Basic Control Loop Components

Front 1

What mechanism is basic to all pneumatic transmitters?

Back 1

Flapper-nozzle-restrictor assembly

Front 2

Draw a basic pneumatic transmitter and explain

Back 2

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Front 3

Draw a differential pressure transmitter and explain

Back 3

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Front 4

Describe Recorders

Back 4

Recorders are used to maintain a permanent record of certain variables for future reference. This collection can be done digitally or using electromechanical devices.

Front 5

What are the pros and cons of using Digital Recorders?

Back 5

Digital or computer monitor data loggers can be very versatile, giving the user much more information and various methods of display. However, these digital information systems can be costly in their initial installation.

Front 6

What are the pros and cons of using Electromechanical Recorders?

Back 6

While of older design and technology, they provide in many instances a self-contained and low cost alternative and are still commonly used.

Front 7

What are the two most common Electromechanical Recorders?

Back 7

1. Circular Charts

2. Strip Chart Recorders

Front 8

What is the Circular Chart Recorder an earlier design of?

Back 8

Strip Chart Recorder

Front 9

Explain how Circular Chart Recorders work

Back 9

A spiral Bourdon tube is connected to a recording pen through a link-lever mechanism. When the pressure in the sensing element increases, the movement of the pen will be proportional to the change in pressure in the Bourdon tube.

Front 10

Describe the chart used in Circular Chart Recorders

Back 10

The chart is in the form of a circular paper disc with a hole in the centre which fits over the chart drive motor. It also has concentric circles that form the scale on which the variable is read. The “time arcs” laid out at uniform distances divide the full or concentric circles into appropriate time intervals of the total period. The chart usually makes one revolution every 24 hours, although some charts rotate in up to 7 days. The clock may be powered by manually wound springs, by electricity, or by pressurized air or gas.

Front 11

What do Strip Chart Recorders have in common with iPhones?

Back 11

There has been a marked trend of size reduction

Front 12

Describe how Strip Chart Recorders work

Back 12

The standard strip chart recorder is characterized by the uniform linear motion of a strip of paper in a vertical or horizontal direction. Instead of placing the primary sensing element directly in the recorder, as in the circular chart type, the output of a transmitter, which is proportional to the process variable, is admitted to the recorder. In most pneumatic recorders of this type, a bellows or capsule is used to position the recording pen.

Front 13

Describe CRT/LCD Trend Displays

Back 13

Computerized control systems can display process conditions on the monitor or screen (often referred to by its technical name, a Cathode Ray Tube or CRT, or a flatscreen Liquid Crystal Display (LCD)). The display looks similar to a strip chart record; several trends can be placed on the same screen and a printout can be taken if desired.

Front 14

What are the advantages of using CRT/LCD Trend Displays?

Back 14

The operator has the flexibility to assign any transmitter output from the process to the trend display - provided, of course, that the transmitter is connected to the computer system. This means the operator can see the trend on almost any process condition; including the trend on things which are not normally recorded, such as valve positions, ambient weather conditions, and machinery speeds. Time periods can be assigned for the graph, stretching or compressing it, to get a closer look at rapidly changing conditions. Graph limits can be set higher or lower, so that small changes can be closely watched. The operator can also select the process conditions that appear on the screen, placing one above or below the other. This makes it possible to diagnose operating problems, since, by visual comparison, the first upset condition and then the subsequent effects can be determined.

Front 15

What is the major limitation of using CRT/LCD Trend Displays?

Back 15

The major limitation of this system is that the computer can only show one screen at a time. If it is necessary to access other displays, such as control loops, the operator may need to go back and forth between screens; this is a time consuming procedure when plant conditions are upset or changing quickly.

Front 16

How is the major limitation of using CRT/LCD Trend Displays remedied?

Back 16

Computer control panels are often supplied with a few dedicated recorders, which monitor critical operating conditions on a continual basis.

Front 17

What are the two categories of Controllers?

Back 17

1. On-off, two-position, and multiposition
controllers.

2. Modulating, including proportional, integral, and derivative controllers

Front 18

Describe On-Off, Two-Position, and Multi-Position Controllers

Back 18

These controllers compare actual conditions to setpoint values, and respond by either starting or stopping a final control element. Two-position and multi-position controllers react the same way, except that their output signals may be high-low, or high-medium-low, rather than on-off.

Front 19

Describe how Proportional Controllers work

Back 19

tI sends a variable signal to the control valve, causing it to be open, closed, or somewhere in between. The control valve position is modulated. The controller output signal will vary depending on how much the actual conditions differ from setpoint. The controller output is thus proportional to the amount of error. The output signal from a pneumatic controller to a control valve will be between 20 - 100 kPa (3 - 15 psi). Both electronic analog and computer type controllers send out 20 - 100 mA signals.

