Study Unit 7

Question 1

Typical FCE’s used in the minerals processing industry

Answer 1

Conveyor belts
Mass flow feeders
Variable speed pumps
Control valves

Question 2

Typical FCE’s used in the chemical industry

Answer 2

Variable speed pumps

Control valves

Question 3

Ball valves

Answer 3

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.

Question 4

Why should ball valves not be used in a partially open condition?

Answer 4

Misalignment between flow direction and opening causes a large pressure drop.

Question 5

Plug valves

Answer 5

Quarter-turned operated valves.

Consists of conical/cylindrical plug. Valve is open when plug’s slit is in line with the flow.

Question 6

Gate valve

Answer 6

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

Question 7

Why should gate valves not be used as throttling valves?

Answer 7

In partly open conditions erosion of the gate might take place.

Question 8

Why can gate valves not be open and closed quickly/frequently?

Answer 8

There is a considerable amount of friction on the valve when opening and closing

Question 9

Butterfly valve

Answer 9

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.

Question 10

Why are butterfly valves easy to install?

Answer 10

The valve is compact, resembling a metal disk

Question 11

Why can butterfly valves be used to deliver slurries and fluids with suspended solids?

Answer 11

There are no cavities for deposition of solid particles inside the valve body

Question 12

Globe valve

Answer 12

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.

Question 13

How does a control valve work?

Answer 13

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.

Question 14

What is the purpose of a control valve?

Answer 14

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.

Question 15

Why is the concept of Cv purely theoretical?

Answer 15

Differential pressure across the valve can never be constant for all valve openings

Question 16

Why is Cv important?

Answer 16

Cv coefficient of a valve is used by suppliers to qualify the valve

Question 17

What do we look at to obtain inherent characteristics?

Answer 17

Graph of Cv (which is also flow, but with a constant pressure drop) versus valve opening

Question 18

Quick opening valve curve shows that …

Answer 18

… 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

Question 19

Linear valve curve shows …

Answer 19

… a linear relationship between the valve opening and the flow development

Question 20

Equal percentage valve curve shows …

Answer 20

… that flow development takes longer, with only 20% of the flow occurring at 56% valve opening

Question 21

What do we look at to obtain operating characteristics?

Answer 21

Flow (% of maximum) vs lift (% of maximum)

Question 22

What does the β parameter tell us?

Answer 22

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)

Question 23

The smaller the β value …

Answer 23

…the more the operating characteristics will exhibit quick opening like behaviour

Question 24

What are the consequences of a small β value?

Answer 24

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.

Question 25

What are the reasons for a small β?

Answer 25

• The valve is too big for the control application. This usually happens when the pump was sized for the pipeline without taking valve pressure drops into account, and a control valve is installed afterwards. A large valve usually has less differential pressure, but also a very small β as a consequence.
• The wrong valve type was chosen for the control application, leading to typical quick opening valve behaviour (due to a small β).

Question 26

Valve size

Answer 26

Flow channel diameter through the valve

Question 27

Rule of thumb when choosing valve size

Answer 27

The rule of thumb states that valve size should be smaller than the pipe diameter, but larger than 50% of the pipe diameter in order to ensure a beta value >= 30%

Question 28

Gain

Answer 28

The variation in the output of a control loop element divided by the variation in input to that element.

Question 29

Valve used if there is a decrease in process gain

Answer 29

Equal percentage valve

Question 30

Valve used if there is an increase in process gain

Answer 30

Linear valve

Question 31

Valve used in a linear process

Answer 31

Equal percentage valve

Question 32

In their operating characteristics equal percentage valves will become more …

Answer 32

… linear

Question 33

In their operating characteristics quick opening valves will become more …

Answer 33

… even more quick opening

Question 34

In their operating characteristics linear valves will become more …

Answer 34

… quick opening

Question 35

Valve used for open/close control

Answer 35

Quick opening valve

Question 36

Cavitation

Answer 36

Actual gas bubbles will form when the pressure drops to below the vapor pressure. At a point where the pressure increases to above the vapor pressure, these bubbles will “collapse” again (return to liquid), and the volume will punctually decrease rapidly, leading to high metal erosion of the valve body as small metal particles are aggressively pulled from the metal valve body.

Question 37

Flashing flow

Answer 37

A consequence of low delivery pressure to the valve, leading the formed gas bubbles to not “collapse” due to valve exit pressures staying below the vapor pressure of the working fluid. This can also lead to problems with downstream process units, or lower volumes of fluid in the pipes, and thus reduces production.