In process plants, the production of the final product is the result of processing raw materials and passing raw materials through different processing units. Process plants require numerous types of equipment to produce the required products from the feed or raw material. This may involve moving gases, liquids and sometimes solids, and carrying out reaction, distillation or separation, cooling, heating and storage of feed, products, byproducts and supporting systems. Process equipment may include columns/towers, reactors, pumps, vessels/drums, compressors, furnaces, heat exchangers, gas or steam turbines, etc. Supporting equipment may include tanks, cooling towers, water treatment equipment, boilers, chemical storage/handling, fuel systems, air systems, flaring systems, fire protection, wastewater treatment equipment, fire protection equipment, loading/unloading arms, etc. process engineers or chemical engineers design the transition stages of raw materials from different units to the production of the final product in Piping and Instrumentation Diagram (P&ID) drawings. P&ID drawings use specific symbols to indicate the connections of equipment, sensors, valves, etc. in different process units. In general, these symbols can display actuators, sensors, and controllers.
One of the important documents in the design, construction and implementation of process plants projects are P&ID drawings. In fact, in general, the design of factories and process industries and the implementation of industrial projects are divided into the following three phases.
1- Conceptual Design and Feasibility Design
In the conceptual design phase, different dimensions of the project are examined in terms of technical and economic justification. The design and execution of the next stages of the project depends on the approval of both technical and economic conditions. If a project does not meet one or both of these conditions, it is not justifiable and will not be done. Due to the importance of the conceptual design phase, the duration of this phase will vary depending on the type of project.
2- Basic Design
In the basic design phase, with the development of conceptual design, the general process from the import of raw materials to the production of the final product are reviewed and defined. And the main equipment, project facilities, support departments and etc. are designed. Initial documentation and drawings prepared at this phase include Piping and Instrumentation Diagram (P&ID), Process Flow Diagram (PFD), Project Layout drawings (including project overview, routes, main equipment And etc.), preparing the initial list of materials and main equipment of the project and etc.
3- Detail Design
In the detailed design phase, all the various components and parts of the project are designed with all the details. In fact, the detailed design is a bridge between the design phases and the project implementation phase. Therefore, in the detailed design, the details of the construction process of the industrial plant should be considered. So that there is no confusion in the construction, implementation and commissioning phases. At this phase, the design of each of the different sections of the project is done from electrical, mechanical, chemical and etc. and detailed documents and drawings Piping and Instrumentation Diagram (P&ID), Process Flow Diagram (PFD), drawings Isometric, main equipment list (Main & Bulk) and etc. are prepared.
P&ID drawings are used by experts in instrumentation, control, mechanics, hydraulics, pneumatics, process and etc. to design different sections of the project. For example, piping experts to prepare isometric and detailed piping documents, Instrumentation experts to select sensors and actuators and prepare a list of instrumentation equipment and Hock up drawing, Industrial automation experts to select and design the control system and prepare the control philosophy and programming of PLC, HMA, DCS and etc. In fact, P&ID drawings are considered as a common language between engineering units to discuss different sections of the industrial plant. Therefore, the following items should be displayed on P&ID drawings:
The standard notation, varying from letters to figures, is important for engineers to understand because it a common language used for discussing plants in the industrial world. The ISA standard can be mentioned as one of the most important standards in designing P&ID drawings. Other standards such as DIN and ETC are also used to design P&ID drawings.
There are standard symbols used to represent the components in these drawings. It is important to note that these symbols are not to scale and are not dimensionally accurate. They are merely used to represent a certain type of component. These symbols are also labeled with words, letters, and numbers to further identify and specify the components that they are representing. Another important consideration is that the drawings do not always represent the physical locations and proximity of each component. And only shows the general process and various connections between pipes and fittings (Pipe), instruments and other equipment such as tanks, pumps, towers and etc.
One of the most important and widely used standards in the process of designing, manufacturing and implementing control systems and instrumentation, as well as preparing technical and engineering documents, is the ISA (Instrument society of America) standard set. The ISA Association has provided a set of symbols required for engineering drawing and control loop design in ISA S5.1. Below are the important symbols in P&ID drawings.
In P&ID drawings, the circle represents separate instrumentation equipment, such as transmitters, sensors, and detectors.
The square surrounding the symbols represents the instrumentation that is displayed and the control function is applied to it. In fact, for instrumentation equipment that has a transmitter and sends instantaneous parameter values for monitoring in the control room.
The hexagon represents the computer and control functions used in control systems. These symbols have very limited use in P&ID drawings.
The square surrounding a rhombus represents a logic in the control system program (PLC/DCS). In fact, in general, the rhombus represents the interlock functions in the controller program. For example, if the pressure measured by the pressure gauge sensor exceeds the specified range, switches must be operated and valves must be opened or closed to reduce or increase the system pressure to protect the system.
The following symbols show examples of different valves depending on their type of actuator.
Symbols of pipes and fittings are shown with different lines.
Other connection symbols are shown in the figure below.
Identification letters on symbols in the ISA standard include the following:
The following table shows the ISA identification letters used on symbols in P&ID drawings.
For example, the TI on the symbol of a device indicates the temperature display (Temperature Indication). The following figure shows several examples of equipment specifications in P&ID drawings.
Process equipment, valves, instruments and pipe lines are tagged with unique identification codes, set up according to their size, material fluid contents, method of connection (screwed, flanged, etc.) and the status Valves (Normally Closed / Normally Open). These tag numbers are placed on the symbols.
There are different types of symbols depending on the type and specifications of the valves. The following figure shows the most common symbol valves in P&ID drawings:
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