Understanding Process Diagrams

Dr. AA

PiCS, UTM

Introduction to FlowsheetingIntroduction to Flowsheeting

Now that I are an Inguneer, all I do is right!Now that I are an Inguneer, all I do is right!

Roughly speaking, communication is paramount ~ As an Engineer, transfer the most amount of information with the least amount of effort on the part of the reader.

Flowsheets are the pictorial representation of theprocess.

STM

STM

CWS

CWS

T-1

F-1

E-1

E-2

E-3

E-4

P-1A/B

F-2

Goal:Transmit the most amount ofinformation with the least amount of effort on the part of the reader!

FlowsheetingFlowsheeting

Is a diagrammatic model of the process

• Shows the arrangement of equipment selected to carry our the process

• Includes

stream connections, names of streams, stream flow rates

compositions and operating conditions (temp., Pressure)

instruments

optional details of streams molar compositionsphysical data (, ),code no. of streams,Enthalpies of streamsBrief description of stream

Flowsheet

Block Diagram

Material Flow Sheet

Energy Flow Sheet

Process Flow Diagram

Information Flow Diagram

Piping & Instrumentation Diagram

Type of Diagrams

Block

Process

Piping and Instrumentation

Incr

easi

ng D

eta

il

6 7

FCV218

Type of Flowsheets

Conveys what is achieved in a given step, not how it is achieved

• Particularly useful in initial studies

• Useful for conveying information to management

• Useful to show inter-relation of streams

• Useful in conveying global, not detailed, material balance

Process

This particular block flow could be used to obtain (assist) in the developmentof an initial material balance for streams crossing the battery limits. Youmight use it in your input/output structure development during synthesis.

Feed 1Feed 2Feed 3

Product 1Product 2Product 3

Block Diagram

Example:

Low TempClean-up

Separation

Reaction &Cooling

Compression

RecycleCompression

Major steps are conveyed but no equipment details. Sometimes the blocksmay be process flow diagram symbols. Sometimes steps may includeequipment if it is particularly important to the communication.

RefineryGas

Air

Byproduct toRefinery

Oxygen

Product

Purge

Block Flow Diagram

I find block flows to be particularly useful for setting up (identifying) the key streamsto solve the material balance around the major blocks. I subsequently solve thematerial balance inside each block.

F-101

T-105T-104

T-103T-102

T-101

F-105

101

113

104

103

108

106

109

105

146

140

160

159111

150

151

147

153

134

128

114

Block Flow Diagram

Material Flow Sheet

• Is drawn with standard symbols and labeled and include all data obtained

• If no space available to label equipment, a equipment key can be drawn at the bottom of the flow sheet

• Following data should be shown on the flow sheet in deferent ways– flow rate of each component– total stream flow rate– percentage composition

C1 R2

Equipment key

C1 Column

R2 Reactor

N Component Flow rate T P

N Component Flow rate T P

Simplest method suitable for simple processes with few equipments: tabulate the data in blocks alongside the process stream

Better method applicable for all complex processes: stream line is numbered and the data tabulated at the top or bottom of the sheet

Energy Flowsheet• Energy balance is to determine the energy

requirement of the process• In energy flow sheet Inlet and outlet energy

flow rates should be shown separately for each piece of equipment. Include:– energy of each component in all (inlet and outlet)

streams– process conditions of all (inlet and outlet) streams– process conditions of the specific equipment

Energy Balance Sheet

Information Flow Diagram (IFD)

• Is used in simulation programs • Is presented by blocks• Each block represents a calculation module in the simulation

program, usually a process unit or part of a unit• units in which no change of composition or temperature or

pressure occurs are omitted from IFD• But other operations not shown on the process flow diagram as

actual pieces of equipment, but which can cause changes in the stream composition must be shown.

• Flow of information should be shown with lines and arrow

Process Flow Diagrams

Dr. AA

PiCS, UTM

Conveys the major processing steps represented by the equipment

• Useful for conveying the heat and material balances

• Useful for conveying major pieces of equipment

• Useful for conveying processing conditions

• Useful for conveying utilities

There are no hard and fast rules but Howat Standards include • essentially every major piece of equipment• every flow• every temperature• every pressure• auxiliary services (utility flows)• equipment sizes• process control

Process Flow Diagram

ReactorHeat exchange

Tray columnFluid contacting column

Sealed tank

Standard symbols BS 8888 ; BS 1553

Material Flow Sheet

• Is drawn with standard symbols and labeled and include all data obtained

If no space available to label equipment, a equipment key can be drawn at the bottom of the flow sheet

Following data should be shown on the flow sheet in deferent ways

flow rate of each component

total stream flow rate

percentage composition

Simplest method suitable for simple processes with few equipments,

tabulate the data in blocks alongside the process stream Slide 14

Better method applicable for all complex processes,

stream line is numbered and the data tabulated at the top or bottom of the sheet (above and below the equipment layout) Slide 15

The following diagrams are examples of class and commercial process flow diagrams (PFD’s). The content depends on the goals for the communication.

