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Drawing essentials · Written guide · 2 min read

Airlock, baglock, NPU and route symbols explained

An enclosure drawing needs a key that tells the reader what each symbol represents. Use these examples from the AsbestoPlan library to distinguish access chambers, equipment and routes, then check the labels against the written plan for the job.

By AsbestoPlan · Published 2026-06-16 · Updated 2026-10-01 · Planning support only — not compliance advice

Airlock and baglock symbols

Blue 2D library symbol with three airlock chambers
Airlock: a generic three-stage library example.
2D library symbol for a three-stage baglock
Baglock: label the material-transfer arrangement separately from personnel access.

An airlock symbol identifies the planned personnel access arrangement; a baglock identifies the material-transfer arrangement. The library thumbnails below distinguish these categories. Once placed, chamber size, stage count and layout are configurable: check the drawing itself, not just the thumbnail.

Use the same equipment name and identifier in the drawing key and the written documents. The pictured layouts are generic planning symbols, not recommended arrangements for a particular site.

NPU and roving-head symbols

2D library thumbnail of a generic negative pressure unit
NPU: identify the selected unit in the drawing key.
2D library thumbnail of the separate Roving Head asset
Roving Head: a distinct planning asset, not another NPU.

NPU means negative pressure unit. Its symbol marks the modelled equipment position. AsbestoPlan also includes a separate Roving Head asset; show that item and its label where required by the agreed drawing, rather than using an extra NPU symbol to represent it.

A symbol does not establish airflow, equipment suitability or the required number of units. The Extraction planning panel estimates from entered enclosure volume and assigned NPU figures; it does not measure airflow or simulate pressure. Use the site-specific design and competent review for those decisions.

DCU symbols

Top-down library thumbnail of a generic blue decontamination unit
DCU: show its planned position and use an unambiguous name.

DCU means decontamination unit. The library has several generic unit representations. Choose the visual that matches the information you are communicating, then check the model dimensions and label. A DCU and an enclosure airlock are separate items; do not give them the same identifier.

Routes and the drawing key

Exported drawing key with a dotted route, two brick finishes and object counts
An exported key from the fictional sample drawing.

A route line explains movement through the drawing. Give each route a clear purpose and distinguish routes with labels and line styles as well as colour. Follow each line from its origin to its destination and check that another reader can do the same.

This actual key excerpt shows how line style and object appearance are explained together. It comes from a fictional demonstration drawing; it is not a standard set of safety signs.

  • Match equipment identifiers and route names to the current written document.
  • Check that the visible key describes the symbols actually used on this sheet.
  • Read the exported file at the size the recipient will use.
  • After moving an item or changing a route, export a fresh file and replace the old attachment.

Further reading and examples

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