Specifying control valves and control system achitecture

Question 2

Control the flow

Answer 2

Control valves

Question 3

Ensure there is no backflow

Answer 3

Non return valves

Question 4

Perform a safety function

Answer 4

Safety critical valves

Question 5

Signal from the controller

Answer 5

4-20mA analogue

Question 6

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

Answer 6

Valve plug/disc

Question 7

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

Answer 7

Actuator

Question 8

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

Answer 8

Body

Question 9

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

Answer 9

Stem

Question 10

Closure for valve body, through which the stem passes

Answer 10

Bonnet

Question 11

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

Answer 11

Seat

Question 12

Surrounds the plug and has openings that determine flow

Answer 12

Cage

Question 13

Specifying a control valve

Answer 13

• 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

Question 14

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

Normal range of pneumatic pressure is 3-15psig

Answer 14

I/P (current pressure) converters

Question 15

The valve is closed as pressure is applied

Normally open/fail open if air fails

Answer 15

Direct acting

Question 16

The valve is opened as pressure is applied

Answer 16

Reverse acting

Normally closed/fail closed if air fails

Question 17

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

Answer 17

Pneumatic diaphragm

Question 18

For rotating valves such as butterfly valves

Answer 18

Rotary control valves

Question 19

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

Answer 19

Piston actuator

Question 20

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

Answer 20

Electric Control Valves

Question 21

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

Answer 21

Handwheels

Question 22

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

Answer 22

up

Question 23

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

Answer 23

Down

Question 24

Three different “speeds” of control valve opening

Answer 24

• Quick opening
• Linear
• Equal percentage

Question 25

Causes the difference in valve opening

Answer 25

- shape of the plug - seat arrangement

Question 26

- Provides a large part of its flow capacity with relatively small lift as the valve opens - Slower thereafter

Answer 26

Quick opening

Question 27

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

Answer 27

Linear

Question 28

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

Answer 28

Equal percentage

Question 29

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

Answer 29

20-70%

Question 30

Ratio of normal maximum operating flow to minimum controllable flow

Answer 30

Turndown ratio

Question 31

Important factor used for sizing valves

Answer 31

Cv of the valve Cv = 1.16Q √ SG/ Delta P ## Footnote SG = specific gravity of the fluid

Question 32

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

Answer 32

Control valve cams

Question 33

Non return valve types

Answer 33

- Swing check valve - Wafer type check valve - Disc check valve - Flap type check valve - Spring lift check valve - Lift check valve

Question 34

Types of safety critical valves

Answer 34

- Block valves - Emergency Block Valves - Zone valves - Fail safe valves - ESD valves - Excess flow valves

Question 35

Separate one item of equipment and seals against liquid flow

Answer 35

Block valves

Question 36

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

Answer 36

Emergency Block Valves (EBV)

Question 37

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

Answer 37

Zone valves

Question 38

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

Answer 38

Fail safe valves

Question 39

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

Answer 39

ESD valves

Question 40

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

Answer 40

Excess Flow Valves