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Instrumentation > Control Valve > Flashcards

Control Valve Flashcards

1
Q

What does a control valve assumptions typical consist of

A

Valve body
Internal trim parts
Actuator

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2
Q

What is the actuator responsible for?

A

Moving the valve stem up and down

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3
Q

What happens when the valve stem is moved up and down

A

The opening Through which flow is allowed changes

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4
Q

An activator is something that converts

A

Energy into motion

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5
Q

What are the four types of actuators

A

Pneumatic, hydraulic, electric and manual

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6
Q

How are manual actuators powered?

A

Manual actuators are powered by hand and uses levers gears or wheels to move the valve stem

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7
Q

What are the advantages of manual actuators?

A

They are inexpensive, self contained, and easy to operate

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8
Q

What are the disadvantages of a manual actuator

A

Some large valves are impossible to operate manually and can be located in remote, toxic or hostile environment to humans

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9
Q

How are pneumatic activators powered and how?

A

Air or another gas pressure is the power source
The air pressures acts on a piston or Bellows diaphragm creating a linear force on the valve stems

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10
Q

What are two ways and pneumatic actuator could be arranged

A

Springed open or springed close

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11
Q

How are hydraulic actuators powered? And how?

A

The powered by fluid pressure they do this by allowing the fluid pressure to act on a piston which provides linear, trust in gate or globe valves

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12
Q

How are electric actuators powered?

A

By any electric motor type provides torque to operate valve

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13
Q

What are the advantages of an electric actuator?

A

They are quiet, non-toxic and energy efficient

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14
Q

What is a disadvantage of electrical actuators?

A

They require electricity and sometimes electricity is not available, but they can work on batteries as well

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15
Q

Operation of a pneumatic control valve

A

Most valves consist of a plug on the end of a stem, the plug open or closes our office opening as system is raised or lowered. This stem is attached to diaphragm, which is driven by air pressure above or below diagram when the air pressure opposes the spring, it causes the valves, stem to move up or down, therefore, opening or closing the office opening and therefore restricting, allowing the floor of the fluid.

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16
Q

What are the types of control valves action?

A

Air to open
Air to close

17
Q

what is an air to close valve?

A

Valves where increasing air pressure psi closes the valve.

18
Q

When is an air to close valve used?

A

In processes where flow/supply is required, even if there is a fail in the system, resulting in no air pressure

19
Q

What is an air to open valve?

A

A valve where increasing air pressure opens the valve

20
Q

What is the fail safe in a air to close valve?

A

Fail to open this is important why you never want to stop flow in a system such as a reactor or oil system

21
Q

Where is the spring place in an air to close valve and where is the air input?

A

The spring is Below the diaphragm and the air input is above the diaphragm

22
Q

What is the fear safe operation of an air to open valve?

A

Fail to close

23
Q

Where is the spring located and the input located in the air to open valve?

A

The spring is above the diaphragm and the air input is below the diaphragm

24
Q

In what kind of processes is the air to open valve used? Give an example

A

Processes where you were once the valve to remain closed if there is a fail in air pressure or PSI
Gas station

25
Q

Explain the operation of an air to close valve

A

The air input is above the diaphragm, whilst the spring is below the diaphragm when an air pressure is applied that can overcome the spring force spring is contracted causes linear downward movement in its stem which causes the valve to be closed when there is no air in the system this spring will overcome the air pressure and expand, causing valve to return to open

26
Q

Explain the operation of an air to open valve

A

The spring is placed above the diaphragm, and the air input is placed below the diaphragm when air pressure is increased enough to overcome the spring force the spring is contracted causing a linear upward movement in the valves them which causes the valve opening at the oriface to open

27
Q

What happens when there’s no air in the air to open valve

A

The valve closes

28
Q

What is direct actuator ?

A

A diaphragm activator that extends the actuator stem when diaphragm pressure increases

29
Q

What is a reverse actuator?

A

Diaphragm activator that retracts the actuator stem with increasing diaphragm pressure

30
Q

What is a double acting actuator

A

An actuator in which power is supplied in either direction

31
Q

What is the flow characteristic of a control valve?

A

Do relationship between the floor is true valve and valve travel as the valve travel is varied from 0 to 100%

32
Q

What is it important to consider when selecting a valve?

A

Valve size
Valve style
Inherent characteristics

33
Q

What is one reason is it important to properly size the valve?

A

For economic purposes, because if the valve is oversize, it will not have resistance except for limited parts in its stroke, and if it is too small, it will not pass in necessary flow, even if it is wide open. this will cause unnecessary expenses since a big five is expensive and a small valve would have to be discarded for large valve

34
Q

What is another reason it is important to properly size the valve

A

For control purposes, if the valve is undersized, it never delivered the full flow and the controllable flow range will be narrow. If it is oversize the full range of the valve will not be utilized and maximum flow rate may occur close to the close position

35
Q

What is flashing

A

Flashing occurs when a liquid enters a valve, and its static pressure drops below vapour pressure, causing bubbles to form when the liquid remains under vapour pressure after downstream recovery. The bubbles formed at the vena contracta will remain in the downstream system, and the process is said to have flashed

36
Q

What does flashing cause

A

It causes serious erosion damage to the valve trim and its parts, which is characterised by a smooth polished appearance

37
Q

How is erosion minimised

A

By prevent or reducing liquid particle impact with the valve and making these surfaces as hard as possible
Lowing the velocity of the erosive flow

38
Q

What is cavitation

A

When liquid that enters a valve, static pressure drops below vapour pressure which causes bubbles and then the downstream pressure recovery is enough to raise the the liquids above vapour pressure causing the bubbles at the vena contracta to collapse/implode

39
Q

What does capitation cause

A

If the bubbles pop in close proximity to the valve surface, the energy released will gradually tear away the material, leaving a rough cinder like surface

Instrumentation (10 decks)

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