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

Step 1: Decode the instrument tag "PAH" using ISA instrumentation nomenclature.
Process and Instrumentation Diagrams label every instrument bubble with a short letter code following the ISA S5.1 standard. Here "P" stands for Pressure, the measured variable, and the following letters "A" and "H" stand for Alarm and High respectively.
Step 2: Combine the letters into the instrument's purpose.
Reading "PAH" as Pressure-Alarm-High means this instrument monitors the pressure inside the connected equipment and raises an alarm whenever that pressure rises above a preset high limit. It does not itself take any control action on a final control element; it only signals the operator.
Step 3: Match this against the loop shown in the P&ID.
PAH-101 sits on the gas storage tank T-101 and is connected by a dashed signal line, confirming it is an instrument signal, separate from PIC-101 (the controller that actually regulates pressure) and separate from V-101 (the downstream flow-regulating valve).
Step 4: Rule out the other options.
Option (A) describes a pressure controller, the role of PIC-101, not PAH-101. Option (C) describes a plain local pressure indicator (PI), not what the "A" for Alarm denotes. Option (D) describes regulating downstream flow, the role of the control valve V-101.\[ \boxed{\text{PAH is a Pressure Alarm High: it alerts when high pressure is detected in the tank, option (B)}} \]