Upload
others
View
10
Download
0
Embed Size (px)
Citation preview
Control valve basics…
I.S.A (The International Society ofAutomation) Defines Control Valve as a PowerOperated Device that Modulates the FluidFlow Rate in a Process Control System
Control Valve is an assembly that includes, at minimum, a valve body assembly (Globe, butterfly, ball) and an actuator (manual, pneumatic, hydraulic, electrical Etc).
Actuator
Valve assembly
Positioner
Control valve - Definition
CONTROLLER
P
PR XTR
4- 20 mA
Pneumatic
P1=20 bar.g P2=10 bar.g
Supply air
Simple control loop
Why Control Valve
Manual On/Off Valve Manual control valves
Valve Types
Linear action control valves Rotary action control valves
Globe valve
3 Way valve
Angle valve Ball valveButterfly valve
V-ball valveGate valve Plug valve
Valve Types
Linear action control valves Rotary action control valves
Ball valves
Butterfly valves
Globe valves
The flow path of Globe valve is relatively more complex than ball valve or
butterfly valve hence Globe valve Cv is lower than ball & butterfly valves
Valve Types
Piston Diaphragm
Linear Rotary
Pneumatic Hydraulic Electric
Actuators
Actuator Types
Actuator Types
Pneumatic actuators
Electrical actuators
Hydraulic actuators
100
80
60
40
20
020 40 60 80 100
Valve Plug travel
Flo
w r
ate
Trim characteristics
Quick opening
Linear
Equal percentage
Plug Types
Quick opening Linear Equal percentage
Leak path
Unbalanced design
The fluid pressure is not balanced undershut-off condition in unbalanced design.This force assumes major significance inlarge size valves.
-Advantage: Better shut-off
-Disadvantage: Higher actuator force.
Balanced design
The fluid pressure acting on the plug isbalanced as the effect of pressure actingbelow the plug (upward force) iscountered by the pressure acting abovethe plug head (downward force) .
-Advantage: Lower actuator force
-Disadvantage: Additional leak path isintroduced
Leak path
Trim Types
Contoured Trim Caged Trim
Top Guiding
Cage Guiding
Cage Types
Ported cageEqual percentage
Drilled hole cagesEqual percentage
Drilled hole cagesLinear characteristic
Drilled hole cages, Multiple cage designs
Variable resistance cage
Flow Coefficient (Cv)
Water Flow
Oil Flow
Honey Flow
→ Valve Flow coefficient (Cv) = The number of US gallons / minute of 60 deg.F water
that will flow through the valve with 1psi Pressure drop.
→ Basic equation :
→ Check for choked flow :
Pvc = FF x Pv
(Pv = Fluid Vapor Pressure, FF = Critical Pressure Ratio , FF = 0.96-0.28 Pv/Pc )
→ Compensate for the swage factors / Consider viscosity
Liquid sizing
PChoked = FL2 (P1-Pvc)
→ It is very vital to understand the Bernoulli's principle - to understand valve sizing- to understand Cavitation - to understand flashing
→ v is the the fluid flow velocity→ g is the acceleration due to gravity→ z is the elevation→ p is the pressure→ ρ is the density of the fluid
From the above equation we could infer that if the velocity increases the pressure would decrease and vice versa
Bernoulli’s principle
------ equation1
Bernoulli’s principle / Continuity equation→ The continuity equation is
Q (flow rate) = A1 X V1 = A2 X V2 (area X velocity) ------ equation2
By applying equations 1 & 2 in a control valve and taking into consideration the pressure loss due to flow turbulence we can infer the following…
area-A1
Pressure- P1
velocity -V1
area-A2
Pressure- P2
velocity -V2
area-Avc
Pressure- Pvc
velocity –Vvc
P1 Inlet Pressure
P2 Outlet Pressure
V2 Outlet VelocityV1 Inlet Velocity
PVc
VVc
Valve inlet Valve outlet
Pv Vapour Pressure
Single Stage Letdown
Liquid service – pressure recovery curve
P1
P2
V2V1
PVc
VVc
Outlet Velocity
Outlet Pressure
Valve inlet Valve outlet
Pv Vapour Pressure
Inlet Velocity
Inlet Pressure
Cavitating service
P1
P2
V2V1
PVc
VVc
Outlet Velocity
Outlet Pressure
Valve inlet Valve outlet
Pv Vapour Pressure
Inlet Velocity
Inlet Pressure
PVc
Flashing service
→ Cavitation & Flashing are liquid phenomena
→ There is bubble formation when the static pressure of the fluid falls below the vapour pressure.
