Control Valves

Ball type –Selection,

Performance and

Accuracy

By Kyle Rayhill

May 21, 2015

Agenda

• Valve characteristics

- Inherent and installed

• Process characteristics

- Defined and undefined processes

- Gain

• Control loops and process variability

• Ball type control valves

• Noise and cavitation

Control Valve The foundation of reliable flow control

• The foundation of reliable, long-lasting flow

control is the control valve

- not just the valve itself, but also the

actuator, positioner and linkages between

the components

• Well engineered valve design

- minimized friction and backlash between

components

- avoid cavitation and reduce noise

- minimize fugitive emission

- not prone to impurities

- can handle process changes

Control Valve Performance Standards

• IEC 60534-4 Industrial process control valves –

Inspection and routine testing

• IEC 61514 Methods of evaluating the performance

of valve positioners with pneumatic outputs

• ISA 75.25 Test Procedure for Control Valve

Response Measurement from Step Inputs

Characteristics of Valve VALVE INHERENT CHARACTERISTICS

• LINEAR:

- Flow increases linearly with valve travel.

• QUICK OPENING:

- Flow increases substantially within the first

30% of valve travel.

• EQUAL PERCENTAGE:

- Equal changes in travel produce equal

percent changes in flow. Flow increases

substantially within the last 30% of valve

travel.

Flow Characteristics

Relative Travel (h)

Rela

tive

Flo

w C

oe

ffic

ient (Q

)

Characteristics of Valve LINEAR VALVE CHARACTERISTICS

Linear: Opening the valve 50% would produce

50% of your rated flow

1

Rela

tive

Flo

w C

oe

ffic

ient (Q

)

Relative Travel (h)

0 .2 .4 .6 .8 1 0

.2

.4

.6

.8

Characteristics of Valve QUICK OPENING VALVE CHARACTERISTICS

Re

lative

Flo

w C

oe

ffic

ient (Q

)

Relative Travel (h)

0 .2 .4 .6 .8 1 0

.2

.4

.6

.8

1

Quick Opening: Opening the valve 40% would

produce 80% of your rated flow

Characteristics of Valve and Process EQUAL PERCENTAGE VALVE CHARACTERISTICS

Equal Percentage: Opening the valve 70%

would produce 40% of your rated flow

Rela

tive

Flo

w C

oe

ffic

ient (Q

)

Relative Travel (h)

0 .2 .4 .6 .8 1

0

.2

.4

.6

.8

Characteristics of Valve EQUAL PERCENTAGE VALVE CHARACTERISTICS

In the table below we have an example of what the flow conditions would look like in

an equal percentage flow characteristic.

% Open 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Flow (GPM) 1 1.50 2.25 3.38 5.06 7.59 11.39 17.09 25.63 38.44

In this specific example, every time the valve opens 10%, the flow increases by 50%

0

5

10

15

20

25

30

35

40

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

If we were to plot the values in the table above, they

would look like the chart to the right.

Equal percentage characteristic is designed to linearize

the installed flow characteristic in normal control valve applications, where the available ∆P to the control valve

decreases with the opening of the valve. This is the case

with the majority of control valve applications.

Characteristics of Valve and Process INSTALLED CHARACTERISTICS

• Describes the control behavior of a valve when it is operating in the process.

• Installed characteristics is important indication of controllability.

To understand how a pump and pipe system work we are going to relate it to a

garden hose. When you put your thumb over the end of the hose the water

travels farther out the end because you’ve increased the pressure inside the

hose. Though the water will travel farther, you will not get as much “flow” out of

the hose. By increasing the back pressure in the garden hose, we decrease

the flow.

Defined vs. Undefined Process WHAT DOES A DEFINED PROCESS LOOK LIKE

Process Model and DPm WHAT DOES A DEFINED PROCESS LOOK LIKE

∆Pm = Differential pressure at max flow

∆P = Differential pressure at Normal flow

∆PO = Differential pressure across closed valve

• In order to calculate the installed characteristics of a valve we must first have

a model of how the process behaves. If two flow conditions are provided,

sizing programs can calculate how the process will behave (process model).

