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.
Basic Information:
The basic information provided by a PFD can be categorized into one of the following:
Process Topology:
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 essentially provides 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.
Stream Information:
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.
Equipment Information:
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.
