Process Flow and Instrumentation Flashcards

A differential pressure transmitter installed on an orifice plate flowmeter reads correctly at low flow rates but consistently under-reads at high flow rates. The most likely cause is:

Answer: The transmitter's square root extractor is disabled, causing a linear instead of differential output

Orifice plate flowmeters generate a differential pressure proportional to the square of the flow rate. The transmitter must apply a square root extraction to convert ΔP to a linear flow signal. If this function is disabled or bypassed, the output is linear with ΔP rather than with flow — at low flows the error is small, but at high flows the under-reading becomes severe because the relationship is nonlinear.

In a P&ID, a dashed line connecting an instrument bubble to a process line most commonly represents:

Answer: A pneumatic signal carrying 3–15 psi air

Per ISA 5.1 P&ID symbology, a dashed line denotes a pneumatic signal (typically 3–15 psi or 20–100 kPa). A solid line with slashes indicates an electrical signal, a line with dots indicates a data/software link, and a filled thermal (capillary) system uses its own distinct symbol. Misreading this convention leads to incorrect troubleshooting of signal type.

A control valve with an air-to-open (fail-closed) actuator is specified for a cooling water service. The process engineer changes the valve to air-to-close (fail-open). Which statement best describes the impact on the controller action required?

Answer: The controller must be switched from reverse-acting to direct-acting to maintain closed-loop stability

The overall loop gain sign must remain negative for stable feedback control. Reversing the valve action (from air-to-open to air-to-close) inverts the process gain sign. To compensate, the controller action must also be reversed — from reverse-acting (which was needed with the original valve) to direct-acting — so the product of gains around the loop stays negative and the loop remains stable.

A thermowell installed in a high-velocity steam line begins to vibrate destructively. The maintenance engineer calculates the Strouhal frequency of vortex shedding matches the thermowell's natural frequency. The BEST long-term corrective action is:

Answer: Replace the thermowell with one of larger bore and shorter insertion length to raise its natural frequency

Thermowell destruction by vortex-induced resonance (Von Kármán vortex street) occurs when the Strouhal shedding frequency equals the thermowell's natural frequency. The natural frequency of a cantilever increases with larger diameter (higher stiffness) and shorter insertion length. Increasing the bore and shortening the insertion shifts the natural frequency upward, away from the shedding frequency, without compromising process operation. Reducing velocity is operationally unacceptable; increasing length worsens the resonance.

A CORIOLIS mass flowmeter is used to measure the flow of a slurry. Over time, the meter reports increasing zero offset errors even after re-zeroing at no-flow. The most probable root cause is:

Answer: Erosion of the flow tubes altering their wall thickness and changing the tube stiffness asymmetrically

Coriolis meters measure mass flow by sensing the phase difference between two vibrating flow tubes induced by Coriolis acceleration. Asymmetric erosion of the tube walls (common with abrasive slurries) changes the stiffness of one tube relative to the other, introducing a permanent phase offset even at zero flow. This manifests as a zero drift that worsens progressively and cannot be corrected by re-zeroing alone — it requires tube inspection or meter replacement.

A level transmitter using a bubbler (dip tube) system in a tank of hot caustic solution reads consistently 8% higher than the actual level. The instrument technician confirms the purge gas flow rate and DP transmitter calibration are both correct. The most likely explanation is:

Answer: The specific gravity value entered in the transmitter configuration is lower than the actual process fluid SG

In a bubbler level system, the back-pressure of the purge gas equals the hydrostatic head: P = ρ·g·h. The transmitter converts measured pressure to level using the configured fluid specific gravity. Hot caustic solutions have a higher SG than water (often 1.3–1.5 depending on concentration), and their SG also changes with temperature. If the SG entered in the transmitter is lower than the actual value, the transmitter will calculate a higher level than actually exists to account for the same measured pressure, producing a consistent positive bias.