If only one flow condition is given, we use a process pressure ratio factor

DPm to estimate a process model

Process Model and DPm WHAT DOES A DEFINED PROCESS LOOK LIKE

∆Pm = Differential pressure at max flow

∆PO = Differential pressure across closed valve

Pressure

source

50 psig

330’ 1.5” pipe

P

0

psig

Characteristics of Process

Flow

(gpm)

Pipe

Loss

(psi)

10 1

20 4

30 9

40 16

50 25

60 36

• When a fluid is travelling through a pipe system,

the piping itself will cause a pressure loss.

• Loss is caused by factors such as friction, the

length of the pipe, smoothness of the inside of the

pipe, viscosity of the media, etc.

• Fluid dynamics tells us that the pressure loss in

the pipe is proportional to the change in flow

squared.

Characteristics of Process

• When we plot what the pipe loss looks like in relationship to flow we

see a quick opening flow characteristic.

• This is how most systems behave. It is possible to have a linear

behaving process, for example, with very short runs of piping, or

special pumps.

Fluid dynamics tells us we can expect a process to behave in a certain

manner. If a process doesn’t behave in this manner, we call it an undefined

process. The consequence to us is that sizing programs cannot graph the

installed characteristics. First we will go over what a defined process is. A

picture of a simple process is shown below.

6” Sch. 40

580’ (Equiv. pipe & fittings)

230’

70F

Water

10 P2 P1 PP

FC

Defined vs. Undefined Process WHAT DOES A DEFINED PROCESS LOOK LIKE

Using the numbers from our process example, we can see that the service

conditions follow the characteristics of a defined process.

Defined vs. Undefined Process WHAT DOES A DEFINED PROCESS LOOK LIKE

Pre

ss

ure

Undefined Process Models

Flow

Pre

ssu

re

Defined Process Models

P1

P2

Flow

P1

P2

Process Models

q

Installed Gain

1.0

2.0

3.0

4.0

Relative Travel, h Gain = q / h = SLOPE

Installed Characteristic

A graphical interpretation of GAIN is the SLOPE of the installed characteristic. The slope

of a line is defined as the RISE divided by the RUN. In the case of a control valve, the

RISE is the change in flow and the RUN is the change in stem position.

50%

50%

40%

Inst

all

ed G

ain

1.0

2.0

3.0

4.0

Installed Characteristic And Gain

Relative Travel, h

q

40%

10%

Gain = Output / Input

Here we have two valves with straight line installed characteristics, one with a very steep

slope and one with a very shallow slope. It turns out that neither of these valves would make a

very good control valve.

Large changes in

valve position

produce small

changes in flow

Small changes in

valve position cause

very large changes

in flow

10%

Ins

tall

ed

Gain

3.0

2.0

4.0

1.0

q max q min

Within the specified control range:

1. Gain 0.5

2. Gain 3.0

3. Gain (max) / Gain (min) 2.0

4. As constant as possible

5. As close to 1.0 as possible

This is why it’s

so important to

have at least 2

flow conditions

Control Loop Requirements

Static performance

(deadband, hysteresis...)

of loop components

Control valve

installed characteristic

and gain

Loop design,

component selection

Positioner gain

Dynamic performance

(deadtime, time constants)

of loop components

Control valve

positioning

accuracy

Loop

components’

maintenance

Loop tuning

Factors affecting to the loop

performance:

Piping design

What is Process Variability?

• What is Process Variability?

- Any unwanted variation in whatever it is you are controlling (flow, pressure,

temperature, etc)

• What is Process Variability Undesirable?

- These changes show up as changes in the properties or quality of the end

product.

