Process Control Automation in oil & gas (Downstream): Process Control Automation is the latest and innovative value-adding service in oil & Gas downstream. In the oil and gas industry, it is crucial to regulate process variables to achieve the task of process automation. In the oil and gas Sector, especially downstream, different chemical processes take place to refine crude oil and to produce Liquid Petroleum Gas (LPG), Diesel, Kerosene oil, Petrol, Heavy Gas Oil, Gasoline, Naptha, Lubricating oil, Residual and different carbon components from crude oil.
There are specific chemical processes for converting crude oil into these products, which require proper technology-based control systems. These control systems consist of sensors, transmitters, controllers, control valves, and final control elements to regulate process variables within the desired range and successfully carry out the operation. In oil & gas downstream, the critical process variables which are very important to control include pressure, level, flow, temperature, and density. There is a complete closed-loop of a control system, as shown in the figure, which is used for chemical plant processing and control applications.
Process Control Automation in Oil & Gas (Downstream): Pressure Measurement in Oil & Gas Downstream
- Pressure is the most critical process variable in the oil and gas Industry. It can be described as “Force acting per unit area” and is represented as P = F / A. In this expression, P = Pressure, F = Force, A = Area.
- Pressure is measured in psi (Pounds per square inch) in British units and as Pa (Pascal) in the Metric system. The System International (SI) unit of Pressure is Pascal or N/m2, but in the oil and gas Industry, we use bar or psi.
- The relationship between different pressure units is:
- Unit psi is equal to 7000 Pa
- The unit bar is equal to 14.504 psi
- The unit bar is equal to 100000 N/m2 or Pa
- Unit hPa (hector-Pascal) is equal to 1 bar (milli-bar)
- Pressure is a crucial factor. If we know the pressure of a specific process, we can easily find other process variables. Refinery Chemical processes require that the system operating pressures be controlled to specific pressures for the systems to operate correctly.
- There are three types of pressure:
- The difference between a fluid’s pressure and absolute zero of pressure is known as Absolute Pressure; it can also be referred to as vacuum pressure. Zero reference point of the absolute pressure scale shows the full vacuum.
- Gauge pressure can be defined as the difference between atmospheric pressure, gauge pressure, and the fluid’s pressure, which changes with the change of atmospheric pressure.
- Differential pressure is used to express the difference between two absolute pressure values.
- In Oil & Gas downstream, we use these sensors for the measurement of pressure:
- Bourdon tube sensing element
- Bellow Sensors
- Diaphragm Sensors
- Pressure transmitter
- Load Cell
- Fibre Optic Pressure Sensor
- Capacitive Pressure Sensor
- Rosemount is the most popular vendor, designing Oil & Gas downstream pressure measurement equipment.
Process Control Automation in Oil & Gas (Downstream): Flow Measurement in Oil & Gas Downstream
- Flow is also an essential process variable in the oil and gas downstream. To achieve the goal of automation in an oil refinery, it is necessary to control the flow of pipelines and chemical process plants. Many instruments measure Oil and Gas flow, and flow measurement accuracy varies for each instrument and application.
- There are several factors that affect the flow of fluids through the pipe and the measurement of oil & gas flow. These include
- Velocity of fluid
- Pipe Size
- Friction due to the pipe
- Viscosity of fluid
- The specific gravity of the fluid
- Velocity Profiles
- Fluid, with its volume passing through a specific point in a certain amount of time, is called flow rate. It can be measured in gallons/minute (GPM) or litres per minute. However, in the oil and gas industry, we mostly use gallons per minute.
- Flow rate varies from pipe to pipe, 1 gpm = 6.309 * 10-5 m3/s.
- The complete flow is the volume of fluid passing over an extended period of time, and it can be calculated in different units, such as litres, gallons, cubic feet, etc.
- Measuring the volume and mass flow rates is used to measure the rate at which the fluid flows through a closed pipe. The mass of substance passes through a surface per unit time, known as the Mass flow rate. The unit is mass over time, kg/t. The volume of fluid that passes through a particular surface per unit time is known as the Volume flow rate. Units: e.g., m3/s. A volumetric flow meter measures volume directly, while mass flow meters measure mass directly.
- In Oil & Gas downstream, we use these sensors for the measurement of flow:
- Differential pressure flow meters
- Orifice Plate
- Venturi-tube
- Turbine flow meters
- Electromagnetic flowmeters
- Ultrasonic flowmeters
- Bluff Body flowmeters
- Positive Displacement flowmeters
- Flow Transmitter
- Microprocessor-based volumetric flowmeters
- Coriolis flowmeters
Level Measurement in Oil & Gas Downstream
The level is a critical process variable of oil & gas downstream. The position of an interface between two different or the same mediums, such as liquid and gas or between two liquid mediums, is known as Level Measurement. For controlling and accounting at the level is a crucial variable used in every oil & gas Industry. We measure level in meters or percentage; sometimes we also measure level in feet. The level is the name of two interfaces. We can also calculate level by calculating density in a few applications.
