59
Slide 11.2
Task Checklist
Introduction
Basic Concepts
Field Measurement Devices
Example Orifice Calculation
Control Valves
Example Control Valves
Basic Control Schemes
Protective Systems
Slide 11.3
Purposes of Process Control
With few exceptions, new process units are equipped with a digital control system (DCS) that provides advanced control capabilities and interfaces to other systems, including management information and accounting systems. The driving force behind automation is economics – to achieve the desired production at the lowest cost. Typically, control objectives include:
– Lowering labor costs– Reducing or eliminating human error– Reducing the size and space requirements of process equipment– Improving product quality– Reducing energy consumption– Reducing raw material consumption– Elimination of product giveaway– Elimination of off-spec products
Slide 11.4
Basic and Corollary Functions
Basic Functions of Process Control Systems are:Maintaining Stability of the operating conditions at key points in the process.Providing the operator with information on these operating conditions and the means for adjusting them.
Corollary Functions of Process Control Systems are:Automating operations to reduce the demand for continuous operator attention as dictated by economics and management information requirements.Insuring that operations are safe for personnel and equipment and meet all environmental or other regulatory requirements.Maintaining product quality while minimizing operating costs.
Slide 11.5
Process Control Architecture
Business Network
SecureGateway
Process Control Network
ProcessControlSystem
ProcessMonitoring
System
Leve
l 4Le
vel 3
Leve
l 2Le
vel 1
Process stream
Leve
l 0
Pro
cess
Con
trol
envi
ronm
ent
Busi
ness
envi
ronm
ent The Process Control Architecture recognizes the following 5 levels:
Level 4: Business Environment
Level 3: Control Applications
Level 2: Protective Systems
Level 1: Basic Control and Monitoring Systems
Level 0: Process interface (field instrumentation).
Slide 11.6
Basic Control
The process designer develops the basic control scheme with input from the process control specialist and manufacturing organization.A basic control loop includes the following elements:
Process
Sensor
Controller
Transmitter Transducer
Final ControlElement
I P
CF230
Set Point
Slide 11.7
Basic Types of Control
The basic types of control are:
Flow
Level
Pressure
Temperature
Analysis
Slide 11.8
Flow Instruments
The proper selection of a flow meter for a particular application requires considerable evaluation of tradeoffs.Most frequently, a orifice meter is selected for economical reasons;
– Total installed cost– Maintenance cost– Operating cost
Other key factors that drive the selection are;– Accuracy– Rangeability– Fluid state– Flowing conditions– Reynolds number– Density– Mechanical installation constraints
Slide 11.9
Flow Instruments
The accuracy of flow instruments in plant applications is classified into 3 categories:Class I Sales, Custody Transfer, and other applications requiring high accuracyClass II Material Balance, and Equipment Performance applicationsClass III General Purpose
The system accuracy achievable for typical meter types is as follows:
ACCURACY ± % OF MEASURED VOLUMETRIC FLOW RATE
METER TYPE CLASS I CLASS II CLASS III
Positive Displacement 0.2 0.2 0.5
Turbine 0.2 0.2 -
Orifice (1) 1.0(2) 2.0 5.0
Vortex Shredding Not Recommended 0.75 1.5
Coriolis (3) 0.2 0.2 0.5
Notes:(1) Accuracy of orifice meter is listed as ± % of full scale.(2) ExxonMobil Corporate Measuring Guidelines allow orifice meters to be used in Class I applications for steam and gases only.(3) Accuracy of coriolis meter is listed as ± %of mass flow rate.
Slide 11.10
Flow Instruments
Rangeability is a flow meter’s ability to cover a range of flow rates within specified accuracy limits. For differential pressure type flow meters, because of the square root relationship between flow rate and differential pressure, flow rangeability is usually severely limited without taking certain measures. A change in differential pressure from 100% down to 10% of full scale (about the limit for reasonable accuracy with conventional transmitters), the change in flow rate is only from 100% to 30% of full scale.
– In other words: orifice plates have only a 3:1 range– Late breaking news: Orifice plates with smart
transmitters can achieve 10:1 rangeStraight run requirements for flow instruments differ by flow meter type. General “rule of thumb” is 20 pipe diameters upstream, and 5 pipe diameters downstream.
– Late breaking news: “Conditioning Orifice Plate”and “V-Cone” can be installed with just 1 diameter upstream and 1 downstream!
