for desalination and water treatment projects
Introduction into PFD and P&ID
Piman is inseparably
and rigidly tied to a number of diagrams: the plant general layout (GL),
one-line electical diagram (OLD), process flow diagram (PFD) and process
and instrument drawing (P&ID), sometimes called piping and instrumentation
drawing. This collection of diagrams is collectively called P&ID throughout
the Piman project. Below are the sample diagrams used in a SWRO Demo Project.
Piman defines PFD as a deliverable set of documents defining the constituent
processes functionality and process sequencing (flow of processes) necessary
to guarantee the required product quality. PFD is a synopsis of the project
and declaration of the design targets set. PFD may be interpreted as a
set of guaranteed figures for the whole plant and its main and auxiliary
systems.
The constituent process description shall include required production
rates, product quality, energy efficiency, process dependability, process
flexibility and maintainability, and potential risks and environment impact.
Usually the PFD set of documents includes a graph of processes and the
process annotations. For desalination plants featuring high energy consumption
and handling continuous streams of fluid, PFD is accompanied by the Mass
and Energy Balance tables as part of the stream conditions definition,
entering and exiting the process.
What other purposes does PFD serve?
- PFD is the specification for P&ID development.
- PFD is the basic input for the Functional Requirement Specification as the major equipment modules (blocks) of the plant are already there.
- PFD has enough information to produce the GL draft.
- PFD is the input for Work Flow Diagram - the sequence of the project major activities.
For P&ID drafting any drawing program may be used: MS Visio, Adobe Illustrator, AutoCad or even MS PowerPoint. Visio is inexpensive and produces good-quality graphics in all known image formats: gif, tiff, jpg, png. Illustrator, renowned for the best quality graphics, is my favorite. The last version of AutoCad is expensive and obviously an overkill. Besides it doesn't produce good-quality images needed for internet-linked activities. (P&ID images generated with these programs are the input to Piman.)
Sometimes engineers especially those not versed in the data management
try to use specialized packages linking AutoCad to the raw database like
MS Access or Oracle. On the first glance those graphics driven programs
seem appealing: symbol and some textual data become bi-directionally linked.
But the end is the same - customization failure. The main reason is that
the P&ID graphic symbols are an abstraction - like a top of an iceberg
- hiding the wealth of information beneath the water level. This information
hierarchy and laws are unknown to the graphics driven programs. Their
customization in practice turns into hard work from scratch. Trying to
plug these programs into already existing ERP
system may be very expensive enterprise doomed to failure as the programs
in question are not built for networking.
Another point to be reckoned with is poor maintenance and service of these
programs as the dedicated staff usually does not have any knowledge in
the process engineering. Serious problem associated with the above programs
is the lack of qualified users - the engineers avoid practicing in drafting
by all means, and the draftsmen are not qualified enough to manage the
drawings database. As a consequence many engineering companies in water
industry have no internal graphics standards. Not seldom "improved" PFDs
are used instead of P&IDs for small-capacity-unit projects. As a rule
such P&IDs have no one-to-one correlation between the diagram symbol
and the database entry. It is said they don't fit into WISWIG - "What
I See What I Get" - mainstream concept of the programs in question.
And the legacy P&ID problem is still left unsolved.
There are numerous graphics-driven programs for P&ID development available
on the market
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In the lack of means for the P&ID data management, P&ID itself
turns into the project data main repository. It becomes overloaded with
various kinds of seemingly unrelated information pieces - the pump performance
data, piping and fitting sizes, ground and water levels, valve design
details (rotary valve of ball type or butterfly one?), number of solenoids
in the pneumatically actuated valves, and even the assembly drawings of
the injection nozzles for chemical solution. Attempts are being made to
standardize this information chaos - more details may be found in the
presentation
by Jeff Ratush.
The solution to this problem is straightforward once one accepts the fact
that P&ID is the worst kind of database ever known as its data cannot
be read by computer.
P&ID development criteria adopted in Piman
| Task | Task weight, % | Task criterion |
| Learning process basics | 20 | More general and simpler symbols are better |
| Equipment procurement and mechanical design | 30 | P&ID is a browser of database |
| Control design | 40 | Affinity to control logic concepts and entities |
Piman
uses multilayered P&ID approach - the upper (graphical) layer contains
only conceptual and general information about the process and its means.
The P&ID symbols provide effective shortcuts to other layers of information
- item geometry, construction and materials, control and electrical data,
and the product information. The upper layer is a "common denominator"
readily understandable by all personnel involved in the project - process
engineers, electrical and control engineers, project managers, mechanical
engineers, unit operators, technicians and drafters. It can be said that
P&ID is a roadmap of the project.