CONTROL VALVE HANDBOOK Episode 1 Part 5 ppt

Positioners Pneumatically operated valves de-pend on a positioner to take an input signal from a process controller and convert it to valve travel.. Modern Control Valves Utilizing Digit

Chapter 4

Control Valve Accessories

This chapter offers information on

digi-tal valve controllers, analog

position-ers, boostposition-ers, and other control valve

accessories

Positioners

Pneumatically operated valves

de-pend on a positioner to take an input

signal from a process controller and

convert it to valve travel These

instru-ments are available in three

configura-tions:

1 Pneumatic Positioners—A

pneu-matic signal (usually 3-15 psig) is

sup-plied to the positioner The positioner

translates this to a required valve

position and supplies the valve

actua-tor with the required air pressure to

move the valve to the correct position

2 Analog I/P Positioner—This

posi-tioner performs the same function as

the one above, but uses electrical

cur-rent (usually 4-20 mA) instead of air

as the input signal

3 Digital Controller—Although this in-strument functions very much as the Analog I/P described above, it differs

in that the electronic signal conversion

is digital rather than analog The digi-tal products cover three categories

D Digital Non-Communicating—A current signal (4-20 mA) is supplied to the positioner, which both powers the electronics and controls the output

D HART—This is the same as the digital non-communicating but is also capable of two-way digital commu-nication over the same wires used for the analog signal

D Fieldbus—This type receives digitally based signals and positions the valve using digital electronic cir-cuitry coupled to mechanical compo-nents An all-digital control signal

re-Chapter 4 Control Valve Accessories

68

Figure 4-1 Modern Control Valves

Utilizing Digital Valve Controllers

W8861

W8119-1

places the analog control signal

Additionally, two-way digital

commu-nication is possible over the same

wires Fieldbus technologies benefit

the end user by enabling improved

control architecture, product capability

and reduced wiring

Use of a single, integrated analog I/P

positioner or digital controller (figure

4-1) instead of a combination of

pneu-matic positioner and transducer (two

instruments) results in lower installed

cost

The ability to embed software

com-mands into the memory of the device

represents the real difference

be-tween digital and analog I/P

seg-ments This allows automatic

configu-ration and setup of the valve when

equipped with a digital controller Most importantly, it allows two-way commu-nication for process, valve, and instru-ment diagnostics

Users purchase digital valve control-lers for several reasons:

D Reduced cost of loop commis-sioning, including installation and cal-ibration

D Use of diagnostics to maintain loop performance levels

D Improved process control accu-racy that reduces process variability Two aspects of digital valve control-lers make them particularly attractive:

D Automatic calibration and config-uration Considerable time savings are realized over traditional zero and spanning

D Valve diagnostics Through the Distributed Control System (DCS), PC software tools, or handheld communi-cators, users can diagnose the health

of the valve while it is in the line FIELDVUER instruments enable new diagnostic capabilities that can be ac-cessed remotely This single element requires a look at the potential impact

of the technology as it applies to con-trol valves

An in-plant person, with the aid of the FlowScannert system, can diagnose the health of a valve through a series

of off-line tests The FlowScanner sys-tem consists of a portable, ruggedized computer and travel and pressure sensors The sensors are connected

to the valve to enable diagnostic tests, which are conducted with the valve off-line A skilled maintenance techni-cian can determine whether to leave the valve in the line or to remove the valve for repair

Digital instruments allow an extension

of this service with added enhance-ments:

D Because sensors are part of the instrument, tests can be run easily at appropriate times

