet is usually a self-acting control valve, or a pneumatic diaphragm control valve equipped with a pressure transmitter, controller, and air supply. Oil discharge valves are typically pneumatic diaphragm control valves or float-type level regulators that are operated in conjunction with associated level sensors, controllers, and air sources as part of the oil discharge control system. Some oil-gas two-phase separators use pneumatic diaphragm control valves to regulate the pressure in the separator, and float level controllers to operate the oil outlet valve and control the liquid level in the separator. 1.2 Oil-Gas-Water Three-Phase Separator: The oil-gas-water three-phase separator enables the separation of gas and liquid from oil well outputs, while also separating some of the water contained in the crude oil. With the development of oil fields, the water content in the fluid produced by oil wells has gradually increased, leading to a greater use of three-phase separators. Due to differences in structure, the control methods for three-phase separators also vary. The control principles of the two typical separators are as follows: (1) After the oil-gas-water mixture enters the separator, the inlet splitter divides the mixture roughly into vapor and liquid phases, with the liquid phase entering the liquid collection section. The collection section has sufficient volume to allow free water to settle to the bottom and form a water layer, with crude oil and an emulsified oil layer containing smaller water droplets above it. Crude oil and emulsified oil overflow from the baffle. The oil level downstream of the baffle is maintained at a constant height by the oil outlet valve, which is controlled by a level controller. Water is discharged from the outlet located upstream of the baffle, and the oil-water interface controller adjusts the opening degree of the drain valve to maintain the oil-water interface at a specified height. The pressure of the separator is controlled by a valve installed on the natural gas pipeline. (2) The separator is equipped with an oil tank and a water baffle. Crude oil overflows from the oil baffle into the oil tank, and the oil level in the tank is controlled by an oil outlet valve operated by a level controller. Water flows under the oil tank and enters the water chamber through the water baffle; the liquid level in the water chamber is controlled by a discharge valve operated by a level controller. (II) Problems existing in the level and pressure control of traditional separators: In the constant-pressure control of separators, pressure control valves on the natural gas pipeline throttle the natural gas to a certain extent in order to maintain stable pressure inside the separator. When the gas volume decreases or the pressure at the gas outlet drops, the throttling degree of the valve increases ； Conversely, the degree of throttling by the valve decreases. In the separator level control, the oil-water outlet valves also throttle the liquid. As the liquid volume increases, the degree of throttling decreases ； When the liquid level is low, the throttling is increased to keep the liquid level stable. To ensure proper drainage even when there is a large volume of liquid, the separator has a high pressure. However, as the liquid level decreases, it is necessary to throttle the liquid through the oil-water outlet valve to prevent the liquid level from dropping too low. Therefore, in production, the separator generally operates at high pressure, with the liquid-phase valve in a throttling state. Excessively high separator pressure affects the inflow to the separator, resulting in increased backpressure at the output ports of the transfer station or metering station as well as at the wellhead, which hinders oil transportation. At present, most oil wells in our country use mechanical extraction methods, which leads to an increase in backpressure at the wellhead and thus raises the energy consumption required for oil extraction. Furthermore, at higher pressures, the saturated dissolved gas in the oil separates from it when the pressure drops due to throttling by the oil outlet valve, which can easily cause gas contamination in the oil pumps in the downstream process. Therefore, a higher separator pressure not only affects the separation efficiency of oil and gas and increases production energy consumption, but also impacts safe production. (III) Variable-pressure level control: The float level controller drives two control valves; one valve controls natural gas, while the other controls crude oil, thereby enabling simultaneous regulation of the valves at the outlets for crude oil and natural gas. When the float rises, the linkage mechanism reduces the opening of the air circuit control valve and increases the opening of the oil circuit control valve ； Conversely, when the float descends, the linkage mechanism will increase the opening of the air circuit control valve and decrease the opening of the oil circuit control valve. The liquid level of the separator is controlled by changing the opening degree of the control valve, thereby altering the relative flow rates of natural gas and crude oil. This control method does not perform fixed-value control on the pressure of the separator; the pressure of the separator is equal to the sum of the pressure at the natural gas outlet or the liquid outlet and the pressure difference before and after the natural gas control valve or the liquid control valve. When the gas and liquid volumes as well as the pressure downstream of the separator change, the pressure of the separator also changes; therefore, this control method is known as variable-pressure control. 3.1 Application of variable-pressure level control in oil-gas two-phase separators: When the volume of liquid and gas entering and leaving the separator remains constant, the liquid level stays at a certain position ； When the amount of liquid or gas entering the separator changes, causing the liquid level to rise, the float linkage mechanism will reduce the opening of the natural gas control valve and increase the opening of the crude oil control valve, thereby reducing the amount of gas discharged and increasing the amount of liquid discharged. Once the amount of liquid and gas entering the separator is equal again, the liquid level will stabilize at a higher position than before ； When the amount of liquid or gas entering the separator changes, causing the liquid level to drop, the float linkage mechanism will increase the opening of the natural gas control valve and decrease the opening of the crude oil control valve, thereby increasing the exhaust volume while reducing the liquid discharge volume. Until the amount of liquid and gas entering and leaving the separator is equal, the liquid level will stabilize at a lower position than before. In this way, as the amount of liquid or gas entering the separator changes, the float linkage mechanism drives the control valve to perform corresponding actions, thereby keeping the liquid level relatively stable. 3.2 Application of variable-pressure level control in oil-gas-water three-phase separators (1) Application of variable-pressure level control in oil-gas-water three-phase separation. The control of the crude oil level is the same as that of the liquid level in the oil-gas separator; the oil-water interface is controlled by a drain valve operated by an oil-water interface controller. (2) Application of variable pressure level control in oil-gas-water three-phase separators. The liquid level in the oil tank is controlled by the crude oil control valve and the natural gas control valve, which are operated by the liquid level controller; the liquid level in the water tank is controlled by the outlet control valve and the natural gas control valve, also operated by the liquid level controller. According to CNPC News, a device for efficient four-phase separation of oil, gas, water, and sand has been successfully developed recently at the Henan Oilfield, and tests on-site have shown good results. The produced fluids from heavy oil fields have poor physical properties: high levels of gel and asphaltenes, high density, high viscosity, large sand content, and difficulty in dewatering, which can easily cause equipment blockages. Previous surface gathering and transportation processes employed multiple stages, which were complex; they resulted in high energy consumption and low efficiency during system operation ； The gathering and transportation system is not airtight, the equipment operates unevenly, resulting in high losses of oil and gas. The most critical issue is the low efficiency of the dehydration equipment, which fails to effectively address the problem of sand clogging. The four-phase separator builds on the three-phase separator by incorporating technologies such as pre-degassing, water washing for demulsification, rectification, and interface control used in three-phase separation. It also features a new type of coalescing filler – a one-way flow channel micro-porous rolled corrugated plate – which overcomes the problems of clogging caused by sand-containing heavy oil fillers and the inability to use fillers for the dehydration of heavy oil. The four-phase separator significantly improves the equipment’s dewatering efficiency and enables sand removal without shutting down production. The new generation of high-efficiency separators also features a compact structure, high processing capacity, well-defined functions for each component, high effective utilization of the equipment, excellent separation performance, and a high degree of automation. Their processing capacity per unit volume is 6 to 8 times that of traditional devices. The new equipment utilizes layer heating technology, enabling the successful degassing and dewatering of heavy oil with high water content in a single stage; this simplifies the process and reduces engineering costs.