process design,cv,line size,hazard,safety valve guide
DESCRIPTION
Process Design,CV,Line Size,Hazard,Safety Valve GuideTRANSCRIPT
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Conceptual Process Design
An introduction atwww.cpcsimulation.com
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Basic process steps
Basic Engineering package FEED Residual process engineering Detail engineering Basic core process documents
Process design basis, PFD, P &ID, Datasheets, Line lists, Safety study
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Agenda for this presentation
Workflow for conceptual process design PFD Development P & ID Development Line sizing Control Valve sizing Pump Head calculations PSV sizing Area classification
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Conceptual process designDESIGN BASIS Why to prepare a design basis? Any guess on what this document consists of? Can a P&ID be made without knowing basis? Two different plants same product. Can P & IDs
be absolutely same? Two different plants same product. Can control
valve and safety valves be exactly same? If Yes why and if no why? Same product with same capacity: plant in India
and plant in Russia- will it be exactly same?
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Conceptual process designDESIGN BASIS P & IDs for same product are different
because. Different licensor philosophies and design
specifications Different Legends Different plant capacity Different climatic conditions Different utility availability Different logistics requirements Different plot layouts Different equipment/ Instrument manufacturers
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Conceptual process designDESIGN BASIS Equipment design margins Standard design philosophy Controls and instrumentation levels
(DCS/PLC, SIL study) Safety philosophy: (Fire, F&G, Flare) Isolation and sparing philosophy Battery limit conditions Environmental norms
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Conceptual process engineering Focus on process flow diagram
development (Example.reflash column) Generate a mass and energy balance Chemicals and catalyst summary Development of conceptual P & IDs Critical equipment sizing Estimate utility requirements Operating instructions
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Conceptual process engineering Not part of conceptual engg.. Pump head checks Control valve sizing Safety valve sizing Safety valve sizing Vendor package information Utility distribution drawings HAZOP study
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DOUGLAS METHODOLOGY
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BATCH VS. CONTINUOUS Production rate: > 45000 T/yr, use continuous
process Product demand: seasonal products batch Multiproduct plant Batch plant Significant gas recycle continuous plant Reaction behavior: High temperature or highly
exothermic reactions, continuous plant Significant opportunities of heat integration
continuous plant
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RECYCLE STRUCTURE
Decide reactants to be recycled / purged
Decide purity / separation feasibility
Check effect of recycle on reaction chemistry e.g. recycle of poisons etc
Check effect of compression cost on feasibility
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BLOCK FLOW DIAGRAM
Major equipments on block flow diagramEvaluate cost based on major equipments and
approximate operating costUse ratio method for cost estimationShortlist alternatives for further evaluationProcess simulation will help fix process
conditions, utility loads etcPinch analysis for energy and area saving, but
keep operational issues in mind
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Any Questions so far?
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PFD developement
Show only process flow lines Start up lines, pump recycle lines need
not be shown Utility streams shown in short Standby equipment and working
equipment depiction Which streams to number? Where to place stream number
diamond?
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PFD Development
All equipments tagged Equipment title block added Exchanger symbol is generic and not as
per its TEMA type Start/stop switches for motors not shown All bays of air cooler are not shown Local temperature and pressure guages
not shown
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Control valve Selection and SizingControl valve Selection and Sizing
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F (kg/hr)Ti (oC)
To (oC)
Ms (kg/hr)TS (oC)
Thermometer
Human Brain
Hand
COMPONENTS OF CONTROL SYSTEM
Measuring element (thermometer)
Controller Final control
element (control valve)
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V1
Process
__1__p + 1
Final ControlElement
__1__v + 1
Controller
Kc {1+(1/ I)+ d}
MeasuringInstrument
__1__m + 1
eR
C
P C
CONTROL SYSTEM BLOCK DIAGRAM
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Controller sends the output signal of the order of 4 to 20 mA.
The i/p converter changes the signal to 3 to 15 psi and sends to Actuator.
The Actuator moves the plug relative to the stationary valve seat.
