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Running head: PROCESS CONTROL

Process Control

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PROCESS CONTROL

Question 1

Process control involves the gathering of different information from sensors in the plant.

Other devices are also used in this process of control hence controlling plant operations. In the

past, process control was done using pneumatic devices, but currently, electronic devices are

used. The main objective of process control is to ensure to ensure a process continues under

certain conditions. Some of the variables which can be controlled in this process are operating

levels of pumps and valves, fluid flow rates, pressure, and temperatures (Ray 1981).

Question 2.

Primary measurements are the values derived from a direct comparison of value to the

reference standard. Quantities such as time, length and mass are primary measurements. In this

case, the quantities required are pressure, temperature and operating levels of pumps and valves

(Ray 1981).

Indirect measurement obtains secondary measurements. In this case, indirect

measurements include fluid flow rates and amount of fluid produced.

Question 3.

Input variable can be categorized as either disturbance or manipulated variables.

Manipulated inputs are those which can be changed by a control system. Disturbance inputs are

those which can affect the output levels, but control systems cannot change them. Some of these

inputs are the amount of pressure and temperature levels (Ray 1981).

Feedback control strategy

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Feedback systems do feedback control, and it is the process of analyzing signals.

Feedback control systems can be constructed using discrete components such as capacitors,

resistors, and transistors. There are two types of feedback strategies. The first one is a closed

loop system. Closed loop systems are systems where the input is not affected by the output. The

purpose of control systems is to control, monitor and measure a process. This can only be done

by checking the output of the process. Closed loop systems are developed to ensure the desired

output is maintained. The control system generates an error signal which shows the difference

between the out and the reference unit. Feedback control strategy has its benefits, and one of

them is accuracy. Since these systems are automatic, the level of accuracy in this systems is very

high. They have a complex design which makes them more accurate compared to open loop

system which has no feedback. Additionally, closed-loop control systems can reduce noise.

Since this system has a feedback mechanism, they can eliminate errors between output and input

signals. This helps in reducing the effects of noise from external sources (Johnson 1993).

Limitations

Feedback control strategy has its limitations. One of the limitations is stability. Closed

loop systems are not as stable as open loop systems, but this disadvantage can be eliminated by

reducing the sensitivity of a system making the system stable. The overall gain of this system is

reduced since the system creates an oscillatory response. The feedback loop causes the

oscillatory response. This system is complex to design and construct. The complexity of design

makes it expensive to construct such a system (Johnson 1993).

Cascade multi-loop process

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Cascade control systems are used in cases where multiple sensors are used to measure

conditions in a control process. In a cascade control block diagram, there are two sensors, two

controls and one actuator in a double process which is in series. A master or primary controller

creates a control effort which acts as the setpoint for a slave or secondary controller. The actuator

is used by the controller to apply a control effort on the secondary action. The secondary process

creates a secondary process variable, and it acts as a control effort for the master process. The

block diagram has a geometry which explains an inner loop which contains the slave controller.

The outer loop contains the master controller. In this case, the inner loop is like a tradition closed

loop control system which has a controller, a process variable, and a setpoint. The process uses

the controller as an actuator. The inner loop functions the same as the outer loop, the only

difference being that the inner loop is used as an actuator (Johnson 1993).

Cascade control block diagram

A good example is in heat exchangers. In the heat exchanger, steam is used to heat the

water. To compare the temperature of water at the outlet and the one used as a setpoint a

feedback temperature controller is used. The control valve is used to regulate the flow of steam

through the opening and closing the valve. The steam flow rate may change if there is a change

in upstream steam pressure without changing the position of the control valve. The temperature

PROCESS CONTROL

of water at the outlet will change, and amount of heat exchange will also change. Detection and

correction of these changes will need some time but use cascade control systems; this challenge

can be overcome (Ogata 1970). This can be done by measuring the disturbance which is the

change in steam flowrate due to a change in upstream pressure. The corrective action taken

would ensure that a constant flow rate of steam is maintained. The flow controller which is an

additional controller is the setpoint, and the temperature controller regulates it.

