DESALINATION

Extensions are data not explicitly keyed to the P&ID symbols or items. Two extensions are discussed below – P&ID spare items and P&ID instrument hookup schematics.

Spare parts & Items

Against the client expectations, P&ID items and the plant subsystems differ substantially in reliability. Spare Parts List (SPL) refers to P&ID items that are planned for repair on site. On the contrary Spare Items List (SIL) aims at the item replacement as faster and/or more economical alternative. SPL is compiled by the equipment and instrument vendors, while SIL is a direct responsibility of the plant designer.
Curiously enough, SIL has been discovered neither by the engineering science nor by EPC contractors. The desalination plant tenders do not mention SIL objectives, to say nothing of the project budget.
CP considers SIL a critical extension of P&ID and part of the plant overall redundancy strategy; without SIL identification the plant design is not considered complete. The latter may be illustrated by the following example. The MTBF value of the standard non-return valve drops by a factor of 5 if it is installed at the discharge of the DWEER energy-recovery device producing cyclic loads. Therefore, SIL should include spare valves quantity sufficient for the plant long-term operation.
CP analyzes P&ID, PFD and the plant control based on a set of predefined rules, auto-generates SIL, and trims it according to the designer preferences - budget, warranty period, etc.. Total costs of SPL and SIL may reach 6 - 8% of the installed equipment costs.

Instrument hookup

Another important extension of P&ID is Instrument Hookup Drawings and BOM (bill of materials); their scope is explicitly defined by P&ID. Current practice is to put all the information about the hookup fittings (tubing, elbows, connectors, and others) and P&ID instruments into the AutoCAD drawing. This information may refer to the assembly and manufacturing drawings, the manufacturer catalog datasheets and the project specification of instrumentation and shall be verified before submitting the hookup drawings to the subcontractor. By the author estimate, nearly half of all the 'as-made' hookup drawings (already used more than once) contain errors or misprints.
CP not only auto-generates the hookup BOM and the hookup specifications with all relevant information on demand, but also checks the applicability of the hookup drawing to the specific instrument, the fluid conditions and the materials.

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