Control Valve Selection [PDF]

Zitiervorschau

COMPRESSIBLE FLUIDS Subcritical Cv

Q*—(GTZ 295*—VP(P1+P2) Q increases, Cv increases VP,P1,P2increases, Cv reduces

INCOMPRESSIBLE FLUIDS

Subcritical Cv

1.16Q*—Gf —VP

Q increases, Cv increases

VP increases, Cv reduces

open position such that it passes 12gpm of water with 1 psi pressure drop.

Eg. : A control valve which has a Cv of 12 has an effective port area in the full

a given flow restriction with a pressure drop of 1 psi.

Cv is “the number of U.S. gallons per minute of water which will pass through

CONTROL VALVE FLOW COEFFICIENT (Cv)

STEP 1. DETERMINE THE REQUIRED CV

E’ R I C

B A L L

Shutoff (ANSI Class IV & VI)

ANSI Pressure Ratings (typically 150# - 600#)

Good for Fibrous Fluids

Low - Med Pressure drop capabilities

Shutoff (ANSI Class IV & VI)

Suitable for higher velocities

ANSI Pressure Ratings (typically 150# - 600#)

x

x

x

x

x

x

x

Valve Cv's higher than Butterfly or Globe

x

High Cv Capacity

x

F L Y

Cost (cheaper than other styles)

Lightweight Valve

B’ x

x

High Performance

x

ANSI Pressure Ratings (typically 150# - 600#)

Range of Alloy Materials

x

x

Severe Service Trim for high velocity,noise,cavitation&erosion

x

Shutoff (ANSI Class IV & VI)

ANSI Pressure Ratings (typically 150# - 4500#)

x

x

High Pressure Drops

x

G L O B E

Shutoff (ANSI Class IV, V & VI)

x

choosing a valve.

following table lists just a few of them, together with some considerations for

There is a wide range of control valves styles available on today's market. The

STEP 2. SELECT THE VALVE TYPE

3

2

Top & Bottom guiding provides adequate support against side loads.

Simple Construction.

Balanced Construction, sizes upto 20” with std. actuators.

Anti Cav/Lo-dB, Multi Cage designs available to handle severe service conditions.

High allowable Pressure Drops.

LIMITATIONS

Complex Construction, compared to single seated valves.

ADVANTAGES

Better Leakage Class. Lower Pressure drop capability. 41000 Series Heavy duty balanced cage guided valves (1” to 14”, 150# to 2500#) & (16” to 20”, 150# to 600#) Used for Critical applications on Power & Fertilizer sectors and in other sectors for high pressure, high pressure drop, large size applications.

LIMITATIONS

Unbalanced construction.

ADVANTAGES

Higher 'P, 'P Shut off, Cv and pressure recovery factor than Single Seated Valves. 21000 Series Single ported top guided valves (1/2” to 2.5”, 150# to 2500#) & (3” to 8”, 150# to 600#) Most popular control valve used in all industries for standard applications.

V-Ported plug design available for high pressure drops and cavitation control

LIMITATIONS

For standard valves, seat leakage is limited to Class II.

ADVANTAGES

GLOBE TYPE CONTROL VALVES: MIL RANGE 1 10000 series Double ported top & bottom guided valves (1” to 16”, 150# to 1500#) One of the early designs used for high capacity flow, now mainly limited to refinery/UOP applications.

Low Pressure Recovery, eliminates Cavitation.

Pressure Drop in stages, Can handle high pressure drops without Cavitation, erosion, vibration & noise.

FEATURES

78000 Series Anti-cavitation & low noise multistage valve (1/2” to 6”, 600# to 2500#) Mainly in power sector for BFP & Spray systems. Used for Very high pressure drop, Anti cavitation applications in other industries.

26000 SERIES SPLIT BODY VALVES FOR CORROSIVE FLUIDS. 29000 SERIES MICROPAK VALVES FOR MICROFLOW CONTROL. 50000 SERIES CRYOGENIC VALVES.

