Process Flow and Instrumentation Flashcards

A differential pressure transmitter connected to an orifice plate reads higher than expected even though the actual flow rate has not changed. Which condition is MOST likely responsible?

Answer: Partial blockage of the low-pressure impulse line

Blocking the low-pressure (downstream) impulse line traps a static head in that leg, artificially raising the differential pressure reading and causing the transmitter to report a higher-than-actual flow. A high-pressure line blockage would cause the reading to drop. Viscosity changes affect the discharge coefficient but not dramatically in turbulent flow, and pipe diameter is a fixed installation parameter.

In a P&ID, a dashed line connecting an instrument bubble to a control valve represents:

Answer: An electrical signal

Per ISA 5.1 symbology, a dashed line between instruments denotes an electrical signal. A solid line represents pneumatic, a line with double dashes or hatching represents hydraulic, and a line with filled circles or filled squares denotes a capillary/filled-system connection. Misreading these line types is a common source of installation errors.

A control loop is operating in cascade mode. The primary (outer) controller output is feeding the setpoint of the secondary (inner) controller. If the secondary controller is placed in manual mode, the PRIMARY controller will:

Answer: Continue adjusting its output, but the secondary controller will ignore the new setpoints

When the secondary controller is placed in manual, it stops responding to setpoint changes sent by the primary controller. The primary controller, still in auto, keeps calculating and adjusting its output (the secondary setpoint), but those setpoint changes have no effect on the final control element. This is why proper cascade implementation includes bumpless transfer logic — without it, the primary output may drift to an extreme value. The primary does NOT automatically go to manual; that would require external interlock logic.

A vortex flowmeter is installed on a steam line and begins reporting erratic, low-frequency pulses well below the meter's stated minimum Reynolds number. The BEST corrective action is:

Answer: Reduce the pipe diameter at the meter location to increase velocity and Reynolds number

Vortex meters require a minimum Reynolds number (typically Re > 10,000–20,000) to shed stable vortices. Below that threshold, the Strouhal-based output is unreliable. Reducing the pipe bore at the meter (using a reducer/expander pair) increases local fluid velocity for the same mass flow, raising the Reynolds number into the meter's valid operating range. Switching to a PD meter is often impractical for steam. Increasing downstream pressure affects density but complicates the system. Using a larger bore lowers velocity and makes the Re problem worse.

On a P&ID, an instrument tag reads 'FIC-204'. The letter 'I' in the middle of the tag designation indicates:

Answer: The instrument provides an indication (local or remote display) in addition to control

In ISA 5.1 instrument tag nomenclature, the first letter is the measured variable (F = Flow), subsequent letters are modifiers and functions (I = Indicate, C = Control), and the number is the loop identifier. 'FIC' means Flow Indicating Controller — the loop measures flow, displays it, and controls it. 'Inferred,' 'interlock,' and 'intrinsically safe' are not part of standard ISA tag letter coding.

A pneumatic control valve with a fail-closed (air-to-open) actuator is specified for a cooling water service. During an instrument air failure, the valve will close, stopping cooling flow. Which justification BEST explains why an engineer would still choose this failure mode for this application?

Answer: The process equipment being cooled would be damaged more by flooding than by overheating, so shutting off cooling is the safer failure state

Valve failure mode selection is always driven by process safety analysis — specifically, which failure state causes less harm. If the cooled equipment (e.g., a water-jacketed reactor with an exothermic reaction) could be flooded or over-pressured by uncontrolled cooling water flow more dangerously than by a controlled temperature rise, fail-closed is the correct choice. This is uncommon but valid. The other options describe cost, cavitation, or regulatory arguments that are not standard justifications for overriding the typical 'fail-open for cooling' convention.