→The bubbles formed collapse when the static pressure increases above the vapour pressure. This is cavitation.
→The bubbles formed are carried downstream if the outlet pressure is less than vapour pressure. This phenomena is flashing.
→Both Cavitation and Flashing can cause excessive erosion on Trim parts, body and downstream pipe.
→Cavitation can be fully eliminated by offering suitable trim but not flashing.
→Low recovery valves are less prone for cavitation
Cavitation and Flashing
Pressure
P1
P2
V2V1
VelocityPVc
FlashpointPv
VVc
VVc
VVc
PVc
PVc
3 cage pressure & velocity profile
Cavitation
Pressure & velocity profile in a 3 cage trim
1 cage pressure profile
Outer Cage
Mid Cage
Inner Cage
Valve Plug
Flow
P1
Pvc(P2)
Pvc(P2)
P?
P?
Multiple stage letdown
FLOW Pvc(P2)
P1
P?
P?
Valve Inlet P1?
The Theory3 - Cages
V trimexit?
Multiple stage letdown
P1
V1 V2
P2
Inlet Outlet
VALVE
Pressure
Velocity
Flashpoint
Pv
InletPressure
Inlet Velocity
Outlet Velocity
Outlet Pressure
The TheoryDisk Stack
Multiple stage letdown
Multiple stage letdown
Pressure
P1
P2
V2V1
Velocity
FlashpointPVc
Flashing
VVc
InletPressure
InletVelocity
Vc =Vena Contracta
Outlet Velocity
Outlet Pressure
Inlet Outlet
Valve
Pv Vapour Pressure
Single StageLetdown
Flashing service – Pressure & velocity profile
Flow over + 1CC Satellite FC Angle valveSeat exit diffuser / baffles By choice of material Liner for the body Can also use 2cage / 3 cage or stack designEccentric rotary plug valve
Flashing service solution
Flo
w
Choked Flow
∆p
∆p allowable
∆p actual
Actual flow
Flow estimated with ∆p actual
Chocked flow
Cv = W
27.3 Fp Y (X P1g1)
W = Mass Flowrate (kg/hr)
Fp = Piping Geometry Factor
Y = Expansion Factor
X = Pressure Drop Ratio
P1 = Upstream Pressure (bar A)
g1 = Specific Weight (kg/m3)
Valve sizing coefficient (Cv) - Gas
Control valve noise
Noise is a random mixture of sound waves of varying amplitude and frequency
Rapid expansion of gases across the seat and turbulence is the primary source of the control valve noise.
Other sources of noise are …
Cavitation / Flashing in liquids, Resonance, Plug instability etc.
60
70
80
90
100
110
120
010
016
025
040
0
630H
z 11.
62.
5 46.
3 10
15KHz
LpA
in d
B
Frequency
Typical spectrum of control valve noise
dB
–“A
wei
ghti
ng
corr
ecti
on
Frequency in Hertz
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
5
20 50 100 200 500 1000 2000 5,000 10,000
Audible control valve noise
Source treatments
→Shifting control valve noise from audible frequency to high frequency by flowing through numerous small holes
→Creating flow turbulence and resistance
- dividing the flow into numerous flow streams
- changing the direction of flow / flowing through a torturous path
- allowing small streams to impinge against each other and lose energy
→By controlling fluid velocities
Path treatments
→ Sound transmission from the valve & pipe line to surrounding environment can be reduced by heavy pipe wall / insulation etc.
