Table of Contents: –
Process Control Page
Section 1. Introduction 03
Section 2. Terminology 05
Section 3. Sources of Information 07
Section 4. Types of Process Control 08
Section 5. Manual Control 09
Section 6. Automatic Process Control 10
Section 7. Control Loops 11
Section 8. Transmitters 13
Section 9. Open Loop 14
Section 10. Flow Recorder Controller 15
Section 11. Pressure Recorder Controller 17
Section 12. Temperature Recorder Controllers 19
Section 13. Level Recorder Controller 23
Section 14. Surge Tank Level Control Logic 24
1. Introduction
The concepts and principles you will learn about instrumentation and process control are applicable to almost every task you undertake in the oil and gas industries.
Instrumentation and control processes are found in all the systems you will face, from the simplest valve to the most complex processing facility. In fact, instrumentation and process control is found in almost every process or devices you will ever face, on or off the job.
An example of a simple process control and instrumentation concept would be someone filling a tank with water from a tap to some preset level. The instrumentation part of this task would be the tank with markings on the side, indicating volume. The process control part of the task would be the control of water flow from the tap (a simple valve). As the level of the water reaches some predetermined point on the tank, commonly called the set point, the flow of the water is gradually decreased and ultimately stopped.
Another example would be someone maintaining a certain speed on a speedometer in a car. The instrumentation part of this process would be the speedometer. The process control part of this task would be the operation of the accelerator pedal, which controls the flow of petrol to the engine. The driver pushes down or lets off of the accelerator pedal to maintain the speed setting at a predetermined speed.
Auto Speedometer
In both of these examples, a person acts as a controller in the process. The person visually gathers information about the flow rate of water or the speed of the automobile and performs a controlling action. This controlling action will be either turning the valve handle or pressing the accelerator pedal to make changes in the process to bring it in line with the desired result.
While there are process control systems, which have to be adjusted by a person reading a gauge, or any indication, many instrumentation and process control systems used in industrial settings are automated. In many areas of our industry, the trend is towards fully automated systems using digital controllers monitored and operated by computer.
Process control and instrumentation is used in all aspects of the oil and gas industries. From production to processing to storage and export, process instrumentation and control plays a critical part in assuring the flow rates, temperatures, pressures and liquid levels in the desired ranges. This ensures that the processes we are working with are operating at safe, and accurate.
2. Terminology
• PV (Process variable)– PV is the process parameter like, flow, level, pressure, temperature, speed, etc that we measure from a process (field).
• SP (Set point)– SP is the desired value of a process parameter set by the operator to control a process.
• Deviation– Deviation is also called OFFSET. It is the difference between the PV and SP.
• Output- Output is normally called for the output signal from a controller.
• Controlled variable– The process variable that we control is the controlled variable.
• Manipulated variable– The variable that we adjust to control another variable is called a manipulated variable, e.g. controlling the flow of hot water through a heater to control the temperature of the gas passing through it. Here the controlled variable is temperature and manipulated variable is hot water flow.
• Transducers– Transducers are signal converters used to convert signals from one form to another form. I / P converter is a transducer converting the current input to equivalent air output and P / I converter is converting air input to equivalent current output.
Parameters normally measured and controlled in a process industry are: –
• Pressure
• Temperature
• Flow
• Level
• Speed
• pH, conductivity
• Composition
• Density
• BS & W
• Dew point
• Oxygen content (injection water), etc.
The devices that are used to monitor and control these process variables are generally known as process instrumentation or simply instrumentation.
Instruments have two basic jobs, and may combine both tasks into one device.
Firstly they measure and indicate the value of a process variable, which may then show up as information on a gauge, recorder, etc. Secondly, after comparing the variable reading to some predetermined value (or set point), they send out a controlling signal to another element in the system. Typically this might be a valve or heating element, which causes a change in the operation.
Such a change might be a valve opening or closing, or a heating element increasing or decreasing in temperature. This has the effect of changing the process enough to bring the process variable in line with the set point.
