What L means on a P&ID.
A typical column or vessel carries three level instruments per service. An LT for control, an LG sight glass for local operator indication, and an LSHH switch for SIS-classified overfill protection. All three may share a loop-number variant, LT-301, LG-301A, LSHH-302. Don't deduplicate them in the I/O list. They're distinct devices doing distinct jobs, and the SIS one is wired to a different cabinet entirely.
Also stands for.
- Liquid level
- Interface level, when service is two-phase
Common second letters.
| Tag | Meaning |
|---|---|
| LT | Transmitter, LT. Level transmitter |
| LI | Indicator, LI. Level gauge or indicator |
| LIC | Indicator plus controller, LIC |
| LG | Glass, sight gauge, LG. Mechanical level glass |
| LSH | Switch high, LSH |
| LALL | Alarm low-low, LALL. SIS trip on low level |
| LC | Controller, LC, sometimes used standalone for older drawings |
Example tags.
| Tag | Meaning |
|---|---|
| LT-301 | Level transmitter on loop 301 |
| LIC-301 | Level indicating controller for the same loop, often inside the BPCS |
| LG-305 | Sight glass on a tank or vessel, no electrical connection |
| LSHH-302 | Level switch high-high, SIS trip on overfill protection |
Typical placement.
On vessel sides at the operating level, at vessel boots and sumps for low-level protection, on column sumps for reflux control. Sight glasses, LG are mechanical and not wired. They appear in the instrument index but not the I/O list. SIS-classified overfill protection, LSHH, LALL gets separate cabling and trip valves on safety-critical service.
Common questions
What does the ISA 5.1 first letter L stand for?
How is letter L typically used on a P&ID?
What second letters commonly follow L in ISA 5.1 tags?
What are example ISA 5.1 tags starting with L?
Run it on your drawings.
Upload a P&ID set and see every tag read in the convention the drawing already uses.