CONTROL VALVE HANDBOOK Episode 1 Part 8 potx

Control Valve SelectionActuator Sizing Actuators are selected by matching the force required to stroke the valve with an actuator that can supply that force.. For rotary valves a similar

Trang 1

Representative Sizing Coefficients for Rotary Shaft Valves Valve Size

Degrees of Valve

90

15.6 34.0

0.86 0.86

0.53 0.42

90

28.5 77.3

0.85 0.74

0.50 0.27

2 V−Notch Ball Valve

High Performance Butterfly Valve

60 90 60 90

59.2 132 58.9 80.2

0.81 0.77 0.76 0.71

0.53 0.41 0.50 0.44

0.49 0.70

60 90 60 90

120 321 115 237

0.80 0.74 0.81 0.64

0.50 0.30 0.46 0.28

0.92 0.99 0.49 0.70

60 90 60 90

195 596 270 499

0.80 0.62 0.69 0.53

0.52 0.22 0.32 0.19

0.92 0.99 0.49 0.70

60 90 60 90

340 1100 664 1260

0.80 0.58 0.66 0.55

0.52 0.20 0.33 0.20

0.91 0.99 0.49 0.70

90

518 1820

0.82 0.54

0.54 0.18

0.91 0.99

Trang 2

Representative Sizing Coefficients for Rotary Shaft Valves (continued) Valve Size

Opening Valve Style

60 90 60 90

1530 3980 2500 5400

0.78 0.63

0.49 0.25

0.92 0.99 0.49 0.70

60 90 60 90

2380 8270 3870 8600

0.80 0.37 0.69 0.52

0.45 0.13 0.40 0.23

0.92 1.00

Trang 3

Chapter 5 Control Valve Selection

Actuator Sizing

Actuators are selected by matching

the force required to stroke the valve

with an actuator that can supply that

force For rotary valves a similar

pro-cess matches the torque required to

stroke the valve with an actuator that

will supply that torque The same

fun-damental process is used for

pneu-matic, electric, and electrohydraulic

actuators

Globe Valves

The force required to operate a globe

valve includes:

D Force to overcome static

unbal-ance of the valve plug

D Force to provide a seat load

D Force to overcome packing

fric-tion

D Additional forces required for

certain specific applications or

constructions

Total force required = A + B + C + D

A Unbalance Force

The unbalance force is that resulting from fluid pressure at shutoff and in the most general sense can be ex-pressed as:

Unbalance force = net pressure differ-ential X net unbalance area

Frequent practice is to take the maxi-mum upstream gauge pressure as the net pressure differential unless the process design always ensures a back pressure at the maximum inlet pressure Net unbalance area is the port area on a single seated flow up design Unbalance area may have to take into account the stem area de-pending on configuration For bal-anced valves there is still a small un-balance area This data can be obtained from the manufacturer Typi-cal port areas for balance valves flow

up and unbalanced valves in a flow down configuration are listed below;

Typical Unbalance Areas of Control Valves Port Diameter

Unbalance Area Single seated unbalanced valves

Unbalance Area Balanced Valves

Trang 4

130

Figure 5-3 Minimum Required Seat Load for Metal-Seated Valves for Improved Seat Life for Class II-V and Recommended Seat Load for

Optimum Performance in Boiler Feedwater Service

900

800

700

600

500

400

300

200

100

0

SHUTOFF PRESSURE DROP, PSI

A2222−4

CLASS V (METAL SEAT)

CLASS IV

CLASS III

CLASS II

1000

CLASS V (METAL SEAT WITH C−SEAL TRIM)

CLASS V (METAL SEAT FOR OPTIMUM PER-FORMANCE AND LIFE IN BOILER FEEDWATER SERVICE.

