P&ID Take-off                                                                                                  latest update: 2008-03-06

Introduction

Piping & Instrument Diagrams (P&ID) are the core documents of any plant. Their primary role is to represent the configuration of the plant with:

  • all plant objects with their primary class (the class that you can find in ISO 15926-4)
  • how these objects are interconnected
  • how these objects are composed (e.g. piping lines with their components, instrument loops with their components)
  • how certain objects are contained in other objects (e.g. streams in pipes and equipment, but also vessel and tank internals).

In this topic we will work out a small part of a P&ID. It shows a flow control loop that serves to keep the flow of concentrated brine (in the horizontal line in the diagram below) constant by regulating a bypass flow (the vertical line in the diagram below).

Flow Control loop F-2258

The instrument loop consists of the following components:

  • FE-2258 - a magnetic flow meter
  • FIT-2258 - a transmitter with an indicator
  • FIC-2258 - an indicating controller with High and Low alarm, mounted in the control room
  • FV-2258 - a control valve with pneumatic actuator and with a positioner

It should be noted that the representation of instrument loops greatly differs for the various plant owner/operators, and only very seldomly shows all the components. The exact configuration of instrument loops is represented on loop diagrams. The reason why they are on P&IDs is to provide an functional overview, and as an interface with:

  • the Process Dept. regarding the data of the streams through the in-line mounted components (here the flow meter and the control valve)
  • the Piping Dept. regarding these in-line mounted components (e.g. face-to-face dimensions, piping class)
  • the Electrical Dept. regarding (here) the electrical tracing of the in-line mounted components.

Since the in-line components are actually of prime importance there are P&IDs where any other set of instrument components is represented with just one balloon.

For lines (piping systems) the same applies as for instrument loops, in that P&IDs very seldomly show all its components (pipe fittings). So what we see on a P&ID is enough to understand the configuration of the plant. Details of piping systems are, for the most, given on 3D models and on Isometrics, such as:

 

What else do we see? Well:

  • the electrical tracing (will not be worked out further here)
  • two (piping) lines, the horizontal one and the vertical one

These lines consist of "segments". The ones shown are:

  • for the horizontal line, which we give the line number B14-RW-0008-4"-AC-HC1.0"E :
    1. a segment upstream of FE-2258
    2. a segment between FE-2258 and the tee-junction

      

       Note - this line number means here:

       B14 = System Locator Code

       RW = Fluid Code

       0008 = Line Sequence No.

       4" = Diameter

       AC = Line Class

       HC = Insulation Purpose

       1.0" = Insulation Thickness

       E = Heat Trace Media
  • for the vertical line, which we gave the line number B14-RW-0011-4"-AC-HC1.0"E :
    1. a segment upstream of FV-2258
    2. a segment downstream of FV-2258

These segments are, for the most, chosen for representing their physical dimensions (from the 3D model), in some cases for linking them individually to their test certificate (required in Germany and other places), etc.. In this case we ignore the segments between the control valve and the block valves up- and downstream of it. We also will not store any data yet to the segments that we did declare.

The stream data follow the line segments. Streams as objects cannot be shown on P&IDs, other than by labelling next to the (piping) lines. This is normally not done (perhaps in the future when ISO 15926 has been adopted :-)  ). Let's give the two streams the following numbers:

  • in the horizontal line: S-B14-RW-0008-0
  • in the vertical line: S-B14-RW-0011-0

System IDs

The line and stream numbers shown above are merely labels, that can (and may) change. For internal use in the Façades we need IDs that will NOT change and that only can have combinations of the numbers 0 - 9, the letters a - z and A - Z, and . - and/or _   and that shall start with a letter or an underscore _  . This is an XML requirement.

In this case we use the first part of the line numbers (e.g. B14-RW-0008) and the stream numbers as-is. The instrument loop and its components get an ID F-2258, F-2258-1, -2, etc. But you may use anything you like within the constraints shown above. What is important is that the IDs are unique in the context of the Façade they belong to.

The segments then have the ID of the line or stream plus a suffix, such as -001. Example: segment 001 of the horizontal line has a system ID of: B14-RW-0008-001 .

Normally the system IDs should not be the same as the tag, line, or stream numbers, because these may change. It is better to have totally different IDs. But in this case we have not followed that rule, so that you, as a reader, may easier recognize the objects when reading the RDF listings.

Tag, line, and stream numbers

The tag numbers, line numbers, and stream numbers are identifiers given by and meant for the users. They are attributed to "temporal parts" of the objects (see below), and may change over time. In that case the temporal part that was valid for the old number is "terminated", and the new number is attributed to a new temporal part. The system ID of the "temporal whole" remains unchanged.

Take-off

When we make a take-off of the above P&ID snippet at a certain date and time the RDF/XML listing as shown here is generated automatically.

Other information about the taken off objects is added later, mainly taken off from other applications. This is shown in a separate listing that can be found for the Facade for the :

Eventually all these sets are being consolidated in one Project Façade, and from there to the applicable Facility Façade of the plant owner/operator.

This is shown in the diagram below (click on a green Façade to get to its contents *)  ):

*) what you see then is an RDF/XML listing of a Transfer File, the actual contents are stored in triples.

Design Considerations

For this take-off the assumption was made that the P&ID software does not have a system of keeping track of changes. This means that all components and their information, for as much that information can possibly be extracted from the P&ID, are being taken off in cases where it is known to have changed or where changes are not recorded by the P&ID system.

It is important that once a system ID for the WholeLifeIndividual has been given to an object, this ID will immutably remain connected to that object. This may sometimes present a problem, for example when a piping system is modified. Whenever an object is being added  to the P&ID the software shall give it an ID that is unique in that system.

For a take-off we did define one single date-time, that is the start date-time for all temporal parts. This means that during the population of the System Façade normally only duplicate instances of WholeLifeIndividual will occur.

For each take-off we create a set of new temporal parts as if we analyze the drawing for the first time. Then there are two ways to further handle this data:

  • we simply store whatever is being taken off; depending on the frequency of take-off (which is normally high) this will result in a very large set of triples in the System Façade
  • we can also design software that compares the template instances in which the previous temporal parts play a role with the newly taken off ones. If they contain the same information (e.g. the line segment still has a size of 4"), then the newly taken off one is ignored (note that the P&ID software has no knowledge of temporal parts, so this ignoring doesn't have any adverse consequences).