Block Flow Diagrams • Plant Design is made up of – Words, numbers and pictures • Engineers think in terms of – Sketches and drawings – Starts with block to show entering and leaving streams • Sketches develop into Flow Diagrams or Flow Sheets – What do they show?? • information on every piece of equipment from a simple valve to larger equipment items such as chemical reactors and distillation columns. • Flow diagrams (three types) – Block, Process and Mechanical (P&I)
Transcript
Block Flow Diagrams
• Plant Design is made up of– Words, numbers and pictures
• Engineers think in terms of – Sketches and drawings– Starts with block to show entering and leaving streams
• Sketches develop into Flow Diagrams or Flow Sheets– What do they show??
• information on every piece of equipment from a simple valve to larger equipment items such as chemical reactors and distillation columns.
• Flow diagrams (three types)– Block, Process and Mechanical (P&I)
Block Flow Diagrams
• Takes its name from the block or rectangle used to represent a unit operation. – Eg. range from a simple mixer to a more complicated distillation
column.
• The blocks are connected by straight lines which represent the process flow streams which flow between the units.
• These process flow streams may be mixtures of liquids, gases and solids flowing in pipes or ducts, or solids being carried on a conveyor belt.
Simple hypothetical process
• Separate streams containing chemicals A and B are mixed before being fed to a reactor. In the reactor C is produced by the reaction:
A + 2 B ==>6 C
• Inside the reactor the reaction does not go to completion and not all the A and B are consumed. The product stream leaving the reactor therefore contains C as well as the unreacted reactants A and B.
• How do we show this in the form of a Block Diagram??
Simple hypothetical process (contd.)
Simple Block Flow Diagram
Two streams leave the separator, one containing A and B, and the other containing C alone.
Observations:•Three unit operations - mixer, reactor and separator•All represented by simple rectangles. The size and shape of the rectangle does not relate in any way to the actual physical size or shape of the unit. •The streams flowing into and out of the units are denoted by simple straight lines. The direction of flow of each of the streams is clearly indicated by arrows. •Each stream has also been numbered in a logical sequence starting with the feed streams. •The diagram has been neatly drawn with the material flowing from left to right.
Block Flow Diagram Rules
• In order to prepare clear, easy to understand and unambiguous block flow diagrams a number of rules should be followed:
– unit operations such as mixers, separators, reactors, distillation columns and heat exchangers are usually denoted by a simple block or rectangle.
– groups of unit operations may be noted by a single block or rectangle. For example a distillation column, its reboiler unit and condenser may all be represented by a single block.
– process flow streams flowing into and out of the blocks are represented by neatly drawn straight lines. These lines should either be horizontal or vertical. Inclined lines are usually not permitted.
Block Flow Diagram Rules
– the direction of flow of each of the process flow streams must be clearly indicated by arrows.
– streams should be numbered sequentially, in a logical order.
– unit operations (i.e., blocks) should be labelled.
– where possible the diagram should be arranged so that the process material flows from left to right
• with upstream units on the left and downstream units on the right.
Typical illustrations • Process stream lines crossing on a diagram
– (a) the streams meet– (b) the streams do not meet.
– where the lines representing two process streams must cross on a diagram it should be indicated that the streams do not physically meet.
– Figure (a) shows a point where several streams meet– Figure (b) illustrates two process flow stream lines which cross on the diagram without
physically meeting.
Simple unit operations • Many different types of unit operations may be found in any industrial
process. – Ranging in complexity from simple flow splitters to very large and complex
distillation column • all these units may be represented by a simple rectangle in a block flow diagram. • as an introduction to the range of unit operations we may encounter, we will now
consider a few different types of units.
• A mixer is a unit which mixes two or more process streams together to produce a single mixed stream. – The flow rates and compositions of the streams entering the mixer may all be
different. In practice a mixer may be a large unit containing a powered mechanical stirrer, or it may be as simple as a junction in a pipe network.
• As mixers are relatively simple units compared to the other types of units we will encounter, often in a block flow diagram they are not represented by a block, but as a simple junction between the lines of the process flow streams as shown in Figure.
Simple unit operations
A mixer may be represented by
(a) a rectangle or block or
(b) a simple junction between the process flow streams.
Simple unit operations
• A splitter is a unit which splits a process flow stream into two or more streams.
– While the flow rates of the streams leaving the splitter may differ considerably, the compositions will be the same as the composition of the stream entering the splitter.
– This is an important point to remember.
Simple unit operations
A splitter may be represented by (a) a rectangle or block or (b) a simple junction between the process flow streams.