CONTROL VALVE SYSTEM
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POSITIONER
Measures valve stem position and compares with set point
Makes sure control valve opening is as required by controller
Corrects for effect of packing friction due to dirt, lack of lubrication etc
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VALVE TYPES AND SELECTION
Globe valve Workhorse of industry Multiple ports available Large pressure drop Size limited to about 12
Butterfly valves Low pressure drop Large sizes available
Ball valves Low leakage Quarter-turn
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CONTROL VALVE SIZING- 1 Need to fix following:
Minimum, normal and maximum flow Corresponding pressure drop Required flow characteristics i.e. linear or quick
acting etc Control valve flow coefficient Cv used as sizing
parameter
Cv is flow of water in m3/hr through valve at 1 bar pressure difference across valve
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HOW TO ASSIGN PRESSURE DROPS? For pump systems:
50-60% of total frictional loss excluding control valve OR
15% of pump differential head OR 0.7 kg/cm2
For process systems Difference of pressure between upstream and
downstream system Should give better controllability Smaller the pressure drop, bigger the valve
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Linear
Quick opening
Equal %
FLOW CHARACTERISTICS Inherent characteristics Quick acting / linear / equal percentage
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SELECTING CHARACTERISTICS: LEVEL
Level control: very slow response (remember tanks in series experiment)
Mostly linear
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SELECTING CHARACTERISTICS: FLOW
Whenever in doubt, use equal percentage
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CAVITATION AND FLASHING Cavitation is collapse of
bubbles Damages trim Special trim designs
available Flashing not as dangerous
as cavitation Needs to be specified to
vendor
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CHOKING Occurs for gaseous systems when downstream
pressure less than about 50% of inlet pressure Can occur at much higher % for two-phase
systems Fluid accelerated to sonic velocity in valve Formation of shock waves Avoided by use of multiple valves or use of ROs
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FAIL- SAFE POSITION Action for valve when air supply fails Three options
Fail open Fail close Fail last
Energy adding valves to be fail closed e.g. steam Energy removing valves to be fail closed e.g.
cooling medium
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Line sizing
An introduction
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Flow rate basis for line sizing
PFD and mass balance Utility summary and utility flow diagrams Design margin: General
considerations For all process lines 10% Start up, intermittent flow 5% Utility lines 15% Utility main header 20%
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Line sizing basis
For revamp, basis differs. Circulating fuel oil system-design ring main for
125 to 150% of flow Pipe wall roughness basis Carbon steel, new- 0.047 mm Brass, aluminum, Copper, Plastics, Glass
0.03 mm Stainless steel 0.025 mm Rusty steel 0.2 mm Galvanized steel 0.13 mm
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Normal pipe size
Avoid 30NB,65NB,125NB, 550NB Nominal pipe size for process lines shall
be 25mm Pipe rack lines shall be minimum 40NB Schedule number as per pipe
specification index Piping length estimate based on project
status-preliminary or plot plan/equipment layout based or isometrics based
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Equivalent length calculations: Le/D values Globe valve 450 Angle valve 200 Gate valve 13 Ball Valve 18 90Deg Elbow 30 Conventional check valve 135 Globe lift check valve 450 45 degree elbow 16 Flow through run 20 Flow through branch 60
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Pressure drop calculations
Preliminary line size based on velocity criteria Actual pressure drop to be verified with
allowable pressure drops as per process design General criteria: Pump suction line delp is
0.085kg/cm2 per 100m of pipe General criteia: Pump discharge line delp is
0.15 to 0.6 kg/cm2 per 100 mtr pipe Reynolds number: Try to be in turbulant region
or laminar region. Avoid transition region(2300 to 4000 Nre)
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Cooling water lines
DeltaP per 100 mtr is generally 0.3 kg/cm2 Flow possible through various pipes- in m3/h based on
above criteria 25 1.5 40 3.8 50 8.5 80 26 100 51 150 160 200 350 250 640 300 1000
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Gravity lines
The maximum recommended velocity for gravity lines is 1 m/s
The minimum line size for gravity lines within process area to be 40mm
Sizing of self venting lines- velocity at about 0.3 to o.7 m/s
Column draw off lines based on static head available
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Compressible fluid line sizing
If pressure drop is less than 10% of the upstream pressure, use density and average linear velocity based on either inlet or outlet conditions
If pressure drop is between 10%-40% of the upstream pressure, use density and average linear velocity based as averages of inlet and outlet conditions
First estimate- v= c/row; c is 13 to 24
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Tower overhead lines
For pressure between, 0 to 0.7 kg/cm2g, use velocity range as 40 to 60 m/s and max allowable pressure drop of 0.011 kg/cm2 for 100 mtr pipe
For pressure between, 0 to 3.5 kg/cm2g, use velocity range as 18 to 30 m/s and max allowable pressure drop of 0.033 kg/cm2 for 100 mtr pipe
For pressure between, 3.5 to 15 kg/cm2g, use velocity range as 12 to 15 m/s and max allowable pressure drop of 0.15 kg/cm2 for 100 mtr pipe
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Special services
Recommended velocity in m/s for special services:
Concentrated H2So4 1.2Salt water 1.8Caustic solution 1.2Plastic pipe 4.5Liquids with suspended solids 0.9 min.Liquid lines to chillers 1.8Refrigerant lines 1.2Liquid from condenser 2.1Reboiler trap out 1.2
