che 462 diaphragm control valves

7/29/2019 ChE 462 Diaphragm Control Valves

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Diaphragm Control Valves

Just what is a control valve?

An automated valve used to restrict

flow to the process We can

manipulate the process

through Bernoulli


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Purpose of a Control Valve

The purpose of a control valve is to throttle flow. The pressure drop

P across the control valve required to do this is an energy expense.

For Liquid flow:

FL

= CV

sqrt [PV

/ SG]

Where:

FL = Liquid flow (gpm)

CV = Control valve flow coefficient per characteristic curve (gpm/psi1/2)

PV = Pressure drop across the valve (psi)

SG = Liquid specific gravity

Cv is proportional to cross sectional area for flow. An increasein valve capacity (Max Cv) will decrease the valve pressure drop

Its relationship to stem position can be linear or nonlinear

For linear trim, Cv is proportional to position. Its response to a

signal is adversely affected by stick-slip and dead band.


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Control Valve, Two Basic Designs:

Sliding Stem, globe gate, needle etc.

Rotating Stem, Ball, Plug, Butterfly, Disk, etc

Sliding StemRotating Stem


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Sliding Stem (Globe) Control Valve

Stem guiding helps keep plugaligned for high pressure drops

Integral flange

Seat ring - designs to make the sealing of the surfacesbetween the plug and seat tighter reduces leakage whenvalve is closed (tighter shutoff) but increase friction thatopposes stem movement. The plug is stuck in the seat.

When enough pressure builds up in the actuator, the plugbreaks free, jumps, and overshoots the desired position

Plug stem connectedto actuator shaft thatmoves up and downto stroke the valve

Stem packing - tightened andrough surface to reduce escapeof process increases friction

that opposes stem movement

Flow in Flow out

Path for escaping process vapors


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Severe Service Options

(high pressure drop, noise, and cavitation)

Plug guidingprevents

deflection

Balanced

plug reducesactuator sizebut increaseleakage

Whisper trim reduces noise

but watch out for occlusionof narrow slots and tiny holesfrom steam and process residue!


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Single Port Double Port

FISHER A Body, older style


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Rotary actuatoreliminates linkageneeded to translateup and down shaftmovement to rotarymotion of plug stem

Lowest cost for 1- 6 size valvesbut it has a very limited choice ofmaterials for corrosive service

Tight plug stem andactuator shaft connection

reduces dead band

Digital valve positionermeasures and controlsactuator shaft rotation

Rotary Eccentric Plug Valve Design


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Rotary Eccentric Plug Valve Cross Section

Stepped

Seat Ring

(less friction

near the seat)

Optional

attenuator

disc to reducenoise

Integral flange

reduces potential

for leaks and the

installation and

maintenance cost

Contoured

eccentric plug

improves plot

of flow versus

stem positionnear the seat

Diaphragm actuator an


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V-Ball with

a diaphragmactuator

Dead band(lost motion)V-ball < 0.5%Other 2 - 8%

Tight ball, stem, andshaft connectionsreduces dead band

Short shaftreduces twist(stick-slip)

Tight linkage design totranslate up-down

actuator shaft movementto rotary ball motion

Unfortunatelymost positionersmeasure actuatorinstead of ballor disc position

V provides better

flow characteristicand shear of pulp

Diaphragm actuator - anincrease in air pressureto top presses on flexiblediaphragm that causes

shaft to move down. Theminimum pressure toopen the valve is set byforce of opposing spring.For very large actuatorsvolumes, the time to strokefull scale can become large


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Butterfly Valve

(with Rotary Piston Operator)

These are often called high

performance valves becauseof high temperature and low

leakage ratings but they are

low performance so far as

stick-slip particularly near

the closed position because

of very high seat/seal friction

These valves are ideally suited

for on-off service as isolation orblock valves for interlocks and

batch sequences - just dont use

them as control valves

Butterfly valves are muchmuch less expensive than

globe valves for large sizes

(>6) since there is much

less metal and machining

Flangeless wafer style

saves on price but it

must be carefully fitted

between piping flanges


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Valves can get Damaged

Flashing and Cavitation


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Various trim designs to select based on process characteristics

Want to achieve linearly installed characteristics


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Valve Actuator


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Bonnet Designs


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Actuator Failure

Failures, FO, FC - Fail Open or Closed

ATO - Air To OpenATC - Air To Close

FLP Fail Last Position


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The controller output signal needsto move the valve; how does it do

that?

Need to convert the process signal to a

change in air pressure.


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Nozzle Baffle Pneumatic Valve


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An element or device which receives information in the form of one quantityand converts it to information in the form of the same or another quantity. ISA-S51.1 1976

Example: Converting different signal types; example: I/P current to pneumatic;

4 to 20 mA to 3 to 15 psig.

I/P based on the DArsonval principal, the same principal in a moving coil meter. A coil placed between the poles of a permanent magnet.

I/P, Current to Pneumatic, Transducer


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Fisher Type 546, 546S, and 546NS Electro-Pneumatic Transducers


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Electropneumatic positioners combine the function of a current-to-pressure

transducer with thoseof a positioner. It receives an electronic input signal

and ensures valve position by adjusting output pressure.

(Fisher Controls International, Inc.)


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Volume Booster Valve Postioner

Volume Booster: Used to increase the strokingspeed. These are pneumatic device that has a

separate air supply to deliver higher volume ofair to move the actuator faster.

A Positioner is actually a feedback controller in

itself. Frequently fast flow loops can interact withthe dynamics of the positioner.

Do not confuse a positioner with a volume

booster! A Positioner willcontrol the valveposition while a volume booster will justprovide more air supply to the actuator.


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Figure 1 Definition of Dead band and Stick-Slip

dead band

Dead band

Stick-Slip

Signal

(%)

0

Stroke

(%) Digital positioner

will force valve

shut at 0% signal

Pneumatic positionerrequires a negative %

signal to close valve

The dead band and stick-slip is greatest near the closed position

Dead band is 5% - 50%

without a positioner !


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Figure 2 Response Time for Different Types of Final Elements

0.1 1.0 10.0 100

200

20.0

2.0

0.2

63 ResponseTime (sec)

Step Size (%)

2

3 4 5

6

1 - variable speed drive with dead band adjustment set equal to zero

2 - sliding stem valve with diaphragm actuator and a digital positioner

3 - sliding stem valve with diaphragm actuator and pneumatic positioner

4 - rotary valve with piston actuator and digital positioner

5 - rotary valve (tight shutoff) with piston actuator and pneumatic positioner

6 - large valve or damper with any type of positioner7 - small valve with any type of positioner

1

All valves look good

for about a 10% step

7


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Pressure Regulators

Provides Pressure Control

Downstream pressure regulation Upstream pressure regulation, a special

case is a relief valve

Self Contained

Pilot Operated


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Pressure Regulator

Spring size determines range

Set screw preloads spring to set pressure Control not exact, pressure will droop

example for a pressure reducing valve, pressure will be

reduced 20% from set point at maximum flow

Common uses in utility distribution


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che 462 diaphragm control valves

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