Three main principles that Level Measuring Devices follows:
- Position of the liquid surface
- Pressure Head
- Weight or density of the material
Direct Method
- The high point above a zero point is measured by the direct method of level measurement through different methods. The actual level is mainly focused here; it is convenient in various facets such as its high reliability and reasonable price for any industry. These methods are especially suitable for hazardous areas due to their advanced safety measurements. Therefore, its usage is highly recommended in oil & gas level measurement. These methods include:
- Dipstick method
- Weighted Gauge Tape
- Sight Glass
- Floats
Indirect Method
Indirect level measurement refers to the measurement of level through some property or parameter of liquid, like pressure, density, conductivity. These methods include
- Hydrostatic Pressure Method
- Hydrostatic Differential Pressure Method
- Dry leg method
- Wet Leg Method
- Torque tube Method
- Conductivity Level Method
- Level Transmitters
Temperature Measurement in Oil & Gas Downstream
Temperature is also an important process variable in oil and gas downstream. Controlling the temperature of the chemical process is crucial to achieving the goal of automation—in oil and gas downstream, all chemical processes are heat-dependent. Different specialized instruments are used to measure temperature in industry. These include thermocouples and a thermistor. We measure temperature in Kelvin, °C, or °F.
Thermocouples are the most widely used temperature sensors in the chemical industry and oil & gas downstream; they are junctions or combinations of two different conductors or metals that are welded together. A thermocouple produces a small voltage in milli-volts when the temperature of the process changes. It transfers heat change into electric volts. Thermocouples are cheap and flexible, but their limitation is their measurement accuracy. Its range is around -250 °C to 2500 °C. Thermocouple operates on the Seebeck effect; it states that “If two metals are joined together and heated, then it generates an EMF”. When joined or welded together at one end, two different metallic conductors and an EMF are produced between the open ends of a thermocouple, depending upon the junction’s temperature. The thermocouple is divided into different categories based on different materials and temperature range; its types include Type E, Type T, Type K, and Type J.
- A thermistor is widely used for temperature measurement in oil and gas downstream. It is a resistance variable, as its resistance depends upon temperature. It can be measured from 90 °C to 130 °C. Thermistor is composed of a mixture of metal oxides, such as copper, cobalt, iron, manganese, and uranium.
A thermistor is of two types;
- Positive temperature coefficient (PTC)
- Negative temperature coefficient (NTC).
PTC is temperature dependent; the resistance increases with the increase in temperature. While in NTC, it is also temperature dependent, but opposite in proportionality, the resistance decreases as the temperature increases. Thermistor generally behaves with a high negative resistance temperature coefficient. Its sharp sensitivity makes it very useful for precise temperature measurement, control & compensation. Its resistance ranges from 0.5 Ohm to 0.75 milli-Ohms. It is also used for temperature compensation in sophisticated electronic equipment and measurement of thermal conductivity.
Area of Application
- The area of applicability of Process Control Automation is oil and gas downstream. It has vast applications in downstream, from the first pump of the refinery, where we give input as crude oil, to the last stage, where we get the final product, depending upon the process control system. All stages of the oil refinery, like the Distillation Column, have a Process control automation system to regulate process variables like pressure, level, flow, and temperature.
Summary of Process Control Automation
- Process Control automation in refinery work in such a way that a sensor measures or detects a state or parameter of the real-world, like pressure, level, temperature, flow, mass or light rays, transforming the state into an analogue or digital signal form. Sensors measure the physical quantity or process variable. Then, the transmitter sends the signal in the form of a current or pneumatic signal to the Controller after the process of signal conditioning. Then the Controller takes a decision based on this signal and compares its value to the set point, then after that the controller forwards the command signal to the last controlling element. The sensor is the first step of a closed-loop process, an essential part of process automation.
- Transmitter has three-step functions:
- Measurement/Sensing (detection of process variable)
- Signal Conditioning (Conversion of a non-standard electric signal into a standard electric signal of 4 – 20 milli-amperes)
- Transmit (Transmission of a standard electric signal to Controller)
- We use this signal for information purposes. It can be sent to a PLC, SCADA, or DCS system, where it can be decoded into a readable format and controlled.
Process Control Automation in Oil & Gas (Downstream): Benefits Due to Upgradation in Technology
- Before process control automation, we needed many engineers and human labour to control the chemical process of a refinery. They manually measured process variables through different instruments and controlled the chemical process manually by opening and closing valves and different final control elements, this manual system was inefficient, and there were chances of mistakes. However, in process control automation, every refinery process is monitored by different sensors and controlled by specialized controllers like DCS or PLC. We can say that all refinery processes are automatically operated through this technology, and it also reduces the cost of many engineers & labour.
These companies offer services for Process Control Automation:
- Emerson
- Endress-Hauser
- Rosemount
- GE
- ABB
- Schneider Electric
- Siemens
- Rockwell
- Hitachi
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