7.0 Process Drawings

Engineers document their work using many complementary methods. Certainly, written documentation is important. However, drawings play an equally important place in documentation because of the complexity of the systems being documented and the precise and easily read nature of process drawings. This section provides a brief explanation of the major categories of drawings and WWW links for further information.

The major categories of drawings are introduced in the following.

· Block Flow	This drawing shows only the major units and process flows. The general goal is to provide an overview of the process to build understanding. An entire plant can be shown in a single drawing. Typically, each unit is shown as a rectangle (block), and the blocks are connected by solid lines with the flow direction indicated with arrows. No special symbols or guidelines are used in developing these drawings.
· Process Flow Diagram (PFD)	This drawing shows all major equipment and process flows in a single drawing. Symbols and identifications (e.g., T-201) are used to represent unit operations, and each stream is designated with a number. The stream properties are given in associated tables, but utility flow rates (e.g., air, steam, fuel, etc.) are typically not given. In some drawings, a few basic control loops are provided to indicate how process flows and other key variables are maintained at desired values. The drawing is not to scale.
· Piping and Instrumentation Drawing (P&ID)	These drawings give details on all piping, including valves (automated and manual), by-pass lines, pipe sizes, sample points, etc. In addition, all instrumentation is shown. A moderate level of mechanical detail is provided for process equipment, so that the piping and instrumentation can be precisely documented. International symbol standards are used for piping, equipment and instruments (ISA, 1992). Many pages of drawings are required for a single unit. The drawings are not to scale.
· Equipment Mechanical Drawings	Each equipment (e.g., drum, fired heater, etc.) is constructed to the specifications in the Equipment Mechanical Drawings and associated text explanations.
· Three-dimensional Layout	Equipment must be located so that they do not interfere with each other and so that space is available for people to travel through the process and perform tasks, such as maintenance. To ensure the layout is adequate, a three-dimensional representation is required. In the past, physical models were constructed to scale; however, a 3-D graphical representation is the standard technique used today.

While these general categories are used widely, each organization applies its own modifications. Therefore, every set of drawings should be accompanied by a key that defines the use of symbols and other drawing standards.

Many other drawings are used to document special issues. Examples are given in the following.

· Details of piping, sampling, etc.

· Loop drawings of connections between a sensor and valve in each control loop

· Logic diagrams for safety and other discrete control systems

· Plot plans of the entire site

In addition, documentation is required for cost estimation for the purchase, construction and start up of the process. This information is usually included in tables; some typical contents are pump/motor power, vessel size, piping diameter and lengths, pressures, temperatures, and materials of construction. Also, written documentation is required to purchase every one of the thousands and thousands of items. For example, a specification sheet is required for every sensor, transmitter, and valve.

Links to WWW Resources

Process Drawings: This sample chapter from Turton, et. al. (1998) discusses the range of drawings used in process design and presents a few examples.

Process Drawings: This five-part WEb lesson gives guidance on reading and preparing P&I Drawings..