Chapter 4 Control Valve Accessories

69

Figure 4-2 Positioner Schematic for Diaphragm Actuator

OUTPUT TO DIAPHRAGM

RELAY

INSTRUMENT

BELLOWS

FEEDBACK AXIS PIVOT NOZZLE

FLAPPER

DIRECT ACTION QUADRANT INPUT AXIS

CAM

REVERSE ACTION QUADRANT BEAM

ACTUATOR

VALVE STEM

CONNECTION

SUPPLY

22A7965-A

A2453-2 / IL

D It is now possible to diagnose

the health of a valve remotely via

HART or Foundation fieldbus

D On-line diagnostics enable

pre-dictive maintenance without disrupting

the process

These additional elements are

ex-tremely important The remote

capa-bility allows monitoring valves Those

who make, supply and service valves

for a living now assist the customer in

the diagnosis of valve condition to a

level never before possible Predictive

maintenance offers additional savings

for the customer It is now possible to

see the performance of the valve as it

operates Watching performance

de-cline over time enables the user to

predict when replacement or repair is

necessary

Other Control Valve Accessories

Figure 4-5 illustrates a top-mounted handwheel for a direct-acting dia-phragm actuator This unit can be used as an adjustable travel stop to limit travel in the upward direction or

to manually close push-down-to-close valves

Figure 4-6 illustrates a top-mounted handwheel for a reverse-acting dia-phragm actuator This unit can be used as an adjustable travel stop to limit travel in the downward direction

or to manually close push-down-to-open valves

Limit Switches

Limit switches operate discrete inputs

to a distributed control system, signal lights, small solenoid valves, electric relays, or alarms The cam-operated type (figure 4-7) is typically used with

Chapter 4 Control Valve Accessories

70

Figure 4-3 Positioner Schematic for Piston Actuator

A1304/IL

INPUT SIGNAL

BYPASS RESTRICTION ADJUSTING SCREW BYPASS RESTRICTION

SUPPLY PORT

OUTPUT TO ACTUATOR SUPPLY

EXHAUST

EXHAUST PORT DIAPHRAGMS

W0679-1/IL

Figure 4-4 Volume Booster

Chapter 4 Control Valve Accessories

71

Figure 4-5 Top-Mounted

Hand-wheel for Direct-Acting Diaphragm

Actuator

W0368-1/IL

Figure 4-6 Top-Mounted

Hand-wheel for Reverse-Acting

Dia-phragm Actuator

W0369-1/IL

two to four individual switches

oper-ated by movement of the valve stem

An assembly that mounts on the side

of the actuator houses the switches

Each switch adjusts individually and

can be supplied for either alternating

current or direct current systems

Oth-er styles of valve-mounted limit

switches are also available

Figure 4-7 Cam-Operated Limit Switches

A7095/IL

W2078/IL

Solenoid Valve Manifold

The actuator type and the desired fail-safe operation determine the selection

of the proper solenoid valve (figure 4-8) The solenoids can be used on double-acting pistons or single-acting diaphragm actuators

Supply Pressure Regulator

Supply pressure regulators (figure 4-9), commonly called airsets, reduce plant air supply to valve positioners and other control equipment Com-mon reduced-air-supply pressures are

20, 35 and 60 psig The regulator mounts integrally to the positioner, or nipple-mounts or bolts to the actuator

Chapter 4 Control Valve Accessories

72

Figure 4-8 Solenoid Valve

W7007/IL

Figure 4-9 Supply Pressure Regulator

with Filter and Moisture Trap

W0047/IL

Pneumatic Lock-Up Systems

Pneumatic lock-up systems (figure

4-10) are used with control valves to

lock in existing actuator loading

pres-sure in the event of supply prespres-sure failure These devices can be used with volume tanks to move the valve

to the fully open or closed position on loss of pneumatic air supply Normal operation resumes automatically with restored supply pressure Functionally similar arrangements are available for control valves using diaphragm actua-tors

Fail-Safe Systems for Piston Actuators

In these fail-safe systems (figure 4-11), the actuator piston moves to the top or bottom of the cylinder when supply pressure falls below a pre-de-termined value The volume tank, charged with supply pressure, pro-vides loading pressure for the actuator piston when supply pressure fails, thus moving the piston to the desired position Automatic operation sumes, and the volume tank is charged when supply pressure is re-stored to normal

