Valve Sizing Considerations

8/10/2019 Valve Sizing Considerations

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Valve Sizing ConsiderationsSteven Hocurscak

Metso Automation



© Metso

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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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=




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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




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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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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

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Valve Sizing Considerations