PROCESS CONTROL AND INSTRUMENTATION
8.1 TYPES OF CONTROL.
Water treatment plant processes may be controlled by manual, semiautomatic or automatic methods, which are defined as follows.
a. Manual control. Manual control involves total operator control of the various water treatment processes. The personnel at the water treatment plant observe the values of the different variables associated with the treatment processes, and make suitable adjustments to the processes.
b. Semiautomatic control. Semiautomatic control utilizes instruments to automatically control a function or series of functions within control points that are set manually. The operator manually starts the automatic sequence of operations. An example of semiautomatic control is the automatic backwashing of a filter after operator initiation of the program,
c. Automatic control. Automatic control involves the use of instruments to control a process, with necessary changes in the process made automatically by the controlling mechanisms. When a process variable change, the change is measured and transmitted to a control device which adjusts the mechanisms controlling the process. Automatic control systems have been developed which are reliable, but provision for emergency manual control must be included.
8.1 WHY CONTROL?
The overall plant objective is to convert raw water to treated water in the most economical way. During this operation, the plant must satisfy several requirement imposed by the designer and the general technical, economic, and social conditions in the presence of ever-changing external influences (disturbance). The control system is called to satisfy the following needs (Stephanopoulos, 1996)
1. Suppressing the influence of external disturbances
2. Ensuring the stability of the process
3. Optimizing the performance of the process
8.2 OPERATIONAL OBJECTIVES
The instrumentation and control requirement in this work is carried out to suppress instability as well as attain maximum safety of both the both whole plant and its workers.
8.3 CONTROL OBJECTIVES
The objectives of control in this work are:
1. To maintain the volume (height) of water in the tanks at desired values.
2. To maintain a constant flow rates in pipes.
3. To ensure that our treated water meets World Health Organizations Quality standards by ensuring correct dosage proportions (concentrations) of chemicals, and
4. To determine the rate of backwash.
8.4 MANIPULATED VARIABLES
These are parameters that can be adjusted in order to achieve the control objective which are: height (h), inlet flow (Fi), outlet flow (F)
8.5 DISTURBANCE
These are variables in which we have no control on them (i.e. they cannot be stopped from occurring). The major disturbance that are likely to be observed in this case is inlet flow (Fi).
8.6 CONTROL MECHANISMS
The control mechanisms of the process, consist of, a sensor to detect (monitor) the process variables, transmitter to convert the sensor’s signal into an equipment signal, controller which may consist the comparator that compares the process signal with the desired set point which from then produces an appropriate controller output signal to the final control element that implement the adjustment.
8.6.1 Sensors
The devices to be used for the online measurement of the process variables are:
• Flow sensors: In this case an orifice is recommended (for its low cost)
• Level sensors: Float actuated devices are sufficient.
• Concentration sensors: Gas Chromatograph or Infrared Spectrometer could be used
Alarms are to be employed, so as to alert the process operators of a process that requires immediate attention, instead of individual issuing point alarm. All alarms associated with a certain aspect of the process are to be simply wired to give a single trouble alarm.
There are various control methods used in chemical engineering such as feed forward control, feedback control and cascade control. For the purpose of this design, a feed back control system will be used except where otherwise stated.
8.7 CONTROL SYSTEMS
8.7.1 Reservoir Tank Control
• Control objective: The primary objective is to maintain the specified volume or level of water in the tank.
• Measured variable: Height
• Manipulated Variable: Flow rate
• Control Configuration: Feedback controller
• Control element: valve
• Control Design: This is given in Figure 8.1
Figure 8.1: Reservoir Tank: Valve Feedback Control System
8.7.2 Disinfection Tank Control
• Control objective: The primary objective is to control the quality of water by controlling the flow rate of calcium hypochlorite dosage.
• Measured variable: Concentration of chlorine, and flow rate
• Manipulated Variable: Amount of Chlorine and flow rate
• Control Configuration: Feedback controller
• Control element: Venturi meter for flow rate control (FC) and gas chromatograph or infrared spectrometer for concentration control (CC).
• Control Design: This is given in Figure 8.2
Figure 8.2 Control system for chlorine dosage and effluent water flow rate.
8.7.3 Control of Filter Unit
The objective is to determine the appropriate head needed for backwash.
• Measured variable: Level
• Manipulated variables: Height
• Control configuration: Feedback controller
• Control element: Venturi meter
8.7.4 Control of Sedimentation Unit
The control objective is to ensure that sludge is removed from sedimentation tank after the sludge level or height has reached the set point.
• Control objective: The primary objective is to maintain the specified volume or level of sludge in sedimentation tank in the tank.
• Measured variable: Height
• Manipulated Variable: Flow rate
• Control Configuration: Feedback controller
• Control element: Auto desludge actuated butterfly valve.
• Control Design: This is given in Figure 8.3
Figure 8.3: Sedimentation tank Actuated Butterfly Valve Feedback Control System
8.7.5 Units with Volume and Flow Rate Controllers.
Volume and flow rate control will be applied to the following units: sedimentation tank, filtration tank, flocculation tank, aeration tank.
• The objective is to control the volume of water in the tanks.
• Measured variable: height
• Manipulated variable: flow rate.
• Control configuration: feedback controller
• Controllers: Venturi meter for flow rate control (FC) and a Differential Pressure Cell for Volume control (VC).
8.8 ALARMS AND SAFETY TRIPS
Alarms are used to alert the plant operators of serious and partially hazardous deviations in process conditions. Key instruments are fitted with switches and relays to operate audible and visual alarms on the control panels and annunciator panels. The instrument is fitted with a trip system to take action automatically to avert hazard where delays and lack of response by the operator is likely to lead to rapid development of a hazardous situation.
