Short answer: Regulators, filters and mass flow controllers are drawn as in-line equipment symbols on the process line. Instruments such as pressure transducers and gauges are drawn as circles ("bubbles") with letter codes, following the ISA-5.1 identification letters (for example PT for pressure transmitter, PI for pressure indicator, FIC for flow indicating controller). Confirm every symbol against the drawing's legend.
Steps
1. Separate equipment from instruments. Equipment sits on the process line. Instruments are bubbles connected to it by a short line.
2. Regulator. This is usually a valve body with a spring or diaphragm head and a sensing line, often with a gauge next to it. It may be tagged PR, PCV or by company code.
3. Filter. This is a box or diamond with a dashed internal line, or a company-specific symbol. Some legends mark the filtration rating on the symbol or in a note.
4. Mass flow controller (MFC). Often a rectangle labeled MFC or with an FIC/FC tag, plus a signal line to the controller.
5. Instrument bubbles. Read the first letter as the measured variable (P, F, T, L) and the following letters as the function (I indicate, T transmit, C control, S switch). A line across the bubble or a different border shows where the instrument is located (field or panel), per ISA-5.1.
6. Note set points and ranges written next to the symbols, and keep them with the component data.
Worked example (generic)
The line reads: filter F-1 → regulator PR-1 with gauge PI-1 → transducer PT-1 → MFC FC-1.
- PI-1 is a local gauge, so an operator can read the regulated pressure.
- PT-1 sends a pressure signal to the controller, which is useful for interlocks.
- FC-1 controls flow from a set point.
Together this tells the mechanical designer which components need front access (the gauge face and regulator knob) and which need cable routing (PT-1 and FC-1).
Common mistakes
- Treating instrument bubbles as in-line components that need their own fittings. Some do, such as an in-line transducer, and some don't.
- Mixing up PI and PT. The difference matters for wiring and access.
- Losing the set point notes when building the BOM.
- Ignoring the flow direction arrows on the MFC and filter. Both are directional.
More steps: building a component list from the symbols
1. Walk the line in flow order and write one row per item: tag, symbol type, in-line or instrument, set point or range, and any note reference.
2. Mark the directional items (regulators, filters, MFCs, check valves, some transducers). Their orientation matters in the 3D model.
3. Mark the items that need signals: transducers, MFCs, switches. These need cable or tubing routes and connector clearance.
4. Mark the items that need operator access: gauges, regulator knobs, manual valves. Layout puts these at the front.
5. Link each row to the legend entry you used, so a reviewer can check your interpretation.
6. Flag symbols you couldn't identify and ask the drawing owner. Don't guess.
Edge cases
- Gauges with isolation. A gauge drawn with a small valve under it means a gauge isolation valve, which is a separate item.
- Dual-purpose instruments. A transducer with a local display may be drawn as one bubble with combined letters (for example PIT). It is still one physical item.
- Shared instruments. A pressure switch may be drawn once with lines to two branches. Confirm whether it is really one item or a drafting shortcut.
- Purifiers and point-of-use filters. These sometimes look like filters but have their own tags and replacement intervals. Treat them as separate parts.
- Panel-mounted vs field instruments. ISA-5.1 bubble styles show location, for example a horizontal line for a primary panel location. A panel-mounted indicator doesn't sit on the gas stick at all, so don't place it there in 3D.
- Set points in notes. Ranges are often written in a table on the drawing instead of next to the symbol. Collect them from the table.
A second worked example (generic)
A stick shows a bubble tagged PIT-2 next to the regulator outlet and a separate bubble tagged PSL-2 on the same line.
- PIT-2: pressure indicating transmitter. It is a single in-line device with a display and a signal output.
- PSL-2: pressure switch, low. It is a separate device that trips an interlock if pressure falls.
The component list therefore has two instruments, both needing signal wiring, and PIT-2 needs its display facing the operator. The note table gives the PSL-2 trip setting, which goes into the part description so the correct variant is ordered and set. Missing PSL-2 because it sits next to PIT-2 is an easy error that drops a safety interlock from the build.
FAQ
Where do the letter codes come from?
ISA-5.1 defines the identification letters. Companies may add codes of their own.
Is a purifier shown like a filter?
Often it has its own legend symbol. Check the legend.
Do I need the instrument range on the BOM?
Usually yes, as part of the part selection or description, so the right variant gets ordered.
What does a letter "S" in an instrument tag mean?
In ISA-5.1, a switch function. It is usually paired with H or L for high or low.
Do I model instrument bubbles in 3D?
You model the physical device. The bubble is only the drawing notation.
What if the legend and ISA-5.1 disagree?
The drawing's legend governs that drawing set. Note the difference for reviewers.