8/7/2019 Valves Instumentation and Control
1/43
Valves, Instrumentation,and Control
Process DesignCEN 574
Spring 2004
Chemical Engineering and Materials Science
Syracuse University
8/7/2019 Valves Instumentation and Control
2/43
O utline
T ypes of valvesComponents of valves
I nstrumentation: on-
line metersProcess controlreview
8/7/2019 Valves Instumentation and Control
3/43
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
8/7/2019 Valves Instumentation and Control
4/43
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.
8/7/2019 Valves Instumentation and Control
5/43
8/7/2019 Valves Instumentation and Control
6/43
8/7/2019 Valves Instumentation and Control
7/43
Valve Types B all Valve B utterfly Valve G ate Valve G
lobe ValveCheck Valve
8/7/2019 Valves Instumentation and Control
8/43
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.
8/7/2019 Valves Instumentation and Control
9/43
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
8/7/2019 Valves Instumentation and Control
10/43
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
8/7/2019 Valves Instumentation and Control
11/43
B all Valve
8/7/2019 Valves Instumentation and Control
12/43
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
8/7/2019 Valves Instumentation and Control
13/43
G ate Valve
8/7/2019 Valves Instumentation and Control
14/43
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
8/7/2019 Valves Instumentation and Control
15/43
B utterfly Valve
8/7/2019 Valves Instumentation and Control
16/43
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.
8/7/2019 Valves Instumentation and Control
17/43
Check valve
8/7/2019 Valves Instumentation and Control
18/43
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
8/7/2019 Valves Instumentation and Control
19/43
G lobe Valve
8/7/2019 Valves Instumentation and Control
20/43
R upture Disk
(not a valve ruptures ata set pressure)
8/7/2019 Valves Instumentation and Control
21/43
Valve Sym bolsB utterfly Valve
B all Valve
G ate Valve
G lob e Valve
8/7/2019 Valves Instumentation and Control
22/43
8/7/2019 Valves Instumentation and Control
23/43
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.
8/7/2019 Valves Instumentation and Control
24/43
InstrumentationMost commonly measured process
conditions: temperature, level, pressure, flow rate, pH.
Other variables include
composition, moisture, specificgravity, viscosity, dissolvedoxygen.
8/7/2019 Valves Instumentation and Control
25/43
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)
8/7/2019 Valves Instumentation and Control
26/43
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.
8/7/2019 Valves Instumentation and Control
27/43
O rifice Meter
8/7/2019 Valves Instumentation and Control
28/43
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
8/7/2019 Valves Instumentation and Control
29/43
Level Meters F low Meters
8/7/2019 Valves Instumentation and Control
30/43
P rocess Control: A R eview
A ssignment: R ead the Smitharticle and T urtons chapter 15about process control.
8/7/2019 Valves Instumentation and Control
31/43
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.
8/7/2019 Valves Instumentation and Control
32/43
Controlled Varia b lesControlled variables are variables
that we want to maintain atconstant or specified values ( T , P,flow rate, level, etc.).
8/7/2019 Valves Instumentation and Control
33/43
8/7/2019 Valves Instumentation and Control
34/43
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).
8/7/2019 Valves Instumentation and Control
35/43
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
8/7/2019 Valves Instumentation and Control
36/43
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)
8/7/2019 Valves Instumentation and Control
37/43
8/7/2019 Valves Instumentation and Control
38/43
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.
8/7/2019 Valves Instumentation and Control
39/43
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.
8/7/2019 Valves Instumentation and Control
40/43
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.
8/7/2019 Valves Instumentation and Control
41/43
Controlled variables
Control loops?
W hat happens if thelight component
concentration in thefeed decreases?
8/7/2019 Valves Instumentation and Control
42/43
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
8/7/2019 Valves Instumentation and Control
43/43