HL,HLL,LL,LLL

From the "Glossary" located under "Tools, General"
**LL - Liquid Level (** Denotes suffix (es), for high, low, high high, low low, etc.) i.e. LLLL - Low Low Liquid Level

Added information about the different "Levels" found on P&IDs:
- HHLL = High High Liquid Level (This would normally be the highest point where an Alarm would ring or flash on the control panel in the Control room)
- HLL = High Liquid Level (this would be the top of the normal range)
- NLL = Normal; Liquid Level (this would be the mid-point of the normal range)
- LLL = Low Liquid Level (this would be the bottom of the normal range)
- LLLL = Low Low Liquid Level (This would normally be the lowest point where an Alarm would ring or flash on the control panel in the Control room)

Your next question might be; "Why does a piper need to know this?"
Piping is responsible for the proper configuration of all Gage Glass, Level Controller, Level Alarm bridle piping for all the vessels. If you don't do it right then the plant does not run properly.