Process Control Automation in Oil & Gas (Downstream)

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 & Gas Industry, it is crucial to regulate process variables to achieve the task of Process automation. In Oil & 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 different Sensors, Transmitters, Controllers, Control Valves and different final control elements to regulate process variables within the desired range to carry out the operation successfully. In Oil & Gas downstream, the critical process variables which are very important to control include pressure, level, flow, temperature, 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

• The pressure is the most critical process variable in the Oil & Gas Industry. Pressure can be described as “Force acting per unit area”, it is represented as P = F / A. In this expression P = Pressure, F = Force, A = Area.
• The pressure is measured in psi (Pounds per square inch) in British units, and it is measured as Pa (Pascal) in Metric system units. The System International (SI) unit of Pressure is Pascal or N/m2, but in Oil & Gas Industry we use bar or psi.
• The relationship between different pressure units are :
  • 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)

• The pressure is a crucial factor. If we know the pressure of a specific process, we can find other process variables easily through it, and Refinery Chemical processes require that system operating pressures must be controlled to specific pressures for systems to operate correctly.
• There are three types of pressure:

1. 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 absolute pressure scale shows the full vacuum.
2. Gauge Pressure can be defined as the difference between atmospheric pressure, gauge pressure and fluid’s pressure changes with the change of atmospheric pressure.
3. 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:

1. Bourdon tube sensing element
2. Bellow Sensors
3. Diaphragm Sensors
4. Pressure transmitter
5. Load Cell
6. Fibre Optic Pressure Sensor
7. Capacitive Pressure Sensor

• Rosemount is the most popular vendor company, which designs Oil & Gas downstream pressure measurement equipment.

Process Control Automation in Oil & Gas (Downstream): Flow Measurement in Oil & Gas Downstream

8. Flow is also one of essential process variable of Oil & Gas downstream. To achieve the goal of automation in an oil refinery, it is necessary to control the flow of pipelines and chemical process plant. There are many instruments for measuring Oil and gas flow. The accuracy of flow measurement is different for each instrument and different for each application.
9. There are factors which affect the flow of fluids through the pipe and also the measurement of Oil & gas flow. These include
10. Velocity of fluid
11. Pipe Size
12. Friction due to pipe
13. Viscosity of fluid
14. The specific gravity of the fluid
15. Velocity Profiles

• Fluid, with its volume passing through a specific point in a certain amount of time, is called Flow rate and can be measured in gallons/minute (GPM) or litre per minute. But in the Oil & Gas industry, we mostly use gallons per minute.
• Flow rate vary pipe to pipe,1 gpm = 6.309 * 10^-5 m³/s.
• The complete flow is the volume of fluid passing over an extended frame of time and can be calculated in different units such as litres, gallons, cubic feet, etc.
• Measuring the volume flow rate and measuring the mass flow rate is used to measure the rate at which the fluid flows through a closed pipe. Mass of substance, which passes through a surface per unit time known as Mass flow rate. Unit is mass over time kg/t. The volume of fluid, which passes through a particular surface per unit time, is known as Volume flow rate. Units: e.g. m³/s, volumetric flow meter measures volume directly while mass flow meters measures mass directly.
• In Oil & Gas downstream, we use these sensors for the measurement of flow:

1. Differential pressure flow meters
2. Orifice Plate
3. Venturi-tube
4. Turbine flow meters
5. Electromagnetic flowmeters
6. Ultrasonic flowmeters
7. Bluff Body flowmeters
8. Positive Displacement flowmeters
9. Flow Transmitter
10. Microprocessor-based volumetric flowmeters
11. 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 level is a crucial variable used in every Oil & Gas Industry. We measure level in meter 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:

1. Position of the liquid surface
2. Pressure Head
3. Weight or density of the material

Direct Method

1. 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 its advance safety measurements. Therefore, its usage is highly recommended in Oil & Gas level measurement. These methods include:
2. Dipstick method
3. Weighted Gauge Tape
4. Sight Glass
5. 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

1. Hydrostatic Pressure Method
2. Hydrostatic Differential Pressure Method
3. Dry leg method
4. Wet Leg Method
5. Torque tube Method
6. Conductivity Level Method
7. Level Transmitters

Temperature Measurement in Oil & Gas Downstream

Temperature is also an important process variable of Oil & Gas downstream. It is crucial to control the temperature of the chemical process to achieve the goal of automation – in Oil & Gas downstream, all chemical processes are heat dependent. There are different specialized instruments which are used to measure temperature in industry. It includes thermocouple and thermistor. We measure temperature in Kelvin or o C or o F.

Thermocouples are the most widely used temperature sensors in Chemical Industry & Oil & Gas downstream; it is a junction or combination of two different conductors or metals that are welded together. A thermocouple produces a small voltage in milli-volts when the temperature of process changes. It transfers heat change into electric volts. Thermocouples are cheap and flexible, but its limitation is its measurement accuracy. Its range is around -250 o C to 2500 o C. Thermocouple operates on the See-beck effect; it states that “If two metals joined together and heated then it generates an EMF”. Two different metallic conductors when joined or welded together at one end, and EMF produced between open ends of thermocouple, it depends upon the temperature of the junction. The thermocouple is divided into different categories based on different material 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 & Gas downstream; it is resistance variable as it has resister, which depends upon temperature. It can be measured from – 90 ^o C to 130 ^o Thermistor is composed of a mixture of metal oxides such as following: copper, cobalt, iron, manganese, uranium.

A thermistor is of two types;

• Positive temperature coefficient (PTC)
• Negative temperature coefficient (NTC).

PTC is temperature dependable; the resistance increases with the increase in temperature. While in NTC, it is also temperature dependable, but opposite in proportionality, the resistance decreases as the temperature increases. Thermistor generally behaves with high negative resistance’s temperature co-efficient. Its sensitivity is very sharp, which 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 & Gas downstream. It has vast applications in downstream, From the first pump of the refinery, where we give input as crude oil till the last stage where we get final product depends upon process control system. All stages of oil refinery like Distillation Column has Process control automation system to regulate process variables like pressure, level, flow, temperature.

Summary of Process control Automation

• Process Control automation in refinery work in such a way that a sensor measure or detects a state or parameter of real-world, like pressure, level, and temperature, flow, mass or light rays and transform the state into an analogue or digital signal forms. Sensors measure the physical quantity or process variable, and then the transmitter sends the signal in the form of current or pneumatic to Controller after the process of Signal conditioning. Then Controller takes a decision based on this signal and compares its value to set point, then after that controller forward command signal to the last controlling element. The sensor is the first step of a closed-loop; it is an essential part of Process automation.
• Transmitter has three-step functions:

1. Measurement/Sensing (detection of process variable)
2. Signal Conditioning (Conversion of a non-standard electric signal into a standard electric signal of 4 – 20 milli-amperes)
3. Transmit (Transmission of a standard electric signal to Controller)

• We use this signal for information purpose, and it can be sent to PLC, SCADA or DCS system, where it can be decoded into a readable format, and we can control process.

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 measured process variables manually through different instruments and control the chemical process manually by opening and closing valves and different final control elements, this manual system was not efficient, and there were chances of mistake. But in process Control automation, every process of the refinery 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 & labours.

These Companies offers services for Process Control Automation:

• Emerson
• Endress-Hauser
• Rosemount
• GE
• ABB
• Schneider Electric
• Siemens
• Rockwell
• Hitachi

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