A separator is used to separate a process flow stream into two or more streams of different compositions. In this manner, separators are different to splitters. Distillation columns, flash units and filters are just three examples of separators used in industry. All may be represented by a simple block. A separator may have more than one inlet stream.
Simple unit operations
Descriptive names are often given to certain process streams. While the process shown in the block flow diagram of Figure is unlikely to ever exist in practice, it will serve to illustrate the application of some of the more important descriptive names given to processes.
Simple unit operations A feed stream (FS) is any stream which enters a process unit. In Figure we see that stream 4 is the reactor FS while stream 6 is the feed stream to the separator. Stream 1, the process FS is also known as the fresh feed. It is the FS to the entire process. A product stream (PS) is any stream that leaves a process unit. We see that stream 5 is the reactor PS while streams 2 and 3 are PS’s of the first splitter. The term product is sometimes used to describe the stream leaving the process which contains the desired chemical products. The effluent is another name for a PS.
Steam 2 is an example of a bypass stream (BS). This is a stream which bypasses around one or more process units before rejoining the main process stream. In Figure stream 2 bypasses both a mixer and the reactor.
Simple unit operations
• A recycle stream is often used to improve the process performance. – As an example, the reactor efficiency may be improved by returning
unreacted reactants to the reactor. A recycle stream takes material from a point downstream in the main process and returns it to a point further upstream. With a recycle in place it is possible for a portion of the material to flow through the recycle loop several times before leaving.
• A purge stream is a stream used to prevent the accumulation of inerts or contaminants within a system, particularly a system which includes a recycle loop. – It is important to note that while the purpose of the purge stream is to remove
inert and other unwanted contaminants from a system, often valuable reactants and products are also lost from the system in the purge. Effort should always be expended to minimize the amount of valuable material lost in the purge stream.
Simple unit operations
• Some unit operations have special names associated with their feed or product streams.
– For example, a separator such as a distillation may have distillate and residue product streams. The distillate is the product stream which leaves at the top of the column, while the residue stream leaves from the bottom of the column. The distillate is sometimes also known as the overheads while the residue is also known as the bottoms.
Simple unit operations
• An important point to remember is that many process streams may be described in two ways. – Considering the process shown in Figure we see that stream 4 could be
described as being both the product stream from the first mixer and the reactor feed stream. In a similar manner we could describe stream 8 as being one of the separator product streams as well as being the feed stream to the second splitter. Both descriptions are valid.
Typical Flow diagrams
Typical Flow diagrams
Typical Flow diagrams
Flow diagram of a typical amine treating process used in industrial plants
• P & ID stands for Piping & Instrumentation Diagram– they do not show operating conditions or compositions or flow quantities.
• For processing facilities, it is a pictorial representation of – key piping & instrument details; – control & shutdown schemes; – safety & regulatory requirements; key constructability guidelines; &– basic start-up & operational information of all the systems, equipment and
their interconnectivities.
• It is the key engineering document of a processing facility.
• It is the “ DNA or finger print “ of a processing facility.
Who requires P & ID??
• Every one who is involved in any processing aspect of the facility.
• Conceptual design engineer- preliminary development
• FEED engineer – converting the design intent into an engineering document.
• Detailed design engineer – “breathing life” into the P&ID document by incorporating every bit of detail and uses it to develop all other key engineering documents like layouts, isometrics, hook-up drawings (provides a utility that manages the installation details and components as well as dynamic bill of materials for instrumentation), MTOs (multimodal transport operators dealing with carriage of goods).
• Safety engineer- uses it as a guiding tool to ensure design and operational safety of the facility
• Regulatory bodies- To review and satisfy that the design of the facility meets all regulatory requirements.
Who requires P & ID??
• Procurement engineer – uses it to procure key piping and valves.
• Construction engineer- Uses it as base document to develop construction philosophies for the facility.
• Insurance authorities – To evaluate the state of facility and its risk factor to arrive at a premium.
• Commissioning engineer – uses it to conduct integrity tests of facility and commissioning the facility in logical groupings.
• Start-up engineer – uses it to identify safe and appropriate way to initiate operation of the facility.
• Operational engineer – uses it to “line-up” the facility and operate the facility to meet the design intent.
• Maintenance engineer – uses it for isolating a part or complete section of a facility to perform maintenance on the equipment and re-instate to its desired operational state.
Various stages in P & ID development • Conceptual stage
• FEED stage (Front End Engineering Design) - is a broad-encompassing term that includes all engineering design activities for the project.
– By breaking these activities out and executing them earlier in the project, one can take advantage of better scope definition to reduce risk and the cost associated with the project.