Low Process Variability is Important

High variability in process control is expensive:

$ Energy usage

$ Feedstock costs

$ Scrap costs

$ Product quality

$ Lost production

$ Waste handling cost

$ Lower efficiency of the Process

$ High variability in one loop can cause problems in other loops,

if the fist loop output is used as input in another. “Loops are

Dancing Together”

Reduction is achieved by:

Correct selection of Valve, Actuator and Instrumentation

Ball type control valves Full Bore vs. reduced bore

• Full Bore

- Internal diameter of flow path is equivalent to the

attached piping. A 6” full bore valve would have

the same internal diameter as a 6” pipe.

- Also referred to as “Full Port” or “Line Size”

• Reduced Bore

- Internal diameter of flow path is less than that of

the attached piping. The internal parts (ball,seat

etc..) are typically one valve size smaller. A 6”

reduced bore valve might have 4” internals.

- Also referred to as “Standard Bore” or “Standard

Port”

Reduced Bore

Full Bore

Ball type control valves Trunnion vs. seat supported

• The type of construction of metal seated ball valves can be split into

two general categories, seat supported and trunnion mounted.

- In a seat supported valve the ball is allowed to “float” and is held in place and

supported by the seats.

- The ball in a trunnion mounted valve is held in place and supported by bearings

mounted in the body.

seat supported ball, used

mainly for On/Off application

due to higher friction

trunnion mounted ball can be

used also in control

applications due to less

friction

Downstream

Sealing

Upstream

Sealing

Ball type control valves

• Seat supported ball valve

- Full and reduced bore

- Higher torque requirements

• Poor control performance (stiction)

- High Cv capabilities

- Tightest shutoff

• Trunnion ball valve

- Full and reduced bore

- High Cv capabilities

- Lower torque requirements then seat

supported

- Tight shutoff

• Segmented ball valve

- Trunnion

- Reduced bore

- Special trims

- Control valve shutoff (ANSI Class IV)

Valve Noise Generation

Sudden compression and expansion at vena contractaSevere turbulence

Turbulence generates sound waves

Cavitation noise

• 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.

Cavitation

Water

(Liquid)

Water

Vapor

(gas)

Ice

(solid)

.006 atm

.01°C

1 atm

100°C

Temperature

Pre

ssu

re

Triple Point

Cavitation

Pressure

Velocity

Vapor Pressure

Because of the law of conservation of energy, when the velocity of the liquid

increases through the valve, the pressure must decrease

P1 P2 PVC

A valve is represented as a simple orifice plate to represent a

restriction in the flow path

Cavitation

Cavitation occurs when the pressure inside the valve drops below the vapor

pressure of the liquid, allowing the liquid to turn into a gas.

P1 P2 PVC

Pressure

Velocity

Vapor Pressure

P1

P2

Cavitation Damage

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100

Percent Open

Re

co

ve

ry F

ac

tor,

FL

Globe

Ecc. Plug

Ball

H.P. Butterfly

P1

P2

PV

Vapor Pressure

Pressure Recovery

Large FL

(Low Recovery)

Small FL

(High Recovery)

FL, Liquid Pressure Recovery Factor

• Special valve trims can stage the pressure drop into smaller drops, with the

pressure at the last dip not getting as close to the vapor pressure as it would

with a single pressure drop.

P2

With Q - Trim

Cavitation Solution Special valve trims

∆P1 ∆P2 ∆P3 ∆P4 ∆P5

P1

P2

Without special trim

With special trim

Summary

• Ball valves are commonly used in control

applications

- Mostly segmented and trunnion ball valves

- Avoid seat supported ball valves in control

applications due to high stiction

• Control performance

- Ball valves typically equal percent inherent flow

characteristic

- High rangeability

- Tight shutoff

- Severe service

• Slurry, high noise, cavitation, etc…

Questions?

• Contact

Kyle Rayhill

Director, Global Oil & Gas Business Line

Metso Automation

(508) 852 - 0215 ext. 6124

kyle.rayhill@metso.com