Valves Instumentation and Control

8/7/2019 Valves Instumentation and Control

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Valves, Instrumentation,and Control

Process DesignCEN 574

Spring 2004

Chemical Engineering and Materials Science

Syracuse University


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O utline

T ypes of valvesComponents of valves

I nstrumentation: on-

line metersProcess controlreview


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Valves, Instrumentation, and

Control Learning Obj ectivesA t the end of this section, you should be able to

Describe the major types of valves, their operationand characteristics, and draw their standardsymbols on a P& I D.Select an appropriate valve for a specific

application. I dentify the function of the major components of avalve.Select trim and K


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List the four most commonly measuredvariables and draw the symbols for those

meters on a P& I D.Describe the principals of measurement.Define the variables involved in an P I Dloop and draw appropriately on a P& I D.Define tuning.

L ist the common process problems that leadto unstable P I D loops.


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Valve Types B all Valve B utterfly Valve G ate Valve G

lobe ValveCheck Valve


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Parts of a Valve

1. Closure member: part of the valve

that closes flow (disk, ball, gate, etc.).2. A ctuator: means of operating thevalve hand, gear, chain wheel, motor,

solenoid, pressure and flow of themedia, air pressure.


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3 . End fitting: must be specified when

buying the valve - butt weld end,compression flange, pipe thread,quick disconnect

4. Material: closure member, housing,seat stainless steel

5. Packing/seals: seals stem, replaced6. Seat: where the closure members

seals against the valve housing


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B all ValveSphere with a port in a housing, rotate toexpose channel.

y A pplications: Flow control, pressure

control, shutoff, corrosive fluids, liquids,gases, high temp.

y A dvantages low pressure drop, lowleakage, small, rapid opening

y Disadvantages seat can wear if used for

throttling, quick open may cause hammer


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B all Valve


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G ate ValveSliding disk, perpendicular to flow

A pplications: Stop valves, (not throttling), high pressure and temp, not for slurries, viscousfluids

A dvantages low pressure drop when fully

open, tight seal when closed, free of contamination buildup

Disadvantages vibration when partially open,slow response and large actuating force


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G ate Valve


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B utterfly Valverotating disk on a shaft, in a housing

L ow pressure, large diameter lines where

leakage is unimportantA dvantages low pressure drop, smalland light weight

Disadvantages high leakage, highactuation forces so limited to low

pressures


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B utterfly Valve


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Check Valvesallows flow in only one direction

Swing valve similar to butterfly excepthinged along one edge rather than rotateabout the diameter, used primarily for check

valves.


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Check valve


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G lobe Valvethree types (globe, angle and Y), disc or plug

moved perpendicular to flow and closes on a ringseat

T hrottling, general purpose flow control valve

A dvantages faster than gate, seat less wear andtear, high pressure drop for pressure control

Disadvantage high pressure drop, requireconsiderable power to operate (gears and levers),heavy


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G lobe Valve


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R upture Disk

(not a valve ruptures ata set pressure)


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Valve Sym bolsB utterfly Valve

B all Valve

G ate Valve

G lob e Valve


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Instrumentation

- Form groups of two.- L ist as many process conditions thatcan be measured on-line as you can.

- L ist the four most common.


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InstrumentationMost commonly measured process

conditions: temperature, level, pressure, flow rate, pH.

Other variables include

composition, moisture, specificgravity, viscosity, dissolvedoxygen.


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TemperatureThermocouple

T wo wires of different metals welded together at the end W hen the junction is heated a small electric current

proportional to the junction temperature is generated T ypes of thermocouple refer to the types of metal

common J,H, K T hermowell is a sleeve that the thermocouple sits in I ssues low current, fouling of thermowell

Thermister/ R esistive thermal detectors ( R TD)Consists of a resister that have a high coefficient of resistance

R esistance = f(temperature)


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Flow

R ate

Historically the most common methodto measure flow rate is the orifice

plate.However, more sophisticated methods

are becoming more common becausethey have less pressure drop.


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O rifice Meter


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Calculating pressure drop across and orifice meter

( P (psi) = K*( V/1726)(velocity2

)K=orifice coefficient usually 0.6-0.8V=density (lb/ft 3 )velocity=velocity through the orifice ft/sfor estimation use orifice of the pipediameter Example: assume 4 inch line, 200 gpm, orifice

diameter = 2 in, orifice radius = 1 in = 0.08 3 ftvelocity=(200gal/min)(ft 3 /7.48gal)(min/60sec)(1/pi(0.08 3 2ft2)=20.6 f/s

( P (psi)=0.7*(62.4)(20.62

)/1726=10.7 psi


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Level Meters F low Meters


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P rocess Control: A R eview

A ssignment: R ead the Smitharticle and T urtons chapter 15about process control.


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Process Upsets

T , F, reaction rate, P etc. maychange and effect product quality.T hese variables may change due toequipment failure, intentional

production rate changes, coolingwater changes, etc. T hese changesare called process upsets.


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Controlled Varia b lesControlled variables are variables

that we want to maintain atconstant or specified values ( T , P,flow rate, level, etc.).


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Measured Varia b lesMeasured variables are variables

that we measure with a meter (often often the controlled variableor a variable that we use to

calculate the controller variable).


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ControllerA unit that reads an I NPU T signal anda SE T PO I NT , compares the two,

performs a calculation on thedifference, and sends out an OU T PU T signal.

Common type of controller: P ID = proportional, integral, derivative


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Controller

controller meter valve

(Electrical or pneumatic signalindicated by dashed line.)

I NPU T OU T PU T

SET PO I NT

(commonly fromthe operator)


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Types of Control:

F eed B ack W hen the controlled variable changes or

is different than the set point, thecontroller adjusts the manipulatedvariable to bring the controlled variable

back to the set point value.

T he controlled variable must change (bedifferent than the set point) for controlaction to be taken.


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F eed F orward T he measured variables are used to change

manipulated variables before changes in thecontrolled variable takes place. Especiallyuseful when there are long lag times in the

process.A ll factors likely to cause a change in thecontrolled variable must be taken intoaccount.Some type of model of process

behavior/dynamics must be known, and theaccuracy of control is directly linked to theaccuracy of control.


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CascadeUses controllers in series. T he firstcontrollers output is the set point tothe second controller.

R educes lags and allows finer controls,

but is more complicated than singleloops.


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Controlled variables

Control loops?

W hat happens if thelight component

concentration in thefeed decreases?


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Distillation ControlPurity of distillate will decrease and amount inoverhead will decrease.Valve CV-1 will open to increase reflux rate inresponse to concentration change.

L evel in reflux drum will drop, so CV- 3 will closeto reduce overhead flow rate.Pressure will decrease causing CV-2 to close andcooling water flow rate to decrease.

L evel and purity in the bottoms will increasecausing CV-5 to open increasing bottoms flow,and CV-4 will close reducing steam


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Valves Instumentation and Control