8.1 TYPES OF CONTROL.
Water treatment plant processes may be controlled by manual, semiautomatic or automatic methods, which are defined as follows.
a. Manual control. Manual control involves total operator control of the various water treatment processes. The personnel at the water treatment plant observe the values of the different variables associated with the treatment processes, and make suitable adjustments to the processes.
b. Semiautomatic control. Semiautomatic control utilizes instruments to automatically control a function or series of functions within control points that are set manually. The operator manually starts the automatic sequence of operations. An example of semiautomatic control is the automatic backwashing of a filter after operator initiation of the program,
c. Automatic control. Automatic control involves the use of instruments to control a process, with necessary changes in the process made automatically by the controlling mechanisms. When a process variable change, the change is measured and transmitted to a control device which adjusts the mechanisms controlling the process. Automatic control systems have been developed which are reliable, but provision for emergency manual control must be included.
8.1 WHY CONTROL?
The overall plant objective is to convert raw water to treated water in the most economical way. During this operation, the plant must satisfy several requirement imposed by the designer and the general technical, economic, and social conditions in the presence of ever-changing external influences (disturbance). The control system is called to satisfy the following needs (Stephanopoulos, 1996)
1. Suppressing the influence of external disturbances
2. Ensuring the stability of the process
3. Optimizing the performance of the process
8.2 OPERATIONAL OBJECTIVES
The instrumentation and control requirement in this work is carried out to suppress instability as well as attain maximum safety of both the both whole plant and its workers.
8.3 CONTROL OBJECTIVES
The objectives of control in this work are:
1. To maintain the volume (height) of water in the tanks at desired values.
2. To maintain a constant flow rates in pipes.
3. To ensure that our treated water meets World Health Organizations Quality standards by ensuring correct dosage proportions (concentrations) of chemicals, and
4. To determine the rate of backwash.
8.4 MANIPULATED VARIABLES
These are parameters that can be adjusted in order to achieve the control objective which are: height (h), inlet flow (Fi), outlet flow (F)
8.5 DISTURBANCE
These are variables in which we have no control on them (i.e. they cannot be stopped from occurring). The major disturbance that are likely to be observed in this case is inlet flow (Fi).
8.6 CONTROL MECHANISMS
The control mechanisms of the process, consist of, a sensor to detect (monitor) the process variables, transmitter to convert the sensor’s signal into an equipment signal, controller which may consist the comparator that compares the process signal with the desired set point which from then produces an appropriate controller output signal to the final control element that implement the adjustment.
8.6.1 Sensors
The devices to be used for the online measurement of the process variables are:
• Flow sensors: In this case an orifice is recommended (for its low cost)
• Level sensors: Float actuated devices are sufficient.
• Concentration sensors: Gas Chromatograph or Infrared Spectrometer could be used
Alarms are to be employed, so as to alert the process operators of a process that requires immediate attention, instead of individual issuing point alarm. All alarms associated with a certain aspect of the process are to be simply wired to give a single trouble alarm.
There are various control methods used in chemical engineering such as feed forward control, feedback control and cascade control. For the purpose of this design, a feed back control system will be used except where otherwise stated.
8.7 CONTROL SYSTEMS
8.7.1 Reservoir Tank Control
• Control objective: The primary objective is to maintain the specified volume or level of water in the tank.
• Measured variable: Height
• Manipulated Variable: Flow rate
• Control Configuration: Feedback controller
• Control element: valve
• Control Design: This is given in Figure 8.1
Figure 8.1: Reservoir Tank: Valve Feedback Control System
8.7.2 Disinfection Tank Control
• Control objective: The primary objective is to control the quality of water by controlling the flow rate of calcium hypochlorite dosage.
• Measured variable: Concentration of chlorine, and flow rate
• Manipulated Variable: Amount of Chlorine and flow rate
• Control Configuration: Feedback controller
• Control element: Venturi meter for flow rate control (FC) and gas chromatograph or infrared spectrometer for concentration control (CC).
• Control Design: This is given in Figure 8.2
Figure 8.2 Control system for chlorine dosage and effluent water flow rate.
8.7.3 Control of Filter Unit
The objective is to determine the appropriate head needed for backwash.
• Measured variable: Level
• Manipulated variables: Height
• Control configuration: Feedback controller
• Control element: Venturi meter
8.7.4 Control of Sedimentation Unit
The control objective is to ensure that sludge is removed from sedimentation tank after the sludge level or height has reached the set point.
• Control objective: The primary objective is to maintain the specified volume or level of sludge in sedimentation tank in the tank.
• Measured variable: Height
• Manipulated Variable: Flow rate
• Control Configuration: Feedback controller
• Control element: Auto desludge actuated butterfly valve.
• Control Design: This is given in Figure 8.3
Figure 8.3: Sedimentation tank Actuated Butterfly Valve Feedback Control System
8.7.5 Units with Volume and Flow Rate Controllers.
Volume and flow rate control will be applied to the following units: sedimentation tank, filtration tank, flocculation tank, aeration tank.
• The objective is to control the volume of water in the tanks.
• Measured variable: height
• Manipulated variable: flow rate.
• Control configuration: feedback controller
• Controllers: Venturi meter for flow rate control (FC) and a Differential Pressure Cell for Volume control (VC).
8.8 ALARMS AND SAFETY TRIPS
Alarms are used to alert the plant operators of serious and partially hazardous deviations in process conditions. Key instruments are fitted with switches and relays to operate audible and visual alarms on the control panels and annunciator panels. The instrument is fitted with a trip system to take action automatically to avert hazard where delays and lack of response by the operator is likely to lead to rapid development of a hazardous situation.
Superb posts with lots of information!!! This is the most miraculous blog site dude…. Waste Water Solutions Company
ReplyDelete