OIL AND GAS SEPARATION PROCESS

1. introduction

When oil and gas are extracted from a well, they are accompanied by various other substances such as condensates, water, and contaminants. To separate these components and obtain desirable fractions, an oil/gas separator is employed. This pressure vessel can be found in onshore processing stations or offshore platforms, and it can be categorized as a horizontal, vertical, or spherical separator based on its configuration. The separators are further classified as two-phase separators (gas/liquid) or three-phase separators (oil/gas/water). In this article, we will focus on the three-phase separator and discuss its Piping and Instrumentation Diagram (P&ID) in detail.

The first stage of the separation process involves reducing the pressure in multiple stages to enable controlled separation of volatile components. This step is crucial for achieving maximum liquid recovery, stabilizing the oil and gas, and separating the water. A single separator with a large pressure reduction can lead to flash vaporization, which can cause instabilities and safety hazards. The first stage separator typically has a retention period of 5-8 minutes, allowing the gas to bubble out, water to settle at the bottom, and oil to be extracted from the middle. The water content in the first stage separator is usually reduced to less than 10-5%.

2. Piping

Piping is a system of interconnected pipes that transport fluids, including liquids and gases, from one location to another. The design of the piping system aims to ensure the efficient and reliable transportation of fluids.

2.1. Inlet Pipe for the Well Fluids

The well fluids, comprising oil, gas, and water, enter the first stage separator vessel through the inlet pipe. A gate valve is installed along the pipeline at the entrance of the vessel to control the flow of the fluid. Figure 3 illustrates the flow diagram of this pipe.

2.2. Flare System (Gas Section)

When the pressure inside the separator vessel exceeds the set limit, the relief valve opens to release a portion of the fluid, which can be gas or a liquid-gas mixture. This diverted fluid is then directed through a piping system known as a flare header or relief header to a central elevated gas flare. In the flare system, the fluid is usually burned, and the resulting combustion gases are released into the atmosphere. This process helps maintain the safety of the separator system. (Refer to Figure 4 for the diagram of the flare system.)

2.3. Outlet Pipe for the Separated Gas (Vapor)

The gas separated from the well fluid mixture is directed through a pipe equipped with various valves, including a pressure valve that acts as an actuator valve. The actuator valve works in conjunction with a pressure transmitter (PT) and a control (PC) to monitor and regulate the pressure of the gas flowing through the piping system.

2.4. Outlet Pipe for the Separated Oil

The separated oil from the well fluid mixture is directed through a pipe equipped with several valves, including a level valve that acts as an actuator valve. The actuator valve works in conjunction with a level transmitter (LT) and a control (LC) to monitor and regulate the flow of oil through the piping system. The same system is applied to the outlet pipe for the separated water.

4. Separator Vessel

The separator vessel is a critical component of the oil and gas separation process. It consists of several major components that facilitate efficient separation. Figure 8 shows the external view of the separator vessel, while Figure 9 illustrates its internal view with the major components.

The separator vessel includes a baffle slug catcher at the crude entrance to mitigate the effect of slugs (large gas bubbles or liquid plugs). Some turbulence is desirable in this section as it helps release gas

5. System Control and Instrumentation

In the oil and gas separation process, various control and instrumentation components play a crucial role in ensuring the proper functioning and monitoring of the system. Here are some key components involved:

5.1. Gate Valve

Gate valves are primarily used to start or stop the flow of well fluids when a straight-line flow with minimum flow restriction is required. They are typically kept fully open or fully closed during operation. When the gate valve is opened, the disc is completely removed, allowing the contents to pass through. Although gate valves provide good shut-off properties and are bidirectional, they cannot be quickly opened or closed and are sensitive to vibration when open. On the other hand, a spectacle blind flange is a safety device used for isolating a section of a pipeline or equipment during inspection or maintenance. It differs from a valve as it is a permanent or long-term isolation device, with one end having an opening for flow during operation and the other end being solid to block flow during maintenance.

