To successfully work with (and design) control systems, it is essential to understand the documents that are typically used to illustrate process control and associated field instrumentation.
The documentation of process control and associated field instrumentation is normally created by the engineering firm that designs and constructs the plant. The company that commissioned the plant may have an internal documentation standard the engineering firm will be required to follow.
The documentation procedure for designs and constructs a new process control industry are done through Plot plan, Process flow diagram, Piping & Instrumentation Diagram and Loop Drawing, the details are below.
It is often helpful to look at the plot plan to get an overview of how a plant is physically organized. By examining the plot plan, it is possible to get an idea of where a piece of equipment is located in the plant.
Process Flow Diagram
A process engineer will select the type of chemical or mechanical processing that best meets the planned production, quality, and efficiency targets. The process engineer then documents the design in a process flow diagram (PFD). The process flow diagram typically identifies the major pieces of equipment, the flow paths through the process, and the design operating conditions—that is, the flow rates, pressures, and temperatures at normal operating conditions and the target production rate.
Since the purpose of the process flow diagram is to document the basic process design and assumptions, such as the operating pressure and temperature of a reactor at normal production rates, it does not include many details concerning piping and field instrumentation. In some cases, however, the process engineer may include in the PFD an overview of key measurements and control loops that are needed to achieve and maintain the design operating conditions.
A Process Flow Diagram (PFD) is a diagram commonly used in chemical and process engineering to indicate the general flow of plant processes and equipment. The PFD displays the relationship between major equipment of a plant facility and does not show minor details such as piping details and designations. Another commonly used term for a PFD is a flowsheet.
Typical content of a process flow diagram
Typically, process flow diagrams of a single unit process will include the following:
• Process piping
• Major equipment items
• Control valves and other major valves
• Connections with other systems
• Major bypass and recirculation streams
• Operational data (temperature, pressure, mass flow rate, density, etc.), often by stream references to a mass balance.
• Process stream names
Process flow diagrams generally do not include:
• Pipe classes or piping line numbers
• Process control instrumentation (sensors and final elements)
• Minor bypass lines
• Isolation and shutoff valves
• Maintenance vents and drains
• Relief and safety valves
Process flow diagrams of multiple process units within a large industrial plant will usually contain less detail and may be called block flow diagrams or schematic flow diagrams.
Piping and Instrumentation Diagram
The instrumentation department of an engineering firm is responsible for the selection of field devices that best matches the process design requirements. This includes the selection of the transmitters that fit the operating conditions, the type and sizing of valves, and other implementation details. An instrumentation engineer selects field devices that are designed to work under the normal operating conditions specified in the process flow diagram. Tag numbers are assigned to the field devices so they may be easily identified when ordering and shipping, as well as installing in the plant.
The decisions that are made concerning field instrumentation, the assignment of device tags, and piping details are documented using a piping and instrumentation diagram (P&ID). A piping and instrumentation diagram is similar to a process flow diagram in that it includes an illustration of the major equipment.
A piping and instrumentation diagram/drawing (P&ID) is a diagram in the process industry which shows the piping of the process flow together with the installed equipment and instrumentation.
Contents and Function
A piping and instrumentation diagram/drawing (P&ID) is defined by the Institute of Instrumentation and Control as follows:
1. A diagram which shows the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on International Society of Automation (ISA) Standard S5. 1.
2. The primary schematic drawing used for laying out a process control installation.
P&IDs play a significant role in the maintenance and modification of the process that it describes. It is critical to demonstrate the physical sequence of equipment and systems, as well as how these systems connect. During the design stage, the diagram also provides the basis for the development of system control schemes, allowing for further safety and operational investigations, such as a Hazard and operability study commonly pronounced as HAZOP.
For processing facilities, it is a pictorial representation of
• Key piping and instrument details
• Control and shutdown schemes
• Safety and regulatory requirements
• Basic start up and operational information
List of P&ID items
• Instrumentation and designations
• Mechanical equipment with names and numbers
• All valves and their identifications
• Process piping, sizes and identification
• Miscellanea – vents, drains, special fittings, sampling lines, reducers, increasers and swaggers
• Permanent start-up and flush lines
• Flow directions
• Interconnections references
• Control inputs and outputs, interlocks
• Interfaces for class changes
• Computer control system input
• Identification of components and subsystems delivered by others
The piping and instrumentation diagram identifies, but does not describe in detail, the field instrumentation that is used by the process control system. Many of the installation details associated with field instrumentation, such as the field devices, measurement elements, wiring, junction block termination, and other installation details are documented using a loop diagram. ISA has defined the ISA-5.4 standard for Instrument Loop Diagrams.
A loop diagram, also commonly known as a loop sheet, is created for each field device that has been given a unique tag number. The loop diagrams for a process area are normally bound into a book and are used to install and support checkout of newly installed field devices. After plant commissioning, the loop Diagrams provide the wiring details that a maintenance person needs to find and troubleshoot wiring to the control system.
The loop diagram typically contains a significant amount of detail. For example, if a junction box is used as an intermediate wiring point, the loop diagram will contain information on the wiring junctions from the field device to the control system.
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