L - Level
Reference for the ISA 5.1 first letter L. 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.
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Also stands for.
- Liquid level
- Interface level, when service is two-phase
Common second letters.
Transmitter, LT. Level transmitter
Indicator, LI. Level gauge or indicator
Indicator plus controller, LIC
Glass, sight gauge, LG. Mechanical level glass
Switch high, LSH
Alarm low-low, LALL. SIS trip on low level
Controller, LC, sometimes used standalone for older drawings
Example tags.
Level transmitter on loop 301
Level indicating controller for the same loop, often inside the BPCS
Sight glass on a tank or vessel, no electrical connection
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.