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HAZARDOUS AREA CLASSIFICATIONHAZARDOUS AREA CLASSIFICATION
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OBJECTIVE OF AREA CLASSIFICATION Fire triangle: Oxygen / Fuel / Ignition source Oxygen: Air abundantly available Plant leak: Fuel supply started A minor spark and EMERGENCY Different fluids, different thresholds Area classification makes sure that ignition
source is not available
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ZONES: HOW TO DECIDE Zone 0 normally means vapour space above continuous
vents, storage tanks, open tanks of volatile materials Zone 1 normally have following locations
Frequent maintenance, leakage prone area Areas where frequent upstream upsets mean downstream
released to atmosphere Location adjacent to zone 0 area
Zone 2 normally means areas like Accidental failure of gaskets/packings etc Zone adjacent to zone 1 area
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FUEL PROPERTIES
Classification based on minimum ignition energy Fluids classified as either IIC or IIB or IIA IIC fluids very sensitive. Explode at slightest spark
energy. E.g. acetylene, hydrogen IIB / IIA rather less sensitive No difference between electric equipments for IIB
and IIA Basically fluid group either IIC or IIA/IIB
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TEMPERATURE CLASS Makes sure that
temperature of electrical equipment surface does not exceed fluid autoignition temperature
85185T6
100212T5
135275T4
200392T3
300572T2
450842T1
oCoFTemperature Code
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EXTENT OF ZONES Defined by codes Depends on whether source of hazard is lighter or heavier
than air Depends on location of source i.e. near ground / above
ground, open or closed space etc
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ELECTRIC EQUIPMENT SPECIFICATIONS Method of containment: contain explosion within
confines of electric equipment e.g. Flameproof apparatus (Ex d)
Method of segregation/separation: Separate spark from fuel e.g. Oil immersion (Ex o)
Method of prevention: Provide just sufficient energy and reduce faulty conditions e.g. Increase Safety (Ex e)
Identified on electric equipment by notations
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Pressure Relief
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WHAT IS THE ROLE OF PSV?
Makes sure that vessel pressure does not exceed vessel design pressure
Pressure increase in a system occurs due to imbalance of either energy or flow i.e. input is not equal to output Relief events
Example of energy imbalance = Fire Example of material imbalance = Blocked outlet Imbalance corrected by releasing material out of the
system (to atmosphere or flare)
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STEPS IN PSV SYSTEM SIZING
Identify relief events Calculate relieving capacity required Select type of valve to be used Calculate PSV orifice area Select Standard orifice having area more than calculated
orifice area Fix PSV inlet and outlet line size based on rated capacity Design flare system (if any) considering worst case
scenario for entire plant
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ACCUMULATIONAccumulation allowed by API:
3% for fired and unfired steam boilers 10% for vessels equipped with a single pressure
relief device in non-fire case 16% for vessels equipped with multiple pressure
relief devices 21% for fire contingency
PSV overpressure should be such that at no instant relieving pressure is reached
Consider vessel operating at 1.5 bar (g)
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What are Relief Events? External fire Control valve failure leading to uncontrolled
flow of fluid or energy Blocked outlet Cooling water failure Power failure Exchanger tube rupture
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OTHER RELIEF EVENTS Blocked outlet case: relief flow is flow into the system
(credit can be taken for pump curve shift) Control valve failure case: Vendor certified full open
control valve flow (corrected for relieving pressure) Cooling water / power failure: uncondensed vapour
minus vapor normally exiting the system Tube rupture: Calculated based on orifice theory All calculations at relieving pressure
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RELIEF DEVICE TYPES Pressure Safety Valves
Conventional spring operated valves Balanced bellows valves Pilot operated valves
Rupture Devices Rupture discs Rupture pin devices
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CONCEPT OF BACKPRESSURE Backpressure = pressure from valve outlet which tries to
close the valve i.e. forces disk down
Superimposed backpressure: backpressure on disc before valve opens
Superimposed backpressure can be constant or variable
Built-up backpressure: backpressure developed due to flow after valve opens
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BALANCED BELLOWS VALVE
Balanced bellows work on similar principle of spring tension, but bellows takes care of problems of backpressure
Conventional valves okay if backpressure limited to 10% of set pressure
Balanced bellows can go easily upto 30% backpressure and upto 50% at reduced capacity
Problem of bellow rupture due to fatigue etc
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RUPTURE DISC Nonreclosing device Once ruptured, material
loss continues Used in place of or
upstream of PSV in corrosive, viscous services etc
Reduce capacity of valve if upstream of PSV
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PSV INLET AND OUTLET LINE
PSV inlet line size should atleast be equal to PSV inlet flange
PSV inlet line size pressure drop limited to 3% of set pressure avoids chattering
3% limit not applicable to pilot operated valves PSV outlet line size should atleast be equal to PSV outlet
flange PSV outlet line size selected to meet backpressure
requirements 10% of set pressure for conventional valves 30 50% of set pressure for balanced bellow valves Max. 90% for pilot operated valves possible
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THANK YOU