H and HH

Instrumentation & Control

On an instrument tag, H is a high alarm asking an operator to act; HH is a high-high trip that acts on its own through the safety system. PAHH is a pressure trip, not a louder alarm — the distinction is the difference between a warning and a protective function.

Explained in depth

Reading a P&ID — the drawing every discipline works from

Related terms

PFD
4–20 mA
Alarm flood
Cause and effect chart
Cv
Direct and reverse acting
Ex i
Fast loop
Hook-up drawing
Impulse line
Loop check
Manifold