→ Proving in-line diffusers / baffle plates / silencers
→Proving vent silencers
Noise reduction techniques
Disk Stack Trim Designs
Noise control – source treatments
Multiple cage Trim Design
Noise control – path treatments
Vent silencer
Inline silencer
Diffuser
60
70
80
90
100
110
120
0 1 2 3 4 5 6 7 8
Allo
wab
le (
dB
A)
Permissible Exposure (Hours in a Day)
Duration of exposure Vs allowable noise
Control Valve Datasheet
Specification sheet
Windows Platform
Download via www.servernglocon.com
Sizing program
IEC 60534 - 8.3 Control valve aerodynamic noise predictionIEC 60534 - 8.4 Control valve hydrodynamic noise predictionANSI/ISA-75.01 Flow equations for sizing control valvesANSI/ISA-75.02 Control valve capacity test procedureANSI/ISA - 75.08.01 Face to Face Dimensions for Integral Flanged GlobeANSI/ISA - S75.19 Hydrostatic Testing of control valvesAPI Specification 6D Petroleum and Natural Gas Industries Pipeline ValvesAPI STANDARD 598 Valve Inspection and TestingASME B 16.34 Valves - Flanged Threaded , and Welding EndASME B16.10 Face to Face and End to End Dimensions of ValvesASME B16.20 Metallic Gaskets for Pipe FlangesASME B16.47 Larger Diameter Steel Flanges (NPS 26 Through NPS 60)ASME B16.5 Pipe Flanges and flanged fittings (NPS 1/2 Through NPS 24)ASME B16.9 Factory - Made Wrought Steel Butt welding FittingsASME QME-1 Qualification of Active Mechanical Equipment Used in Nuclear Power PlantBS 6364 Specification for Valves for cryogenic serviceISO 15848 Part 1 & 2 Fugitive Emmission - Proto Type Test / Production testMSS SP-55 Quality standard for Steel Castings for valvesNACE MR0175/ISO 15156 General Principles for selection of cracking-resistant materials ASME PBVC - Section II Part D Material PropertiesASME PBVC Section V NDE ASME PBVC Section VIII Pressure Vessels – Divisions 1 and 2ASME PBVC Section IX Welding & Brazing
Standards
Actuator Sizing
Noise Calculations
Velocity Calculations
Warning Alerts
Detailed Specification Build
Sizing program
SG Mobile App Sizing OutputsSizing Inputs
Severn Glocon Android Mobile App
CONTROLLER
P
PR XTR
4- 20 mA
Pneumatic
P1=20 bar.g P2=10 bar.g
Supply air
Simple control loop
Positioner:
Valve positioner delivers pressurized
air to the valve actuator so that the
position of the valve stem or shaft
corresponds to the set point from the
control system. Valve positioners are
typically pneumatic or analog I/P or
smart.
Positioner require position feedback
from the valve stem or shaft and
deliver pneumatic pressure to the
actuator to open and close the valve
Positioner
Need for positioner:
• More accurate control: Since valve positioners know the valve’s exact position, they
provide more precise control.
• Faster control: Positioners help control valves respond faster to changes in the process
variable, minimizing the amount of time the system is operating above or below the set
point.
• Consistent valve position: A positioner is one solution to help stabilize valve position.
• More flexible configurations and functions:
• Minimize the effects of friction. Friction in the valve stem packing contributes to both
hysteresis and dead band, which reduce productivity
Positioner Need
Types
• Pneumatic devices send and receive pneumatic signals. They are intrinsically safe and can
provide a large amount of force to close a valve.
• Electro-pneumatic valve positioners convert current control signals to equivalent
pneumatic signals. They use a mixture of electricity and air.