70000 SERIES ANGLE VALVES.

81000 SERIES THREE WAY VALVES FOR DIVERTING AND COMBINING APPLICATIONS. 525/535 SERIES SELF ACTUATING VALVES FOR UPSTREAM / DOWNSTREAM PRESSURE CONTROL. SPECIAL VALVES FOR NUCLEAR / DEFENSE APPLICATION. MATIRX SERIES CONTROL VALVES WITH MULTI-STAGE, MULTIPATH, AXIAL FLOW, ANTI-CAVITATION CONTROL VALVE

C D E

F

G

I J

H

B

22000 SERIES HAND OPERATED BELLOW SEALED VALVES FOR HANDLING TOXIC / COSTLY FLUIDS. 25000 SERIES TRIP VALVES FOR OIL FIRING SYSTEMS.

A

OTHER CONTROL VALVES IN MIL PRODUCT RANGE

4

HARDEN ED

10

6

5

3

5

3

2

STEAM IN VENT / INTERMITTENT SERVICE (MACH NUMBER 16” ½” to 2”

13

8

STANDARD

HARDENED

10

7

*RECOMMENDED FLUID VELOCITY (m/sec) 3” to 6” 8” to 14” >16” ½” to 2”

HARDENED

STANDARD

LIQUIDS WITH PARTICLES

CAGE GUIDED

SINGLE SEATED

CLEAN LIQUIDS

velocity considerations.

Manufacturers Cv table for the body style selected, keeping in mind the

The body size can be chosen by checking the calculated Cv value against the

STEP 3. SELECT THE VALVE SIZE

10% open 5% open

Linear

Equal percentage

70% open

50 % open

25% open

Normal

90% open

80% open

50% open

Maximum

18:1

8:1

5:1

IN ADDITION, WE HAVE CUSTOMISED CHARACTERISTICS CATERING TO SPECIFIC CONTROL REQUIREMENTS.

LINEAR ONLY

Lo dB / ANTICAVITATION DESIGN:

LINEAR ONLY

MULTI STEP / THREE WAY VALVES:

LINEAR / EQUAL % / ON-OFF

SINGLE/DOUBLE SEATED/CAGE GUIDED VALVES:

STANDARD TRIM CHARACTERISTIC FOR MIL VALVES

10% open

Quick opening

Minimum

Rangeability

Trim Characteristic - Equal percent, linear, quick open etc.

x

Example Control Valve Flow Characteristics

Trim size for future conditions.

recommended)

Valve % open at operating condition/s (generally 70 - 80% is

x

x

consider the following when selecting a trim size:

Some valve styles have a number of trim sizes to choose from. You may

STEP 4. SELECT TRIM SIZE (RATED Cv)

FLOW TO OPEN ONLY

Lo - dB DESIGN

FLOW TO CLOSE ONLY

ANTI CAVITATION DESIGN

FLOW TO OPEN ONLY

MULTISTEP VALVES

FLOW TO CLOSE (Liquids)

FLOW TO OPEN (Gas / Steam)

CAGE GUIDED VALVES:

FLOW PASSING INTO SEATS

DOUBLE SEATED VALVES:

FLOW TO OPEN

SINGLE SEATED VALVES:

STANDARD FLOW DIRECTION FOR CONTROL VALVES

STEP 5. SELECT FLOW DIRECTION

DUE

TO

CONTAMINANTS

(H2S,

be specified to prolong the valve life.

the process fluid. Where erosion exists, harder or more durable materials can

process, special alloys should be selected based on material compatibility with

Where carbon steel or stainless steel materials are unsuitable for corrosive

problems can combine to destroy or consume incorrect specified materials.

corrosion, high velocity, erosion, entrained particles, cavitation or other

Correct material selection is essential for reasonable valve life. Process fluid

6. TYPE OF EROSION OCCURRING OR EXPECTED (ABRASIVE PARTICLE, CAVITATION, EROSIVE-CORROSIVE, OR HIGH LIQUID VELOCITY IMPINGEMENT).