Class I As required by user specification, no factory leak test required

Class II 20 pounds per lineal inch of port circumference

Class III 40 pounds per lineal inch of port circumference

Class IV

Standard (Lower) Seat only—40 pounds per lineal inch of port circumference (up through a 4-3/8 inch diameter port)

Standard (Lower) Seat only—80 pounds per lineal inch of port circumference (larger than 4-3/8 inch diameter port)

Class V Metal Seat—determine pounds per lineal inch of port circumference from

figure 5-3

Class VI Metal Seat—300 pounds per lineal inch of port circumference

B Force to Provide Seat Load

Seat load, usually expressed in

pounds per lineal inch of port

circum-ference, is determined by shutoff

re-quirements Use the following guide-lines to determine the seat load required to meet the factory accep-tance tests for ANSI/FCI 70-2 and IEC

Trang 5

534-4 leak classes II through VI See

table for recommended seat load

Because of differences in the severity

of service conditions, do not construe

these leak classifications and

corre-sponding leakage rates as indicators

of field performance To prolong seat

life and shutoff capabilities, use a

higher than recommended seat load

See Figure 5-3 for suggested seat

loads If tight shutoff is not a prime

consideration, use a lower leak class

C Packing Friction

Packing friction is determined by stem size, packing type, and the amount of compressive load placed on the pack-ing by the process or the boltpack-ing Packing friction is not 100% repeat-able in its friction characteristics Live loaded packing designs can have sig-nificant friction forces especially if graphite packing is used The table below lists typical packing friction val-ues

Trang 6

132

Typical Packing Friction Values STEM SIZE

RIBBON/ FILAMENT

-3/8

125 150 250

-125

-190 600

900 1500

250 320 380

1/2

125 150 250 300

-180

-230 600

900 1500 2500

320 410 500 590

5/8

125 150 250 300 600

-218

-290 400

3/4

125 150 250 300

-350

-440 600

900 1500 2500

660 880 1100 1320

1

300 600 900 1500 2500

610 850 1060 1300 1540

1-1/4

300 600 900 1500 2500

800 1100 1400 1700 2040

2

300 600 900 1500 2500

1225 1725 2250 2750 3245

Values shown are frictional forces typically encountered when using standard packing flange bolt torquing procedures.

D Additional Forces

Additional forces may be required to

stroke the valve such as: bellow

stiff-ness; unusual frictional forces

result-ing from seals; or special seatresult-ing forces for soft metal seals as an ex-ample The manufacturer should ei-ther supply this information or take it into account when sizing an actuator

Trang 7

Actuator Force Calculations

Pneumatic diaphragm actuators

pro-vide a net force with the additional air

pressure after compressing the spring

in air to close, or with the net

precom-pression of the spring in air to open

This may be calculated in pounds per

square inch of pressure differential

For example: Suppose 275 lbf is

re-quired to close the valve calculated

following the process described

earli-er An air-to-open actuator with 100

square inches of diaphragm area and

a bench set of 6 to 15 psig is one

available option The expected

operat-ing range is 3 to 15 psig The

precom-pression can be calculated as the

dif-ference between the lower end of the

bench set (6 psig) and the beginning

of the operating range (3 psig) This 3

psig is used to overcome the

precom-pression so the net precomprecom-pression

force must be;

3 psig X 100 sq in = 300 lbf

This exceeds the force required and is

an adequate selection

Piston actuators with springs are

sized in the same manner The thrust

from piston actuators without springs

can simply be calculated as:

(Piston Area)(Minimum Supply

Pressure) = Available Thrust

(be careful to maintain compatibility of

units)

In some circumstances an actuator

could supply too much force and

cause the stem to buckle, to bend

suf-ficiently to cause a leak, or to damage

valve internals This could occur

be-cause the actuator is too large or the

maximum air supply exceeds the mini-mum air supply available

The manufacturer normally takes re-sponsibility for actuator sizing and should have methods documented to check for maximum stem loads Manufacturers also publish data on actuator thrusts, effective diaphragm areas, and spring data

Rotary Actuator Sizing

In selecting the most economical ac-tuator for a rotary valve, the determin-ing factors are the torque required to open and close the valve and the torque output of the actuator

This method assumes the valve has been properly sized for the application and the application does not exceed pressure limitations for the valve

