Study Unit 7 Flashcards
Typical FCE’s used in the minerals processing industry
Conveyor belts
Mass flow feeders
Variable speed pumps
Control valves
Typical FCE’s used in the chemical industry
Variable speed pumps
Control valves
Ball valves
Quarter-turned operated valves
Valve is open when the ball’s hole is in line with the flow and closed when the ball is pivoted 90 degrees by the valve handle (hole is perpendicular to the flow).
Mostly used for shut-off applications.
Why should ball valves not be used in a partially open condition?
Misalignment between flow direction and opening causes a large pressure drop.
Plug valves
Quarter-turned operated valves.
Consists of conical/cylindrical plug. Valve is open when plug’s slit is in line with the flow.
Gate valve
Sliding type valve
Gate slides down to close valve. When fully open, the flow area is equal to the area of the pipe, thus there is negligible pressure drop in valve.
Mainly used as on-off valves
Why should gate valves not be used as throttling valves?
In partly open conditions erosion of the gate might take place.
Why can gate valves not be open and closed quickly/frequently?
There is a considerable amount of friction on the valve when opening and closing
Butterfly valve
Quarter turning valve (quick operation)
There is minimum obstruction of the fluid flow through the valve as the flow passes around the disk aerodynamically. This results in a very low pressure drop across the valve.
Valve can be operated easily without requiring high torque and without wear-and-tear.
Why are butterfly valves easy to install?
The valve is compact, resembling a metal disk
Why can butterfly valves be used to deliver slurries and fluids with suspended solids?
There are no cavities for deposition of solid particles inside the valve body
Globe valve
Used for regulating flow in a pipeline, consisting of a movable disk type element and a stationary ring seat in a generally spherical body. The body is split into two parts by an internal baffle. This forms a “seat” on which a movable plug can be screwed in to close the valve. This plug is connected to a stem which is operated by screw action using a hand wheel.
How does a control valve work?
In essence, the pneumatic valve receives an amount of compressed air at the top( see loading pressure connection) which forces the actuator stem downwards against the actuator spring. This valve plug stem is connected all the way through the valve into the valve body where it ends in a valve cage. The cage then moves into the flow path of the fluid, indicated by an arrow inside the valve body, to regulate the flow through the valve. Releasing some air from the diaphragm casing will allow the actuator spring to force the actuator stem and case upwards, with the effect of more fluid that can pass through the valve.
What is the purpose of a control valve?
The valve and cage creates a pressure drop in the working fluid across the valve body. When closing or opening the path through which the liquid or gas (the fluid) must flow through the valve, this pressure drop is manipulated, which subsequently manipulates the amount of fluid that will pass through the valve.
Why is the concept of Cv purely theoretical?
Differential pressure across the valve can never be constant for all valve openings
Why is Cv important?
Cv coefficient of a valve is used by suppliers to qualify the valve
What do we look at to obtain inherent characteristics?
Graph of Cv (which is also flow, but with a constant pressure drop) versus valve opening
Quick opening valve curve shows that …
… a large percentage of theoretical flow occurs already at relatively low valve openings, e.g. it is seen that 50% flow occurs at roughly 27% valve opening
Linear valve curve shows …
… a linear relationship between the valve opening and the flow development
Equal percentage valve curve shows …
… that flow development takes longer, with only 20% of the flow occurring at 56% valve opening
What do we look at to obtain operating characteristics?
Flow (% of maximum) vs lift (% of maximum)
What does the β parameter tell us?
It is an indication of the pressure differential across the valve when the valve is open at its maximum (100% valve lift) divided by the pressure differential at a fully closed valve (0% valve lift)
The smaller the β value …
…the more the operating characteristics will exhibit quick opening like behaviour
What are the consequences of a small β value?
A large flow profile development is observed at initial valve lifts and at later valve lifts, this flow profile development is reduced.
This may lead to increased valve erosion as the valve is constantly operated at low valve lift percentages.
