Control Valve Flashcards
What does a control valve assumptions typical consist of
Valve body
Internal trim parts
Actuator
What is the actuator responsible for?
Moving the valve stem up and down
What happens when the valve stem is moved up and down
The opening Through which flow is allowed changes
An activator is something that converts
Energy into motion
What are the four types of actuators
Pneumatic, hydraulic, electric and manual
How are manual actuators powered?
Manual actuators are powered by hand and uses levers gears or wheels to move the valve stem
What are the advantages of manual actuators?
They are inexpensive, self contained, and easy to operate
What are the disadvantages of a manual actuator
Some large valves are impossible to operate manually and can be located in remote, toxic or hostile environment to humans
How are pneumatic activators powered and how?
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
What are two ways and pneumatic actuator could be arranged
Springed open or springed close
How are hydraulic actuators powered? And how?
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
How are electric actuators powered?
By any electric motor type provides torque to operate valve
What are the advantages of an electric actuator?
They are quiet, non-toxic and energy efficient
What is a disadvantage of electrical actuators?
They require electricity and sometimes electricity is not available, but they can work on batteries as well
Operation of a pneumatic control valve
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.
What are the types of control valves action?
Air to open
Air to close
what is an air to close valve?
Valves where increasing air pressure psi closes the valve.
When is an air to close valve used?
In processes where flow/supply is required, even if there is a fail in the system, resulting in no air pressure
What is an air to open valve?
A valve where increasing air pressure opens the valve
What is the fail safe in a air to close valve?
Fail to open this is important why you never want to stop flow in a system such as a reactor or oil system
Where is the spring place in an air to close valve and where is the air input?
The spring is Below the diaphragm and the air input is above the diaphragm
What is the fear safe operation of an air to open valve?
Fail to close
Where is the spring located and the input located in the air to open valve?
The spring is above the diaphragm and the air input is below the diaphragm
In what kind of processes is the air to open valve used? Give an example
Processes where you were once the valve to remain closed if there is a fail in air pressure or PSI
Gas station
Explain the operation of an air to close valve
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
Explain the operation of an air to open valve
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
What happens when there’s no air in the air to open valve
The valve closes
What is direct actuator ?
A diaphragm activator that extends the actuator stem when diaphragm pressure increases
What is a reverse actuator?
Diaphragm activator that retracts the actuator stem with increasing diaphragm pressure
What is a double acting actuator
An actuator in which power is supplied in either direction
What is the flow characteristic of a control valve?
Do relationship between the floor is true valve and valve travel as the valve travel is varied from 0 to 100%
What is it important to consider when selecting a valve?
Valve size
Valve style
Inherent characteristics
What is one reason is it important to properly size the valve?
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
What is another reason it is important to properly size the valve
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
What is flashing
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
What does flashing cause
It causes serious erosion damage to the valve trim and its parts, which is characterised by a smooth polished appearance
How is erosion minimised
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
What is cavitation
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
What does capitation cause
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