Slide 11.11
Flow Instruments
Orifice Meter - Flow measurement by pressure drop over a flow restriction in a pipe
β ratio (d/D) limited to 0.25-0.70
Slide 11.12
Flow Instruments
Overall permanent pressure loss through various primary elements:
Orifice Permanent ∆P= 0.6*100” H2O=60” H2O=2 psi (0.14 kg/cm2)
Venturi Permanent ∆P= 1-3” H2O (25-76 mm H2O)
Dall Flow Tube has lower permanent ∆P than Venturi, but more prone to plugging
Slide 11.13
Flow Instruments
- Expensive- Must be clean- 8-10 psi ∆P (0.6 kg/cm2)- Proving required- Low viscosity fluids
+ Very accurate+ Good rangeability
(10:1)
- Expensive- Must be clean- Proving required- 3-7 psi ∆P (0.2-0.5 kg/cm2)
+ Very accurate+ Good rangeability (10:1)+ Better for viscous liquids (> 10 cp)
- Poor rangeability(3:1)
- 10,000 Re min.
+ Lowest cost+ 2 psi ∆P (0.14 kg/cm2)
- Liquid must be conductive
- Expensive
+ No ∆P
- Expensive- Proving may be
required
+ No ∆P+ Wide range (100:1)+ Provides density/MW
- Liquid meter cannot handle vapor
- Proving may be required
- 8” max now
+ Highest ∆P+ Mass reading+ Density reading+ Very accurate
Slide 11.14
Problem – Flow meters
What is the most common flow meter?What type of flow meter use for custody transfer?What type of meter use for many chemical’s applications?Where use a magnetic flow meter?Where use an ultrasonic flow meter?What meter options if 10:1 turndown required? How are you going to point this out?
Slide 11.15
Level, Temperature and Pressure Instruments
Types of Level Measuring Devices:Differential Pressure Type (d/p Cell)Displacer TypeCapacitance ProbesBall FloatNuclearGauge Glass and Magnetic Gauges for local indication.Radar
Pressure measurements uses electronic pressure transmitters (highly accurate) and local pressure gauges. Pressure switches are no longer preferred.
Temperature measurement normally through Thermocouples:Type K (Chromel: NiCr-Alumel: NiMnAlSi) (0/2000F) (-20/1090C)Type T (Copper-Constantan: NiCu) (-300/200F) (-195/95C)Other types for specific temperatures, materials or process conditions
Slide 11.16
Temperature Instrument Notes
Temperature control points: Consider including “Check TI”Tankage: Include safety critical priority 1 alarm Towers: Note where thermocouple is located (vapor space or DC)
Slide 11.17
Level Instruments
• Used for alarms/shutdowns
• Used for solids or viscous/fouling
• Can be used for large spans
• Set spans: 14, 32, 48”
Slide 11.18
Level Instruments (cont.)
• Simple device• Used in open systems• Measure pressure of
nitrogen or air bubbled through liquid
MAGNETIC LEVEL GAUGE• Now preferred for high pressure,
toxic, etc.• Can send level signal• CAUTION: If fluid density
drops below specified density, gauge will read zero! (float sinks)
LEVEL INSTRUMENT ARRANGEMENT• Provide independent alarms instruments if
required• Level gauge must cover all level instrument spans
Slide 11.19
Level Instruments (cont.)
Guided Wave Radar (GWR)– Starting to be used in many installations due to advantages:
Independent of specific gravityCan give both total level and interface levelNo moving parts
– However, there are disadvantages that have to be considered:Need to know dielectric constant of fluidDoes not handle foam wellProbe cannot touch vessel wallSolids cause incorrect readingsCalibration problems– May read zero after level exceeds 100%!
Dead zone at top and bottom of standpipe
Slide 11.20
Problem – Level Instruments
Compared to a standard design (horizontal drum with a displacer type level instrument and level gauges), how will design be different?– Cooling tower basin– High pressure, high H2S vessel– 3 phase separator with emulsion– Water K.O. Drum with MAG level gauge– Level span is 20”– Solids in fluid can cause plugging
Slide 11.21
Types of Control
Types of Control include:
– Cascade Control
– Feed Forward Control
– Ratio Control
– Split Range Control
– Override Control
Slide 11.22
Cascade Control
Cascade Control uses the output of the primary controller to manipulate the setpoint of the secondary controller.Allow faster secondary controller to handle disturbances, avoiding upsets.Example: Steam rate is flow controlled (fast), with temperature (slow) resetting the flow control set point.
Cascade Control uses the output of the primary controller to manipulate the setpoint of the secondary controller.Allow faster secondary controller to handle disturbances, avoiding upsets.Example: Steam rate is flow controlled (fast), with temperature (slow) resetting the flow control set point.