Torque Equations

Rotary valve torque equals the sum of

a number of torque components To avoid confusion, a number of these have been combined and a number of calculations have been performed in advance Thus, the torques required for each valve type can be repre-sented with two simple and practical equations

Breakout Torque

TB = A(nPshutoff) + B

Dynamic Torque

TD = C(nPeff) The specific A, B, and C factors for each valve design are included in fol-lowing tables

Trang 8

134

Typical Rotary Shaft Valve Torque Factors V−Notch Ball Valve with Composition Seal VALVE

SIZE,

INCHES

VALVE

SHAFT

DIAMETER,

INCHES

A

B

M T D , LBFSIN Composition

Bearings

60 Degrees

70 Degrees

2

3

4

6

8

1/2 3/4 3/4 1 1-1/4

0.15 0.10 0.10 1.80 1.80

80 280 380 500 750

0.11 0.15 1.10 1.10 3.80

0.60 3.80 18.0 36.0 60.0

515 2120 2120 4140 9820 10

12

14

16

18

20

1-1/4

1-1/2

1-3/4

2 2-1/8

2-1/2

1.80 4.00 42 60 60 97

1250 3000 2400 2800 2800 5200

3.80 11.0 75 105 105 190

125 143 413 578 578 1044

9820 12,000 23,525 23,525 55,762 55,762

High Performance Butterfly Valve with Composition Seal

VALVE

SIZE,

INCHES

SHAFT

DIAMETER

INCHES

INCH-POUNDS

B Dynamic T D

Maximum Rotation

Maximum rotation is defined as the

angle of valve disk or ball in the fully

open position

Normally, maximum rotation is 90

grees The ball or disk rotates 90

de-grees from the closed position to the

wide open position

Some of the pneumatic spring-return

piston and pneumatic

spring-and-dia-phragm actuators are limited to 60 or

75 degrees rotation

For pneumatic spring-and-diaphragm

actuators, limiting maximum rotation

allows for higher initial spring

com-pression, resulting in more actuator

breakout torque Additionally, the

ef-fective length of each actuator lever

changes with valve rotation Published

torques, particularly for pneumatic

pis-ton actuators, reflect this changing le-ver length

Non-Destructive Test Procedures

Successful completion of specific non-destructive examinations is required for valves intended for nuclear service and may be required by codes or cus-tomers in non-nuclear applications, particularly in the power industry Also, successful completion of the ex-aminations may permit uprating of ASME Standard Class buttwelding end valves to a Special Class rating The Special Class rating permits use

of the butt-welding end valves at

high-er pressures than allowed for Stan-dard Class valves Procedures re-quired for uprating to the Special Class are detailed in ASME Standard B16.34

Trang 9

While it is not feasible to present

com-plete details of code requirements for

non-destructive examinations, this

book will summarize the principles

and procedures of four major types of

non-destructive examinations defined

in ANSI, ASME, and ASTM standards

Magnetic Particle (Surface)

Examination

Magnetic particle examination can be

used only on materials which can be

magnetized The principle includes

application of a direct current across a

piece to induce a magnetic field in the

piece Surface or shallow subsurface

defects distort the magnetic field to

the extent that a secondary magnetic

field develops around the defect If a

magnetic powder, either dry or

sus-pended in liquid, is spread over the

magnetized piece, areas of distorted

magnetic field will be visible,

indicat-ing a defect in the piece in the area of

distortion After de-magnetizing the

piece by reversing the electric current,

it may be possible to weld repair the

defect (normal procedure with

cast-ings) or it may be necessary to

re-place the piece (normal procedure

with forgings and bar stock parts)

Af-ter repair or replacement, the

magnet-ic partmagnet-icle examination must be

re-peated

Liquid Penetrant (Surface)