• Digital or “smart” devices use a microprocessor to position the valve actuator and monitor
and record data. They are very accurate, use less air than analogue positioners, and allow
for online digital diagnostics
Type of Positioner
Electro-pneumatic positioner Digital or smart valve controllerPneumatic positioner
APS Actuator Positioning Sensor
SPS Spool Positioning Sensor
I2PCurrent to Pressure convertor
LUI Local User Interface
Ps,P1,P2 Pressure sensors
µC Microcontroller
S Supply pressure
Schematic of Digital Positioner
Actuator
Major position feed back sensor
Spool position feed back sensor
Primary controller
Secondary controller
SpoolI2P+-
+-
ActuatorSet point
SpoolSet point
Positioner Control Structure
What are the consequences of getting the Technical selection process
Wrong?
Control Valve (Trim) Erosion Damage
#1 Damage to Seat Ring
#3 Damage to Inner Cage
#2 Damage to Seat Ring /Cage interface
#5 Damage toInner Cage
#6 Damage to Plug
#4 Damage to Seat Diffuser
Control Valve (Trim) Cavitation Damage
#1 Damage to Seat Ring
#2 Damage to Cage /Seat Interface
#3 Damage toIntermediate Cage
#4 Damage to Inner Cage
#5 Damage to Plug
#6 Damage to Seat Ring
Choke Valve (Trim) Erosion Damage
#1- Erosion to Hardened Flow Sleeve
#3 - Erosion toStellite Cage
#2 - Erosion to Carbide Plug & Flow Sleeve
#4 - Erosion to CarbidePlug / Sleeve
#5 - Erosion to CarbideDisk Stack assembly
#6 - Erosion to CarbidePlug profile
Fatigue / Vibration Failure
#1 - Fracture failureof Carbide
Plug / Sleeve
#2 - Partial detachment of Carbide Plug nose
#5 - Fatigue failure of Plug Stem
#4 - Fatigue failure ofPlug ‘Balance Seal’
Ring#3 - Total detachment
of Carbide Plug / Sleeve
Valve SelectionValve Applications Required Specification
Compressor Anti-surge valves Quick Opening within the range of 0.5 to 2 seconds
Pump Recycle Valves Handle very High Pressure Drop & Cavitation Service
Separator Level control Valve Linear Flow characteristics, Erosive Process fluid, Hardened Trim Materials
Gas to Flare Valves High Aero Dynamic Noise, Source Treatment + Path Treatment
Seawater overboard dump valve
Very High Pressure drop and possibility of vacuum in downstream of valve
Pipeline Booster station valves Electro-Hydraulic Actuated Valves
Gaseous Oxygen Valves High Pressure Gaseous Oxygen, Fire retardant material
Cryogenic Valves LN, LOX, LNG Process fluids. Valves with extended bonnet design to avoid cold leakage.
Letdown valves Flashing Service & Angle Valves
Mixing & Diverting 3-Way Valves
Valve SelectionValve Applications Valve Type
High Cv & Low dP Butterfly Valves
High dP Globe Valves
Pulp & Slurry Applications Ball Valves & Plug Valves
Low Pressure drop Valves Globe & Top Guided Valves
Suspended Particles Globe & Top Guided Valves
High Pressure Drop Valves Globe & Cage guided Valves
Cavitation Service Globe & Multistage Trim Design
Aero Dynamic Noise service Globe & Multistage Trim Design
High Outlet Velocity Larger Globe valve with reduced Trim Size
High Rangeability in Flow Globe with Eq% or customized characteristics. Sometime two valves in split range to be offered
Constant Pressure drop Globe Valve with Linear Flow Characteristics
Liquid Level control Globe Valve with Linear Flow Characteristics
Globe ValvesEccentric
PlugBall Butterfly
GeneralService
HighPerformance
SevereService
Valve type vs Application category
Thank You