What is Instrumentation documentation?


Instrumentation documentation consists of drawings, diagrams and schedules. The documentation is used by various people for different purposes. Of all the disciplines in a project, instrumentation is the most interlinked and therefore the most difficult to control.

The best way to understand the purpose and function of each document is to look
at the complete project flow from design through to commissioning.
• Design
• Design criteria, standards, specifications, vendor lists
• Construction
• Quantity surveying, disputes, installation contractor, price per meter, per installation
• Operations
• Maintenance commissioning

Instrument list

This is a list of all the instruments on the plant, in the ‘List’ format. All the instruments of the same type (tag) are listed together; for example, all the pressure transmitters ‘PT’ are grouped together.

  1. Instrument index lists– Associated documentation such as loop drawing number, datasheets, installation details and P&ID.
  2. Loop List -The same information as the instrument list but ordered by loop number instead of tag number. This sort of order will group all elements of the same loop number together.
  3. Function- Gives a list of all the instrumentation on the plant and may include ‘virtual’ instruments such as controllers in a DCS or PLC.
  4. Tag No- The instrument tag number as defined by the specification.
  5. Description -Description of the instrument as denoted by the tag number.
  6. Service Description -A description of the process related parameter.
  7. Functional Description –The role of the device.
  8. Manufacturer- Details of the manufacturer of the device.
  9. Model- Details of the model type and number.

Instrument location plans

The instrument location drawing is used to indicate an approximate location of the instruments and junction boxes. This drawing is then used to determine the cable lengths from the instrument to the junction box or control room. This drawing is also used to give the installation contractor an idea as to where the instrument should be installed.

Cable racking layout

Use of the racking layout drawing has grown with the use of 3D CAD packages; this drawing shows the physical layout and sizes of the rack as it moves through the plant

Cable routing layout

Prior to the advent of 3D CAD packages, the routing layout used a single line to indicate the rack direction as well as routing and sizes and was known as a ‘Racking & Routing layout’.

Block diagrams – signal, cable and power block diagrams

Cable block diagrams can be divided into two categories: Power and Signal block diagrams. The block diagram is used to give an overall graphical representation of the cabling philosophy for the plant.

Field connections / Wiring diagrams

Function :To instruct the wireman on how to wire the field cables at the junction box.

Used by: The installation contractor. When the cable is installed on the cable rack, it is left lying loose at both the instrument and junction box ends. The installation contractor stands at the junction box and strips each cable and wires it into the box according to the drawing.

Power distribution diagram

Function: There are various methods of supplying power to field instruments; the various formats of the power distribution diagrams show these different wiring systems.

Used by: Various people depending on the wiring philosophy, such as the panel wireman, field wiring contractor.

Earthing diagram

Function: Used to indicate how the earthing should be done. Although this is often undertaken by the electrical discipline, there are occasions when the instrument designer may or must generate his own scheme – Eg. for earthing of zener barriers in a hazardous area environment.

Used by: Earthing contractor for the installation of the earthing. This drawing should also be kept for future modifications and reference.

Loop diagrams

Function :A diagram that comprehensively details the wiring of the loop, showing every connection from field to instrument or I/O point of a DCS/PLC.
Used by :Maintenance staff during the operation of the plant and by commissioning staff at start up.

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