Line List, Valve List and Instrument Index Explained

Three engineering lists sit beside every P&ID (piping and instrumentation diagram). The line list records every pipe line: its number, size, fluid, start and end points, piping class, operating and design conditions, insulation and test requirements. The valve list records every manual valve: its tag, size, pressure rating, type, materials and the line it sits on. The instrument index records every instrument tag: its service, type, location, range, signal, loop and datasheet. Procurement turns them into purchases: bulk piping materials from the line list, valve packages from the valve list, and instrument packages from the instrument index.

Together with the equipment list, they turn a drawing into quantities a supplier can price.

Why this matters now

Two things are changing how these lists are made. The instrument tags they rely on follow ISA 5.1, the International Society of Automation's standard for identifying instrumentation and control functions, which was updated in 2024 with a new title, new symbols and its guidance annexes moved into separate technical reports [1]. And design data is becoming machine readable: DEXPI, the open exchange format for P&IDs, carries piping topology and attributes, not just graphics [2], so lists can be generated from the design instead of typed from it. Our P&ID reading guide covers the symbols themselves.

The line list

Process engineers usually start the line list from the P&IDs and the heat and material balance; piping engineers complete it. The Process Industry Practices standard for P&IDs, PIC001, sets out what information belongs on the drawing itself [3]; the line list holds the rest in table form.

Two columns carry regulatory weight. The process piping code ASME B31.3 has separate chapters for Category M fluid service, high pressure piping and high purity piping [4], so the fluid service category on the line list decides which rules apply. In Great Britain, the pressure equipment regulations classify piping by fluid group, maximum allowable pressure and nominal size [5], which the line list must make possible to work out.

The valve list

The valve list is usually owned by the piping engineer and generated from the P&IDs and piping classes. Control valves and relief valves often sit in the instrument index or their own lists, but are cross referenced here.

The instrument index

The instrument engineer owns the index. Tags follow ISA 5.1 conventions, where the letters show what is measured and what the instrument does: in FT 101, F is flow and T is transmitter. Each instrument then gets a datasheet; ISA publishes standard specification forms for process measurement and control instruments [6].

How procurement uses the three lists

Line list to bulk piping. Piping take offs by piping class turn lines into quantities of pipe, fittings, flanges, gaskets and bolts. Buy these as bulk packages by class, with a margin for site losses agreed with construction.

Valve list to valve packages. Group valves by type, rating and material, so each supplier quotes what it makes well. Ask for unit prices, because valve quantities change with every P&ID revision.

Instrument index to instrument packages. Group by measurement type, such as pressure, temperature, flow, level and analysers, with control valves as their own package. Each line goes out with its datasheet.

Practical example

In this illustrative example, a 12 sheet P&ID set for a solvent recovery unit produces these lists.

When revision C of the P&IDs adds six pressure instruments and one line, the index grows to 91 tags and the line list to 141 lines. Because each list has a status and revision column, procurement can see at once which packages need an addendum and which have not yet gone out.

Where ChemCapital fits

ChemCapital's P&ID to RFQ builds these lists from the design. Its DEXPI import reads equipment, instruments, valves, nozzles, pipe segments and signals where the file follows supported structures, and existing Excel or CSV lists can be imported with column mapping. Every extracted tag gets a confidence score and a review step, more than 300 tag patterns are recognised as standard, accepted items are grouped into procurement packages, and a revised drawing shows what changed side by side.

Ready to turn your P&IDs into purchase packages? Start free with ChemCapital.