Indicators, Recorders, Gauges, and Digital Readings
Instrumentation includes those devices that are monitored by operations personnel to assure that the various processes are working safely and efficiently.
Gauges, sight glasses, analog and digital readings, recorders and indicator lights are a few of the devices which give information about process variables to an operator.
Based upon the information gathered from the visual readings, an operator may change some aspect of the process to bring the process to the predetermined set point (e.g., opening or closing a valve, increasing or decreasing the pressure).
In some facilities, particularly facilities where operators may be required to work in widely separated parts of a facility, we may find audible indicators such as horns, whistles, or buzzers used to give information on the state of a process. These signals are especially important where process variables such as level, flow, temperature or pressure are critical.
3. Sources of Information
We receive information about a process from: –
• Gauges
• Recorders
• Indicators
• Sight glasses
• Audible alarms and
• Visible alarms (flashing lights).
Requirements of a process control system: –
• To receive information about a process.
• To control parameters like flow, temperature, pressure, level, etc.
• To maintain the quality of products.
• To operate a process plant safely.
• To protect the asset, personnel and environment.
4. Types of Process Controls
1. Manual.
• with controller
• without controller
2. Automatic control system.
• ON-OFF control system
• Gap control system
• PID control system
• Cascade control system
• PLC based control system.
Requirements of a Control System: –
• In the above picture, the level (process variable) is controlled by opening and closing
the valve.
• When inlet flow to the tank changes, the level varies.
• Also when outlet flow from the tank changes, the level varies.
• To maintain the proper level in the tank the valve must be adjusted.
• It can be done either by an operator or by an instrument.
• In modern installations the control is by instruments.
• The controller goes into action when there is a process change.
5. Manual Control (without a controller)
This is done by opening and closing the valve manually. This needs the full-time attention of an operator. Control will not be steady.
Manual Level Control of a Tank: –
• The operator observes the level in the tank.
• Then he compares the level with the desired value.
• He goes to the valve and adjusts it.
• Again he watches the level to see the action is correct or not.
• If required again he adjusts the valve.
• This is a typical manual control (without a controller).
• Here the operator is the controller.
6. Automatic Process Control
In a simple situation, it may be possible for one operator to manually deal with all the process variables and control them, but as process becomes more complex, the number of process variables and upsets that must be dealt with increases. Additionally, as in the case of oil and gas processing facilities, these processes are physically spread out over a large area.
In such cases, automatic process control systems are installed. An automatic process control system does exactly what the name implies; it automatically controls a process and its variables.
Needs of automatic process control: –
• Manual control is possible only in simple process.
• Manual control needs full-time attention of the operator.
• Critical control systems cannot be controlled manually.
• Manual control is not possible when stations are far away.
• Manual control needs more manpower.
• On manual control, process will not be steady.
• On manual control, the process will not be safe.
• It will be difficult to maintain the product quality on manual control.
7. Control Loops
A Control loop is the combination of all the instruments in a particular order to achieve the monitoring and control of a process variable.
A Control loop can be: –
• Open loop or
• Closed loop.
Depending on the location where they are installed, automatic process controls will vary in their shape, size, and components. However, all automatic process control systems contain the following main elements:
• A primary measuring element
• A transmitting element
• A controller, and
• A final control element.
Working of a control loop: –
• Primary elements sense the process parameter fluctuations and supply this signal to the transmitter.
• The transmitter sends this signal to the controller normally in the form of current (4 – 20 mA)
• The controller then compares the desired value (set point) and actual value (PV) and re-adjusts the output as per the deviation. The output from the controller also will be current (4 – 20 mA). This current is converted into equivalent pneumatic output in the I / P converter (transducer).
• This signal is sent to the final control element for final adjustment of the process parameter.
In the above example the primary element is an orifice plate.
It senses the difference in the upstream and downstream pressures. The transmitter
receives this DP (differential pressure) signal and transmits it to the controller.
The output from the controller is sent to the final control element. Most of the final
Control elements are valves which operate on air pressure (pneumatic). In such
cases this output is to be converted into equivalent pneumatic output by using a transducer.