Electro-Pneumatic Transducers

Figure 4-12 illustrates an electro-pneumatic transducer The transducer receives a direct current input signal and uses a torque motor, nozzle-flap-per, and pneumatic relay to convert the electric signal to a proportional pneumatic output signal Nozzle pres-sure operates the relay and is piped to the torque motor feedback bellows to provide a comparison between input signal and nozzle pressure As shown, the transducer can be mounted directly on a control valve and operate the valve without need for additional boosters or positioners

Chapter 4 Control Valve Accessories

73

Figure 4-10 Lock-Up System Schematic for Piston Actuator

35A6998-C

A2285-4/IL

Figure 4-11 Typical Schematic of a “Fail-Safe” System

35A6996-C

A2283-4/IL

Chapter 4 Control Valve Accessories

74

Figure 4-12 Electro-Pneumatic

Transducer Mounted on a

Dia-phragm-Actuated Control Valve

ELECTRO-PNEUMATIC

TRANSDUCER

W8723−1

Electro-Pneumatic Valve

Positioners

Electro-pneumatic positioners (figure

4-13) are used in electronic control

loops to operate pneumatic

dia-phragm control valve actuators The

positioner receives a 4 to 20 mA DC

input signal, and uses an I/P

convert-er, nozzle-flappconvert-er, and pneumatic

relay to convert the input signal to a

pneumatic output signal The output

signal is applied directly to the

actua-tor diaphragm, producing valve plug

position that is proportional to the

in-put signal Valve plug position is

me-chanically fed back to the torque

com-parison of plug position and input

signal Split-range operation capability

can provide full travel of the actuator

with only a portion of the input signal

range

Diagnostics

Digital valve controllers incorporate

predefined instrument and valve

diag-Figure 4-13 Electro-Pneumatic Positioner on Diaphragm Actuator

W4930/IL

nostics within firmware to provide alerts if there are problems with instru-ment mounting, electronics, hardware

or valve performance

HART-based handheld field communi-cators when connected to the digital valve controllers enable user-config-ured alerts and alarms These flags provide notification of current status and potential valve and instrument problems Typical alerts include travel deviation, travel limit, cycle count and travel accumulation

AMS ValveLinkR software allows tests that identify problems with the entire control valve assembly Using the valve stem travel feedback, actua-tor pressure sensor and other sensors

on the instrument, the health of the control valve can be evaluated while the valve is still in service and fully op-erational This helps to pinpoint prob-lems before the equipment fails, with-out disrupting the process

Chapter 5

Control Valve Selection

Control valves handle all kinds of

fluids at temperatures from the

cryo-genic range to well over 1000_F

(538_C) Selection of a control valve

body assembly requires particular

consideration to provide the best

available combination of valve body

style, material, and trim construction

design for the intended service

Ca-pacity requirements and system

oper-ating pressure ranges also must be

considered in selecting a control valve

to ensure satisfactory operation

with-out undue initial expense

Reputable control valve

manufactur-ers and their representatives are

dedi-cated to helping select the control

valve most appropriate for the existing

service conditions Because there are

frequently several possible correct

choices for an application, it is

impor-tant that all the following information

be provided:

D Type of fluid to be controlled

D Temperature of fluid

D Viscosity of fluid

D Specific gravity of fluid

D Flow capacity required (maxi-mum and mini(maxi-mum)

D Inlet pressure at valve (maxi-mum and mini(maxi-mum)

D Outlet pressure (maximum and minimum)

D Pressure drop during normal flowing conditions

D Pressure drop at shutoff

D Maximum permissible noise

lev-el, if pertinent, and the measurement reference point

D Degrees of superheat or exis-tence of flashing, if known

Chapter 5 Control Valve Selection

76

D Inlet and outlet pipeline size and

schedule

D Special tagging information

re-quired

D Body Material (ASTM A216

grade WCC, ASTM A217 grade WC9,

ASTM A351 CF8M, etc.)

D End connections and valve

rat-ing (screwed, Class 600 RF flanged,

Class 1500 RTJ flanges, etc.)

D Action desired on air failure

(valve to open, close, or retain last

controlled position)

D Instrument air supply available

D Instrument signal (3 to 15 psig, 4

to 20 mA, Hart, etc.)