5. INSTALLATION ENVIRONMENTAL CONDITIONS

4. QUANTITY AND TYPE OF SOLIDS (SAND, SILICA, CATALYST etc.)

3. CORROSIVE PROPERTIES CHLORIDES, ETC.)

2. OPERATING PRESSURE / TEMPERATURE

1. FLUID COMPATIBILITY

CONSIDERATIONS

STEP 6. MATERIAL SELECTION

Nickel

Monel

Hastelloy B

Hastelloy C

Alloy -20

(Stainless Steel)

ASTM A 351 Gr CF8M

(Stainless Steel)

ASTM A 351 Gr CF8

(Alloy Steel)

ASTM A 217 Gr WC9

(Alloy Steel)

ASTM A 217 Gr WC6

(Low Carbon Steel)

ASTM A 352 Gr LCB

(Carbon Steel)

ASTM A 216 Gr WCC

Body Material (High)

-195°C

-195°C

-195°C

-195°C

-45°C

-254°C

-254°C

-27°C

-27°C

-45°C

260°C

485°C

371°C

371°C

315°C

815°C

815°C

565°C

537°C

343°C

427°C

(Low) -27°C

Temp. Range

Temp. Range

SELECTION OF BODY MATERIAL

phosphoric, wet hydrogen

or

Hot concentrated caustic, alkaline or neutral salts and reducing

Alkalis, Salts, Foods, Organics, air free acids, reducing agents.

Hydrochloric, sulphuric acids; chloride gas.

Free Cl2 or acids solutions cupric salts; oxidizing agents.

Oxidising agents, all suplphuric acids at room temperature.

Modification of CF8 with Molybdenum added. Due to it’s superior corrosion resistance, CF8M is widely used in chemical processing industries

Basic low carbon stainless steel used for majority of applications involving corrosive fluids.

High temperature applications involving high pressure steam & other erosive service like flashing.

High temperature applications involving high pressure steam & other erosive service like flashing.

For low temperature application.

Suited for use in air, saturated/superheated steam & non corrosive oil & gases at moderate temperatures.

Application

ASTM A 182 Gr. F11 ASTM A 182 Gr. F22 ASTM A 182 Gr. 5a ASTM A 182 Gr. F316

ASTM A 217 Gr. WC9 ASTM A 217 Gr. C5 ASTM A 351 Gr. CF8M

Forgings ASTM A 105

Castings ASTM A 216 Gr. WCB / WCC ASTM A 217 Gr. WC6

Hardness

Corrosion resistance

Erosion resistance

Abrasion resistance

High temperature suitability

Hardened trim for severe service applications

x

x

x

x

x

x

applications requiring the following characteristics:

control valves. However, other materials may need to be considered for

Stainless steel is generally used as the standard plug and seat ring material in

SELECTION OF TRIM MATERIAL

Alloy Steel, 1 ¼ Cr – ½ Mo Alloy Steel, 2 ¼ Cr – 1 Mo Alloy Steel, 5 Cr – ½ Mo Ty. 316

General Classification Carbon Steel

CASTING V/S. FORGING SPECIFICATIONS

Non-Erosive, Corrosive, Mod. Press. Drop. Most corrosion resistant of 300 series

Corrosive & NonCorrosive Service

Corrosive & NonCorrosive Service

Slightly Erosive & Corrosive Service Erosive & Corrosive Service Erosive & Corrosive Service. Corrosion resistance similar to Inconel. Erosive & Corrosive Service. Corrosion resistance similar to Inconel.

14 HRC max.

35 HRC min.

31-38 HRC

38-47 HRC 38-47 HRC 45-50 HRC

400ºC

400ºC

650ºC

650ºC

No.5 Colmonoy Hard Facing

No.6 Colmonoy Hard Facing

650ºC

56-62 HRC

General Application

Hardness

Max. Temp. Limit. 593ºC

650ºC

Hard Facing

Bar Stock: ASTM A 582 Ty. 416

Bar Stock: ASTM A 479 Ty. 316 Castings: ASTM A 351 Gr. CF8M Bar Stock: ASTM A 479 Ty. 410

ASTM Code

No.6 Stellite Solid (< 2”)

Ty. 410 Stainless Steel (Hardened & Tempered) Ty. 416 Stainless Steel (Hardened & Tempered) No.6 Stellite

SS 316

General Classfn.