8. Transmitters
For purpose of instrumentation, a transmitter is an element in a control system that accepts information at some point in the system and conditions it for transmission to a control centre, remote terminal unit, or other appropriate location. A transmitter usually conditions a signal to a standard form – 4 to 20 milliamperes and 3 to 15 psi are examples.
This standard signal is transmitted to any one or combinations or all of the following: –
Controller
Recorder
Indicator
Alarms
Interlocks.
9. Open Loop
In the open loop system, control of the process variable is not done. There is no automatic feedback from the controller to the process or from the process to the controller, only we can monitor the process variable and do the control manually.
All process parameter indicators, recorders and alarms without a controller are open loop. Flow recorder or indicator without a controller is a typical example of the open loop.
The picture shown above is that of an open loop. In this we are only measuring the
Flow. There is no automatic adjustment on the flow rate as we seen in the previous
example of a closed loop. In the open loop system shown above, we can only monitor
the changes in the flow (PV). Flow rate can only be adjusted by opening or closing the manual valve.
10. FRC (Flow Recorder Controller)
(Closed Loop)
• First the flow is measured by a primary measuring element, e.g. orifice plate, venturi tube, D/P cell, turbine meter etc.
• A transmitter transmits the measured value to the controller. This can be either an electric or pneumatic signal. In most cases the measured value is given to a recorder as well. Then the instrument loop is called a Flow Recorder Controller (FRC). In some cases the measured value is given to an alarm also, to warn the operator. In such cases the instrument is called a Flow Recorder Controller Alarm (FRCA).
• The controller receives the measured value (PV). The controller has a set point (desired value).
It compares the PV with the set point and the output is readjusted according to the
deviation.
This output can be either an electric or pneumatic signal.
• The output signal is sent to the flow control valve (final control element) and adjusts it, to get the desired flow. Thus the flow is maintained always at the desired value.
• In the flow control loops the position of the measuring element will be at the upstream of the control valve. If the pressure at the upstream and downstream of the orifice plate fluctuate, the DP measured will not be correct. Pressure fluctuations will be more at the downstream of the valve than at up stream.
11. PRC (Pressure Recorder Controller)
• In this pressure control loop first the pressure is measured by a primary measuring element, e.g. bourdon tube, diaphragm type, bellows type etc.
• A transmitter transmits the measured value to the controller. This can be either an electric or pneumatic signal. In most cases the measured value is given to a recorder as well. Then the instrument loop is called a Pressure Recorder Controller (PRC). In some cases the measured value is given to an alarm also, to warn the operator. In such cases the instrument is called a Pressure Recorder Controller Alarm (PRCA).
• The controller receives the measured value (PV). The controller has a set point (desired value). It compares the PV with the set point and the output is readjusted according to the deviation. This output can be either an electric or pneumatic signal.
• The output signal is sent to the pressure control valve (final control element) and adjusts it, to get the desired pressure. Thus the pressure is maintained always at the desired value.
12. TRC (Temperature Recorder Controller)
There are many types and below shown are the 2 types commonly used
• In the above control loop, the process variable the temperature is measured by a primary measuring element.
e.g. bimetallic, RTD, thermocouple etc.
• The temperature transmitter element transmits the measured value (PV) to the controller. This can be either an electric or pneumatic signal. In most cases the measured value is given to a recorder as well. Then the instrument loop is called a Temperature Recorder Controller (TRC). In some cases the measured value is given to an alarm also, to warn the operator. In such cases the instrument is called a Temperature Recorder Controller Alarm (TRCA).
• The controller receives the measured value (PV). The controller has a set point (desired value). It compares the PV with the setpoint and the output is readjusted according to the deviation. This output can be either an electric or pneumatic signal.
• The output signal is sent to the temperature control valve (final control element) and adjusts it, to get the desired temperature. Thus the temperature is monitored, controlled and maintained always at the desired value.