Examination

This examination method permits

detection of surface defects not visible

to the naked eye The surface to be

examined is cleaned thoroughly and

dried The liquid penetrant dye, either

water or solvent soluble, is applied by

the applicable light source (Some de-velopers require use of an ultraviolet

or black light to expose defective areas) If defects are discovered and repaired by welding, the piece must

be re-examined after repair

Radiographic (Volumetric) Examination

Radiography of control valve parts works on the principle that X-rays and gamma rays will pass through metal objects which are impervious to light rays and will expose photographic film just as light rays will The number and intensity of the rays passing through the metal object depend on the

densi-ty of the object Subsurface defects represent changes in density of the material and can therefore be photographed radiographically The piece to be inspected is placed be-tween the X-ray or gamma ray source and the photographic film Detail and contrast sensitivity are determined by radiographing one or more small flat plates of specified thickness at the same time the test subject is exposed The small flat plate, called a penetra-meter, has several holes of specified diameters drilled in it Its image on the exposed film, along with the valve body or other test subject, makes it possible to determine the detail and contrast sensitivity of the radiograph Radiography can detect such casting defects as gas and blowholes, sand inclusions, internal shrinkage, cracks, hot tears, and slag inclusions In cast-ings for nuclear service, some defects such as cracks and hot tears are ex-pressly forbidden and cannot be re-paired The judgment and experience

of the radiographer is important

Trang 10

be-Chapter 5 Control Valve Selection

136

Ultrasonic (Volumetric)

Examination

This method monitors sound wave

re-flections from the piece being

in-spected to determine the depth and

size of any defects Ultrasonic

ex-amination can detect foreign materials

and discontinuities in fine-grained

metal and thus lends itself to

volumet-ric examination of structures such as

plate, bar, and forgings The test is

normally conducted either with a

spe-cial oil called a coupler or under water

to ensure efficient transmission of

sound waves The sound waves are

generated by a crystal probe and are

reflected at each interface in the piece

being tested, that is, at each outer

face of the piece itself and at each

face of the damaged or malformed

in-ternal portion These reflections are

received by the crystal probe and

dis-played on a screen to reveal the

loca-tion and severity of the defect

Cavitation and Flashing

Choked Flow Causes Flashing

and Cavitation

The IEC liquid sizing standard

calcu-lates an allowable sizing pressure

drop, nPmax If the actual pressure

drop across the valve, as defined by

the system conditions of P1 and P2, is

greater than nPmax then either

flash-ing or cavitation may occur Structural

damage to the valve and adjacent

pip-ing may also result Knowledge of

what is actually happening within the

valve will permit selection of a valve

that can eliminate or reduce the

ef-fects of cavitation and flashing

The physical phenomena label is used

to describe flashing and cavitation

be-cause these conditions represent

ac-tual changes in the form of the fluid

media The change is from the liquid

state to the vapor state and results

from the increase in fluid velocity at or

just downstream of the greatest flow

restriction, normally the valve port As

liquid flow passes through the

restric-Figure 5−4 Vena Contracta Illustration

RESTRIC-TION

VENA CONTRACTA FLOW

A3444/IL

tion, there is a necking down, or con-traction, of the flow stream The mini-mum cross−sectional area of the flow stream occurs just downstream of the actual physical restriction at a point called the vena contracta, as shown in figure 5−4

To maintain a steady flow of liquid through the valve, the velocity must

be greatest at the vena contracta, where cross sectional area is the least The increase in velocity (or ki-netic energy) is accompanied by a substantial decrease in pressure (or potential energy) at the vena

contrac-ta Further downstream, as the fluid stream expands into a larger area, ve-locity decreases and pressure in-creases But, of course, downstream pressure never recovers completely to equal the pressure that existed up-stream of the valve The pressure dif-ferential (nP) that exists across the valve is a measure of the amount of energy that was dissipated in the valve Figure 5−5 provides a pressure profile explaining the differing perfor-mance of a streamlined high recovery valve, such as a ball valve, and a valve with lower recovery capabilities due to greater internal turbulence and dissipation of energy

Regardless of the recovery character-istics of the valve, the pressure differ-ential of interest pertaining to flashing and cavitation is the differential be-tween the valve inlet and the vena contracta If pressure at the vena con-tracta should drop below the vapor pressure of the fluid (due to increased fluid velocity at this point) bubbles will form in the flow stream Formation of

Định dạng
Số trang	20
Dung lượng	281,4 KB