• For HAZOP (HAZard and OPerability study)
• For design purposes – ‘AFD’ Stage (Approved For Design - which means that they are completed in all
aspects except for what will be decided in the Detailed Design phase)
• Incorporating vendor details
• For construction purposes – ‘AFC’ Stage (Approved For Construction)
• Final ‘As-built’ stage
• Typically, there could be several revisions in each stage of development.
Information required in the P & ID “Equipment- centric” development ie., P&ID is developed around anindividual or group of related equipment.
P&ID, as a minimum indicates key information on:
• Equipment basic size & basic design information
• Each pipe connected to the equipment
• Each and every instrument connected to the equipment
• Control of the equipment- both manual & automatic
• Safety features of the equipment
• Emergency protection features of the equipment
• Guidance on construction & installation of the equipment
• Information on start-up & shutdown of the equipment
• Information on the maintenance of the equipment
Design & layout of P & ID • Key is to ensure that the P&ID is not clustered with information.
Typically,– One large or complicated equipment per P&ID
– Sometimes another one or two small equipments for easy understanding and to avoid interruption in the interpretation.
– Equipment representation & piping designation differs based on developers practices. Therefore a ‘legend’ for P&ID is provided.
– Equipment representation (& therefore the P&ID series) normally follow the most regular ‘flow-path’ of the processing fluid. Parallel trains are normally bunched to keep the flow path.
– Includes information to guide the user from one P&ID to another to allow chasing of a stream or a pipe.
Design & layout of P & ID • Key is to ensure that the P&ID is not clustered with information.
Typically,– One large or complicated equipment per P&ID
– Sometimes another one or two small equipments for easy understanding and to avoid interruption in the interpretation.
– Equipment representation & piping designation differs based on developers practices. Therefore a ‘legend’ for P&ID is provided.
– Equipment representation (& therefore the P&ID series) normally follow the most regular ‘flow-path’ of the processing fluid. Parallel trains are normally bunched to keep the flow path.
– Includes information to guide the user from one P&ID to another to allow chasing of a stream or a pipe.
Basic symbols• Control valves
• Actuators
• Instrument lines
• Failure mode
Basic symbols• General instrument and control symbols
• Distributed control systems
• Instrument lines
Type of Instrument
• Indicated on the circle representing the instrument-controller by a letter (see Table)• First letter for property• Subsequent letters indicate the function
– For eg. I – Indicating and RC – Recorder controller
Typical P&ID’s
Typical P&ID’s
Closer view ofOne section
Typical P&ID’s
Usefulness of P & ID
• The P&ID is the last stage of process design and serves as a guide – for those responsible for final design and construction.
• Mechanical engineers and civil engineers – design and install pieces of equipment.
• Instrument engineers – specify, install, and check control systems.
• Piping engineers – develop plant layout and elevation drawings.
• Project engineers – develop plant and construction schedules.
Block Flow Process Diagram for the Production of Benzene via the Hydrodealkylation
Chemical Plant Operations• Different operations take place in a chemical plant such
as – control of heat exchangers, chemical reactors, distillation columns
etc.
• How we regulate each of the basic quantities in a process? Control, Regulate, Adjust
• These quantities are:
– Flow
– Inventory – level or pressure
– Temperature, Composition
– Pressure – two phase
A flow measuring device or Flowmeter. This consists of two parts
Firstly an Orifice Meter. This is shown in the diagram by two parallel lines.
This is connected to a sensor or Flow Transducer labelled FT in the figure.
An adjustable valve or Control Valve which alters the flowrate. This is shown by its conventional flowsheet symbol. Finally these are connected by the Controller itself identified by the element FC.
Flow Control System
Inventory Control Systems
Most basic requirement in a plant
is a control system to regulate the amount of material or
inventory in an item of equipment or over part of the process.
Inventory may be measured in a number of ways.
Mass holdup may sometimes be determined directly, but
usually volume is measured.
In liquid systems volume is measured by level.
In gas or vapour systems pressure is used as a measure of
inventory.
Level Control Systems
Here we will consider simple feedback control of the level in a tank. This being the case it is necessary to measure the level directly and adjust the flow into or out of the tank to keep it constant.
Control system consists of A Level Transducer denoted by LT in the diagram.
A Control Valve.
A Level Controller denoted by LC
Pressure Control Systems
In gas or vapour systems we regulate inventory as pressure.
A typical system is shown below.
Both the inlet and outlet are gas or vapour. Therefore if the control valve is shut then the pressure in the tank will rise and vice versa.
Temperature Control Systems
To change temperature - necessary to add or take away energy.