Slide 11.23
Control Loop Speed
Fast Response– Reboiler steam pressure
Closing control valve will result in immediate drop in pressure,which will immediately lower temperature
– Fuel gas rateClosing control valve will result in immediate firing decrease
Slow Response– Reboiler level
Closing control valve will slowly raise reboiler level as condensate fills reboiler
– Reboiler/furnace outlet temperatureClosing control valve will change duty which will slowly result in outlet temperature drop
Slide 11.24
Feed Forward Control
Feed Forward Control measures a disturbance and feeds a signal to an earlier part of the control loop so that corrective action can be initiated as the disturbance is taking.Example: Feed rate increases, raising the cooling water set point, avoiding temperature from rising.
Slide 11.25
Ratio Control
Ratio control is used to ensure that two or more flows are kept at the same ratio even if the flows are changing.
Slide 11.26
Split Range Control
Split Range Control is two or more control valves are controlled by one control loop.Example: When temperature is below 200ºF, add steam. When temperature is above 215ºF, add cooling water.
Slide 11.27
Override Control
Override Control normally uses the primary controller to control.A secondary controller takes over if it goes out of a set range.Example: Normally steam rate is temperature controlled, unless steam header pressure drops. Then the steam header pressure will control the steam rate.
Slide 11.28
Control Basics
Come up with basic control system.Provide necessary instruments and control valves.Review with Controls Engineer to be sure your system will work.Avoid common problems:– Two control valves in series– Control valves on cooling water– HX bypass control valve with insufficient pressure drop in HX– Improper range for instruments
Orifice meters: 3:1 turndown, 25 pipe diameter run length, βratio 0.25-0.70, not mixed flowControl valves: 8:1 turndown, but could be less depending on actual valve oversizeSpecifying COT control on a recirculating reboiler
Slide 11.29
What Do We Typically Control?
Streams in and out of unitUtility streams– Fuel gas, steam
Pump and compressor– Pump discharge to control rate– Compressor suction/spillback to contol rate or pressure
Fractionator pressureUnit internal streams
Slide 11.30
Fundamental Types of Control
FEED
FIC
FIC
PIC
FIC
TIC
LIC
LIC
GAS
DISTILLATE
STEAM
FEED
BOTTOMS
FC
FC
FC
FC
FC
FO
Slide 11.31
Typical Furnace Control Scheme
OIL
FIC
FIC
TI
FL(CO)
FL(CO)
TI
PI
TI
TI
TI
TI
TO TOWER/RX
TI
PILOT GAS
FUEL GAS
PIC
FURNACESHUTDOWN
SYSTEM
PHAPLA
AL
AL
TI
Slide 11.32
Centrifugal Compressor Control Scheme
Slide 11.33
Problem - Control
When do fired heaters require pass flow control?Where is control valve?– Centrifugal pump– Reciprocating pump– Centrifugal compressor– Reciprocating compressor
How temperature control cooled product?
Slide 11.34
Fractionator Control Principles
Objectives of fractionator control– Maintain desired separation– Provide means for operator to adjust separation as product specification
demands change
Constant pressure required for satisfactory control
Two types of fractionator control– Material balance– Composition
+ Analysis+ Temperature (inferred)
Slide 11.35
Fractionator Pressure Control
Partial Condensation of Overhead Product
Slide 11.36
Fractionator Pressure Control
Drumless Flooded Condenser System (Total Condenser)
Overdesign exchanger by 10%
Pump low flow recycle required
Slide 11.37
Fractionator Pressure Control
Flooded Condenser Method with Drum (Total Condenser)
Slide 11.38
Fractionator Pressure Control
Hot Gas Bypass Arrangement (Total Condenser)
Slide 11.39
Composition vs. Material Balance Control
Temperature Controlled Split - Control of Heat Input
Use when there is a good correlation between tray (vapor) temperature and composition.
Slide 11.40
Composition vs. Material Balance Control
Material Balance Controlled Split (distillate draw flow controlled)Example: Diesel stripper, chemical purifier
Distillate and steam rate ratio controlled with feed rate.
Slide 11.41
Control Valve Specification
Specify operating conditions– Max, normal, and min rates– Pressure drop at normal rate– Upstream Temp., Press., S.G. or Mol Wt.
Is fluid: Corrosive? Contains solids?– Failure to list solids may result in a valve plugging (see next slide)
Valve body type and flow characteristics– Only if important. Usually allow Instrument Engineer to define.
