Roymech engineering encyclopedia

Process Diagrams and Documents

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Process Diagrams and Drawings


There are a number of diagrams & drawings associated with process engineering most of which are involved to some level with mechanical engineering.   BS 5070 Pt 1 :1988 provides useful guidance on the different types of drawings.

A list of the diagrams covered by these notes is provided below

  1. Block Diagram
  2. Mass Flow Diagram (not Mentioned in BS 5070)
  3. Process Flow Diagrams
  4. Process Data Sheets
  5. System and Piping Diagrams
  6. Piping and Instrument Diagrams
  7. Mechanical Flow Diagrams (not Mentioned in BS 5070)
  8. Equipment Location Diagrams
  9. Pipe Arrangement Drawings (not Mentioned in BS 5070)
  10. Piping Isometrics (not Mentioned in BS 5070)
  11. Installation Drawings

Block Diagram

This is a simple diagram to show the essentials of an installation in sufficient block outline to indicate the main design features.   This will use lines and simple geometric shapes

Mass Flow Diagram

This is a simple block diagram which identifies the mass flow of "all" matter through the process.   This document is essential in any process design and must be updated as the design progresses

Process Flow Diagram

The Flow diagram shows all the essential parts of the process and items of equipment in sufficient detail to enable the analysis and calculation of the physical characteristics of the system to be undertaken

A Process Flow Diagram is a diagram of a fluid flow system showing the equipment items connected by the major process pipes and containing data on the essential process control circuits or major process requirements. The drawing is not to scale and the the equipment items are represented by symbols.  The main equipment items and flow streams should be identified and included in tables which identify process requirements in sufficient detail to enable production of the Piping and Instrument Diagrams (Engineering Line Drawings).

Process Data Sheets

The process data sheets (PSD) are generally A4 documents with a number of sheets.   A PSD relates to a single item of equipment and contains the essential process data for initiating the detail design of an item.   It includes the overall size, number, approximate geometry and identification of the connections, material of construction and the full range of operating conditions.  The Process Data sheet generally includes a simple diagram.

The process design also generally includes for Instrument Process Data Sheets which provides a similar level of detail.

Circuit Diagrams

The circuit diagrams are used for pneumatic and hydraulic systems to show the various control valves, fluid pipeline equipment, and pneumetic /hydraulic line interfaces.   The valve systems are specific to the fluid power industry.

The system and piping diagrams

This show detailed connections between the components and items of equipment.   The main purpose of these diagrams are to identify pipe routings and initial pipe lengths for producing estimates.

Piping and Instrument Diagrams

The P & I diagram (Engineering Line Diagram) allows the design to progress from the "Process Flow Sheet / System and Piping diagram" to the final system design and installation stage. The P&ID are definitive and comprehensive diagrams showing all of the equipment, piping, valves and instrumentation. All items to be identified using a standard numbering systems. This normally entails having a unique plant item number for each item of equipment, valve, instrument and line. Ideally the line number should include a size, material and fluid contents identifier to enable the anyone reading the drawing to obtain this information without having to refer to other documents.

An Engineering line should include.

  • Mechanical equipment with names and numbers
  • Instruments with identification and necessary interfaces with control loops
  • Interconnecting piping, sizes and identification
  • Valves with identifications
  • Vents, drains, special fittings, sampling lines, reducers and increasers
  • Flow directions
  • Interface tags to other ELD's
  • Control interfaces, inputs and outputs.
  • Main plant interlocks.
  • Identification of symbols used
  • Reference list including identification of relevant PFD's

The symbols used to be in accordance with identified standards and should clearly indicate the type of component, the method of connection (screwed, flanged etc) and and the status (Valves - Normally Closed, Normally Open). The method of operating equipment items should be clearly identified (electric motor, pneumatic actuator). It is not generally necessary to identify services and electrical supplies to the operators.

Mechanical Flow Diagram

This type of diagram is required to clearly show a system involving moving mechanical components which need to be interfaced and interlocked. All sequenced movements have to be identified including the method of initiating and terminating the movements. A mechanical flow diagram is generally a block diagram.

Equipment Location Diagram

The design process includes for provision of equipment location diagrams which identify is some detail the locations of all the equipment items used in the process.   These diagrams also provide local floor loadings, static and dynamic and proposed fastening arrangements. This information is required to enable the structural design of the building to be progressed.

Piping Arrangement Drawings

These drawings are generally completed using 3 D Cad systems. They are either Isometric drawings or orthographic or a combination of the two.   These drawings are completed to identify, in detail the pipe runs and to show the interfaces with the equipment and the various other services and to identify the access allowances.

Piping Isometrics

Each pipe line is detailed by at least one isometric. This need not be to scale but should show all material, size, weld, and fitting information.   The isometric includes all pipe lengths and all necessary datums identifying the pipe location in the region where it is to be installed.   The isometric is the pipe manufacturing drawing and includes a part list identifying all component parts of the pipe being detailed.

Installation Drawings

These drawing provide instructions of the installation requirements and often are based on the equipment location drawings and the piping arrangement drawings.   The drawings often provide information on the sequence of equipment and piping installation by provision of a number of drawings of the same area at different stages of the installation.