Final Control Element

Actuator System

• Control Valve– Valve body– Valve actuator

• I/P converter

• Instrument air system

Typical Globe Control Valve

Cross-section of a Globe Valve

Types of Globe Valves

• Quick Opening- used for safety by-pass applications where quick opening is desired

• Equal Percentage- used for about 90% of control valve applications since it results in the most linear installed characteristics

• Linear- used when a relatively constant pressure drop is maintained across the valve

Inherent Valve Characteristics

0

0.5

1

0 20 40 60 80 100Stem Position (% Open)

f(x)

=%

QOLinear

Typical Flow System

C.W.

FT

Pressure Drop vs. Flow Rate

0

5

10

15

20

25

0 50 100 150 200Flow Rate (GPM)

Pre

ssur

e D

rop

(psi

)

Line Losses

Pump Head

Valve P

Installed Flow Characteristic

0

50

100

150

200

0 20 40 60 80 100Stem Position (% Open)

Inst

alle

d F

low

Rat

e (G

PM

)

Linear Valve

=% Valve

Slope of Installed Characteristic

0

12

34

56

7

0 20 40 60 80 100Stem Position (% Open)

Slo

pe o

f th

e In

stal

led

Flo

w C

hara

cter

isti

c Linear Valve

=% Valve

Effect of Linearity in the Installed Valve Characteristics

• Highly nonlinear installed characteristics can lead to unstable flow control or a sluggish performance for the flow controller.

Flow System with Relatively Constant Valve Pressure Drop

30 ftFT

Pressure Drop vs. Flow Rate

0

5

10

15

0 100 200 300 400 500 600Flow Rate (GPM)

Pre

ssur

e D

rop

(psi

)

Line Losses

Valve P

Hydrostatic Head

Installed Valve Characteristics

0

100

200

300

400

500

600

0 20 40 60 80 100Stem Position (% Open)

Inst

alle

d F

low

Rat

e (G

PM

) Linear Valve

=% Valve

Analysis of These Examples

• Note the linear installed valve characteristics over a wide range of stem positions.

• If the ratio of pressure drop across the control valve for the lowest flow rate to the value for the highest flow rate is greater than 5, an equal percentage control valve is recommended.

Control Valve Design Procedure

• Choose a control valve so that the average flow rate results when the valve is 2/3 open.

• After the valve has been sized, check to ensure that the maximum and minimum flow rates will be accurately metered.

Additional Information Required to Size a Control Valve

• CV versus % open for different valve sizes.

• Available pressure drop across the valve versus flow rate for each valve. Note that the effect of flow on the upstream and downstream pressure must be known.

Valve Sizing Example

• Size a control valve for max 150 GPM of water and min of 50 GPM.

• Therefore, choose the valve size so that valve is approximately 67% open at 100 GPM.

Determine CV at 100 GPM

• Use the valve flow equation (Equation 2.1) to calculate Cv

• For P, use pressure drop versus stem position (e.g., Table 2.2)

/)(

PK

FxC m

v

Cv versus % Valve Travel for Different Sized Valves

• Body % Valve Opening

• Size in 50 60 70

• 1 3.63 5.28 7.59

• 1.5 4.30 6.46 9.84

• Cv 2 11.1 20.7 32.8

• 3 21.7 36.0 60.4

• 4 31.2 52.6 96.7

Check Max and Min Flows

• Ensure that the flow rate will be accurately controlled at the maximum and minimum flow rates.

• At minimum flow rate valve should be at least 10-15% open.

• At maximum flow rate the valve should be at most 85-90% open.

Valve Deadband

• It is the maximum change in instrument air pressure to a valve that does not cause a change in the flow rate through the valve.

• Deadband determines the degree of precision that a control valve or flow controller can provide.

• Deadband is primarily affected by the friction between the valve stem and the packing.

For Large Diameter Line (>6”), Use a Butterfly Valve

Valve Actuator Selection

• Choose an air-to-open for applications for which it is desired to have the valve fail closed.