• This temperature control valve is a 3-way valve in the process line where the process variable (temperature) is to be controlled. The temperature is controlled by bypassing the required amount of oil/gas around the cooler. The primary measuring element and the final control element are in the same process line. The cooler has a by-pass line. The flow through the cooler is controlled by the 3-way valve according to the temperature deviation. The balance of the total flow will pass through the by-pass line. Hence the flow is not restricted, but a small pressure drop may occur. Many examples of this type are available in operations. Some compressor lube oil coolers and some of the temperature control valves used in gas treatment are few examples.
TRC (Temperature Recorder Controller)-2
• In the above control loop, the process variable the temperature is measured by a primary measuring element, e.g. bimetallic, RTD, thermocouple etc.
• The temperature transmitter element transmits the measured value (PV) to the controller. This can be either an electric or pneumatic signal. In most cases the measured value is given to a recorder as well. Then the instrument loop is called a Temperature Recorder Controller (TRC). In some cases the measured value is given to an alarm also, to warn the operator. In such cases the instrument is called a Temperature Recorder Controller Alarm (TRCA).
• The controller receives the measured value (PV). The controller has a set point (desired value). It compares the PV with the set point and the output is readjusted according to the deviation. This output can be either an electric or pneumatic signal.
• The output signal is sent to the temperature control valve (final control element) and adjusts it, to get the desired temperature. Thus the temperature is monitored, controlled and maintained always at the desired value.
• In the above picture a hot medium is cooled in a cooler with the help of a cooling medium. The main process is controlling the temperature of the hot medium. The temperature (process variable) is measured at the outlet of the cooler. This temperature is to be monitored and controlled. The output of the controller is given to the control valve, which is placed in the cooling medium. The flow of the cooling medium is adjusted or manipulated to control the temperature of the other medium. Here the cooling medium flow is the manipulated variable. In other words the main process variable (temperature) is controlled by adjusting the flow of the manipulated variable. The control valve is a normal 2-way valve in the cooling medium and no valve is provided in the measuring medium. Gas compressor cooler temperature control is a typical example. Here the cooler exit temperature is controlled by manipulating the air flow through the cooler (speed control or on/off control of the cooler fans). Heating the oil with hot water is another example.
13. LRC (Level Recorder Controller)
• In the above controller the level is measured by a primary measuring element.
e.g. displacer, D/P cell, capacitance device etc.
• A level transmitter transmits the measured value to the controller. This signal can be either electrical or pneumatic signal. In most cases the measured value is given to a recorder as well. Then the instrument loop is called a Level Recorder Controller (LRC). In some cases the measured value is also given to an alarm to warn the operator. In such cases the instrument is called a Level Recorder Controller Alarm (LRCA).
• The controller receives the measured value (PV). The controller has a set point (desired value). It compares the PV with the set point and the output is readjusted according to the deviation. This output can be either an electric or pneumatic signal.
• The output signal is sent to the level control valve (final control element) and adjusts it, to get the desired level in the vessel. Thus the level is maintained always at the desired value. This is the way an automatic level controller works. Many of your level controllers are working according to the above principles.
- Surge Tank Level Control Logic
The purpose of surge tanks is to remove complete gas from the liquid coming from the production/test separators. Maintaining the correct level in the tanks is very important to avoid gas going to pump suction causing vapour lock and losing the pump discharge flow (low low level) or oil going to flare (high high level).
- In the previous page’s picture two identical surge tanks are connected together.
- Both tanks receive liquid from the separators.
- Gas phase is connected to the flare.
- The stand pipe in the middle of the tanks is connected to both the tanks.
- Both the tank levels will be equal always, due to the interconnection line.
- The desired level (SP) is 50%
- Normally the level controller controls the level at 50% by adjusting the level control valve (LCV).
- Due to rapid changes in the incoming or out going flow or due to malfunction of the level control instruments the tank level may go high or low. In such cases to safe- guard the process the following actions are designed to take place.
- High (H) level initiates the alarm & starts the stand-by pump.
- Low (L) level initiates the alarm & stops the stand-by pump.
- Low Low (LL) level initiates the alarm & stops both pumps.
- High High (HH) level initiates the alarm and shuts down the entire station.