SELECTION OF TRIM MATERIAL

Bar Stock: ASTM A276 Ty.440C

Ty. 440C Stainless Steel. Hardened 400ºC

650ºC

Max. Temp. Limit. 400ºC

58 HRC

700-1000 HV After Nitriding

28 HRC min.

MULTISTEP VALVE

Plug : 17.4 PH / CA6NM Plug : 440 C Seat : SS316 + Stellite Seat : SS316 + Stellite Cage : CA 6 NM Nitrided Liner/Spacer : 17.4 PH Other common trim material offered by MIL: SS304L/316L/316L Urea Grade/ HAC, HAB, Alloy20, Monel, HVD1, Feralium, Inconel, APX etc.

CAGE GUIDED VALVE

CAGE GUIDED / MULTI STEP VALVES: High Performance material is standard as these valves are designed specifically for severe service conditions.

SINGLE SEATED / DOUBLE SEATED VALVES: PLUG / SEAT : SS 316 or SS 316 with Stelliting on seating surface or SS 316 with complete stelliting.

Very Erosive & NonCorrosive Service

Erosive & Corrosive Service. With case hardening, excellent for high temperature service.

Erosive & Corrosive Service

40 HRC min (H900) 32 HRC min (H1075)

General Application

Hardness

STANDARD TRIM MATERIAL FOR MIL VALVES

Casting: ASTM A743 Gr. CA6NM

Castings: ASTM A 747 CB7CU1

Bar Stock: ASTM A 564 Gr. 630

ASTM Code

CA6NM Stainless Steel Heat Treated

17.4 PH Stainless Steel

General Classfn.

Most common in Carbon Steel and Chrome-Moly steel valves. Most common in Stainless Steel valves

440 C

Stellite 6

All services except molten alkali, hot hydrofluoric acid and oxygen

-27 deg C to 180 deg C

-195 deg C to 232 deg C -27 deg C to 400 deg C

-195 deg C to 566 deg C

PTFE in standard bonnet

PTFE in Extended bonnet

Graphite in standard bonnet

Graphite in Extended bonnet

In high temperature service, standard bonnet with Grafoil is less expensive than extended bonnet with Teflon.

Other considerations: Lubricator & Isolation valve: essentially obsoleted by modern self-lubricating packing.

All services except strong oxidizers

Application

Temp. Range

Material

GLAND PACKING

Guides generally should differ in hardness by min. 5-10 HRC from the stem material

Carbon Steel / Chrome-Moly steel valves in high temperatures.

Application

Material

GUIDE BUSH

-27 deg C to 400 deg C

-195 deg C to -27 deg C

Standard bonnet

Extended bonnet

Other considerations: Finned bonnets for high temperature service are being obsoleted in favour of plain extension bonnet which has equivalent heat dissipation properties and is less complicated and expensive.

- 400 deg C to 566 deg C

Temp. Range

Type

STEP 7. SELECT BONNET TYPE

204 200 195 705 515 1410 1025 2115 1540 3530 2570 5880 4280

260 170 170 665 480 1330 955

316 140 140 605 450 1210 900

371 110 110 570 430 1135 870

50 20

427 80

482 50

0

0

0

0

0

00 537 20

2

0

1

149 230 215 730 560 1455 1120 2185 1680 3640 2795 6070 4660

2

0

1

93

2

0

1

ANSI 2500#

50

350 105

170 415 345

410 420 825

2. ASTM A 351 Gr. CF8M

1. ASTM A 216 Gr. WCC

80

700

830

845

155

515

1050 260

1245 860

1750 430

2915

2075 1430 3460

1235 1265 2060 2110 3430 3520

1705 1305 2840 2170 4730 3620

1815 1355 3025 2255 5040 3760

1995 1435 3325 2390 5540 2980

260 235 750 620 1500 1240 2250 1860 3750 3095 6250 5160

290 275 750 720 1500 1440 2250 2160 3750 3600 6250 6000

2

2

ANSI 1500#

38

2

1

ANSI 900#

0

1

1

ANSI 600#

qC

ANSI 300#

ANSI 150#

Working Pressure in psig

F

Temp.

