Graphics plays a key role in the process project engineering. Crenger is inseparably tied to a number of diagrams: the plant general layout (GL), 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.
PFD may be defined 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 the 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 Crenger.)
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
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 Crenger
|Task||Task weight,%||Task criterion|
|Learning process basics||30||More general and simpler symbols are better|
|Detail design||30||P&ID is a browser of database|
|HMI & SCADA design, commissioning, operation||40||Affinity to control logic concepts and entities|
Crenger 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.
To most of the P&ID experts the task to teach everybody to read P&ID (upper layer) may seem absurd bearing in mind recent heavy updates of the industry standards introducing more complexity and new abstractions (in a single layer) and a work grandiosity - IEC guys invested more than 7 million US$ and 12 years of hard work into ISO 14617 "Graphical symbols for diagrams" (ISO Bulletin March 2003).
Use-case analysis shows that the personnel with basic technical backgrounds easily handle the task of deciphering P&ID compiled of 67 generic symbols carefully sorted out by Crenger and denoting already known common-knowledge abstractions. For comparison, similar complexity abstractions - road signs include 47 warning signs, 68 direction signs, 61 information signs and 20 road markings.