This can be achieved either by:
Transfer energy indirectly, using a second stream, through coils, tubes, jackets etc. The second stream could be, for eg, steam, cooling water, etc.
Mix in a second stream directly. This stream will have a different energy content from the original.
Composition Control Systems
Control of composition - probably the most important objective in the chemical industry (due to specification on products).
To illustrate composition control consider the simplest process in which composition can be changed, namely blending.
Here two streams of different compositions are mixed together e.g. a concentrate and a diluent as shown in the diagram.
Process Flow Diagram (PFD) for the Production of Benzene via the Hydrodealkylation
Process Flow Diagram (PFD) for the Production of Benzene via the Hydrodealkylation
What these designations of unit operations in PFD refer to?For eg. P-101A/B and what each number or letter means.
P-101A/B identifies the equipment as a pumpP-101A/B indicates that the pump is located in area 100 of the plant
P-101A/B indicates that this specific pump is number 01 in unit 100.
P-101A/B indicates that a back-up pump is installed. Two identical pumps P-101A and P-101B. One pump will operate while the other is idle.
PFD for the Production of Benzene via the Hydrodealkylation
Part 1
Part 2
PFD for the Production of Benzene via the Hydrodealkylation
(Rationale behind placement of flags for T, P and MF)
Why T flag here??
In the benzene process, the feed to the reactor is substantially hotter than the rest of the process and is crucial to the operation of the process. In addition, the reaction is exothermic, and the reactor effluent temperature must be carefully monitored. For this reason Stream 6 (entering) and Stream 9 (leaving) have temperature flags.
Benzene reactor
PFD for the Production of Benzene via the Hydrodealkylation
(Rationale behind placement of flags for T, P)
Why P flag here??
The pressures of the streams to and from R-101 in the benzene process are also important. The difference in pressure between the two streams gives the pressure drop across the reactor. This, in turn, gives an indication of any maldistribution of gas through the catalyst beds. For this reason, pressure flags are also included on Streams 6 and 9.
Avoid unnecessary stream numberingFollow Stream 13 leaving the top of the benzene column. This stream passes through the benzene condenser, E-104, into the reflux drum, V-104. The majority of this stream then flows into the reflux pump, P-102, and leaves as Stream 14, while the remaining noncondensables leave the reflux drum in Stream 19. A flag giving the temperature (112°C) was provided on the diagram indicating condensation without sub-cooling. An additional flag, showing the pressure following the pump, is also shown. In this case the entry for Stream 14 could be omitted from the stream table, because it is simply the sum of Streams 12 and 15, and no information would be lost.
Piping and Instrumentation Diagram for Benzene Distillation
Piping and Instrumentation Symbols
Piping and Instrumentation Symbols
Last three figures – one can see gradual development of a process from a simple BFD to PFD and finally to the P&ID. Each step - additional information. Let’s follow the distillation unit1. Block Flow Diagram (BFD): The column was shown as a part of one of the three process blocks.
2. Process Flow Diagram (PFD): The column was shown as the following set of individual equipment: a tower, condenser, reflux drum, reboiler, reflux pumps, and associated process controls.
3. Piping and Instrumentation Diagram (P&ID): The column was shown as a comprehensive diagram that includes additional details
Consider the benzene product line leaving the right-hand side of the P&ID in Figure. The flowrate of this stream is controlled by a control valve that receives a signal from a level measuring element placed on V-104. Cast your eyes on the sequence of instrumentation.
A level sensing element (LE) is located on the reflux drum V-104. A level transmitter (LT) also located on V-104 sends an electrical signal (designated by a dashed line) to a level indicator and controller (LIC). This LIC is located in the control room on the control panel or console (as indicated by the horizontal line under LIC) and can be observed by the operators.
From the LIC, an electrical signal is sent to an instrument (LY) that computes the correct valve position and in turn sends a pneumatic signal (designated by a solid line with cross hatching) to activate the control valve (LCV). In order to warn operators of potential problems, two alarms are placed in the control room. These are a high-level alarm (LAH) and a low-level alarm (LAL), and they receive the same signal from the level transmitter as does the controller.
This control loop is also indicated on the PFD.
However, the details of all the instrumentation are condensed into a single symbol (LIC), which adequately describes the essential process control function being performed.
The control action that takes place is not described explicitly in PFD drawing.
However, it is a simple matter to infer that if there is an increase in the level of liquid in V-104, the control valve will open slightly and the flow of benzene product will increase, tending to lower the level in V-104.
For a decrease in the level of liquid, the valve will close slightly.Lec4-mat-of-construc.ppt