Once you have chosen the body material you can select the ANSI body rating required by checking the process temperature and maximum process pressure against the appropriate ANSI B16.34 table.

STEP 8. SELECT BODY RATING

ANSI

DIN

Threaded ** - BS, JIS STANDARDS CAN BE GIVEN

WELD END

FLANGED

ANSI

6,10,16 25,40 64 100 160,250 320,400

Socket WELD

BUTT WELD

PN PN PN PN PN PN

RF RTJ T&G FF

Common in low pressure rotary valves.

Ring Joint (In all sizes, ANSI 150#-ANSI 2500#, recommended for higher pressure service)

Raised Face (In all sizes, ANSI 150#-ANSI 2500#, excellent for general application and most common connection used)

Most common connection due to easy installation / removal.

Butt weld (Above 2”, specify pipe schedule)

END CONNECTIONS FOR MIL VALVES

Flangeless

Flanged

Most applicable to high pressure, high temperature, hazardous fluid or vibrating systems.

Welded Socket weld (Up to 2”, specify pipe schedule)

Available through 2”, 600# lb, limited to corrosive service.

Threaded

STEP 9. SELECT END CONNECTION TYPE

when used with a metal seat and metal or resilient seals.

NOTE: Pressure-balanced valves typically meet Class II and Class III leakage

with Class II, but with a higher degree of seat and seal tightness.

This class establishes the maximum permissible leakage generally associated

Class III

port control valves with a piston ring seal and metal-to-metal seats.

with commercial double-port, double-seat control valves or balanced single-

This class establishes the maximum permissible leakage generally associated

Class II

required.

as the basic class, but by agreement between user and supplier, no test is

A modification of any Class II, III or IV valve where design intent is the same

Class I

eg. High temperature steam.

applications where a soft seat cannot be used due to high temperature.

ANSI Class V shutoff can be specified for applications requiring tight shutoff in

Tighter shutoff can be achieved by using a soft seat (ANSI Class VI).

leakage. ANSI Class IV is standard for most metal seated control valves.

control valve. Leakage class per ANSI Standard B16.104 typically rates seat

Control valve seat leakage can be an important factor in the selection of a

STEP 10. SELECT SHUT-OFF CLASS REQUIRED

single port with O-rings or similar gapless seals.

associated with resilient seating control valves either unbalanced or balanced

This class establishes the maximum permissible seat leakage generally

Class VI

used with a soft seat and exceptionally tight resilient seals.

NOTE: Pressure-balanced trim can achieve Class IV or Class V leakage when

seal tightness.

seat control valves or balanced single port designs with exceptional seat and

class is generally associated with metal seat, unbalanced single-port, single

requires special manufacturing assembly and testing techniques. This

of time with high differential pressure across the seating surfaces. It

control valve, without a blocking valve, may be required for long periods

This class is usually specified for critical applications where a closed

Class V

limitations to remember, such as temperature and pressure.

(commonly referred to as a soft seat). However, there are some soft seat

Class IV leakage can be specified using an elastomeric insert configuration

means and metal-to-metal seats. With globe or rotary valve, a bubble-tight

balanced single-port control valves with extra tight piston rings or other sealing

with commercial unbalanced single-port, single-seat control valves and

This class establishes the maximum permissible leakage generally associated

Class IV

Pressure applied to valve inlet after filling entire body cavity and connected piping with water and stroking valve plug closed. Use net specified maximum actuator thrust, but no more, even if available during test. Allow time for leakage flow to stabilize. Actuator should be adjusted to operating conditions specified with full normal closing thrust applied to valve plug set. Allow time for leakage flow to stabilize and use suitable measuring devices.

Maximum service pressure drop across valve plug, not to exceed ANSI body rating. 100 psi (6.9 bar) pressure drop minimum. 50 psig (3.4 bar) or maximum rated differential pressure across valve plug, whichever is lower.