• Choose an air-to-close for applications for which it is desired to have the valve fail open.

Optional Equipment

• Valve positioner- a controller that adjusts the instrument air in order to maintain the stem position at the specified position. Greatly reduces the deadband of the valve. Positioners are almost always used on valves serviced by a DCS.

• Booster relay- provides high capacity air flow to the actuator of a valve. Can significantly increase the speed of large valves.

Photo of a Valve Positioner

Control Relevant Aspects of Actuator Systems

• The key factors are the deadband of the actuator and the dynamic response as indicated by the time constant of the valve.

• Control valve by itself- deadband 10-25% and a time constant of 3-15 seconds.

• Control valve with a valve positioner or in a flow control loop- deadband 0.1-0.5% and a time constant of 0.5-2 seconds.

Sensor Systems

• Sensor– temperature sensors– flow sensors– level sensors– pressure sensors– composition analyzers

• Transmitter

The Control Relevant Aspects of Sensors

• The time constant/deadtime of the sensor

• The repeatability of the sensor

Sensor Terminology

• Span

• Zero

• Accuracy

• Repeatability

• Process measurement dynamics

• Calibration

Span and Zero Example

• Consider a case in which the maximum temperature that is to be measured is 350ºF and the minimum temperature is 100ºF.

• Then the zero is 100ºF and the span is 250ºF

• In addition, if the measured temperature is known at two different sensor output levels (i.e., ma’s), the span and zero can be calculated directly.

Smart Sensors• Sensors with onboard microprocesssors that offer a

number of diagnostic capabilities.

• Smart pH sensors determine when it is necessary to trigger a wash cycle due to buildup on the electrode surface.

• Smart flow meters use statistical techniques to check for plugging of the lines to the DP cell.

• Smart temperature sensors use redundant sensors to identify drift and estimate expected life before failure.

Temperature Sensing Systems

• RTD’s are an order of magnitude more precise but are less rugged and cost more than thermocouples (TC’s).

• Typical dynamic response time constant is 6-20 seconds for both RTD’s and TC’s.

• Additional thermal resistance on inside or on the outside of the thermal well can result in an excessively slow responding temperature measurement.

Pressure Measurements

• Usually based on mechanical balance bars

• Very fast measurement dynamics

• Repeatability less than ±0.1%

Flow Measurements

• Orifice plate/DP cell most common approach. Good repeatability and fast dynamic response.

• Magnetic flow meters and vortex shedding flow meters are also used in certain situations. They are more expensive but more reliable and require less maintenance.

• A straight run of pipe required for good accuracy for all flow meters.

Orifice Plate/DP Cell Flow Indicator in a Flow Control Loop

FC

FT

Flow Setpoint

Paddle Type Orifice Plate

Sizing an Orifice for a Differential Pressure Flow

Indicator is the ratio of the orifice diameter to the pipe diameter.

• 0.2 < < 0.7• Pressure drop at minimum flow should be

greater than 0.5 psi.• Pressure drop across the orifice should be less

than 4% of the line pressure.• Choose the maximum value of that satisfies

each of the above specifications.

Example of a Magnetic Flow Meter

Example of a Vortex Shedding Meter

Level Sensors

• Usually based on the hydrostatic head in a vessel measured by the differential pressure.

• Has a repeatability of about ±1% with a time constant less than 1 second.

• Level measurements based upon a float or x-rays are also used in special situations.

Typical Differential Pressure Level Measurement

DPT

VaporDiaphragm

Lower Tap

Upper Tap

Liquid

Analyzer Sensor Systems• GC- most common composition analyzer. Based

on plug flow of a volatile sample through a packed bed--behaves as deadtime. Deadtime and repeatability depend on the particular components being measured.

• Radiation absorption- infrared, ultraviolet, and visible. Can be effective for certain components.

• Sample system can affect dynamics and reliability of composition measurement.

Table 2.3

• Lists the control-relevant aspects of actuators and sensors in the CPI:– Time constant– Valve deadband or repeatability– Turndown ratio, rangeability, or range