In addition the following information

will require the agreement of the user

and the manufacturer depending on

the purchasing and engineering

prac-tices being followed

D Valve type number

D Valve size

D Valve body construction (angle,

double-port, butterfly, etc.)

D Valve plug guiding (cage-style,

port-guided, etc.)

D Valve plug action

(close or

push-down-to-open)

D Port size (full or restricted)

D Valve trim materials required

D Flow action (flow tends to open

valve or flow tends to close valve)

D Actuator size required

D Bonnet style (plain, extension,

bellows seal, etc.)

D Packing material (PTFE V-ring, laminated graphite, environmental sealing systems, etc.)

D Accessories required (positioner, handwheel, etc.)

Some of these options have been dis-cussed in previous chapters of this book, and others will be explored in this and following chapters

VALVE SELECTION PROCESS

DETERMINE SERVICE CONDITIONS

S (P1, ∆ P, Q, T1, Fluid Properties, Allow-able Noise, etc).

S Select appropriate ANSI Pressure Class required for valve body and trim.

CALCULATE PRELIMINARY Cv REQUIRED

S Check noise and cavitation levels

SELECT TRIM TYPE

S If no noise or cavitation indication, choose standard trim.

S If aerodynamic noise is high, choose Whis-per Trim R.

S If liquid noise is high and/or cavitation is in-dicated, choose Cavitrol R III trim.

SELECT VALVE BODY AND TRIM SIZE

S Select valve body and trim size with re-quired Cv

S Note travel, trim group, and shutoff options.

SELECT TRIM MATERIALS Select trim materials for your application; make sure trim selected is available in the trim group for the valve size selected.

OPTIONS Consider options on shutoff, stem packing, etc.

Valve Body Materials

Body material selection is usually based on the pressure, temperature,

Chapter 5 Control Valve Selection

77

corrosive properties, and erosive

properties of the flow media

Some-times a compromise must be reached

in selecting a material For instance, a

material with good erosion resistance

may not be satisfactory because of

poor corrosion resistance when

han-dling a particular fluid

Some service conditions require use

of exotic alloys and metals to

with-stand particular corrosive properties of

the flowing fluid These materials are

much more expensive than common

metals, so economy may also be a

factor in material selection

Fortunate-ly, the majority of control valve

ap-plications handle relatively non-corro-sive fluids at reasonable pressures and temperatures Therefore, cast carbon steel is the most commonly used valve body material and can pro-vide satisfactory service at much

low-er cost than the exotic alloy matlow-erials Specifications have been developed for ordering highly corrosion resistant, high nickel alloy castings These specifications represent solutions to problems encountered with those al-loys These problems included unac-ceptable corrosion resistance compared to the wrought materials, poor weldability, poor casting integrity

Designations for the High Nickel Alloys Casting

Designations

Equivalent Wrought Tradenames

Generic Designations

UNS Numbers for Wrought Equivalents

CK3MCuN Avesta 254 SMO (1) Alloy 254 S31254

CN7M Carpenter 20Cb3 (2) Alloy 20 N08020

CU5MCuC Incoloy 825 (3) Alloy 825 N08825

CW12MW Obsolete Hastelloy C (4) Alloy C N10002

CW2M New Hastelloy C (4) Alloy C276 N10276

CX2MW Hastelloy C22 (4) Alloy C22 N06022

CW6MC Inconel 625 (3) Alloy 625 N06625

CY40 Inconel 600 (3) Alloy 600 N06600

CZ100 Nickel 200 Alloy 200 N02200

M25S S-Monel (3) Alloy S

M35-1 Monel 400 (3) Alloy 400 N04400

N12MV Obsolete Hastelloy B (4) Alloy B N10001

N7M Hastelloy B2 (4) Alloy B2 N10665

1 Trademark of Avesta AB

2 Tradenames of Carpenter Technology

3 Tradenames of Special Metals Corp.

4 Tradename of Haynes International