Action on failure
Flashing service– Specify vapor and liquid rates and conditions– Sonic differential pressure
Slide 11.42
Anti-Cavitation/Anti-Noise Alert
Valves in cavitation or high noise service will normally be fitted with special trim– Trim is a cylinder with many little holes in it– Will plug unless stream is clean or strainers provided (GP 15-9-1)
Always specify solids when presentAlways note when standard anti-cavitation or anti-noise trim is not allowedSome sites (BMRF) do not allow due to reliability problems– Not allowed in compressor spillback in Upstream installations
Slide 11.43
System Head - Capacity Relationship
Slide 11.44
Control Valve Pressure Drop
Allocation of Pressure Drop to Control Valve
- Circuit Differential Pressure: Pressure at source minus Pressure at destination +/- static head effect
- Static Head Constant:∆P = 20% of overall circuit differential pressure @ max flowrate
- Static Head Variable:∆P = 25-30% of overall circuit differential pressure @ max flowrate
For 3-Way valves - Use 50% of exchanger DP for exchanger port
Slide 11.45
Control Valve Sizing
Control Valve Sizing Calculations may be done manually by utilization of the equations provided in the Design Practices or by computer software (Pegasys).
The results of specific control valve sizing calculations should be used to select a control valve that provides the calculated Cv at an opening that represents 80% of the full open capacity of the valve. For butterfly valves the calculated Cv should occur at an opening of 60° from full closed.
Check rangeability of control valve meets the required rangeability for the minimum and maximum flow rate.
Check that valve size is not greater than line size
Slide 11.46
Control Valve Sizing
Method 1 - Liquid to Liquid
where: q = volumetric flow rate (gpm, m3/h)p1 = absolute inlet pressure (psia, kPa)p2 = absolute outlet pressure (psia, kPa)Gf = liquid specific gravity at upstream conditions referenced to
water at 60 °FN1 = 1.0 for Customary units; = 0.0865 for Metric units
Alternative equations available in Design Practice XII-F
)( 211 ppG
NqC f
v −=
Slide 11.47
Control Valve Sizing
Method 2 - Gas/Vapor to Gas/Vapor
where: xT = the pressure drop ratio factor (empirically determined for each valve)
w = mas flow rate (lb/h, kg/h)p1 = absolute inlet pressure (psia, kPa)p2 = absolute outlet pressure (psia, kPa)k = the ratio of specific heats (Cp/Cv)M = Molecular weightT1 = absolute upstream temperature (°R, °K)N8 = 19.3 for Customary units; = 0.948 for Metric units
Alternative equations available in Design Practice XII-F
MxT
YpNwCv
1
18
=Tkx
XY142.2
1−=1
21
pppx −
=
Slide 11.48
Control Valve Characteristics
Equal Percentage (logarithmic) provided unless specified otherwise.Linear characteristic specified when pressure drop is constant.
Slide 11.49
Types of Control Valves
Linear actuated (globe, cage, angle)– Double seated (balanced seat forces, but not tight shutoff)– Single seated (tight shutoff, but may require larger/piston actuator)– Cage (easy trim removal, can be balanced plug, not tight shutoff)– Angle (To avoid impingement on valves surfaces, fluids with solids)
Rotary actuated (butterfly, eccentric plug, ball)– Butterfly (high capacity, but torque limitations, not tight shutoff)– Ball (high capacity or pressure drop applications, tight shutoff)
Three-way valves (mixing or diverting)
Slide 11.50
Types of Control Valves
Double-seated rarely purchased now due to GP control valve leakage requirement
Butterfly and ball valves normally have poor controlling characteristics.
Be sure site approves use of 3-way valves. Difficult/impossible to take out of service.
CAMFLEX/V-BALL
Rotary valve quickly replacing double-seated valve as valve of choice.
Slide 11.51
Control Valve Tightness
Class III (TSO3)– Leaks 0.1% of rated Cv– Requires Owner’s Engineer approval (GP 15-9-1)– Double seated control valve are Class III
Class IV (TSO4)– Leaks 0.01% of rated Cv– Standard for control valves
Class V (TSO5)– Minimum required for “tight shut off” (GP 15-9-1)
Class VI (TSO6)– “Bubble tight”– Have a good reason to specify this
Slide 11.52
High Performance Control Valve Recommendations
Here are some recommendations given to improve control valve performance. Consider these when you are reviewing control valve selections. (EE.53E.2005)
Slide 11.53
Control Valve Fail Safe Position
Choice of Failure PositionWhen selecting the positions to which the valves go on air or power
failure the goal is:to require minimum operator attention to put the unit in the safest possible standby condition;to minimize upsets to associate units;and to ease the problem of returning to service when the failure is corrected.
