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Valve Sizing ConsiderationsSteven Hocurscak
Metso Automation
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Oversizing Your ValvesIssue with oversized valve
• A valve that is oversized can cause premature failure of valve trim or
body
- Oversized valves have low opening angles, and lower opening angles increase
the velocity of the liquid
- Properly sizing control valves can dramatically improve performance and life
E, loss of materialE, loss of material
E R
O S I O N , E
FLOW VELOCITY, v2
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Guideline for Proper Valve SizingSolution to oversizing your valves
• Maintaining larger opening angles (above 20%) is ideal for flow
conditions that are seen often
- If the condition is a startup case that will only be seen a few times a year, it’s okay
to size the valve in this application.
- On a normal flow condition ideally we want the valve to be open around 50%
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Guideline for Proper Valve SizingDeath by oversizing
• Seat is eroded away
• Segment shows
signs of erosion
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Guideline for Proper Valve SizingDeath by oversizing
• Hole in body
• Seat is eroded away
• Erosion pattern in
body
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CavitationDefinition of Cavitaion
• Each liquid has a different pressure and temperature at which it boils (or
vaporizes). Water for example at atmospheric pressure will boil at 100°C
(212°F).
• Dropping the pressure inside of a valve for example can effect the
temperature at which a liquid will boil.
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Water
(Liquid)
WaterVapor
(gas)
Ice
(solid).006 atm
.01°C
1 atm
100°C
Temperature
P r e s s u r e
Triple Point
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Cavitation
Pressure
Velocity
Vapor Pressure
Because of the law of conservation of energy, when the velocity of the l iquid
increases through the valve, the pressure must decrease
P1 P2PVC
A valve is represen ted as a simple orif ice plate to represent a
restriction in the flow path
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Cavitation
Cavitation occurs when the pressure inside the valve drops below the vapor
pressure of the liquid, allowing the liquid t o turn into a gas.
P1 P2PVC
Pressure
Velocity
Vapor Pressure
P1
P2
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Pressure Shock Generation Mechanisms
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• The damage caused by cavitation is very rough and pitted and can occur quickly.
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Cavitation
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The Pressure Recovery Factor of the valve indicates the size of the pressurerecovery relative to PVC and P2.
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Pressure Recovery Factor FL
P2
PVC
P1 – P2
Pressure Recovery
P2 – PVC
P1
P1 P2PVC
A valve is represented as a simple orifice plate to represent a restriction in the flow path
P1 - P2
P1 - PVCFL=
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• A high recovery valve (similar to the red line) has a greater “recovery” fromPVC to P2. This can mean the valve is more likely to experience problems
with cavitation.
• The higher the pressure recovery factor (FL) the lower the recovery of the
valve and the better the valve is able to avoid cavitation and noise issues.
P2
PVC
Pressure Recovery
P2 – PVC
P1Higher FL
(Low Recovery)
Lower FL
(High Recovery)
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Pressure Recovery Factor FL
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PVC = 5
P1 - P2
Pressure Recovery
P2 - PVC
P1 = 15
P2 = 10
FL=P1 - P2
P1 - PVC
15 - 10
15 - 5= 5
10= = .7.5=
Pressure Recovery Factor FL
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PVC = 5
P1 - P2
P1 = 15
P2 = 10
FL=P1 - P2
P1 - PVC
15 - 1015 - 8
= 57
= = .85= .714
PVC = 8P2 - PVC
Pressure Recovery Factor FL
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• Turbulence and friction cause permanent pressure loss
P1
PVVapor Pressure
P2
P1
PVVapor Pressure
P2
VELOCITY CHANGE DIRECTION CHANGE
Valve Geometry Effects FL
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P1
P2
P1
P2
MOSTLY VELOCITY CHANGE DIRECTION & VELOCITY CHANGE
Valve Geometry Effects FL
PVVapor Pressure
PVVapor Pressure
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Pressure Recovery Factor FL
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80 90 10
Percent Open
R e c o v e r y
F a c t o r , F L
Globe
Ecc. Plug
Ball
H.P. But terfly
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p2
Pi
P1
P2
Vapor pressure
Lowest system
pressure w/o plate
Lowest system pressure
with plate
Sizing w/ Orifice Plates
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Design Features
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Variable Resistor in Valve
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Design Features
• A typical defined process tells us the largest dP a valve willexperience will typically be on the minimum flow
• A fixed resistor must be sized for the maximum flow, so it’s most
effective on lower dP’s
• If a liquid application is going to cavitate often times they do on a
minimum / startup flow
Variable Resistor vs. Fixed Resistor
Pressure
Pump CurvePOPm
P
System Curve
System Pressure Drop
Q maxQ minFlowrate
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FlashingIssue with oversized valve
• Caused when the downstream pressure is below the vapor pressureof the liquid.
• Similar to cavitation, vapor bubbles form, but unlike cavitation the
vapor pressure never recovers high enough for the vapor bubbles to
implode
• Vapor bubble remain mixed in the media and travel downstream
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Flashing (Liquid)
Velocity
P1
P2Vapor pressure
Pressure
P1 P2Pvc
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• Typically, damage potential of f lashing is smaller than in
cavitation.• Damages are erosion type wear, smooth grooves and
cavities.
Flashing Damage
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Flashing SolutionsIt’s important to understand flashing is caused by the
process, not the valve. Therefore there isn ’t anything you
can do to stop the valve from flashing, it has to be aprocess change.
To prevent flashing damage to the body we turn the valve
to the FTC or non-preferred direction. When flashing
occurs it w ill h it the seat of the valve, instead of the body.
The seat is made of a harder material, and easier to
replace, thus less damaging.
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• Rotary valves FTC direction
• Hardened trim, SST body
• Discharging into vessel if possible
• Wide valve openings
• Limit flow velocities
• Enlarger downstream pipe
• No Q-trim in clear flashing service (P2 << Pv)
Flashing - conclusions
FTC direction
Valve Discharging Directly into Vessel27