0.0005 liters per minute of water per inch (mm) of port diameter per psi (bar) differential.

VI

V

Not to exceed amounts shown in Class VI Seat Leakage Allowance table based on port (orifice) diameter.

As above

0.01% of As above As above rated capacity

IV

Air or nitrogen at 50 to 125 ºF (10 to 52 ºC)

Water at 50 to 125 ºF (10 to 52 ºC)

As above

As above As above

Pressure is applied to valve inlet, with outlet open to atmosphere or connected to a low head-loss measuring device, full normal closing thrust provided by actuator.

Testing procedures required for establishing rating

0.1% of rated capacity

II

45 to 60 psi (3.1 to 4.1 bar)or maximum operating differential whichever is lower

Test pressure

III

Test medium Air or water at 50 to 125 ºF (10 to 52 ºC)

Maximum leakage allowable 0.5% of rated capacity

Leakage class designation

Seat Leakage Classifications (In accordance with ANSI / FCI 70.2)

STANDARD : CLASS V OPTIONAL : CLASS VI

MULTISTEP VALVES:

STANDARD : Class III,*Class IV (Elastomer Seals) OPTIONAL : CLASS V

CAGE GUIDED VALVES:

STANDARD : CLASS II OPTIONAL : CLASS IV & CLASS VI

DOUBLE SEATED VALVES:

STANDARD : CLASS IV OPTIONAL : CLASS V & CLASS VI

SINGLE SEATED VALVES:

LEAKAGE CLASS FOR MIL VALVES (AS PER ANSI/FCI 70.2)

SPRING DIAPHRAGM

PNEUMATIC ACTUATOR

x High Performance Servo Actuator

x Manual Hand Wheel

x Electro-Hydraulic

x Electrical

x Piston Cylinder

x Pneumatic Operated Spring Diaphragm

is to be supplied to.

Fail Open

Stay put (Using Air Lock)

Fail Close

Spring less Type

Spring Return Type

may depend on the application requirements and/or the type of plant which it

close as required. Different types of actuator available. The one you select

The purpose of an actuator is to provide a force to allow the valve to open and

STEP 11. ACTUATOR SELECTION

Simple in construction, no moving parts, much less maintenance.

Piston requires frequent changes of ‘O’ rings and seals, In the event of any O ring damage, air will leak.

Any slight moisture content in the air can corrode the cylinder inside, any pitting in the cylinder will start air leak and damage the seals.

Fail safe action inherent for diaphragm actuators, Backup system required for piston cylinder. Back up system with volume tank, 3-way transfer valves, NRV, etc. complicates the instrumentation system.

Only spring diaphragm has ability to act without a positioner. Spring ranges that match controller output signal ranges doesnot require positioner.

x

x

x

x

x

ADVANTAGES OF DIAPHRAGM ACTUATORS OVER PISTON CYLINDER ACTUATORS

A. SPRING DIAPHRAGM ACTUATORS: are common on globe-style control valves requiring infinite positioning. They usually include an opposing spring to provide fail-safe action.

Most plants have air supply lines running throughout the plant making it relatively easy to connect a new actuator.

The simplicity of pneumatic actuators makes them easy to adapt to fail-open or fail-closed configurations. However, depending on the pneumatic actuator type, fail-in-place configurations can be complex.

With no electricity, and thus no spark potential, deployment of pneumatic actuators in hazardous classified areas is a popular choice; but don’t ignore that many control valves are accessorized with solenoid valves, limit switches, or electronic controllers.

Pneumatic actuators are generally simpler, less costly, and easier to install and maintain.

PNEUMATIC ACTUATOR

Sometimes employed with control valve bodies for a precise manual control or when a pneumatic actuator is to be added later.

F. HANDWHEEL

Double acting servo-valve loads and unloads hydraulic pressure from both sides of a piston plate. Advantages are fast response and high pressure drop capability. Disadvantage is external hydraulic source requirement.

E. ELECTRO HYDRAULIC

Similar to piston actuator but generally utilizes a double acting positioner to vary the pneumatic pressure on both sides of the plate. Advantages and disadvantages are the same as the piston actuator.

