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Separator process control
A separator operates through a continuous, rather than a batch, process. This means that the inlet stream constantly flows into the separator and that the gas and liquid must be removed at the same rate.
For liquids, this is done by means of a level controller and level valve. The traditional level controller consists of a float on a spring. As the liquid level in the separator rises, the float rises until it closes a switch, which then opens the level valve to let out some liquid. When the level falls back down to the normal operating level, the switch opens again and drives the level valve closed. A two-phase separator uses a single liquid-level controller and level valve; a three-phase separator will have both an oil outlet with an oil-level controller and level valve and a water outlet with a water-level controller and level valve.
If the level valves control the liquid coming out of the separator, how is the gas controlled? Because the liquid is incompressible and the liquid level in the separator remains fairly constant, the gas is contained in an approximately constant volume. As more gas enters the separator, the pressure rises. A pressure controller is mounted on the separator-gas space or on the outlet-gas piping. The controller sends a signal to the pressure-control valve in the gas-outlet piping telling it to open when the pressure is higher than the set point. Pressure-control valves are usually modulating, which means that they gradually open wider as the pressure rises to a value higher than the set point and close as the pressure falls to a value lower than the set point.
In short, whatever amount of liquid comes into the separator, an equal amount must exit through the level-control valve. The level controller senses whether the liquid level is high or low and adjusts the level valve accordingly. Whatever amount of gas that comes in the inlet of the separator, an equal amount of gas must exit through the pressure-control valve. The pressure controller senses pressure in the separator, opening the pressure-control valve if the pressure gets higher than the desired set point and closing it if the pressure gets lower than desired. If the inlet stream shuts off, the outlet valves would all close, maintaining the pressure and level in the separator.
Detailed information on instrumentation and controls, including control-valve selection.
If the process-control system operates correctly, operators use all manual valves correctly, and nothing breaks, there is no need for a safety system. However, controllers malfunction, valves leak, and operators make mistakes. The safety system is there to prevent:
- overpressure and possible rupture of equipment
- injury to personnel
- damage to equipment
RP 14C provides a systematic way to ensure that all necessary safety equipment is in place. Two levels of protection normally exist in a safety system: primary and secondary.
The primary protection is usually a sensor or switch on the equipment that detects the undesirable event. For example, equipment may have a pressure, level, or temperature switch to detect values that are too high or too low, based on the normal operating ranges. Once the undesirable event is detected, a safety shutdown system is required to shut down flow into the affected equipment.
In the event the primary protection fails to operate or operates too slowly to correct a problem, there is secondary protection consisting of a pressure safety valve (PSV) to prevent overpressure. A PSV is designed to open, relieving overpressure in a vessel or piping through “relief header” piping that directs the relieved fluids to a safe place for retrieval or disposal. Alternatively, secondary protection may consist of redundant sensors or switches, such as those used for primary protection, which may be located on downstream equipment or on the equipment in question.
A separator with a given operating pressure will have a “design” pressure or “maximum allowable working pressure” (MAWP) sufficiently greater than the operating pressures to prevent small fluctuations in the process from causing overpressure of the pressure vessel. As an example, in the staged-separation process, the operating pressure of each downstream separator will be lower than that of the separator flowing into it. This allows the system-design pressure to be reduced as well. When a higher-design-pressure system flows into a lower-design-pressure system, there is potential for overpressuring the downstream, lower-pressure-rated system. With multistage separators, the different operating pressures often lead to a different design pressure for the HP, IP, and LP separators and their associated piping. This introduces a hazard commonly referred to as “gas blowby.” For example, if the liquid-level valve were to stick open, the liquid would flow out of the separator and the gas would “blow by” the liquid-control valve until the pressure equalized between the upstream and downstream separators. This equalized pressure could be higher than the design pressure of the downstream separator.
Safety systems must be designed to protect the lowest-pressure system in situations like the one outlined previously. Relief valves are normally provided on pressure vessels to protect against overpressure caused by “blocked discharge,” which occurs when all outlets to the vessel are closed because of blockage or system shutdown. Relief valves must also be adequately sized to protect against overpressure caused by blowby. The gas-blowby rate may exceed the HP-system inlet flow rate for a short time because the HP separator is being blown down, in an uncontrolled manner, to the lower-pressure system. The flow rate must be calculated based on the upstream pressure, the control valve capacity at full open, any other flow restriction in the piping, and the downstream-vessel relief-valve set pressure. The calculated flow can then be used to adequately size the relief valve.
If the pressure difference between the two vessels is very large, the blowby rates will be correspondingly large. Consider, for example, an HP separator with a 1,480-psig MAWP in which the liquid flows to an atmospheric storage tank. The absolute pressure in the HP separator is 100 times that in the atmospheric storage tank (14.7 psia). Gas blowby from the HP separator expands to 100 times the original gas volume when it goes to atmospheric pressure. If the liquid-control valve from the separator has a 2-in.-diameter opening (3.14 in.2), the vent on the tank must have 100 times the area to pass the same amount of gas (3.14 in.2 or a 20-in. diameter vent). It is not a good idea to have an HP vessel dumping liquid to an atmospheric tank. This demonstrates yet another advantage of staged separation—reducing the amount of gas blowby possible between any two pressures.
In addition to primary and secondary protection for the process, an emergency support system is used to minimize the effects of escaped hydrocarbons. This system includes combustible gas detectors, fire detectors, smoke detectors, a containment system to collect leaking liquid hydrocarbons, and an emergency shutdown system to provide a method for the process-control system to initiate a platform shutdown. An in-depth discussion is presented in Safety systems.
- API RP 14C, Recommended Practice for Analysis, Design, Installation, and Testing of Basic Surface Safety Systems for Offshore Production Platforms, seventh edition. 2001. Washington, DC: API.
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