Control Valve

Question 1

What does a control valve assumptions typical consist of

Answer 1

Valve body
Internal trim parts
Actuator

Question 2

What is the actuator responsible for?

Answer 2

Moving the valve stem up and down

Question 3

What happens when the valve stem is moved up and down

Answer 3

The opening Through which flow is allowed changes

Question 4

An activator is something that converts

Answer 4

Energy into motion

Question 5

What are the four types of actuators

Answer 5

Pneumatic, hydraulic, electric and manual

Question 6

How are manual actuators powered?

Answer 6

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

Question 7

What are the advantages of manual actuators?

Answer 7

They are inexpensive, self contained, and easy to operate

Question 8

What are the disadvantages of a manual actuator

Answer 8

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

Question 9

How are pneumatic activators powered and how?

Answer 9

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

Question 10

What are two ways and pneumatic actuator could be arranged

Answer 10

Springed open or springed close

Question 11

How are hydraulic actuators powered? And how?

Answer 11

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

Question 12

How are electric actuators powered?

Answer 12

By any electric motor type provides torque to operate valve

Question 13

What are the advantages of an electric actuator?

Answer 13

They are quiet, non-toxic and energy efficient

Question 14

What is a disadvantage of electrical actuators?

Answer 14

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

Question 15

Operation of a pneumatic control valve

Answer 15

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.

Question 16

What are the types of control valves action?

Answer 16

Air to open
Air to close

Question 17

what is an air to close valve?

Answer 17

Valves where increasing air pressure psi closes the valve.

Question 18

When is an air to close valve used?

Answer 18

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

Question 19

What is an air to open valve?

Answer 19

A valve where increasing air pressure opens the valve

Question 20

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

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

Fail to close

Question 23

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

Answer 23

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

Question 24

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

Answer 24

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

Question 25

Explain the operation of an air to close valve

Answer 25

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

Question 26

Explain the operation of an air to open valve

Answer 26

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

Question 27

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

Answer 27

The valve closes

Question 28

What is direct actuator ?

Answer 28

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

Question 29

What is a reverse actuator?

Answer 29

Diaphragm activator that retracts the actuator stem with increasing diaphragm pressure

Question 30

What is a double acting actuator

Answer 30

An actuator in which power is supplied in either direction

Question 31

What is the flow characteristic of a control valve?

Answer 31

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

Question 32

What is it important to consider when selecting a valve?

Answer 32

Valve size Valve style Inherent characteristics

Question 33

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

Answer 33

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

Question 34

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

Answer 34

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

Question 35

What is flashing

Answer 35

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

Question 36

What does flashing cause

Answer 36

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

Question 37

How is erosion minimised

Answer 37

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

Question 38

What is cavitation

Answer 38

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

Question 39

What does capitation cause

Answer 39

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