BLOCK FLOW DIAGRAMS (BFD):
The BFD can take one of two forms. A block flow diagram may be drawn for a single process. Alternatively a block flow diagram may be drawn for a complete chemical complex involving many different chemical processes. In each of the cases the high level process is represented by a block with no physical equipment.
We differentiate between these two types of diagrams by calling the first a block flow process diagram and the second a block flow plant diagram.
PROCESS FLOW DIAGRAMS (PFD):
The PFD represents a quantum step up from the BFD in terms of the amount of information that it contains.
There are no universally accepted standards and PFD’s from one company will probably contain slightly different information than the PFD’s for the same process from another company. Having made this point, it is fair to say that most PFDs convey very similar information.
A typical PFD contains the following information:
- All the major pieces of equipment in the process will be represented on the diagram along with a description of the equipment.
- Each piece of equipment will have a unique equipment number and a descriptive name.
- All process flow streams will be shown and identified by a number.
- A description of the process conditions and chemical composition of each stream will be included.
- The data will be displayed either directly on the PFD or included in an accompanying flow summary table.
- All utility streams supplied to major equipment that provides a process function will be shown.
- Basic control loops, illustrating the control strategy used to operate the process during normal operations will be shown.
The basic information provided by a PFD can be categorized into one of the following:
Equipment is represented by “symbols” that identify specific components.
Each major piece of process equipment is identified by a number on the diagram. A list of the equipment numbers along with a brief descriptive name for the equipment is printed along the top or bottom of the diagram, but could also be represented in a separate document. When located on the drawings the equipment numbers and names roughly correspond to the horizontal location of the corresponding piece of equipment.
During the life of the plant, many modifications will be made to the process, often it will be necessary to replace or eliminate process equipment. When a piece of equipment wears out and is replaced by a new unit that provides essentially the same process function as the old unit, then it is not uncommon for the new piece of equipment to inherit the old equipment’s name and number.
On the other hand, if a significant process modification takes place, then it is required to use new equipment numbers and names. Once a piece of equipment is retired the number will not be reassigned to new equipment.
Each process stream is identified by a unique number and the flow direction is indicated by one or more arrows.
The process stream numbers are used to identify streams on the PFD and the type of information that is typically given for each stream can be seen in the next section.
For small diagrams containing only a few operations, the characteristics of the streams such as temperatures, pressures, compositions and flow rates can be shown directly on the figure, adjacent to the stream.
It is not practical for a more complex diagram. In this case, only the stream number is provided on the diagram. This indexes the stream to information on a flow summary or stream table, which is often provided below the process flow diagram.
Stream information that is normally provided in a flow summary table can be seen below
- Stream number
- Pressure (kPa)
- Temperature (°C)
- Total mass flow rate (kg/h)
- Fluid Density (kg/m3)
- Flow rate – Design (m3/h)
- Flow rate – Maximum (m3/h)
- Flow rate – Minimum (m3/h)
With information from the PFD and the flow summary table, material balances and other problems are easily analysed.
The final element of the PFD is the equipment summary. This summary typically provides sufficient information necessary to estimate the costs of equipment and furnish the basis for the detailed design of equipment.
PIPING AND INSTRUMENT DIAGRAM (P&ID)
The P&ID or mechanical flow diagram (MFD) provides information needed by engineers to form the basis of the design of the plant.
A P&ID should contain the following information:
- Equipment – indicated by type, reference tag numbers and basic design data.
- Lines – reference tag numbers, piping material class, line size, fluid service, insulation type and thickness.
- General piping requirements – such as slope, special insulation such as heat tracing, minimum / maximum piping distance requirements etc.
- Manually operated piping valves – valve type (ball valve, gate valve, check valve etc.), valve size, locked closed/open, normally closed/open etc.
- Fittings – reducers/expanders, spectacle blinds, spacers, strainers.
- Drains and Vents – are usually indicated using typical symbols along with their size and type.
- Automated valves – shutdown valves (SDV), control valves, blow down valves (BDV) are indicated with sizes. Also fail position (fail open/fail close/fail in position) is indicated for each of these valves. Actuator connection and type is indicated. Reference instrument tag numbers are attached to every automatic valve.
- Safety Valves – pressure relief valve (PRV) or Temperature relief valve (TRV) are indicated with their instrument tag numbers, set points, types are indicated by different symbols.
- Instruments – gauges, transmitters, local indicators, DCS indicators, interlocks and other functions are indicated in detail on the P&ID. Interconnection between these elements has to be indicated by different types of instrument signals (hardwired signal, soft signal, pneumatic or hydraulic signal etc.). The location of the instrument elements (field mounted or DCS) is indicated by difference between symbols.
- Notes – are written wherever required to improve clarity for anyone referring to the P&ID. ‘Hold’ is used to indicate uncertainty about relevant data.
- Interconnections (OPC) – are shown between lines (piping OPC) and instrument signals (Instrument OPC) present on two different P&ID sheets.
The P&ID includes every mechanical and control aspect of the plant, but typically exclude the following:
- Operating condition data (Pressure, Temperature etc.)
- Stream flow
- Equipment location
- Pipe routing
- Pipe lengths and pipe fittings
- Supports, structures and civil details.
Each PFD could be represented by many P&ID’s and P&ID’s generally focus on 2-3 major pieces of equipment per drawing.
All process information that can be measured (instrumentation) in the plant is shown on the P&ID by elongated circular flags. This includes the information to be recorded and used in process control loops. The flags on the diagram indicate where the information is obtained in the process and identifies the measurements taken and how the information is dealt with.
The abbreviations used to identify the type of measurement are in accordance with the ISA 5.1 standard.
The P&ID is the last stage of process design and serves as a basis of the detailed plant and piping design .
Based on this diagram:
- Mechanical engineers and civil engineers will carry out detail design.
- Instrument engineers will specify and check control systems.
- Piping engineers will develop plant layout and elevation drawings.
- Project engineers will develop plant and construction schedules.