Selective override control

In override control systems, one output variable is used to control the manipulating

variable. This happens in cases where there are one manipulating variable and many output

variables. In most cases, only one output variable is regulated, but the control system has to

ensure that the remaining variables do not go beyond the safe limits. A selector switch is used to

ensure a second controller controls the other variables so that they do not go beyond the safe

limit. This can be made possible using low selector switch or high selector switch. When the

variable should not cross the upper limit, the high selector switch is used (Ogata 1970).

A good application is in a boiler. The steam pressure of a boiler is controlled by

regulating the flowrate at the discharge point in normal circumstances. The pressure controller

and pressure transmitter control the pressure. However, the level of water in the boiler should be

maintained at a specific level since it is important to ensure that the heating coil is continuously

immersed in water. Having it immersed in water ensures that the coil does not burn out. The

PROCESS CONTROL

lower limit switch is used to maintain the level of water, and it acts as an override control.

Split range control.

This control system is used when there are only one output variable and multiple

manipulated variables. A split range control is used to coordinate the many manipulated

variables. A common application is in the steam headers. Steam comes from different boilers,

and it is combined in the steam header. It is essential to maintain a constant pressure in the

header, and this is done using a pressure control loop. From the pressure controller, a command

is sent to control the flow rates of steam simultaneously from the boilers which are in parallel.

From this system, it is evident that there are many manipulating variables and one output which

PROCESS CONTROL

is the header (Ogata 1970).

Ratio control.

It is a type of feedforward control and is mainly used in industries. The main objective of

this control system is to ensure that the ratio of two processes is maintained. In most cases, the

variables are the disturbance and flow rates. One application is in blending processes (Ogata

1970).

A good application of ratio control is in cases where an industry wants to control two

reactants. In most cases, one of the flow rates is calculated, but it is allowed to float. This flow

rate is normally not regulated. The outer flowrate is calculated and regulated providing a control

ratio. The flow rate of the first reactant is calculated and added according to the required amount

of the second reactant. Consequently, the controller reacts to the input signal through adjusting

PROCESS CONTROL

the valves in the second line.

Adaptive control

Adaptive control systems use information such as data from online sources to improve its

performance or change itself. This can be simply explained by saying this systems changes

depending on the feedback loop, and over time it becomes a better system. Compared to other

controls, this system has to change parameters hence it is a system which depends on how it was

designed before the control loop operation took place (Ogata 1970).

A good example is in an industry where energy consumption has to be monitored. There

is an estimation of the energy consumption coefficient. After estimation, a linearized version of

the plant is created. The linearized version of the plant is used in placing poles when calculating

the gains in the last controller. The main challenge is to ensure that the performance of the plant

PROCESS CONTROL

is maintained even in cases where the estimation is not correct.

Discussion.

Process control involves various devices, and its main objective is to ensure production

processes run consistently and efficiently with minimal variations. In most industries, process

control systems are installed to maintain energy efficiency and to ensure the operation is safe,

and they make profits. Process control systems control, monitor, and measure manufacturing

activities and processes. They observe and correct variations either automatically or manually.

The main objective is to ensure consistency and little or no energy is lost. Benefits of control

systems include improved environmental performance, lower costs of production, production of

quality products which are consistent, ensure safety in the industry and save energy. Well-

designed control systems can help achieve around 15% of savings on energy (Ogata 1970).

Conclusion

It is evident that control systems have numerous advantages especially if industries rely

on feedback control strategies. Different devices can be used to achieve this, and it is important

PROCESS CONTROL

for designers to identify the best control system depending on the number of inputs and number

of outputs. The success of control systems depends on the devices selected and the mode of

operation. Control systems may be the solution to reducing consumption of energy in most

industries which will translate to higher profits and better performance of plants.

PROCESS CONTROL

References

Johnson, C. D. (1993). Process control instrumentation technology. Prentice Hall PTR.

Ogata, K., & Yang, Y. (1970). Modern control engineering.

Ray, W. H. (1981). Advanced process control. McGraw-Hill Companies.

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