This post illustrates the steps from the initial block and flow diagrams to achieve the final Piping & Instrumentation Diagram (P&ID) diagrams required for the interpretation and governance of process plants, as well as to highlight and report on the video pages of the Distributed Control System (DCS) measurement and control loops suitable for the management of the plants.
Normally the process scheme is a unified graphic representation of a system to be implemented, developed during the design phases and is divided into:
- Block Flow Diagram (BFD)
- Process Flow diagrams (PFD)
- Process & Instrumentation Diagram (P&ID or P&I)
- Flow & Instrumentation Diagram (F&ID or FID)
Block Flow Diagrams (BFD)
The Block Flow Diagram has the purpose of illustrating the logical flow of the processing phases of the product, highlighting the processing sequence, their concatenations and the types of treatment plant: each block represents a treatment unit of the process plant.
Process Flow Diagram (PFD)
The Process Flow Diagram examines the single unit or the plant itself, and it represents the main equipment, the connections between them (pipes), highlights the main tools (equipment, instruments, and valves), and the essential settings for a correct operation and a correct management of the plant itself, as the following nominal operate conditions:
- Chemical composition
Process & Instrumentation Diagram (P&ID or P&I)
The Process & Instrumentation Diagram is the logical continuation of the flow scheme because the latter is supplied with all the information necessary for the construction and operability of the plant. Each specialized engineering unit completes and/or develops, using an internationally recognized symbology (for Example, ISA S series 5), the parts of its competence by inserting in the scheme:
- Data and characteristics of the equipment
- Numbering and classification of pipes
- Instrumentation in the control room
- Instrumentation and valves in the field
- Construction and installation notes
For a simple e completed P&ID see Figure 1 that show a simple Wessel level control consisting of:
- an electrical control loop, with level transmitter LT 1, level controller LIC 1, I/P converter LY 1 and level control valve LCV1;
- an electrical safety loop, with a level switch of high-level LSHH 2 with level alarm LAHH 2 and safety valve LSV 2 auctioned by the solenoid S;
and with the necessary different Supply, i.e. Air (AS), Electrical (ES) and Water (WS).
Example of a measured and controlled Wessel with discrete electronic instrumentation:
- Identify the control loop
- Identify the security loop
Flow & Instrumentation Diagram (F&ID or FID)
From the P&ID, additional utility schemes are obtained that are necessary for the construction and operation of the plant, such as:
- Simplified diagrams for video pages
- Simplified diagrams for the operating manual
- Roadmaps for hydraulic or pneumatic piping test circuits
Normally from the Piping & Instrumentation Diagram (P&ID), it is possible to obtain the Flow & Instrumentation Diagram (F&ID), also called Scheme of March which briefly highlights:
- Process lines
- Main instrumentation
- Control and security loops
suitable for the construction of video pages and the management of the Distributed Control System (DCS).
For a simplified F&ID see Figure 2 that shows a typical 3 Phases Separator of crude oil coming from the extraction wells, in the three phases:
- Gas, top right, controlled by the PC controller
- Oil, lower on the right, controlled by a first LC level controller
- Water, down on the left, controlled by a second LC level controller
Complete the control of the separator, the safety devices to prevent high pressure:
- The Pressure Safety Valve (PSV)
- The Rupture Disk, normally named Pressure Safety Element (PSE)
The scheme is indeed simplified as a demonstration to present the main elements of:
- a) The measure, i.e. monitoring of the main process variables
- b) Regulation, or control of the main process variables
- c) Actuation, or modification of the main process variables
- d) Safety, control, and blocking of the process for exceeding the process pressure max
Figure 2 Simplified example of a 3 Phases Separator: Gas, Oil, and Water
P&ID vs PFD
A process flow diagram (PFD) shows, generally, only the major equipment used in a chemical process and the logic flow of the chemicals treated within the plant. PFDs are developed in the initial stages of plant engineering, or for training, maintenance and operation purposes, to outline the basic characteristics of the processes that have to be managed by the plant. A PFD generally does not show the relieving system, pumps minimum flows, compressor anti-surge flows, and the isolation system.
A P&ID drawing, instead, is a more detailed representation of the processes, including any minor or minor flow, any piping and instrumentation equipment, and the control loops that govern the system. P&IDs diagrams are developed by an intense cooperation of instrumentation, electrical, mechanical, safety engineers.
The process to achieve a final P&ID is described in this article.
In a P&ID, all components are numbered and defined in terms of service and specifications, noozles, internals, and manholes are indicated, and the piping connection types are clearly defined.
More in detail, P&ID shows (with regards to piping components):
- any startup, production, shut-off, emergency, and maintenance line
- any piping component with explanatory comments
- process line information, such as line number, size, fluid symbol, applicable piping class, and insulation requirements
- any configuration requirement, like number of pockets and free draining
Regarding instrumentation systems, P&ID drawings show:
- any instrument to control, monitor, shut down, interlock or automate the plant
- all field instruments
- the control and shutdown valves including their type, failure status, and sizing information
- the required pressure relief valves with indications about the set over-pressure and sizing
- the types of signals, such as pneumatic, electric, hydraulic, soft links, digital etc.
Lastly, P&ID diagrams should show:
- the battery limit between the EPC contractor and the End User
- the are battery limit
- process packaged equipment
- line break information, as line number, piping class, and insulation