D.DOUBLE ACTING CYLINDER

Pneumatic pressure acts on the cylinder plate causing stem movement, an opposing spring provides returning force. Primary use 'is for on-off service. Advantages are high power-to-size ratio and positive fail safe action. Disadvantage is throttling requires double acting positioner.

C. CYLINDER

Pneumatic pressure is set on one side of the piston plate and varied on the other to effect motion. Advantages are fast response, flexibility high power-tosize ratio. Disadvantage is positioner prerequisite and lack of inherent fail safe action.

B. PISTON

Maintain position regardless of changing forces

Handle high air pressures

Increase speed of operation

Permit faster speed of response

Change characteristics

Provide simple adjustments including split ranging

x

x

x

x

x

x

: Mounted on or off valve

C. Liquid level controller : Mounted on liquid containing vessel

B. Temperature Controller : Mounted on or off valve

A. Pressure Controller

3. CONTROLLER: It is a device whose input is indicative of the valve of and process variable and whose output initiates valve position changes to maintain the set-point valve of that process variable.

The pneumatic output signal is used either to directly actuate the valve or is used as a positioner input signal.

Used to convert a 4-20 mA electrical input signal to either a 3-15 psi or a 6-30 psi pneumatic output signal.

2. ELECTROPNEUMATIC TRANSDUCER (Mounted on or off the valve)

B. Electropneumatic - Operates on a 4-20 mA input signal

Usually, operates on a 3-15 psi input signal (3-9,3-27,6-30,9-15 also used)

A. Pneumatic - most common type:

Permit greater accuracy & process control

x

Positioners may be used for the following reasons:

A positioner is a device, pneumatic, electro-pneumatic or digital, which, by using a control signal precisely positions the moving parts of a control valve in accordance with the signal value.

1. POSITIONER (Valve mounted)

STEP 12. SELECTION OF ACCESSORIES.

Mounted on or off the valve; capacity tanks or accumulators used to store air pressure f or transfer to a springless actuator to ensure proper valve failure during loss of plant air.

9. VOLUME TANK

Pneumatic relay used in conjunction with a volume tank to transfer volume tank pressure to a springless actuator to ensure proper valve failure during loss of plant air.

8. TRANSFER VALVE (Valve mounted)

Pneumatic relay which locks in the actuator pressure during loss of plant air supply causing the valve to fail-in-last-position.

7. LOCK UP VALVE (Valve mounted)

Supply air pressure reducing regulator (with internal filter and relief valve) used in conjunction with any of the above air consuming devices. Air filters are used upstream of the positioner to remove oil, moisture and foreign material from the process air entering the positioner.

6. AIR SET (Valve mounted)

Pneumatic device whose output volume is greater than its input signal thus increasing valve stroking speed.

5. VOLUME BOOSTER (Valve mounted)

Solenoid valves are used in as the control mechanism to open or close the valve in on/off applications where a positioner is not required.

Commonly 3-way type valve used in on-off service: when solenoid is energized, air supply is applied directly to the actuator completely opening or closing the valve: when de-energized, actuator pressure is vented to atmosphere allowing the actuator spring to fully stroke the valve to the opposite position.

4. SOLENOID VALVE (Valve mounted)

Handwheels allow the valve to be switched from automatic to manual control. Sometimes used as a built-in travel stop.

13. HANDWHEELS (Valve mounted)

Mounted downstream of valve multistage, multi port devices which effectively reduce valve noise upto 20 dbA by increasing the valve outlet pressure.

12. LO-DB PLATE & CARTRIDGES

Mounted in the actuator; mechanical stops which valve travel, generally to prevent full closure

11. TRAVEL STOP

Valve mounted: Mechanical device which trips electrical switches at set position of valve stroke.