Front 20

Describe Proportional Plus Integral (Reset) Controllers

Back 20

Proportional controllers are not always capable of operating the process exactly on setpoint, although they are generally close. To operate exactly on setpoint, a modification called integral, or reset, action is required.

Front 21

Describe as a modification Proportional Plus Integral plus Derivative Controllers

Back 21

This enables the controller to respond more quickly, almost like a temporary on-off controller, if the process begins to deviate widely from the setpoint. Derivative action is usually restricted to slow-responding processes such as heating systems.

Front 22

Describe Auto/Manual Control

Back 22

Most (but not all) controllers have the option of selecting either automatic or manual control. Automatic means the controller responds to differences between setpoint and actual conditions. Manual control means that the operator selects the controller output; the controller does not respond on its own to the process. Manual control is necessary, for example, when the control loop transmitter is being repaired or calibrated.

Front 23

Describe Feedback/Feedforward Control

Back 23

Most control loops respond to a specific process condition after the condition has changed. Since this information is fed back to the controller, it is called a feedback system. In some specialized control systems, the controller is alerted ahead of time that a change in the process is about to occur. The controller responds before the change takes place. This is referred to as feedforward control.

Front 24

Describe Cascade Control

Back 24

In some control systems the output signal from one controller becomes the setpoint signal for another controller. Cascade systems are sometimes used in level/flow combinations, such as in the water level control for large boilers, and in the speed control of turbines and engines.

Front 25

Describe Split Range Control

Back 25

In this arrangement, a single controller controls two or more control valves, operating them in sequence, e.g. operating one valve between 0-50% output and another valve between 50-100% output.

Front 26

Describe High/Low Select Control

Back 26

This is a specialized control system in which two controllers are connected, through a select switch, to a single control valve. The select switch chooses which controller will send its signal to the control valve.

Front 27

What are some examples of final control elements?

Back 27

They could be a motor, a speed controller, a fan damper, or some other device. However, the majority of final control elements are control valves.

Front 28

What are Control Valves

Back 28

A control valve may be described as a variable resistance or restriction placed in a
flow line to control the flow of fluid or gas in a process.

Front 29

What are the components of Control Valves?

Back 29

It consists of two main components, called an actuator and a valve body, that are joined to move together vertically. An actuator consists of a molded diaphragm installed in a diaphragm case, and a diaphragm plate connected to an actuator stem. The valve body contains the inlet and outlet openings, seat ring, and the valve plugs connected to the valve plug stem. The yoke of the actuator is connected to the top of the valve stem by means of a stem connector and locknut.

Front 30

How do Control Valves work?

Back 30

An increase in controller output pressure will force the diaphragm downward against the opposing force of the actuator spring, causing the valve stem and valve plug to move closer to the seat and restrict the flow. With a decrease in controller output, the actuator spring will force the diaphragm upward to increase the flow rate. A pressure drop or differential must always exist between the valve inlet and discharge to maintain a flow through a valve; as the valve opens further, this pressure drop will decrease.

Front 31

What is the most common Control Valve Body?

Back 31

The most common control valve body style is in the form of a globe that can be either single or double-seated.

Front 32

When are Single-Seated Globe Valves used and when are Double-Seated Globe Valves used?

Back 32

Single-seated globe valves are usually used when tight shutoff is required. The double-seated valve is quite popular when throttling action is required, because less actuator force is required to move the valve plugs.

Front 33

What is the difference between direct-acting and reverse-acting single-seated globe valves?

Back 33

In direct-acting valves, the downward movement of the valve plug reduces the rate of flow. In reverse-acting valves, the downward movement of the valve plug increases the rate of flow.

Front 34

How do Double-Seated Valves work?

Back 34

The fluid pressure acts on both plugs and the opposing forces almost cancel each other. A disadvantage of this type of valve is that a certain amount of leakage can be expected when the valve is in the closed position, as it is almost impossible to fully close both ports at the same time.

Front 35

Describe Actuators

Back 35

The actuator is the part of a control valve that is connected to the valve stem. It varies the valve opening according to the magnitude of the applied signal from the controller. The actuator must generate sufficient force to overcome the unbalanced forces acting on the valve plug. Actuators are classified according to the input signal and output power used, and may be operated mechanically, pneumatically, electrically, or electro-hydraulically.

Front 36

How are Solenoid Valves considered?

Back 36

Solenoid operated valves are one of the simplest and most popular type of control valves for applications where the valve must be either in the fully open or closed position.

Front 37

How do Solenoid Valves work?

Back 37

The valve disc is held firmly on its seat by the line pressure. When the solenoid is energized, the magnetic field within the coil will draw the plunger upwards into the coil, thus opening the valve. If the solenoid coil becomes deenergized, the closing action of the valve is achieved by the mass of the plunger, valve stem, and disc. Once the disc is moving toward the seat, the pressure differential across the valve will snap the valve tightly shut.