Line Designation

Written by Jerry Ratzlaff. Posted in Designer

Flow lines should be labeled to show pipe size, line service identification, line number, line material, piping class and the line pressure rating.  

Numbering Process

Most companies have their own method of numbering lines.  Typically, a line number will be assigned to a primary flow line and then incremented as it branches.  If there is no in house process to number the lines, start at the upper left line (on the P&ID) and use that as the lowest number.  Assign numbers up to each piece of equipment.  Do this for all lines.  Next start with the outlet lines. Once all the primary lines are numbered, secondary lines should be numbered with the same methodology.

Line Numbering Example

A typical line number or line designation would be as follows:

3" PV-500-040-A-2"HC-ST


3 = Nominial diameter of the line

PV = Service Classification

500 = Line Number

040 = Pipe Schedule

A = ANSI Pressure Rating

2"HC = Coating or Insulation thickness and function.  This example shows 2" insulation for the purpose of heat conservation.  Other examples might be CC for cold conservation, FP for fire protection or PS for personnel safety.  

Some coatings might be C for cement lined pipe, F for fiberglass, G for galvanized, P for plastic.

This is an optional item and is typically left off if there are no entries on it.

ST = Type of heat tracing to be used. This example shows steam tracing as the type of heat tracing.  Other examples might be ET for electrical tracing.

Helpful Hints

  • Each company follows its own set of rules when assigning line numbers and line designation.  This section has general information related to numbering lines.  
    The line number should be continuous from one piece of equipment to the next.  This would not be applicable if the piping class does not change.  One line number may indicate more than one line size but not more than one class.  Whenever the piping class changes, a new line number will be assigned.
  • If the specification of a line changes downstream of a control valve or control station, a new line number with a lower piping class should be assigned downstream of the valve.  The higher piping class should be carried through the control valve up to the outlet of the downstream block valve and downstream of the bypass valve.
  • If lines with different numbers and piping classifications join at a valve, the block valve should be given the higher class.  This is to ensure that the valve provides safe shutoff without violating ANSI Classifications.
  • Headers will have line numbers separate from each branch. 
  • Short connections such as vents or drains can be included under the same valve number of the line from which it ties into.