Unless there are reasons to the contrary, the standard is:

• All major equipment• All major process lines• All major utility lines involving material flow• All stream numbers, temperatures, pressures, flows• All major process controls and valves• All equipment sizes with relevant MEB information as required• All equipment names and numbers

Process Flow Diagram

The goal is to present the most amount of information with the least amount of effort on the part of the reader.

• The flowsheet should generally flow from left to right.• The flowsheet should not be cluttered - use multiple sheets.• The flowsheet should be in landscape with the bound edge at top. • The equipment should be drawn in approximately relative size, e.g. towers larger than drums, exchangers larger than pumps etc.• The major towers and reactors are generally on one, or nearly one, level. • The reader should be able to follow it with his or her eye.• The streams should have the minimum of direction changes.• The streams that enter across the battery limits should be on the left. • The streams that leave across the battery limits should be on the right.• The streams that move to the next sheet should leave on the right.• The streams that recycle to earlier sheets should leave on the left.

Process Flow Diagram

T-104109

105

146

140

153

160

The process flowsheet shown below is onepossible expansion of the block flow for T-104. Whenthis PFD was drawn, the process was not completeas is evident by the missing equipment sizes, pumpdischarge pressures etc. In this case, the processcontrol scheme was not included. It generally is,however.

FlowsheetingFlowsheeting

The following process flow is an approximate rendition of a refinery alkylation unit. This is however not as complete as it is required in industry practice

Note that equipment sizing is not included.

The material balance grid is included. Many companies require the material balance to be imposed on the diagram. In those cases, the stream numbering may not be as extensive as we typically use in design. Or, only a limited number of streams are included in the material balance grid.

Process Flow Diagram

Process Flow Diagram

Another example from design. This includes the equipment sizing and material balance block. Note the symbol key is included. This is frequently necessary when client standards differ.

The following two sheets are examples of a commercial PFD developed prior to construction. Note that the content is, yet again, different. In this case, the control scheme is included as is the legend key. There are some differences. Note that there are three different feed points shown on the diagram. It is typical to have multiple feed points for a column but unusual to show them on the PFD. They were shown here because it was critical for discussing different feedstocks in the process design report. The process description which accompanied the PFD described the reason for the multiple feeds. On the second diagram, you will note the pressure control. This is a split range controller with the primary control being a flooded condenser and the secondary control being a vent. Note that the pumps show operating and design conditions.

Process Flow Diagram

The process description will sometimes dictate the content of the process flow diagram. For most purposes, the process control scheme will be included in our work. Multiple feed points or side stream points will not be shown unless it is critical to the process. We will typically not be showing the future equipment such as the reboiler shown on the left of the column.

When you examine the flowsheet, you should be able to deduce the type of equipment.

Tower type?Repoiler type?Pump type?

This is the other part of the previous flowsheet. Because of scan limitations, I’ve broken the flowsheet into two parts. However, the original is all one flowsheet drawn in landscape.

In this flowsheet, you will not the original reboiler is on flow control reset by temperature. The primary measured variable is steam flow, the manipulated variable is steam flow and the controlled variable is steam flow. The secondary measured variable is temperature. What is the temperature indicative of? Why above the bottom?

This process flow diagram is another commercial example. This client standard had two PFD’s per page, each shown in landscape, one on top of the other on the same page. I have taken the top PFD and split in two. (This is the basis for the following EFD’s.)

This was a revamp. Shaded equipment is new, unshaded equipment is existing. Note that the control scheme is included. There are additional symbols which indicate that the controls are connected to a digital distributed control system. The tags at the edge of the page indicate connections to other PFD’s.

Process Flow Diagram

This is an extractive distillation tower. There are additional reboilers here (type?) that are in place for heat recovery. The shaded equipment is new. The unshaded equipment was existing. The equipment might be in a new service, however. We will generally not show instrument connections to the DCS (distributed control system). The instruments will be shown but the connection will be implied.

This is the overhead system for the extractive distillation column. Note that there is a vent condenser from F-204. Why might we add a vent condenser? What is the purpose? Note that the F-204 Reflux Drum has a ‘boot’. What might that be for? The circle symbol in the center bottom is a professional stamp of a licensed engineer. He/she is signifying that the engineering integrity of the process.