5.2. Relief Valve:

Relief valves are essential for controlling or limiting the pressure within a system or vessel to prevent it from exceeding safe limits. These valves are designed to open at a predetermined set pressure to protect pressure vessels and other equipment from excessive pressures caused by process upsets, instrument or equipment failures, or fires.

5.3. Actuators (Diaphragm or Globe):

Actuators are mechanisms used to automate valves, eliminating the need for manual operation. They can be remotely controlled and serve as shutdown mechanisms in emergency situations where human interaction could be dangerous. Diaphragm-style actuators consist of a rubber diaphragm and stem housed in a circular steel housing. They are suitable for valves that require shorter travel, such as diaphragm valves and globe valves.

5.4. Ball Valve (Normally Closed):

Ball valves are commonly used in the system, and they typically operate in a normally closed configuration. An increase in fluid pressure to the actuator is required to open the valve.

5.5. Pressure Transmitter:

Pressure transmitters are devices that sense pressure and convert it into a proportional current signal. The signal is then transmitted to a monitor or controller, providing continuous pressure measurement and feedback.

5.6. Pressure Controller (mounted in the control room):

Pressure controllers receive data readings from pressure transmitters and compare them to a programmed setpoint. Based on this comparison, the pressure controller signals a control element, such as an actuator valve, to either open or close, depending on the corrective action required to maintain the desired pressure levels.

5.7. Level Transmitter:

Level transmitters measure the well fluid level inside the separator vessel and provide an electrical output proportional to the measured level. They can operate in different modes, such as providing a point level output when a specified value is reached or continuously measuring and transmitting the level data.

5.8. Level Controller (mounted in the control room):

The level controller receives the level readings from the level transmitter and compares them to a specified setpoint. Based on this comparison, the level controller signals the actuator valves to open or close, regulating the well fluid level within the separator vessel.

6. Additional Points

In addition to the essential components discussed earlier, there are several crucial aspects to consider when it comes to control and instrumentation systems in oil and gas separation processes. These aspects encompass safety considerations, automation and advanced technologies, environmental concerns, continuous process improvement, and regulatory compliance. Understanding these factors is essential for achieving efficient and sustainable separation operations. Let's explore them further:

6.1. Safety Considerations:

The safety of personnel, equipment, and the environment is of paramount importance in oil and gas separation processes. Control and instrumentation systems are designed to incorporate various safety features and protocols to prevent accidents and ensure the well-being of workers. These systems incorporate relief valves that control pressure levels and protect vessels and equipment from exceeding their design limits. Regular maintenance, inspections, and adherence to safety standards are critical for identifying potential hazards and mitigating risks associated with the separation process.

6.2. Automation and Advanced Technologies:

Advancements in automation and digital technologies have revolutionized oil and gas separation processes. Control and instrumentation systems now leverage sophisticated automation features, enabling remote monitoring, data analytics, and predictive maintenance. Real-time monitoring of the separation process allows for prompt detection of anomalies, ensuring quick responses and minimizing downtime. Additionally, advanced technologies optimize system performance, enhance productivity, and contribute to cost-effectiveness by streamlining operations and reducing human error.

6.3. Continuous Process Improvement:

Continuous process improvement is an ongoing goal in the oil and gas industry. Control and instrumentation systems facilitate this by providing accurate data and insights for analysis. Operators utilize this information to identify areas of improvement, optimize resource utilization, and enhance operational efficiency. Continuous process improvement initiatives lead to reduced costs, increased production capacity, and the sustainable development of oil and gas resources. By embracing innovation and refining separation processes, the industry strives for excellence and competitiveness.

6.4.Regulatory Compliance:

Oil and gas separation processes are subject to rigorous regulatory requirements. Control and instrumentation systems must adhere to these regulations to ensure operational safety, environmental protection, and compliance with industry standards. Stringent safety protocols, equipment certifications, and regular inspections are crucial aspects of regulatory compliance. By upholding these standards, the industry ensures the well-being of workers, minimizes environmental impact, and builds public trust.

By considering these crucial factors, control and instrumentation systems in oil and gas separation processes contribute to safer operations, improved efficiency, reduced environmental impact, continuous optimization, and adherence to regulatory frameworks.