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Instrumentation, Measurement and Control > Specifying control valves and control system achitecture > Flashcards

Specifying control valves and control system achitecture Flashcards

1
Q
A
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2
Q

Control the flow

A

Control valves

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3
Q

Ensure there is no backflow

A

Non return valves

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4
Q

Perform a safety function

A

Safety critical valves

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5
Q

Signal from the controller

A

4-20mA analogue

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6
Q

The part that modulates (obstructs) the flow rate with either linear or rotary motion

A

Valve plug/disc

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7
Q

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

A

Actuator

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8
Q

The pressure containing part of the valve. Contains the components that contact the fluid

A

Body

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9
Q

The connector from the actuator to the inside of the valve - transmits force to move the plug/disc

A

Stem

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10
Q

Closure for valve body, through which the stem passes

A

Bonnet

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11
Q

The surface that the valve plug contacts when the valve is closed, thus forming the seal

A

Seat

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12
Q

Surrounds the plug and has openings that determine flow

A

Cage

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13
Q

Specifying a control valve

A
  • Type of fluid (Viscosity, SG of fluid)
  • Maximum and minimum flow-rate
  • Input P at maximum and minimum flowrates
  • Pressure drop at specified max and min flowrates
  • Valve body and seal material (contact materials)
  • Valve type (globe, diaphragm, gate, or pinch)
  • Valve flow characteristic
  • Fail position. (fail closed, open or in position)
  • Mechanical stops
  • Temperature of fluid
  • Connections (Inlet and outlet pipe size)
  • Certification required
  • Type of seals
  • Tagging requirements
  • Named manufacturer
  • Manual, air actuated (3-15 psig), 4-20 mA, Hart
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14
Q

Used to convert a signal in electrical current to a pneumatic (pressure) signal

Normal range of pneumatic pressure is 3-15psig

A

I/P (current pressure) converters

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15
Q

The valve is closed as pressure is applied

Normally open/fail open if air fails

A

Direct acting

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16
Q

The valve is opened as pressure is applied

A

Reverse acting

Normally closed/fail closed if air fails

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17
Q

Used by most control valves which is spring loaded. Air displaces it in proportion to the air supply pressure

For large valves considerable forces
are sometimes required. In this
case, higher pressures can be used
with an I/P or P/P converter used

A

Pneumatic diaphragm

18
Q

For rotating valves such as butterfly valves

A

Rotary control valves

19
Q
  • Pneumatically controlled piston for large vales that require greater force to move
A

Piston actuator

20
Q

Eliminate the need for a compressed air system and avoid inefficiencies due to frictional losses in transmission. They can be more reliable, require less maintenance and be more tolerant to environmental temperature and pressure variations. They offer superior control and are very easy to integrate into new control systems.

Fewer limitations on size of valve

A

Electric Control Valves

21
Q

For diaphragm actuators are often used as adjustable travel stops. They also provide a ready means of positioning the control valve in emergency

A

Handwheels

22
Q

Closing of the valve moves the system curve … the Head vs Flow curve

A

up

23
Q

Opening of the valve moves the system curve … the Head vs Flow curve

A

Down

24
Q

Three different “speeds” of control valve opening

A
  • Quick opening
  • Linear
  • Equal percentage
25
Q

Causes the difference in valve opening

A
  • shape of the plug
  • seat arrangement
26
Q
  • Provides a large part of its flow capacity with relatively small lift as the valve opens
  • Slower thereafter
A

Quick opening

27
Q

Valve plug is shaped so that flowrate is directly proportional to the valve lift (% travel)

A

Linear

28
Q

Valves have a plug shape so that each increment in valve lift (% travel) increases the flowrate by a certain percentage of the previous flow
Rate of change in flowrate increases with valve opening

A

Equal percentage

29
Q

Normally the valve only controls the flow rate correctly at openings of between ….. of the fully open position.

A

20-70%

30
Q

Ratio of normal maximum operating flow to minimum controllable flow

A

Turndown ratio

31
Q

Important factor used for sizing valves

A

Cv of the valve
Cv = 1.16Q √ SG/ Delta P

SG = specific gravity of the fluid

32
Q

By using this, the valve stem travel is no longer a linear function of the pressure which changes the valve opening characteristic

A

Control valve cams

33
Q

Non return valve types

A
  • Swing check valve
  • Wafer type check valve
  • Disc check valve
  • Flap type check valve
  • Spring lift check valve
  • Lift check valve
34
Q

Types of safety critical valves

A
  • Block valves
  • Emergency Block Valves
  • Zone valves
  • Fail safe valves
  • ESD valves
  • Excess flow valves
35
Q

Separate one item of equipment and seals against liquid flow

A

Block valves

36
Q

Separate one item of equipment from another
during fire or other emergencies. These are usually arranged as fail safe emergency block valves (FSEBV)

A

Emergency Block Valves (EBV)

37
Q

Isolate units; generally located at the end of pipe racks and at ground level. Also known as battery limit valves.

A

Zone valves

38
Q

Fail in a fully opened or fully closed position upon loss of power. The correct failsafe position must be determined by process hazard evaluation

A

Fail safe valves

39
Q

Ball valves that close rapidly; and seal in
both the up and down stream directions. Same as block or zone valves. Important for SIL

A

ESD valves

40
Q

Close automatically if fluid flow rates exceed a critical value. The critical flow rate of excess flow valves depend upon the physical characteristics of the fluids as well as flow rates

A

Excess Flow Valves

Instrumentation, Measurement and Control (12 decks)

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