Unless stated otherwise, the target content for PFD’s is:

All major process equipmentAll major process and utility streams – all numberedAll major process controls necessary to operate the processAll operating temperaturesAll operating pressuresAll operating flowsAll equipment sizes

Process Flow Diagram

Piping and Instrumentation Piping and Instrumentation DiagramDiagram

Dr. AA

PiCS, UTM

Piping and Instrumentation Diagram

• Similarly to electrical schemas, the control industry (especially the chemical and process industry) describes its plants and their instrumentation by a

• P&ID (pronounce P.N.I.D.) (Piping and Instrumentation Diagram), sometimes called P&WD (Piping and wiring diagrams)

• The P&ID shows the flows in a plant (in the chemical or process industry) and the corresponding sensors or actors.

• At the same time, the P&ID gives a name ("tag") to each sensor and actor, along with additional parameters.

• This tag identifies a "point" not only on the screens and controllers, but also on the objects in the field.

Piping & Instrumentation Diagram (P & I)

• P & I should be included with – All process equipment identified by equipment number– All pipes identified by a line number. Pipe size and material of

construction should be shown (material may include as a part of the identification number)

– All valves with an identification no. along with their type & size should be shown

– Ancillary fittings that are part of piping system such as inline sight glasses, strainers and stream traps with an identification no.

– Pumps identified by a suitable code no.– All control loops and instruments with identification

Measured Measured VariableVariable

Type of Type of ConditionerConditioner

Type of Type of ComponentComponent

F = FlowF = Flow R = RecorderR = Recorder T = TransmitterT = Transmitter

L = LevelL = Level I = IndicatorI = Indicator M = ModifierM = Modifier

P = PressureP = Pressure C = ControllerC = Controller E = ElementE = Element

Q = QuantityQ = Quantity A = AlarmA = Alarm   

T = TemperatureT = Temperature      

Instrument Identification

P&IDThe P&ID mixes pneumatic / hydraulic elements, electrical elements and instruments on the same diagram

It uses a set of symbols defined in the ISA S5.1 standard.

Examples of pneumatic / hydraulic symbols:

pipe

valve

binary (or solenoid) valve (on/off)

350 kW heater

vessel / reactor

pump, also

heat exchangeranalog valve (continuous)

one-way valve (diode)

Instrumentation identification

V1528

FIC

S

tag name of the corresponding

variable

function (here: valve)

mover(here: solenoid)

The first letter defines the measured or initiating variables such as Analysis (A), Flow (F), Temperature (T), etc. with succeeding letters defining readout, passive, or output functions such as Indicator (I), Record (R), Transmit (T), and so forth

ISA S5.1 General instrument or function symbols

 Primary location

accessible to operator

Field mountedAuxiliary location

accessible to operator

Discrete instruments

Shared display, shared control

Computer function

Programmable logic control

1. Symbol size may vary according to the user's needs and the type of document.2. Abbreviations of the user's choice may be used when necessary to specify location.3. Inaccessible (behind the panel) devices may be depicted using the same symbol but with a dashed horizontal bar.Source: Control Engineering with data from ISA S5.1 standard

Example of P&ID

FT101 is a field-mounted flow transmitter connected via electrical signals (dotted line) to flow indicating controller FIC 101 located in a shared control/display device

Square root extraction of the input signal is part of FIC 101’s functionality.

The output of FIC 101 is an electrical signal to TY 101located in an inaccessible or behind-the-panel-board location.

The output signal from TY 101 is a pneumatic signal (line with double forward slash marks) making TY 101 an I/P (current to pneumatic transducer)

TT 101 and TIC 101 are similar to FT 101 and FIC 101 but are measuring, indicating, and controlling temperature

TIC 101’s output is connected via an internal software or data link (line with bubbles) to the setpoint (SP) of FIC 101 to form a cascade control strategy

The ISA code for instrument type 

First letter 

Measured or initiating variable Modifier

A Analysis 

B Burner, combustion 

C User's choice 

D User's choice DifferentialE Voltage

 

F Flow rate Ration (fraction)G User's choice

 

H Hand 

I Current (electrical) 

J Power ScanK Time, time schedule Time rate of changeL Level

 

M User's choice MomentaryN User's choice

 

O User's choice 

P Pressure, vacuum 

Q Quantity Integrate, totalizerR Radiation

 

S Speed, frequency SafetyT Temperature

 

U Multivariable 

V Vibration, mechanical analysis 

W Weight, force 

X Unclassified X axisY Event, state, or presence Y axisZ Position, dimension Z axis

Common connecting linesConnection to process, or instrument supply

Pneumatic signal

Electric signal

Capillary tubing (filled system)

Hydraulic signal

Electromagnetic or sonic signal (guided)Internal system link (software or data link)Source: Control Engineering with data from ISA S5.1 standard

Many Standards

• DIN• ISA• etc