CCT Installation Procedures 4 — Questions and Answers
Question 1: When installing a vortex flow meter, what installation condition will prevent the meter from producing any output signal?
- Insufficient upstream straight run
- Reynolds number below the meter's minimum detectable threshold (Correct answer)
- Excess downstream backpressure
- Grounding the meter body to the process pipe
Correct answer: Reynolds number below the meter's minimum detectable threshold
Vortex meters require a minimum flow velocity (minimum Reynolds number) to generate detectable vortex shedding; below this threshold, output is zero.
Question 2: During installation of a pressure gauge on a steam line, what accessory is installed to protect the bourdon tube from direct contact with high-temperature steam?
- Pulsation dampener
- Root valve with bypass
- Siphon (pigtail) or condensate pot (Correct answer)
- Dual-block-and-bleed valve
Correct answer: Siphon (pigtail) or condensate pot
A siphon or condensate pot fills with condensed water that acts as a thermal buffer, preventing live steam from entering the gauge's bourdon tube.
Question 3: What grounding requirement is critical when installing a Coriolis mass flow meter to prevent measurement interference?
- The meter housing must be grounded to an isolated instrument earth separate from all other grounds
- The meter must be isolated from the process pipe to eliminate stray current paths through the fluid
- The meter flanges must be bonded to the process piping to equalize potential and prevent stray currents through the sensor (Correct answer)
- No grounding is required because Coriolis meters use mechanical vibration, not electrical fields
Correct answer: The meter flanges must be bonded to the process piping to equalize potential and prevent stray currents through the sensor
Equalizing potential between the meter and process pipe via bonding prevents stray electrical currents from flowing through the process fluid and disturbing the Coriolis drive or pick-up coils.
Question 4: A technician installs a thermowell by threading it directly into a threaded process connection. What additional step must be verified before committing the installation to service?
- Apply PTFE tape to all NPT threads and torque to 20 ft-lb
- Verify the thermowell wake frequency calculation (Strouhal number analysis) confirms no resonance with process flow-induced vibration (Correct answer)
- Paint the thermowell exterior with high-temperature coating to prevent surface oxidation
- Install a check valve downstream of the thermowell to prevent backflow
Correct answer: Verify the thermowell wake frequency calculation (Strouhal number analysis) confirms no resonance with process flow-induced vibration
Flow-induced vibration can excite a thermowell at its natural frequency, leading to fatigue failure; ASME PTC 19.3 TW wake frequency analysis must confirm adequate safety margin.
Question 5: When installing a differential pressure transmitter below the process taps on a liquid service, which configuration of impulse lines is correct?
- Slope the lines upward from the taps to the transmitter so gas can vent back to the process
- Slope the lines downward from the taps to the transmitter so liquid drains toward the transmitter and fills the lines completely (Correct answer)
- Run the lines horizontally with vent valves at every high point
- Run both legs through a common manifold before splitting to the high and low ports
Correct answer: Slope the lines downward from the taps to the transmitter so liquid drains toward the transmitter and fills the lines completely
On liquid service with below-tap transmitter mounting, lines sloped downward ensure liquid-filled impulse lines free of gas pockets that would create measurement errors.
Question 6: What is the significance of specifying the 'wetted materials' of a pressure instrument during installation engineering?
- Wetted materials determine the instrument's maximum ambient temperature rating
- Wetted materials must be compatible with the process fluid to prevent corrosion, contamination, and safety hazards (Correct answer)
- Wetted materials affect only the instrument's response time, not its accuracy
- Wetted materials must match the color coding standard of the plant piping system
Correct answer: Wetted materials must be compatible with the process fluid to prevent corrosion, contamination, and safety hazards
Incompatible wetted materials can corrode in service, contaminating the process, causing measurement drift, and potentially causing catastrophic seal failure.
Question 7: During commissioning of a newly installed 4-20 mA loop, the technician reads 3.8 mA with the transmitter at zero-percent input. What is the most likely cause?
- The loop power supply voltage is too high
- The transmitter zero is not trimmed, or there is excess resistance causing a voltage drop that reduces loop current (Correct answer)
- The transmitter is wired in three-wire mode instead of two-wire mode
- The analog input card on the DCS is configured for voltage input instead of current input
Correct answer: The transmitter zero is not trimmed, or there is excess resistance causing a voltage drop that reduces loop current
A reading below 4 mA at zero input typically indicates the transmitter zero is trimmed low or excess loop resistance is causing a voltage drop that the transmitter cannot overcome.
CCT Installation Procedures 4
When installing a vortex flow meter, what installation condition will prevent the meter from producing any output signal?