Question

In the P&ID shown below, which one of the following is the function of PAH in the process?

• A. to maintain a constant pressure in the vessel by throttling the pump outlet
• B. to alert when high pressure is detected in the tank
• C. to measure the average pressure in the vessel and display it locally
• D. to regulate the flow from the vessel to downstream users

Hint:

Decode the ISA tag letter by letter: P = Pressure, A = Alarm, H = High, and match that purely-alerting role against the loop shown in the diagram.

Answer 1

The Correct Option is B

P&ID instruments can be grouped by the layer of protection they belong to: the Basic Process Control System (BPCS), which continuously measures and adjusts a variable, and independent alarm or safety layers, which only watch a variable and inform, without themselves adjusting anything.

PIC-101, together with PY-101 and PY-102, forms the BPCS pressure-control loop that continuously measures tank pressure and adjusts a final control element.

PAH-101, by contrast, is drawn as a separate instrument bubble connected only by a dashed signal line, with no connection to any final control element. Under ISA nomenclature, P-A-H expand to Pressure, Alarm, High: this instrument's job is to independently watch the tank pressure and trigger an alarm when it exceeds a preset high threshold.

Matching this role against the four options confirms that PAH alerts the operator when high pressure is detected in the tank, option (B).