Transcript Issues on Control Valves - Universiti Teknologi MARA

Control Valves
• Purpose
• Terminologies
• Construction
• Selection criteria
• Sizing calculation
• Issues on control valve
Cheer up. It is not
as difficult as
you thought!
Introduction
• Purpose
to regulate flow of any phase (liquid, gas, fluid
and mixed phases) that could pass through it.
At the moment, solid phase (such as sand,
flour, sugar) is not possible.
• Chemical process applications
>80% of final control element found in chem.
proc. ind.
• Symbols
=
Construction
Actuator
Body
Seat
Plug
Selection Criteria
Valve action: ATO vs. ATC
 Air-to-closed (ATC) or Fail open (FO).
Use to allow flow of fluid during loss of
pneumatic air supply. This action may
save overheating of temperature related
processes such as catalyst heating or
heating of polimerization process.
 Air-to-open (ATO) or Fail closed (FC).
Use to prevent flow when no pneumatic air
supply. This action may prevent hazardous
chemical or flammable fluid from
endangering lives.
compare
compare
compare
compare
Selection Criteria
Valve action: ATO vs. ATC
The selection of control valve action would depend upon safety of personnel,
action time and process hazard upon loss of pneumatic signal.
Prevent
pressure
build-up
Preventing
reaction
upstream.
Prevent
material
from overheating.
(Smith & Corripio, 3rd. Ed., p.158)
Selection Criteria
Capacity and sizing
P in psi
Q  Cv
P
S .G.
Fluid S.G.
Q
Flowrate
In gpm
Example
Calculate the Cv of a control valve
when a 12.5 gpm fluid having a SG of
0.8 pass through it. The pressure
drop across the control valve is 14
psi.
Cv 
Q
12.5

3
P
14
S .G.
0.8
Selection Criteria
Valve characteristics: Linear, equal percentage and quick opening.
Selection Criteria
Valve characteristics: Linear, equal percentage and quick opening.
What will be the flowrate when the
indicator shows 0.75?
Quick opening
Flow coef. = 0.96
Flowrate = 0.96 x 8 = 7.7 gpm
Linear
Flow coef. = 0.75
Flowrate = 0.75 x 8 = 6 gpm
Equal percentage
Flow coef. = 0.3
Flowrate = 0.3 x 8 = 2.4 gpm
Example
8 gpm
6 gpm
4 gpm
2 gpm
0 gpm
Selection Criteria
Control valve size selection
Select an equal percentage control valve for the following condition.
Flow condition
Flowrate, gpm
Pressure drop, psi
Valve lift, %
Maximum
120
10
80
Normal
42
15
50
Minimum
13
20
20


Flow condition
Flowrate,
gpm
Pressure
drop, psi
Valve
lift, %
Cv
calculated
Cv
actual
Maximum
120
10
80
37.9
37.3
Normal
42
15
50
10.8
10.8
Maximum
13
20
20
2.9
2.93
Contiunue on next page for valve rangeability calculation…
Selection Criteria
Control valve rangeablity
Check for valve rangeability
Rinstalled
Rshelf
Q
 max
Qmin
Pmin rate
Pmax rate
 13.0
Cv , max
37.9


 13.06
Cv , min
2.9
Small difference. Selection 2” okay to use.
Issues on Control Valves
• What make a control valve becomes linear,
equal percentage or quick opening control
valve?
• Why flashing and cavitation are serious
problem in control valve?
• What causing flashing and cavitation in a
control valve?
• What is the current solution to flashing
and cavitation in control valve?
What make a control valve becomes linear, equal
percentage or quick opening control valve?
The amount of flow produced with
respect to steady increment in
stem travel.
Why flashing and cavitation are serious
problem in control valve?
Erosion
What causing flashing and cavitation
in a control valve?
Hear sound of cavitation
and flashing.
What is the current solution to flashing and
cavitation in control valve?
Read p.135 of Fisher Control Valve Handbook