10. LIMIT SWITCHES

 HIGH VELOCITY  HIGH NOISE  CAVITATION  FLASHING

P1

PVC

PV

P2

A liquid flow can generally be treated as being incompressible if there is no vapour formation. However, vapour bubbles are produced if the local static pressure falls below the fluid vapour pressure. In the event that the pressure recovery is sufficient to raise the static pressure above the vapour pressure, then the vapour bubbles will collapse this process being known as cavitation. Collapsing vapour bubbles release extremely high levels of energy and noise. If these bubbles implode in close proximity to a solid surface then the energy released tears away the material leaving a rough, pitted surface. From the outside, this effect can sound like a high-pitched hissing for incipient cavitation, to a metallic rattling sound for fully developed cavitation.

CAVITATION

Severe service generally applies to applications showing any one or more of the above characteristics. Severe service applications can be detrimental to your valve and surrounding equipment, and greatly reduce the valve service life, if they are not taken into account when sizing and selecting a valve.

HIGH PRESSURE DROP EXTREME TEMPERATURE EROSIVE

SEVERE SERVICE CONSIDERATIONS

Noise & vibration

Corrosion

Valve failure

A combination of the above

x

x

x

x

PVC

P2

PV

Use hardened materials in valve construction

x

Special Valve design ie. expanded outlet

Control velocities

x



If the downstream pressure on the process fluid is equal to or less than its vapour pressure, the vapour bubbles created at the vena contracta do not collapse, resulting in a liquid-gas mixture downstream of the valve. This is commonly called flashing. The result is a two-phase mixture at the valve outlet and in the downstream piping. Velocity of this two-phase flow is usually very high and results in the possibility of erosion of the valve and piping components. Flashing damage is characterised by a smooth, polished appearance. Flashing damage may be controlled through use of the following:

P1

FLASHING

Cavitation damage may be controlled by using hardened materials in valve construction or by using a cavitation reducing/eliminating trim in the valve.

Pitting and erosion

x

The effects of cavitation (depending on its severity) include the following:

vibration

mechanical

of

valve

Major

of

components,

sources and

valve

noise

hydrodynamic

control

and

are

fatigue

failure

of

the

valves

components.

Noise

produced

by

to reduce the SPL (sound pressure level).

to reduce physical damage to valve components and the piping system, and

the vena contracta. Reduction or elimination of cavitation is usually necessary

the fluid stream is highly localised to the region immediately downstream of

Cavitation noise similar to a rattling sound, as if gravel were being carried in

may result to the vibrating part/s.

mechanical rattling. Mechanical noise is usually secondary to the damage that

mechanical vibration is usually well below 100 dBA and is described as a

produce

natural frequency. Resonant vibration produces high levels of stress that may

Vibration can also be produced by valve components resonating at their

fluctuations within the valve body and fluid impingement upon the valve plug.

Mechanical noise can result from vibrations caused by the random pressure

the

system.

aerodynamic fluid noise.

piping

downstream

turbulence created in the valve and radiated to the surroundings by the

atmosphere. In fact, this is not the case: Control valve noise is generated by

It is commonly thought it is the control valve which radiates noise to the

NOISE

The cage guided design of trim is used extensively on high duty flashing applications whereby flow is directed over the plug to dissipate the energy within the confines of the trim. Experience has also shown that in case of flashing it is good practice to use a single stage of pressure letdown.

the valve.(see below)

cavitation can be reduced or eliminated by staging the pressure drop through

100 dBA and reach as high as 150 dBA in certain services. Noise and

velocity may be as low as Mach 0.4. Aerodynamic noise levels can be above

vena contracta, high noise levels can be generated even though the outlet

rate. As the gas flow through a control valves accelerates as it approaches the

The noise level is generally a function of flow stream velocity and mass flow

Aerodynamic noise is the major source of valve noise for gaseous service.

usually not a noise problem.

downstream of the vena contracta. Liquid flow noise is generally low and is

result from the rapid deceleration of the fluid as the flow area increases

Hydrodynamic noise is caused by turbulent liquid flow velocity fluctuations that

problem

noise is significantly lower than cavitation noise, but erosion is often a serious

deceleration and expansion of the two-phase flow stream. Generally, flashing

subsequent bubble collapse that occurs in cavitation. Noise results from the

Flashing noise occurs when a portion of the fluid vaporises without the