General rules to follow:Close valves feeding heat and material to the unitClose valves on streams leaving the unitOpen valves on heat absorbing circuits (furnaces, heat exchangers, reflux streams, pumparounds)
Slide 11.54
Control Valve Location
Like to place in “cooler” portion of line– Lower cost, easier to work on, less chance of cavitation
Feed upstream of preheat exchangersProduct downstream of coolers
May want immediately after pump– Minimizes equipment that must have “max-max” design pressure– Allows designing for “fail closed”
If fail closed control valve can block in HX, must protect for fire
Slide 11.55
Control Valve Primer (GP 15-9-1)Standard was double ported globe valve
– Balanced forces best, especially for high pressure drop– Not being specified much due to higher leakage– A “balanced” valve, Camflex, and V500 rotary plug are almost as good – Butterfly and ball valves normally not recommended unless:
Butterfly: low pressure drop, large linesBall: very high turndown, TSO, dirty service
Equal % standard, since makes linear relationship when include pump curve and frictional pressure drop
– Cv increases slowly until valve mostly open, then opens quickly– Linear used for constant pressure drop
Bypass valve same Cv as control valve– Normally use globe, since can throttle. – Add gate if TSO/isolation required.– Pressure drop above 100 psi requires special bypass valve
Size for >15% open at min, valve Cv > 1.25x Cv required at max rate– Do not oversize, can result in valve damage and fire– Take into account piping configuration (expanders/reducers will limit flow)
Check with manufacturer or Instrument Engineer if calculating Cv from % open of a Camflex valve– Linear valve, but readout is designed to be Equal %
Slide 11.56
Protective Systems
A protective system is a component, group of components, or system that reduces risk by preventing, or mitigating the consequences of hazardous incidents.A protective system responds to a process demand and brings the process to a safe state. System may be either manually or automatically initiated.Protective System components shall be separate from the control systemcomponents.Protective Systems are designed to be fail-safe (Fail-Action).The typical protective system consists of:
– Sensors, e.g transmitters, switches (possible voting redundancy)– Programmable Electronic Logic, e.g. Triconex, TMR– Final elements, e.g. valves
During design/detailed design, Protective Systems are defined by:– Cause and Effect Diagram, and– Availability (probability system will work, Spurious Trip Rate, and Testing
Frequency.
Slide 11.57
Shutdown Systems (GP 15-7-2)
Minimum valves required to test valves or allow maintenance?
– Also decide limit switches and double blocks?
Sensing Element– 1oo1, 1oo2, 2oo2, 2oo3 set by unit
reliability (not shutdown inadvertently) and availability (will work when needed)
– Historically used 1oo1, but now both requirements pushing toward 2oo3
– Need pre-alarm and usually a control sensing element
– With 2oo3, can duplicate the signal for pre-alarm and control
Other instrument stuff– System bypass alarm (Priority 1)– Fault and tripped alarms– Manual reset on trip valve– Hand switch shutdowns
Slide 11.58
Alarm Systems
Priority 1 alarms include all SHE-critical alarms and a limited number of other high priority alarms that do not meet the definition of SHE critical alarms but which nonetheless require immediate action or attention by the console operator. Priority 1 alarms shall be operable when the control system operator interface fails.
Priority 2 alarms indicate the existence of an abnormal condition which may lead to a serious process upset. Prompt operator action or attention is required; however, the alarm point should be set to allow for sufficient time to determine the correct course of action. A priority 2 alarm is typically less critical, and need not be operable when the control system interface fails.
Priority 3 alarm indicate the existence of an abnormal condition which requires operator attention, but does not present an immediate threat to continued operation and/or plant safety.
Slide 11.59
Separation Requirements
SHE Critical Alarm sensors shall be independent from the Control Systemsensors. SHE Critical Alarm Display shall be independent from the Control System. Manual trip shall be independent from the Control System, such that the unavailability of the Control System shall not affect operation of Manual trip. Manual trip shall be a hardwired pushbutton, located at a continuously manned location such as an Operator Console. Protective System trip valves shall be independent from the Control System. Provide ability to test and maintain Protective System trip valves. Block valves around trip valves shall either have alarmed limit switches or be car sealed. – Alarmed limit switch required for “automated sequential interlock
system” such as a burner management system
Late breaking news: 2oo3 trip sensors can also be used for control and alarms. See GP 15-7-2.
