Control, signals and introduction to valves and switches

Question 1

region over which a quantity may be received (input range) or transmitted (output range)

Defined by stating lower and upper range values

Answer 1

Range

Question 2

Also known as full scale deflection (FSD)
Magnitude of the range of the instrument

Answer 2

Span

Question 3

Ratio of maximum capacity to lowest capacity

Answer 3

turndown ratio

Question 4

ratio of change in magnitude of output signal corresponding to change in magnitude of input signal

Answer 4

sensitivity

Question 5

minimum difference in values of a quantity that can be measured by a device

Answer 5

Resolution

Question 6

Signal is from the control unit

Answer 6

Output

Question 7

signal is to the control unit

Answer 7

Input signal

Question 8

Signal over a range

Answer 8

Analogue

Question 9

Signal is discreet (on or off)

Answer 9

Digital

Question 10

Sensor output is often a ____which is converted to a proportional DC-current

Answer 10

voltage

Question 11

Sensor output is a voltage and this is converted to

Answer 11

a proportional dc-current

Question 11

Why is voltage converted to current?

Answer 11

Cannot transmit sensor output voltage over long distances

Question 12

allows for self monitoring

Answer 12

Live zero

Inherent error detection
Current outside range indicates a fault: zero mA means line is broken, without this zero mA could mean empty tank or broken sensor

Question 13

What does a valve do?

Answer 13

• Turns flow ON or OFF
• Diverts flow or non-return
• Regulates flow of a manipulated variable
• Safety functions such as pressure relief

Question 14

Often the “final control element”

Answer 14

Control valve

Question 15

Components of a control valve

Answer 15

• Valve plug, disc etc.
• Actuator
• Housing and transducers etc.

Question 16

A pneumatic, electrically or hydraulic, powered device that supplies force and motion to open or close a valve

Answer 16

Actuator

Question 17

Types of valves

Answer 17

• Linear motion
• Rotary motion
• Pinch and diaphragm valves

Question 18

Example of linear motion valve

Answer 18

Globe valves

Question 19

Example of rotary motion valves

Answer 19

Ball valves
Butterfly valves

Question 20

Air pressure causes a stem to move up or down
Plug moves in and out of a “seat” to prevent or allow flow respectively
Commonly used for control
Flow path is changed by valve event when open
Associated pressure drop

Answer 20

Globe valves

Question 21

Opens by lifting a plate or wedge out of the fluid path
Typically limited to ON/OFF applications

Answer 21

Gate Valve

Question 22

Ball with hole through it rotates around vertical axis
When bore is aligned in the pipe flow direction the fluid flows
Quartner turn 90 degrees bore is perpendiculaar to flow path and fluid cannot pass.
The valve actuator is designed so that an axial movement of a piston is transformed into a 90 degrees rotation of a haft. The actuator is bracketed to the valve.
Mainly used for ON/OFF (imprecise flow control)

Answer 22

Ball valves

Question 23

Axial movement of the piston is transformed into a rotation of a shaft. The actuator is bracketed to the valve

Answer 23

Valve actuator

Question 24

Operation similar to ball valve Disc rotates around vertical axis When disc is aligned perpendicular to pipe flow direction the fluid flows Quarter turn (90o) bore is parallel to flow path and fluid cannot pass Mainly used for ON/OFF (imprecise flow control)

Answer 24

Butterfly valves

Question 25

Advantages of butterfly valve over ball valve

Answer 25

- Butterfly valve is cheaper and lighter weight than ball valve - Butterfly valve is hygienic

Question 26

- Diaphragm is pushed into contact with the valve body to close off the flow - Straight pipe or sear - Material chosen for chemical resistance - Traditionally used for ON/OFF but newer models are used for control - Good for low pressure applications

Answer 26

Diaphragm valve

Question 27

Opening or closing of valves is usually performed using ...

Answer 27

pneumatic or electrical actuators ## Footnote Pneumatic (air driven) actuators are most common due to simplicity and low cost For ON/OFF actuators a digital 24Vdc signal can be sent to a solenoid to either send or not send air to the valve

Question 28

Valves require air pressure to stay closed If air pressure is removed, the valve will naturally open Under a power outage, the source of the air pressure would be lost and the valve would "fail" in the open position Designed to allow flow in an emergency

Answer 28

Normally open/fail open

Question 29

Valves require air pressure to stay open If air pressure is removed, the valve will naturally close Under a power outage, the source of the air pressure would be lost and the valve would "fail" in the closed position Designed to stop flow in an emergency

Answer 29

Normally closed/fail closed

Question 30

This applies to a control valve such as ball or butterfly that does not react to loss of power This would be useful when the process cannot be shut down or where it is unnecessary to halt the process

Answer 30

Fail in place

Question 31

Operate either in fully open or fully closed states To operate this valve and force it to its driven position compressed air (or current) must be supplied to the actuator

Answer 31

ON/OFF Valves

Question 32

Use analogue outputs from the control system to allow a certain degree of valve opening raher than ON/OFF

Answer 32

Control valves ## Footnote Sometimes higher pressures are required to actuate the valve: An additional penumatic positioner can be used. This has its own higher pressure air supply.

Question 33

A device which will open or close a circuit when a parameter reaches a certain level

Answer 33

Switch

Question 34

A device which sends a signal of the magnitude of the parameter

Answer 34

Transmitter

Question 35

allow a 24Vdc signal input to the control system, typically confirming the valve is in its driven position.

Answer 35

valves with single feedback

Question 36

allow a 24V dc signal indicating that the valve is open or indicating that the valve is closed

Answer 36

valves with double feedback

Question 37

digital input to the control system

Answer 37

feedback

Question 38

What feedback is provided using and are fitted to the brackets that connect the valve to the actuator.

Answer 38

Proximity switches ## Footnote output 24V

Question 39

What feedback is provided using and are fitted on top of the actuator

Answer 39

Micro switches ## Footnote output 24V

Question 40

phenomenon where a fluctuating current which by defition has a magnetic component induced an electromotive force in a target object.

Answer 40

Inductance

Question 41

required so that the control system has verification of the valve status

Answer 41

valve feedback

Question 42

type of signal feedback is

Answer 42

digital

Question 43

a form of safety override to prevent dangerous or damaging situations.

Answer 43

Software and hardware interlocks

Question 44

uses the control system wiring/signals. For example, a discahrge pump cannot empty a vessel if the tank outlet valve upstream of the pump inlet is closed

Answer 44

Software interlock

Question 45

does not use the software or control system wiring

Answer 45

hardware interlock ## Footnote For example a pressure switch is fitted downstream of a pump. The switch output would be linked to the pump electrical supply breaker and the pump would cut off when the switch was activated.